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Economic Geology Vol. 72, 1977, pp. 925-930

Age of lgneous Activity and Mineralization, Cerro de Pasco, Central Peru

MILES L. SILBER•IAN AND DONALD C. NOBLE

Abstract

K-Ar ages of mineral separates of biotite, plagioclase, and sanidine from one of a group of volcanic domes of quartz latite to dacite composition and two quartz latite dikes, all of •vhich predate mineralization at the Cerro de Pasco mining district, are in the range of 14 to 15 m.y., with an overall average of 14.5 ñ 0.2 m.y. A large ((5 cm long) sanidine phenocryst from one of a group of late dikes that cut the major mineral deposits gives a K-Ar age of 15.2 ñ 0.2 m.y. This age, slightly but apparently significantly, greater than those of the older volcanic rocks that predate mineralization suggests that the sanid{ne crystal contains about 1 X 10 -n moles/g of extraneous radiogenic argon.

The radiometric ages establisln the thne of volcanism and mineralization at Cerro de Pasco as middle Miocene and suggest that the various igneous and hydrothermal events took place within a very short period of geologic time. Available geological and radiometric data support the concept of an intimate genetic relation between hydrothermal activity and the pulse of magmatic activity represented by the exposed volcanic rocks.

Introduction

CERRO DE PASCO is the largest and best known of a large group of base and precious metal deposits in the Andes of central Peru (Petersen, 1965). The purpose of this paper is to present potassium-argon ages of volcanic rocks from the district, which fix the time of volcanic activity and of genetically associated mineralization.

Geological Setting and Nature of Mineralization

For more than 50 years Cerro de Pasco (Fig. 1) has been the focus of many geological, mineralogical, and geochemical studies. Recent SUlnmaries, geological maps and sections, and comprehensive bibliog- raphies are given by Ward (1961), Petersen (1965), and Rivera (1970).

The base and precious metal deposits at Cerro de Pasco are intimately related to shallow igneous acti¾- itv. Volcanic rocks are found xvithin a north-south

elongate volcanic vent having a maxilnum diameter of ahnost 3 km (Fig. 2). The location of the vent itself appears to be controlled by a north-south-trending high-angle reverse fault--the longitudinal fault-- that is tangent to the eastern edge of the vent area (Fig. 2). Mineralization, concentrated in the vicinity of the longitudinal fault, is found both in the volcanic rocks and in Mesozoic carbonate rocks outside of the vent.

The oldest volcanic unit, the Rumiallana Aggloln- erate, is composed of abundant angular fragments of nearby pre-Cenozoic rocks set in a matrix of biotite- and plagioclase-bearing, silicic, pyroclastic materiM.

The unit, best exposed in the eastern part of the vent area, possesses well-developed cross-bedding, dune structures, and other features typical of base or ground surge deposits (Moore, 1967; Fisher and \Vaters, 1970; Crowe and Fisher, 1973; Sheridan and Updike, 1975). These observations support the conclusion of Lacy (1957) that explosive vol- canisInn characterized the early stages of igneous activity at Cerro de Pasco. However, the subhorizontal nature of the bedding and other internal structures of the agglomerate cast doubt on the earlier hypothesis that a large volcanic cone once was present above the vent area. The original structure of the pyroclastic vent probably was more silnilar to the diatreme model presented by Hearn (1968).

In the western and southern parts of the vent the Rumiallana Agglomerate is intruded by felsic rocks comlnonly termed "Cerro •uartz Monzonite Por- phyry." Many of these rocks, well exposed on the surface, possess well-developed flow foliation and very fine grained groundmass materials. They are interpreted as a nulnber of interpenetrating cumulo- domes and plug dolnes of dacitic to quartz latitic composition. The presence of volcanic domes with locally subhorizontal flow foliation would argue that the present level of exposure is not appreciably lower than that at the time of volcanic activity and mineralization. These domes and the Rumiallana Ag- glomerate are cut by a number of generally east-west- trending dikes of distinctive porphyritic quartz latite or quartz monzonite porphyry (Fig. 2).

Along the eastern margin of the vent, igneous activity was followed by hydrothermal alteration and

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14' 78 ø

i I

LIMA

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0 50 100 I I I

KILOMETERS

©Cerro De Pasco

ß La Oroya Morococha

ß Huancayo

ß Yauricocha

ß Huancavelica

ß Julcani

Huachocolpa ß ßAyacucho ß Atunsulla

I I 76 ø 74 a

FIG. 1. Map of central Peru showing locations referred to in text.

replacement of both Mesozoic limestone and the volcanic rocks of the center, producing an immense body of silica-pyrite rock. Still later, a complex system of veins and replacement bodies containing copper-lead-, zinc-, and silver-bearing minerals were formed.

