Epithermal Alteration and Mineralization in the Comstock District, Nevada - Donald M. Hudson (2003)

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IntroductionTHE COMSTOCK district lies within a northwest-trending beltof Miocene andesitic rocks in western Nevada (Fig. 1) and isone of the more celebrated epithermal districts in the world.Although production from the district is not exceptionallylarge—approximately 257 tonnes (t) of gold and 6,000 t of sil-ver (Bonham and Papke, 1969; Smith and Tingley, 1998)—the district was known for rather high grades. The Con Vir-ginia orebody (Big Bonanza) alone produced 1,131,900 t ofore with a calculated average gross assay grade of 87.4 g/t Auand 1834.3 g/t Ag (R.E. Kendall, unpub. report, 1968). Placergold was discovered in 1850 and lode gold-silver depositswere discovered in 1859 (Smith and Tingley, 1998). The prin-cipal period of production was 1860 to 1880, but productioncontinued intermittently until the 1990s.

The Comstock district was one of the first epithermaldistricts described in the United States (Blake, 1864;Richthofen, 1865). Richthofen (1865, 1868) first applied theterm “propylite” to certain rock units in the Comstock dis-trict. Becker (1882) realized that propylite was altered an-desite and described one of the most common hydrothermalalteration assemblages worldwide. King (1870), Church(1879), Becker (1882), and other early writers described thegeology and orebodies of the district in terms of the under-standing of the day. Gianella (1936) defined much of thestratigraphy in the southern part of the district, and Calkins(1944) expanded upon this work. Later studies relied heavilyon the earlier investigators for the geologic setting for theirinterpretations. Ore mineralogy was investigated by Terrill(1914) and Bastin (1923); fluid inclusions and stable isotopes

were determined by Vikre (1989), Criss and Champion(1991), and Criss et al. (2000).

This paper describes the geology of the district on the basisof new mapping and incorporates previous work to interpret

Epithermal Alteration and Mineralization in the Comstock District, Nevada

DONALD M. HUDSON†

1540 Van Petten Street, Reno, Nevada 89503

AbstractThe precious and base metal deposits of the world-renowned Comstock district of western Nevada consti-

tute one of several superimposed Miocene hydrothermal systems. In the district, Mesozoic metasedimentaryand igneous rocks are overlain by Oligocene to Miocene ash-flow tuffs. A thick sequence of middle Mioceneandesitic volcanic rocks and intrusions host the bulk of the hydrothermal alteration and ore deposits. Some ofthe magmatic events were directly associated with hydrothermal activity. Secondary biotite alteration locally af-fects preore dioritic intrusions. Preore barren quartz, alunite, pyrophyllite, and kandite alteration (high-sulfi-dation style) is zoned outward from mostly discontinuous, crudely radial, fractures associated with andesitic in-trusions. Blanket-like cristobalite, alunite, and kaolinite alteration exposed at the periphery of the district maybe linked to preore alteration near the paleowater table. Later, large volumes of subore-grade massive quartzveins and quartz-adularia stockworks were deposited in steeply to moderately east-dipping sets of north- andnortheast-striking faults that also localized the major precious metal-bearing lodes. Quartz, chlorite, illite, andvery localized muscovite alteration (deep low-sulfidation) developed coeval with vein deposition. Epidote-bear-ing propylitic alteration forms halos around the higher-temperature parts of the lodes. Adularia and rare bladedcalcite indicate boiling during deep low-sulfidation hydrothermal activity, but little is apparently associated withore deposition. The main stage Au-Ag-Cu-Zn-Pb ore was deposited late in the deep low-sulfidation system inirregularly spaced lenticular dilatant zones or structural intersections. Massive, nonbladed(?) calcite was de-posited just below and in the lower parts of orebodies in the Comstock lode, but it was deposited in the upperparts of orebodies or was barren elsewhere in the district. Pliocene to Holocene reactivation of faults has dis-rupted the district, redistributing the relative positions of many of the orebodies and alteration assemblages.

Economic GeologyVol. 98, 2003, pp. 367–385

† E-mail, [email protected]

OLas Vegas

0 100 km

N

OReno

ComstockDistrict

NEVADA

O Elko

120oW 114oW

42oN

CALIFORNIA36oN

FIG. 1. Location map of the Comstock district within the belt of 6 to 17Ma andesitic volcanism in western Nevada and eastern California. AfterStewart and Carlson (1976).

a complex hydrothermal and geologic history. Geologic andalteration mapping of about 31 km2 in the Comstock districtwas conducted in 1983 and 1984 for United Mining Corpora-tion at 1:2400 scale. In 1985, as part of Westley Explorations’activity in the district, I reviewed the rock collection (referredto herein as the Becker collection) made by George Becker in1879 and 1880 for his U.S. Geological Survey Monograph 3(1882); the collection is now stored at the National Museumof National History in Washington, D.C. The collection con-sists of geographically documented wall rocks collected frommany of the underground workings. By 1986, a further 21km2 had been mapped at 1:6000 scale in the district north ofthe Lyon County line in the Virginia City area. The area southof the Lyon County line was mapped in 2000 as part of aNevada Bureau of Mines and Geology project covering theVirginia City 7.5' Quadrangle. More than 600 thin sectionswere made of rocks from the study area, and more than 350X-ray diffraction analyses of altered rocks were made to de-termine clay mineral species, from both the surface and sub-surface. Additional subsurface information was compiledfrom numerous unpublished reports and maps in the Com-stock mining district files of the Nevada Bureau of Mines andGeology and the University of Nevada-Reno libraries, SpecialCollections department.

StratigraphyThe oldest rocks exposed in the Comstock district are

siltstone and fine-grained feldspathic sandstone with minorinterbedded limestone of the Lower Jurassic GardnervilleFormation (Fig. 2). The broadly folded Gardnerville Forma-tion is intruded by a thick lopolith(?) of fine- to medium-grained pyroxene gabbro (Fig. 2) that crops out intermittentlythroughout an area of more than 30 km2. Intruded into thatsequence are Cretaceous diorite, granodiorite, and granitestocks and dikes, few of which are exposed at the surface. Inthe southern part of the district, steep-walled to broad chan-nels, which cut down as much as 200 m into Mesozoic rocks(primarily gabbro), are filled with a sequence of Oligocene toearly Miocene felsic ash-flow tuffs. The ash-flow tuffs appar-ently are absent in the northern part of the district.

Unconformably overlying the units described above is athick sequence of probably locally erupted andesite of theearly Miocene Alta Formation (Fig. 2), the major host rock ofthe district. The lower member consists of up to 300 m of in-terbedded hornblende-augite, augite, and hornblende an-desite flows, flow breccias, mudflow breccias, and minor vol-caniclasitic sedimentary rocks. The overlying Sutro Memberconsists of lacustrine siltstone, sandstone, and conglomerate,0 to 30 m thick. The upper member consists of more than 700m of hornblende and hornblende-pyroxene andesite flowsand rare andesitic breccias. The age of the Alta Formation isnot well constrained; K-Ar ages range from 14.4 to 20.1 Ma(Fig. 3; Vikre et al., 1988), and the oldest isotopic ages arefrom flows high in the stratigraphic sequence. Several phasesof medium-grained, equigranular quartz diorite to andesiteporphyry compose the Davidson Diorite, which intruded theAlta Formation. Stocks, dikes, and plugs of the DavidsonDiorite are extensively exposed at the surface and in the sub-surface in the Jumbo district 6 km west of Mt. Davidson(Hudson et al., 2002) and in the Flowery district 6 km east of

Mt. Davidson (Becker collection). On the basis of this distri-bution, the irregular intrusive complex appears to be as muchas 12 km long, at least 5 km wide, and elongated in a roughlyeast-west direction. The Davidson Diorite intrudes most ofthe exposed section of the Alta Formation but has yielded awide range K-Ar and fission-track ages that overlap the K-Arages of the Alta Formation (Fig. 3; Vikre et al., 1988).

Disconformably overlying the Alta Formation is the KatePeak Formation, informally divided into a hydrothermally al-tered lower member and an essentially unaltered uppermember of similar composition and texture. The formationconsists of hornblende andesite to dacite porphyry, commonlywith phenocrysts of augite and/or biotite, and rarely traces ofquartz. The lower member (Fig. 2) consists of numerousdikes and plugs that crop out in much of the district but areconcentrated near the productive part of the Comstock lodeand as a few erosional remnants of flows east of the Occiden-tal lode. Early(?) intrusive phases (primarily plugs) of thelower member appear to lack strong structural control. Manyof the dikes, however, parallel or intrude the Comstock, Sil-ver City, and Occidental lodes (Fig. 2), suggesting concurrentfaulting and intrusion initiated later during Kate Peak For-mation magmatism. Following extensive alteration related tothe emplacement of the lower member, the area was erodedand an unknown thickness of rock was removed. Nearly all ofthe flows of the lower member of the Kate Peak Formationand part of the Alta Formation were eroded. The uppermember of the Kate Peak Formation was emplaced on whatappears to be a surface of relatively low relief. More than 500m of flows and lahars of the upper member are preserved infault blocks east of the Occidental lode, but in most of the dis-trict only a few erosional remnants remain. Much of the al-tered rocks of the district were covered by a flow of theKnickerbocker Andesite less than 40 m thick. The Knicker-bocker Andesite lies either at the base of the upper memberof the Kate Peak Formation or in a window eroded into theupper member of the Kate Peak Formation. K-Ar ages of theKnickerbocker Andesite overlap those of the upper memberof the Kate Peak Formation (Fig. 3), and field relations do notmake the relative ages clear.

