Fluid inclusion evidence for a genetic link between simple antimony veins and giant silver veins in the Coeur d'Alene mining district, ID and MT, USA



The US Antimony mine in west central Montana is the United States' second largest quartz–stibnite vein deposit with production and reserves of about 15.4 kt of antimony. It is in the Mesoproterozoic Belt Basin on the eastern side of the Coeur d'Alene mining district that is known for its world class Ag, Pb, and Zn veins. To advance understanding of the origin and relation of the simple quartz–stibnite veins in the Prichard Formation to the giant Ag-rich tetrahedrite veins in the Revett and St. Regis Formations, fluid inclusions in quartz from the US Antimony mine were characterized using a variety of single inclusion and bulk analysis techniques. The measured Sb concentrations in fluid inclusions (approximately 2000 ppm) agree with equilibrium models for stibnite-saturated fluids at the measured homogenization temperatures (224–263°C), salinities (approximately 5 wt.% NaCl), H2S concentrations (<0.001 mole%), and near neutral pH indicated by sericitic alteration. The aqueous-carbonic fluid inclusions in quartz from the US Antimony mine have P-T-X that are similar to previous results on fluid inclusions from the giant silver veins in the district. The assemblage and abundance of trace elements and ore metals (Sb > As > Fe > Pb > Zn > Cu > Ag) in fluid inclusions from the US Antimony mine support previous interpretations, based upon Pb isotopes, that the quartz–stibnite veins are genetically related to the Ag-rich tetrahedrite veins. In particular, the measured range of Ag concentrations in US Antimony fluid inclusions brackets that calculated on the basis of equilibrium with tetrahedrite in the silver veins. These data clearly show that hydrothermal fluids in the Prichard Formation contained metals that were fixed at shallower levels in the Revett and St. Regis Formations. We surmise that both the antimony and silver veins formed in response to Cretaceous magmatism, prograde metamorphism of the lower Prichard Formation, and episodic discharge of metamorphic fluids along dilatant faults. Ascending fluids deposited quartz and stibnite in the upper Prichard Formation and siderite, quartz, and tetrahedrite at shallower levels in the Revett and St. Regis Formations in response to cooling, decompression, phase separation, and mixing with external fluids.