Mineralogy, petrology, and genesis ofthe Lucifer manganese deposit, Santa

Rosalia area, Baja California Sur, Mexico

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Authors Freiberg, Daniel Arthur

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MINERALOGY, PETROLOGY, AND GENESIS OF

THE LUCIFER MANGANESE DEPOSIT,

SANTA ROSALIA AREA, BAJA CALIFORNIA SUR, MEXICO

byDaniel Arthur Freiberg

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCEIn The Graduate College

THE UNIVERSITY OF ARIZONA

1979

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of re­quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg­ment the proposed use of the material is in the interests of scholar­ship. In all other instances, however, permission must be obtained from the author.

SIGNED:

APPROVAL BY THESIS DIRECTORThis thesis has been approved on the date shown below:

WILBERTGeosciences

7 / ? 7 ?Dare

ACKNOWLEDGMENTS

My foremost acknowledgments and deepest gratitude go to my

parents, Dr. and Mrs. Andrew Freiberg, for their unfailing support, both

financial and moral, which made this thesis possible.

- I would like to thank Dr. John Guilbert, my major advisor, for

introducing me to the problems of the Boleo district and for his assis­

tance and helpful criticism. I am also grateful to Dr. Richard Beane

and Dr. Arend Meijer for helpful suggestions and many useful consulta­

tions. It was upon the advice of Dr. Beane that my study of the Boleo

district focused upon the Lucifer deposit.

The Compahla Minera de Santa Rosalia was extremely helpful and

cooperative during my visits to Santa Rosalia, providing me with, among

other things, maps, an assistant, lodging and, more than once, help with my truck. I am especially grateful to Ing. Pedro Ortiz.

Use of the scanning electron microprobe quantometer was made

possible through the kindness of Dr. Michael Drake and the Department of

Lunar and Planetary Sciences of The University of Arizona. Tom Teska provided much valuable assistance in its use. I owe special thanks to Hort Newsom for the many long and tedious hours spent showing me how to use the microprobe.

Among the many students of the Department of Geosciences of The

University of .Arizona who have helped me out during the course of this project, I would especially like to thank Fleet Koutz, Bob Brakenridge,

iii

Rick Gottschalk, and Greg McNew for valuable suggestions and for reading

early drafts of the manuscript.

iv

TABLE OF CONTENTS

LIST OF ILLUSTRATIONS.......... vii

LIST OF T A B L E S ............................................... x

A B S T R A C T .................................- ................ xi

INTRODUCTION................................................. 1

Location, Production, and Access ......................... ' 2Previous Studies . . . . ................................. 5

STRATIGRAPHIC SETTING AND PETROLOGY OF THE LUCIFERMINE REGION.................... 6Quartz Monzonite (Cretaceous) ............................. 6Comondd Volcanics (Middle(?) and Late Miocene) .......... 9Boleo Formation (Early Pliocene) ......................... 19

Basal Conglomerate................................... 20Tuffaceous Limestone ................................. 21Tuffs and Tuffaceous Conglomerate ..................... 23

Pliocene and Quaternary Sedimentary Units Overlyingthe Boleo Formation.......... 33

Tres Virgenes Volcanics (Pleistocene and Recent) ........ 34STRUCTURAL GEOLOGY ........................................... 37

General Structural Features of the Santa Rosalia(Boleo) Region ....................................... 37

Structural Features of the Lucifer M i n e .................. 38MANGANESE D E P O S I T S ................ 46

General Features of the Lucifer Manganese Deposit ......... 46Mineralogy of the Lucifer Deposit ......................... 56

General Considerations ............................... 56Composition of the Cryptomelane M i n e r a l s ............ 68Paragenetic Relationships in the Lucifer

Manganese O r e ..................................... 75

ALTERATION IN THE LUCIFER A R E A ............................... 78

Alteration of Mafic Silicates . . . . . . . ........ . . . 78Alteration Associated with Manganese Mineralization . . . . 83

Page

v

TABLE OF CONTENTS— Continued

Page

Whole-rock Compositional Analyses of Fresh andAltered Rocks.................. 84

DISCUSSION....................................................... 94

ORIGIN OF THE LUCIFER DEPOSIT.................................... 102

Geologic Criteria ....................................... . 102Tectonic Setting and Related Processes of Ore

Deposition................................. 112Geochemical Processes of Formation ..................... 118

RELATION OF THE LUCIFER DEPOSIT TO COPPER DEPOSITS AND OTHER MANGANESE MINERALIZATION IN AND NEAR THE SANTA ROSALIA (B0LE0) DISTRICT .............................. 134

APPENDIX I: GENERAL MICROPROBE INFORMATION .............. 142

APPENDIX II: MICROPROBE ANALYSES (WEIGHT PERCENT) OFLUCIFER ORE SAMPLES............................. 145

APPENDIX III: MOLECULAR PROPORTIONS OF CRYPTOMELANE"A"-SITE CATIONS AT 8 (Mn + Fe + Al) ........ 159

APPENDIX IV: " LOCATIONS OF SAMPLES DISCUSSED IN TEXT . . . . . 162

LIST OF REFERENCES ............................................... 163

vi

LIST OF ILLUSTRATIONS

1. Location Map of Area Studied........................... 3

2. Northern Portion of the Boleo District ................. 4

3. Correlation of Lucifer and Boleo StratigraphicSections . . . . . ............ 7

4. Geologic Map of the Lucifer Mine and SurroundingArea, Baja California, M e x i c o .............. .. . . I n pocket

5. Geologic Cross Section of the Lucifer Areaalong Line A - A * ................................... 8

6. Boleo Basal Conglomerate Overlying ComondtiAgglomerate........ .............................. 11

7. Photomicrographs of Comondu Vitrophyric-texturedVolcanic Rock (Sample LZ22) ........ . . . . . . . 14

8. Photomicrographs of Comondil Felsophyric-texturedVolcanic Rock (Sample LX10) 17

9. Boleo Tuffaceous Limestone Overlying BasalConglomerate...................... 22

10. Remnant of the Manganese Ore Manto OverlyingBoleo T u f f s ....................................... 27

11. Photomicrographs of Boleo Tuff with Original,Non-intraformational Texture (Sample LP3A) . . . . 29

12. Photomicrograph of Boleo Tuff with IntraformationalTexture (Sample LZ20B) ............................. 30

13. Quaternary Conglomerate Overlying StratifiedManganese O r e ..................................... 35

14. . Exposure of Partly Buried Ridge of Comondu Volcanics . . 39

15. Lucifer Mine, Looking West up Wash in the NortheastPart of the Study A r e a ............................. 40

Figure Page

vii

viii

16. Dip-histogram Rosette for Bedded Comondu VolcanicUnits............ 42

17. Strike-histogram Rosettes for Fractures in theLucifer A r e a ............................ 44

18. Lucifer Ore Sample with Distinctive ClasticAppearance.................................... 49

19. Detail Map of the Exposure of the Ridge of ComonduVolcanics near the Southern End of the Orebody . . . 51

20. Contact of Mineralized Boleo Basal Conglomerate andComondtS Breccia at the Exposure of the ComonduRidge........................ 53

21. Mineralized Comondd Bedded Volcanics Grading intoBreccia........................................ 54

22. Manganiferous Laharic Breccia ............ 54

23. Boleo Tuffaceous Conglomerate Overlying Manganese Ore . . 55

24. Stratified Manganese Ore Overlying Altered BoleoBasal Conglomerate ............................. 57

25. Photomicrograph of Lucifer Ore Polished SectionIllustrating the Growth of Pyrolusite AroundCryptomelane ....................................... 61

26. Photomicrograph of High-K Cryptomelane SurroundingEarly Coronadite.............................. 63

27. Photomicrograph of Botryoidal Colloform Cryptomelane . . 65

28. Photomicrograph of Pisolitic Cryptomelane Broken bySiliceous Gangue ................................... 65

29. X-ray Backscatter Scanning Images of LuciferManganese O r e .................................. 66

30. Plot of Univalent vs. Divalent Cations in LuciferCryptomelane "A" Sites

'LIST OF ILLUSTRATIONS— Continued

Figure Page

71

lx

LIST OF ILLUSTRATIONS— Continued

Figure Page

31. Pb-K-Ba Ternary Diagram ................................. 72

32. Na-K-Ca Ternary Diagram ................................. 74

33. Paragenetic Sequence of Mineral Formation inLucifer O r e ........................ 76

34. Photomicrograph of a Sample of the Unaltered Core ofa Boulder of Comondu Volcanics in MineralizedBoleo Basal Conglomerate (Sample L X 1 ) ................ 90

35. Photomicrograph of a Sample of the Altered Rim of aComondu Volcanic Boulder in Mineralized BoleoBasal Conglomerate (Sample L X 3 ) ..................... 92

36. Hypothetical Phase Relations Involved in the Alterationof Comondu Rocks at the Lucifer M i n e .............. 124

37. Activity Diagram Depicting Phase Relations Involvedin the Alteration of Comondti Rocks in the Lucifer Area in Terms of the Variables Log (a^ Qa2+/and Log (a^+/ay+) at Quartz and H^O Saturation . . . 127

38. Eh-pH Diagram Depicting Stability Relations amongSome Manganese Compounds . . . ............. 131

LIST OF TABLES

1. Chemical Composition, Molecular Norm, and ModalComposition of Selected Comondii Volcanic Rocks . . . 13

2. Chemical Composition (Weight Percent) of TwoSamples of Boleo T u f f ............................. 32

3. Average and Range of Lucifer Manganese-oxideMicroprobe Analyses ............... 59

4. Average and Range of Molecular Proportions of"A"-site Cations in Lucifer Cryptomelane Mineralswith Respect to 8 [Mn + Fe + Al] .................. 69

5. Microprobe Analyses of Fresh and Altered Mafic SilicateGrains in Comondu Volcanic Rocks (Weight Percent) . 80

6. Microprobe Analyses of Two Partly Altered AugiteGrains............................................. 82

7. Microprobe Analyses of Groundmasses of SeveralSamples of Comondii Volcanics (Weight Percent) . . . 85

8. Uncorrected Chemical Composition (Weight Percent)of Comondd Volcanic R o c k s ........................ 86

9. Corrected Chemical Composition (Weight Percent)of Comondii Volcanic R o c k s ......................... 87

10. Chemical Composition of Comondu' Volcanic RocksExpressed in Grams Per Cubic Centimeter . . . . . . 88

11. Weight Percent Fe, Mn, and Fe/Mn of Unaltered andAltered Comondu Volcanic Rocks, Boleo Tuffs,and Lucifer O r e ................................... 119

12. Equations and Slopes of Field Boundary Reactionsfor the System Ca0-K20-Al20g-Si02-H20 126

13. Microprobe Analyses for Potassium, Barium, and Leadin Comondu Volcanic Rocks (Weight Percent) ............ 129

14. Partial Average Compositional Analyses (Weight Percent)of Lucifer Manganese Ore and Boleo Copper Ore . . . 139

Table Page

x

ABSTRACT

The Lucifer manganese mine is situated in the northwest part of

the Boleo copper district. The manganese horizon generally occurs near

the top of the lower Pliocene Boleo tuffs, to which it is conformable

and probably syngenetic. The ore consists principally of pyrolusite

and minerals of the cryptomelane-hollandite-coronadite isostructural

series. Structural features and the geometry of the deposit, as well

as manganese mineral compositions, indicate that the manganese emerged

from a hot spring source located to the south, where the orebody and

hosting sediments onlap a mineralized remnant of the Miocene Comondu volcanics.

Interaction of hot acid solutions with Comondu rocks presumably

resulted in leaching of manganese, silicon, potassium, and other elements

to contribute to the formation of the manganese deposit. .Depletion of

manganese in alteration products of Comondu mafic silicate minerals sug­gests that they may have been the major source of manganese. The Comondu

volcanics were subjected to structural deformation probably related to rifting of the proto-Gulf of California which resulted in an extensive fracture system that apparently served as channel-way for the ore solutions.

xi

INTRODUCTION

The Lucifer manganese deposit is a generally stratiform manto of

high-grade manganese oxide ore deposited upon and within a sandy tuff

unit of the lower Pliocene-age Boleo Formation. The purpose of this

study is to examine the mineralogy and petrology of the orebody and host

rocks at the Lucifer mine, with the ultimate goal of interpreting the

genesis of the deposit in light of contemporary concepts of tectonic set­

tings and mineralization processes.

In a major study of the Lucifer deposit conducted in the 19401s,

Wilson (1949) concluded that the deposit originated by epigenetic hydro-

thermal replacement of the tuffs which host the manganese. The alterna­

tive possibility that the deposit is largely syngenetic with the

sediments in which it occurs, having originated from a hot spring source,

is considered in this study.

Considerable evidence in support of a syngenetic-sedimentary

origin for the Lucifer deposit stemmed from re-evaluation of strati­

graphic, structural, and geometrical features of the deposit, as pre­sented in Wilson1s study, and from remapping the mine region. The pos­sible genetic significance of both the tuffs which host the manganese oxides and the formations which underlie the tuffs warranted detailed mineralogic and petrologic examination of these units involving thin

section and electron microprobe studies and detailed mapping of a promi­

nent mineralized exposure of the Miocene volcanics. Another major as­pect of this study has been an investigation of the manganese mineralogy

1

involving microprobe analysis as well as polished section and x-ray

examination in the hope that further insight into the genesis of the

Lucifer deposit would result.

A final consideration of this study was the tectonic setting of

the Lucifer deposit and its possible bearing on the ore depositional

processes. This setting, along an active spreading center, was be­

lieved to be especially worthy of consideration in light of current in­

terest in mineralization processes that occur in the Red Sea, the

African Rift, and along mid—ocean ridges.

Location, Production, and AccessThe Lucifer manganese mine is situated 12 km northwest of the

town of Santa Rosalia, Baja California Sur, Mexico, approximately 6 km inland from the Gulf of California (Figure 1). The mine was in operation

from 1941 until the late 1950*s, producing more than 300,000 tons of man­

ganese (Wilson, 1956a). The Lucifer mine is located in the extreme

northwestern part of the significant Boleo copper district, of which

Santa Rosalia is the center. The location of the Lucifer mine with re­

spect to the currently active copper mines in the district, and to the town of Santa Rosalia, is shown in Figure 2. The total extent of the mineralized area in the vicinity of Santa Rosalia is approximately 150 square kilometers. Since production began in the 1870’s, the Boleo dis­trict has yielded over 700,000 tons of copper, as well as subordinate

zinc and silver (Wilson, 1955).The Lucifer mine may be reached by a dirt road through Arroyo

del Infierno intersecting the Trans—peninsula (Baja) highway north of

2

3

28--

oreo inFigure 2

Cerro de |$l:La Relorma _Tertu|i

Lit Tret & \Vfigenei

|y >-LUCIFER< ^Ly^TMANGANESE $# Ignacn MINE

SANTA ROSALIA

lsl> San Mircos

Punta Chixti-- 27'

Penta Concepelfn

10 I SO Kllemeters

____ SO Miles

Figure 1. Location Map of Area Studied. — Modified from Wilson (1955).

Figure 2. Northern Portion of the Boleo District. — Modified after Wilson (1955, Plate 2).

5

Santa Rosalia. The road leads to the remnants of the old town of Luci­

fer, from which the mine may be reached by climbing up the side of the

Arroyo.

Previous Studies

Investigations of the Boleo district as a whole, and the Lucifer

deposit in particular, conducted by Wilson of the U. S. Geological Sur­

vey in the 1940's and early 1950's are the major source of information

on the historical background, stratigraphy, structure, and economic ge­

ology of the district (Wilson, 1948, 1949, 1955). Wilson recognized the

importance of the pre-ore topography in controlling the distribution of

the ore-bearing sediments and the mineralization itself in the district.

However, his hydrothermal replacement theories for the origin of both

the copper deposits and manganese deposits of the district fail to take .

into account some characteristics of the deposits in a satisfactory

manner (Nishihara, 1957; Pelissonnier, 1965; Schmidt, 1975). These lat­

ter authors presented theories on the Boleo copper deposits based upon

current concepts of ore deposition in sedimentary environments. None of

these investigators, however, presented new data on the Lucifer deposit, nor did they propose origins specifically for that deposit.

Discussions of the origin of manganese mineralization which ap­peared in the late 1950's and I960's and suggested modifications of Wilson's theories on the origin of Lucifer were presented by Park (1956),

Hewett and Fleischer (1960), and Hewett (1966).

STRATIGRAPHIC SETTING AND PETROLOGY OF THE LUCIFER MINE REGION

The stratigraphic column of the Lucifer mine region correlated

with the general stratigraphy of the Boleo copper district is presented

in Figure 3. A revised geologic map of the Lucifer mine area is pre­

sented in Figure 4 (in pocket), modified after Wilson (1949, Plate 40).

Figure 5 is a cross section constructed along line A-A* in Figure 4.

Descriptions of the stratigraphy of the Boleo district are to be found

in the studies by Wilson (1948, 1949, 1955). The discussion which fol­

lows emphasizes results of mapping and petrologic studies conducted in

the course of this investigation of the Lucifer deposit.

Quartz Monzonite (Cretaceous)

The basement in the vicinity of Santa Rosalia is composed of quartz monzonite, of affinity to the Mesozoic Peninsula batholith com­

plex which comprises the main plutonic mass of Baja California (Wilson, 1955; Gastil, Phillips, and Allison, 1975). A Cretaceous K-Ar date of

91.2 ±2.1 m.y. obtained from the nearby La Reforma volcanic complex (Figure 1) is consistent with dates determined elsewhere for the age of

the Peninsular batholith (Schmidt, 1975). Only three small outcrops

were known to Wilson in the district, all of which are located in Arroyo de las Palmas in the northeastern part of the district (Figure 2). Pre­

sumably, the batholithic complex underlies the entire district.

6

STRATIGRAPHIC SECTION LUCIFER MANGANESE DEPOSIT

GENERALIZED STRATIGRAPHIC SECTION BOLEO COPPER DISTRICT

*1ALLUVIAL a COLLUVIAL -

SEDIMENTS TRES VIRGENES VOLCANICS

“ a!

LUZUJoo

BOLEO FORMATIONTuf la c eo u s co n g lo m erate

w i th l u l l lenses Basal , u ( , _ M a n a anei e_or*v=4j

Tullaceous llmeslon Basal conqlomerale

COMONDU VOLCANICS

intermixed basaltic- andesitic lava flows,

volcanic breccia, agglomerate, crystal tuff, ash flows

* — conglomerate B sandstone C *-C R E TA C E O U S

TRES VIRGENES VOLCANICS Ai i uv.r.a i u\7b LAKE sfo. SANTA ROSALIA FORMATION •INFIERNO FORMATION conglomerate 8 sandstone

GLORIA FORMATION conglomerate B sandstonetu ffore bed 0 conglomerate O

k tu ff x ore bed I •conglomerate I - tuff

<-ore bed 2 •conglomerate 2

BOLEO FORMATION

f-ore bed 3 -conglomerate 3,gypsum

fCore bed 4 -conglomerate 4-tuffaceous limestone -basal conglomerateCOMONDU VOLCANICS

mainly basalt 8 andesite flows 8 pyroclastic volconlcs

QUARTZ M O NZO NITE

siSImUJzUJoo□CL

htr<ui

UJ

sog5

Figure 3. Correlation of Lucifer and Boleo Stratigraphic Sections. — Modified after Wilson (1949; 1955, Figure 9).

ELEV

ATI

ON

A

BO

VE

SE

A

LE

VE

L

(fe

et)

1300

1200

"-■eIOOO

900- -

800- -

700- :

600--

i S S

EXPLANATION QuatemoryA lluvial-C olluvial Sediments

Tires Vlrgenes Vo Iconics

___ PlioceneTUffaceous Conglomerate

T u ffs\m Basal Conglomerate

Miocene |> % Z| Comoodd Volcanics

____tMain Manganese Ore Horizon

100 0

Figure 5. Geologic Cross Section of the Lucifer Area along Line A-A'.

,9

Comondu Volcanics (Middle(?) and Late Miocene)

The Comondu volcanics are the oldest rocks significantly exposed

throughout most of the Boleo district. At the type area, near the town

of Comondu, 150 km south of Santa Rosalia (Figure 1), the formation con­

sists mainly of terrestrial sediments, but grades gulfward into predomi­

nantly basaltic breccia. The ComondCi volcanics reach an overall

thickness of at least 500 m in the Boleo district. In the high ranges

south and west of Santa Rosalia, well over 1000 m of Comondu volcanics

may be present.

Depositional trends within the Comondu are suggestive of a source

to the east, probably near or within the present area of the Gulf of

California, as indicated by: 1) the westward thinning of the formationacross the peninsula, and 2) lithologic changes from predominant volcan­ic flows and pyroclastics along the gulf coast to sandstone and conglom­

erate to the west (Wilson, 1955; Karig and Jensky, 1972).The age of the Comondu is based mainly upon their stratigraphic

position between the lower Miocene Isidro Formation and lower Pliocene

Boleo Formation, both dated by fossil evidence (Wilson, 1955). There

are indications, however, that related volcanism occurred as early as

upper Oligocene; that is, greater than 28 m.y. (Karig and Jensky, 1972).

Comondu volcanics are exposed over large parts of the Lucifer mine region, in particular, to the south and east of the orebody (Figure

4). Over 350 m of Comondu volcanic rocks are exposed at Lucifer; as indicated in Figure 3, this unusually large section is at the expense

of the upper units of the overlying Boleo Formation and more recent

10

sediments which either never were deposited in the region or were re­

moved by erosion.

The Lucifer deposit lies along the north flank of a partly bur­

ied ridge of Comondta volcanics, an apparent offshoot of Cerro del In-

fierno, located to the southeast of the mine (Figure 2). Although not

mineralized to an economic extent, the Comondu is mineralized and capped

by manganese oxides where the orebody abuts them.

The Comondti, at Lucifer, consists of a variety of lava flows and

pyroclastic flows, tuffs, agglomerates, and breccias displaying a wide

variety of colors and weathering characteristics. These lithologic

types are complexly interlayered, with contacts that vary, both vertic­

ally and horizontally, from sharp to gradational, irregular, or obscure.

The implication is that the Comondu volcanics have had a rather complex

history of cooling and syndepositional weathering and alteration which

has played a large part in influencing present-day outcrop and petro­

graphic characteristics. Comondii agglomerate overlain by Boleo Forma­

tion basal conglomerate is illustrated in Figure 6.Petrographic studies indicate that two distinct lithologic types

occur within the Comondu volcanics at Lucifer, based on textural and min-

eralogical criteria. One type, most aptly described as felsophyric- textured, consists predominantly of generally highly aligned plagioclase

microlites with relatively little interstitial material and few pheno- crysts. The second lithologic type may be described as vitrophyric—

textured, and is composed of large plagioclase and pyroxene grains in a glass-rich matrix that constitutes most of the rock.

11

v* % < . \j- . - v ' - . y

’ ' Avr

Figure 6. Boleo Basal Conglomerate Overlying Comondu Agglomerate. — The irregular erosional nature of the pre-Boleo surface is apparent.

12

Analyses of representative samples of these lithologic types,

selected as among the least altered specimens available, are presented .

in Table 1, along with molecular normative mineralogies calculated from

these analyses using the method of Barth (1962) with modifications and

assumptions as stated. Petrographic modal analyses are also presented

in Table 1. Compositionally, all of the Lucifer Comondti volcanic sam­

ples analyzed are most aptly described as low-silica, or basaltic ande­

sites (Williams, Turner, and Gilbert, 1954; Taylor et al., 1969).

