FLUID INCLUSIONS IN QUARTZ AND STIBNITE FROM THE YELLOW PINE DEPOSIT OF IDAHO

Fluid inclusion studies in quartz and stibnite from the Yellow Pine (YP) Au-Sb-W district, east central Idaho reveal an extended period of magmatic and hydrothermal activity. The district is in the Atlanta lobe of the Cretaceous Idaho batholith to the west of the Eocene Thunder Mtn. Caldera. The deposits are localized by second order faults and breccia zones associated with the regional Meadow Creek fault. The ore is hosted in the batholith as well as in metamorphosed Lower Paleozoic sedimentary rocks of a roof pendant. Quartz and stibnite occur in veins and breccia. Four vein types were observed in drill core (1) pre-ore quartz veins with brassy, medium- to fine-grained pyrite, (2) ore stage veinlets filled with dark, fine-grained pyrite ± quartz, (3) late ore stage, coarse-grained stibnite-quartz veins, and (4) post-ore, pink dolomite or calcite veinlets. This abstract addresses quartz and stibnite paragenesis and fluid inclusion microthermometry.

Cathodoluminescence (CL) textures show a paragenesis beginning with igneous phenocrysts cut by pre-ore aqueous-carbonic inclusions, followed by a complex series of hydrothermal events involving intermittent quartz + sulfide ± sericite deposition within well zoned hydrothermal quartz, overprinted by bright CL quartz associated with stibnite. The inclusions in quartz related to metal bearing hydrothermal activity yield T_h of 175-233°C, P_h of 9-28 bars, salinities of 6-9 wt. % NaCl, and low contents of CO₂ ± CH₄.

The stibnite is commonly highly fractured, in which case it generally lacks visible inclusions. In unfractured stibnite crystals, four inclusion populations have been observed at room temperature; including one with both liquid- and vapor-rich inclusions. Microthermometry on what are considered primary inclusions in stibnite give T_h of 160 to 189°C, P_h of 6-11 bars, salinities of 4-11 wt. % NaCl, and no gases.

The homogenization temperatures and pressures from ore-stage quartz and stibnite indicate that mineralization at YP formed in a shallow epizonal environment. The differences between inclusions in quartz and stibnite indicate that decompression and cooling caused stibnite precipitation.