Abstract Several gold deposits discovered since 1990 in the central Pequop Mountains of Elko County, northeastern Nevada, make up the new Pequop mining district. The most advanced projects, including Long Canyon and West Pequop, have a combined resource exceeding 42.5 tonnes Au and growing. Favorable open-pit mining economics are generated by high-grade, oxidized gold deposits above the water table. The deposits exhibit characteristics typical of Carlin-type gold deposits, including limestone and calcareous siliciclastic host rocks, collapse breccias, and <5 micron gold grains in rims of oxidized arsenian pyrite grains. Host rocks are decalcified, argillized, and locally silicified (jasperoid). Some gold mineralization, particularly at Long Canyon, occurs along the margins of competent blocks of Cambrian Notch Peak dolomite in contact with limestone. The Pequop mining district lies outside the well-known Nevada gold trends. In contrast to many Carlin-type deposits, mineralization is hosted by the Cambrian and Ordovician miogeoclinal sequence of interbedded platform carbonate and siliciclastic rocks. The degree of penetrative deformation and metamorphism is unusually high due to extensive crustal thickening and deep burial during the Jurassic Elko and Cretaceous Sevier orogenies. Zircon U-Pb dates show that the Pequop Mountains were the site of Jurassic (162–154 Ma), Cretaceous (85–70 Ma), and Eocene (41–39 Ma) intrusive activity, which is observed in other Carlin-type districts. Jurassic mafic to felsic dikes and sills, particularly lamprophyres, form passive hosts to mineralization. Eocene felsic dikes on the western side of the Pequop Mountains are unaltered and unmineralized, they lie within a northeast-trending corridor of gold anomalies, older dikes, and positive aeromagnetic anomalies, which is permissive evidence for an Eocene age of mineralization. Geophysical anomalies suggest the Pequop district may lie above a prominent break in the continental crust. It is near a west- to northwest-trending conductor, defined by magnetotelluric surveys that may mark the transition between rocks of the Archean Wyoming Province and the Paleoproterozoic Mojave Province. Aeromagnetic data suggest the district is astride a northeastern alignment of intrusions that extends from the Bald Mountain district, located to the southwest, and can be traced northeast to the Tecoma district. Low-frequency filtering of gravity data reveals a distinct northwest-trending boundary that coincides with a similarly oriented trend of barite vein occurrences. These data, along with the ages of intrusions, suggest the district may be underlain by a deep magmatic plumbing system.

Abstract The Tuscarora volcanic field (TVF), the largest Eocene volcanic field in Nevada, lies just north of major gold deposits of the Carlin Trend and west of deposits in the Independence Mountains (Fig. 1). Ongoing detailed mapping documents at least five voluminous volcanic-intrusive centers in the southeastern part of the TVF (Figs. 1, 2). 40 Ar 39 Ar dates show that the five centers developed between 39.9 and 39.2 Ma during a brief, intense period of magmatism. Precious-metal mineralization at Tuscarora formed at about 39.3 Ma, contemporaneous with a major intrusive episode, and is the oldest Tertiary volcanic-hosted epithermal deposit in Nevada. Compilation of 40 Ar 39 Ar dates indicates that magmatism was particularly intense between 39 and 40 Ma throughout northeastern Nevada. Given that several Carlin-type deposits are now interpreted to have formed in the Eocene (Hofstra, 1995~ Emsbo, 1996~ Leonardson and Rahn, 1996~ Phinisey et al., 1996~ Rota, 1996), contemporaneous with this activity, we suggest that Eocene magmatism was directly or indirectly the heat source to drive hydrothermal circulation that generated Carlin-type deposits (see also Hofstra, 1995).

Mesozoic structures in the Dolly Varden Mountains include low-angle normal(?) faults at high angles to bedding, near-bedding-parallel faults which cut out strata, large-scale folds, east-striking normal faults, and north-striking normal faults. Mesozoic deformation in the Dolly Varden Mountains is constrained to be no younger than 165 ± 3 Ma based on a U-Pb zircon age obtained for the Melrose intrusion. In the nearby Currie Hills, north- and northwest-trending folds deforming Paleozoic and Mesozoic strata are likely correlative with contractional features in the southern Pequop Mountains. Near the town of Currie, formations as young as Lower Jurassic appear to be included in the folding. Considering the age of these formations, and the lack of evidence for deformation prior to their deposition, it appears that Mesozoic structures in the Dolly Varden Mountains formed between the Early Jurassic and 165 Ma, or probably within the Middle Jurassic. The age of folds in the Currie Hills cannot be as tightly constrained. No strong evidence for correlating them with folds in the Dolly Varden Mountains exists. Therefore, it can only be said that they probably formed after the Lower Jurassic and before Tertiary volcanism.

