Evidence for exsolution of Au-ore fluids from granites crystallized in the mid-crust, Archaean Louis Lake Batholith, Wyoming

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.

路易斯湖岩基与怀俄明州南帕斯绿岩带中的太古代脉金矿床及次要的铜矿化细脉系统同期形成，并毗邻而立，因而可能向这些热液系统贡献了流体。该岩基由花岗闪长岩至花岗岩组成，缺乏极端分异，并证实了不混溶的早期硫化物相的早期形成和后期替代。镁铁质包体中金和铜的异常浓度表明，金属可能来自混合的镁铁质岩浆。磷灰石中的 SO3 和 Cl 值暗示了在分异过程中，富含硫和氯的流体相对较早地分离出来。流体包裹体岩石学表明，可能的不混溶的高盐度和低盐度流体分离，尽管结晶压力大于 3 kbar。低盐度包裹体与造山型金系统中的包裹体具有相似性，但含有显著更高的 Pb 和 Zn 浓度。高盐度包裹体与侵入中心矿床的卤水具有相似的元素组成。有观点提出，硫化物熔融体在岩浆分异早期分离，吸附了 Au 和 Cu，但后来可能由于 fO2 的增加而变得不稳定，从而释放了吸附的金属。金属通过低盐度和高盐度的硫和氯含热液流体从岩体中迁移，其中 Au 更倾向于在低盐度流体中迁移。