INSIGHTS FROM PARTITIONING THEORY INTO THE ZINC CONTENT AND ORIGIN OF MINERALIZING FLUIDS IN THE LEMHI PASS TH-REE DISTRICT, IDAHO-MONTANA

The Lemhi Pass district represents a major crustal enrichment of thorium and rare earth elements with minor accompanying base metals near the central Idaho-Montana border. The mineralization is highly fracture-controlled, hosted by Proterozoic quartzites and siltites, and is hydrothermal in origin. To understand the origin of the Lemhi Pass mineralization requires knowledge of the composition of the hydrothermal fluids. Partitioning theory offers a way to obtain some compositional information, provided that the appropriate partition coefficient values are known. In the present study, the zinc concentration of the hydrothermal fluid as a function of temperature was calculated from measurements of the zinc concentration in hydrothermal calcite. The results fit best with a hydrothermal fluid that had a temperature of around 100° C, significantly lower than the general 350 to 535° C temperature inferred for mineralization in the district from previous studies. The results indicate that the hydrothermal fluid was anomalously Zn-rich compared to most crustal fluids, with concentrations varying widely between about 200 and 30,000 ppm. The higher Zn concentrations in this range are much higher than has been observed in sedimentary brines. However, the higher Zn concentrations in the present study do overlap the range that has been observed in magmatic fluids, including specifically fluids exsolved from carbonatite, which previous studies have hypothesized to be the source of the rare earth elements and thorium in the district. If the hydrothermal fluid was so rich in Zn, then this raises the question of why comparatively little Zn mineralization occurs in the Lemhi Pass district. A clue may lie in the prevalence of hematite and barite in the district. This may indicate that the fluid was oxidizing enough for sulfate to predominate over sulfide in solution, greatly increasing the solubility of Zn and inhibiting its precipitation.