A comparison of the compositions of the vitrophyric-textured

samples (LX11 and LZ22 in Table 1) with the felsophyric-textured sam­

ples (LP5A and LX10) indicates considerably greater concentrations of

CaO and MgO in the vitrophyric samples; the felsophyric samples have

somewhat higher Na20 and AI2O3 contents. These compositional differences

are reflected in the normative and modal analyses presented in Table 1.

Vitrophyric-textured rocks tend to predominate lower in the Co-

mundu section at Lucifer. They occur as poorly stratified flows, mas­

sive crystal tuff units, and as the clasts of agglomerates apparently derived from these lithologies.

Microscopically, the vitrophyric flows, massive tuffs, and ag­

glomerates are all extremely similar. They are composed of highly fresh, often very large crystals up to 7 mm long, of labradorite or calcic ande-

sine, augite, and orthopyroxene in a dark colored glassy matrix (Figure

7). The grains are generally well formed and euhedral, although some­

what rounded in some cases. Ophitic intergrowths of pyroxenes may be seen partially altering to limonite and various silicate alteration

13

Table 1. Chemical Composition, Molecular Norm, and Modal Composition of Selected ComondCi Volcanic Rocks. ___

LP5A LX10 LX11 LZ22Chemical Composition3 (oxides, weight %)

Si02 54.20 55.80 52.00 55.20AI9O3 17.30 16.90 14.80 16.00CaO 7.11 6.46 7.64 7.01MgO 4.44 3.83 6.43 5.34Na20 3.73 4.09 2.78 3.50k 2o 0.86 1.71 0.99 2.34FeO 7.34 6.66 6.96 6.56MnO 0.11 0.10 0.13 0.12Ti02 1.05 1.19 0.94 1.00P2°5 0.41 0.48 0.30 0.36L.O.I. 3.53 3.15 5.76 3.08Sum. 100.80 101.20 99.50 101.30Ba(ppm) 1100 1400 800 1300

quartzNorm6" (%)

9.20 8.30 8.80 6.00orthoclase 5.00 10.50 6.50 14.00albite 34.50 37.50 27.00 32.00anorthite 29.00 23.00 26.50 21.50diopside 4.00 4.80 10.00 9.20enstatite 10.40 8.60 14.20 10.60magnetite 3.90 2.70 3.60 3.00ilmenite 1.40 1.80 1.40 1.40hematite 1.30 1.70 1.50 1.50apatite 0.80 1.10 0.50 0.80

plagioclase feldspar0Mode (volume %)

70-80 70-80 20-25 15-20orthopyroxeneclinopyroxene 1-3 4-6 6-10

6-10 10-15disseminated opaque material 3-5 6-10 1 6-10

groundmass^ 10-15 8-12 55-65 55-65altered silicates 3-5 3-5 1-2 2-4aThese and all subsequent whole-rock analyses analyzed at X-ray AssayLaboratories, Ltd., Don Mills, Ontario, Canada. Samples sent as pulpsor crushed. LP5A = felsophyric-textured. volcanic breccia clast; LX10 =felsophyric-textured bedded flow volcanic; LX11 = vitrophyric--texturedagglomerate clast; LZ22 = vitrophyric-textured bedded flow volcanic. All samples collected at least 100 m from nearest existing outcrop of manganese ore horizon.^After method of Barth (1962), modified to present normative diopside rather than Barth's WO component. Assumed FegOgiFeO = 2:1, based on average Comondu "low-Si" andesites of Demant (1975).cIncludes phenocrysts and microlites.^includes fresh, altered, and limonitic glass and clay.

14

Figure 7. Photomicrographs of Comondu Vitrophyric-textured Volcanic Rock (Sample LZ22). — Note that fresh augite occurs adja­cent to and rims altered portions of grains. Diameter of field is 3.82 mm.

15

products. The crystals, crystal fragments, and alteration products make •

up from one-fifth to greater than one-third of some specimens. The

groundmass, which comprises most of the rock, generally consists of brown

or gray siliceous glass partially altering to clay. In thin sections of

some vitrophyric-textured samples, ubiquitous limonitic disseminations

are distinguishable and are particularly abundant around altered mafic

silicate minerals. In other thin sections, individual limonite grains

cannot be distinguished, and much of the groundmass takes on a nearly

opaque appearance, due presumably to uniform distribution of submicro-

scopic iron oxides mixed with the volcanic glass of the groundmass. The

groundmass may contain abundant, tiny, oriented plagioclase crystallites

which tend to display definite flowage around large grains.

Poorly-bedded vitrophyric-textured units appear to be the most

common type among the Comundfi volcanics present in the Lucifer region.

Of possible ash flow origin, they tend to be highly fractured, with many

of the fractures running subparallel to barely discernible flow bands.

Crosscutting of fractures causes these units to weather with a blocky appearance. In many cases, the blocks apparently became separated and

enmeshed within a tuffaceous matrix to take the appearance of volcanic

breccia. The poorly-bedded vitrophyric units range in color from dark gray to dark brown, or they may take on a reddish-brown appearance.

They are generally more vesicular than the well-bedded felsophyric flows,

and are more likely to be weathered with a rough, gnarly appearance.

Pyroclastic units with vitrophyric-textured clasts display con­

siderable variation in outcrop appearance, particularly in coloration.

16

The clasts may range from gravel to boulder-sized, and may actually

grade into a grayish-brown, knobby-weathering rock unassociated with a

tuffaceous matrix. The clasts are usually highly vesicular, and in many

cases are scoriaceous. The vesicles may be filled with clay minerals,

calcite, or zeolite minerals.

The agglomerate may be encrusted by crystal tuff masses several

meters thick which are very similar petrographically to the agglomerate

clasts. These encrustations lack the orthogonal fracturing and blocky

appearance of the bedded vitrophyric units. They have a distinctive,

highly weathered appearance; and possibly due to vulnerability to weath­

ering, outcrops of this lithology are rare and scattered in the Lucifer

area. They tend to be highly vesicular, even scoriaceous in places.

Crystals are clearly visible in hand sample, with lengths up to 0.75 cm.

Felsophyric-textured units appear to predominate higher in the

Comondu section at Lucifer, and are the units overlain by the Boleo For­

mation, including the manganese orebody. Felsophyric-textured rocks oc­

cur as probable lava flows, as well as the clasts of agglomerates and breccias apparently derived from these flows.

In thin section, the felsophyric-textured volcanics are charac­

terized by generally trachytic or pilotaxitic textures, as illustrated

in Figure 8. The plagioclase, which for the most part is extremely

fresh, was determined to be labradoritic in composition. The feldspar

microlites tend to be elongate, averaging 0.05 mm wide and up to 1.0 mm

long. Interstitial material constitutes less than 15% of the rock and

is composed largely of dark glass altering to palagonite or smectitic

17

Figure 8. Photomicrographs of Comondu Felsophyric-textured Volcanic Rock (Sample LX10). — Diameter of field is 3.82 mm.

18

clay. Mafic silicate minerals, most or all of which have been altered to

to limonite and various silicate alteration products, rarely constitute

greater than 5% of the rock. Whenever unaltered mafic silicates are

seen associated with these alteration products, they appear to be pyrox­

enes; no olivine was conclusively identified in any of the thin sections

examined. Minute disseminated opaque material, probably titanium-rich

magnetite or hematite, constitutes perhaps as much as 10% of some samples.

Beds in the well-stratified felsophyric-textured units commonly

average 2-6 cm in thickness. They tend to weather light gray or tan,

although their true color is considerably darker. Their fabric tends to

be smooth and tight, although some vesicularity is not uncommon. A pro­

gressive increase in fracturing from nearly unfractured stratified rock

to a resulting breccia is often discernible over as short a distance as

1 m. The breccias and agglomerates are considerably more widespread

than the bedded flows. These pyroclastic rocks are poorly indurated and

are composed of angular or rounded clasts up to 0.1 m in diameter set in a coarse, sandy, tuffacecus matrix. The clasts resemble the bedded flows

in hand sample and in thin section. The degree of rounding of the clasts,

their fabric, sorting, and weathering characteristics are all highly

variable, but microscopic characteristics are fairly uniform. Agglomer­

ates and breccias occurring relatively far from flows appear to demon­

strate greater textural variety, possessing textures that may range

from trachytic or pilotaxitic to hyalopilitic.

19

Fractures and bedding planes in both the vitrophyric-textured

units and the felsophyric-textured units may be filled by calcite or,

less commonly, by chrysocolla and manganese oxides. The matrix of ag­

glomerates or breccias may be impregnated by similar mineralization, and

even clasts may be slightly mineralized along cracks or vesicles.- Miner­

alization of this nature only occurs on a significant scale where the

Lucifer manganese orebody terminates against Comondu volcanics near the

top of the Comondu section. There, the felsophyric flows and breccias

have been pervasively mineralized by manganese oxides as will be dis­

cussed later in this report.

Boleo Formation (Early Pliocene)

The Pliocene sediments in the vicinity.of Santa Rosalia and else­

where in Baja California are separable into three distinct formations

distinguished by unconformities and characteristic faunal assemblages

that are believed to be of early, middle, and late Pliocene age (Figure

3). Only the early Pliocene Boleo Formation is present at the Lucifer

mine, and it is the only economically mineralized formation in the Santa

Rosalia area. The middle Pliocene Gloria Formation and late Pliocene

Infierno Formation can be found overlying the Boleo Formation east and

south of Lucifer.The Boleo Formation overlies the Comondu volcanics with marked

angular unconformity throughout most of the Santa Rosalia region. The

formation extends inland from the gulf coast for a maximum distance of

6-10 km, where it onlaps Comondu volcanics in the Sierra de Santa Lucia

20

(Figure 1). The Boleo Formation extends northward into the La Reforma

volcanic complex (Schmidt, 1975) (Figure 1). The formation may be traced

southward at least 30 km to the San Bruno Plain (Wilson, 1955).

The Boleo Formation is less than 100 m thick at Lucifer.

Throughout the Boleo district, the formation ranges in thickness from

50-250 m (Wilson, 1955). The base of the formation, which generally is

less than 20 m thick, is composed of a basal conglomerate, a thin, but widespread tuffaceous limestone, thick scattered lenses of gypsum, and a

few fossiliferous sandstone lenses. The last two are absent in the Luci­

fer mine area. The main part of the Boleo Formation overlies the basal

units and consists of cyclically deposited tuffs and tuffaceous conglom­

erate, of which only the lowermost cycle is present at Lucifer.

Basal Conglomerate

The basal conglomerate occurs in scattered lenses throughout the

Boleo district, generally overlying the steep slopes associated with

areas of high relief in the.pre-Pliocene paleosurface (Wilson, 1955).

A conglomeratic unit which Wilson (1949) correlated with the basal con­

glomerate of the Boleo Formation is present at Lucifer (Figure 6). The

clasts are mainly fairly well-rounded boulders that reach diameters of

greater than 1 m. A locally occurring upper unit with smaller, pebble­

sized, highly angular clasts is distinguished in the detailed geologic

map of the exposure of the Comondu ridge which is discussed below (Figure

19, p. 51).The pebbles and boulders which constitute the basal conglomer­

ate at Lucifer all appear to have been derived from erosion of Comondu

21

volcanics. The matrix is generally tan-colored, but is often distinc­

tively orange-stained due to iron oxide. The conglomerate is poorly

sorted and poorly indurated. According to Wilson (1949), the basal con­

glomerate reaches thicknesses of up to 10 m in the Lucifer area, and is

approximately 4 m thick at the center of the mine.

Tuffaceous Limestone

The Boleo tuffaceous limestone reaches thicknesses of up to 5 m.

The unit is the oldest known deposit of marine origin in the Santa

Rosalia area (Wilson, 1948). Locally, the detrital Content of the lime­

stone is sufficient to justify its being considered conglomeratic

limestone or calcareous tuff. Lower Pliocene fossils in the unit are

the basis for dating the Boleo Formation (Wilson, 1948).

In the Lucifer area, outcrops of the limestone unit occur north

and east of most outcrops of manganese ore (Figure 4). The extent of ■

the unit is rather limited in the Lucifer area, and locally, Boleo tuff

overlies Boleo basal conglomerate. Although limestone is absent from

most localities where economic manganese mineralization.occurred, in

the northwest part of the area about 1 m of generally low-grade manga-

niferous tuff is seen to overlie limestone, which itself appears to be

slightly manganiferous. A typical exposure of the tuffaceous limestone overlying basal conglomerate is illustrated in Figure 9.

Wilson (1949) reported the limestone to be 1-2 m thick in the

Lucifer area, reaching a maximum thickness of 4 m. The limestone is

poorly bedded; where distinguishable, beds vary from 1 to 3 m. The color varies from pink or light orange to gray or brown.

22

Figure 9. Boleo Tuffaceous Limestone Overlying Basal Conglomerate.

23

In thin section, the limestone appears to consist either of

light brown,,fine-grained calcite, or a mosaic of coarse grains of spar­

ry calcite. The latter may be the result of recrystallization. Fossils

are rather abundant, although poorly preserved. In samples examined,

they appeared to consist mainly of limonite-rimmed gastropods. Patches

of montmorillonite and limonite occur in varying amounts, as do detrital

grains, which consist mainly of feldspar. Limonite veinlets are common.

Tuffs and Tuffaceous Conglomerate

The major part of the Boleo Formation is the result of five depo­

sitions! cycles during which conglomerate was successively overlain first

by clayey tuff, the principal copper-bearing unit, and then in turn by

sandy tuff, tuffaceous sandstone, and finally by conglomerate of the

next cycle. From youngest to oldest, the five cycles are numbered 0, 1,

2, 3, and 4 (Wilson, 1955). The manganese orebody probably occurs in

tuffs of the oldest cycle (Wilson, 1949).

The tuffs are andesitic to latitic in composition, consisting

largely of fragments of plagioclase, hornblende, biotite, pyroxenes, and volcanic shards in a montmorillonite matrix. Quartz is extremely rare.

The montmorillonite content increases toward the base of the tuffs.

The conglomerates consist of Comondu boulders and pebbles set in moder­ately to poorly cemented tuffaceous matrix.

At no single location are all five cycles of deposition present.

Progressively younger sediments may be observed to onlap the Comondu volcanics or basal Boleo units, proceeding from the northwestern part of

24

the district toward the southeast (Wilson, 1955). Thus, in the north­

ern parts of the district, for example at the Lucifer mine, strata of

cycles 3 and 4 onlap the Comondd, and strata of younger cycles are ab­

sent (Figure 3). In the southeastern parts of the Boleo district, sedi­

ments of cycles 3 and 4 are absent, and only the younger cycles are

represented. This trend is due in part to post-depositional erosion

which accompanied or followed southeastward tilting. Wilson (1948,

1955) presented evidence, however, that the actual locus of sedimenta­

tion migrated southeastward during the time that the Boleo Formation

was being deposited by demonstrating the southeastward migration - of- the

thickest part of each successively younger conglomerate unit. In addi­

tion, each progressively younger tuff unit appears to achieve maximum

thickness farther to the southeast. The locus of ore deposition accom­

panied the southeastward migration of the locus of sedimentation,such

that mines in the northwestern part of the district occur in ore bed 4

within the tuff overlying the oldest conglomerate, and are located in

progressively younger tuff units toward the southeast.

The origin and source of the Boleo tuffs are uncertain. Schmidt (1975) presents evidence that the tuff units may have been derived from

volcanic activity occurring to the north of the Boleo district in the La

Reforma volcanic complex (Figure 1). Wilson (1955) provides evidence, on the other hand, for at least one episode during which tuff originated

to the west of the district, which would be in accord with the observed

southeastward migration of the locus of sedimentation.

25The conglomerates typically occur in elongate lenses with pro­

nounced central bulges surrounded by thinner flanks (Wilson, 1955).

They become finer grained toward the gulf, eventually wedging out into

tuffaceous sandstone of probable marine origin, implying derivation from

the west. A major change in depositional trends from the time of deposi­

tion of the Miocene Comondu volcanics is thus indicated.

The Boleo conglomerate units and their gulfward sandy equivalents

probably are the result of a nearshore environment of deposition inter­

rupted by periodic pyroclastic eruptions responsible for the Boleo tuffs

(Wilson, 1955). The tendency of the conglomerate lenses to bulge toward

the Gulf of California may indicate strandline deposition, perhaps re­

lated to a delta front, a conclusion substantiated by marked gulfward

inclination of cross-bedding.

In the Lucifer area, the members of the Boleo Formation which

overlie the basal conglomerate and limestone consist of a basal tuff

unit, the main manganese host, overlain by tuffaceous conglomerate with

tuff lenses. Correlation of these units with the rest of the Boleo dis­

trict is somewhat problematical (Figure 3). If the tuffaceous conglom­

erate overlying the manganese orebody is correlative with the oldest

conglomerate of the five conglomerate-tuff cycles, then the tuff unit hosting the manganese deposit cannot be correlated with the tuff units

that host copper mineralization, but is, instead, older. On the other

hand, correlation.of the Lucifer orebed with Boleo copper ore bed No.

4, as suggested by Wilson (1949) and indicated in Figure 3, implies

that Conglomerate No. 4 is absent from the Lucifer area, and that the

26

conglomerate overlying the Lucifer deposit is correlative with Conglomer­

ate No. 3.

The tuff and conglomerate units present at Lucifer are discussed below in detail.

Basal Tuff. The lowermost tuff unit at Lucifer attains thick­

nesses of up to 15 m. Maximum thicknesses are achieved in the northern

parts of the mine area; toward the south the tuffs wedge out against the

Comondti paleotopographic ridge. The tuff is normally tan or pink. It

is generally well stratified, with strata varying from 0.1 to 10 cm in thickness. Compositionally, the tuff varies from clayey with few visi­

ble crystal fragments to sandy or pebbly. The typical upward coarsening trend is not found in the Lucifer area; coarse- and fine-grained strata

seem randomly interlayered. The tuff is poorly indurated and forms cov­

ered slopes. The unit is concretionary in many places, which tends to

obscure stratification.

The basal tuff is particularly well-exposed near the head of the tram, where the contact between the tuff and the overlying manganese ore

lens is seen .to be exceedingly sharp, the manganese forming a ledge above the tuff (Figure 10). The tuff is 5.6 m thick, of which the top

0.3 m is a zone of mainly concordant manganese oxide and calcite vein- lets. The tuff is stained by iron and manganese oxide about 1 m into

the unmineralized tuff. The 5.3 m of unmineralized tuffs consist of

interlayers of sandy tuff and pebbly tuff. The lower contact with basal

conglomerate is covered.

27

i

Figure 10. Remnant of the Manganese Ore Manto Overlying Boleo Tuffs. — Tuffs in the background are displaced upward by a fault. Basal conglomerate is exposed near the left side of the photograph. The conglomerate overlying the tuff unit is post-Boleo Formation in origin.

28

In thin section, the Boleo tuffs at Lucifer are essentially

light to dark brown montmorillonitic clay with varying amounts of crys­

tal and lithic fragments. Two contrasting textures were encountered,

original and intraclastic.An unmineralized sample collected from about 1 m below the man­

ganese ore horizon in the vicinity of the tram head (sample LP3A) with

an original texture consists of a dark brown matrix and 30-40%

fresh, angular, and poorly sorted crystal and lithic fragments. Photo­

micrographs of this sample are presented in Figure 11. The crystal frag­

ments consist mainly of andesine plagioclase, with subordinate brown

biotite, green biotite possibly altering to chlorite, and rare horn­

blende and quartz. Volcanic fragments make up less than 1% of the rock.

The groundmass is composed of volcanic glass, smectite, and limonite.

About 5% of the matrix was impregnated by calcite. Limonite is also present in veinlets.

Sample LZ25, collected from the east-central part of the study

area (Appendix II), is composed of well-rounded intraformational grains 1 or 2 mm in diameter composed mainly of buff-colored clay and varying amounts of highly angular glass and crystal shards. A photomicrograph of this sample is presented in Figure 12. One to two percent of the

sample consists of moderately well-rounded fragments of plagioclase and

minor unaltered hornblende biotite, chlorite, and an unknown fibrous ma­

terial. Some of the fragments approach the size of the intraclasts.

The intraclasts are tightly packed, with interstitial material relative­

ly scarce. The interstitial material is similar to intraclast material.

29

A. Nicols uncrossed.

B. Nicols crossed.

Figure 11. Photomicrographs of Boleo Tuff with Original, Non-intraformational Texture (Sample LP3A). — Diameter of field is 3.82 mm.

30

Figure 12. Photomicrograph of Boleo Tuff with Intraformational Tex­ture (Sample LZ20B). — Diameter of field is 3.82 mm.

31although generally lighter in color. In the specimen discussed, chryso-

colla occurs as interstitial material.

A specimen of Boleo tuff directly overlying the ore horizon was

described by Wilson (1949, p. 191) as a-bentonitic tuff composed largely

of montmorillonite mixed with fragments of glass, biotite, feldspar, and

other tuffaceous material. It thus apparently resembles the tuff lying

below the manganese lens.

Chemical compositions of two samples of Boleo tuff from the

Lucifer area, LP3A described above, and LZ20B, a noncalcareous, concre­

tionary sample collected from the central part of the study area (Ap­

pendix II), are presented in Table 2.

In the northern parts of the Lucifer area, the lower tuff unit

is seen to be overlain by tuffaceous conglomerate. The manganese hori­

zon consists of a zone of interbedded tuff and manganese oxides, with

abundant calcite veinlets also generally concordant with the stratifi­

cation of the tuffs, thus resembling the manganiferous tuffs below the

manganese lens. Both manganese oxides and tuff display convolute bedding and other soft—sediment deformation characteristics.

Tuffaceous Conglomerate and Tuff Lenses. The tuffaceous conglom­

erate and interbedded tuff lenses which overlie the basal tuff at Lucifer

attain thicknesses of up to 65 m in the immediate mine area, with tuff

layers constituting up to 15 m of the total (Wilson, 1949). The tuff

lenses resemble the basal tuff and bear a few thin, discontinuous, low-

grade manganese deposits (e.g., Figure 4, north-central part of map).

Table 2. Chemical Composition (Weight Percent) of Two Samples of Boleo Tuff.

LP3AaSample Number

LZ20BbSi02 47.70 48.80

“ 2°3 12.70 14.40

CaO 12.40 1.72

MgO 3.41 10.30

Na20 2.77 0.94

k 2o 1.00 0.95

FeO 5.22 4.57

MnO 0.27 0.19

Ti02 0.66 0.70

P2°3 0.12 0.16

L.O.I. 14.16 16.51

Sum. 101.00 99.70Ba(ppm) 1200 2300aSample collected in vicinity of exposure of main ore horizon at head of tram; contains abundant crystal fragments and calcite veinlets.

^Sample collected about 75 m northwest of south- easternmost exposure of main ore horizon; sapo- nitic, noncalcareous, crystal fragments rare.

33The conglomerate clasts resemble Comondu volcanic rocks present

in the Lucifer region. The sandy, tuffaceous matrix is light gray or

tan, and both matrix and clasts are commonly stained a distinctive white

color. Tuffaceous conglomerate clasts typically are smaller than basal

conglomerate clasts, generally less than 10 cm in diameter, although boulders of up to 50 cm may be seen. The clasts are generally fairly

well sorted and rounded. The clast volume to matrix volume ratio is

distinctively high, although patches of nearly pebble-free tuff occur.

The matrix may be crustified and calcareous, but the conglomerate is

poorly indurated.

