The oil-bearing, carlin-type gold deposits of Yankee basin, Alligator Ridge district, Nevada

The Carlin-type gold orebodies of Yankee basin, in the Alligator Ridge dist rict of northeastern Nevada, are unique for this deposit type in that they contain abundant oil. This liquid hydrocarbon occurs as (1) primary and sec ondary fluid inclusions in calcite +/- realgar vein networks encircling the orebodies in variously fresh-appearing to strongly decalcified, silicified , and collapse-brecciated Mississippian to Devonian pilot Shale; and (2) vu g- and fracture-filling free oil proximal to the orebodies in scattered, re sidual pods and lenses of unoxidized, basal pilot limestone. The fluid-incl usion and free oils are geochemically equivalent and have similar thermal m aturities (early to peak oil-generation stage). Timing of entrapment of the fluid-inclusion oils is paragenetically constrained as dominantly preminer al and synmineral. Associated free oil could have arrived at any time prior to, during, or after mineralization but before late, oil-barren, spelean c alcite vein-mineralization and subsequent supergene oxidation. Biomarker fi ngerprints and carbon isotope signatures indicate that the oils were self-s ourced from the pilot Shale; their concentration in the organically lean ba sal limestone suggests derivation from carbonaceous siltstones higher in th e formation, but in structurally lower configurations. The Yankee fluid-inclusion oils were clearly involved in the gold-mineraliz ing hydrothermal system but were not thermally degraded to pyrobitumen, the analogous solid hydrocarbon characteristic of Carlin-type gold de posits. This relationship suggests that the Yankee system was cooler than the 175 d egrees to 250 degrees C widely cited as typical for such mineralization, a contention supported by independent geothermometers. The oil-bearing fluid inclusions all have homogenization temperatures lower than 150 degrees C, w ith most less than 120 degrees C. Temperature-sensitive biomarker transform ation ratios of the oils, expressed as equivalent vitrinite reflectance (R- o; 0.75-0.95%) suggest peak paleotemperatures no higher than about 145 degr ees C. These implied low system temperatures are consistent with the lack o f evidence for a contemporaneous igneous heat source. The fossil Yankee basin hydrothermal system is similar in many ways, such a s paleotemperature, host rocks, hydrocarbons, hydrothermal alteration, and geochemistry, to nearby, active, moderate-temperature (120 degrees- 130 deg rees C) but gold-poor systems which encompass producing oil reservoirs. Num erous such warm systems have likely existed in the past in this region. We suggest that many of them formed small Carlin-type gold deposits and/or oil reservoirs that still await discovery.