A geochemical characterization of cold-water natural acid rock drainage at the Zn-Pb XY deposit, Yukon, Canada

摘要

Acid rock drainage (ARD) is considered to be temperature-limited due to the diminished activity of Fe(II)-oxidizing microbes at low temperatures. Nonetheless, ARD streams are present in cold climates. This study presents a geochemical characterization of a cold climate ARD creek at the Zn-Pb XY deposit in Yukon, Canada, which showed highly elevated concentrations of dissolved zinc (up to 475 mg/L). Acid rock drainage at the XY deposit is likely generated via subsurface abiotic and biotic oxidation of sulfide minerals, and then exits as seeps at the headwaters of the creek. The uppermost reaches of the creek have the lowest pH levels (pH 3.3) and highest metal concentrations, with prolific precipitation of iron-hydroxysulfate and -oxyhydroxide mineral precipitates (schwertmannite, jarosite, and goethite), present as terraced iron formations (TIFs) at one sampling location. The lower reaches of the creek show a progressive pH increase (up to pH level 4.9) which occurs due to Fe(III)-and Al-hydrolysis, the neutralizing influence of carbonate-rich strata and/or ground waters, and dilution by surface waters entering the creek. Progressive pH neutralization causes a change in precipitate mineralogy to X-ray amorphous Al-hydroxysulfates, with a composition similar to aluminite and hydrobasaluminite, and amorphous Al(OH)(3). Natural attenuation of Cd, Zn, and Pb occurred downstream from the headwater seeps, which was likely influenced by adsorption reactions involving both metal-sulfate anions and metal-sulfate ternary complexes. Generally, the concentrations of Cd, Zn, and Pb in the mineral precipitates followed trends in pH, aqueous concentration, predicted speciation, and mineralogy of the precipitates. However, anomalous trace metal concentrations were observed in the precipitates at 120 m downstream and potentially reflect a local ground water input. Overall the geochemistry of this cold climate ARD creek was similar to ARD in more temperate climates, although low temperatures appear to retard the kinetics of ferrous iron oxidation and seasonal hydrodynamics may influence the formation of TIFs in these highly saturated waters. (C) 2015 Elsevier Ltd. All rights reserved.