Pliocene and Quaternary Sedimentary Units Overlying the Boleo Formation

As indicated.in Figure 3, conglomerate and sandstone sequences

of middle and late Pliocene and Pleistocene ages overlie the Boleo For­

mation in various parts of the Boleo district, although they are absent

in the Lucifer area. These sequences tend to become finer grained toward

the gulf and appear to be largely composed of Comondu erosional products.

As much as 20 m of Quaternary sediments blanket the mesa and

terrace tops near Lucifer and elsewhere in the Santa Rosalia region

(Wilson, 1949). The Quaternary deposits generally consist of unconsoli­

dated, poorly-sorted pebbles and boulders, but relatively well cemented

conglomerate occurs as well.

A poorly—sorted, poorly-stratified, loosely consolidated conglom­

erate is widely exposed just north of the map area (Figure 4). A few

scattered remnants occur in the mine area. The conglomerate is

.34

distinguished on the detailed map of the southern part of the Lucifer

mine (Figure 19, p. 51) where it is about 10 m thick and overlies

Comondtj volcanics, Boleo basal conglomerate, Boleo tuffaceous conglom­

erate, or the manganese horizon, as shown in Figure 13. The clasts

range up to boulder-sized, tend to be highly angular, and are set in an

orange or pink sandy matrix. The conglomerate seems to be overlain by

Tres vCrgenes volcanics and does not appear to contain clasts of Tres

Virgenes origin. The conglomerate may be a type of alluvial fan deposit

filling channels cut into the Boleo tuffaceous conglomerate.

The youngest sediments present in the Lucifer area include talus

and rockslide accumulations with.boulders up to several meters across,

as well as sheetwash and arroyo deposits. Much of the cover consists of

man-made debris and tailings.

Tres Virgenes Volcanics (Pleistocene and Recent)

The lava flows and pyroclastic rocks which comprise the Tres Virgenes volcanics cover large areas of the mesa tops north and west of

the Lucifer mine, but extend to only small areas south of Arroyo del In-

fierno. The major centers of eruption appear to have been the Tres Vir­

genes volcanic cones, located northwest of the Boleo district, and the

Reforma volcanic complex, located to the north (Figure 1) (Wilson, 1955;

Schmidt, 1975). Eruptions of the Tres Virgenes have reportedly occurred

during historic times.

35

Figure 13. Quaternary Conglomerate Overlying Stratified Manganese Ore.

36The Tres vlrgenes volcanics sharply overlie the Boleo Formation

near Lucifer, capping and filling gaps in the erosional surface forming

the top of the Boleo conglomerate. A small tongue of the Tres v£rgenes

extends into the southwestern part of the study area (Figure 4); there

the formation consists of a thin basal welded tuff unit overlain by vit-

ric tuff making up the major part of the formation. The basal welded

tuff is approximately 2 m thick, and the vitric tuff is estimated to be 10-15 m thick.

The basal welded tuff is dark gray and consists of 4 or 5% lith—

ic and crystal fragments set in an extremely well-laminated rock. Thin

section examination revealed a classic welded tuff vitroclastic texture

of exceedingly fresh, interlaminated, flattened glass shards set in a

glassy groundmass. The crystal and lithic fragments are elongated in

the direction of lamination. The crystal fragments consist mainly of

oligoclase feldspar and scattered green augite, hornblende, and opaque grains.

The vitric tuffs which overlie the welded tuff form-the actual

surface of the mesas above Lucifer. They are light gray and appear

thickly bedded on a large scale, although fine lamination is implied by

lineation of vesicles. The vitric tuffs are hard, brittle, and weather

resistant, and tend to break up into huge blocks as large as several

meters across. In thin section, the vitric tuffs are seen to be made

up of 3 or 4% crystals and crystal fragments set within a gray, glassy

matrix. The crystals consist mainly of plagiociase feldspar, orthoclase

feldspar, and diopsidic augite commonly rimmed by iron oxide.

STRUCTURAL GEOLOGY\

General Structural Features of the Santa Rosalia (Boleo) Region

The Santa Rosalia area has been subjected to what may be con­

sidered two major periods of structural deformation (Wilson, 1949, 1955);

namely, 1) a late Miocene-early Pliocene period of normal faulting and

moderately steep tilting which affected the Comondu volcanics; and 2)

structural activity during the Pliocene and post-Pliocene time, and

probably continuing to the present, during which time gentle tilting and

further normal faulting has occurred. The early episode of deformation

was responsible for the marked angular unconformity separating Miocene

volcanics from younger sediments which occurs throughout the district.

The later period of structural activity is responsible for the numerous

less distinctive disconformities and minor angular unconformities that separate the post-Miocene formations.

The late Miocene-early Pliocene deformation produced mainly

westward-dipping normal faults with displacements commonly of 50-100 m, which resulted in a series of eastward-tilted blocks (Wilson, 1955).

Erosion accompanied or followed this initial period of structural ac­

tivity, producing a highly irregular topography which apparently pro­

vided the primary structural controls on post-Comondu strata (Wilson,

1948, 1955). Wilson demonstrated that steep dips in the Boleo Formation

and younger strata are mainly the consequence of the relief of the sur­

face of deposition rather than of later deformation.37

38

Both periods of structural deformation resulted in faults which

strike predominantly 10° to 45° northwest, generally paralleling the

coast (Wilson, 1955). A small number of faults in the district follow

a second, northeasterly trend. Virtually all faults in the district are

normal faults. The maximum known vertical displacement by faults in the district is 250 m. The largest faults have lengths of 3 km or more, but

in general, faults cannot be traced for more than a few hundred meters.

The faults commonly occur in en echelon patterns (Wilson, 1955).

Structural Features of the Lucifer Mine

The lucifer region is specifically influenced by the ridge or

promontory-like erosional remnant of the Comondu volcanics lying to the

south of the orebody and the bulk of the Boleo Formation sediments pres­

ent in the region. Boleo sediments, as well as the manganese horizon,

appear to wedge out against this ridge, and can be seen to dip away from

it in generally northeasterly directions at steep angles (Figure 14).

Adjacent to the ridge, to the north, and also believed to be a paleotopographic remnant, is a west-northwest striking structural ter­

race dipping generally 5° to 10° to the northeast (Wilson, 1949, Plates 49 and 52). The Boleo tuffs deposited on the terrace are seen to be

nearly flat-lying (Figure 14). To the north of the structural terrace,

Boleo sediment dips steepen considerably (Figures 4 and 15).

These structural and stratigraphic relationships are illustrated

in the cross section of the Lucifer area (Figure 5).

Northeast tilting of the Comondu volcanics in the Lucifer area

is indicated by dip directions of the Comondu bedded units, which have

39

Figure 14. Exposure of Partly Buried Ridge of Comondu Volcanics. — Note that the sediments in the background dip steeply off the ridge, whereas the tuffs in the foreground, which are deposited near the center of the structural terrace, are nearly flat-lying. Gray cliffs on the right are Boleo tuffaceous conglomerate. The brown conglomeratic unit in center of photograph is Quaternary conglomerate, and ap­parently was deposited within a channel cut into Boleo sediments. Manganese ore occurs to the left of the Quater­nary conglomerate. The highest cliffs are of Tres Virgenes volcanics.

40

i

Figure 15. Lucifer Mine, Looking West up Wash in the Northeast Part of the Study Area. — Note the steep dips of limestone units on the hill slope to the left.

41

been plotted on a rose diagram (Figure 16). The angle of dip was found

to be somewhat greater in the Lucifer area than the 20° to 40° reported

as typical of the Santa Rosalia area in general by Wilson (1955). Dip

angles of between 40° and 50° are most commonly encountered in the Co-

mundu bedded units, and may range to nearly vertical. Host of the-ob­

served tilting of the Comondtj volcanics is probably related to late

Miocene-early Pliocene fault movement, but the unusually steep dips

observed in the Lucifer mine region for both the Comondu volcanics and

Boleo sediments imply additional regional tilting that probably affected

the area sometime prior to deposition of the Tres Virgenes volcanics,

which overlie an erosional surface formed within the Boleo sediments

and are seemingly unaffected by this regional tilting. In a few cases,

small-scale fold-like structures are encountered within the Comondu vol­

canics which probably formed prior to.solidification of the lavas, and

are thus not of tectonic origin. Irregularities in the surface upon

which the Comondu volcanic units were deposited may also help account

for some of the unusual dip measurements and other structural peculiari­

ties observed within the Comondu.

In addition to the presumed large-scale faulting responsible for

the northeasterly tilting encountered in the Lucifer area, the region

has also been affected by smaller scale faulting, mainly post-ore in

age. Although not apparent in the geologic map of the mine region

(Figure 4) because of large areas covered by alluvium, colluvium, and

man-made rubble, the region is broken by numerous short, discontinuous

faults that generally strike to the northwest, with small displacements

42

N

Figure 16. Dip-histogram Rosette for Comondu Volcanic Flows, four measurements.

— Thirty—

43either to the east or west (Wilson, 1949, especially Plate 44). Several

faults are apparent on the surface in the region, generally affecting

Boleo sediments. The largest of these faults is seen in the northern

part of the geologic map, to the north and east of the orebody. This

westward-dipping normal fault is responsible for 15-20 m of displacement,

exposing Comondu volcanics on its up, or eastern, side. To the north

and south, the fault appears to break up into smaller, subparallel

faults.

Another fault, near the head of the tram, is responsible for 4

or 5 m of displacement and removal of part of the outcrop of manganese ore exposed there; the effects of this fault may be seen in Figure 10.

Wilson (1949) reported the maximum displacement of any fault

actually affecting the orebody to be 8 m. Also reported was a fairly

widespread, nearly flat fault that dips to the northeast at 0° to 30°.

Where it occurs, it forms either the upper boundary of the manganese

deposit or else lies a short distance above it. Wilson attributed this

fault to slumping or sliding of sediments away from the adjoining Co- mundu ridge.

A study of fracture orientations in the Lucifer area, including

mineralized veinlets, was conducted in the hope that such a study might

provide insight into how manganese-bearing solutions may have been chan­

nelled, to eventually lead to the deposition of an economic mineral de­

posit. Rose diagrams are presented in Figure 17 showing all fracture

and veinlet strike orientations measured in the course of this study

(Figure 17a), and strike orientations of fractures and veinlets which

A. Mineralized and unmineralized fractures; B. Mineralized fractures from all Luciferwhite = fractures in Boleo Formation (18 units (32 measurements).measurements); black = fractures in Comondu volcanics (121 measurements).

Figure 17. Strike-histogram Rosettes for Fractures in the Lucifer Area.

■c-

are mineralized (Figure 17b). In Figure 17a, orientations of fractures

in Boleo sediments are distinguished from thoie in the Comondu volcanics.

Figure 17a and 17b indicate that the predominant strike of both mineral­

ized and unmineralized fractures in the Lucifer region is north-northwest,

paralleling the general strike of bedding and the predominant fault trend

in the region. Figure 17a indicates that the Comondu also shows a defi­

nite second, considerably less-pronounced northeast fracture trend. Al­

though it is not certain from the data presented if this second trend is

absent in Boleo sediments, the northeast trend is probably related to

the major N30°E strike trend noted by Schmidt (1975) to be characteristic

of Late Miocene deformation in the La Reforma complex, and thus may pre­

date the Boleo Formation. The principal NNW-SSE trend is characteristic

of both the late Miocene-early Pliocene deformational event and later

structural movements (Wilson, 1949, 1955). Thus, the Comondu volcanics

may have been affected by the intersection of two pronounced fracture

trends prior to deposition of the manganese ore.

The greater number of readings from Comondu volcanics in Figure

17a is a function of the greater tendency to fracture observed in those

rocks with respect to the Boleo sediments. Within the Comondu, the ma­jority of readings were obtained from the more brittle and competent

bedded lithologies. Because the Boleo sediments predominate in the

northern parts of the study area, most readings were obtained from the

southern parts of the study area, where the Comondu volcanics are well

exposed, although the attempt was made to obtain readings which are rep­resentative of the entire study area.

45

MANGANESE DEPOSITS

General Features of the Lucifer Manganese Deposit

The main ore horizon, or manto, at Lucifer achieved thicknesses

of greater than 6 m (Wilson, 1949). These maximum thicknesses occurred

in the center of the structural terrace discussed in the section on

structural features of the Lucifer region. The structural terrace

formed the general axis of maximum thickness of the deposit, and appar­

ently defined its shape. Wilson (1949, Plates 49 and 52) showed that

the geometry of the manto was such that both the structure contours of

the base of the manto, as well as the axis of maximum thickness of the

ore, parallel a bend in the Comondu ridge, changing from a northwesterly

bearing in the southeastern part of the deposit to one nearly due west.

Where the manganese manto terminates against the ridge of Co­

mondu volcanics, it thins considerably and its dip steepens (Figure 14).

In the opposite direction, to the north of the structural terrace, the

manto also becomes thinner and lower grade, as manganese oxides become

intermixed with Boleo tuff, and dips of the manto as well as those of the hosting Boleo sediments become somewhat steeper. The Comondu ridge

would thus appear to have been structurally the highest part of the

manganese deposit, as well as of the Boleo sediments, implying that the

manganese orebody and hosting tuffs were influenced in the same manner

by the paleotopography.

46

47

Typically, the manganese ore occurred near the top of the lower­

most tuff unit of the Boleo Formation (Wilson, 1949). In the south part

of the mine, where the tuff unit pinches out against the basal conglomer­

ate of the Boleo Formation, the manto overlies the conglomerate. Where

the basal" conglomerate wedges out against the ridge of Comondu volcanics,

the manto is found to overlie Comondu. Both the basal conglomerate and

Comondu volcanics may be mineralized where overlain by the manto.• The manto terminated to the southeast and east by outcropping,

whereas to the north and northwest, the extent of economic mineraliza­

tion was determined by ore grade or thickness, the ore horizon eventually

becoming little more than a zone of slightly mineralized tuff which thins

to a fraction of a meter where seen in the northernmost parts of the

study area.

The main workings extended for about 600 m in an east-west direc­

tion parallel to the Comondu ridge, and for the most part, extended no

more than 100 m from where the manto terminates against the ridge. Out­

crops of the orebody were worked from open-cuts and shallow adits as far

as 500 m north of the Comondu ridge (Wilson, 1949, Plate 52).

The manto is apparently conformable with the hosting tuff. Where presently observable, both upper and lower contacts of the manto are al­ways rather sharp (Figures 10 and 23, p. 55). In places, the manto itself

displays poorly developed stratification and soft-sediment deformation

features (Figures 13; Figure 24, p. 57). Crosscutting of host strata by

the manganese strata, or indications of mineralization by replacement or

impregnation of hosting tuff are typically absent.

48The color of the manganese mineralization ranges from jet black

to dark gray or brown; lower grade, ferruginous ore may be reddish-

brown in color. Interlayered with the manganese minerals, mainly pyro-

lusite and cryptomelane-hollandite minerals, are lenses of jasperoidal

silica, generally brown or reddish-brown in color. Limonitic lenses may take on brilliant hues of orange or red. Although not restricted to the

uppermost part of the manto in any stratigraphic sense, jasper is often

seen to cap the manganese ore. Banded intergrowths of silica and manga­

nese oxides give the appearance of being the manganese equivalent of

"iron-formation".

The luster of the ore ranges from shiny and metallic to vitreous

or dull and earthy. The manganese minerals vary from dense, compact, and

hard to powdery and crumbly. The ore generally is massive, but visible

crystals of pyrolusite may be seen in hand. samples in many cases. Com­

monly, the ore has a highly clastic appearance, as may be seen in Figure

18, as though composed of detrital manganese oxide grains within a matrix

of colloform manganese oxide and gangue, and may be indicative of an intraformational origin.

Remnants of the manganese manto may still be seen in a few expo­sures, but a complete, truly representative cross section can no longer

be found exposed. One of the best remaining exposures of the manto oc­

curs near the southeastern edge of the mine, near the head of the tram

which connected the mine with the town of Lucifer in Arroyo del Infierno

(Figure 4). Over 2 m of the manto remain, forming the cap of the

Figure 18. Lucifer Ore Sample with Distinctive Clastic Appearance.

50

exposure (Figure 10). The mineralization is mainly jet black, hard, and

dense.

Concretions up to cobble-sized are present and are exceptionally

hard. Patches of brown, ferruginous, low-grade ore are also present.

Veinlets of calcite are abundant, and gypsum veinlets are also seen.

Below this distinct ore horizon is the zone of stringers of man­ganese oxides and later calcite veinlets, both generally concordant with

the bedding of the tuffs. A sample from the stringer zone, LP3B, was

examined in thin section and was found to be similar in appearance to

unmineralized tuff samples examined. Crystal and lithic fragments com­

pose only about 1 or 2% of the sample, but the fragments appear fresh.In addition to veinlets of manganese oxides, abundant calcite veinlets

are present, which, for the most part, subparallel the manganese vein-

lets, both of which appear to follow along laminae. Calcite may also

be seen to cut across manganese veinlets. In spots, fairly large portions

of the groundmass appear to have been impregnated by calcite.

The depth to which the Comondii volcanics are mineralized where

the manto terminates against the ridge is unknown. Similar mineraliza­

tion is reported for a span of at least 100 m northwest of the exposure of the ridge (Wilson, 1949), where it is buried by post-Boleo age sedi­

ments and volcanics. A detailed map of the westernmost exposure of the

Comondu ridge is presented in Figure 19.

The basal conglomerate of the Boleo Formation can be seen at this

exposure to wedge out against the Comondu volcanics, which are composed

largely of felsophyric-textured volcanic breccia and agglomerate

Otology napped or gr id base by 0. F r e i l e r g May 1171

Figure 19. Detail Map of the Exposure of the Ridge of Comondu Volcanics near the Southern End of the Orebody.

52

(Figure 20). Both basal conglomerate and Comondtf pyroclastics appear to

be mineralized in a similar fashion, mainly involving impregnation of

matrix material by oxides of manganese and iron and jasper. Manganese

oxides are also seen to have entered fractures and vesicles within

clasts, as well as fractures and bedding planes of stratified volcanic

units (Figure 21).

Actual transport of volcanic and conglomerate clasts may have

occurred down the paleoslope of the Comondu ridge, giving rise to an ex­

tremely chaotic lahar-like breccia, composed in part of accumulations

of rock fragments and boulders with very little matrix material, as well

as patches of manganiferous tuff-bearing dispersed rock fragments (Fig­

ure 22).

Overlying the mineralized conglomerate and volcanics are incrus­

tations of manganese oxide ore, the fringe of the manganese manto. The

incrustations are up to 2 m thick, and are observed to dip to the north or northeast at angles of up to 40° (Figure 19). The incrustations oc­

cur as conformable dip slopes where they overlie bedded volcanic units.

In the northwestern part of the exposure of mineralized Comondtf (Figure 19), the incrustations are distinctly overlain by Boleo Formation tuf-

faceous conglomerate (Figure 23). Elsewhere in the exposure, the man­

ganese incrustations are seen to have been covered by post-Boleo

sediments (Figure 13).

Sporadically occurring manganif erous patches, generally 1 or 2

square meters in size, are seen in the tuffaceous conglomerate of the

Boleo Formation and in younger clastic sediments, either directly

53

Figure 20. Contact of Mineralized Boleo Basal Conglomerate and Comondu Breccia at the Exposure of the Comondu Ridge.

54

Figure 22. Manganiferous Laharic Breccia.

55

Figure 23. Boleo Tuffaceous Conglomerate Overlying Manganese Ore. —The steep dip of the base of the conglomerate is an indica­tion of the slope of the paleotopographic surface.

56overlying the incrustations or related to joints. This mineralization

may in part be the result of remobilized manganese oxides impregnating

the overlying clastic units, and may be partly detrital in origin, hav­

ing been derived from erosion of the original manganese deposit. It in­

cludes pebble-sized concretions which may also be detrital in origin.

The manganese grains from sample LP15, one such concretion collected

from within the post-Boleo conglomerate, were found by microprobe analy­

sis to be highly siliceous, but otherwise to be compositionally very

similar to manganese minerals in samples from the main ore horizon.

Clasts within the mineralized portions of the basal conglomerate

of the Boleo Formation, and to a lesser extent, breccia and agglomerate

fragments of the Comondti volcanics in the mineralized zone and elsewhere

in the Lucifer region, tend to have altered rims up to several centi­

meters thick that have a distinctive bleached appearance (Figure 24). Small pebbles may be completely altered. The alteration, which essen­

tially involved formation of clays from the glassy groundmass of the

clasts and deposition of zeolites and other authigenic minerals in ves­

icles, will be discussed in greater detail below.

Mineralogy of the Lucifer Deposit

General Considerations

X-ray diffraction studies, polished section examinations, and

electron microprobe analysis indicate that economic mineralization at

the Lucifer mine consists predominantly of pyrolusite and minerals of

the cryptomelane-hollandite-coronadite isostructural series. Minor

57

I

Figure 24. Stratified Manganese Ore Overlying Altered Boleo Basal Conglomerate.

58amounts of the minerals todorokite and nsutite also appear to be present

in several samples of the Lucifer ore examined. Gangue consists pre­

dominantly of jasperoidal silica or crypto-crystalline quartz, and is

argillaceous as well as ferruginous. Hematite and goethite are abundant

in iron-rich portions of the deposit. Veinlets of calcite commonly are

present; veinlets of gypsum are occasionally encountered as well. Smec-

titic clays are commonly associated with the manganese mineralization,

and the occurrence of halite is also indicated. At one location, chryso—

colla is found associated with the manganese mineralization.

The results of microprobe analyses of Lucifer ore and a discus­

sion of experimental procedures are presented in Appendix %. Table 3 is

a summary of the data in Appendix II for the major mineral types present

in the Lucifer ore.

The cryptomelane-hollandite-coronadite isostructural series has

the general formula A2_yBg_zXi6. "A" represents K+ , Ba^+ , or Pb^+ as

the respective end members of the series. Ca^, Na+ , and, to a -lesser ex­

tent, other cations of similar size may substitute into the "A" site as

well (Bystrom and Bystrom, 1950; McKenzie, 1971; Burns and Burns, 1977).

"B" most commonly represents Mn^+ , with substitutions by Mn^** and less 3+ 34.commonly by Fe or A1 , maintaining the charge balance, "y" in the

formula is generally very close to 1, and "z" ideally is equal to 0."X" represents 0“ or OH to a lesser extent. Water is generally present

in cryptomelane minerals, often in amounts exceeding 1% (Gruner, 1943).