Abstract Interstate 80 between Elko and Carlin, Nevada, follows the valley of the Humboldt River. The Humboldt drains a 17,000 square mile basin along its 275 mile course, located entirely within the State of Nevada. While barely a trickle of water in late summer, spring floods may swell the river well over its banks. The average annual discharge is 256,000 acre-feet, although this varies from 25,000 acre-feet to 635,000 acre-feet annually. The Humboldt River played an important part in the history of the American West during the early years of westward settlement. In 1928, Peter Skene Ogden led a party of Hudson’s Bay Company trappers south from Idaho, following the Humboldt to the present location of Winnemucca. Ogden trapped there briefly, returning to Utah for the winter. The following spring, he returned to follow the Humboldt to the marshes west beyond the Stillwater Range, where the Humboldt spreads and disappears into the desert sand. For ten years, British and American trappers worked the river, then called Mary’s River. By 1838, the beaver were largely trapped out, the market for beaver felt hats was waning, and the American west was about to change. Three years later, the first party of immigrants to California, on the advice of mountain men, followed the Humboldt west from near its source to the Humboldt Sink, then crossed the Forty-Mile Desert to reach the Carson River and the Sierra Nevada. In 1845, John C. Fremont began his explorations and publicized the desert country and the Humboldt Trail.

For several decades it has been recognized that the extent to the east of lower Paleozoic eugeosynclinal rocks (Fig. 1) is greater in northern Elko County than elsewhere in Nevada. It was early recognized that these rocks are allochthonous; in some places it was demonstrable that they form the upper plate of the Roberts Mountains thrust and rest on eastern-facies (carbonate-clastic) lower Paleozoic rocks. Recent detailed mapping has shown that the Roberts Mountains thrust is not deflected to the east in central Elko County, as had been earlier supposed, but continues with a generally northward trend through the Mountain City quadrangle. Western-facies and overlap-assemblage rocks present to the east are the result of younger thrusting southward and eastward. Ages of younger thrusting include late Paleozoic, early Mesozoic, and post-Early Jurassic. These observations render superfluous explanations that require a deformation of the Roberts Mountains thrust to bring it to a northeasterly trend through north-central Elko County. We propose to account for the presence of the Roberts Mountains thrust in southern Twin Falls County, Idaho, by the presence of an east-trending right-lateral rift. This rift is now buried beneath Tertiary lava, with about 120 km of post-Devonian displacement, in southern Owyhee County, Idaho. Subsequent to the right-lateral movement, we propose that the Idaho block was translated still farther east and rotated counterclockwise about 30°. The compression in northern Elko County resulting from this rotation may be responsible for the east-west trends of the formations just south of the Idaho line, which condition the east-west trend of Mesozoic plutons in northern Elko County. Since these plutons are as old as Jurassic, the tectonism responsible may be Late Triassic and Early Jurassic. It is also responsible for the presence of western-facies and overlap-assemblage Paleozoic rocks thrust over eastern-facies and overlap-assemblage rocks in parts of northern Elko County in post-Antler time.

The Circle Creek Rhyolite is a multiple-source mass of fayalite-ferro-augite rhyolite, forming an extensive complex in northern Elko County, Nevada, with a diameter of about 8 miles and an exposed area of about 36 square miles. From the structure of the flow layering and the complex sequence of microbrecciation structures, it seems to be a relatively thick mass that rose passively through many fissures to flood a sag basin in the older rocks, which are as young as the early Pliocene Idavada Volcanics, a sequence of ignimbrites and tuffs. Chemical analyses, thin-sections, and X-ray diffractometer studies show that the Circle Creek Rhyolite is a two-feldspar rhyolite. The interrelations of the pyroxene phases are unusually complex. The Circle Creek is partly concealed by later tuffs, gravels, and olivine basalt and is cut by two sequences of high-angle faults of different ages.