FLUORINE CONCENTRATIONS AND PH OF ORE FLUIDS IN THE LEMHI PASS TH-REE DISTRICT (IDAHO-MONTANA) DETERMINED FROM SEM-EDS ANALYSIS OF EVAPORATE SOLUTE MOUNDS DERIVED FROM DECREPITATED FLUID INCLUSIONS

The Lemhi Pass district is the fifth-largest rare earth element (REE) and the largest thorium occurrence in the United States, part of a 115 km-long trend of REE-Th mineralization along the central Idaho-Montana border. Although the Lemhi Pass mineralization has long been recognized to be hydrothermal in origin, important uncertainties about the composition of the hydrothermal ore fluids remain, such as the pH. The purpose of the present research was to determine the pH of the ore fluids through SEM-EDS analysis of evaporate solute mounds sourced from decrepitated fluid inclusions in quartz. Because the ore fluid F concentration can be determined from the evaporate mounds and because the Ca concentration of the ore fluid is already known from previous studies, then if the fluid was also saturated with respect to fluorite, which is part of the Lemhi Pass ore mineral assemblage, the pH of the ore fluid can be calculated.

A total of 30 evaporate solute mounds were analyzed by SEM-EDS. The SEM-EDS analyses were calibrated with analyses of evaporate solute mounds derived from standard solutions with variable concentrations of F. The Lemhi Pass fluid inclusions were found to contain high fluorine concentrations between 5500 and 21,800 ppm. This corresponds to an ore fluid pH between about 0.6 and 1.05 for temperatures between 250 and 300°C, respectively. This temperature range overlaps the homogenization temperature range of 267 to 294° C found for fluid inclusions analyzed by SEM-EDS in this study. The actual ore formation temperatures could be significantly higher than the fluid inclusion homogenization temperatures because the pressure of ore formation is not well known. If the ore formation temperature was significantly higher than 300° C, then the pH could be significantly higher than 1.05. Nonetheless, the present study suggests that the Lemhi Pass hydrothermal ore fluids were acidic, which fits with the very high base metal concentrations of thousands to tens of thousands of ppm in fluid inclusions reported in previous studies and low base metal sulfide mineral abundances in the deposits.