Scaling mine pit highwall sulfide reactivity based on humidity cell tests and a 13-year field oxidation test

Abstract

The reliability of kinetic humidity cell tests (HCTs) to predict sulfide oxidation and leachate chemistry in mine highwalls was assessed using long-term field oxidation data from the Gold Quarry mine in Nevada, USA. HCTs use finely crushed material (<6.35 mm), which may overestimate weathering rates compared to field conditions where rock is coarser and exposed to natural conditions. Four material types, ranging from highly acid-generating to highly acid-neutralizing, were crushed to < 2, 2–4, 4–16, and 16–64 mm fractions and subjected to both laboratory HCTs (for up to 22 weeks) and a 30-week field weathering test in 1996–1997, with a 1000 cm³ block also deployed in the field. In 2011, these materials were retrieved and run in HCTs to assess long-term effects. The leachate pH and sulfate in the 2011 HCTs closely matched the terminal 1997 HCT data, confirming that the 1997 HCTs had reached equilibrium. HCT sulfide oxidation rates were 2.1–4.7 times higher than the corresponding field tests for 2–64 mm fractions and up to ∼1600 times higher than those of the blocks, highlighting the need for empirical scaling. Mineralogical analysis identified dissolution and precipitation reactions, confirmed by geochemical modeling. The sulfate release and scaling factors correlated with pH and particle size, providing a robust, field-calibrated framework to scale laboratory sulfide reactivity data to more accurately predict pit lake and waste rock draindown chemistry, necessary for Environmental Impact Statements and mine closure planning.