StructureRocks of the Comstock district were cut by numerous Late

Cenozoic normal faults before, during, and after mineraliza-tion (Table 1). The major premineralization and synmineral-ization faults are the east-dipping Comstock, Silver City, andOccidental fault zones. Numerous other faults localize minoralteration and small orebodies.

The Comstock fault zone is the main structure in the dis-trict, traceable for more than 15 km along strike (Fig. 2). Themain mineralized part of the Comstock fault zone strikesabout N 15° E, from near the junction with the Silver Cityfault at the Overman cut and extending north. The Comstockfault zone turns abruptly west about 500 m south of the junc-tion, where it has a strike of about N 65° E along the northedge of American Flat, and then turns abruptly south with astrike of about N 5° W. Almost the entire length of the faultzone in the district is bounded by nearly parallel faults, re-ferred to by miners as the “west wall” and the “east wall”(King, 1870; Becker, 1882). Most of the Comstock lode is

368 DONALD M. HUDSON

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GEOLOGY AND EPITHERMAL MINERALIZATION OF THE COMSTOCK LODE, NEVADA 369

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FIG. 2. Simplified geologic map of the Comstock district. BF = Buckeye fault, CFZ = Comstock fault zone, EVZ = Eastvein zone on 2000-ft level, GHF = Grizzly Hill fault, HF = Haywood fault, OL = Occidental lode, SCL = Silver City lode,WV = Woodville vein.

confined between these bounding faults, which are traceablefrom south of American Flat to just north of Cedar Hill. Thefault zone below a depth of about 125 m dips about 40° E andgradually flattens to about 35° E about 600 m depth (Fig. 4;Becker, 1882; Nevada Bureau of Mines and Geology miningdistrict files). The true thickness between the east and westwalls, at depth, varies between 0 and 50 m. The east and westwalls meet with only a thin seam of clay marking the faultzone in parts of the Chollar and adjoining Potosi mines, be-tween about 200 and 500 m below surface (King, 1870;Nevada Bureau of Mines and Geology mining district files).The east and west walls are separated elsewhere along the ex-plored parts of the fault zone to at least 900 m below surface.Above about 125 m depth, the west wall dips between 38° Eand 45° E, but the east wall bends along the entire mineral-ized length to nearly vertical or steeply west dipping at thesurface (King, 1870). The horizontal distance between theeast and west walls at the surface varies from 20 to 300 m andis commonly more than 150 m. This upper zone forms awedge tapering downward (Fig. 4) with numerous small

faults occurring within the wedge. The fault or fissure con-taining the Con Virginia orebody lies in the hanging wall ofthe main Comstock fault zone (Fig. 4). From the main fault,the nearly vertical structure strikes northeast for about 200 mthen turns north to about N 15° E and is traceable for aboutanother 300 m. Drilling over and tunneling above the ConVirginia orebody apparently did not reveal the upper contin-uation of this structure.

At the southern end of the productive part of the Comstocklode at the Overman mine, the Silver City lode (Fig. 2) strikesabout N 50° W for about 2,700 m and then irregularly curves toa north strike that continues for more than 4 km. The Silver Citylode is bounded by two parallel faults north of the change instrike, similar to the faults bounding the Comstock fault zone.These faults dip as much as 65° NE at the surface but quicklyflatten to about 40° NE at depth (Gianella, 1936). South of theLucerne cut, the main Silver City lode narrows to a single struc-ture with numerous splays in the footwall and hanging wall.

The Occidental lode (Fig. 2) roughly parallels the Com-stock fault zone, having a general strike of N 15° E and a dip


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201816141210 22

Kate Peak Fmlower member

Alta Formation

Davidson Diorite

KnickerbockerAndesite

Alunite

Age (Ma)

Kate Peak Fmupper member

Muscovite

Adularia

FIG. 3. Summary of K-Ar (diamonds) and fission track (squares) ages from the Comstock district with analytical uncer-tainty at one standard deviation (from Vikre et al., 1988). Arranged in approximate order from oldest (bottom) to youngest(top) on the basis of stratigraphic relations. Filled symbols from Edwin H. McKee (writ. commun., 1987).

TABLE 1. Summary of Orientations and Displacements of Major Faults in the Comstock District

Fault General strike Dip at surface Dip at depth Displacement (m)

Comstock N 15° E 50° E–80° W 35°–40° E 500–900Occidental N 15° E 45° E 35° E 0–300Silver City N 50° W to N-S 60°–40° E 35°–40° E 100–500East Vein N 50° E ? 70°–55° SE <100 ?Haywood N 70° E 70° S ? 250 ?Grizzly Hill N 70° E 55° N ? <70 ?Buckeye N 15° E to N 20° W 50°–55° E ? >300 Woodville N 45° E 80° N ? 200

? = unknown


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South LateralSutro Tunnel

1000 m

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Sutro Tunnel120 m N of

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900 m

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FIG. 4. Geologic cross sections through the Comstock lode using data from the Becker collection and drill holes. Modi-fied from Becker (1882) and D.M. Hudson (unpub. data). Horizontal and vertical scales equal.

of about 40° E (Fig. 4). Unlike the Comstock fault zone, theOccidental fault is a single fault for most of its length butsplays into several branches at its southern end. Most of thedisplacement appears to have been in the central and south-ern parts. The northern end is covered by unfaulted flows ofthe upper member of the Kate Peak Formation.

Numerous vein-filled normal faults (Fig. 5) strike generallyN 10° W to N 15° E and about N 65° E to N 75° E in thesouthern part of the district. A north-northeast–strikinggraben with veins along both bounding faults lies in the foot-wall of the Silver City lode. The Buckeye fault, parallel to theOccidental fault, drops the upper member of the Kate PeakFormation down to the east by up to 300 m. The normal faultsin the southern part of the district generally dip 45° to 60°, ex-cept for the nearly vertical Woodville vein.

Only a few veins are known between the Comstock and Oc-cidental lodes north of the Woodville vein. The East vein zone(Fig. 5) strikes N 45° E to N 60° E and dips 50° to 78° SE(Nevada Bureau of Mines and Geology Comstock mining dis-trict files). To the north of the East vein zone (Fig. 5), theHardy vein strikes N 70° E to N 78° E and dips 85° S (RobertKendall, written report, 1952, University of Nevada-Reno li-braries, Special Collections department). The intersectionbetween the East vein zone and a nearly vertical split of themain Comstock lode appears to have localized the Con Vir-ginia orebody. The widest part of the orebody formed at thisintersection. The Garfield lode (Fig. 6), discovered in thesouth lateral of the Sutro Tunnel, strikes N 42° E and dips 72°NW (Stoddard and Carpenter, 1950). There appears to be lit-tle displacement on any of these faults.

Both premineralization and postmineralization displace-ment can be documented on most of the faults in the districtbut only total displacements can be calculated (Table 1).Gouge zones containing uncemented mineralized rock, slick-enlines on vein margins, and crosscutting clay seams are evi-dence for postmineralization displacements. The steep es-carpment west of the Comstock fault zone probably indicatesrecent renewed displacement also. Faulting appears to havebeen accompanied by slight westward tilting as shown by lessthan 10° W dips of the Knickerbocker Andesite, although thebasal contact of the upper member of the Kate Peak Forma-tion is commonly almost horizontal. Only close to the majorfaults do the Knickerbocker Andesite or Pliocene-Pleistocenefan gravels dip more steeply westward. Although all of thefaults are dominantly dip-slip faults, some apparently havestrike-slip components, including the Occidental, Comstockand Silver City fault zones. Slickenlines are only rarely ob-served and, in a single exposure, are commonly contradictoryabout the lateral component.

Hydrothermal AlterationTwelve hydrothermal alteration assemblages or subassem-

blages are recognized in the Comstock district (Table 2). Theassemblages overlap spatially and formed during severalMiocene hydrothermal events. Most of the assemblages canbe assigned to deep low-sulfidation alteration or intermedi-ate-depth high-sulfidation alteration, using the classificationscheme of Hedenquist et al. (2000). The intermediate-depth,high-sulfidation alteration includes the alunitic, alsic, andkaolinite assemblages. Sericitic and illitic assemblages can

form in either alteration style (Hedenquist et al., 2000) and,where events overlap spatially, they are difficult to assign toeither style unless clear lateral zoning patterns exist.

Three subassemblages of propylitic alteration are recog-nized in the district (Table 2) that are readily mappable basedmostly on the presence or absence of visible epidote. Whereepidote is present, calcite rarely forms more than 3 percent ofthe rock volume (the assemblage is herein referred to as thepropylitic-e assemblage—chlorite-epidote-albite). Chloritewith minor epidote replaces mafic minerals and much of thematrix glass. Plagioclase is partially to completely replaced byalbite, minor illite, and epidote. Epidote fracture fillings andrarely quartz fracture fillings occur erratically in the assem-blage. Disseminated pyrite is minor or absent. This assem-blage exists in nearly all intermediate-composition rocks inthe footwall of the Comstock lode (Fig. 5); the abundance ofepidote (up to 20%) is greatest near the lode and decreases tothe west. Although rare in andesites, actinolite has replacedessentially all of the pyroxenes in surface exposures of theDavidson Diorite, where epidote is uncommon. The propy-litic-e assemblage forms a wide belt at the surface, parallelingthe Comstock lode in its hanging wall; however, epidoteabundance does not form a clear pattern. A narrow belt ofpropylitic-e assemblage rock occurs on either side of the Oc-cidental lode (Fig. 5), and the abundance of epidote increasestoward the lode.