Some water may actually substitute into the "A" site, particularly in

the mineral referred to as "psilomelane" (or "romanechite"), the name

59

Table 3. Average and Range of Lucifer Manganese-oxide Microprobe Analyses1."High--potassium11 Cryptomelane "Barium" and "Lead"-Cryptomelane Coronadite ! Pyrolusite

Average RangeNumber of Analyses Average Range

Number of Analyses Average Range

Number of Analyses Average Range

Number of Analyses

Mn 56.51 52.89-59.14 13 52.91 48.10-56.34 21 50.71 48.62-52.32 9 54.73 44.82-59.96 10Fe 0.37 0.03-1.78 13 0.79 0.09-4.39 21 0.36 0.05-1.07 9 0.24 0.04-0.49. 10A1 0.28 0.09-0.59 6 0.16 0.00-0.78 20 0.11 0.09-0.13 3 0.13 0.00-0.64 5

Ba 0.37 0.00-1.02 10 3.36 0.00-6 .'63 21 1.51 0.00-2.66 6 0.05 0.00-0.09 8

Pb 0.85 0.30-1.34 10 1.81 0.03-4.57 21 12.19 10.91-14.82 6 0.05 0.00-0.22 7

K 4.56 3.58-5.03 10 2.19 0.84-4.20 21 0.67 0.48—0.86 6 0.08 0.03-0.31 8Na 0.36 0.17-0.78 6 0.38 0.20-0.87 20 0.28 0.18-0.33 3 0.01 0.00-0.02 4

Ca 0.24 0.10-0.38 6 0.46 0.14-0.87 20 . 0.42 0.31-0.48 3 0.15 0.11-0.22 4Si 0.34 0.08-0.93 7 0.20 ' 0.08-0.40 12 0.12 0.06-0.16 3 5.64 2.91-10.51 8Mg 0.13 0.01-0.33 3 0.31 0.03-0.70 6 — — 0 — — 0Cu 0.02 0.00-0.04 4 0.18 0.03-0.24 4 0.03 0.02-0.05 5 0.03 0.02-0.04 2Zn 0.20 0.00-0.52 4 0.28 0.20-0.37 3 0.24 0.00-0.64 5 0.16 0.06-0.25 2Co 0.04 0.00-0.14 4 0.02 0.01-0.03 3 0.03 0.00-0.09 5 0.06 0.00-0.12 2Ni 0.01 0.00-0.03 ' 4 0.01 0.00-0.01 3 0.05 0.00-0.10 5 0.00 0.00-0.00 2Ag 0.005 0.00—0.02 4 0.01 0.01-0.02 3 0.01 0.00—0.03 5 0.03 0.00-0.05 2S 0.02 0.00-0.05 4 — " — . 0 0.04 0.01-0.08 ! 3 0.00 0.00-0.00 2see Appendix II for individual analyses. Highly siliceous cryptomelane mineral analyses (Si greater than 1%) excluded from this table. Sample LP3E also ex­cluded because of apparent impure nature of material, as discussed in text.

60

applied to a barium-bearing mineral with an essential 1^0 content (for­mula (Ba,K,Mn,Co)2^n5®10* x ^0) (Wadsley, 1953; Burns and Burns, 1977). Although small amounts of psilomelane may be present in Lucifer ore,

x-ray diffraction and microprobe studies gave no definite indication of

its presence, and therefore it will not be specifically referred to in

any of the following discussions on the mineralogy of the Lucifer deposit.Pyrolusite is the second major manganese mineral species

present in Lucifer ore. The presence of todorokite ((Na,Mg,Ca,K,Ba,Mn)

MnsOiz-SHzO), and nsutite ((Mn24Hn3-t>In'4+)(0,0H-)2) and possibly birnes- site ((Na,Ca)Mnj0^^‘H20) was indicated in x-ray analyses or microprobe analyses of samples LP3E, Lxl3B, and BS5. Several microprobe analyses of

grains in these samples indicated the presence of significantly higher

concentrations of calcium, magnesium, sodium, and aluminum than would be

expected in cryptomelane-type minerals or pyrolusite (Appendix II).

Polished section studies revealed clear distinctions between the

various manganese ore minerals, as well as between ore and gangue phases.

Cryptomelane minerals appear distinctly bluish-white with respect to

pyrolusite, which tends to be yellowish in reflected light. Pyrolusite

was also found to have considerably greater bireflectance than the cryp­

tomelane minerals. Both have extremely high reflectivities when well

polished, allowing the manganese minerals to be readily distinguished

from gangue in the Lucifer ore. Pyrolusite is quite often found to be

idiomorphic, forming delicate growths which resemble pine-cones that

grew into what apparently were once open spaces, later filled with sil­

ica and iron oxides (Figure 25). Pyrolusite grains in Lucifer ore

61

i

Figure 25. Photomicrograph of Lucifer Ore Polished Section Illustrating the Growth of Pyrolusite around Cryptomelane. — Idiomorphic pyrolusite is growing into open spaces later filled by sili­ceous gangue. Diameter of field is 2.00 mm.

62

samples often display distinctive concentric cracking patterns. Where

poorly formed, pyrolusite tends to be broken, pitted, and otherwise

blemished to a much greater extent that the cryptomelane minerals. As

apparent from Figure 25, pyrolusite always seems to form rims about

cryptomelane grains, and shows evidence of replacing them in some in­

stances. Either may be the dominant manganese phase in a sample. Both

pyrolusite and the cryptomelane minerals may occur as grain-like patches

or segregations surrounded by gangue.

In general, it was found that in samples in which more than one

type of cryptomelane mineral could be distinguished, the darker colored,

harder, more reflective, generally less bireflectant grain proved to con­

tain a higher lead or barium content. Increased barium content, in par­

ticular, was found to impart a bluish coloration to the grain. The most

striking example of contrasting appearance of cryptomelane minerals of

different compositions was observed in sample LP2B, from the zone of min­eralized Comondu volcanics (Appendix IV) (Figure 26). The grains deter­

mined by microprobe analysis to be coronadites are characteristically

bright, well-polished, and have high relief. Intergrown with the corona-

dite, and in sharp contrast, a cryptomelane type which was determined by

microprobe analysis to have a very high potassium content and low barium

and lead contents is considerably more pitted and broken-surfaced.

A full spectrum of colloform textures is displayed both by the

ore minerals and gangue minerals. These include concentric banding,

botryoidal-reniform textures, and spheroidal growths, some of which ap­

pear to be pisolitic in origin. Good examples of these textures can be

63

Figure 26. Photomicrograph of High-K Cryptomelane Surrounding Early Coronadite. — Diameter of field is 1.04 mm.

64

seen in Figures 27 and 28. Syneresis cracks, presumably due to shrink­

age upon dehydration, are common in the manganese minerals; the cracks

are generally filled by silica or calcite, but may be found to be filled

with manganese minerals as well. Manganese minerals are broken up in

many cases to the point that the texture may appear fragmental or even

breccia-like (Figure 28). The clastic appearance of some of the ore has been discussed, and may be indicative of actual physical tearing up and

subsequent recementing of earlier formed grains. Replacement of crypto- melane minerals by silica, or replacement of hematite in iron-rich sam­

ples, has resulted in myrmykitic-like intergrowths of silica and replaced

mineral, seen in several samples.In many instances, microprobe analysis of apparently homogeneous

grains revealed compositions that implied mixing occurred, either be­

tween two manganese oxide phases, or of silica, manganese oxide, and

iron oxide minerals, on a microscopic or even submicroscopic scale.

Definite evidence of the second type of mixing is indicated by x-ray

backscatter scans such as are illustrated in Figure 29 which reveal

zones of concentration of manganese, iron, and silica, as well as zones

of concentration of two or all three of these elements together. The

fact that the cryptomelane 1:8 ratio of "A"-site cations to "B,,-site

cations is still maintained in analyses of the siliceous zones in many

cases suggests that mixing of minerals, rather than the formation of new phases, occurred.

Inspection of the polished sections at high magnification re­vealed that many of the apparently homogeneous cryptomelane grains have

65

Figure 27. Photomicrograph of Botryoidal Colloform Cryptomelane.

Figure 28. Photomicrograph of Pisolitic Cryptomelane Broken by Sili­ceous Gangue. — Diameter of field in Figures 27 and 28 is 4.20 mm.

A. Iron.

B. Silicon.

Figure 29. X-ray Backscatter Scanning Images of Lucifer Manganese Ore. — Magnification 200 diameters.

ISIS;

s i aV

C. Manganese.

Figure 29, Continued.

.68

a wispy fabric composed of cryptocrystalline spherules and tiny, deli­

cate fibers, with many broken patches that may account for the silica

and other impurities determined in some of the microprobe analyses.

Composition of the Cryptomelane Minerals

Because the cryptomelane-hollandite minerals, together with pyro-

lusite, are the most abundant manganese phases in Lucifer ore, and be­

cause preliminary studies indicated compositional characteristics of

possible interest, an intensive investigation of the compositions of the

cryptomelane minerals was undertaken, utilizing the electron microprobe.

Analyses of major and minor elements in cryptomelane minerals and other

manganese minerals in several samples collected from various parts of the

Lucifer orebody are presented in Appendix II. Sample locations are indi­

cated on the Sample Location Map (Appendix IV). In Table 3, the average

value and range of major and trace components of Lucifer cryptomelane-

hollandite-coronadite minerals and pyrolusite are presented, based on the data in Appendix II.

Molecular proportions of the major "A"-site cations of the cryp­

tomelane minerals (barium, lead, potassium, sodium, and calcium) were

calculated with respect to 8 moles of the "B"-site cations ((Mn + Fe +Al) = 8, or (Mn + Fe) = 8, where .no analysis of aluminum was made), as called for in the ideal unit cell formula. These data are presented in

Appendix III for individual microprobe analyses. Table 4 is a summary

of the data in Appendix III.

Table A. Average and Range of Molecular Proportions of "A"-site Cations in Lucifer Crypto- melane Minerals with Respect to 8[Mn + Fe + Al].f

"Pb" and "Ba" Cryptomelane (Type I)

"1High-K" Cryptomelane (Type II)

Coronadite (Type III)

Avg. RangeNo. of

Analyses Avg. RangeNo. of Analyses Avg. Range

No. of Analyses

Ba 0.21 0.00-0.41 27 0.02 0.00-0.05 14 0.10 0.00-0.16 6Pb 0.07 0.00-0.19 27 0.04 0.00-0.09 14 0.52 0.46-0.63 6K 0.47 0.18-0.83 27 0.90 0.71-1.03 14 0.15 0.11-0.20 6Na 0.13 0.04-0.25 26 0.10 0.04-0.27 8 0.10 0.07-0.12 3

Ca 0.10 0.03-0.17 26 0.04 0.02-0.08 8 0.09 0.07-0.11 3

0.98 0.82-1.12 26 1.10 1.06-1.21 8 0.96 0.89-1.02 3aSee Appendix III for individual analyses. Some ratios based on 8[Mn + Fe] , where no analy­sis of Al was available. Cryptomelane mineral types are discussed in the text.

The fundamental constraint on the composition of the cryptome-

lane minerals at Lucifer appears to be the maintenance of an "A"-site

to "B"-site cation ratio of very close to 1:8, as indicated by Figure

30, which is a plot of total moles of univalent cations in the "A" sites

of Lucifer cryptomelane minerals against total moles of divalent cations,

all mole values proportioned to 8 (Mn + Fe + Al). Line A-A1 repre­sents the plot which would result for an "ideal" cryptomelane maintain­

ing an exact 1:8 "A"-site to "B"-site ratio.

A ternary plot of lead-potassium-barium molecular proportions

is presented in Figure 31, emphasizing the fact that whereas lead, po­

tassium, and barium may substitute for one another to a considerable ex­

tent in the Lucifer cryptomelane minerals, there is a distinct gap be

between low lead and high lead (coronadite) compositions. Low-lead

varieties of cryptomelane appear to be characteristic of Lucifer ore,

and form a continuous series from high-barium to high-potassium compo­

sitions, designated as "Type I" in Figure 31. Analyses of nearly "pure"

K-cryptomelane, approaching the ideal formula KMngO^g apparently form a

cluster designated as "Type II" in Figure 31. High-potassium (Type II)

cryptomelanes were detected only in four samples, LP2B, LP15, LP16B,

and BS2, collected respectively from the zone of mineralized Comondu

breccia, nearby mineralized Quaternary sediments and Boleo basal conglom­

erate, and from the main ore mass, within 100 m of the mineralized Co­

mondu (Appendix IV). High-lead type cryptomelane (Type III) was found only in sample LP2B.

70

MO

L E

S

Ba

+

Pb +

Ca

1.0

\

.8--

.6—

.4 —

.2 —

Figure 30.

\\\\

ideal ^ \cr yp I o m e l a n e ^ ^ser ies \

\

• LF2D* LF13C O LFl 5 x LF16B A L7.3h ■A LZ39 o B51■ BS2 O BS5

\A

\ x \ □

A\ *\\\\ A a\

\\ ■\ \

\4 .6

M O L E S Na + K

.8 1.0

Plot of Univalent vs. Divalent Cations in Lucifer Cryptomelane All values proportioned to 8(Mn + Fe + Al) or 8(Mn + Fe) if no available for Al.'

"A" Sites, data

K CRYPTOMELANETYPE H -

Hlgh-K - CryptomelaneLP2B

* LPI3C

x LPI6B

6 L Z 34

4 LZ39

■ BS25 0 TYPE I -

° BS5Cryptomelane

TYPE H - y Coronadite

Ptr--CORONADITE

--xBaHOLLANDITE

Figure 31. Pb-K-Ba Ternary Diagram. — Cryptomelane phases only.

73

A ternary diagram of calcium-potassium-sodium molecular propor­

tions is presented in Figure 32. All cryptomelane analyses for which

sodium and calcium values were determined are included in Figure 32, as

well as several analyses from samples LP3E and BS5 which, based on un­

usually high sodium or calcium values and also magnesium content, where

available, probably represent mixing of cryptomelane minerals with

other phases. The likely presence of todorokite and other minerals with

high sodium, calcium, or magnesium contents has been discussed. •

It may be seen in Figure 32 that most analyses plot in an arbi­

trarily drawn, but nevertheless restricted high-potassium field (Field

I). The only analyses plotting outside this field are from samples

LP2B," LP3E, LP16B, and BS5. Sample LP3E is a concretion collected from

the main ore horizon near the head of the tram, approximately 100 m from the exposure.of the ridge of Comondu-volcanics (Appendix IV). The other

samples were collected from the zone of mineralized Comondu breccia

(LP2B and BS5) or from manganese-impregnated Boleo basal conglomerate (LP16B).

The analyses from LP3E and BS5, the coronadite analyses from LP2B, and several analyses from LP16B lie within a low-potassium field

(Figure 32, Field II). Other analyses from LP2B and LP16B, however,

plot well within the range of general values (Field I), including all

analyses from sample BS2, which is a sample of the main ore mass inwhich LP3E occurred.

K

/ x x

v V /

Figure 32. Na-K-Ca Ternary Diagram. — Includes cryptomelane and mixed phases.

LP2B

LP3E

LPI3C

LP I5

LPI6B

L Z 3 4

L Z 3 9

BSI

BS2

BS5

Paragenetic Relationships in the Lucifer Manganese Ore

The proposed sequence of mineralization, as determined from ex­

amination of polished sections of Lucifer ore, is presented in Figure 33.

The gangue minerals in the ore all appearto have formed after the man­

ganese oxide minerals. The earliest formed manganese minerals presum­

ably were "protocryptomelane" minerals, such as nsutite, birnessite, and possibly other poorly crystalline phases not specifically identi­

fied, from which the cryptomelane minerals, pyrolusite and todorokite

may have formed, based on studies of supergene deposits and experimental

studies (Bricker, 1965; Roy, 1968; McKenzie, 1971). Manganese minerals

in concretions may, in general, have formed earlier than minerals pres­

ent in interconcretion ore.

Textural features in samples collected near the mineralized Co-

mondu volcanics suggest that the unusual range of cryptomelane composi­

tions in these samples may be the consequence of more than one stage of

cryptomelane mineral formation. Thus, in LP2B, later high-potassium

cryptomelane consistently appears to nucleate cores of coronadite, and

in some cases, actual replacement may have occurred (Figure 26). In

LP16A, high-potassium cryptomelane nucleates and possibly replaces both

high-barium and high-lead varieties of cryptomelane. It is not known

when coronadite formed with respect to barium-rich phases.

Pyrolusite consistently appears to have formed after the crypto­

melane minerals present. Colloform iron oxides and silica, for the most

part, seem to fill the open spaces into which pyrolusite can be observed

to have grown, often maintaining well-formed, unobstructed growths.

75

76

+■ Early____________ _̂____________Late

"Proto-cryptomelane" -1-2-

Coronadite -----

Ba-cryptomelane 2-2—2—1_2_

High-K Cryptomelane -----

Pyrolusite ' -----

Iron-oxides _____

Silica --- .

Calcite _____

Chrysocolla

Figure 33. Paragenetic Sequence of Mineral Formation in Lucifer Ore.

77

Veinlets of amorphous iron oxides and silica may also be seen to cut

across manganese minerals. Late-stage, fairly pure silica is found to

cut across both manganese oxides and iron-rich gangue.

Calcite occurs in discontinuous patches and veinlets which can

be seen to cut across all prior-formed minerals discussed above. More

commonly, calcite is found to occur in the center of silica and iron-

oxide patches and replacements. The remaining gangue minerals, mainly

clay minerals and halite, may in part have been present prior to depo­

sition of the ore minerals, or their formation may have been contempo­

raneous with deposition of other minerals in the ore to some extent.

Chrysocolla veinlets are abundant in the manganese ore at one

location in the western part of the area included in Figure 4, where

sample LZ39 was collected. The chrysocolla in this sample occurs within

calcite veinlets and replacements, and would appear to be contemporane­

ous with calcite deposition. The deposition of calcite and chrysocolla

apparently .represent the last stage of mineralization affecting the

Lucifer deposit. Microprobe analysis of sample LZ39 indicated that

much of the cryptomelane had somewhat higher than normal copper contents (Appendix II).

f

ALTERATION IN THE LUCIFER AREA

The Comondti volcanics and the Boleo sediments have been sub­

jected to several stages of alteration. Alteration involving destruc­

tion of mafic silicate minerals in Comondu volcanic rocks, also observed

in clastic sediments derived from Comondu volcanics, apparently occurred

prior to mineralization and associated alteration in the district. Sub­

sequent argillization of the Comondu and derived sediments and formation

of various authigenic minerals, was the likely consequence of their in­

teraction with solutions associated with the manganese mineralization.

The formation of smectitic clays in the Boleo tuffs was probably mainly

related to typical post-depositional interaction of volcanic glass with

seawater, apparently leaving mafic silicate minerals and feldspars un­

affected. This last type of alteration is believed to be unrelated to

the formation of the Lucifer deposit, and need not be discussed further.

Alteration of Mafic Silicates

Most samples of Comondd rocks examined in thin section can be

seen to have undergone at least some alteration of mafic silicate miner­

als, generally pyroxenes, although olivine may have been involved as

well. The intensity of alteration may vary from grain to grain and from

sample to sample. Some samples apparently have no unaltered pyroxene

remaining, whereas in others, pyroxene phenocrysts are altered, but

fresh pyroxene can still be identified in the groundmass.

The alteration generally involved: 1) intense limonitic stain­

ing, which often obscures optical properties; 2) change in color to78

79

red-orange or brown; and 3) changes in texture to a fibrous or a wispy-

amorphous appearance. In many samples, orange-red, euhedral, aniso­

tropic grains with parallel extinction appeared to be pseudomorphs

after orthopyfoxene, or perhaps olivine, and seemed to closely fit the

description of "iddingsite" (Gay and Le Haitre, 1961).

The altered grains frequently appear zoned with either core or

rim consisting of limonite and altered or unaltered material. Altera­

tion may also occur as patches within large pyroxene phenocrysts, often

associated with cleavage fractures.

The well-bedded felsophyric-textured units appear to be the rock

type most affected by the mafic silicate alteration process. The pro­

cess appears to'have affected felsophyric-textured rocks more extensive­

ly than vitrophyric rocks in general, regardless of proximity.to the

manganese mineralization or to the degree of weathering. Thus, altered

samples associated with manganese mineralization may still have fairly

fresh-looking pyroxenes. Both felsophyric and vitrophyric pyroclastic

units are characterized by fresher mafic silicate minerals than their

bedded equivalents, although they are much more vulnerable to normal weathering.

Microprobe compositional analyses of mafic silicates in Comundti

volcanic rocks and their alteration products are presented in Table 5.

The alteration products of three different samples are listed separately;

apparently the alteration products in each of these samples have charac­

teristic compositions.

Table 5. Microprobe Analyses of Fresh and Altered Mafic Silicate Grains in Comondd . Volcanic Rocks (Weight Percent).

_____________ Unaltered Grains______________Orthbpyroxenes Cllnopyroxenes ________________ ________ Altered Grains________________________(two analyses) (eight analyses) "Chlorophaeite*'(7) ‘‘ChlorophaeiteM(7) hIdding8iteM(?)Analyses from Analyses from LP6A (three analyses) (six analyses) (four analyses)

m & LX3 LX3, LP17 & LZ22 LZ22 • LP17 LP6AAvg. Range Avg. Range Avg. Range Avg. Range Avg. Range

S102 50.02 48.58-51.46 52.06 49.93-53.41 42.61 40.30-45.28 51.53 45.95-55.33 32.59 29.98-34.04

A12°3 6.93 6.16- 7.70 2.00 0.91- 3.77 1.94 1.58- 2.34 6.15 4.74- 7.74 2.88 2.77- 2.99

Na20 0.04 0.03- 0.05 0.26 0.15- 0.43 0.04 0.00- 0.09 0.17 0.12- 0.34 0.01 0.00- 0.04

K2° 0.07 0.06- 0.08 0.03 0.01- 0.10 0.79 0.64- 0.95 1.91 1.18- 2.90 0.63 0.50- 0.73

CaO 1.42 1.36- 1.47 18.12 10.17-22.43 0.62 0.60- 0.65 1.81 1.34- 2.49 0.72 0.66- 0.83

MgO 22.65 21.88-23.42 15.29 13.31-17.79 9.63 8.37-11.57 11.50 6.32-14.10 6.75 5.75- 7.40

FeO 15.63 14.38-16.87 10.62 3.53-17.90 25.27 20.36-31.84 15.94 10.58-20.83 44.72 42.44-47.63

MnO 0.35 0.33- 0.36 0.26 0.11- 0.38 0.03 0.01- 0.05 0.01 0.00- 0.04 0.06 0.00- 0.09

t i o 2 0.20 0.15- 0.24 0.53 0.12- 0.87 0.003 0.00— 0.01 0.07 0.00- 0.23 0.05 0.02— 0.08

Cr-0- 0.03 0.02— 0.03 0.13 0.00- 0.37 0.03 0.02- 0.04 0.02 0.00- 0.06 0.01 0.00— 0.02

Sum. 97.34 94.61-100.01 99.30 97.85-100.97 80.96 77.28-87.45 89.11 80.12-95.03 88.42 87.84-89.23

81

The alteration products of sample LP6A appear to be "iddingsite" based on their optical properties and on their compositions (Table 5,

col. 5), as compared to analyses presented in other papers on the sub­

ject (Wilshire, 1958; Gay and Le Maitre, 1961). The compositions deter­

mined for the mafic silicate alteration products of sample LP6A imply derivation from olivine, although no unaltered olivine was identified in

the sample. The alteration products in samples LZ22 and LP17 both ap­pear to have been derived from augite, as no orthopyroxene was identi­

fied in either sample, and the analyses include altered inclusions

within augite grains and altered cores rimmed by augite. Based on their

composition, the alteration products from these samples are probably

"chlorophaeite" rather than "iddingsite" (Wilshire, 1958).

The chemical exchanges involved in the alteration of mafic sili­

cate minerals in the Comondd volcanics are indicated more clearly in

Table 6, which compares an analysis of unaltered augite in sample LZ22 to an altered inclusion within that augite grain, and compares an unal­

tered rim of an altered augite grain to the altered core of the grain.

The data in Tables 5 and 6 indicate that the mafic silicate al­teration process is always characterized by an increase in KgO and FeO/

MgO, and by losses of MgO, TiOg, and MnC^. The low analysis totals of

altered grains suggest that the alteration process also involved sub­

stantial hydration. The consistent, appreciable loss of manganese in­

volved in the alteration process may be of significance to the origin

of the Lucifer deposit, and is therefore especially noteworthy.

