FLUID INCLUSION EVIDENCE FOR THE TEMPERATURE AND COMPOSITION OF ORE FLUIDS IN THE LEMHI PASS AND DIAMOND CREEK REE-TH DISTRICTS, IDAHO-MONTANA

The Lemhi Pass and Diamond Creek districts contain numerous hydrothermal rare earth element (REE) and thorium (Th) deposits located along the central part of the Idaho-Montana border. Lemhi Pass is the larger of the two districts and represents the largest known occurrence of thorium and the fifth largest occurrence of REE’s in the U.S. The much smaller but geologically similar Diamond Creek district is located about 28 km northwest of the Lemhi Pass district and thought to be genetically related. Most of the REE-Th mineralization occurs in quartz-hematite veins hosted in Proterozoic metasedimentary rocks. Previous studies of the mineralogical composition of the Lemhi Pass deposits have yielded insights into the characteristics of the ore-forming hydrothermal fluid. In particular, this previous research suggests that the hydrothermal fluid was derived in part by exsolution from magma, had a temperature between 350 and 525° C, had near neutral acidity, was oxidizing, and was relatively rich in phosphate and Mg but poor in K and Ca. The purpose of the present study was to extend this work through an investigation of fluid inclusions.

Most of the usable fluid inclusions found thus far have been in fluorite, a late stage ore mineral, from Diamond Creek. During microthermometry, many of these fluorite-hosted fluid inclusions decrepitated before homogenizing, but some homogenized from liquid-vapor to liquid at temperatures ranging from 107 to 259° C. The salinity of these fluid inclusions ranged from about 13 to 21 equivalent weight percent (eq. wt. %) NaCl. Fluid inclusions in quartz from both Diamond Creek and Lemhi Pass had a narrower salinity range of 12 to 15 eq. wt. % NaCl. Most quartz-hosted fluid inclusions decrepitated around 320-330° C before homogenizing, but some homogenized from liquid-vapor to liquid between 365-371° C. The quartz-hosted fluid inclusions also show evidence of fluid immiscibility with liquid-rich and vapor-rich inclusions trapped in the same fluid inclusion assemblages, as well some liquid-rich inclusions with multiple daughter minerals. Raman spectroscopy consistently showed both fluorite- and quartz-hosted fluid inclusions to be rich in CO₂ and N.