The Sunbeam and Grouse Creek epithermal gold-silver deposits lie within the Custer graben of the Eocene Challis volcanic field in central Idaho. The Custer graben is a major northeast-trending volcano-tectonic depression that forms part of the trans-Challis fault system, a 220-km-long by 65-km-wide zone defined by regional high-angle faults, grabens, and cauldron subsidence features. The relationship between the cauldrons and grabens along the trans-Challis fault zone northeast of the deposit area bears a strong resemblance to that observed in the East African rift system. The Challis volcanic field consists of a lower sequence of andesitic lavas and an upper sequence of quartz latitic and rhyolitic ash-flow tuffs, capping lacustrine and fluvial sedimentary rocks, and late-stage rhyolitic dikes, domes, and flows.The Sunbeam deposit is hosted by a subaerial rhyolitic flow dome complex and underlying pyroclastic rocks. Ore grades occur principally along dominant northeast- and subordinate northwest-trending fractures that are focused in silicified and argillized zones. Individual mineralized fractures manifested as either iron-rich clayey zones with silicified cores and/or margins, or stockworks with anastomosing veinlets of silica and pyrite. Hydrothermal breccias are common and follow the regional structural fabric. Gold and silver occur primarily as electrum; silver sulfides and sulfosalts are minor. Pyrite and arsenopyrite are the dominant sulfide minerals. The silver to gold ratio is 6:1. Argillically altered areas consist primarily of illite with less than 15 percent interlayered smectite. Pyrophyllite is present in the interior of the ore zones; smectite exceeds 15 percent beyond the ore zones. Kaolinite is rare and alunite is absent. Oxidation is supergene in origin and forms a thin 12- to 30-m (40-100 ft) cap to the deposits.The Grouse Creek deposit is hosted by subaqueous volcaniclastic and pyroclastic rocks that are located within a subbasin, immediately adjacent to and northwest of the Sunbeam rhyolite flow dome. The deposit is interpreted to be partly epigenetic and partly syngenetic. Ore occurs dominantly in silicified zones invading the same rhyolitic pyroclastic unit that hosts part of the Sunbeam deposit, in hydrothermal volcanic breccias, and in the basal portion of the carbonaceous black shale which caps the sequence. Northeast-and northwest-trending fractures form the feeders for the hydrothermal system but have less influence on precious metal deposition than at the Sunbeam deposit. Ore minerals are similar to those at the Sunbeam deposit; however, the silver to gold ratio is roughly 43:1. Argillic alteration and silicification encase the ore zones. Silicification is closely associated with ore grades, whereas argillically altered rocks extend beyond the limits of ore grade. Propylitic alteration is widespread in the footwall of the ore zone.Regional structural and lithologic investigations and detailed geologic studies of both deposits indicate that the two deposits are contemporaneous but were deposited in different host environments. The Sunbeam deposit formed in a subaerial rhyolite flow dome that flanks the southern margin of the northeast-trending rift-valley lake in which the Grouse Creek deposit formed. Geochemical and mineralogical patterns in the host sedimentary sequence at the Grouse Creek deposit indicate that hydrothermal activity occurred during sediment accumulation. Most of the precious metals were deposited below the sediment-water interface. However, warmer metal-rich brines penetrated the sediment surface to form hydrothermal plumes, which provided nutrients for biological blooms near the vent areas. Subsequent volcaniclastic sedimentation covered the deposit and its near-surface manifestation. The geologic environment of the Custer graben and the Sunbeam and Grouse Creek area bears strong resemblance to intracratonic rift environments, such as the East Africa rift system.

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