82

Table 6. Microprobe Analyses of Two Partly Altered Augite Grains.________________ Sample LP17___________ ________Sample LZ22________

Unaltered Altered Inclusion Unaltered Altered Core Part of Grain in Grain Rim of Grain of Grain

Si02 52.90Weight Percent

55.33 52.23 40.34

A1203 0.91 5.95 1.65 2.34

Na20 0.15 0.34 0.24 0.09

k 2o 0.06 2.50 0.04 0.64

CaO 10.17 1.47 22.43 0.60

MgO 17.79 11.54 16.58 11.57

FeO 17.90 17.77 3.53 31.84

MnO 0.38 0.00 0.11 0.05

Ti02 0.71 0.10 0.35 0.01

^r2°3Sum.0.00

100.970.0395.03

0.3597.85

0.0287.45

Si 7.8678Molecular Proportions 8.4614 7.7967 7.4501

A1 0.1600 1.0732 0.2894 0.5091

Na 0.0429 0.0994 0.0696 0.0312

K 0.0113 0.4879 0.0071 0.1502

Ca 1.6198 0.2400 3.5875 0.1188

Mg 3.9447 2.6319 3.6907 3.1880

Fe 2.2265 2.2729 0.4403 4.9221

Mn 0.0477 0.0000 0.0138 0.0073Ti 0.0790 0.0116 0.0390 0.0016

Cr 0.0000 0.0036 0.0823 0.0035

0 24.0000 24.0000 24.0000 24.0000

Alteration Associated with Manganese Mineralization

83

The conspicuous bleaching of pebbles and rims of boulders of

the Boleo basal conglomerate where it is overlain and impregnated by

manganese oxides and associated iron oxides and jasper (Figure 24), and the occurrence of authigenic minerals in vesicles and fractures of the

boulders and pebbles are believed to be the consequence of the interac­

tion of ore-bearing solutions with the rocks. Similar alteration is seen in Comondu rocks throughout the district, but generally is much

less pronounced.

Vesicles and fractures in the altered rocks are filled with

smectitic clay, "chlorophaeite"(?), limonite, rare quartz crystals,

possibly adularia, and zeolite, identified petrographically and by x-ray

diffraction as epistilbite (Cag^AlgSi^gOgz) * lOH^O).Examination of basal conglomerate pebbles and boulders from the

mineralized zone in thin section indicates that the alteration mainly

involved argillization of glassy groundmass. Mafic silicates apparently

were affected principally where greatest alteration of the groundmass

could be observed. Apparently, destruction of previously altered pyrox­

enes was enhanced by interaction with the ore solutions in many cases,

based on the much more pronounced mafic silicate destruction in

vitrophyric-textured rocks in the immediate vicinity of the manganese

mineralization. Plagioclase microlites and phenocrysts were only

slightly affected, involving for the most part their breakdown to clay

along cleavages and fractures. Neither petrographic examination

nor microprobe analysis revealed any indication of albitization of

plagioclases.

Microprobe analyses of the glassy groundmasses of three rela­

tively unaltered samples, LZ22, LP17, and LX1, and the clay-rich ground-

mass of sample LX3 are presented in Table 7. X-ray diffraction of the

groundmass of LX3 indicates the occurrence of smectitle clay and a mod-oerately strong 7.2 A line suggesting the occurrence of kaolinite or

chlorite, although the former is more likely based on the disappearance o oof the 15 A peak upon heating to 200 C and based on petrographic

characteristics.

84

Whole-rock Compositional Analyses of Fresh and Altered Rocks

Whole-rock compositional analyses were determined for Comondu

volcanic samples collected from original depositional locations and from

the basal conglomerate of the Boleo Formation. Uncorrected analyses are

presented in Table 8. In Table 9, the weight percent of all non-volatile

components except manganese have" been normalized to 100% to present a clearer indication of the relative abundance of the components in each

sample by compensating for the large addition of water and other vola­

tiles into the altered samples, as implied by the large "loss on igni­

tion" values obtained for these samples, and addition of manganese into

the mineralized samples. The product of the specific gravity of each

sample and the weight percent of each component is presented in Table

10, thus permitting comparison of the weight of each component per equal volume, 1 cubic centimeter, of rock.

Table 7. Microprobe Analyses of canics (Weight Percent)

Groundmasses of Several Samples of Gomondu Vol-

LZ22 LZ22 LP17 • LXl LXl LX3 LX3Si°2 63.59 65.76 73.01 80.24 80.61 48.56 46.30

M 2°3 14.20 17.07 12.67 12.45 11.66 18.58 18.45

Na20 3.44 4.15 0.32 0.71 0.08 0.16 0.25

k 2o 5.38 5.92 1.60 1.49 1.12 0.29 0.34

CaO 3.08 2.43 0.82 0.59 0.40 1.57 1.63

HgO 1.25 0.40 0.75 0.00 0.18 4.02 4.56

FeO 0.90 0.55 4.01 1.05 1.03 1.80 1.76

MnO 0.02 0.00 0.01 0.00 0.00 0.00 0.00t i o2 0.11 0.10 1.64 0.69 0.70 0.17 0.66

Cr2°3 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sum. 91.98 96.40 94.84 97.21 95.78 75.15 73.95

Table 8. Uncorrected Chemical Composition (Weight Percent) of Comondu Volcanic Rocks. — Columns 1-5 are original Comondu volcanic rocks; columns 6-8 are Comondu rocks in

__________ the Boleo basal conglomerate. ______________1

LP5A2

LX103LP2A

4LX13A

5LX13B

6LX1

7LX3

8LX7

Si02 54.20 55.80 54.40 47.90 49.00 64.10 52.00 52.20

A12°3 17.30 16.90 18.80 16.80 17.70 15.90 19.10 20.50

CaO 7.11 6.46 4.97 5.45 5.55 4.55 4.84 4.84

MgO 4.44 3.83 1.48 1.81 1.73 2.41 2.96 2.06

Na20 3.73 4.09 4.42 3.64 3.96 3.88 3.64 4.48

KgO 0.86 1.71 2.47 2.34 2.23 2.09 0.35 0.60

FeO 7.34 6.66 7.18 7.66 7.41 4.15 5.71 6.95

MnO 0.11 0.10 0.07 5.95 3.97 0.08 0.21 0.07

Ti02 1.05 1.19 1.33 1.23 1.27 0.59 0.75 0.77

P2°5 0.41 0.48 0.51 0.54 0.49 0.17 0.08 0.03

L.O.I. 3.53 3.15 4.62 4.68 4.55 2.72 11.17 7.94

Sum. 100.80 101.20 101.00 98.90 98.70 101.10 101.50 101.20

Ba (ppm) 1100 1400 1300 2200 1300 1300 1100 1000

Table 9. Corrected Chemical Composition (Weight Percent) of Comondti Volcanic Rocks. — Values __________ neglect MnO and L.0.!♦ values and are reproportioned to 100% total._________________

LP5A LX10 LP2A LX13A LX13B LX1 LX3 LX7Si02 56.10 57.40 56.80 54.70 54.80 65.40 58.10 56.40

AlgOg 17.90 17.40 19.60 19.20 19.80 16.20 21.30 22.20

CaO 7.36 6.64 5.19 6.22 6.21 4.64 5.40 5.23

MgO 4.60 3.94 1.55 2.07 1.94 2.46 3.31 2.23

Na20 3.86 4.21 4.62 4.15 4.43 3.97 4.07 4.85

K2° 0.89 1.76 2.58 2.67 2.50 2.14 0.39 0.65

FeO 7.60 6.85 7.50 8.74 8.30 4.24 6.38 7.52

Ti02 1.09 1.22 1.39 1.40 1.42 0.60 0.84 0.83

P2°5 0.42 0.49 0.53 0.62 0.55 0.17 0.09 0.03

BaO 0.12 0.14 0.15 0.25 0.16 0.14 0.12 0.12

oo

Table 10. Chemical Composition of Comondu Volcanic Rocks Expressed In Grams Per Cubic Centimeter.

LP5A2.1*

m o2.4*

LP2A2.0*

LX13A2.0*

LX13B2.0*

1X12.4*

1X31.7*

1X71.3*

Si02 1.14 1.34 1.09 0.96 0.98 1.54 0.88 0.68

ai203 0.36 0.41 0.38 0.34 0.35 0.38 0.32 0.27

CaO 0.15 0.16 0.10 0.11 0.11 0.11 0.08 0.06

MgO 0.09 0.09 0.03 0.04 0.03 0.06 0.05 0.03

Na2° 0.08 0.10 0.09 0.07 0.08 0.09 0.06 0.06

k2o 0.02 0.04 0.05 0.05 0.04 0.05 0.01 0.01

FeO 0.15 0.16 0.14 0.15 0.15 0.10 0.10 0.09

MnO 0.002 0.002 0.001 0.12 0.08 0.002 0.004 0.001

Ti02 0.02 0.03 0.03 0.02 0.03 0.01 0.01 0.01

P2°5 0.01 0.01 0.01 0.01 0.01 0.004 0.001 0.000BaO 0.003 0.004 0.003 0.005 0.003 0.003 0.002 0.001

L.O.I. 0.07 0.08 0.09 0.09 0.09 0.07 0.19 0.10^Specific gravity (grams per cubic centimeter)

89

The five original Comondu samples are felsophyric-textured; the

basal conglomerate samples are vitrophyric-textured. Two of the samples,

LX10 and LP5A, were described in the section on petrology of the Co-

mondu volcanics; these samples were collected from locations well re­

moved from exposures of manganese ore. The other samples were collected

from locations at the exposure of mineralized Comondu volcanics mapped

in detail and discussed above (Appendix IV).

Sample LP2A is a Comondu breccia fragment collected from the

mineralized zone, where it was completely enmeshed in manganese oxides.

The sample is texturally and mineralogically similar to samples LX10 and

LP5A, except that no unaltered pyroxenes can be identified. The plagio-

clase microlites appear to be extremely fresh, giving no indication of.

having been albitized or otherwise altered. Samples LP13A and LP13B

are agglomerate clasts also collected from the mineralized zone. Manga­

nese oxides occupy some of the vesicles and fractures, but these sam­

ples are otherwise very similar to LP2B in that the plagioclase

microlites appear extremely fresh, as indicated by petrographic exami­

nation and substantiated by microprobe analysis, while no unaltered

mafic silicates were to be found.

Samples LX1 and LX3 are from the same boulder embedded within

altered tuff, iron-rich jasper, and manganese oxides in the mineralized

portion of the Boleo basal conglomerate where it wedges out between the

manganese manto and mineralized Comondu. LX1 is an extremely fresh sam­

ple of the core of the boulder; photomicrographs of the sample are pre­

sented in Figure 34. Sample LX3 is from the altered rim, and differs

90

A. Nicols uncrossed.

Figure 34. Photomicrograph of a Sample of the Unaltered Core of a Boulder of Comondtf Volcanics in Mineralized Boleo Basal Conglomerate (Sample 0X1). — Diameter of field is 3.82 mm.

91

from LX1 in that its groundmass has been largely altered to clays (Fig­

ure 35). Slight breakdown of the mafic silicate minerals, both ortho­

pyroxenes and clinopyroxenes, is also apparent. Sample LX7 is from an

altered pebble collected from the mineralized basal conglomerate at the

same location as LX1 and LX3, to which it is similar texrurally. It

differs mineralogically, as it contains rare amphiboles. Both its

groundmass and its mafic silicate minerals appear to have been altered

to a considerably greater extent than those of LX3.

The whole-rock compositional differences presented in Tables 8,

9, and 10 between the slightly altered felsophyric-textured samples, LP5A

and LX10, and the highly altered sample from the mineralized zone, LP2A,

may be primarily related to the mafic silicate alteration process, based

on similar chemical differences between fresh and altered grains indi­

cated by the microprobe analyses presented above. A comparison of the

whole-rock analyses of LP5A, LX10, and LP2A with those of the two

felsophyric-textured mineralized samples, LX13A and LX13B, suggests

that mineralization involved little more than a slight loss of silica;

otherwise, the mineralized samples appear to be very similar composi-

tionally to LP2A, implying that most of the compositional differences

of the mineralized samples with respect to LX10 and LP5A may also be re­

lated primarily to alteration of the mafic silicate minerals.

The corrected and uncorrected whole-rock analyses of samples

LX1, LX3, and LX7 presented in Tables 8, 9, and 10 indicate that inter­

action of these rocks with the manganese solutions involved consider­

ably different chemical exchanges than those which involved the

t

B. Nicols crossed.

Figure 35. Photomicrograph of a Sample of the Altered Rim of a Comond(f Volcanic Boulder in Mineralized Boleo Basal Conglomerate (Sample LX3). — Diameter of field is 3.82 mm.

93felsophyric textured rocks. Substantial hydration was apparently in­

volved, implied by the considerable increases in "loss on ignition"

values of the altered samples with respect to LX1. The hydration and

loss of silica and KgO in particular would seem to be directly related

to the alteration of the groundmass to clays, as discussed above, which

would appear to have been the principal effect of interaction of Comondu

rocks with the manganese solutions. The considerably greater volume of

groundmass in the vitrophyric rocks may explain why these rocks were ap­

parently more intensely effected by interaction with manganese solutions

than were f elsophyric-textured rocks.

A comparison of the whole-rock analyses of samples LX3 and LX7

indicates that these two samples have very similar chemical composi­

tions, despite the fact that they are different mineralogically, and

that LX7 has a much more altered appearance.

DISCUSSION

Stratiform manganese deposits are abundant in the geologic rec­

ord from the late Precambrian to the present, comprising the most com­

monly encountered type of manganese ore deposits (Roy, 1968; Park and

MacDiarmid, 1970). Stratiform manganese deposits may occur within a

variety of depositional environments and tectonic settings. They con­

sistently display a variety of sedimentary characteristics that give

rise to theories of syngenetic origins contemporaneous with hosting

strata. Although the largest terrestrial manganese reserves occur in

stratiform deposits not clearly associated with volcanic activity, those

of conclusive volcanic affiliation are most widespread, and many others

have been found to be associated with at least small amounts of tuffa—

ceous material, to give support to theories of volcanic-related hot

spring origins (Hewett, 1966; Stanton, 1972).

There are many noteworthy examples of stratiform manganese de­

posits which, like Lucifer, occur within tuffs or volcanic-derived

elastics. Among those which may have particular bearing on the origin

of the Lucifer deposit are the early Cretaceous deposits of Coquimbo

Province, central Chile (Aguirre and Mehech, 1964); the mainly Eocene

age deposits of Oriente Province, Cuba (Simons and Straczek, 1958); the

mid-Tertiary San Francisco deposit, Jalisco state, Mexico (Zantop, 1978)

the lower Pliocene(?) Artillery Peak deposit, northwestern Arizona

(Lasky and Webber, 1949); and the Pliocene(?) age Three Kids deposit,

southern Nevada (McKelvey, Wiese, and Johnson, 1949). In addition.

94

95

Quaternary manganese deposits in northern Chile and the Afar Rift region

in Ethiopia are significant because of their relation to ongoing geolog­

ical processes (Cruzat Ossa, 1970; Bonatti et al., 1972).

All these deposits consist, for the most part, of beds of man­

ganese mineralization interlayered with clays, altered andesite tuffs,

and clastic volcanic debris. The manganese may occur within a single lens or in groups of lenses. The manganese-bearing units occur within

extensive volcanic piles deposited either in continental lacustrine or

playa environments (Artillery Peak, Three Kids, San Francisco) or in

shallow marine basins. The bulk of the mineralization consists of-man­

ganese oxides in highly oxidized states, such as the cryptomelane min­

erals or pyrolusite. Manganese carbonates occur only rarely in deposits

of this type.

Jasperoidal silica is the major gangue mineral. The jasper gen­

erally occurs interlayered with the manganese oxides, as at Lucifer,

but in some cases it occurs as a stratigraphically discrete horizon.

Iron oxides and, less commonly, calcite are also present in the gangue.

The hosting tuffs are, in some cases, altered to montmorillonite, chlor­

ite, and zeolites, as in the manganese deposits of Cuba.

The ore horizons of all these deposits exhibit many characteris­

tics which are typical of sedimentary deposits, in particular: 1) tex­

tural and, in some cases, chemical stratification which may parallel host

rock bedding down to a microscopic scale; 2) soft sediment deformation;

and 3) the common occurrence of intraformational conglomerate containing

manganese fragments in upper layers of the ore horizon. In addition,

96

the San Francisco deposit contains fossil remains (Zantop, 1978), and in

several deposits of Cuba, clastic dikes composed of tuff or limestone

from overlying beds are to be found within the ore horizon (Simons and

Straczek, 1958). At Artillery Peak, the sedimentary features include mudflakes, scour and fill surfaces, and drying cracks filled with silt

from overlying layers (Lasky and Webber, 1949). The manganese horizons

at Artillery Peak are characterized by tremendous stratigraphic persis­

tence over thousands of meters across the basin of deposition and in­

volve many different lithologic types within that stratigraphic horizon,

ranging from conglomerate to fine-grained tuff.

Paleotopographic and paleoenvironmental factors appear to have

exerted a major control on the form and composition of many of these de­

posits. Thus, in the Elqui River district of central Chile, a bed of

manganese occurring within highly altered tuffs associated with impure

limestone is traceable for 20 miles along strike, although no more than 1 mile wide across strike. The manganese bed has been interpreted to be

the result of deposition parallel to an ancient shoreline (Park and

MacDiarmid, 1970). In other districts in central Chile, lava flows ap­parently deposited in a shallow marine depositional basin created iso­

lated troughs with lagoonal environments in which manganese deposition

occurred. The characteristics of the individual troughs appear to have

determined the variability in thickness of the mineralized beds, prob­

ably as well as other characteristics of the deposits (Aguirre and Mehech, 1964).

97

In the Lake Mead-Three Kids region, manganese deposition was

generally found to have occurred near the margins of structural basins

adjacent to mountain uplifts, the long axes of the deposits striking

parallel to the borders of the basins (McKelvey et al., 1949). Proxim­

ity to shore, depth of water, and proximity to the presumed hot spring

source are believed to have been significant factors controlling local­

ization of the manganese ore.

The geometry of the depositional basin with respect to location

of a presumed source of hydrothermal solutions apparently was a signif­

icant factor in the formation of the San Francisco deposit, Jalisco,

Mexico. The deposit displays marked zonation into an iron-rich,

manganese-poor northwestern region and manganese-rich, iron-poor south­

eastern region where the deposit is thickest and widest, and where clas­

tic layers and admixtures are least abundant. The layers of manganese

oxides and iron oxides interfinger laterally, but contacts are extremely

sharp. The zonation is believed to be best explained as the consequence

of mineralizing solutions, bearing both M n ^ and Fe^+ , entering the depo­

sitional basin at the northwestern edge via hot springs. In response to

the tendency of iron oxides to precipitate before manganese oxides upon

a gradual increase in Eh and pH, iron oxides precipitated closer to the

source than the manganese oxides. The decrease of clastic admixtures

toward the southeast is believed to be supportive of the postulated di­

rection of transport of the solutions (Zantop, 1978).

Deposition of manganese in the Arica province of northern Chile

has been taking place since the Pleistocene, corresponding to the onset

98

of the latest phase of volcanic activity (Cruzat Ossa, 1970). Hydro­

thermal manganese has been deposited around hot springs, within lacus­

trine basins along with clastic materials and lesser amounts of

colloidal silica, and in subsurface veins and lenses. Fumaroles and

solfataras are still active in the district, and one warm spring (28°C)

is currently depositing manganese in a swamp.

The Pleistocene-age iron-manganese-barium deposit at En Kafala,

in the northern Afar region of Ethiopia, is situated within what is be­

lieved to be an active spreading center, the continuation of the Red Sea

Rift (Bonatti et al., 1972). Many indications exist of exhalative-

hydrothermal activity connected with volcanism that is still intensely

active. Fumaroles and hydrothermal-springs;-some of which feed saline

lakes, are common along faults and fissures running parallel to the axis of the Afar rift.

The manganese deposit at En Kafala wedges out against a reef- limestone deposit and is believed to be of sumbarine origin, having

formed while the Afar rift was submerged by Red Sea water. Lying below

the orebody is a tuffaceous basalt with a vitroclastic texture typical

of subaqueous volcanic eruptions. Manganese is present both in the

groundmass and in veinlets. Extreme separation of iron from manganese

occurred, resulting in basal layers which are iron-rich and manganese-

poor overlain by strata which are manganese-rich and iron-poor.

The mineralogy at En Kafala consists mainly of pyrolusite, bir-

nessite, and todorokite. Barium is present mainly as strontium-rich

barite. Silica is largely present within iron-rich montmorillonite of

possible authigenic origin in the iron-rich strata; opal is rare.

According to Bonatti et al. (1972), the deposit originated by

the introduction of iron- and manganese-bearing solutions from below,

through the sea floor and into the bottom sea water where fractionation

into iron oxide and manganese oxide precipitates occurred. The source

of the iron and manganese is believed to be basaltic rocks associated

with volcanic activity in the region which were leached by hot, saline

brines presumably derived from sea water circulating through fractures

and fissures (Bonatti et al., 1972; Rona, 1978).

Active deposition of manganese oxides by hot springs occurs in

the Arica province in northern Chile (Cruzat Ossa, 1970); the Hokaido

district, Japan (Hewett, 1966); and elsewhere. Analyses of manganese­

bearing springs and sinter aprons deposited about such springs are pre­

sented by Hewett and Fleischer (1960) and Hewett, Fleischer, and Conklin

(1963). Recent formation of manganese oxides by submarine hydrothermal

exhalations has been investigated at various locations, including Matu-

pai Harbor, New Britain (Ferguson and Lambert, 1972), the volcano Banu

Wuhu (Zelenov, 1964), the Red Sea brines (Bischoff, 1969), and at numer­

ous seamounts and other locations associated with mid-ocean spreading centers (Rona, 1978).

Manganese oxides deposited by hot springs in continental ter­

rains generally are accompanied by travertine deposits composed largely

of calcium carbonate (White, 1955; Hewett and Fleischer, 1960). Silica

apparently is a much more common product of submarine exhalative

99

100activity, as are iron hydroxides and authigenic iron-smectite (nontro-

nite) (Bonatti et al., 1972; Rona, 1978; Snyder, 1978). The manganese

of continental hot spring deposits is most commonly present in the min­

erals pyrolusite, the cryptomelane minerals, psilomelane, or wad (Hewett

and Fleischer, 1960); birnessite and todorokite are more frequently en­

countered in submarine hydrothermal deposits (Rona, 1978). Barium and

strontium are characteristically present in notable abundances, both as

product of submarine hydrothermal exhalations and continental hot spring

manganese deposition.

White (1955) discussed qualitative aspects -of thermal springs

related to mineralization in general. He observed that near-surface

wall rocks in volcanic spring systems are likely to show little or no

alteration unless the waters are rich in sulfuric acid. At depths great­

er than 100 ft, nearly all rocks subjected to aqueous solutions heated

to temperatures close to boiling show the presence of alteration minerals,

including clays, silica, adularia, chlorite, zeolite, and pyrite. Simi­

lar alteration in the uppermost zones of large-scale geothermal systems

is discussed, in detail by Miyashiro (1973) and Ellis and Mahon (1977),

and by Mottl and Holland (1978) with respect to geothermal systems in

Japan, New Zealand, and Iceland. At the active geothermal fields of

Wairakai, New Zealand, for example, hydrothermal alteration of rhyolite

tuff and breccia is characterized by glass going to kaolinite or mont-

morillonite and the presence of the zeolites mordenite and heulandite

filling vesicles in the uppermost zones, where temperatures of less than

200°C are recorded (Miyashiro, 1973). Low-temperature alteration of

101basalt in the uppermost zones of the Reykjanes geothermal system, Ice­

land, is characterized by the presence of smectites and the low-

temperature zeolites mordenite, stilbite, and mesolite (Mottl and

Holland, 1978). The analogy with the alteration described at Lucifer is obvious.

