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Comstock |
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Nevada, USA |
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Ag Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Comstock District lies within the Virginia City district of Storey County, south-western Nevada, USA, approximately 30 km to the south-east of Reno, and 350 km from Carlin. The district is famous for the Comstock Lode, a group of bonanza silver-gold shoots of exceptional grade.
The recognition of low grade placer gold in the district in 1850 was followed by the discovery of lode silver and gold in 1859. The main period of production was from 1863 to 1880, which combined with intermittent underground and open pit mining to the present has yielded over 260 t of gold and 6000 t of silver (Hudson, 1987).
The lodes of the Comstock district occur within intermediate calc-alkalic volcanic rocks of Miocene age. Large orebodies were located along the plane of, and in hangingwall splays of, the Comstock Fault, a north trending, east dipping normal structure. Other normal faults in the vicinity were also mineralised, although the Comstock Fault with the greatest length (11 km) and largest offset (more than 750 m), had the largest orebodies (Ashley, 1991).
Geology
The geology of the Comstock District may be summarised as follows (Hudson, 1987), from the lowest member:
Lower Jurassic, Gardnerville Formation - argillite and limestone.
Thrust - of possibly Jurassic age, related to the early Nevadan stages of the Sevier Orogeny.
Mesozoic, Peaville Sequence - meta-andesite, meta-diorite and meta-gabbro.
Unconformity
Oligocene to Miocene Tuffs, dated at 28 to 20 Ma - mainly siliceous ash-flow tuffs, comprising from the oldest to youngest, the Mickey Pass Tuff, the Lenihan Canyon Tuff, the Nine Hill Tuff, the Santiago Canyon Tuff and an un-named tuff unit.
Unconformity
Miocene, Alta Formation, >1000 m thick - composed predominantly of andesitic, trachyandesitic and rhyodacitic porphyritic flows, flow breccias, mud-flow breccias and lacustrine sediments. These are the main host to ore.
Miocene, Hornblende Andesite - dykes and stocks.
Miocene, Davidson Diorite, dated at 17 Ma - occurring as stocks and dykes, ranging in composition from diorite to quartz-diorite, to granodiorite to andesite porphyry.
Miocene, Kate Peak Formation - which commences with a lower member composed of andesitic to dacitic flows with accompanying stocks and dykes. During the latter stages of emplacement of the lower member, normal faulting, mineralisation and alteration took place in the Comstock lode deposits. Adularia from the Comstock and Occidental Lodes yielded K-Ar ages of 13.7 to 12.6 Ma. The upper member of the Kate Peak Formation, which has the same textures and composition as the lower member, was deposited on an erosional surface developed on the altered rocks. The upper member has been dated by K-Ar determinations on hornblende and biotite as 14.9 to 12.3 Ma.
Disconformity - representing a period of erosion and quiescence.
Miocene or Pliocene, Knickerbocker Andesite, up to 20 m thick - a widespread unit over most of the district.
Pleistocene, Olivine Basalt, dated at 1.14 Ma.
Structure
The major structures that predate, or are synchronous with the mineralisation, are the Comstock, Silver City and Occidental Faults. The relationship of alteration, mineralisation and movement in these fault zones implies that mineralisation was contemporaneous with movement on the faults. These relationships include silicified ribs, alunite-quartz bands, and hydrothermal breccias and veins developed within the fractures and small faults of the larger fault zones, and the development of gouge, cemented by vein material, which show numerous episodes of brecciation and re-cementation (Hudson, 1987).
The Comstock and Occidental Faults trend in a general NNE direction, while the Silver City Fault strikes at closer to north-west. All three dip at 35° to 40°, generally to the east, flattening slightly with depth. Near the surface they tend to be close to vertical. These faults have been subsequently rotated to lower angles by regional tilting. Dip-slip displacement on the Comstock and Silver City Faults that predated, or was synchronous with mineralisation, has been estimated at about 400 m. The displacement on the Occidental Fault during the same period is around 100 m. Post mineralisation (Miocene) offsets, based on displacement of the Knickerbocker Andestite, are 400 to 500 m on the Comstock Fault, about 250 m on the Silver City Fault, and 200 m on the Occidental Fault (Hudson, 1987).
The later movement on the Comstock and Silver City fault zones occurred along pairs of parallel major faults which, with few exceptions, straddled the earlier mineralised fault zones on either side, with lesser displacement on faults between, within the mineralised interval. The major bounding faults of the Comstock Fault Zone are about 50 m apart at depth, but splay out to 200 m at the surface. A number of other post-Miocene faults, generally of small displacement, parallel the Comstock fault zone, or have roughly east-west strikes (Hudson, 1987).
Mineralisation and Alteration
Most of the known orebodies occur within the Comstock and Silver City Fault zones. A few, including some bonanza lodes, were found in hangingwall fractures of the Comstock Fault, while a few small orebodies occurred in the footwalls of the Comstock, Silver City and Occidental fault zones (Hudson, 1987).
