P-T-X OF FLUID INCLUSIONS ASSOCIATED WITH HYDROTHERMAL EVENTS IN THE STIBNITE-YELLOW PINE DISTRICT, IDAHO, USA

The Stibnite-Yellow Pine district in Idaho produced Au, W, and Sb that formed in three stages. Invisible gold resides in disseminated and vein pyrite and arsenopyrite, W in scheelite, and Sb principally in stibnite. Infrared and optical petrography, SEM-cathodoluminescence imaging, EMPA, and LA-ICP-MS were used to discern the paragenesis of quartz. Associated fluid inclusion assemblages were characterized by microthermometry, laser Raman spectroscopy, and mass spectrometry.

SEM-cathodoluminescence images show four distinct types of quartz: igneous quartz Qi; and hydrothermal quartz Q1, Q2, and Q3 that precipitated with iron sulfide minerals, scheelite, and stibnite. Trace element chemistry shows that Qi contains Ti and formed at hotter temperatures than Q1-Q3, which lack Ti and contain Li and Al. Textural intergrowths and enrichment in Sb in Q3 confirms that it is cogenetic with stibnite. From Q1 to Q3, moderate salinity fluids evolved from H₂O-CO₂-NaCl ± CH₄ to H₂O-NaCl ± CO₂, to H₂O-CH₄ and decreased in temperature from 233 to 156 °C. Noble gas isotope analyses of fluid inclusions suggest that Qi and Q1 formed from fluids derived from reduced intrusions and metasedimentary country rocks. Minerals associated with Q2 and Q3 contain mantle derived He discharged from oxidized intrusions. Depth estimates based on fluid inclusion data and a transition from lithostatic to hydrostatic regimes decrease from ~5-6 km during Q1 to ~1-2 km during Q2 as the district was progressively exhumed.

The Stibnite-Yellow Pine district is similar to other Au-W-Sb districts around the world in fluid chemistry, having late-stage stibnite, and cooling temperature as a mechanism for Sb deposition. However, our results suggest that Au, W, and Sb precipitated under different conditions during distinct events. The fluid T-X-depth estimates and crustal source of He during Q1 gold mineralization are characteristic of orogenic Au deposits. In contrast, the T-X-depth estimates and mantle source of He during Q2 W and Q3 Sb mineralization suggest much shallower epithermal deposition coincident with Paleocene intrusions and the Eocene Thunder Mountain Caldera.