Geology of the Taylor Silver deposit, White Pine County, Nevada

Edwards, Jeffrey M., author
Thompson, Tommy B., advisor
Vaughn, John, committee member
Johnson, Robert, committee member
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The Taylor silver mine is located 15 miles southeast of Ely, Nevada, on the western flank of the Southern Schell Creek Range. The 1200 ton per day open pit mine produces up to 1,200,000 ounces silver and 1000 ounces gold annually. Preproduction reserves totalled 7 million tons at 3.0 oz Ag/ton and 0.003 oz Au/ton. Soft silver prices since 1982 have forced intermittent shutdowns. The ore body is an epithermal, high-silica, low-sulfide replacement deposit. Brecciated Devonian Guilmette Formation limestones host silver ores immediately beneath unconformable hydrocarbon-rich Mississippian Pilot Shale siltstone. Intrusion of pre-mineral, lower Oligocene fluorine-rich rhyolite dikes and sills, and post-mineral, lower Miocene latite dikes, bracket ore deposition. The structural history of the district is complex. Compressive deformation began in Mesozoic time as west to east thrust faults imbricated portions of the section east of a broad north-south fold in the Taylor mine area. Gravity-driven low angle faults, related to Paleocene through Eocene uplift of the neighboring Snake Range, attenuated and severely brecciated portions of the stratigraphic section. Oligocene compressive tectonism reactivated the Taylor fold and low-angle faults in the district. High-angle reverse faults breached the Taylor anticline, and focussed ore fluids into receptive breccias on and near the fold axis. Jasperoid outcrop characteristics, paragenesis, and trace metal contents serve to define four hypogene mineral stages: (1) weakly-mineralized stage I jasperoids contain elevated Au, up to 1 ppm, and locally over 1000 ppm As; (2) stage II jasperoids, marked by the presence of stibnite, contain up to 18% Sb, up to 1 ppm Au, locally over 5000 ppm Ba, and up to 2.5 oz Ag/ton; (3) stage III jasperoids represent the main Ag-ore-stage and contain up to 1000 ppm Pb, up to 3000 ppm Zn, and Ag:Au ratios over 1100:1; (4) stage IV open space sulfide-calcite veins and breccia fillings locally contain over 50 oz Ag/ton, with up to 4.5% combined base metals, and up to 3000 ppm Sb. Ore-stages III and IV were structurally confined to the immediate mine area. Stage I and II jasperoids form a distinct Sb-Au-Ba halo from 0.5 to 1 mile around the silver deposit. Fluid inclusion analyses demonstrate that Taylor was a prograde system. Stage II quartz precipitated from boiling fluids at 200°C. Ore-stage III accessory fluorite was deposited from boiling fluids at about 225°C. Stage IV vein sphalerite precipitated from non-boiling fluids at 306°C. Jasperoid whole rock vapor phase analyses indicate that the vapor phase was dominated by CH4, C2H6, and C3H8 probably derived from the thermal maturation of organic matter, indigenous to the enclosing Guilmette and Pilot sediments. Exploration drill hole logs define a base of silicification that cross cuts structure and stratigraphy. Lack of evidence for boiling in vein fluorite below this level suggests that the base of silicification may be coincident with the base of boiling. Banded quartz-sulfide-calcite veins found at depth indicate that episodic boiling reached deeper levels but strength and duration were apparently not adequate to promote wall rock silicification. Apparent vertical metal zonation grades from Pilot Shale- and Joanna Limestone-hosted Sb-Au-Ba-As-rich jasperoids down into Guilmette Formation limestone-hosted Ag-Zn-Pb-rich jasperoids, and finally into base metal-Ag-Sb-rich veins and breccia fillings below the base of silicification. District-wide paragenesis, wall rock alteration, and metal zonation thus appears to be related to stratigraphic level, depth, and particularly the boiling of indigenous-organic-derived volatiles. Both stratigraphic reconstruction and estimates from fluid inclusion data suggest depth of formation in the range of 3500 ± 700 feet (base of the Pilot Shale).
1988 Summer.
Includes bibliographical references (pages 176-184).
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Geology -- Nevada -- White Pine County
Mineralogy -- Nevada -- White Pine County
Ore deposits -- Nevada -- White Pine County
Associated Publications