Geology and Mineral Resources of the Sheldon-Hart Mountain National Wildlife Refuge Complex (Oregon and Nevada), the Southeastern Oregon and North-Central Nevada, and the Southern Idaho and Northern Nevada (and Utah) Sagebrush Focal Areas - 2016

Federal & State Resources / US Geological Survey (USGS)

Vikre, and 23 co-authors, reported in 2016 that the U.S. Department of the Interior has proposed to withdraw approximately 10 million acres of Federal lands from mineral entry (subject to valid existing rights) from 12 million acres of lands defined as Sagebrush Focal Areas (SFAs) in Idaho, Montana, Nevada, Oregon, Utah, and Wyoming (for further discussion on the lands involved see Day and others, 2016).

The purpose of the proposed action is to protect the greater sage-grouse (Centrocercus urophasianus) and its habitat from adverse effects of locatable mineral exploration and mining, subject to valid existing rights.

This report addresses the mineral-resource potential of a subset of lands proposed for withdrawal. It summarizes the current status of locatable, leasable, and salable mineral commodities and assesses the potential of selected locatable minerals in lands proposed for withdrawal that span the Nevada, Oregon, Idaho, and Utah borders.

Geologic Framework

The four study areas encompass the northern part of the Basin and Range physiographic province, a large, arid region of alternating north-trending rugged mountain ranges and low-relief intermontane basins. Parts of the study areas also cover the high-lava plateaus of southern Idaho and the Nevada-California-Oregon border region. The Columbia Plateau physiographic province is characterized by subdued topography underlain by widespread, thick deposits of stratified volcanic rocks and isolated volcanic cinder cones.

The study area for the Southern Idaho and Northern Nevada Sagebrush Focal Area includes the southwestern part of the Snake River Plain, a broad northeast-trending basin floored by basaltic lavas. The study areas drain largely to the north and then west via the Snake River, although the south flank of the study area for the Southeastern Oregon and North-Central Nevada Sagebrush Focal Area drains to the south into the interior Humboldt River Basin in central Nevada. Within the four study areas, the oldest rocks are Precambrian quartzite and schist and small quartz monzonite intrusions in the easternmost part of the study area for the Southern Idaho and Northern Nevada Sagebrush Focal Area; similar rocks may underlie Paleozoic shelf-slope sedimentary rocks (mostly carbonates) that crop out widely in the eastern part of this study area. Paleozoic rocks were overthrust, starting in the Late Devonian and continuing through the Mesozoic, by successively younger packages of deepwater marine sediments, which crop out in the central part of the study areas as the Roberts Mountains, Golconda, and Fencemaker allochthons.

The far western part of the study areas is mostly covered by Cenozoic volcanic rocks; basement there probably consists of Paleozoic sedimentary and Mesozoic volcanic-arc rocks of the Black Rock and Pueblo terranes, which crop out between the study areas for the Sheldon Hart Mountain National Wildlife Refuge Complex and Southeastern Oregon and North-Central Nevada Sagebrush Focal Areas. Paleozoic and early Mesozoic sedimentary rocks through out the study areas are intruded by Jurassic and Cretaceous granitic plutons emplaced during subduction of the Farallon Plate beneath western North America. 

Following a Late Cretaceous to early Tertiary hiatus, attributed to a change in subduction zone geometry, magmatism resumed with eruption of Eocene to early Miocene silicic to intermediate composition lava flows and tuffs. Beginning about 17 Ma, magmatism was dominated by bimodal basalt-rhyolite volcanism related to the Yellowstone Hot Spot. Miocene flood basalts, caldera-derived rhyolite ash-flow tuff, and rhyolite lava flows blanket much of the study areas. Extensional faulting has been ongoing since at least the middle Miocene, forming the present-day Basin and Range topography and creating numerous basins filled with sediments shed from the rising mountain ranges.

Locatable Minerals Commodities Potentially Lost

Numerous locatable mineral commodities, including metallic and nonmetallic (also known as industrial) mineral commodities, occur in and near the four study areas. Many of these commodities have been mined, and numerous unmined deposits have been identified (resource; Table 3). Metallic commodities (for example, copper, silver, and lead) commonly occur in several deposit types as determined by comparison to deposit type models, and two or more metals (for example, gold and silver) often have been produced from one deposit. 

Deposit types that occur in the study areas include epithermal gold, silver, and mercury, and gallium, gemstones, lacustrine diatomite, volcanogenic uranium, orogenic-low-sulfide gold-quartz vein, hectorite (lithium-rich clay), specialty clays, zeolites, hydroauthagenic uranium, Carlin-type gold, bedded barite, and numerous intrusion-related deposit types including porphyry copper, porphyry molybdenum, polymetallic skarn, replacement, and vein, tungsten greisen, and distal disseminated silver-gold.

Favorable stratigraphy for lacustrine diatomite, intrusion-related, volcanogenic massive sulfide copper, Carlin-type gold, black shale vanadium, sedimentary exhalative zinc-lead-silver-gold, Mississippi Valley-type lead and zinc, and bedded barite deposits occurs in the study areas.