In the propylitic-c assemblage (chlorite-calcite-albite), cal-cite is a common alteration mineral, making up 5 to 50 per-cent of rock volume, and epidote is absent. Chlorite replacesthe groundmass and some of the mafic minerals but calcite iscommonly dominant. Plagioclase is partially to totally re-placed by mixtures of albite and calcite with minor illite orillite/smectite. The propylitic-c assemblage occurs more dis-tally from the lodes and in much of the southern part of thedistrict. There is the suggestion of vertical zoning betweenthe Comstock and Occidental lodes; the propylitic-c assem-blage is found on hilltops and the propylitic-e assemblage isexposed near the bottom of adjacent drainages. Many of theBecker collection samples from the Sutro Tunnel are of thepropylitic-e assemblage where the propylitic-c assemblage isexposed at the surface directly above. The propylitic-e assem-blage ends to the south in the intermediate composition rocksnear Silver City. The felsic ash-flow tuffs rarely contain epi-dote, even where they directly adjoin epidote-rich andesite.Chlorite usually is sparse in the ash-flow tuffs, primarily re-placing biotite with minor replacement of matrix. Zeolites arepresent locally in the propylitic assemblages (Coats, 1940) butseldom are abundant.

The propylitic-a (chlorite-albite-quartz) assemblage is con-fined to the immediate vicinity of the Comstock lode (Fig. 5and Table 2). This assemblage shares the abundant chlorite ofthe other propylitic assemblages, but it rarely contains epi-dote or calcite. Plagioclase phenocrysts are variably albitizedor are partially to completely replaced by K feldspar. Propy-litic-a assemblage rocks are variably silicified, possibly as anincomplete overprint. Several percent illite or muscovite iscommon in the assemblage, as well as 2 to 10 percent dis-seminated pyrite. The altered rocks may contain many veinsor very few veins. The propylitic-a assemblage apparently en-closed many of the orebodies. Blake (1864), Church (1879),


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Cristobolite + kaolinite + aluniteIlliticKaolinitic, alsic, aluniticSilica + kaolinitePropylitic-c (epidote absent)Propylitic-e (with epidote)Propylitic-a (chlorite + illite + K-spar + albite)SericiticMetamorphic rocksUnaltered units

Illitic along faultsQuartz + adularia veinCalcite + quartz veinConcealed veinStockwork veins

CV

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WV

BF

HF

GHF

CL

CL

GL

OL

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FIG. 5. Map of the Comstock district showing veins and general distribution of alteration assemblages. Same area as Fig.2. CL = Comstock lode, CV = Con Virginia orebody projected to the surface, EVZ = East vein zone on 2000-ft level, GL =Garfield lode on Sutro Tunnel level, HV = Hardy vein on 2000-ft level. Other abbreviations from Fig. 2.


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and Becker (1882) describe the wall rocks of orebodies asgreen, probably from chlorite of the propylitic-a assemblage.

Texturally destructive alunitic alteration, usually less than 2m wide, occurs discontinuously along small, nearly verticalfaults or fractures in the hanging walls of the Comstock andthe Occidental lodes and west of Mt. Davidson (Fig. 5 andTable 2). A few irregular bodies up to 100 m wide also arepresent. Some of the alunitic alteration occurs within localhydrothermal breccias. Alunitic alteration does not appear toextend much more than 100 m below the current surface, onthe basis of drilling and samples from the Becker collection.In this alteration, crystal aggregates of alunite commonly re-place phenocrysts, and micrometer-sized quartz replaces therock matrix. Alunite to natroalunite solid solution ratios varyfrom about 2:3 to 3:2, on the basis of data derived using theX-ray diffraction technique of Parker (1962). Up to 5 percentdisseminated pyrite is found in the assemblage.

An assemblage of pyrophyllite, diaspore, quartz, and pyrite,herein referred to as alsic alteration, is zoned laterally out-ward from the alunitic alteration or occupies fractures alone(Fig. 5 and Table 2). In the hanging wall of the Comstocklode, pyrophyllite and quartz are typically present with littleor no diaspore, usually in linear zones less than 3 m wide butrarely up to 10 m wide. In the hanging wall of the Occidentallode diaspore is more common, and up to 30 percent diasporemay be intergrown with quartz with or without pyrophyllite.Pyrophyllite is known from drilling and from specimens in theBecker collection to persist to at least 300 m below the pre-sent surface in the hanging wall of the Comstock lode.

The kaolinitic assemblage forms zones up to 200 m wide,zoned laterally outward from the alsic assemblage (Fig. 5 andTable 2). The kaolinitic assemblage also occupies hydrother-mal conduits without the alunitic or alsic assemblages. Bothdickite and well-ordered kaolinite are present in the kaoliniticassemblage, but their distribution has not been studied. Thekaolinitic assemblage appears to end within 100 m of the sur-face, on the basis of drilling and the lack of kaolin-group min-erals in rocks of the Becker collection. Part of the kaoliniticassemblage may be supergene, but several samples fromprospects containing kaolin-group minerals with unoxidizedpyrite indicate a that hypogene origin is likely for much of thisassemblage.

The illitic assemblage (Table 2) occurs in zones 1 to 200 mwide that are locally zoned laterally outward from thekaolinitic assemblage (Fig. 5). Similar to the assemblages de-scribed in previous paragraphs, illitic alteration assemblagesmay form proximal to faults and fractures without any of theabove assemblages. Illite and, uncommonly, randomly inter-stratified illite/smectite replace primarily phenocrysts, andquartz commonly is dominant in the groundmass. Much ofthe illitic alteration in the hanging wall of the lodes contains 3to 20 percent disseminated pyrite and minor pyrite fracturecoatings. Original textures are usually well preserved but, lo-cally, illitic alteration is texturally destructive. Specimensfrom the Becker collection demonstrate that illitic alterationpersists to the deepest explored levels of the hanging wall, atleast 900 m. Illitic alteration also is found erratically along thesouthern part of the Comstock lode, along many of the lodes


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TABLE 2. Mineralogy1 of Alteration Assemblages of the Comstock District

High-sulfidation alteration

Alunitic Alsic Kaolinitic Cristobalite-kaoliniteAlunite Pyrophyllite Kaolinite CristobaliteQuartz Quartz Dickite KaolinitePyrite Diaspore Quartz AluniteHematite Pyrite Pyrite Pyrite

Hematite

High- and low-sulfidation alteration Supergene(?) alteration

Illitic Sericitic Silicification Silica-kaoliniteIllite Muscovite Quartz QuartzQuartz Quartz Pyrite Disordered kaolinitePyrite PyriteAnhydriteIllite/smectiteSmectite

Low-sulfidation alteration

Propylitic-e Propylitic-c Propylitic-a PotassicChlorite Chlorite Chlorite BiotiteEpidote Calcite Albite QuartzAlbite Albite Quartz PyriteQuartz Smectite K feldspar PyrrhotiteCalcite Quartz Illite ChalcopyriteZeolites Zeolites PyriteActinolite Muscovite

1 Essential mineral, common mineral, uncommon mineral (italic, bold, normal text, respectively)

in the southern part of the district, and is generally distal tothe footwall in the northern part of Comstock lode. Betweenthe Haywood and Grizzly Hill faults, illitic alteration affectsthe upper part of the silicic ash-flow tuffs in a wide blanket,but it is not found in the directly overlying Alta andesites.

The sericitic assemblage is found at the surface only in thefootwall of the Comstock lode (Fig. 5 and Table 2). The as-semblage consists of 2M muscovite, quartz, and pyrite. Thealteration destroys texture only locally. The sericitic and illiticassemblages are similar in appearance and their distributionis only approximated on Figure 5. Large areas of sericitic al-teration are confined to the area of Cedar Hill, opposite theapparent core of the alunitic and alsic alteration in the hang-ing wall. Small irregular bodies of strong sericitic or illitic al-teration lie within the larger mass of propylitic-a alteration inthe Comstock lode. These zones, only a few of which areknown, locally enclose stoped orebodies and extend 0.5 to 10m from them. Quartz veins range from abundant to absent insericitic or illitic altered rocks.

Subsurface anhydrite-bearing alteration is known only fromdrilling and in several specimens from the Becker collection.Disseminated anhydrite makes up traces to 30 percent of therock in the illitic assemblage. Anhydrite veins are uncommonin specimens from the Becker collection and in the few drillholes that penetrate the assemblage. Several specimens ofquartz-pyrophyllite alteration also contain disseminated anhy-drite. Anhydrite has replaced the cores of plagioclase phe-nocrysts in propylitically altered subsurface specimens in theBecker collection from the vicinity of the Union and Combi-nation shafts. The deepest specimen of anhydrite alterationcomes from the hanging wall of the East vein on the 2300-ftlevel of the Ophir mine. The shallowest specimen is from the400-ft level of the Scorpion mine. Above this particular sam-ple is a northeast-striking, en echelon zone of silica-kaolinitealteration (Fig. 5 and Table 2), consisting of porous, sugaryquartz and disordered kaolinite that may be the weatheredsupergene equivalent of the anhydrite and pyrite-rich illiticalteration at depth. Locally, the anhydrite has been altered togypsum, and gypsum rims on anhydrite or nearly completereplacement of anhydrite is common in subsurface samples.The few veinlets of anhydrite are also largely replaced by gyp-sum. There are numerous gypsum fracture coatings in speci-mens from as deep as the 3000-ft level of the Mexican mine,but their origin is unknown.

