EPISODIC HYDROTHERMAL PROCESSES IN AU-SB-W DEPOSITS OF THE STIBNITE DISTRICT, IDAHO, USA: THE VALUE OF IN-SITU TECHNIQUES

The Stibnite mining district in central Idaho hosts large, structurally controlled antimony and gold (~ 200 metric tons) deposits that Midas Gold has applied for a permit to mine. This historic district also supplied tungsten for the WWII effort. New research and mapping by the Idaho Geological Survey and collaborators (Gillerman, et al., 2014, 2019; Stewart et al., 2016) and colleagues at the US Geological Survey (multiple authors, in preparation) confirm the episodic hydrothermal sequence noted by early workers. Early disseminated gold is associated with muscovite alteration and sulfidation of biotite in granitic or metasedimentary host rocks. Later Au, or W and Sb veins and breccias are more fracture-controlled. Radioisotopic ages document a complex sequence of ore deposition from early Paleocene through mid-Eocene time. Disseminated gold, hosted in arsenian pyrite and arsenopyrite, was earliest, followed by carbonate-quartz-pyrite veins with potassium feldspar alteration. Scheelite and stibnite deposition followed the gold. Epithermal quartz-carbonate-pyrite Au-Ag veins and breccias with potassium feldspar envelopes constitute a younger system, possibly related to the onset of Eocene Challis magmatism. Investigation of the hydrothermal systems at Stibnite has required both old and new tools. Petrographic observations, documented photographically from microscope work, distinguish multiple hydrothermal generations and overprinting. Imaging and in-situ microanalyses of zoned pyrite for Au and trace elements, and measurements of common Pb isotope ratios on mineral separates by standard ID-TIMS plus in-situ LA-ICPMS analysis reveal characteristics of pre-ore and ore-bearing sulfides. Arsenic is the major trace element associated with gold-bearing pyrite, but trace copper and antimony are present. Lead isotopes are highly radiogenic and distinct from juvenile signatures of the Eocene magmatism nearby. The in-situ Pb isotope work, conducted on polished thin sections, shows subtle differences between the early gold and later epithermal system. Modelling of the isotopic evolution suggests hydrothermal fluids interacted with Proterozoic and Archean blocks under central Idaho. Utilization of multiple tools has enabled understanding of the hydrothermal history at Stibnite.