Controls on groundwater selenium, arsenic and base metals in groundwater around a selenium-bearing volcanogenic massive sulfide deposit: constraints from stable isotopes, trace elements and redox controls

Abstract

Understanding the controls on the behaviour of metalloids (Se, As) and metals (Cu, Zn, Pb) in natural aqueous systems is vital to interpreting hydrogeochemical data in environmental and mineral exploration applications. Geochemical, isotopic and redox measurements of a suite of groundwaters sampled from around the ABM zone of the Kudz Ze Kayah (KZK) volcanogenic massive sulfide (VMS) deposit in the Yukon, Canada are presented and contrasted with other case studies from a variety of mineral deposit types. This deposit has atypically high As (up to 4.3 wt%, average 2457 ppm) and Se (up to 2620 ppm, average 157 ppm) contents in the sulfide mineralization. As a relatively undisturbed deposit (unmined), it is an ideal site to study the mobility and solubility of trace metals in groundwaters. Herein we present field measurements (pH, dissolved oxygen, specific conductance, oxidation–reduction potential and temperature), major ion, trace element, anion (Cl, Br, SO4, PO4), and stable isotope (δ2H, δ13CDIC, δ18O, δ18OSO4, δ34S) data. Waters are dominantly low-salinity HCO3 to HCO3–SO4-type waters with variable sulfate (4.83 to 601 mg l−1), Ca (23–235 mg l−1) Mg (3.1–96.8 mg l−1), Na (0.30–66.9 mg l−1) and K (0.55 to 6.25 mg l−1) concentrations. These waters also have variable trace element concentrations that include As (0.01 to 148 µg l–1), Se (<0.02 to 1.01  µg l–1), Fe (0.01 to 3.84 mg l−1), Zn (<0.2 to 1070  µg l–1), Pb (<0.01 to 8.4  µg l–1), Cu (0.03 and 24.5  µg l–1) and Sb (0.01 to 54.4  µg l–1). Some waters also have elevated concentrations (compared to most meteoric waters) of Nb (up to 0.3  µg l–1), Y (up to 1.42  µg l–1), Zr (up to 18  µg l–1), and the rare-earth elements (REEs) (ΣREE up to 2.04  µg l–1). The δ18O (−22.8 to −20.9 ‰) and δ2H (−174 to −158 ‰), together with the δ13CDIC (−10.6 to +1.9 ‰), δ34S (+10 to +12 ‰) and δ18OSO4 (15.5 to −4.75 ‰) all suggest that local meteoric water has interacted with massive sulfide mineralization at the ABM zone. Our results demonstrate the requirement for the use of multiple techniques in hydrogeochemical studies, with dissolved concentrations of major and trace elements coupled with a suite of stable isotopes that help define a larger geochemical footprint for the KZK deposit. Water–mineral interaction between groundwater aquifers and VMS deposits like the ABM zone are distinctly different from dispersion halos described from other deposit types (i.e. Cu porphyry, unconformity U). 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