Summary:
The Denton-Rawhide Mine is located within the western portion of the Basin and Range Physiographic Province, an area of northward-trending mountain ranges separated by alluvial valleys. The Rawhide epithermal gold-silver deposit is located along a northwest-trending structural zone that delineates the northeastern margin of the central Walker Lane.
The oldest rocks exposed near the Denton-Rawhide project area are limestone, shale and dolomite sequences that most likely correlate with the Triassic-age Luning Formation. These sequences, referred to as basement rocks by Gray (1996), occur in the Sand Springs Range north and east of the Denton-Rawhide Mine. Drilling around the open pits within the project area has encountered these older rocks to depths of 120 to 800 feet below the near surface volcanics (BLM, 1996)
The rock types within the Rawhide volcanic center have been subject to extensive faulting and hydrothermal alteration. The three primary units within the Mine area are referred to informally as lithic tuffs, andesite, and stratified tuffaceous sediments and volcanic breccias. The lithic tuffs (Tlt) consist of volcaniclastic and pyroclastic rocks 800 to 2,000 feet thick (Gray, 2000). The unit is locally divided into subunits based on textural and compositional criteria. Irregular flows of andesite (Ta), the principal host for bulk mineable ore, overlie the lithic tuff (Tlt). Thickness of the unit ranges from 0 to 400 feet (Gray, 1996).
Unconformably overlying the andesite unit is a sequence of intercalated lithic tuff-breccia with volcaniclastic sandstone and siltstone (Tst). Individual beds of volcanic breccias have been reported to be more than 30 feet thick. The finely stratified pyritic siltstones are interpreted to have formed during subaqueous hydrothermal venting within a lacustrine environment (Black, 1990). This series of tertiary volcanic rocks overlies a rooted flow dome, which is believed to be the top of a cupola of a larger intrusive pluton which has been mapped in the area. This flow dome, described by Gray (1996), is anticipated to be extensive in nature and is observed blanketing much of the area surrounding the Rawhide volcanic center. A magnetic geophysical survey conducted by KRMC identified this arcuate igneous body trending from the Koegal Hills in an east-northeast direction under the alluvium. The presence of this intrusive body would undoubtedly isolate the Rawhide site from the surrounding areas with regard to groundwater flow. Such rocks act mostly as confining units or barriers to flow. Any groundwater present within such a rock unit would exist only locally in the unit where the rock is fractured.
A rhyodacite (Trd), described in Black (1990) as a hornblende biotite rhyodacite, appears to be an intrusive flow dome, and was the ore host at Hooligan Hill. Trd was encountered as intrusive bodies below Murray Hill, Crazy Hill and Rawhide Wash, but appears to be extrusive north and northwest of Hooligan and Grutt Hills (Gray, 1996). The Balloon Rhyolite (Trp) includes intrusive and extrusive rocks in the Murray Hill Pit and in Balloon Hill. Trp is typically bleached white due to alteration. Local mineralization is strongly associated with silica veining and brecciation (Gray, 1996).
The Balloon Rhyolite (Trp) includes intrusive and extrusive rocks in the Murray Hill Pit and in Balloon Hill. Trp is typically bleached white due to alteration. Local mineralization is strongly associated with silica veining and brecciation (Gray, 1996).
A welded, crystal-rich lapilli ash flow tuff (Tlat) sequence crops out extensively to the north and east of the Mine. This unit is rarely encountered within the mineralized deposit (Gray, 1996). The tuff is older than, and may underlie, the Rawhide volcanic center (Black, 1990).
Mineralization at the Denton-Rawhide Mine is highly structurally controlled. Intersections of major fault structures coincide with bulk-mineable mineralization and orebodies are often bounded by faults. Some faults are steep to vertical for considerable depths, which attests to their importance as major plumbing conduits for hydrothermal events. Right lateral strike-slip faulting of the preTertiary basement is manifested as repeated en echelon, north-south striking right-lateral strikeslip faults in the overlying rocks. Major zones of intersection of these faults with northwesttrending faults correlate well with major zones of mineralization. Repeated post-mineral deformation is evidenced by offset orebodies and alluvium (Gray, 1996).
The ore deposit is located within an area of hydrothermal alteration approximately 3 miles in diameter that encompasses most of the Rawhide volcanic center. Two types of epithermal precious metal mineralization have been described at Rawhide. Porous, poorly welded lithic tuffs and volcaniclastic sediments host disseminated ore with pervasive silicification and adularization. Ore in brittle, non-porous host rocks, such as the andesite unit, is more fracture-controlled and characterized by abundant, closely spaced, sheeted to stockwork quartz-adularia veins (Black, 1990).
Gold occurs as electrum in both oxidized and sulfide ore at Rawhide. Silver occurs primarily as embolite and lesser cerargyrite and embolite in oxide ore, and as silver selenides, sulfides and sulfosalts in sulfide ore (Gray, 2000). Fine-grained pyrite is the most common sulfide mineral, comprising 1-5% of sulfide ore. Black (1990) reported that approximately 75-80% of the ore was oxidized.