Geochemical study of the Louis Lake Batholith and its potential as a fluid source for gold deposits in the Atlantic City District, WY

The Louis Lake Batholith is positioned in the southern tip of the Wind River Range, an upthrust exposure of the Wyoming Craton. Because the Louis Lake Batholith is adjacent to a known gold mineralized area, it was investigated as a potential fluid source for these deposits. The area has experienced very little deformation since the Archean, making it plausible to study any possible links between the granite and the deposits. The South Pass Greenstone Belt contains lode gold deposits and minor copper vein stockwork systems, similar in age to the crystallization age of the batholith. U-Pb titanite ages for shear zones associated with the Atlantic City District gold deposits range from 2635 to 2616 +/- 2 Ma (Frost et al., 2006). Gold mineralization is shear-hosted, and minor copper mineralization is found as lodes and stockworks that cut the gold mineralization (Hausel, 1991).;The Louis Lake Batholith samples range from biotite granite to granodiorite, with localized hornblende. The samples were analyzed using a variety of techniques, including whole rock geochemistry, petrography, mineral chemistry, and studies of fluid inclusions. Whole rock data demonstrate that the batholith was formed in a volcanic arc setting. REE spider diagrams reveal that all phases are genetically related and show a slight negative Eu anomaly, except for a mafic enclave, which has a slight positive Eu anomaly, and thus likely a different source. Aplites show flat REE profiles because they are more highly fractionated. Magnetite is widespread, typically as aggregates or as clusters with titanite, biotite, and hornblende (if present). Euhedral titanites are widespread and can contain inclusions of magnetite. The presence of magnetite and titanite implies a relatively oxidizing environment at the time of crystallization. Apatite grains are widespread and occur as inclusions in mafic minerals and as isolated zoned crystals. Minor amounts of allanite are present, generating radioactive halos in adjacent minerals, such as apatite and titanite. The sulfide content of the melt is marked by trace amounts of chalcopyrite that can be seen as small inclusions in magnetite, and more rarely in quartz grains.;Fluid inclusions were analyzed from a variety of late-magmatic phases: quartz veins and irregular segregations in unaltered host granite, pegmatite dikes, and aplite dikes. Four types of inclusions were found in the Louis Lake samples. Type 1 inclusions contain high salinity, aqueous fluids with varying amounts of daughter minerals and sometimes a few mol % CO2. Type II inclusions are a low to moderate salinity, aqueous-carbonic fluid that sometimes contains small daughter minerals. Type III inclusions are moderate salinity, aqueous-carbonic inclusions with daughter minerals, and are thought to be an intermediate between Types I and II. Type IV inclusions are CO 2-rich, 60-70 mol % CO2, and low salinity. Type I and II fluid inclusions from one sample were analyzed using LA-ICP-MS for a suite of elements.;Upon investigation of the Louis Lake Batholith, several parallels with felsic to intermediate intrusions associated with ore deposits become apparent. Gold-related plutons are comparable to the batholith in this study in that that have a similar oxidation state, lack extreme fractionation, and show evidence for a release of an early-crystallized sulfide phase. The trend of SO3 concentration in apatites implies fluids exsolved relatively early during fractionation of the granite. Geochemical analysis of the mafic enclave indicates that gold and other metals were introduced to the granite by mixing with mafic magmas. The elevated Au and Cu content of enclaves is congruent with that recorded in intrusions associated with some ore deposit types. Types I and II fluid inclusions are Na-dominated, with similarities to hydrothermal fluids in ore deposits. High salinity fluids contain similar metal contents as intrusion-related ore systems, while low salinity fluids share affinities with both intrusion-related ore systems and orogenic gold systems. The model of Core et al (2006) in which magma mixing is an important step in the creation of high copper content of enclaves is therefore also proposed for large porphyry deposits (Core et al, 2006). (Abstract shortened by UMI.)...