A potassic alteration assemblage (Table 2) lies within andadjacent to a part of the Davidson Diorite in the Sutro Tun-nel near the junctions with the north and south laterals of theSutro Tunnel. Becker collected at least 10 specimens alongthe Sutro Tunnel that contain up to 50 percent secondary bi-otite, less secondary quartz, and traces of chalcopyrite andpyrrhotite mantled by pyrite. The alteration appears to be di-rectly related to the intrusion of the Davidson Diorite and isknown only from samples in the Becker collection. The dis-tribution or extent of potassic alteration is unknown.

A poorly exposed assemblage of cristobalite and kaolinite(Table 2) locally directly underlies flows or flow domes of theupper member of the Kate Peak Formation east of the Occi-dental lode and on Mt. Abbie west of the Comstock lode (Fig.5). These blanket-like areas are only a few meters thick andappear to overlie both propylitic-e and propylitic-c alteration

assemblages. The alteration is dominated by cristobalite andquartz with less abundant moderately ordered kaolinite andminor alunite. This assemblage can be assigned to the shal-low, low-sulfidation alteration in the classification scheme ofHedenquist et al. (2000).

Characteristics of the Comstock LodeThe Comstock lode is a combination of small, lenticular, in-

termittent ore shoots contained within a much larger mass ofsubore-grade massive veins, breccias, and stockwork veins(Fig. 6), complexly rearranged by postmineralization faulting.The ore-bearing part of the lode extends about 4,200 m fromCedar Hill to the junction with the Silver City lode (Fig. 6). Ofthe entire extent of the lode, only a very small part was of oregrade. King (1870) estimated that less than 0.2 percent of thevolume of the lode constituted ore. The segment north andwest of American Flat produced little but did contain localsmall pods of ore or base metal sulfides (Raymond, 1877).

Gangue mineralogy

The most common lode material appears to be stockworkveins, primarily in the Alta Formation, but also hosted inDavidson Diorite, Mesozoic rocks, and intrusions of KatePeak Formation (Fig. 4). Becker (1882) showed large areas ofstockwork veins persisting to the deepest explored levels ofthe lode. Stockwork veins are commonly 10 to 50 percent ofthe rock volume but vary along the lode from a few percentto more than 90 percent (Fig. 7). Stockworks persist for up to400 m in the footwall from the west wall but rarely constitutemore than 5 vol percent. Stockwork veins appear absent oruncommon in the hanging wall of the east wall, because noneare mentioned in records of mines south of the Con Virginiamine. The stockwork veins cut the wall rocks at all angles.Possibly there is a preferred strike parallel to the lode withrandom dips, but lack of exposure prevents detailed analysis.In a crosscut in the New Savage mine, more than 20 genera-tions of crosscutting veins were observed on a 4 m2 surface.Each generation of vein may have 1 to 4 depositional bands.In some cases, as many as 40 bands were noted. The mostcommon vein filling appears to be comb and/or mosaic quartzhaving poor or simple banding and grains 0.1 to 2 mm long(Fig. 7). The next most common veins have margins ofroughly equal amounts of 10 to 500 µm grains of anhedral tosubhedral adularia and anhedral quartz in poorly boundedbands usually less than 2 mm wide (Fig. 7E). The interiors ofthese veins are filled with comb and/or mosaic quartz com-monly with no or trace adularia; however, some contain ap-preciable subhedral adularia. Both of these vein types rarelycontain pyrite. Other less common vein types include highlybanded comb and mosaic quartz, quartz-pyrite, adularia-pyrite with little quartz, quartz with adularia centers, andcoarse-grained adularia (up to 1.5 mm grains). Pyrite or epi-dote fracture coatings cutting veins are rare. Whereas nearlyall of the veins lack pyrite, 1 to 8 percent disseminated pyriteis common in the wall rocks. Manganese oxides, not obviouslyafter rhodochrosite, with minor pyrite locally form distinctbands in veins. Individual veins vary from thin fracture coat-ings to 30 cm wide, but the vast majority are 2 to 20 mm wide.Lateral displacements of several centimeters of an earliervein by a later vein are found, but many of the veins appear


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to be simple dilations (Fig. 7C). Some brecciation locally ac-companies the stockworks and vein matter cements wall rockand/or vein fragments. The stockwork veins usually containanomalous precious metals (Church, 1879; various mine su-perintendents’ reports, University of Nevada-Reno libraries,Special Collections department) but were rarely mined.Some of the stockwork zone material was mined in the 1980sor set aside in low-grade stockpiles (Jonathan Sprecher, oralcommun., 1983).

Probably the next most abundant lode material consists ofwhite, massive to stockwork quartz referred to as “red quartz”(King, 1870) or “bastard quartz” by miners (Church, 1879).This quartz is massive, mosaic, and/or comb-textured, and itis usually unbanded to poorly banded and contains multiplegenerations of quartz breccia fragments having crosscuttingveins cemented by quartz. The usually white to clear gray,massive quartz appears to contain little adularia and, in a fewplaces, ghost blade texture after bladed calcite is observed.


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B

C D

FE

a

e

e

e

c c

0.5 mm0.5 mm

a

a

a

s

s

s

s

aa

a

A

FIG. 7. A. Example of brecciated andesite from stockwork in the Imperial mine. White, fine-grained adularia (a) mostlyon vein margins, zoned quartz-amethyst (lower right) in large vein interior. B. Brecciated quartz with black (mostly spha-lerite) cement from Hale and Norcross mine dump. C. Stockwork veins in propylitic-a altered andesite from the Imperialmine. Mostly white fine-grained adularia (a) on vein margins with coarser quartz and some adularia in vein interiors. D. Brec-cia from Con Imperial mine with veined rock fragments and sulfide fragments (s) cemented by fine-grained quartz and adu-laria. E. Photomicrograph of stockwork vein from Imperial mine showing fine-grained adularia-quartz (a) on vein marginnext to silicified and chloritized wall rock (right) and coarser quartz in vein interior. F. Photomicrograph (plane light) of lat-tice blades of quartz after calcite (c) with recrystallized crustiform colloform quartz and opaque minerals (e) of electrum andacanthite from the Chollar mine.

Fragments of wall rocks are rarely found within massivequartz. Weakly limonite-stained massive quartz occurs toabout 120 m depth and contains anomalous precious metalsbut did not make ore (King, 1870). Massive quartz was foundalong the main lode to the deepest explored depths (800 m).Rare descriptions in various superintendent reports (Univer-sity of Nevada-Reno libraries, Special Collections depart-ment) indicate that the deep quartz is hard, locally containsbase metals, and consistently contains precious metals of sub-ore grade. Dump materials from the deeper parts of the lodeconsist of white mosaic to comb quartz with traces of pyriteand base metal sulfides. Adularia and calcite are not knownfrom the deeper parts of the lode.

Ore-bearing lode material is restricted to certain parts ofthe Comstock lode (Fig. 6) and existing descriptions are notclear as to nature of the material. The most detailed descrip-tions come from Church (1879), who recognized a distinctdifference between the nature of rich ore-bearing quartz andbarren massive quartz. The ore-bearing quartz in many of thedeeper bonanza orebodies he described as milky white, veryfriable with a sandy texture that resembled crushed sugar ortable salt, and apparently anhedral. This “sugar quartz” wasloose and crumbly, except where cemented by abundant oreminerals or by later quartz. The quartz contained some frag-ments of propylite, massive quartz, and sulfide-cementedquartz. Orebodies shallower than about 200 m tended to haveless friable quartz. The descriptions of Blake (1864) and King(1870) suggest that some of the ore zones were completelyshattered and broken, possibly by postmineralization faulting.

Examples of the ore-bearing lode material from dumps andmuseums have a variety of textures but, because they are outof context, they are described below only as illustrations ofthe variations. Quartz is by far the most abundant ganguemineral. Mosaic quartz appears to have been most commonin the ore zones, typically with grain sizes of 0.05 to 1 mm, in-tergrown with anhedral ore minerals. The material may havea granular texture with 10 to 90 percent quartz intergrownwith the ore minerals. Some ore zones contain moderately topoorly banded, crustiform to cockade quartz. Mosaic, mas-sive, or crustiform quartz cementing numerous breccia frag-ments of wall rock, sulfides, or vein quartz is common. Sac-charoidal quartz of the type described by Church (1879) isnow rarely exposed. Wall rock fragments within breccias are

commonly a jumble of fragments with propylitic and sericiticor illitic alteration assemblages that were cut by one or moregenerations of veins before brecciation. Finely banded, crus-tiform quartz veins appear to be uncommon in the lode ma-terial, and some are found as breccia fragments cemented bymassive or ore-bearing quartz or by sulfides (Fig. 7B). Insome specimens, irregular angular areas of fibrous and radi-ating off-white or very pale green chlorite up to 1 mm wide(Fig. 8) are found intermittently along bands. In some speci-mens, chlorite and minor quartz, which have an overall gran-ular texture, form cement for the ore minerals. Anhedral tosubhedral, milky white adularia (commonly less than 0.2 mm)is intergrown with ore-bearing quartz; however, its abun-dance appears to be highly variable. Other known forms ofquartz include lattice blade (Fig. 7F), ghost sphere, and mosstextures, but their relative abundance is unknown. All of theabove-described finer-grained textures may or may not be di-rectly associated with ore minerals, but ore minerals appearto be rarely associated with coarser-grained (>1 mm) quartz,although occasional interstitial pyrite is observed. Coarse-grained quartz may alternate with one or more bands of finer-grained quartz. The coarse-grained quartz consists of featheryand locally zonal and flamboyant grains commonly 0.5 to 5mm long. Unlike sulfides in massive quartz, sulfides in ore-bearing quartz are only weakly oxidized near the surface(Blake, 1864; King, 1870; Church, 1879).