The quartz latite dikes are older than the economically important mineralization; west of the silica-

pyrite body they are cut by numerous major trans- verse copper-silver (enargite-pyrite) veins. The copper-silver veins in turn are cut by several dikes of "albitized quartz monzonite porphyry" (actually porphyritic quartz latite petrographically very similar to the older east-west-trending dikes) (Lacy, 1957, fig. 3) which appear to have been emplaced during

I •---• Longitudinal

N • fault

71.. T• c••x•t•xPm I 4

Cerro Quartz Monzonite Porphyry Tcd, dikes of quartz latite Tc, volcanic domes • plug domes

Silico- pyrite body

Rumiallana agglomerate

Rocks of pre-Miocene age

Approximate sample location

I 1500 m

Fro. 2. Sketch map of the Cerro de Pasco mineral district showing generalized geologic relations and approximate sample locations (modified from Rivera, 1970).

IGNEOUS ACTIVITY AND 3IINERALIZATION, CERRO DE PASCO, PERU 927

TABLE 1. Generalized Stratigraphic and Paragenetic Relations of Volcanic Rock Units and Mineralization in the Cerro de Pasco Mining District, Peru

Ages • Rock unit and mineralization Specimen no.

15.2 -b 0.2 m.y. "Albitized quartz monzonite porphyry" dikes 4 Major base and precions metal mineralization Silica-pyrite body

14.8 4- 0.2 14.2 4- 0.4 Quartz latite dikes "Cerro Quartz Monzonite Porphyry" 2, 3 14.4 4- 0.4 Volcanic domes 1

Rumiallana Agglomerate

From Table 2. For samples 1 and 2, the average of the two mineral ages were used.

the very latest stages of mineralization (Rivera, 1970). The sequence of volcanic activity and mineralization is summarized in Table 1.

determined by the lithium-metaborate-fusion tech- nique (Ingamells, 1970), with lithium serving as an internal standard.

Rocks Dated

Three of the four specimens on which ages were measured (specimens 1, 2, and 3, Tables 1 and 2) are from units intruded after eraplacement of the Rumiallana Agglomerate and before the formation of the silica-pyrite body and the various economically important orebodies. The fourth specimen dated was a single, large relict sanidine phenocryst (specimen 4, Tables 1 and 2) from a supergene( ?)-altered dike of quartz latite cutting the silica-pyrite body. The sampled dike was assigned to the group of late "albitized quartz monzonite porphyry" dikes by Lacy (1957, fig. 2), and thus in all probability postdates the major base and precious metal mineralization at Cerro de Pasco.

No specimens of the pervasively altered Rumial- lana Agglomerate suitable for dating were found, nor was it possible to obtain an age on the northwest- trending dike of late "albitized quartz monzonite porphyry" that cuts the copper-silver veins west of the silica-pyrite body (Lacy, 1957, fig. 3). \Vhere observed, the dike is strongly altered by supergene and (or) hypogene solutions.

The specimens dated do show signs of the intense hydrothermal alteration that has affected the rocks farther to the east. These are principally the presence of secondary calcite and the destruction of horn- blende. I-Iowever, the biotite, plagioclase, and sanidine phenocrysts which were dated showed virtually no signs of alteration.

Analytical Methods

The potassium-argon age determinations were lnade in the laboratories of the U.S. Geological Sur- vey, Menlo Park, California. Argon content was determined by standard isotope-dilution methods (Dalrymple and Lanphere, 1969). Argon lnass analyses were done using a Nier-type mass spec- trometer operated in the static 1node. Potassium was

Results and Discussion

,4ge and duration of igneous and hydrothermal activity

Radiometric ages, along with K and Ar analytical data, are presented in Table 2. Ages of mineral pairs are concordant within the limits of analytical uncertainty, and the ages of the two samples that were dated by two different minerals are reported as average ages. The ages of the samples from premineralization units (samples 1, 2, 3, Table 2) are also not distinguishable within the limits of the calculated analytical uncertainty. We are not able to resolve the ages of the volcanic donne and quartz latite dikes within the precision of our isotopic dating techniques.

The K-Ar age of the sanidine crystal from the postmineralization dike, 15.2 ñ 0.2 m.y., is greater than all of the other K-Ar ages, at statistical levels of confidence that range from 0.85 to 0.97 (Table 3). The reported analytical precision of this sample is better than the other individual K-Ar ages because it is the average of four K and three Ar analyses, yielding an overall uncertainty of 1.34 percent.

Although this age difference appears to have statistical significance, it is not geologically reasonable since this dike cuts the silica pyrite body and the main copper-bearing veins.

The large (6 cm in maximum diameter) sanidine crystal dated is not, in our opinion, xenocrystic, which would provide a possible explanation for its anomalously old age. Sanidine is a common phenocryst mineral in all of the dikes at Cerro de Pasco. Margins of these phenocrysts are crystal faces, and the crystals are not broken, fractured, or significantly resorbed. They contain inclusions of the other phenocryst minerals present in the rock.

