Geochemistry of natural acid rock drainage in the Mt Evans area, Anaconda–Pintler Range, Montana, USA

Abstract

This paper investigates natural acid rock drainage in two streams draining either side of Mt Evans, Montana. Bedrock consists of pyrrhotite-bearing schist intruded by granitic dykes 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 (REEs). Copious aluminum precipitates inferred to be hydrobasaluminite coat boulders of both streams as pH rises > 5, with adsorption of copper and REEs. 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 dykes. 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 deposit, a possibility that is unlikely to be tested by drilling given the proximity of the site to a federal wilderness area. Supplementary material: All analytical data related to this project, additional maps and photographs, and selected results from geochemical modelling are available at https://doi.org/10.6084/m9.figshare.c.5649850 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