The vertical extent of the individual orebodies, or shoots, rarely exceeded 150 m, with maximum lateral dimensions of 300 m and mining widths of up to 45 m. Anomalous Ag and Au values, sometimes of mineable grade, occur above and below the main ore horizons as well as along strike. For the most part the Comstock lode is a stockwork zone. Vein densities vary from a few percent to nearly 100% of the rock volume, although the density of veining does not appear to be directly related to grade. Individual veins are commonly 2 to 30 mm wide, rarely exceeding 300 mm (Hudson, 1987).
The mineralogy of the stockwork mineralisation is vertically zoned from a deep quartz, to an intermediate quartz±adularia to a shallower calcite+quartz±minor adularia zone. In the transition from the intermediate to shallow zones, veins contain calcite which is replaced by quartz. The higher level calcite+quartz±adularia veins are rarely preserved at Comstock, but outcrop extensively in the Occidental and Silver City lodes (Hudson, 1987).
The quartz+adularia zone contains many vein types. The most abundant consist of coarse grained quartz with occasional amethyst. Less common are quartz with minor adularia, in which the adularia is in the size range of 5 to 500 micron, occurring as intergrowths with quartz on the vein margins, while the vein centre is occupied by coarse grained quartz. Far less abundant vein types include, quartz+adularia+pyrite, adularia with lesser quartz and quartz+pyrite, all of which are usually fine grained. Veins are often banded, but rarely contain more than four stages of gangue deposition. As many as 30 generations of cross-cutting veins have been recognised (Hudson, 1987).
The ore shoots are located in the upper portion of the quartz+adularia zone and extends into the calcite+quartz±adularia zone. Paragenetic studies indicate a progression from argentite±electrum, galena, chalcopyrite±electrum, sphalerite, with minor pyrite deposited throughout. This sequence occurs within a single band of ore deposition, and may be repeated several times in an individual vein. The period or periods of veining during which the ore minerals were introduced is not known. Associated gangue minerals are quartz, often adularia, while a few specimens contain calcite. Manganese oxides, which are of both hypogene and supergene in origin, are abundant and often carry silver. Other ore minerals include stephanite, pyrargyrite, polybasite, native silver, tetrahedrite and molybdenite (Hudson, 1987). The quartz in the Comstock ores typically appears crushed. Adularia does not occur in the bonanza lode veins (Ashley, 1991).
Wall rock alteration exhibits considerable variation, dependent on depth below the palaeo-surface and distance from a major structure. In the inferred deeper sections of the stockwork mineralisation in the Comstock Lode, an assemblage of sericite+quartz+pyrite commonly grades laterally and along strike into chlorite+illite+pyrite+albite and/or adularia. Both assemblages are associated with ore, although neither is necessarily indicative of ore. At intermediate levels, above the ore, an assemblage characterised by quartz+anhydrite+pyrite+illite/sericite±kaolinite is found. This intermediate level appears to underlie an upward expanding zone of intense alunite and pyrophyllite. The cores of these near surface altered zones formed along faults or large fractures, and consist of quartz+alunite+pyrite, sometimes zoned outwards to pyrophyllite+quartz±diaspore+pyrite. These core assemblages grade laterally to a kaolinite+quartz+pyrite suite which in turn grades into illite+quartz+pyrite±mixed layered illite-montmorillonite assemblages in contact with the regional propylitic alteration. The uppermost zone, which is preserved below the upper member of the Kate Peak Formation, is present as a roughly flat lying layer of cristobalite+opal±pyrite±kaolinite±alunite which may have formed above the water table (Hudson, 1987).
The regional propylitisation, which may be related to the intrusion of the Davidson Diorite, includes two distinguishing suites. These are a generally deeper albite+chlorite+epidote+quartz ±white mica±pyrite, and an overlapping, but shallower zone of albite+chlorite+calcite±white mica±pyrite. Zeolites occur locally (Hudson, 1987).
Limited fluid inclusion data suggests that the bonanza ores were formed at temperatures at 240° to 300°C related to meteoric waters (Ashley, 1991).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1994.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Berger B R, Tingley J V, Drew L J 2003 - Structural localization and origin of compartmentalized fluid flow, Comstock Lode, Virginia City, Nevada: in Econ. Geol. v98 pp 387-408
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Criss R E, Singleton M J, Champion D E 2000 - Three-dimensional Oxygen isotope imaging of convective fluid flow around the Big Bonanza, Comstock Lode Mining district, Nevada: in Econ. Geol. v95 pp 131-142
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Hudson D M 1987 - Summary of the geology of the Comstock district, Nevada: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 413-418
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Hudson D M 2003 - Epithermal alteration and mineralization in the Comstock District, Nevada: in Econ. Geol. v98 pp 367-385
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John, D.A., du Bray, E.A., Henry, C.D. and Vikre, P.G., 2015 - Cenozoic Magmatism and Epithermal Gold-Silver Deposits of the Southern Ancestral Cascade Arc, Western Nevada and Eastern California: in New Concepts and Discoveries, Geological Society of Nevada, 2015 Symposium, Reno/Sparks, Nevada, May 14 to 24, 2015, Symposium Proceedings, pp. 611-645.
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Vikre P G 1989 - Fluid-mineral relations in the Comstock lode: in Econ. Geol. v84 pp 1574-1613
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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