Early evolution of weathering and sulfide depletion of a low-sulfur, granitic, waste rock in an Arctic climate: A laboratory and field site comparison

Abstract

Leachate from a humidity cell experiment provided a geochemical framework to evaluate the early evolution of weathering of the same waste rock in an Arctic environment. Comparison of laboratory and field results indicates the hydrogeochemical system within the higher sulfide (0.01–0.27wt.%S) waste rock pile has attained a peak weathering state, indicated by a shift to acid neutralization by weathering products of Al-bearing minerals, stabilization of the pH near 4.5, mobility of metals from sulfide and Al-bearing minerals, and a highly correlated relation between SO4 release and outflow from the waste rock pile. Further weathering of the waste rock should be driven by external environmental factors of temperature and precipitation/infiltration instead of internal factors of wetting and transient acid-neutralization processes. An evaluation of sulfide depletion indicates that 4% of the sulfide in the <6.3-mm fraction of this waste rock pile, corresponding to 2% of the sulfide in the <50-mm fraction, has been removed through oxidation and leaching as SO4. Weathering is strongly seasonal because of the Arctic climate, which produced a daily sulfide-depletion rate corresponding 0–0.02% and a peak annual depletion of 100kg or 1.5% of the remaining sulfide content in the <6.3-mm fraction. Peak sulfide weathering is expected to continue until about 15% of the available sulfide is depleted, similar to an observed decrease in sulfide weathering in the laboratory. Estimates of the reactive surface area of the sulfide minerals and a peak rate constant were used to evaluate the sulfide percent in the fine fraction of rock in the pile undergoing weathering during the annual freeze-thaw cycle, which can be used to estimate a climate rate factor to adjust the weathering mass through the seasonal changes. For modeling of future leachate, laboratory results indicate that an exponential rate limiting factor is necessary to account for slowing of sulfide oxidation after peak weathering because of the formation of secondary minerals that inhibit element release.