Alteration of basalt under the physical and chemical conditions

prevalent at the discharge zone of a submarine hydrothermal convection system typically results in horizontally and vertically zoned mineral

assemblages characteristic of low- to intermediate-grade metamorphism

(Rona, 1978). Depletion halos have been found to occur in the altered

rocks surrounding submarine hydro thermal deposits and are believed to be

the consequence of the removal of metals by the hydro thermal system to

produce the deposits. The lack of moderate- or high-temperature meta­

morphism or hydrothermal alteration at Lucifer stands in contrast to most

mid-ocean spreading center hydrothermal systems, although not to the presumed rift-related manganese-barium-iron deposit at En Kafala, or to

most other continental manganese deposits.

ORIGIN OF THE LUCIFER DEPOSIT

Geologic Criteria

Four mechanisms for the origin of the Lucifer deposit seem reas­

onable, based on current concepts of ore genesis:

1) The manganese and other metals may have been derived from hydro-

thermal solutions; the Lucifer deposit formed largely by impreg­

nation and replacement of the Boleo tuffs, a preferred host.

This is the epigenetic-replacement origin of Wilson (1949).

2) The metals may have been derived from hydrothermal solutions

feeding hot springs; deposition of the manganese ore occurred

along with deposition of the Boleo tuffs within a shallow marine

sedimentary basin. This is based on the volcanogenic-sedimentary

origin advocated by Hewett (1966) and others for the origins of many manganese deposits.

3) The metals may have been derived from sediments actually occur­

ring in the depositional basin in which the Lucifer deposit oc­

curs; in particular, the Boleo tuffs. Reworking of the tuffs by

sea water or diagenetic remobilization could have released man­

ganese and other metals which either reprecipitated as manganese

minerals within the hosting tuffs, or migrated upward to the

sea floor and precipitated there as manganese minerals. Theories

involving the origin of manganese deposits by reworking of sedi­

ments present in the depositional basin or by diagenetic remobi­

lization of metals are discussed by Park (1956) and Stanton (1972),

102

103

and a similar mechanism for the origin of the Boleo copper beds

has been proposed by Schmidt (1975).

4) The metals may have been derived from remote sources by the

weathering of the volcanic and sedimentary rocks bordering the

depositional basin in which the Lucifer deposit occurs. Trans­

port of the metals to the basin could have been either fluvial

or by groundwater, and the metals could have entered the basin

as either particulate matter or in solution. Weathering solu­

tions are thought to supply the manganese and other metals in

some present-day ferromanganese basins, and have been postulated

to have supplied the metals to several major stratiform manga­

nese deposits thought to be of sedimentary origin, but which

lack clear evidence for association with contemporaneous

volcanic-related hot spring activity, including the Russian de­

posits of Nikopol and Chiatura (Zantop, 1978).

The last three postulated mechanisms for the origin of the Luci­

fer deposit suggest that the deposit is sedimentary or sedimentary-

diagenetic, although (2), like the epigenetic-replacement theory,

involves a hydrothermal source. Several possibilities exist as to the

precise nature and origin of the hydrothermal solutions which may have

been involved in the mineralization at Lucifer, and will be dealt with below.

Wilson (1949, p. 217) concluded that the Lucifer deposit origi­

nated from deposition "by hydrothermal solutions that rose along faults

through the Comondu volcanic rocks and spread out along the bedding

104

planes of the tuff members of the Boleo Formation, impregnating and re­

placing the tuff." Wilson cited the following lines of evidence:

1) the presence of veinlets of manganese oxide and other minerals

within the Comondu volcanics, particularly along faults;

2) the manganese ore is accompanied by jasper which may be seen to

cut across bedding, transgress Comondu volcanics, and locally

may cut across both the overlying and underlying conglomerates;

3) the occurrence of similar local transgression of manganese ox­

ides into overlying and underlying conglomerate units;

4) localization of manganese deposits irregularly throughout the

district within Boleo tuff, most deposits consisting of mere

patches or a complex of veinlets, pockets, and irregular masses,

in many cases clearly related to faults;

5) structural control of ore deposition by a paleotopographic ter­

race, as described above.

The presence of mineralized veinlets in pre-ore rocks need not

be prohibitive of a syngenetic hot spring origin for the major part of

the deposit (Snyder, 1978). A hydrothermal system capable of supplying

enough metals to form a deposit the size of Lucifer would entail a plumb­

ing system of considerable extent, whether mineral deposition were to

occur within pre-existing rocks or at the sediment-sea water interface.

Any available channel might be utilized, including faults, fractures,

and bedding planes, all of which could be the site of mineral deposition

under the right circumstances.

105

The abundance of jasper in the gangue at Lucifer also presents

no obstacle to a hot spring syngenetic origin, and may, in fact, be sup­

portive. Evidence of silica supplied hydrothermally has been reported

at many submarine hydrothermal deposits, including Thera and Stromboli

in the Mediterranean Sea and the Red Sea hot brines (Bonatti et al.,

1972). Often, as appears to have been the case at Lucifer, the silica

is found to co-precipitate with iron. Hot spring waters sampled from

the Galapagos rift are reported to be enriched in Si02 to as much as

four times the level of normal sea water (Snyder, 1978), and analyses

of the silica content of spreading-center hydrothermal deposits from

ten regions ranged up to 52.94% (Rona, 1978). Silica has been reported

to be a major constitutent of several terrestrial hot spring waters and

their sinter aprons as well (Hewett and Fleischer, I960; Stanton, 1972;

Hewett et al., 1963).

Crosscutting relationships of the jasper, at Lucifer, as well as

of manganese oxides, could easily have resulted from downward or lateral

migration into wet, unlithified, or partially lithified sediments at the

time of initial deposition of the manganese ore (Snyder, 1978). The ir­

regular and sporadic indications of upward migration could have been the

result of secondary mobilization due to diagenetic or supergene activity,

a possibility admitted by Wilson (1949, p. 217-218). The feasibility of

secondary.mobilization unrelated to original deposition is emphasized by

the occurrence of mineralized patches seen in talus deposits which have

probably formed since mining activity began.

106

The presence of many small, low-grade manganese deposits near

Lucifer merely indicates that manganese mobility was the result of

regional-scale conditions, probably related to district-wide thermal

activity, fracture favorability, and abundance of source material. The

observation that many of these other deposits probably are of epigenetic

replacement origin need not logically imply that Lucifer had the same

origin.

The structure controls which affected the deposition of the man­

ganese manto at Lucifer were the result of the same paleotopographic fea­

tures that affected all post-Miocene depositional patterns within the

sedimentary basin containing the deposit. Thus, the structural terrace

at Lucifer probably controlled the depositional patterns of the Boleo

tuffs as well as the orebody, as indicated by the rapid pinch-out of

both the tuffs and the orebody within less than 100 m from the center "of

the structural terrace, where the ore reached maximum thickness and the

tuffs are over 5 m thick (Figure 14; Wilson, 1949, Plates 45 and 49).

In addition, the Boleo tuffs and the manganese orebody show the same sig­

nificant steepening of dip to the northeast of the more gently dipping terrace.

The results of this study appear to suggest that, for the most

part, the Lucifer deposit is somehow sedimentary in origin, having

formed syngenetically with the hosting lowermost tuff unit of the Boleo

Formation within a shallow marine basin.

The most compelling line of evidence for a sedimentary origin

to the Lucifer deposit is the stratigraphic persistence of the

107

manganese horizon, despite changing lithologies, traceable for at least

one-half kilometer north of the southernmost limits of the manganese

ore, where laminae of manganese oxides can be seen to be interstratified

with Boleo tuff strata in a zone measurable in centimeters, occurring

just below the contact with overlying Boleo tuffaceous conglomerate. In

general, wherever manganese oxides and Boleo tuff occur together, they

appear to be interstratified, displaying similar soft-sediment deforma­

tion features. Also difficult to explain as the result of replacement

or impregnation of a favored host is the general sharpness of contacts

between the manganese horizon and overlying or underlying strata, which

are generally tuffs or tuffaceous conglomerates, and not observably

different from tuff strata which interbed with the manganese ore.

Various features of the Lucifer deposit are consistent with

other manganese deposits believed to be sedimentary in origin, although

they do not in themselves necessitate such an origin; these features in­

clude the lenticular form; the concordance with stratification of the

hosting rocks, and the apparent structural control of the Lucifer depos­

it by the paleotopographic features of the depositional basin. The man­

ganese ore itself displays a variety of characteristics often indicative

of a sedimentary origin, some of which are difficult to explain other­

wise. Particularly noteworthy in this context are the pronounced strat­ification of the manganese ore and the interstratification of manganese

oxides and jasper, soft-sediment deformational features, and the occur­

rence of pisolitic structures, concretions, and the frequently occurring

intraformational-clastic fabric of the ore.

108

Of the mechanisms invoking a sedimentary origin for the Lucifer

deposit proposed above, it is suggested that an origin by the emergence

of hydrothermal solutions from hot springs best explains the features of

the deposit. Although a weathering origin involving transport of manga­

nese by groundwater might account for most features of the deposit, the alteration of the Comondu volcanics and Boleo basal conglomerate asso­

ciated with the manganese mineralization are probably more characteristic

of temperatures associated with the uppermost zones of hydrothermal sys­

tems, as discussed above, than of normal weathering conditions unaffected

by a thermal source. The contemporaneous deposition of the Boleo tuffs

with the manganese ore would further suggest that the Lucifer deposit

was related to a hydrothermal convection system driven by the magmatic

activity responsible for the Boleo tuffs.

A fluvial origin for the Lucifer deposit seems unlikely because

it would fail to explain the mineralization and alteration noted in the

vicinity of the mineralized Comondu ridge. In addition, there is no

real evidence for a fluvial outlet in the Lucifer area during the time

of deposition of the Boleo tuffs.

The scarcity and freshness of mafic silicate minerals in the

Boleo tuffs argue against their being the source of the manganese, as

proposed for other manganese deposits by Park (1956). In general, it

would seem to be difficult for a theory involving reworking of the Boleo

tuffs to account for the geometric and structural features of the

lucifer deposit, its stratigraphic persistence even where the tuffs

have pinched out, and the alteration, mineralization, and laharic

109features seen in the exposure of the Comondu ridge discussed above. In

addition, evidence that the Comondii volcanics were a more likely source

for the mineralization than the Boleo tuffs will be presented.

Several lines of structural and geometrical evidence suggest

that the source of the manganese was to the south of the manto, presum­

ably related to the mineralization and related alteration of the Comondu

volcanics occurring where the manto onlaps the paleotopographic ridge.

As a structural and topographic highland at the time of deposition of

the Boleo Formation, the ridge was a potential source of sediments to

the depositional basin in which the Lucifer deposit occurs. The shape

of the depositional basin, as well as of the manganese manto, was direct­

ly influenced by the ridge, as indicated by the change in direction of

structure contour trends and isopach trends determined by. Wilson (1949).

The extent of mineralization is such that the thickest, highest grade

ore occurred within 100 m of the ridge, with the mineralization becoming

progressively lower in grade and thinner towards the north, away from the ridge.

The manganese solutions apparently became mixed with Comondu

pyroclastic rocks and Boleo conglomerate, impregnating the matrix, and

locally mineralizing fractures. Where Comondu bedded volcanics occurred,

the manganese solutions flowed over bedding planes, including exposed

dipslopes, and generally covered the Comondu ridge as they flowed to­

wards the adjacent basin. The steep slope of the ridge and the satura­

tion of the Comondii breccia and basal conglomerate matrices by the ore

fluids apparently lead to mudflows which resulted in the chaotic breccia

110seen in the exposure of the ridge (Figure 22). The mudflow may have

been in part subaerial, or entirely subaqueous.

The laharic nature of the mineralized Comondu breccia near the

presumed source of the Lucifer deposit seems to imply that mineraliza­

tion occurred rapidly. Interbedding of manganese with tuffs distal from

the source area would imply that pyroclastic volcanism was ongoing while

manganese was being deposited.

The manganese is believed to have been deposited in a marine

environment because the hosting Boleo tuffs are interpreted to be marine

deposits, and because many .of the sedimentary structures of the manga­

nese ore, such as soft-sediment deformation,- the intraformational tex­

tures, and the presence of pisolites and concretions, are often

associated with subaqueous - environments of deposition. The deposit is

thought to be of shallow marine origin due to its proximity to the pinch-

out of the Boleo Formation against the Comondu ridge, which may have been

subaerial in part, and also because both the Boleo tuffs and the manga­

nese locally display reworked, intraformational fabrics which might in­dicate deposition in an energetic, wave-swept environment.

The postulated emergence of manganese solutions from a single,

hot-spring source located near the zone of mineralized Comondu might

help explain compositional and textural features indicated in the course

of the study of the mineralogy of the Lucifer ore, namely: 1) the re­

striction of coronadite and high-K cryptomelane to samples collected

proximally to the mineralized Comondu (Figure 31); 2) the unusually

large range of cryptomelane compositions determined for samples LP16B

Ill

and LP2B (Figures 31 and 32); 3) the textural evidence for two or more

stages of cryptomelane formation in these samples (e.g.. Figure 26);

4) the restricted range of cryptomelane compositions in samples collect­

ed distally from mineralized Comondu, particularly with respect to lead,

sodium, and calcium content (Figures 31 and 32); and 5) the fact that

samples collected in the vicinity of the mineralized Comondu yield analy­

ses that fit into this restricted range, as well as analyses that do not,

whereas all analyses from samples collected distally fit into the range

(Figures 31 and 32).

The actual mineralization process involved .would have been ex­

ceedingly complex, involving such variables as the temperature and compo­

sition of the ore solutions, the forcefulness of the emission of the ore

solutions, assuming an exhalative origin, the characteristics of the me­

dium into which the ore solutions were emitted, the solubility and dif­

fusion properties of the mineral species involved, and many other factors

(Turner and Gustafson, 1978). It seems reasonable to envision some sort

of process, however, in which solutions capable of precipitating minerals

with the characteristic low-lead, sodium, and calcium, high-barium compo­

sition of the Lucifer ore were able to flow from the zone of mineralized

Comondu, the postulated source area of the deposit, to distal parts of the depositional area. Uncharacteristic phases were deposited closer

to this postulated source, due either to thermodynamic and chemical con­

siderations of the ore solutions and the system as a whole, or simply

to the fact that there was not enough of the solutions which formed them

to make it to the distal parts of the area of deposition before all

possible precipitation of manganese phases had occurred. In either

case, the observed mixing and replacement would be expected to occur

proximally to the source of the ore solutions.

Tectonic Setting and Related Processes of Ore Deposition

The geologic record depicted by the rocks of Baja California

may be divided into Mesozoic, mid-Cenozoic, and late Cenozoic tectonic

frameworks (Gastil et al., 1975). The pre-Cenozoic record exposed in

the Santa Rosalia region is very scant, although Mesozoic batholithic

rocks presumably form the basement upon-which the well—exposed Miocene

and post-Miocene volcanics and sediments were deposited (Wilson, 1955).

The Mesozoic plutonic basement presumably represents remnants of the

roots of a subduction-related dacite-andesite arc complex (Gastil et al.,

1975). Emplacement of the Mesozoic batholithic rocks was apparently

followed by uplift, cooling, and erosion, and eventually by the onset

of the Miocene tectonic regime.

The Comondu volcanics are believed to record a marginal trench-

arc system lying to the west of Baja California prior to 8-10 m.y. ago,

presumably related to subduetion of the Farallon plate beneath the North American plate (Karig and Jensky, 1972; Larson, 1972). Major plate re­

organization apparently occurred 8-10 m.y. ago, leading to the transi­

tion from subduetion activity to transform fault motion along the western

edge of the North American plate. The direction of North American plate

movement changed significantly, assuming a trend closely paralleling the

present-day western coastline of Baja California; that is, approximately

112

11315-20° more northerly than present-day motion (Larson, 1972). These

plate reorganizations probably occurred as a consequence of the colli­

sion of an oceanic rise lying west of the Farallon plate with the sub-

duction zone. The precise timing and details were undoubtedly quite

complex, and appear to have involved the creation of a southward migrat­

ing ridge-transform-trench triple junction (Larson, 1972).

The plate reorganizations which occurred 8-10 m.y. ago would

have been responsible for the early, more severe stage of structural

deformation of the Comondu volcanics, as discussed in the sections on

structural features, including the pre-Pliocene predominant north-

northwest trending faults and fractures. The secondary northeast frac­

ture trend may also, in part, be related to these tectonic events

(Schmidt, 1975).

These plate reorganizations, in addition, would have been respon­

sible for the change from an eastern source for the Miocene volcanics

deposited in Baja California, to western sources for Pliocene and young­

er deposits. The change in sedimentation direction, as well as the late

Miocene-early Pliocene structural activity, are probably associated with

the initial episode of rifting of the Gulf of California, which lead to

the formation of the so-called proto-gulf of California (Larson, Menard,

and Smith, 1968; Moore and Buffington, 1968; Karig and Jensky, 1972;

Larson, 1972; Moore, 1973). The existence of a proto-gulf from 8-10 m.y.

ago to possibly as recently as 4 m.y. ago is based on several different

lines of evidence discussed by the authors listed above. The duration

of the initial episode of extensional tectonics associated with the

114

proto-gulf is not clearly constrained; it is not certain whether spread­

ing ended abruptly significantly before the onset of current extensional

activity, or whether it was continuous but at a slower rate for some

period of time. In either case, the indication is clear that little or

no spreading occurred for a period of time during the 4-8 m.y. interval(Karig and Jensky, 1972). Deposition of the Boleo Formation in the Santa

/Rosalia area in the early Pliocene would appear to have occurred during

this time period (Moore, 1973; Schmidt, 1975).

It is widely held, on the basis of considerable bathymetric and

geophysical data, including fracture zone patterns, magnetic anomaly pat­

terns, and seismic data, that Baja California has been moving northwest

with respect to mainland Mexico at the average rate of 6 cm per year for

the past 4 million years (Larson et al., 1968; Moore and Buffington,

1968; Moore, 1973). This motion is believed to be the consequence of

strike separation along a series of en echelon transform faults and in­

terconnecting spreading centers (Vine, 1966; Sykes,-1968; Moore and

Buffington, 1968). Initiation of the current episode of rifting of the

Gulf of California may have occurred as the result of the Pacific-North

American plate boundary, a probable transform fault, either jumping or

migrating eastward from its original location west of Baja California

to its present location within the Gulf of California. The result was

to leave Baja California attached to the Pacific plate as a single unit (Larson, 1972).

Lucifer and the other ore deposits of the Santa Rosalia region,

which all appear to be somehow related to the deposition of the Boleo

115

Formation, clearly occur in a structural framework, particularly with

respect to fault and fracture characteristics, related to an extensional

tectonic setting that had been in existence for at least 3 million years

before the deposition of the Boleo Formation, and which has continued to

the present.

The significance of rift-related structural features to hydro-

thermal convective systems and examples of the types of rift-related

structures affecting hydrothermal deposition in oceanic spreading cen­

ters are discussed by Rona (1978). Particularly noteworthy in this con­

text is the common occurrence of fumaroles and hydrothermal springs

which stem from faults and fissures that run parallel to the axis of the

Afar rift, noted in connection with the manganese deposits of En Kafala

(Bonatti et al., 1972).

Hydrothermal convection systems in general require a heat source

and circulation system with suitable recharge and discharge (Rona, 1978).

In typical spreading-center hydrothermal systems, such as are believed to be currently active in mid-oceanic ridges and the Red Sea, emplacement of

basaltic magma is believed to provide the necessary thermal.source

(Bonatti, Guerstein-Honnorez, and Honnorez, 1976; Rona, 1978). Cold sea

water is believed to penetrate through fractures caused either by tec­

tonic processes or in the process of magma chilling, whereupon it enters

a convection system generated by the hot basalt. Heated sea water may

interact with basalt and sediments, effectively leaching the rocks with

which it comes into contact with certain elements, principally silica,

calcium, potassium, helium, and arsenopyrite and transition metals.

116

while transferring magnesium, sodium, and oxygen to produce the hydro-

thermal alteration associated with the leaching process. Certain ele­

ments, including barium, may be partly derived from the mantle (Rona,

1978).

The spreading-center hydrothermal convection model may be analo­

gous to the hydrothermal system involved in the formation of the Lucifer

deposit. However, a fundamental difference between the spreading center

model and the hydrothermal system probably involved at Lucifer is the

fact that the likely thermal engine for the system at Lucifer was mag­

matic activity associated with the Boleo tuffs, rather than the genera­

tion of oceanic basalt. The apparent calc-alkalic nature of the Boleo

tuffs, as well as the calc-alkalic volcanism and intrusive activity

known to have occurred throughout the Pliocene in the La Reforma complex,

as described by Schmidt (1975), implies a .type of lag of magmatic compo­

sition behind the extensional stresses responsible for post-Miocene nor­mal faulting and related fractures whichiare suggested to have provided

the plumbing for convecting hydrothermal fluids in the district. Schmidt

postulates reasons for this apparent lag in magmatic composition. Al-

kalic and tholeiitic volcanism, which are more commonly associated with

extensional tectonic environments and generation of sea floor in spread­

ing centers (Christiansen and Lipman, 1972; Carmichael, Turner, and

Verhoogen, 1974) apparently commenced only in the Pleistocene in the

Santa Rosalia region with the eruption of the Tres vfrgenes volcanics

(Demant, 1975; Gastil et al., 1975; Schmidt, 1975).

117

Several mechanisms may be postulated by which the manganese and

other metals present in the Lucifer deposit may have been derived: 1)

high-temperature leaching of either the hosting Boleo tuffs or basement

rocks, either the Comondu volcanics or the Cretaceous batholith rocks,

analogous to the spreading-center convection model; 2) from late magmatic

contributions; and 3) from the mantle, transported as volatiles.

It is suggested that the results of this study point to the Co­

mondu volcanics as having been involved to the greatest extent by the

proposed hydrothermal system, and furthermore, that the Comondu volcanics

probably were the major source of the metals present in the Lucifer depos­

it, as indicated by the following lines of evidence: 1) the Comondu vol­

canics, particularly bedded units, show the greatest fracturability of

pre-ore rocks seen in the Lucifer area; 2) the Comondu display extensive

orthogonal fracturing related, at least in part, to pre-Boleo Formation

stresses; 3) abundant vein mineralization, related to the principal

fracture trend, occurs in the Comondu; 4) the hot spring proposed as

source of the Lucifer deposit appears to have vented from a mineralized

part of the paleotopographic ridge of Comondu volcanics; and 5) altera­tion of the Comondii volcanics appears to have lead to chemical exchanges

which could have produced the type of solutions which may have formed the Lucifer deposit.

Lower Boleo sediments probably were involved in the convective

flow of hydrothermal solutions proposed to have lead to the Lucifer de­

posit, as indicated by the presence of manganese oxide veinlets in the

Boleo tuffaceous limestone and tuffs at Lucifer. However, while metals

118

may have been extracted from the Boleo tuffs to contribute to the min­

eralization, the intrinsic freshness of the tuffs and their high back­

ground manganese values and low Fe:Mn ratio are indications that the

Boleo tuffs probably were not major contributors of metal to the Lucifer

deposit. Data on iron and manganese contents of relatively unaltered

Comondu volcanic rocks, a highly altered sample of Comondu', Boleo tuffs

and Lucifer manganese ore are presented in Table 11.

