Faro Waste Rock Project: Characterizing geochemical heterogeneity in sulfide- and carbonate-rich waste rock

Abstract

Sustained oxidation of sulfide minerals in waste rock generates acid mine drainage for time frames of hundreds to thousands of years. Management to minimize long-term degradation of water supplies and ecosystem health requires a thorough understanding of geochemical processes occurring within the waste rock. A comprehensive field investigation was conducted on sulfide and carbonate-rich waste-rock dumps at the Faro Mine Complex, south-central Yukon Territory, Canada. This investigation included installation of three highly instrumented boreholes, in situ measurements of physical and geochemical parameters (e.g., water content, temperature, air permeability, and pore-gas O2 and CO2 concentration), and collection of pore-water, pore-gas, and solid-phase samples. Field and laboratory measurements indicate the waste rock is lithologically and mineralogically segregated, with sulfide-rich (>20 wt% S) waste rock dominating in the lower benches (20–50 mbgs) and carbonate-bearing waste rock dominating in the upper benches and other parts of the dumps. The observed segregation strongly influences spatial variability of geochemical parameters. Three distinct geochemical zones are observed within the waste-rock dumps, including a rapid O2-supply zone, with near-atmospheric concentrations of O2 (0–30 mbgs); a strong O2-depletion zone (30–50 mbgs), characterized by abundant sulfide minerals and significant O2 depletion to <5 vol%; and a thermally-induced O2-enriched zone (50 mbgs to the pre-mining surface), where thermally-driven gas transport results in replenishment of O2 concentrations (7–18 vol%). These observations indicate sulfide-mineral oxidation and pH neutralization via dissolution of carbonate and aluminosilicate minerals influence pore-water pH and concentrations of dissolved sulfate and metal(loid)s. High concentrations of dissolved metal(loid)s (e.g., up to Fe 36,900 mg L−1, Zn 14,200 mg L−1, Pb 6.16 mg L−1, Al 8,750 mg L−1, As 63.6 mg L−1, Mn 2,170 mg L−1, Cd 21.7 mg L−1, and Cr 1.53 mg L−1) and SO4 (up to 114,000 mg L−1) in pore water correlate with extreme values of pH (as low as 1.4). In the absence of an effective remediation strategy, the oxidation of still-abundant un-oxidized sulfide minerals will continue through vast portions of the waste-rock dumps.