COMPOSITION AND TEMPERATURE OF ORE FLUIDS IN THE LEMHI PASS AND DIAMOND CREEK TH-REE DISTRICTS DETERMINED FROM FLUID INCLUSION EVIDENCE

The Lemhi Pass and Diamond Creek districts, located along the central Idaho-Montana border, represent the endpoints of a 115 km trend of Th and rare earth element (REE) mineralization. These districts rank among the largest Th and REE occurrences in the United States. This research aims to determine the composition and temperature of the hydrothermal fluids responsible for the formation of these deposits through analysis of fluid inclusions using microthermometry and LA-ICP-MS.

This study so far has focused on fluid inclusions hosted by the transparent ore-stage gangue minerals, quartz and fluorite. Two types of fluid inclusions have been identified: (1) aqueous liquid + vapor and (2) aqueous liquid + vapor + halite. The fluid inclusions do not show any evidence of the hydrothermal fluids having boiled. Quartz-hosted fluid inclusions have homogenization temperatures ranging from 267 to 294° C and salinities ranging from 25 to 29.5 equivalent weight % NaCl. Na, Ca, Mg, and K are respectively the most abundant elements detected in the fluid inclusions. Cl was not quantified because of its poor LA-ICP-MS ionization potential but is inferred also to be abundant based on microthermometry. Together, these elements account for most of the dissolved mass. REE concentrations are on the order of 1’s to 100’s of ppm and Th on the order of 10’s of ppm or less. The fluid inclusions are rich in base metals (Zn, Cu, Pb), with concentrations on the order of 100’s to 1000’s of ppm. Fluorite-hosted fluid inclusions have considerably lower homogenization temperatures and salinities ranging from 118 to 143° C and 18.5 to 22 equivalent weight % NaCl, respectively. Na, K, and Mg are respectively the most abundant detectable elements in the fluid inclusions with Cl also inferred to be abundant. The Ca and REE content of the fluid inclusions could not be quantified because of interferences from their high concentrations in the fluorite matrix. REE concentrations in the fluorite matrix range about an order of magnitude higher than average crustal concentrations but are lower than the concentrations in monazite and xenotime, which are the main carriers of REE’s in the district.

The fluid inclusion evidence points to a relatively fertile ore fluid that may have had sedimentary and magmatic components where dilution may have induced REE and Th mineralization.