The dike from which the dated sanidine was ob- tained has been strongly affected by supergene, and

928 M. L. oCILBER31.4N AND D.C. NOBLE

TABLE 2. Potassium-Argon Analytical Data and Ages on Rocks from Cerro de Pasco, Central Peru

40Ar Radiogenic a 40Ar Specimen Mineral Ko•O-*(mole/g Radiogenic Age 4 Average

no. 1 dated (wt %) X 10 -lø) (percent) (m.y.) ages •

1.74 0.367 60.0 14.0 4- 0.4 1 Plagioclase 1.80

Biotite 9.01 1.98 64.2 14.8 4- 0.4 9.01

0.858 2 Plagioclase 0.856 0.190 39.3 15.0 4- 0.5

9.08 1.97 60.4 14.6 4- 0.4 Biotite 9.14 9.42

3 Sanidine 9.47 1.99 81.2 14.2 4- 0.4

11.55 2.64 75.7 4 Sanidine 11.53 2.64 2.61 81.5 15.2 4- 0.25

11.55 2.54 75.0 11.64

14.4 4- 0.4

14.8 4- 0.2

Constants: X• = 0.585 X 10-•ø/yr; Xa = 4.72 X 10-•ø/yr. Atomic abnndance 4øK = 1.19 X 10 -4 mole/•nole K. 1 Specimen 1. Field no. Cerro Dome 1; location: lat 10 ø 39.7' S; long 76 ø 16.6' W.

2. Field no. Cerro N. Dike; location: lat 10 ø 39.8' S; long 76 ø 16.5'.W. 3. Field no. Cerro S. Dike; location: lat 10 ø 40.0' S; long 76 ø 16.5' W. 4. Field no. Cerro Pyrite Dike; location: fat 10 ø 39.9' S; long 76 ø 15.7' W.

2 L. B. Schlocker, analyst. a Argon analysis and age calculation, M. L. Silberman and A. Berry. 4 Plus-minus represents analytical uncertainty only, at one standard deviation, and is approximately 3 percent. 5 Plus-minus on averaged ages is calculated as the standard deviation of the mean (standard error).

possibly very late stage hypogene, solutions. Sani- dine and quartz are the only apparently unaltered major magmatic minerals present. The dated material includes about 5 percent of small plagioclase grains that were incorporated within the large sanidine phenocryst when it grew. This plagioclase now is strongly sericitically altered, showing that solutions were able to gain access to the inner portions of the sanidine crystal, presumably along fractures, during the period of alteration. Nevertheless, we believe that it is unlikely that most of the excess argon is contained within this material, for hydrothermal

T^m.E 3. Statistical Levels of Confidence of Differences in Ages of Individual Samples from Cerro de Pasco

Age

Probo,,ty ,e,,e,

difference

sericite from ore deposits commonly gives meaning- ful radiometric ages. The sanidine, however, is optically homogeneous (2V•---• 10 ø) and, except along fractures, shows no microscopic evidence of hydrothermal or supergene alteration.

The sanidine crystal also contains a very large number of extremely small inclusions of low-refrac- tive-index material. Trains of these irregular inclusions appear to follow crystallographic planes of the sanidine. The inclusions have not been definitely identified, but their relief, and the probable presence of more than one phase, suggests that they consist of glass and perhaps one or more fluid phases. We provisionally interpret the inclusions as primary, although in view of the strong alteration of the host rock, we recognize the possibility that the inclusions may be a result of pervasive solution and redeposi- tion of the potassium feldspar (O'Neil and Taylor, 1967).

It would appear possible that the inclusions contain sufficient extraneous 4øAr to produce the observed age relations. Studies by Fyfe et al. (1969) have demonstrated that argon is soluble in granitic melts and that some of it is retained in the glass resulting from their quenching. Anomalously old K-Ar ages have been reported from nonhydrated volcanic glass from a welded ash-flo•v tuff in southern Nevada

(Marvin et al., 1970); the amount of excess radiogenic argon present is on the order of 0.5 to 1.0 X

IGNEOUS .4CT[V[TY .lIND MINER.4L[Z.4T[ON, CERI•O DE P,45'C0, PERU 929

10 n mols/g. Glass trapped within phenocrysts re- tains large amounts of volatile constituents (Ander- son, 1974), and argon may well be among the•n. The presence of the glass inclusions in the dated sanidine indicates rapid cooling after eraplacement of the dike. If the glass had contained some extraneous argon, it is probable that it did not have enough ti•ne to outgas appreciably.