It must be emphasized that the extent of involvement of the Cre­

taceous batholithic rocks or the importance of direct contributions from

late magmatic or mantle sources are not known and cannot be ruled out,

although they would not appear to be necessary to account for any of the

major constituents of the Lucifer deposit.

Geochemical Processes of Formation

An understanding of the chemical processes responsible for the

formation of the Lucifer deposit involves the recognition that the pre-

ore rocks have apparently been affected by two separate stages of alter­

ation involving considerably different chemical exchanges. Destruction

of mafic silicate minerals in the Comondu* volcanics is believed to have

occurred during the early stage of alteration, whereas the hydrothermal

solutions apparently directly responsible for the formation of the Luci­

fer deposit are believed to be responsible for the later alteration stage.

The processes which may have been involved in the alteration of

mafic silicate minerals to "chlorophaeite" or "iddingsite" are discussed

in some detail by Wilshire (1958) and Gay and Le Maitre (1961). Hydrogen

Table 11. Weight Percent Fe, Mn, and Fe/Mn of Unaltered and Altered Comondtt' Volcanic ___________ Rocks, Boleo Tuffs, and Lucifer Ore.______________________

1Unaltered

Comondu Volcanics32

Sample3

Boleo Tuffk

4Lucifer

Manganese OrecAvg. Range LP2A Avg. Range Avg. Range

Fe 5.37 5.12- 5.73 5.60 3.82 3.56- 4.07 1.10 .75- 2.12

Mn 0.09 0.08— 0.10 0.05 0.18 0.15- 0.21 49.03 37.00-57.70

Fe/Mn 59.70 54.20-67.60 112.00 21.20 19.60-24.40 0.02 — — —

^Includes samples LP5A, LX10, LX11, and LZ22. °Includes samples LP3A and LZ20B. cFrom Wilson (1949, p. 213).

119

120diffusion in a strongly oxidizing environment appears to have been in­

volved, permitting the chemical exchanges involved, oxidation of iron,

and structural disordering of the pre-existing mineral framework.

Microprobe analyses of the alteration products of the mafic

silicate minerals in the Comondif volcanics indicate significant deple­

tion of manganese with respect to unaltered minerals (Tables 5 and 6). This depletion of manganese is probably not typical of either "idding-

site" or "chlorophaeite", based on published analyses presented by

Wilshire (1958) and Gay and Le Maitre (1961), and it is suggested that

it may be an indication that the manganese present in the Lucifer depos­

it was derived from the mafic silicate minerals occurring in the Comondu

volcanics.

The data presented in Tables 5 and 6 also indicate that iron en­richment often accompanied alteration of the mafic silicate minerals.

This apparent iron enrichment may provide an explanation for the mecha­

nism by which manganese fractionation from iron occurred at Lucifer,

allowing for the manganese to be concentrated into - a large economic de­

posit. Investigations into the problem of Fe/Mn fractionation by

Krauskopf (1957, 1967) and subsequent workers (Stanton, 1972; Hajash,

1975; Bonatti et al., 1976) have established that a solution derived by

the leaching or weathering of igneous rocks will have an Fe:Mn ratio

that is dependent on a number of factors, foremost of which are tempera­

ture, water:rock ratio, and pH. Table 11 indicates that fresh Comondu

volcanic rocks in the Lucifer area have an average Fe:Mn ratio of 57.3,

and Boleo tuffs have an average Fe:Mn ratio of 21.2. The significantly

lowered Fe:Mn ratio of 0.02 in the Lucifer manganese ore is thus seen

to be problematical.

Recent water-rock interaction experiments at elevated tempera­

tures and pressures indicate that formation of iron-rich mineral phases

may occur in sea water-reacted rocks if the water:rock ratio is suffi­

ciently low, thus aiding in the retention of much of the iron of the un­

altered rock and allowing for increased Mn:Fe ratios in the reacted sea

water (Seyfried and Bischoff, 1977). The iron enrichment accompanying

alteration of Comondti mafic silicate minerals at Lucifer suggests that

the formation of iron-rich phases as a consequence of the alteration

processes affecting the Comondu may have allowed for similar preferen­

tial segregation of manganese from iron. The apparent absence in the

Lucifer area of either iron sulfides or an iron oxide facies such as is

present at En Kafala or the San Francisco deposit also points to this

conclusion.

There appear to be two likely mechanisms by which manganese

could have been derived from mafic silicate minerals in the Comondu' vol­

canic s and concentrated into an economic deposit, and that would account

for the two stages of alteration that appear to have been involved in

the ore formation process at Lucifer.

1) The earlier stage of alteration may have somehow allowed the

direct removal of manganese from altered grains by later hydro-

thermal ore solutions which were migrating along fractures and

bedding planes in the Comondu', to be transported eventually to

the site of deposition.

121

1222) The manganese may initially have been mobilized during or, more

likely, sometime after the early alteration stage and concen­

trated with other phases in the Comondti, most probably opaque

minerals or limonite, to be removed later by the ore-forming

solutions. This second interpretation is supported by the oc­

currence of manganese in many of the opaque grains in concentra­

tions on the order of 1% or more, indicated by microprobe studies, although acceptable quantitative analyses of the opaque grains

were not attainable.

Leaching of manganese by the ore solutions may have occurred to

a greater extent at depth than within any rocks now exposed in the Luci­

fer area, aided perhaps by higher temperatures and pressures and a lower

f(02). It may be of significance, however, that sample LP2A, the al­

tered f elsophyric-textured- breccia clast collected in the zone of miner­

alized Comondu, has a considerably reduced whole-rock manganese content

compared to relatively fresh Comondif rocks (Table 11). In addition, as

indicated in Table 11, the Fe:Mn ratio of LP2A is approximately double

that of average unaltered Comondu rocks. The tremendous volume of Co—

mondu volcanic rocks in the Lucifer area would virtually eliminate the

necessity to consider the efficiency of the process of manganese remov­

al. It should be noted that manganese-iron separation may have also

occurred in the process of forming the abundant disseminations and vein-

lets of iron oxides seen throughout rocks in the district, as well as

in the alteration of glass to clays or other alteration products.

123

Considerable amounts of iron may have combined with amorphous SiC^, ac­

counting for the abundant jasper seen at Lucifer.

An understanding of the chemical nature of the ore solutions re­

sponsible for the Lucifer deposit is provided by representation of the

phases involved in the presumed hydrothermal alteration of Comondu*rocks

at Lucifer on a ternary composition diagram and an activity-activity

phase diagram (Figures 36 and 37, p. 127). These diagrams are based on

the not-entirely justifiable assumptions that the alteration phases rep­

resent equilibrium assemblages, and that only the groundmass was in­

volved in the alteration process. Nevertheless, they provide insight

into the alteration process and imply that the alteration seen at Lucifer

may be related to the means by which the ore solutions obtained potassium

and possibly other cations occurring with the manganese in Lucifer ore.

Figure 36, a ternary composition diagram in the system CaO-AlgOg-

K2O, conveniently represents all the major phases involved in the alter­ation process seen in Comondu rocks at Lucifer. Ubiquitous SK^ and 1^0

are assumed. The average of the compositions of the groundmasses of

samples LX1 (unaltered) and LX3 (altered) are indicated on the diagram

(data from Table 7). They are assumed to be in equilibrium with K-

feldspar and the zeolite epistilbite, respectively, and the intersection

of these tie—lines indicates compatibility of both with smectite of one

specific composition under the proper conditions. In reality, the

groundmass of LX1 is probably an impure cryptocrystalline mixture of K-

feldspar and silica, while that of LX3 is probably a mixture of smectiteand kaolinite.

KaollnlteKaollnite

a ,2°3___

Smectites

Plus quartz and HgOCaO<-Calclte

Muscovite

Eplstllblte K- feldspar

Figure 36. Hypothetical Phase Relations Involved in the Alteration of Comond(f Rocks at the Lucifer Mine.

tZT

125

A schematic activity-activity phase diagram for potassium-

calcium alteration minerals found at Lucifer is presented in Figure 37.

The diagram was constructed assuming the stability of the montmorillon-

ite of composition indicated by the intersection of the tie—lines in

Figure 36, as well as the other phases indicated in that figure. No

effort was made to quantify the stability field sizes, and the diagram

is strictly qualitative in its usefulness. The slopes of the reaction

lines are based on the equations presented in Table 12. The relative

sizes of the stability fields are based on the relative sizes of the

fields at 260°C presented in a similar.diagram by Ellis and Mahon (1977).

Possible trends of the ore solutions as they attained equilib­

rium with K-feldspar are indicated. These trends are based on: 1) the

apparent stability of smectitic clays in both the fresh and altered

groundmasses; 2) the results of whole-rock and microprobe analyses of fresh and altered samples which indicate that the alteration involved

substantial loss of silica and potassium, and only a relatively slight

gain in calcium, if any; 3) the assumption that the solutions had an initial activity near the kaolinite-montmorillonite (smectite)—

epistilbite junction, based on the presence of these three phases in

altered rocks; and 4) the absence of sericite among the alteration prod­

ucts, which demands that the solutions never crossed into the potassium- mica field.

The reactions indicated by Figure 37 and Table 12 suggest a means

by which the ore solutions may have picked up abundant silica and potas­

sium, as well as possibly barium and lead, which also were involved in

126

Table 12. Equations and Slopes of Field Boundary Reactions for theSystem CaO-KgO-AlgOg-SiOg-HgO*

_______________________ Reaction^__________________________________Slope1) 1.5 Kfeld + H+ =0.5 Muse + K+ + 3Si022) Muse + 1.5 H20 + H+ = 1.5 Kaol + K+3) 10 Mont + 12.5 HgO + SH*" = 22.5 Kaol + 30 Si02 + K+ + Ca2+ -.5

4) 15 Muse + 30 Si02 + 10H20 + Ca2+ + 10H+ = 10 Mont + 14K+ 7

5) 22.5 Kspar + 5 H20 + Ca44- + 20 H+ = 5 Mont + 22K4 + Si02 22

6) Zois + 6 Si02 + 3K+ + H+ = 3 Kspar + H20 + Ca2+ 1.5

7) 2 Kspar + 5 H20 + Ca44 = Epistil + 2K4 2

8) 20 Mont + 120 Si02 + 205 H20 + 41 Ca44 = 45 Epistil +2 K4 + 80 H4 2/41

9) 2 Zois + 12 Si02 + 13 H^0 + 2H+ = 3 Epistil + Ca44 0

^Abbreviations:Kfeld = K-feldspar (KAlSi^Og)

Muse = muscovite mica (KAlgSi^O^gCOH)^Kaol = kaolinite (Al^^O^COH)^)

Mont = montmorillonite (Ko.lCa0.2A14.5Si7.5020^OH^4 * H20^Zois = zoisite (Ca2AlgSig012(°H))Epistil = epistilbite (CaAl2Sig0^g • 5H20)

127

ZOI S I T E

E P I S T I L B I T E

P o s s ib le t r e n d s of

o re s o lu l ions

M O N T M O R I L L O N I T E

K FELDSPAR

K A O L I N ! TE

Figure 37. Activity Diagram Depicting Phase Relations Involved in the Alteration of Comondu' Rocks in the Lucifer Area in Terms of the Variables Log (a2Ca24-/a2H+) and log (a^/a^) at Quartzand H2O Saturation.

128

forming cryptomelane minerals. The reaction paths in Figure 37 indicate

that the solutions that reacted with the Comondtf rocks had a low initial

(aK+ /ajj+) ratio and probably followed a nearly constant trend in ( a ^ ^ V

a +). It is apparent that an increase in a + probably was a major fac- H Ktor in the alteration process, and that the ore solutions had a strong

propensity to remove K+ ions from the rocks with which they reacted in

the process of achieving equilibrium.

Microprobe data and the whole-rock analyses presented in Tables

1 and 8 indicate that unaltered Comondu volcanics contain significant amounts of barium and lead in addition to potassium, particularly in the

groundmass of the vitrophyric-textured rocks, providing a suitable source

for all these elements. Microprobe analyses of potassium, barium, and

lead in the groundmass and in several mineral grains of Comondii rocks

are presented in Table 13, from which it may be seen that up to 0.28%

barium and 0.23% lead were detected in the groundmass of sample LZ22.

It is interesting to note that 500-600 ppm of lead were detected in an

exceedingly low potassium- and low barium-augite phenocryst. The appar­

ent depletion of barium accompanying depletion of potassium in the al­

tered rim of a rock (sample LX3) as compared to the fresh interior of

the rock (sample LX1) is indicated by the data in Tables 8, 9, 10, and may imply that barium and potassium were, in fact, involved in some of

the same chemical processes to account for the presence of both in the Lucifer ore.

The effectiveness of low pH solutions, as the ore—forming solu­

tions at Lucifer appear to have been, in extracting manganese and iron

129

Table 13. Microprobe Analyses for Potassium, Barium, and Lead in ____________Comondu Volcanic Rocks (Weight Percent).______________

Sample No. Type of Grain Potassium Barium Lead1 LZ22 Groundmass — a 0.13 0.23

2 LZ22 Groundmass __a 0.28 0.09

3 LZ22 Groundmass 0.76 0.08 0.04

4 LZ22 Groundmass . 7.54 0.21 0.015 LZ22 Plagioclase phenocryst 0.71 0.06 0.026 LZ22 Plagioclase phenocryst 0.28 0.04 0.007 LZ22 Augite phenocryst 0.02 0.00 0.06

8 LZ22 Augite^ phenocryst 0.03 0.03 0.05

9 LX1 Groundmass 0.52 0.18 0.02

Average

Groundmass

RangeNo. of Analyses

Potassium0 3.0 .52-7.54 8Barium 0.18 .08— .28 5

Lead 0.08 .01- .23 5aQuantitative analyses not made; presence detected by EDS scan. ^Same grain as number 7. cIncludes data from Table 7.

130

from basic volcanic rocks, even at low temperatures and after short time

durations, has been demonstrated experimentally by Krauskopf (1957) and

subsequent investigators. Krauskopf suggested that sea water can achieve

sufficient acidity to produce manganese-bearing ore solutions simply by

reacting with atmospheric COg or organic acids, although the process

would be greatly enhanced by the dissolution of volcanic-related vola­

tiles. Lowering of the pH of sea water can also be achieved by the ef­

fect of increasing temperature on the equilibrium constant of water, and

as a consequence of reactions involving the magnesium and sulfate compo­

nents of sea water (Mottl and Seyfried, 1977). Other research on water-

rock interaction has established that acid conditions may be generated

by the interaction of.sea water and basalt to form magnesium-smectites

and mixed-layer clays, which involves the removal of OH- ions from sea

water in the process (Seyfried and Bischoff, 1977; Seyfried, Bischoff,

and Dickson, 1975). Other pH-dependent equilibria that might be in­

volved are discussed, by-Hajash (1975). Any or all of these mechanisms

could have been instrumental in producing the type of solution necessary

to have leached and transported manganese at Lucifer.

The geochemical environment of deposition of the Lucifer deposit

may be described with an Eh-pH diagram (Figure 38, after Carrels and

Christ, 1965). The stability of manganese species with respect to oxi­

dation conditions (Eh) and hydrogen ion concentration (pH) has been

studied in considerable detail at 25°C by a number of workers (Krauskopf,

1957; Hem, 1964; Bricker, 1965; Carrels and Christ, 1965).

131

>-2 -6

YROLUSITE*

MnjOj ' 'x , MAHGANITE+0.2-

- 0.2-RHODOCHROSITE

ALABANDITE

25 C-0.8-

lo lal ~ 1 aim .

Figure 38. Eh-pH Diagram Depicting Stability Relations among SomeManganese Compounds. — Modified from Carrels and Christ (1965).

132

Standard conditions are assumed in Figure 38; that is, total

pressure = 1 atm and temperature = 25°C. It is also assumed, based on

geological observations discussed above, that the Lucifer manganese ore

formed under normal marine conditions, open to -exchange, and in equilib­

rium with atmospheric CO^. Hence, the partial pressure of COL, would have

been approximately 10 ^ atm. Figure 38 also assumes total dissolved sul­

fur species = 10 \ which is somewhat higher than, but of the same order of magnitude as, the concentration of sulfur in sea water (10 mole/1, calculated from data in Mason, 1966).

Figure 38 emphasizes the necessity for the ore solutions to have2+been acidic in order to allow transport of manganese ions in the Mn

(aq) state. The nature of the alteration reactions listed in Table 12

would also suggest the acidity of the manganese solutions. Figure 38

also indicates that a relatively high pH is required for the stability

of most manganese minerals, implying that precipitation of manganese at

Lucifer may have occurred as a consequence of rising pH upon mixing

with sea water (pH = 8.0-8.5) or sea water-saturated rocks, and in the

process of the manganese solutions reacting with the ComondiT volcanics.

The occurrence of pyrolusite as a major constituent of the ore

suggests a relatively highly oxidizing environment of deposition, which

is in support of the contention that the Lucifer deposit formed in a

near-shore and, possibly in part, subaerial environment. The absence

of manganite (MngOg), rhodochrosite (MnCO^), or related manganese miner­

als at Lucifer implies that oxidizing conditions were prevalent when the

manganese solutions entered the depositional basin.

133

The stability field of pyrolusite probably approximates that of

the cryptomelane minerals, based on the common association of these min­

erals, particularly in the uppermost, most highly oxidized zones of

supergene manganese deposits (Roy, 1968). The occurrence of cryptome­

lane minerals indicates high concentrations of the "A"-site cations in

the ore solutions needed to permit growth of the cryptomelane structure

(Buser, Graf, and Feitknecht, 1954; McKenzie, 1971).

It is conceivable that the apparent formation of pyrolusite

after the cryptomelane minerals in Lucifer ore may be due to the pres­

ence of excess manganese after all available cations needed to form the

cryptomelane minerals were used up.

The occurrence of jasper at Lucifer probably indicates fairly

low mixing ratios with sea water upon discharge of the ore fluids, as

higher mixtures of sea water appear to lead to the formation of iron

smectites, as at the En Kafala deposit, or to the absence of silica (Seyfried and Bischoff> 1977), and may be another indication of rapid

discharge of the ore solutions at Lucifer. Since precipitation is

largely a function of temperature at pH <9.0, the observed late occur­

rence of silica in the paragenetic sequence of Lucifer ore may be due

to the temperature cooling sufficiently to allow the presumably super­

saturated silica to precipitate (Krauskopf, 1967).

RELATION OF THE LUCIFER DEPOSIT TO COPPER DEPOSITS AND OTHER MANGANESE MINERALIZATION IN

AND NEAR THE SANTA ROSALIA (BOLEO) DISTRICT

The manganese oxide manto deposit at Lucifer, which has been the

focus of this study, is the most significant manganese mineralization in

the Santa Rosalia (Boleo) district, and is the most important mineraliza­

tion in the northern part of the district. Elsewhere, throughout most

of the district, copper is the major resource of economic interest. Low-

grade, uneconomic, bedded copper mineralization (mainly chrysocolla) in

the Boleo tuffs and small zones of chrysocolla impregnating the matrix

of Comondu breccia are present at several locations in the Lucifer area

as considered in this study (Figure 4, in pocket). The Lucifer area

also includes low-grade and low-bulk patches of manganese which prob­

ably are unrelated to the main manto deposit, some of which were mineable.

The occurrence of calcite with the copper and manganese described

aboVe, as well as the observed late-stage deposition of calcite and cop­per associated with the main manganese manto, may be indications that

the minor deposits of manganese and copper formed later than the main

manto, perhaps diagenetically as a consequence of mobilization of copper

and manganese by pore waters. Wilson (1949) suggested that much of this

localized, subeconomic mineralization may be related to small faults.

Other low-grade or low-bulk manganese deposits, some of which

were economic, are scattered over an area extending north of Lucifer

for several kilometers and southeast to Canada de la Gloria and Arroyo

134

135

del Boleo, 4.5 km from the Lucifer mine (Figure 2) where the northern­

most economic copper mineralization is juxtaposed with the manganese

mineralization (Wilson, 1949, 1955). Extensive beds of manganese ox­

ides, some containing up to 20% manganese, are found throughout the re­gion of the Boleo copper beds, and abundant manganese oxide occurs

intermixed with the copper mineralization. The manganese appears to be

more abundant in oxidized copper ore, although it is not restricted to

it. Economic manganese mineralization, however, is restricted to the

northernmost parts of the Boleo district in beds probably correlative

with Ore Bed No. 4, occurring within the -lowermost Boleo tuff unit.

Many of the minor manganese deposits in the Caffada de la Gloria-

Arroyo del Boleo area appear to show a-strong relationship to fractures,

either in the Comondu volcanics or the Boleo limestone and tuffs. Most

mineralization is associated with ferruginous, highly altered tuff which

commonly displays soft-sediment deformation that probably is, at least

in part, post-mineral. - Manganese mineralization in this area, as at

Lucifer, is accompanied by jasper,and small, less tuffaceous patches

appear to achieve grades comparable to those at Lucifer. Manganese bod­

ies cutting across gypsum or replacing fossiliferous limestone were re­

ported to occur in the area by Wilson (1949).

Many other manganese deposits, some of economic importance, have

been noted to occur along the eastern coast of the Baja California penin­

sula in a belt extending from the Lucifer region southward to 70 km south

of the town of Muleg^, and including San Marcos Island (Wilson, 1955;

Gonzales Reyna, 1956). The most important of these are veinlet deposits

136

occurring within altered ComondCi flows and pyroclastics at Gavilan and

Guadalupe, located on the Concepcion Peninsula, and at La Azteca, lo­

cated near Mulege' (Gonzales, 1956; Wilson, 1956b).

A pronounced northwestern structure trend controls the orienta­

tion of bedding, fractures, and veinlets in these deposits. The miner­

alogy includes primary manganese silicate and carbonate minerals, as well

as manganese oxide minerals, some of which are believed to be the prod­

ucts of secondary enrichment (Gonzales, 1956). The gangue mineralogy

includes abundant gypsum as well as quartz; calcite is rare. The mafic

silicate minerals of the Comondti volcanics have been intensely altered

to limonitic alteration products, and small amounts of epidote have been

noted as well. It is tempting to suggest that these deposits are genet­

ically related to the Lucifer deposit. The implication is that these

veinlet deposits formed at considerably higher temperatures than Lucifer,

as suggested, although not necessitated, by the occurrence of epidote

and manganese silicate minerals.

The relationship-of copper mineralization to manganese mineral!- •

zation in the Boleo district is one of the intriguing problems of the

district and will be addressed here briefly. Nishihara (1957) and Schmidt

(1975) discussed the significance of the stratigraphic persistence of

the primary, that is, sulfide copper ore beds as well as other sedimen­

tation patterns with respect to the role of sedimentary environment of

deposition on the formation of copper mineralization. Nishihara (1957)

believed that the source of mineralization was leaching of primary cop­

per sulfides in the Comondu volcanics by ground water, whereas Schmidt

(1975) pointed out the association of copper mineralization with the

hosting clayey tuff, which he concluded to be the source of metals.

Guilloux and Pelissonnier (1974) have indicated the apparent enrichment

of the Boleo copper beds in proximity to syndepositional normal faults

and pointed out the importance of pre-ore basement structures to ore

deposition in the district. They concluded that the source of the min­

eralization was leaching of the basement, the leaching having been chan­

neled by submarine hills and the syndepositional fault scarps prior to

burial by post-ore sediments.

