GRANITIC MAGMATISM AS A FLUID SOURCE FOR LODE GOLD DEPOSITS IN THE ATLANTIC CITY-SOUTH PASS CITY AREA, WYOMING

The origin of the ore fluid of lode gold deposits has long been in debate. There are three major models for the origin of the fluids. These models include fluids from the exsolution of a felsic magma, magmatic devolatilization, and circulating meteoric waters. This study examines the model of exsolving fluids from a crystallizing granitic magma.

The Archean age Louis Lake Batholith is intruded into the South Pass Greenstone Belt, which hosts the lode gold deposits near Atlantic City and South Pass City, WY. Because of its proximity and similarity in age to the deposits, the Louis Lake Batholith is a potential ore fluid source. The dominant mineral assemblage is quartz-plagioclase-potassium feldspar-biotite, but in some areas, hornblende is also present. Minor minerals include magnetite, ilmenite, and titanite. Metamict crystals of allanite are also present. Sulfide mineralization has also been noted within the granite.

This metaluminous granite contains numerous pegmatites and aplitic dikes as evidence of late stage fluid movement. Fluid inclusion work on the late-stage dikes has provided evidence of Archean age aqueous-carbonic and hypersaline fluids moving through a crystallizing granite at depth. These fluids provide the potential for carrying metals in solution, because they are similar to fluids found in ore deposits. The aqueous-carbonic fluid is important because of compositional similarities with the fluids of lode gold deposits. The hypersaline fluid provides evidence of hypersaline fluids moving within granites that have crystallized much deeper than levels of water-salt immiscibility. This fluid is compositionally similar to the highly saline fluids of porphyry deposits.