Calcite, which can be barren or closely associated with ore,is apparently much less common than quartz along the Com-stock lode. Masses of barren calcite up to 5 m wide crop outon Cedar Hill, the only known surface exposure. The calciteis blocky with a grain size of 2 to 10 mm. Minor quartz andtraces of adularia are intergrown with the calcite. Specimensof ore from several orebodies contain anhedral to subhedral,white, nonbladed calcite (W.M. Keck Museum, University ofNevada, Reno). Some of this calcite (as much as 25%) is inti-mately intergrown with quartz and ore minerals, whereas inother specimens the calcite forms distinct bands or crosscut-ting veins. In some cases, calcite is found as breccia clasts ce-mented by sulfides and quartz. Bastin (1923) reported thatcalcite was found sparsely or abundantly in all of the orebod-ies and described anhedral to subhedral and nonbladed cal-cite intergrown with ore minerals. Browne (1868) and Ray-mond (1869, 1870, 1873b) noted that calcite was found


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A B

gs

c

a

a

s

g

c

sg

a

c

chc

s

ch

a

s g gs

s

e

cp

c

g

ca

ch s

s

ch sg

s

p g 0.25 mm0.25 mm

q

q q

q

q

FIG. 8. Photomicrographs of vein ore from the Ophir mine. Acanthite (a), chalcopyrite (c), electrum (e), and galena (g),largely replaced by sphalerite (s) in a matrix of poorly banded chlorite (ch, low relief) and quartz (q).

erratically in the ore zones, locally as nonore-bearing bandsalternating with quartz and ore-bearing bands. Browne(1868) and Raymond (1869, 1870) reported that the produc-tive veins gradually change from quartz to calcite or “lime-stone” commonly with considerable “sulphate of lime” (anhy-drite and/or gypsum) beneath the productive zones. Wherecarbonate is more abundant than quartz, the vein matter isnearly devoid of sulfides and is commonly of subore grade;however, abundant pyrite occurs in the adjoining wall rock(Raymond, 1873b, 1875). Massive barren calcite appears tohave been deposited over a short vertical interval, probablyless than 50 m, changing again to quartz with depth (Ray-mond, 1872, 1873a). In the Belcher, Imperial, and Con Vir-ginia mines, some calcite-dominant vein matter that was es-sentially devoid of sulfides did make high-grade ore and hadunusually high gold/silver ratios (Raymond, 1873b, 1875).These veins are separate from the sulfide-rich orebodies andlateral to them. Rhodochrosite has been reported only fromthe Savage mine (King, 1870).

Ore mineralogy

The ore minerals consist of a base-metal–precious-metalsuite. In apparently decreasing order of abundance, the majorore minerals are sphalerite, chalcopyrite, galena, pyrite, acan-thite, and electrum. Stephanite was reported to be the mainsilver mineral in the Con Virginia orebody (Becker, 1882) andwas abundant in the Ophir orebody (Blake, 1864). Also re-ported, but apparently as minor minerals, are aguilarite, jal-paite, miargyrite, pearcite, polybasite, proustite, pyrargyrite,pyromorphite, sternbergite, stromeyerite, tetrahedrite, anduytenbogaardite (Terrill, 1914; Bastin, 1923; Coats, 1936;Barton et al., 1978; Vikre, 1989). Native silver, covellite, chal-cocite, and chlorargyrite have been found in oxidized andpartly oxidized zones (Bastin, 1923). Sulfides make up 50 to90 vol percent; however, some ore is sulfide poor. In most orespecimens, sulfide grains are 0.2 to 2 mm across and usuallyanhedral; they are intergrown with other sulfide grains, simi-lar-sized quartz, and locally with chlorite or calcite (Fig. 8).

There was a large variation in character between and withinorebodies. The orebodies included sulfide bodies lacking ore-grade precious metals, others with intergrown base metal andprecious metal minerals, and still others that essentiallylacked base metals. The Ophir mine had a western vein richin base metals but without ore-grade precious metals,whereas the eastern vein was rich in both base and preciousmetals (Blake, 1864). King (1879) and Raymond (1870,1873a, 1875, 1877) noted several zones in other mines thatwere rich in base metals but below ore grade in precious met-als, as bodies spatially separated from ore zones. Some ore-bodies were nearly devoid of base metals, or base metal con-tents increased considerably with depth. Raymond (1869,1870, 1873a, 1875) noted the latter case in the Ophir, Haleand Norcross, and Savage mines, and in the deeper (eastern)orebodies of the Yellow Jacket, Crown Point, and Belchermines. The top of the Crown Point-Belcher orebody had ahigh gold/silver ratio, and silver, base metals, and calcite in-creased with depth (Raymond, 1870, 1873a). Raymond(1873a, 1875) reported that the gold/silver ratio decreasedwith depth, except that where calcite was dominant there wasa higher proportion of gold to silver. In contrast, gold/silver

ratios varied widely with depth in some orebodies (e.g., Chol-lar and Potosi). Some parts of the Con Virginia orebody werepoor in base metals, whereas other parts were rich in chal-copyrite and galena but poor in sphalerite, and in still otherparts sphalerite was dominant (Raymond, 1875, 1877). How-ever, most of the richest ores were reported to contain appre-ciable chalcopyrite and were relatively poor in sphalerite(Blake, 1864; Raymond, 1875, 1877).

Only a few dozen specimens of precious metal-bearing orefrom dumps or museums were available for petrographicanalysis. These specimens possess a wide range of texturesfrom granular to breccia to massive to highly banded. Brecciaore specimens contain fragments that have up to 90 percentsulfide, with or without precious metals, cut by later genera-tions of veins containing base and precious metals. Specimensof highly banded ore from the shallow Ophir orebody containmore than 20 bands of sulfides. Within each band, galena ap-pears to be early, cut by or partially replaced by electrum andacanthite. However, in some cases galena and acanthite ap-pear to be coeval. Chalcopyrite and pyrite are observed topartially replace the galena and acanthite. Sphalerite appearsto have been deposited late, partially replacing other ore min-erals. A similar sequence is observed in material from severalorebodies in both breccia fragments and banded veins, al-though some variations in the order of replacement are ob-served. Specimens of ore from the Chollar mine contain in-tergrowths of acanthite and electrum or, in other specimens,isolated grains of electrum and acanthite with only traces ofsphalerite and pyrite.

Some ore was disseminated rather than deposited in veins.In samples from the Ophir and Con Virginia orebodies, spha-lerite, chalcopyrite, galena, and pyrite are disseminated in thewall rocks, commonly as replacements of mafic phenocrysts.Up to 20 percent of the wall rock may be replaced by dis-seminated base metal sulfides, but apparently only in orezones. These typically chloritic and moderately silicified wall-rock breccia fragments are cut by or cemented by quartz,some of which contains base and precious metals. The dis-seminated ores do not appear to be selvages around veins;however, later brecciation or veining may have obscured suchrelationships. Becker (1882) noted that a part of the preciousmetal ore in the Con Virginia orebody was disseminated inthe wall rocks. Church (1879) noted small masses of dissemi-nated ore along the Comstock lode.

Forms of orebodies

The orebodies along the Comstock lode formed discretepods of irregular shape (Fig. 6). Two orebodies had west dips,whereas the rest had east to nearly vertical dips (Figs. 4 and6). The Ophir orebody dipped 70° W near the surface (Blake,1864) but rolled to steeply east-dipping with depth (King,1870). The western and shallower of the Belcher orebodiesdipped 45° to 48° W, extending for about 150 m vertically be-fore it was cut off by a clay seam in the west wall (Fig. 4; King,1870). Orebodies along the east wall dipped east from 90° to40° and appear to have developed where flexures in the eastwall were steeper than normal along the dip of the east wall(Browne, 1869; Church, 1879). The orebodies were com-monly concave or convex to the course of the lode and thuscut somewhat diagonally across the general course of the lode


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(King, 1870). Mined horizontal widths were mostly 10 to 17m. Individual orebodies rarely extended more than 150 mvertically and most were less than 150 m long. The cutoff ofore grade along strike of the ore-bearing part of an orebodywas commonly where the quartz mass narrowed and appar-ently feathered into several narrow veins containing far lesssulfide (Church, 1879). Elsewhere, orebodies were cut offabruptly by clay seams (King, 1870).

Occidental LodeOf all of the major lodes in the district, the Occidental

(Brunswick) lode is the best exposed at the surface, but it hasnot been extensively explored at depth. The Occidental lodestrikes and dips about parallel to the Comstock lode and istraceable for nearly 6 km. The southern end splays into sev-eral veins, and the northern end is covered. The bulk of theproduction came from an 1,100-m-long segment near thecenter of the lode along a concave flexure (Fig. 2).

