Abstract

The northeasternmost Great Basin, encompassing western Utah, easternmost Nevada, and part of southwestern Idaho, contains a number of substantial deposits of gold and/or silver. Several districts or deposits have yielded in excess of a million ounces of gold and/or hundreds of millions of ounces of silver. The region is best noted for numerous mines which yielded a major silver co-product from base metal production, plus a few major gold districts. This paper briefly reviews some of these districts, presently or previously productive, citing examples of various types of ore systems.

Districts associated with silicic Cenozoic plutons, some of them altered and mineralized, are well known. Several of these districts have epithermal systems on their flanks. The precise temporal relationship of epithermal vein and bulk-mineable mineralization to intrusive-related manto and “porphyry” ores remains uncertain.

Sediment-hosted precious metals mineralization in the Mercur, Utah, deposits very closely resembles the famous Carlin-type deposits of east-central Nevada. Quartz-adularia veins in volcanic rocks closely resemble similar precious metal ores elsewhere. But in this region, all gold deposits tend to occur near silver-base metal deposits and intrusive bodies. Enargite/tetrahedrite-gold-pyrite subsystems are present in at least two base metal districts localized in carbonate rocks. It is suggested that similar geochemical conditions, developing locally in rather different host rocks, yielded the same mineral assemblages as are known further west in Nevada.

A number of factors, developed through geologic time, helped to localize economic ore bodies of Cenozoic age. Paleozoic effects are largely limited to deposition of favorable sedimentary host rock units, although diagenetic origins are postulated for ores in some Idaho and Nevada basin-related shale units. Belts of Mesozoic igneous and tectonic activity appear to be precursors to younger igneous activity closely associated with major Cenozoic deposits along certain trends. These linear features tend to cut across the regional Cenozoic tectonic pattern. A much closer relationship is observed between ore and small plutons with cogenetic andesitic volcanic piles, and with voluminous ash flow magmatic events in the Cenozoic.

In Utah, a spatial association of sediment-hosted and vein epithermal deposits with centers of intrusive-related ores is notable. It is postulated, however, that the necessary geochemical conditions to form epithermal ores (e.g. the deeper parts of hot spring systems) can occur without the metals emanating from a specific magma. Sources of heat and fluids are the major necessary ingredients.