DESCRIPTIVE MODEL OF COMSTOCK EPITHERMAL VEINS

MODEL 25c

By Dan L. Mosier, and Donald A. Singer, and Byron R. Berger

APPROXIMATE SYNONYM Epithermal gold (quartz-adularia) alkali-chloride type.

DESCRIPTION Gold, electrum, silver sulfosalts, and argentite in vuggy quartz-adularia veins hosted by felsic to intermediate volcanic rocks that overlie predominantly clastic sedimentary rocks, and their metamorphic equivalents (see fig. 106).

GENERAL REFERENCES Buchanan (1980), Boyle (1979).

GEOLOGICAL ENVIRONMENT

Rock Types Host rocks are andesite, dacite, quartz latite, rhyodacite, rhyolite; and associated sedimentary rocks. Mineralization related to calc-alkaline or bimodal volcanism.

Textures Porphyritic.

Age Range Mainly Tertiary (most are 40-3.7 m.y.).

Depositional Environment Calc-alkaline and bimodal volcanism and associated intrusive activity over basement rocks composed of clastic sedimentary rocks and their metamorphic equivalents. Volcanic-related geothermal systems lack access to saline fluids from basement sources.

Tectonic Setting(s) Through-going fracture systems, major normal faults, fractures related to doming, ring fracture zones, joints.

Associated Deposit Types Placer gold and epithermal quartz-alunite Au.

DEPOSIT DESCRIPTION

Mineralogy Argentite + gold or electrum silver sulfosalts laumannite. Galena, sphalerite, chalcopyrite, tellurides, hematite, and arsenopyrite are moderate to sparse. Gangue minerals are quartz + pyrite adularia calcite sericite chlorite. Barite, fluorite, rhodochrosite, kaolinite, and montmorillonite are moderate to sparse. Ore minerals constitute only a few percent of vein.

Texture/Structure Banded veins, open space filling, lamellar quartz, stockwork.

Alteration From top to bottom of system: quartz + kaolinite + montmorillonite zeolite barite calcite; quartz + illite; quartz + adularia illite; quartz + chlorite; presence of adularia is variable.

Ore Controls Through-going anastomosing fracture systems, centers of intrusive activity. Hanging wall more favorable.

Weathering Bleached country rock, limonite, jarosite, goethite, alunite, hematite, argillization with kaolinite.

Geochemical Signature Higher in system Au + As + Sb + Hg or Au + As + Cu; Au + Ag + Pb + Cu; also Te and W.

EXAMPLES

Comstock, USNV (Becker, 1882)

Guanajuato, MXCO (Buchanan, 1980; Wandke and Martinez, 1928)