Experimental Metalloid Mobilisation from a New Zealand Orogenic Gold Deposit

Elevated dissolved antimony (Sb) and arsenic (As) are common environmental issues around orogenic gold mines. The metalloids occur principally in stibnite (Sb2S3) and arsenopyrite (FeAsS), and are typically accompanied by pyrite (FeS2). Samples of arsenopyrite-rich (a parts per thousand 2.5 wt% As) and stibnite-rich (a parts per thousand 10 wt% Sb) ore were collected from the Globe-Progress mine in the Reefton goldfield, New Zealand. Crushed samples (< 5 mm particles) were placed in kinetic leach columns and monitored for a year. Leachate was collected and analysed monthly. After 12 months, only a small portion (< 1 %) of the total As and Sb had leached from the ore samples. Over the course of each month, the pH of all leachates decreased from a parts per thousand 7 to a parts per thousand 3 due to pyrite oxidation. Dissolution of Ca-Fe-Mg carbonates in the host rock was insufficient to neutralize the leachate. Dissolved As concentrations from the arsenopyrite-rich ore sample were initially 16 mg/L, but decreased to a parts per thousand 2 mg/L over 12 months. Dissolved Sb concentrations from the stibnite-rich ore were > 7 mg/L throughout the experimental period, with maxima of 12 mg/L. SEM analysis after 6 months showed secondary arsenolite (As2O3) and Ca-Al-Mg sulphates on arsenopyrite-rich ore surfaces, but an absence of secondary minerals, including Sb oxides, on stibnite-rich ore surfaces. These experiments document geochemical and mineralogical processes associated with short-term (days to weeks) water-rock interactions that yield relatively high concentrations of dissolved metalloids (> 5 mg/L) where localised (1-10 m) acidification and limited oxidation occur in sulphide-rich rocks.