Alternatively, extraneous argon may be •mcluded within the structure of the sanidine itself. Damon

et al. (1967) report anomalously old ages from large phenocrysts in several samples of volcanic rocks, including plagioclase phenocrysts as much as 2 cm long in a basalt from Arizona, which are at- tributable to excess radiogenic argon. The amount present in their plagioclase sample is approximately 2 X 10 -x2 moles/g. A much larger amount (2.7 x 10 •0 moles/g) of excess Ar 4ø was found by Living- ston et al. (1967) in large poikilitic orthoclase grains from a quartz monzonite. Damon et al. (1967) indicate that excess 4ø.*r in large volcanic phenocrysts may be due to incorporation of argon from the magmatic environment into the mineral during intra- telluric growth and failure to completely outgas this extraneous argon after eruption. Diffusion data (Da- mon et al., 1967; Foland, 1974) indicate that large phenocrysts do not have adequate time to outgas completely in a cooling lava when exposed to atmo- spheric temperatures.

An alternate possibility, suggested by P. E. Da- mon (written commun., 1976), is that excess argon was introduced into the sanidine structure from very late stage hypogene solutions. This interpretation is supported by the apparent absence of excess argon in the plagioclase and sanidine from the earlier, unaltered dikes.

X;Vhether in the glass inclusions or within the crystal structure of the sanidine, the amount of extraneous •øAr in Specimen 4 appears to be on the order of 1 x 10 -n moles/g. This assumes that the "albitized quartz monzonite porphyry" dikes are essentially the same age as the other volcanic units. If, however, the dikes are younger than the other volcanic rocks, the amount of extraneous argon present would be correspondingly greater. For example, if the dikes are 1 m.y. younger than the other dated rocks, the amount of excess argon would be about 3 X 10 -xx moles/g.

Detailed geochronological studies of epithermal and polymetallic base and precious metal mineral deposits in the Great Basin of the western United States, such as those at Bodie (Silberman et al., 1972) and Goldfield (Ashley and Silberman, 1976) and Peru (Julcani; M. L. Silberman and D.C. Noble, unpub. data) have documented the temporal association of hydrothermal mineralization and inter-

mediate to silicic volcanic activily. Ilydrothermal activity usually starts during the waning phases of volcanic activity and generally continues for some time after volcanism,, although some late-stage volcanic activity may accompany it. In general, the duration of volcanic activity, directly associated with the alteration and mineralization, has been found to be on the order of less than 1 to 1.5 m.y. By analogy to these results, and by the virtually identical petrography of the albitized quartz monzonite dikes to the dikes that predate mineralization, we suggest that they are not appreciably younger than the premineralization dikes.

As noted above, the "albitized quartz monzonite porphyry" dikes are virtually identical petrographically to the dikes that predate mineralization. We thus believe that it is most probable that the volcanic activity and mineralization at Cerro de Pasco took place within a very short period of geologic time, that a very close genetic relation exists between the igneous activity represented by the exposed •,olcanic rocks and mineralization, and that the amount of extraneous argon in Specimen 4 is not nmch greater than about 1 X 10 -n moles/g.

Regional implications

Igneous and hydrothermal activity at Cerro de Pasco is somewhat oIder than that at other dated

mineral districts in central Peru, which were active between 10 and <:2 m.y. ago (Giletti and Day, 1968; Noble et al., 1974a; Noble et al., 1974b; McKee et al., 1975; Eyzaguirre et al., 1975). The center appears to postdate much of the intense middle Miocene tectonism that affected the Andes of Peru (Noble et al., 1974b; Farrar and Noble, 1976), and thus suggests that much of the deformation took place before 14.5 m.y. ago. Finally, the tuffs of the "Rock Forest" southwest of Cerro de Pasco (McLaughlin, 1924) are almost 10 m.y. younger than the volcanic rocks of Cerro de Pasco (Farrat and Noble, 1976) and thus both cannot have resulted from the same

episode of volcanic activity, as suspected by some geologists who previousIy had worked in the region.

Acknowledgments

We are indebted to Cerro de Pasco Corporation (now Centromin Peru) and Cia. de Minas Buena- ventura, S.A., and particularly to ¾. R. Eyzaguirre and Nelson Rivera, both formerly of Cerro de Pasco Corporation, and Alberto Benavides of Minas Buena- ventura, for facilitating the study. The field and laboratory work was partly supported by National Science Foundation Grant GA-40421. D.W. Charl-

930 31. L. SILBERM.4N .4ND D.C. NOBLE

ton assisted in the field, and A. B. 5Vallace helped with the preparation of the mineral separates. M. L. S.

U.S. GEOLOGICAL SURVEY 345 ]MIDDLEFIELD ROAD

MENLO PARK, CALIFORNI^ 94025

D.C. N. DEPARTMENT OF GEOLOGY AND GEOLOGICAL

ENGINEERING

•IICHIGAN TECItNOLOGICAL UNIVERSITY

HOUG•tTON, MIC•tIGAN 49931 May 26, October 27, 1976

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