Evidence for exsolution of Au-ore fluids from granites crystallized in the mid-crust, Archaean Louis Lake Batholith, Wyoming
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The Louis Lake Batholith is contemporaneous with and sits adjacent to Archaean lode gold deposits and minor copper stockwork vein systems of the South Pass Greenstone Belt, Wyoming Province, and may thus have contributed fluid into these hydrothermal systems. The batholith is granodiorite to granite, lacks extreme fractionation, and contains evidence for early formation and later replacement of an immiscible early sulphide phase. Mafic enclaves contain anomalous concentrations of gold and copper, suggesting that metals were introduced from in-mixed mafic magma. Apatite SO3 and Cl values imply relatively early exsolution of sulphur- and chlorine-bearing fluids during fractionation. Exsolution of possibly immiscible high- and low-salinity fluids is implied from fluid inclusion petrography, even though crystallization pressures were greater than 3 kbar. Low salinity inclusions share affinities with those of orogenic gold systems, but contain notably higher Pb and Zn concentrations. High salinity inclusions have similar elemental compositions to brines of intrusion-centred ore systems.
It is proposed that sulphide melt separated early during magma fractionation, scavenging Au and Cu, but became later destabilized, possibly because of increased fO2, releasing scavenged metals. Metals were transported from the pluton in sulphur- and chlorine-bearing hydrothermal fluids of low and high salinity, with Au being preferentially transported in the low-salinity fluid.