The occurrence of the Lucifer deposit and Boleo copper deposits

within the same early Pliocene tuff units, and the apparent importance

to both deposits of structure and sedimentation patterns strongly con­

trolled by their tectonic setting are compelling indications that the

origins of both deposits are related.

Although the contention of Schmidt (1975) that the Boleo tuffs

originated from the La-Reforma complex seems reasonable for the earliest units, the evidence for the migration of the locus of maximum sedimenta­

tion and other stratigraphic data presented by Wilson (1955) seem to in­

dicate that the actual center of magmatic activity may have been

migrating southeastward as well. With respect to ore deposition in the

Boleo district, it is not certain whether the apparent southeastward mi­

gration of the center of volcanic activity is significant as the engine

driving hydrothermal convection systems, as is suggested to be the case

in the Lucifer area, or whether its significance lies in causing the

southeastward migration of the mineral-bearing Boleo tuffs, the related

137

138

copper deposits forming through normal sedimentary processes. Either

possibility, as well as the importance of syndepositional faulting to

the origin of the Boleo beds, are strong indications that tectonic events

exerted an important influence on mineral deposition in the Santa Rosalia

area.

The ubiquity of manganese throughout the Boleo district raises

the question of whether the manganese and copper are derived from the

same source. Some interesting observations on the chemistry of the man­

ganese ore and copper ore, in connection with the ideas presented in this study, offer a possible answer to this question. Partial analyses of

the average compositions of the Boleo copper ore and Lucifer manganese

ore are presented in Table 14, which is based upon data from Wilson

(1949, 1955) and Hewett (1966).- Table 14 indicates that the enrichment

of manganese at Lucifer is 7.6 times the concentration of manganese in

Boleo ore, whereas Boleo ore contains 17 times more copper than does

Lucifer ore. Table 1.4 also indicates that barium, strontium, and lead

all appear to be enriched in Lucifer ore relative to Boleo ore by about

the same factor as is manganese. Barium, strontium, and lead all are

major constituents of the Lucifer manganese ore, and of manganese ores

in general (Hewett and Fleischer, 1960; Hewett et al., 1963; Hewett,

1966). Bonatti et al. (1972) believe that barium and strontium, in the

forms of barite and strontiobarite, are characteristic constituents of

hydrothermal manganese deposits associated with oceanic spreadingcenters

Table 14. Partial Average Compositional Analyses (Weight Percent) of Lucifer Manganese Ore and Boleo Copper Ore.

Lucifer Manganese Ore__________~ ________________ Boleo Copper OreNo. of

Analyses Average Range SourceNo. of

Analyses Average Range SourceEnrichment

Factor®Hn 51 49.03 37.00-57.70 Wilson (1949) 35 6.42 0.39-22.49 Wilson (1955) +7.6

Cu 15 0.28 0.11- 0.71 Wilson (1949) Many 4.81 0.05-35.00 Wilson (1955) -17.0

Ba 7 0.86 0.06- 1.62 Wilson (1949) __b .OX — Wilson (1955) +1 order of mag.

Sr 4 0.72 0.50- 1.00 Hewett (1966) - b .OX — Wilson (1955) +1 order of mag.

Pb 15 0.59 0.01- 1.74 Wilson (1949) 4 0.06 0.00— 0.23 Wilson (1955) +9.8

Co 4 0.035 0.03- 0.05 Hewett (1966) 126 0.12 0.02- 0.86 Wilson (1955) -3.4

Hi 4 0.0015 0.0007-0.003 Hewett (1966) 7 0.04 trace-0.13 Wilson (1955) -27.0

Zn 44 0.11 0.01- 0.17 Hewett (1966) 130 0.80 0.05- 6.0 Wilson (1955) -7.3aA 4* indicates enrichment at Lucifer; - indicates enrichment at Boleo ^Qualitative spectrographic analysis of composite sample.

139

140

It may also be seen from Table 14 that cobalt, nickel, and zinc

all appear to be enriched in the Boleo ore with respect to Lucifer ore,

although by amounts that vary considerably with respect to either manga­

nese or copper, which stands in definite contrast to the three elements

related to manganese mineralization.

The data in Table 14 suggest that the manganese at Lucifer and

the manganese present in Boleo copper ore may have been derived from a

common source that yielded manganese, barium, strontium, and lead in

apparent characteristic proportions. The results of this study indicate

that this source probably was the Comondti volcanics. The metals associ­

ated with the copper mineralization (cobalt, nickel, and zinc) fail to

follow the same enrichment trends as either manganese or copper, and may

have been derived in part from both a presumably separate copper source

and from the principal manganese source.

The quality of the data presented in Table 14, particularly

barium and strontium values, are such that these conclusions are extreme­

ly tenuous. However, the data seem to support a major contention of

this study; namely, that the origin of the manganese at Lucifer is the

consequence of hydrothermal activity which would have occurred district­

wide in response to district—wide magmatic activity over the course of

time involved in the deposition of the Boleo Formation, itself a product

of this magmatic activity. Furthermore, since the data in Table 14 ap­

pear to suggest that the copper and manganese were, for the most part,

derived from different sources, the contention of this study that the

manganese and related metals at Lucifer and presumably throughout the

141

Boleo district is predominantly derived from the Comondu volcanics need

not conflict with Schmidt's (1975) conclusion that the Boleo tuffs were

the most likely source of copper mineralization in the Boleo district.

APPENDIX I

GENERAL MICROPROBE INFORMATION

Electron microprobe analyses for this study were performed on

The University of Arizona ARL Scanning Electron Microprobe Quantometer,

equipped with a Tracor-Northern automation system for reduction of data using the Bence-Albee correction program for silicate mineral analyses,

and the ZAF correction program for ore mineral analyses and analyses

for barium and lead in silicate minerals. Analyses were carried out at

15 KV, using a tightly focused beam at 30 nanoamps sample current.

Calibration for the 18 elements analyzed were carried out uti-

lizing the following standards:

Mn - manganese metal (ore minerals); rhodonite

K - orthoclase

(silicates) Mg and Si - diopsideNa — albite Ti - spheneCa and A1 — anorthite Ba — benitoiteCr and Fe — chromite Cu - copper metalS - arsenopyrite Ag - silver metalPb - galena

Zn - zinc metalNi - nickel metal

Co - cobalt metal

Elements in silicate minerals were analyzed for the following time intervals:

142

143Na - 10 seconds

Ca - 10 seconds

Si - 10 seconds

Fe - 10 seconds

Ti - 20 seconds

Ba - 30 seconds

Elements in ore minerals were most commonly analyzed for the

following time intervals:

Mn - 10 seconds Si - 10 seconds

Fe - 20 seconds A1 - 20 seconds

K - 20 seconds Na - 20 seconds

Ba - 20 seconds Pb - 20 seconds

Ca - 20 seconds Mg - 20 seconds

S - 30 seconds Cu — 60 seconds

Zn - 60 seconds Ag — 60 seconds

Co - 60 seconds Ni - 60 seconds

A few analyses were performed for different time intervals, but

the results appear to be consistent with analyses conducted for the in­

tervals as listed within the limits of analytical error.

The microprobe was calibrated frequently to assure maximum ac­

curacy and precision. Alternate analyses of groundmasses, fresh sili­

cate minerals and altered silicate minerals were performed, and only

analyses made between quantitatively acceptable analyses of fresh

K - 10 seconds

Mg - 10 seconds

A1 - 10 seconds

Mn - 20 seconds

Cr - 20 seconds

Pb — 30 seconds

144mineral grains which approached 100% totals are included in Tables 5, 6, 7, or 13.

Each polished ore sample analyzed was thoroughly scrutinized us­

ing the x-ray energy dispersive system with which the microprobe is

equipped, in order to get a semi-quantitative indication of the range of

mineral compositions in each sample. When a variation in the EDS pat­

tern was recognized, an analysis of the grain was made. The data pre­

sented thus represent a range of compositions within a sample, and the

analyses from each sample are not limited only to the most abundant

compositions encountered within a sample. Although the quality of the

ore analyses could not be judged on the basis of totals, the consistency

of the data over many microprobe sessions seem to suggest that the qual­

ity of the data presented is high.

APPENDIX II

MICROPROBE ANALYSES (WEIGHT PERCENT) OF LUCIFER ORE SAMPLES

145

Table II-l. Sample LP2B - High-K Cryptomelane

v

1 2 3 4 5 6 7 8 9Mn 56.10 59.14 52.89 57.57 57.30 57.17 58.07 57.31 55.98

Fe 0.15 0.11 0.18 0.20 0.03 0.22 0.15 0.31 0.29

A1 —— — ——— 0.17 0.09 0.21 — — —

Ba 0.15 0.00 0.00 0.09 0.22 0.00 ——— ——— ———

Pb 0.94 1.21 0.80 0.55 1.34 1.29 ——— — ———

K 4.85 4.82 4.48 4.91 4.95 5.03 — -— —--

Na —— ——— ——— 0.23 0.17 0.19 — ——' —

Ca ——— — —— — 0.10 0.14 0.11 —— ——— -—

SiV i —

0.57 0.19 0.19 ——— “—— — —— — — — —

Mg

Cu ——— ——— 0.04 ——— ——— 0.02 0.02 0.00Zn ——— — 0.00 — ——— — 0.18 0.11 0.52

Co ——— 0.14 -— — — — 0.03 0.00 0.00Ni — -— 0.00 -— -— -— 0.01 0.03 0.00Ag ——— -— 0.00 -— — ——— 0.00 . 0.02 0.00S 0.05 0.00 0.01 •——— ——— —— — —— —— — ——— 146

Table II-2. Sample LP2B - Coronadite

1 2 3 4 5 6 7 8 9Mn 51.14 50.90 48.62 52.10 50.61 49.10 49.92 51.64 50.32

Fe 0.05 0.17 0.35 0.19 0.13 0.82 1.07 0.26 0.22

A1 — — —— — ——— 0.11 0.13 0.09 —— — —

Ba 2.66 1.42 1.95 —— — 0.00 0.80 2.25 — —

Pb 11.70 11.18 12.33 ——— 10.91 14.82 12.20 ——— ———

K 0.53 0.68 0.48 ——— 0.86 0.84 0.63 — —

Na ——— ——— ——— — 0.33 0.18 0.32 — —

Ca — — — —- -— 0.48 0.31 0.47 -— —-

SI 0.06 0.15 0.16 ——— -— — —-- —

Mg

Cu 0.02 ——— 0.01 0.05 ——— ——— ——— 0.02 0.02

Zn 0.12 — 0.00 0.64 — -— —- 0.32 0.15

Co 0.05 —- 0.09 0.00 -— —— — — 0.00 0.01

Ni 0.01 —— 0101 0.00 — — -— — — (KOI 0.03

Ag 0.03 — 0.00 0.01 ——— 0.00 0.00

S 0.08 0.02 0.01 ——— ——— ——— ——— ——— 147

148Table 11-3. Sample LP3E — All Analyses.

1 2 3 4 5 6 7 8 9 10 11 12Mn 53.41 51.11 51.26 . 52.31 51.45 54.05 51.93 52.85 ; 52.50 51.60 50.04 50.62Fe 0.47 1.36 1.41 0.21 0.46 0.00 0.89 0.37 ; 0.31 0.28 0.90 0.28A1 - - - — —-- — — 0.08 0.07 0.07 0.07 ; 0.04 0.02 0.06 0.03Ba 3.50 2.66 3.08 3.20 3.96 2.57 0 . 0 0 0100 - 3.42 3.13 1.86 3.53

Pb 0-07 0.96 1.46 0.40 0.64 0.25 0.51 0.25 I 0.66j

0.45 0.56 0.14

K 2.16 0.79 0.74 1.05 0.10 0.95 0.74 1.11 1.14! 1.57 0.82 1.06

Na — — — - - - — 0.71 0.64 1.08 0.69 - 0.66 0.40 0.88 0.65

Ca — — — — — — 0.69 0.66 0.61 0.76 i 0.71 0.53 0.63 0.83

Si 0.08 0.12 0.12 0.11 — — — — — — —! ~

0.05 0.10 0.08

Mg — — — — - - - — — -! ___ 0.30 1.58 0.66

Cu — — — - - - 0.04 — — — — - - -;

1— — — —

Zn — — — 0.08 — - - - — — — ! —

Co — — — — 0.05 — — “ — ! - - - - - — — - - - —

Ni — — — — 0.01 — — — — — — — — — —

Ag — - - - — 0.00 — — — i - - - - - — — — — — —

S 0.03 0.00 0.01 0.00 — — — ™ — — —j — — — —

Table II-5. Sample LP13C - All Analyses.

la 2b 3C 4C 5dMn 28.35 52.90 11.19 2.48 50.76Fe 0.98 1.88 48.34 53.83 0.41A1 0.01 0.04 0.00 0.00Ba 2.31 2.72 1.33 0.00Pb 2.26 3.97 ——— ——— ———K 1.31 3.08 ——— 0.03 0.03Na 0.09 0.25 — 0.03 0.01Ca 0.10 0.18 0.07 0.11SI 21.84 ——— ——— 2.17 5.70Mg — — —- ——— 0.30 0.04Cu 0.01 0.03 0.04 ———Zn 0.10 0.37 0.38 ——— — ——Co 0.03 0.02 0.11 — —Ni 0.00 0.00 0.00 ——— ———Ag 0.01 0.02 0.00 — — —— —S ——— — 0.02 0.00^Siliceous cryptomelane°Cryptomelane**Goethite(?)“Pyrolusite

149

Table II-6. Sample LP15.

l3 2 3 4 5b 7 8Mn 40.20 55.00 48.24 40.14 44.85 51.50 52.42

Fe 0.05 0.22 0.13 0J 09 0.15 0.10 0.11

A1 — — ——— 0.04 0.07 0.12 0.05

Ba 0.36 0.54 0.18 0.25 0.07 0.06 0.00

Pb 0.63 1.01 1.99 1.06 0.06 0.03 0.00

K 3.06 4.39 4.41 3.33 0.31 0.07 0.06

Na ——— ——— 0.09 0.13 -— 0.00 0.01

Ca 0.16 0.12 — 0.15 0.12

Si

MgCu

Zn

11.28

0.00

0.04

0.93 6.47 13.88 10.51 7.86 7.02

Co

Ni

AgS

0.00

0.02

0.00

0.01 0.00

— — —

0.01

— —

aColumns 1-4, siliceous high-K cryptomelane “Columns 5-7, pyrolusite

150

Table II-7. Sample LP16B Analyses. — Hlgh-K cryptomelane (column 1) and Pb- and Ba- _____________ cryptomelane.analyses (columns 2-9).______________________________________

1 2 3 4 5 6 7 8 9Mn 4/.5J 56. UO 55.22 56.32 53.16 54.15 5070/ 51.19 51.49

Fe 0.31 0.77 0.35 0.57 0.58 1.10 0.42 0.72 0.79

A1 ——— — — 0.02 0.00 0.09 0.01 0.01 0.06 0.02

Ba 0.40 2.26 0.00 1.33 6.61 3.09 2.67 6.63 4.70

Pb 0.03 0.03 4.57 1.14 0.88 2.92 4.55 0.72 1.04

K 3.58 2.50 2.67 3.36 0.89 2.56 2.37 0.84 1.17

Na ——— —- 0.30 0.57 0.36 0.39 0.24 0.33 0.34

Ca — ——— 0.15 0.27 . 0.51 0.32 0.34 0.52 0.52

Si 8.17 0.14 — - — — ——— ——— 0.12 0.16 0.15

Mr ——— — »■ — ——— — — — ——— 0.03 0.13 0.16

151

Table II-8. Sample LP16B - Pyrolusite.

Mn 59.96 56.33 57.76 57.99 59.53

FeAT

0.33 0.49 0.40 0.16 0.23/U.

Ba 0.06 0.04 0.08 ——— ———

Pb 0.00 0.00 0.22 ——— —

K

Na

0.06 0.04 0.04 ;;;Ca

Si 3.05 4.36 2.91 — —

Mg

Cu ——— — — — — 0.04 0.02

Zn — — — 0.06 0.25

Co — ——— 0.00 0.12

Ni — —- — 0.00 0.00

Ag —— — ——— 0.05 0.00

S ——— ——— —- — 152

Table II-9. Sample LZ34 - All Analyses.

la 2 3 4 5b37.09 40.73 44.46 36.83 56.26

Fe 1.09 0.53 0.27 0.76 0.22

A1 0.09 0.04 0.04 0.76 ' 0.64

Ba 3.17 3.71 3.49 1.02 0.09

Pb 0.69 0.90 0.80 0.51 0.00

K 1.20 1.94 2.16 2.22 0.03

Na 0.24 0.22 0.31 0.27 0.02

Ca 0.51 0.31 0.31 0.27 0.22

Si 13.78 11.49 8.89 15.88 3.68

Mr 0.13 0.06 0.05 0.03 0.08^Columns 1-4, siliceous cryptoraelane. ^Column 5, pyrolusite.

153

Table 11-10. Sample LZ39.

la 2 3 4 5bMn 35.48 52.03 52.91 53.64 2.43Fe 0.36 0.25 0.59 0.16 49.63A1 0.22 0.04 0.03 0.06 —

Ba 1.88 4.21 4.02 3.92 —

Pb 2.35 2.27 1.88 3.82 -—

K 1.66 1.37 1.71 1.25 —

Na 0.21 0.34 0.34 0.23 —

Ca 0.36 0.60 0.48 0.14 —

Si 14.25 0.40 0.40 0.38 3.92

Mg

Cu 0.39 0.23 0.2C 0.24 0.35

Zn ----— 0.28 — 0.20 0.44

Co — 0.01 — 0.03 0.11

Ni — 0.01 — 0.01 0.00

Ag ——— 0.01 — — — 0.01 0.00

S mmmmmm

^Columns 1-4, cryptomelane. kColumn 5, Iron-oxide. KT

Table 11-11. Sample BS1 - Cryptomelane Analyses.

1 2 3 4Mn 55.65 50.66 56.34 54.25

Fe 1.21 4.39 0.62 0.63A1 0.35 0.78 0.32 0.71

Ba 2.58 1.08 2.23 6.00

Pb 0.90 3.44 0.58 0.83

K 3.72 3.20 4.20 2.49Na 0.33 0.26 0.24 0.20Ca 0.29 0.87 0.23 0.40

Table 11-12. Sample BS2 — Cryptomelane.

la 2 3 4Mn 55.49 54.86 57.72 55.12

Fe 0.33 1.78 0.88 0.75

A1 0.59 0.47 0.15 0.14

Ba 0.80 1.02 0.85 1.91

Pb 0.53 0.58 0.30 0.16

K 3.58 4.38 4.20 2.98

Na 0.78 0.34 0.42 0.87

Ca 0.38 0.35 0.34 0.54

Si 0.08 0.28 0.16 0.12

Mg 0.33 0.06 0.01 0.15^Columns 1-3, High-K cryptomelane

Table 11-13. Sample BS5

la 2 3 4 5b 6m 48.10 48.79 51.59 51.58 58.34 57.70

Fe 0.14 0.09 0.29 0.29 0.04 0.14

A1 0.14 0.17 0.06 0.09 0.14 3.02

Ba 4.52 3.52 3.05 3.49 0.61 0.12

Pb 1.04 0.85 0.95 1.37 0.22 0.00

K 1.16 1.54 1.46 1.54 0.45 0.05

Na 0.39 0.36 0.67 0.57 0.15 0.04

Ca 0.70 0.70 0.79 0.66 0.38 0.21

Si 0.15 0.16 0.08 0.12 0.32 0.54

Mg — — — ■■ — ■■ 0.70 0.67 0.24 0.22^Columns 1-4, cryptomelane DColumns 5-6, other manganese-oxides

157

Table 11-14. Sample LX13B - ______ _______ Mangan es e-oxide.

1 2Hn 53.06 52.36

Fe 0.07 0.03A1 0.09 0.04Ba 0.00 1.30

Pb 0.13 0.04K 1.62 1.30

Na 0.85 4.56

Ca 0.62 1.48

Si 0.02 0.17

Mg 1.71 1.70

APPENDIX III

MOLECULAR PROPORTIONS OF CRYPTOMELANE "A"-SITE CATIONS AT 8(Mn + Fe + Al)a

SampleNo.

AnalysisNo. Ba Pb K Na Ca Sum

CryptomelaneTyped

LP2B la-1* .01 .04 .97 — — — IIla-2* .00 .04 .91 — — — IIla-3* .00 .03 .96 - - — IIla-4 .01 .02 .96 .07 .02 1.08 IIla-5 .01 .05 .97 .06 .03 1.12 IIla-6 .00 .05 .98 .06 .02 1.11 II

LP2B lb-1* .16 .48 .12 IIIlb-2* .09 .47 .15 — — — IIIlb-3* .13 .54 .11 — — — IIIlb-5 .00 .46 .20 .12 .11 0.89 IIIlb-6 .06 .63 .19 .07 .07 1.02 IIIlb-7 .14 .51 .14 .12 .10 1.01 III

LP13C 3-1 .26 .17 .52 .06 .. .03 1.04 I3-2 .16 .15 .64 .09 .04 1.08 I

LP15 4-1* .03 .03 .85 II4-2* .03 .04 .89 — — — II4-3 .01 .09 1.03 .04 .04 1.21 II4-4 .02 .05 .93 .06 .03 1.09 II

LP16B 5a-l* .03 .00 .85 II5a-2* .15 .00 .59 — — — I5a—3 .00 .18 .54 .11 .06 0w89 I5a-4 .08 .04 .67 .20 .06 1.05 I5a-5 .40 .04 .19 .13 .11 0.87 I5a-6 .19 .11 .53 .14 .07 1.04 I5a-7 .17 .19 .53 .09 .07 1.05 I5a-8 .41 .03 .18 .12 .11 0.85 I5a-9 .29 .04 .25 .13 .11 0.82 I

LZ34 6—1 .26 .04 : • 36 .11 ' .15 • 0.92 I. . 6-2 .: . .28 .05 .53 .10 .08 1.04 I '6-3 .26 .05 .54 .14 .08 1.07 I6-4 . . .10 .04 .67 .13 .08 1.02 I

159

160

SampleNo.

AnalysisNo. Ba Pb K Na Ca Sum

CryptomelaneTyped

LZ39 7-1 .18 .14 .53 .12 .12 1.09 I7-2 .26 .10 .36 .12 .12 0.96 I7-3 .24 .07 .36 .13 .10 0.90 I7-4 .24 .15 .27 .08 .11 0.85 I

BS1 8-1 .14 .03 .71 .11 .06 1.05 I8-2 .06 .13 .64 .10 .17 1.10 I8-3 .13 .02 .83 .08 .04 1.10 I8-4 .35 .03 .50 .07 .08 1.03 I

BS2 9-1 .04 .02 .71 .27 .08 1.12 II9-2 .05 .02 .85 .12 .06 1.10 II9-3 .05 .01 .80 .14 .06 1.06 II9-4 .11 .01 .60 .30 .10 1.12 II

BS5 10-1 .30 .05 .27 .15 .16 0.93 I10-2 .23 .04 .35 .15 .16 0.93 I10-3 .19 .04 .32 .25 .17 0.97 I10-4 .22 .06 .33 .21 .14 0.96 I

aCalculated with respect to 8 (Mn + Fe) if no analysis of A1 available. Analyses with no available A1 value indicated by *.^Refers to analyses as presented in Appendix II.^ - indicates value not available.Refers-to Figure 31 (see text).

APPENDIX IV

LOCATIONS OF SAMPLES DISCUSSED IN TEXT

161

T - . t y ? » *fFigure IV-1. Sample Location Map.

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