This paper investigates natural acid rock drainage in two streams draining either side of Mount Evans, Montana. Bedrock consists of pyrrhotite-bearing schist intruded by granitic dikes and plutons of late Cretaceous to Tertiary age. The headwaters of both streams are moderately acidic (pH < 5.0) and carry elevated loads of dissolved sulfate, aluminum, and other trace metals (Cd, Co, Cu, Mn, Zn) as well as rare earth elements (REE). Copious aluminum precipitates inferred to be hydrobasaluminite coat boulders of both streams as pH rises > 5, with adsorption of copper and REE. Concentrations and loads of dissolved sulfate and trace elements are anomalously high in a small tributary that is sourced by meltwater from a rock glacier. The S-isotope composition of dissolved sulfate in both watersheds is similar to that of pyrrhotite in the meta-sediments, but not molybdenite in late porphyry dikes. Calculations of sulfate flux (i.e., sulfate load divided by surface area) indicate a relatively fast rate of sulfide oxidation in the study area, possibly due to exposure of fresh bedrock in the steep and recently glaciated field area. Overall, the geochemistry of the site suggests the possible presence of a metamorphosed sedimentary-exhalative (SEDEX) deposit, a possibility that is unlikely to be tested by drilling given the proximity of the site to a federal wilderness area.
Thematic collection: This article is part of the Hydrochemistry related to exploration and environmental issues collection available at: https://www.lyellcollection.org/cc/hydrochemistry-related-to-exploration-and-environmental-issues