The uppermost part of the lode consists of variably quartz-cemented breccia (1–4 m wide) and weakly veined, illitizedandesite up to 10 m wide (Fig. 9). Mosaics of very finegrained quartz, 0 to 40 percent <100 µm adularia, and somecrustiform, medium-grained, comb-quartz cement the brec-cia. The angular breccia fragments, which make up 40 to 80percent of the lode material, are weakly limonitic and moder-ately silicified and/or illitized. In an irregular but sharp tran-sition, the lode changes from breccia or illitized andesite to acrudely banded vein consisting of 80 to 95 percent calcitewith stringers of quartz, some adularia, and 1 to 20 percentangular breccia fragments of propylitized andesite. The tran-sition zone contains irregular pods of very fine grained quartzthat replaced calcite, locally with hollow casts of calciterhombs. The calcite tends to be white to gray and sparry andhas grains 1 to 50 mm across. Rarely, the calcite is bladed andthe blades are recrystallized to much finer grained, sugarycalcite. The calcite is locally brecciated and recemented alongwith breccia fragments of quartz. Locally, crustiform bandsoccur within the calcite, rarely alternating with bands ofquartz.

The lode varies from 1 to 7 m wide and the orebodies weremined from the center to the hanging-wall side of the lode.Crosscutting calcite and quartz-adularia veins, 1 to 20 mmwide, are oriented subparallel to the strongly epidotized foot-wall. The calcite-dominated part of the lode has an apparent

vertical range of 80 to 100 m; it changes into a dominantlyquartz and adularia vein at depth (Fig. 9). The quartz-adu-laria–dominated part of the lode appears to be cementedbreccia and stockwork that resembles the Comstock lode;however, it may contain appreciable calcite. Ore was minedfrom the calcite-rich zone into the more quartz-adularia–dominated zone, but the uppermost quartz-cemented brecciais subore grade. Little is known about the ore mineralogy, butmuseum specimens contain sphalerite, galena, chalcopyrite,acanthite, electrum, and minor pyrite in both calcite-rich andquartz-adularia–rich lode material. The exposed calcite-richpart of the lode is nearly devoid of sulfides.

Silver City Lode and Southern Part of the DistrictThe Silver City lode and its branches (Fig. 5) contained

mostly small orebodies, commonly located at intersections oflodes (Gianella, 1936). Vein matter consists of banded veins,stockworks, and/or cemented breccias. Stockworks of fine-grained quartz, with or without fine-grained adularia, occuralong the northwest-striking part of the main strand of the Sil-ver City lode. Other lodes, commonly less than 5 m wide,consist of breccias cemented by quartz (± minor adularia).Locally, as in the Woodville vein, the vein material is finely tocoarsely crustiform with only minor brecciation. Irregularpods of coarse-grained, white calcite and manganiferous cal-cite formed along many of the lodes. Interlocking, anhedral tosubhedral equant calcite crystals up to 8 cm across locallyhave multiple growth zones of manganese oxide inclusions.Calcite-bearing material was mined locally (Gianella, 1936)but is mostly subore grade. According to Gianella (1936), thelodes in the Silver City area tend to have low sulfide contents.Only traces of chalcopyrite, galena, sphalerite, and molyb-denite were noted in the ores. Becker (1882), on the otherhand, noted that abundant sphalerite and galena were foundin the Justice mine, and museum ore specimens from otherlodes contain the typical Comstock ore suite. Minor amountsof pyrite, if present, are mostly disseminated in the wall rocks.Zones of strongly illitic and locally silicified gouge and wallrock, 1 to 5 m wide, are found along most of the lodes wherequartz vein matter is absent (or nearly so). These zones com-monly contain pyrite fracture coatings and up to 10 percentdisseminated pyrite. The illitic zones may alternate alongstrike with quartz-rich lode material (Fig. 5) that has little, ifany, illitic or pyritic alteration of the wall rocks.


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1800 m

1600 m

1400 m

OCCIDENTAL BRUNSWICK

BrunswickDecline

Airshaft

NorthBrunswick

Decline

Lower Tunnel

Nev.Rte.341

Sutro Tunnel

? ? ? ? ? ? ? ? ? ?

OccidentalBQ BQ

CCCCCC

QACC

QA

Stope

FIG. 9. Longitudinal projection of the central part of the Occidental lode showing the vertical zonation of gangue miner-alogy relative to mineralization. BQ = quartz-adularia cemented breccia and silicification, CC = massive calcite with minorquartz-adularia, QA = quartz-adularia. Mineralogy of deepest zones unknown. Mine workings modified from J.H.G. Wolf(Nevada Bureau of Mines and Geology mining district files, unpub. map, 1911).

Ages of MineralizationAlthough the relative ages of alteration and mineral deposi-

tion can be determined, only a range of K-Ar ages has beenestablished for the ores. The ages of hydrothermal activity cannot be distinguished from the age of intermediate composi-tion magmatism in the area (Fig. 3). K-Ar isotope ages (Fig.3) on premineralization (Alta Formation and Davidson Dior-ite) and postmineralization or synmineralization (upper mem-ber of Kate Peak Formation and Knickerbocker Andesite)units overlap. The oldest hydrothermal event may be the in-termediate-depth high-sulfidation alteration west of Mt.Davidson (Fig. 5). The Davidson Diorite lacks intermediate-depth, high-sulfidation alteration and appears to intrude al-tered Alta Formation. This center may be related to Alta-agedmagmatism. Potassic alteration is the next oldest hydrother-mal event. It is directly associated with the Davidson Dioriteand assumed to have formed during emplacement of the in-trusion. The intermediate-depth, high-sulfidation alteration,centered near Cedar Hill and lying mostly in the hanging wallof the Comstock lode, is associated with intrusions of thelower member of the Kate Peak Formation and is overlain bythe upper member. K-Ar ages of alunite from this assemblageform a cluster of 15 to 16.3 Ma as opposed to mostly 12.7 to14.1 Ma for the upper member of the Kate Peak Formation(Fig. 3; Vikre et al., 1988). The blanket-like, shallow, high-sul-fidation, cristobalite-kaolinite alteration also formed beforethe deposition of the upper member of the Kate Peak For-mation and may represent the position of the paleowatertable above the Cedar Hill-centered, intermediate-depth,high-sulfidation alteration.

K-Ar ages of 13 to 14 Ma for adularia and muscovite (Fig.3) suggest that the deep, low-sulfidation precious metal de-posit formed roughly 1 m.y. after the Cedar Hill-centered, in-termediate-depth, high-sulfidation alteration. Adularia andmuscovite K-Ar ages are not resolvable from those of theKnickerbocker Andesite or the upper member of the KatePeak Formation. If the upper member of the Kate Peak For-mation and Knickerbocker Andesite were emplaced beforethe precious-metal–bearing hydrothermal event, the exposedparts are surprisingly lacking in hydrothermal alteration. Ifthese units postdated lode mineralization, a period of deeperosion would be required to expose the epidote-bearingpropylitic alteration. The K-Ar ages, however, do not indicatea long time gap between lode formation and the deposition ofthe upper member of the Kate Peak Formation. Therefore,the upper member of the Kate Peak Formation and Knicker-bocker Andesite were probably emplaced before epithermalprecious metal mineralization, but the exposed remnants ofthese units apparently had poor permeability during Com-stock hydrothermal activity and are essentially unaltered.

Surface GeochemistrySurface samples of lode material (Table 3) are noticeably

low in trace elements typically associated with epithermalprecious metal deposits. All of the samples are of lode mate-rial but not of high-grade material. Trace element values aregenerally low, even for Cu, Pb, and Zn, whereas ore-graderock contains abundant chalcopyrite, galena, and sphalerite.

The only consistently anomalous element is thallium. Lessconsistent anomalies are observed in As, B, Cu, Mo, Pb, Sb,Se, Te, and Zn. Several of the samples from the Occidentallode (Table 3: COM-430, 431, 432, 433) were taken from lessthan 10 m updip from the tops of stopes, indicating there isnot a significant geochemical halo above the orebodies. Nobarite or fluorite have been reported from the lodes, and theirabsence is reflected in the low values of fluorine and barium.

DiscussionAt least four Miocene hydrothermal events are recognized

in the Comstock district. The secondary biotite alteration isproximally associated with the intrusion of a phase of theDavidson Diorite. It was probably very localized and wasoverprinted by later hydrothermal activity. The two centers ofintermediate-depth, high-sulfidation alteration (cf. Heden-quist et al., 2000) include alunitic, alsic, and kaolinitic assem-blages, and at least some of the areas of illitic and sericitic al-teration. Alteration in the hydrothermal alteration centerwest of Mt. Davidson is distributed along a crudely radial setof fractures. The other, probably younger, area is centered onnumerous intrusions of the lower member of the Kate PeakFormation at the north end of the district. This alteration isspread along fractures to the northeast, northwest, and south.Intermediate-depth, high-sulfidation alteration was not local-ized along the Occidental fault or the East vein zone, andprobably not along the Comstock fault zone, suggesting thatthese faults formed later. The fractures (or faults) that focusalteration in both areas are short and discontinuous, suggest-ing that there was little displacement along them. Other thanlocally strong pyritization, no known deposition of base orprecious metals accompanied the two intermediate-depthhigh-sulfidation altered areas. The blanket-like cristobalite-kaolinite shallow high-sulfidation alteration probably formedat the paleowater table, in a manner similar to that describedfor nearby Steamboat Springs (Schoen et al., 1974). Therewas probably only minor erosion (<100 m?) of these high-sul-fidation hydrothermal events before the deposition of theoverlying upper member of the Kate Peak Formation.

