GOLD MINERALIZATION IN A CORDILLERAN UPLIFT? A STUDY OF THE LOUIS LAKE BATHOLITH, WIND RIVER RANGE, WY

The Wind River Range, a part of the Wyoming Craton, is composed of Archean age granites and minor supracrustal rocks that were uplifted during the Laramide orogeny along the Wind River Fault as a basement-involved thrust. At the southern end of the range, these granites are in contact with the South Pass Greenstone Belt, which hosts lode gold mineralization of the historically mined fields of Atlantic City and South Pass City. The 2.63 Ga Louis Lake Batholith is the largest of the granites that intrude into the South Pass Greenstone Belt during regional deformation and greenschist- to amphibolite-facies metamorphism. This batholith is a complex, calcalkaline, multistage, granitic to granodioritic body with multiple late phases, including aplites and quartz segregations. Because of its proximity to the deposits and similar age dates between the granite and the gold mineralization, the Louis Lake Batholith is a potential ore fluid source. Fluid inclusions within quartz of quartz veins, pegmatites, and quartz segregations can be grouped into two types: Type 1 and Type 2 inclusions. Type 1 inclusions are low salinity, aqueous-carbonic inclusions. Type 2 inclusions are aqueous inclusions with multiple daughter solids, including K and Ca salts. Fluid densities derived from microthermometry show entrapment at pressures of a few kbars. These inclusion types provide evidence of Archean age aqueous-carbonic and hypersaline fluids derived from a crystallizing granite at depth. These fluids have the potential to carry metals in solution, because they are similar to fluids found in lode gold deposits and porphyry deposits. Hornblende geothermometry and geobarometry confirm the mid-crustal depths of crystallization of the batholith.