Geology and Mineralization of the Grouse Creek Au-Ag Deposit Custer County, Idaho

The Grouse Creek deposit is located within a northwest trending third order basin, which in turn is located within a northeast trending second order basin, that occurs within the Custer graben, a northeast trending first order basin. The deposit is hosted by pyroclastics, carbonaceous and tuffaceous lacustrine sediments, and rhyolites belonging to the Tertiary Challis volcanics. The oldest rock unit observed in the Grouse Creek deposit is a propylitically altered quartz latite pyroclastic, unit Tpp, which is correlative with the tuff of Eightmile Creek. This unit was erupted approximately 48 m.y. ago with initial subsidence of the Custer Graben. The deposition of unit Tpp was followed by a period of erosion and an eruption of a rhyolitic massive pyroclastic, unit Tp. This massive pyroclastic may consist of several relatively indistinct units possibly identifiable only by horizontal silicified zones contained within. Near the end of or soon after the massive pyroclastic eruption and deposition, second and third order basins were formed. Second order basins trend northeast while third order basins trend northwest. Rhyolite flow domes, Trh and Tr, intruded the third order basin margins approximately 45.8 ±2. 3 m.y. ago. Shortly after the intrusion of rhyolite Trh, a vent breccia or hydrothermal explosion breccia, Tp(l), was erupted at or near the surface. Portions of unit Tp (1) was then epiclastically reworked within the third order basin followed by the progressive in-filling of the third order basin with a coarsening upward sequence of carbonaceous black shales, Ts(b), volcaniclastic siltstones and sandstones, Ts, and finally a coarse-grained pyroclastic, Tp(u). During and after the deposition of the upper pyroclastic unit, Tp(u), recurrent movement along the N50-85E trending set of faults occurred with fine-grained rhyolite dike emplacement. The youngest rock unit in the study area is a rhyolite quartz porphyry dike, and represents the last episode of intrusive activity in the Grouse Creek deposit area. The Grouse Creek deposit contains proven plus probable mineable reserves of 5,420,000 tons of ore with an average grade of 0.042 opt gold and 1.97 opt silver for a total of 227,658 ounces of gold and 10,689,108 ounces of silver. Ore horizons are relatively flat-lying tabular zones which occur in favorable stratigraphic horizons, with thicker and/or higher grade zones occurring in and near the dominant N50-85E striking feeding structures. Gold in the Grouse Creek deposit is in the form of electrum, which is very young in the paragenetic sequence, and as inclusions within pyrite. Silver minerals in the Grouse Creek deposit from oldest to youngest include polybasite, galena, tetrahedrite, freibergite, acanthite, stephanite, miargyrite, pyargyrite, native silver, and electrum. Other silver minerals include aguilarite, owyheeite, and naumannite, but are unknown as to their paragenetic position. Other sulfides present in the deposit include pyrite (occurred throughout mineralization), arsenopyrite (occurred through tetrahedrite depostion), cerussite (unknown paragenetic position), boulangerite (unknown paragenetic position), geocronite (unknown paragenetic position), bornite (early in paragenetic sequence), and chalcopyrite (paragenetic position between bornite and polybasite). The origin of the Grouse Creek ore deposit is interpreted to be epigenetic with a syngenetic phase. The epigenetic origin is supported by the strong structurally controlled feeder zones, veining, and open space filling textures. The syngenetic phase is supported by the strong stratigraphic controls away from the feeder structures, massive pyrite lenses up to 2.5cm thick in the lower portion of the carbonaceous black shales, and the presence of framboidal pyrite in portions of the carbonaceous black shales. Structural controls for the Grouse Creek deposit are dominated by the youngest set, therefore the more open or permeable set, of faults striking N50-85E. The older set of faults striking N30-45W also controls the Grouse Creek deposit, but to a lesser extent. The intersections of these two sets of structures were major zones of permeability allowing the upward migration of the mineralizing fluids. The stratigraphic contact between the massive pyroclastics, Tp, and the carbonaceous black shales, Ts(b), is a major zone of mineralization due to the impermeable nature of the shales which stopped the upward migration of fluids, forcing lateral migration along this contact. Analysis of four primary fluid inclusions indicates the ore-forming fluids were relatively low in salinity, 1.81-2.06 wt.% NaCI, and suggest temperatures higher than expected (258.4-274.6 degrees Celsius) for the Grouse Creek deposit. These temperatures are slightly higher than the epithermal (50-200 degrees Celsius) range and fall within the mesothermal (200-300 degrees Celsius) range as defined in Lindgren’s classification of ore deposits (Park and MacDiarmid, 1975). Roedder (1985), however states that epithermal deposits are characterized by inclusion homogenization temperatures generally \u3c300 degrees Celsius, showing examples of epithermal deposits with inclusion homogenization temperatures up to 350 degrees Celsius. The geological setting and the sulfide mineral assemblages present both indicate an epithermal ore forming environment, despite the relatively high homogenization temperatures suggested by fluid inclusions in the Grouse Creek deposit. Based on field relations, the age of mineralization in the Grouse Creek deposit is no older than 45.8 ±2.3 m.y. and no younger than the age of the beginning of Ts volcaniclastic siltstone and sandstone sedimentation. The Grouse Creek ore deposit shares many similarities with rift basin deposits, and indicates that the Custer graben might be a rift basin