CHEMICAL AND PHYSICAL CHARACTERISTICS OF MINERALIZING HYDROTHERMAL FLUIDS IN THE LEMHI PASS TH-REE DISTRICT, IDAHO-MONTANA: IMPLICATIONS FOR DEPOSIT GENESIS

The Lemhi Pass district hosts numerous hydrothermal vein-type Th-REE deposits that collectively comprise estimated resources of 174,000 tonnes of ThO₂ and 200,000 tonnes of REO, the largest and fifth largest in the U.S., respectively. The origin of the mineralization is enigmatic, with features that could be products of sedimentary, metamorphic, or magmatic hydrothermal fluids, including a possible relationship to carbonatite. The purpose of the present study was to characterize the hydrothermal fluids that formed the Lemhi Pass deposits to gain insights into their origin. Fluid inclusion microthermometry and zirconium-in-rutile geothermometry suggest a formation temperature and pressure between about 470° to 540° C and 4.9 to 9.8 kbars, respectively, corresponding to a mid to lower crustal depth. The mineralizing fluids appear to have been saline, mostly in the range of 16 to 25 equivalent weight percent NaCl, with a maximum of about 35. LA-ICP-MS and SEM-EDS analysis of fluid inclusions and their decrepitates show the principal elements in solution to have been Cl, Ca, Na, K, and Mg, in decreasing order of concentration. The mineralizing fluids’ REE concentrations ranged from 1’s to 100’s of ppm and thorium concentrations ranged from 1’s to a high of 12 ppm, which are higher than typical in crustal fluids, though not necessarily fluids associated with carbonatite-REE deposits. The mineralizing fluids were relatively rich in base metals, with Zn, Pb, and Cu concentrations up to 1000’s of ppm. Despite these high fluid concentrations, Lemhi Pass is poor in base metal mineralization, perhaps because of low concentrations of sulfide in the fluid that would have inhibited base metal sulfide mineral precipitation. Base metal sulfide mineral precipitation may also have been inhibited by the mineralizing fluids’ low pH, which was calculated to be as low as 2-3, based on the fluids’ high concentrations of F between about 320 and 19,000 ppm, high concentrations of Ca in some fluid inclusions averaging about 42,000 ppm, and assuming saturation with respect to fluorite, which occurs in some deposits. The high concentrations of fluoride and chloride in the mineralizing fluids likely enhanced the solubility of Th and REE’s and promoted their enrichment in the Lemhi Pass deposits.