The younger, deep, low-sulfidation alteration and preciousand base metal deposition apparently was localized along onlya few of the faults that previously were the focus of the inter-mediate-depth, high-sulfidation alteration. Instead, the deep,low-sulfidation mineralization and alteration formed mostlyalong newly developed faults after what may have been a 1 to2 million-year hiatus in hydrothermal activity. Much of thepresent distribution of the propylitic assemblages appears tobe associated with the deep, low-sulfidation hydrothermalevent, because epidote-bearing propylitization increased inintensity near the lodes and seems to have little spatial asso-ciation with the intermediate-depth, high-sulfidation alter-ation, particularly along the Occidental lode and in the south-ern parts of the district. The presence of epidote indicatesthat these rocks exceeded 230°C (Reyes, 1990) and generallyagrees with fluid inclusion filling temperatures above 230°Cin nearby lodes (Vikre, 1989). Areas of epidote-bearingpropylitization correspond well to more isotopically ex-changed (low δ18O) wall rocks shown by Criss and Champion(1991). The most highly exchanged wall rocks (lowest δ18Ovalues) are in the illitic or propylitic-a assemblages along the


0361-0128/98/000/000-00 $6.00 381


0361-0128/98/000/000-00 $6.00 382

TAB

LE

3. T

race

Ele

men

t Geo

chem

istr

y of

Sur

face

Sam

ples

from

the

Com

stoc

k an

d O

ccid

enta

l Lod

es

Sam

ple

no.

Vein

type

Au

Ag

As

BB

aB

iC

dC

uF

Hg

Mn

Mo

PbSb

SeTe

Tl

Zn

Com

stoc

k lo

de

CO

M-1

91St

ockw

ork

0.65

15.0

30<1

050

0<1

<0.1

30<1

000.

1230

0<5

-10

2<1

.0<0

.05

2.25

40C

OM

-195

Stoc

kwor

k0.

2310

.020

<10

500

<10.

330

<100

0.22

700

<515

6<1

.00.

051.

9070

CO

M-4

20St

ockw

ork

0.65

150.

010

1030

0<1

0.6

3030

00.

2220

0<5

70<2

n.d.

0.05

2.70

95C

OM

-421

Stoc

kwor

k0.

8050

.010

1530

0<1

0.4

70<1

000.

421,

000

<530

4<1

.0<0

.05

1.65

70C

OM

-421

ASt

ockw

ork

1.35

100.

010

1530

0<1

0.2

30<1

000.

6215

0<5

506

1.7

0.10

1.85

40C

OM

-422

Stoc

kwor

k0.

2015

.010

<10

300

<10.

350

<100

0.04

300

1015

0<2

n.d.

0.10

2.35

55C

OM

-423

Stoc

kwor

k0.

3010

.0<1

010

300

<11.

010

0<1

000.

0430

015

150

<2n.

d.0.

051.

4025

0C

OM

-423

ASt

ockw

ork

0.05

10.0

<10

<10

500

<11.

715

0<1

000.

2850

07

200

<2<1

.00.

101.

4530

0C

OM

-429

Stoc

kwor

k<0

.05

1.0

1010

300

20.

330

<100

0.10

150

<550

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051.

1355

CO

M-4

29A

Stoc

kwor

k0.

057.

0<1

010

200

<10.

250

<100

0.36

100

1515

0<2

1.2

0.30

1.02

15C

OM

-772

Stoc

kwor

k<0

.05

10.0

<10

<10

300

10.

830

<100

0.06

200

1070

0<2

n.d.

1.30

1.20

70C

OM

-774

Stoc

kwor

k0.

051.

550

<10

300

<1<0

.120

200

0.08

500

203

<2n.

d.0.

101.

4070

CO

M-7

96St

ockw

ork

<0.0

50.

5<1

015

300

<10.

830

<100

0.04

300

2015

0<2

n.d.

0.15

0.80

95C

OM

-879

Stoc

kwor

k.1

5<0

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0<1

030

0<1

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20<1

000.

0420

0<5

30<2

n.d.

<0.0

5.8

015

CO

M-4

21B

Qua

rtz

1.05

70.0

<10

1070

<10.

420

<100

0.22

50<5

70<2

<1.0

<0.0

50.

3040

CO

M-4

19C

alci

te<0

.05

3.0

2010

700

<10.

130

100

0.12

500

<520

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d.<0

.05

2.25

70

Occ

iden

tal l

ode

CO

M-2

60C

alci

te0.

1020

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050

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3010

00.

0815

015

020

10n.

d.0.

156.

0050

CO

M-2

60A

Cal

cite

0.70

5040

<10

<20

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<100

0.08

1,00

0<5

202

n.d.

<0.0

50.

205

CO

M-2

60B

Cal

cite

<0.0

50.

710

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00<0

.02

500

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0<2

n.d.

<0.0

50.

8015

CO

M-4

34C

alci

te0.

453

<10

<10

70<1

<0.1

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000.

0470

0<5

<10

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d.<0

.05

0.35

5C

OM

-435

Cal

cite

0.35

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0<1

020

0<1

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10<1

000.

141,

000

<5<1

0<2

n.d.

<0.0

50.

5065

CO

M-4

36C

alci

te<0

.05

<0.5

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<10

<20

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<100

0.10

300

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51.

5045

CO

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recc

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03

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2020

00.

56<1

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n.d.

<0.0

50.

205

CO

M-2

96B

recc

ia-q

uart

z<0

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2015

500

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100

0.12

20<5

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d.0.

050.

205

CO

M-2

97B

recc

ia-q

uart

z<0

.05

0.7

40<1

050

0<1

<0.1

15<1

000.

0815

070

20<2

n.d.

0.05

5.00

30C

OM

-430

Bre

ccia

-qua

rtz

<0.0

53

10<1

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0850

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0<2

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<0.0

52.

6015

CO

M-4

31B

recc

ia-q

uart

z0.

053

<10

<10

300

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<100

0.06

100

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<0.0

52.

6030

CO

M-4

32B

recc

ia-q

uart

z0.

107

30<1

030

0<1

<0.1

2010

00.

2420

0<5

<10

<2n.

d.<0

.05

1.30

35C

OM

-433

Bre

ccia

-qua

rtz

<0.0

50.

7<1

0<1

020

<1<0

.130

<100

0.04

500

10<1

0<2

n.d.

<0.0

53.

2045

CO

M-4

37B

recc

ia-q

uart

z<0

.05

1.5

<10

<10

300

<1<0

.17

<100

0.04

500

15<1

0<2

n.d.

<0.0

50.

155

Au,

As,

Bi,

Cd,

Hg,

Sb,

Te,

Tl,

and

Zn b

y at

omic

abs

orpt

ion,

all

othe

rs b

y D

C-a

rc e

mis

sion

spe

ctro

scop

y, U

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eolo

gica

l Sur

vey

labo

rato

ry, D

enve

r; S

e an

alys

es b

y in

duct

ivel

y co

uple

d pl

asm

a, G

eo-

chem

ical

Ser

vice

s In

c., R

eno

Com

stoc

k lo

de s

ampl

es fr

om o

pen

pits

and

out

crop

, Occ

iden

tal l

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sam

ples

from

out

crop

; sam

ple

size

abo

ut 7

kg

All

valu

es in

par

ts p

er m

illio

n; n

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not

det

erm

ined

Comstock lode. At least some of the illitic and sericitic alter-ation along the various lodes took place during deep low-sul-fidation hydrothermal activity, because these assemblages lo-cally form narrow halos around some of the ore shoots, and inthe southern part of the district, illitization occurs discontin-uously along mineralized fault zones. The calcite-rich propyl-itization probably formed in areas of low fluid flow distal fromthe lodes. These are also areas of less isotopically exchangedwall rocks (higher δ18O values) shown by Criss and Champion(1991). The boundary between the propylitic-e and propy-litic-c assemblages may approximate a 230°C isotherm on thebasis of the lower limit of epidote stability (Reyes, 1990). Thelack of appropriate samples makes the timing of anhydrite al-teration difficult to interpret in the context of this paragene-sis.

The deep, low-sulfidation environment existed during ac-tive deformation, when only minor amounts of precious met-als were deposited. Massive quartz was deposited in openfractures that filled and opened repeatedly. Rare bladed cal-cite pseudomorphs and minor adularia within the massivequartz demonstrate at least some boiling during this stage,similar to that reported in modern geothermal areas (Sim-mons and Christenson, 1994; Simmons and Browne, 2000).The wall rocks in the footwall and the lode were repeatedlyfractured during the massive quartz deposition. The fine-grained adularia that commonly was deposited at the edges ofstockwork veins indicates that adularia saturation was reachedat the initial opening of veins and rarely was reached later inthe formation of a particular vein. Adularia is uncommon inmassive quartz bodies but common where veins cut andesite,possibly indicating a buffering effect of the wall rocks. Theadularia in the Comstock lode has a subrhombic crystal formthat Dong and Morrison (1995) suggest is formed at low de-grees of supersaturation. The fine-grained quartz intergrownwith adularia suggests rapid crystallization. In footwall stock-works and in the other lodes, rhombic adularia is more com-mon and possibly formed under conditions of high supersat-uration. Subore-grade precious metals and some base metalswere deposited throughout the explored vertical extent of thelodes (nearly 1 km). Evidence of boiling (e.g., fluid inclusions,presence of adularia or bladed calcite) has not been observedin direct association with subore-grade precious metals, butlocal adularia-bearing veins are found in low-grade stockworkzones.

