Abstract

The Dugway mining district is situated in the northern Dugway Range (NDR) in the eastern Great Basin, Utah. The district was located in 1869, and mining has been sporadic and limited, ceasing in 1956. Production of Cu, Pb, Zn, Ag, and Au was high-graded because of the expensive, long freight-haulage to the nearest railhead. Total district production of ore through 1956 is 10,900 mt.

The basement of the NDR is overlain by perhaps 11 km of middle Proterozoic aulacogenal and late Proterozoic miogeosynclinal sediments. The Early Cambrian Prospect Mountain Quartzite is the oldest exposed unit and is overlain by near-continuous carbonate sedimentation extending into early Triassic time that filled the early Ibex and later Oquirrh basins of the miogeosynclinal shelf. Pennsylvanian, middle-late Permian, and early Triassic sediments are no longer found at the NDR; early Permian sediments unconformably overlying upper Mississippian carbonates. From the late Triassic to the present, the NDR was uplifted above sea level, and remained exposed and eroding.

By 70-65 Ma, Nevadan orogeny magmatism appears to have swept as far east as the NDR with local intrusion of biotite granodiorite. Analysis of biotite from a small body of biotite granodiorite present in the NDR provided an age date 65 Ma. About the same time, eastward directed Sevier thrusting in the NDR created an imbricate thrust belt atop a seismically indicated, northerly-aligned fold axis of the west-plunging glide plane of the thrust belt. The westerly-dipping imbricate plates underlying the NDR may constitute most of the estimated 15-km-thick upper crust, which means the basal plane of decollement is at the brittle-ductile transition zone.

About 45 Ma, arc magmatism moved from north to south in western Utah and included emplacement of batholiths at high crustal levels with related stocks and plugs, extensive volcanism including caldera formation, production of vast amounts of pyroclastic rocks as well as installation of deep- and shallow-seated geothermal cells and hydrothermal systems. The NDR is at the center of a belt of calc-alkaline rocks known as the Thomas Range-Tintic volcanic field that is dated at 43 to 34 Ma. Although igneous dikes exposed throughout the district, few large intrusive bodies are observed at the surface or in subsurface drilling at the NDR. From 43 to ~25 Ma, volcanism was dominated by andesitic to dacitic flows and dacite to rhyolite ash-flow tuffs. Volcanic activity diminished after ~25 Ma, but basalt and later rhyolite-basalt volcanism renewed about 17 Ma, contemporaneous with initiation of the Basin and Range extension in the western United States. Reported age dates of orthoclase from two rock samples are 18 Ma and 13 Ma. Up to 100 m thickness of volcanic rocks are present at the NDR; these rocks are spatially related to vents.

During the Basin and Range period, the Dugway Range was uplifted and tilted in a westerly direction at about 35°. Tectonic extension, or magmatic intrusion, also resulted in formation of a north-northwesterly aligned central horst bordered by grabens on the east and west. Preserved within the grabens, are Upper Mississippian and Upper Devonian carbonates which unconformably overly Cambrian Prospect Mountain Quartzite. The results of recent drilling indicate the unconformity is generally a bedding-parallel, rubble-filled flat fault zone, herein termed the “flat-fault unconformity” (FFU), perhaps resulting from pre-Basin-Range compression and thrusting.

Generally coincident with the previously mentioned north-aligned Sevier fold zone of the Dugway Range are magmatic centers with precious and base metals, Fl and Be, and other mineralization. Base and precious metal mineralization, as pyritic Cu or Pb-Zn-Ag, is present throughout the district as fissure veins in quartzite, and fissure veins and lesser replacement deposits in carbonate rocks. High-angle faults bounding the central horst, as well as subsidiary parallel and transverse faults, demonstrate strong spatial correlation with the distribution of mineralization.

Carbonate terrane of the south half of the central horst, identified herein as the “south horst zone” (SHZ), exhibits strong local Au values. In addition, Cambrian Prospect Mountain Quartzite hosts Cu-Au-bearing Fe-quartz stockwork subjacent to the FFU. Some intrusive rocks also exhibit elevated precious and base metal values.

Hydrothermal alteration of the igneous rocks at the Dugway district consists of propylitic and phyllic alteration. Carbonate in the NDR rocks have been altered by dolomitization, bleaching, decalcification, silicification including formation of punky and dense jasperoid and quartz veining, recrystallization, argillization, sericitization, calcite veining, and locally distal to outer skarn formation. The basal quartzite exhibits quartz veining and local Fe-quartz stockwork. The results of fluid inclusion study indicate hydrothermal fluids were epithermal to mesothermal with homogenization temperatures of up to 210-216° C, with 8.6 to 10.2 equivalent weight percent NaCl. Boiling is not indicated and CO₂ is absent.

The district was explored for porphyry Cu-Mo deposits from the 1920s through 1970s, and for other base metals in the 1960s. In the late 1980s, the U.S. Geological Survey (USGS) and Gold Standard, Inc. completed comprehensive lithogeochemical surveys. During the early 1990s, 45 reverse circulation drill holes were completed to depths up to 245 m in search of economic Au mineralization in the Cambrian Prospect Mountain Quartzite-hosted Fe-quartz stockwork subjacent to FFU.

The mean Au abundance for samples of all rock types colleted by the USGS is 0.87 ppm, compared to 0.55 ppm for those collected by Gold Standard, Inc. The highest mean Au content was found in samples restricted to the SHZ at 1.05 ppm. Some of these samples contained up to 8.5 ppm Au. Mean base metal content of the samples from the SHZ is 0.15% Cu, 1.7% Pb, 2.0% Zn and 73 ppm Mo. Multivariate R-mode factor analysis of 196 samples from the SHZ for nine elements resulted in a three-factor model that explains 77% of variability with one factor, Ag-Au-Mo plus barite, galena, and jasperoid in silicified carbonate rocks, averaging 3.73 ppm Au. Eighteen centers of carbonate-hosted mineralization are found in the SHZ, of which the three most productive were the Yellow Jacket, Raymond-Bald Eagle, and Buckhorn mines.

Grab samples of the quartzite from throughout the district were found to contain up to 11.5 ppm Au. In some samples, the Fe-quartz stockwork exhibits fine-grained disseminated sulfides and is cut by later barite and fluorite veinlets. Microscopic logging of the quartzite indicates common propylitic alteration with sparse bleaching, argillization, calcite veining and strong hematite, pyrite, quartz veining, and other silicification.

In the drill holes, Au mineralization ranging from 30 m of 0.3 ppm Au, at 24 m depth, to 6 m of 4.5 ppm Au, at 14 m depth, was found. Locally abundant Ag, As, Bi, Mo, Cu, Pb, Zn, and Fe, accompanies gold. Copper is very strong with the highest-grade drill hole containing 17 m of 0.74% Cu at the surface, including 7.6 m of 1.23% Cu at 9 m depth.

The quartz stockwork Au mineralization is a newly recognized mineral model in the district and has been identified as a “flat-fault Au deposit” model. Another mineral model suggested to match the observed NDR geology and geochemistry in the Dugway district is the USGS’s eastern Great Basin, Au-rich, carbonate-hosted, base metal replacement deposit-related, intrusive porphyry system model. The model’s geometry consists of zoned mineralization with epithermal polymetallic veins and pods distal to deeper disseminated Au spatially associated with a concealed porphyry body. Evidence for the USGS model in the Dugway district includes favorable fracture patterns and other structure, igneous petrology, geophysical magnetic response, and geochemistry.