VIRGINIA CITY
Virginia City Mining District is located in west-central Nevada, about 27km
southeast of Reno (Fig. 1) is the site of the famous Comstock Lode.
Approximately 225000 Kg of Gold and 7 million Kg of silver were produced from
the district (1862 – 1953) (Vikre, 1989). The area is mountainous with elevations
ranging from 1280 m in the valley to 2,360m on Mt. Davidson. It is characterized
by a semi-arid climate.
The oldest exposed lithologies are Mesozoic metasedimentary and
metavolcanic rocks, which are intruded by Cretaceous granodiorite These units
are unconformably overlain by Oligocene and early Miocene silicic ash-flow tuffs,
thick andesite flows and associated breccias of the Miocene Alta Formation.
Overlying the Alta Formation are andesite flows, breccias, and accompanying
dikes and stocks of the Kate Peak Formation. The Alta Formation is the main
host of orebodies in the district (Thompson, 1956) and is the unit most affected
by hydrothermal alteration. The alteration assemblages are propylitic and argillic
(Thompson, 1956; Vikre, 1998).
Figure 1 – Area Location and alteration zones.
Mineralization and hydrothermal alteration of the Comstock Lode are
generally associated with the North-South trending Comstock, Silver City and
Occidental faults. Occurences of finely grained alunite and kaolinite resulting
from supergene processes (oxidation of pyrite, formation of low pH fluids and
alteration of rocks to alunite and clays) may be noted within the district
(particularly along Geiger Grade). It is important to note that some of the areas
described as “bleached” by earlier workers are quartz-alunite and clay mineral
assemblages that are hypogene and a result of alteration by fluids lower in pH
than those that have formed The Comstock deposits (Vikre, 1998). The main ore
zones are located along the Comstock fault and associated cross faults. Thick
veins of crushed quartz with silver sulfosalts, native silver and native gold are
found in proximity to the faults. The main ore mineralization episode at
Comstock is middle Miocene (Vikre, 1989). Vikre (1989) also suggests that the
high-sulfidation mineralization is older than the main Lode mineralization, while
Hudson (1987) suggests closer time and genetic relationship of quartz-alunite
alteration to main Comstock ore (Fig.2). Main alteration assemblages present in
the district can be summarized as following: widespread propylitic assemblage,
not spatially associated with ore and most likely the oldest, zeolitic assemblage
superimposed in propylitic, erratically distributed quartz-alunite alteration (high
sulfidation), quartz-sericite-montmorillonite-pyrite alteration assemblage found
peripheral to the gold-silver veins during mineralization (Vikre 1989).
Figure 2 – Idealized cross section of Comstock ore mineralization and alteration assemblages.
(after Hudson, 1986).
LIST OF PERTINENT REFERENCES
Boardman, J. and Huntington, J. ,1997, Mineralogic and Geochemical mapping
at Virginia City, Nevada Using 1995 AVIRIS Data.– Proceedings of the 12th
ERIM Conference on Applied Remote Sensing, Denver, CO., V.1, p 191-199.
Hudson, D. M.,1987, Summary of the Geology of the Comstock District, Nevada
in Johnson, J.L. ed. Bulk Mineable Precious Metal Deposits of the Western
United States. Guidebook for fieldtrips: Geological Society of Nevada, p.413418.
Hudson, D. M.,1986, Comstock Lode Fieldtrip. Geological Society of Nevada
Special Publication No. 4
Hutsinpiller, A. and Taranik, J.V. ,1988, Spectral Signatures at Virginia City. In
Schafer et al. Bulk Mineable Precious Metal Deposits of the Western United
States. Symposium Proceedings. Geological Society of Nevada, p. 545-551.
Kruse, F. A., and Huntington, J. H., 1996, The 1995 Geology AVIRIS Group
Shoot: in Proceedings, 6th JPL Airborne Earth Science Workshop: Jet
Propulsion Laboratory Publication 96-4, v. 1, p. 155 - 166.
Spatz, D. and Taranik, J. ,1988, Identification and mapping of volcanic lithologies
genetically or spacially associated with the precious metal deposits of the
western U.S. using Landsat TM imagery. In Schafer et al. Bulk Mineable
Precious Metal Deposits of the Western United States, Symposium
Proceedings. Geological Society of Nevada.
Thompson, G.W. (1956) Geology of the Virginia City Quadrangle. USGS Special
Paper 1024 C, p.64.
Vikre, Peter G. ,1998, Quartz-Alunite alteration in the Western Part of the Virginia
Range, Washoe and Storey Counties, Nevada. Econ. Geology V.93, p.338 343.
Vikre, Peter G. ,1989, Fluid-mineral relations in the Comstock Lode. Economic
Geology, V.84, p.1574-1613.
Wally, Jansen, 1993. Using remotely sensed hyperspectral data to identify and
map minerals in mine wastes in the Virginia City, Nevada region. In:
Association of Engineering Geologists 36th annual meeting; El Agua y La
Tierra; program and abstracts. Keaton-Jeffrey-R (president) Association of
Engineering Geologists. 36; Pages 57. 1993.