The main-stage ore was deposited late in the history of lodeformation, on the basis of crosscutting relations and the smallvolume of postore mineral deposition (King, 1870; Church,1879). The lenticular shapes and low-angle crosscutting of thestrike of the Comstock lode by many of the orebodies sug-gests late episodes of localized dilation. Small flexures in thelode may have localized the dilatant zones (Fig. 6). Whetherthere was appreciable displacement on the faults where theorebodies formed is unknown. However, multiple stages ofbrecciation and veining indicate that deformation continuedduring mineralization. Fluid inclusion evidence of boiling hasrarely been observed in any of the lode material (Vikre, 1989),but minor adularia intergrown with ore minerals and cement-ing clasts of ore-bearing material suggest that there was atleast some boiling during and after ore formation. Assumingthat nonbladed calcite is not associated with boiling (Sim-

mons and Christenson, 1994), then much of the barren mas-sive calcite underlying orebodies within the Comstock lodemay have formed by descent of shallow, CO2-rich, steam-heated waters heated to calcite saturation (Simmons et al.,2000) and deposited in narrow zones beneath the orebodies.In contrast to calcite in the Comstock lode, massive calcitewas deposited within the upper parts of the ore zone in theOccidental lode. Small amounts of intergrown adularia sug-gest at least some boiling during this calcite deposition. Cal-cite-rich lode material is either ore-grade or strongly anom-alous in precious metals indicating coeval deposition of calciteand ore minerals. Some of the coarse-grained, nonbladed cal-cite in the Silver City lode and its branches is ore bearing, in-dicating at least some mixing of CO2-rich fluids during min-eralization, but most appears to be unrelated to metaldeposition. The bulk of the calcite in the Silver City lode isbarren and may have been deposited during late collapse ofthe hydrothermal system (Simmons et al., 2000). The distrib-ution of the orebodies (Fig. 10) may be best explained bycomplex interaction of boiling, ascending metalliferous fluidsinteracting with descending, CO2-rich, steam-heated water inlocalized dilatant zones within the lodes. In a few instances,orebodies formed at fault intersections, such as in the ConVirginia orebody and at many places in the Silver City area.

Vertical zoning patterns within the Comstock lode can onlybe approximated. The deepest levels appear to contain mas-sive and stockwork quartz veins with few sulfide minerals andsparse metals. Upward from this, more massive quartz andsome quartz-adularia stockwork occur with low preciousmetal content in the nonore zones. The orebodies appear tohave formed within a restricted part of the quartz-adulariazone (Fig. 10). In these ore zones, massive quartz appears tograde upward for a short distance to a small vertical intervalof massive, largely barren, calcite. The calcite grades upwardto highly brecciated and veined sulfide-rich ore. The discretevertical interval of the ore zones is highly variable in contentof both base and precious metal as well as in abundance ofquartz, adularia, calcite, and chlorite. The nature of the zoneabove the orebodies is more speculative because the tops ofthe lodes are eroded along most of the length of the Com-stock lode. Additionally, the tops of the downdropped ore-bodies in the northern part of the lode are concealed beneathcover or by intermediate-depth, high-sulfidation alteration.Drilling (1977, Rosario Exploration) above and just west ofthe Con Virginia orebody (Fig. 4) encountered only propyli-tized rock with rare quartz stringers. Notes by the superin-tendent of the Ophir mine (University of Nevada-Reno li-braries, Special Collections department) record quartzstringers west of the northern end of the Con Virginia zonefor about 100 m above the level of the top of the orebody, butthe stringers all but disappeared at 200 m above. A crosscutfrom the Union shaft above the East vein zone, 300 m updipfrom the ore horizon, encountered a 30-m-wide zone of clayand small seams of quartz and gypsum (Union mine superin-tendent notes, 1903, University of Nevada-Reno libraries,Special Collections department). Drilling (1986, Westley Ex-plorations) in the same area intersected zones of quartz–illite-pyrite-anhydrite alteration with no veining.

The vertical distribution of the various orebodies can beexplained by postmineralization faulting that has dismem-


0361-0128/98/000/000-00 $6.00 383

bered the Comstock lode, by multiple periods of ore deposi-tion at different elevations, or some combination of both.Given the variety of ore and gangue mineralogy described forthe different orebodies, different mineralizing events for var-ious orebodies seem likely. However, all of the ore depositionappears to have formed late in the history of the lode. Post-mineralization, renewed movement within the Comstockfault zone appears to have dropped orebodies near the eastwall down relative to those near the west wall. In some cases,orebodies were split in two as indicated in the diagrams ofBecker (1882) for the Savage orebody. Because of postminer-alization movement along the east wall, the Con Virginia ore-body, East vein zone, and Hardy vein probably were droppeddown to the east, and the East vein zone was further dis-membered by many crosscutting, east-dipping faults (NevadaBureau of Mines and Geology mining district files). Thus, areconstruction of the Comstock lode before Late Cenozoicreactivation may place the orebodies at about the same hori-zon (Fig. 10). Renewed uplift and reactivation of many of themajor mineralized structures probably began about 3 Ma aspart of accelerated deformation in the Sierra Nevada-Basinand Range transition zone (Henry and Perkins, 2001). Thisrenewed faulting and erosion has exposed deep levels of thesystem in the footwall of the Comstock lode, whereas the ex-humed pre-upper Kate Peak Formation/Knickerbocker An-desite erosional surface in the hanging wall has been onlyslightly eroded.

A close spatial relationship between low-sulfidation andhigh-sulfidation epithermal environments, such as in theComstock district, has rarely been documented. Similar su-perposition of hydrothermal environments is reported at Lep-anto, Philippines (Disini et al., 1998), Mount Skukum,Canada (Love et al., 1998), Emperor mine, Fiji (Eaton and

Setterfield, 1993), and the Bahcecik prospect, Turkey (Yigit etal., 2000). The superposition of alteration styles in the Com-stock district obscures the deep low-sulfidation alteration as-sociated with the precious metal deposits, concealing it withinthe much more obvious intermediate-depth, high-sulfidationalteration. Alteration associated with the East vein zone iscompletely obscured by alunitic, alsic, and kaolinitic alter-ation, and other veins also may be concealed by intermediate-depth, high-sulfidation alteration elsewhere in the district.

AcknowledgmentsI am indebted to the Nevada Bureau of Mines and Geology

for the use of facilities and to Larry Garside for many usefuldiscussions. Some funding came from the U.S. GeologicalSurvey STATEMAP program (contract 00HQAG0048).Thanks are due also to the National Museum of Natural His-tory, which provided 46 polished thin sections of selectedsamples as well as permitted X-ray diffraction analysis of al-teration mineralogy. Dave Smith of the U.S. Geological Sur-vey provided the geochemical data. Thanks to Jon Sprecherfor the opportunity to start working on the geology of the dis-trict. Larry Garside, Chris Henry, Dave John, and Stuart Sim-mons gave constructive reviews of the manuscript.

REFERENCESBarton, M.D., Kieft, C., Burke, E.A.J., and Oen, I.S., 1978, Uytenbogaardite,

a new silver-gold sulfide: Canadian Mineralogist, v. 16, pt. 4, p. 651–657.Bastin, E.S., 1923, Bonanza ores of the Comstock lode, Virginia City, Nevada:

U.S. Geological Survey Bulletin 735, p. 41–63.Becker, G.F., 1882, Geology of the Comstock lode and the Washoe district:

U.S. Geological Survey Monograph 3, 422 p.Blake, W.P., 1864, Notes on the geology and mines of Nevada Territory

(Washoe region, U.S.): Quarterly Journal of the Geological Society of Lon-don, v. 20, p. 317–327.


0361-0128/98/000/000-00 $6.00 384

CedarHill

Ore body

Massive calcite

Quartz-adularia stockworkand massive quartz

Quartz stockwork andmassive quartz

S N

Illitic alteration+/- anhydrite?

Sericitic alteration

200 m

FIG. 10. Schematic longitudinal vertical projection along the plane of the Comstock lode during mineralization showinga possible structural reconstruction of the positions of orebodies, vein mineralogy, and general distribution of alteration as-semblages. Near-surface alteration is omitted owing to lack of data.

Bonham, H.F., and Papke, K.G., 1969, Geology and mineral deposits ofWashoe and Storey counties, Nevada: Nevada Bureau of Mines and Geol-ogy Bulletin 70, 140 p.

Browne, J.S., 1868, The Comstock lode: Mineral resources of the states andterritories west of the Rocky Mountains: Washington, U.S. Treasury De-partment, p. 341–380.

Calkins, F.C., 1944, Outline of the geology of the Comstock lode district,Nevada: U.S. Geological Survey Open-File Report 45-29, 35 p.

Church, J.A., 1879, The Comstock lode, its formation and history: New York,John Wiley, 226 p.

Coats, R.R., 1936, Aguilarite from the Comstock lode, Virginia City, Nevada:American Mineralogist, v. 21, p. 532–534.

——1940, propylitization and related types of alteration on the Comstocklode: ECONOMIC GEOLOGY, v. 35, p. 1–16.

Criss, R.E., and Champion, D.E., 1991, Oxygen isotope study of the fossil hy-drothermal system in the Comstock lode mining district, Nevada: Geo-chemical Society Special Publication, v. 3, p. 437–447.

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Epithermal Alteration and Mineralization in the Comstock District, Nevada - Donald M. Hudson (2003)

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