The Pequop Mining District, Elko County, Nevada: an evolving new gold district

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. Aeromag-netic 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.