Abstract
Mine waters at the Reefton orogenic gold mine (active from 2007 to 2016) in southern New Zealand contained dissolved arsenic (As) and antimony (Sb) up to 5 mg/L during production of a sulfide concentrate that included arsenopyrite (FeAsS) and stibnite (Sb2S3). Ferric oxyhydroxide adsorption extracted As down to < 0.1 mg/L but dissolved Sb remained elevated due to adsorption competition with As. The Reefton sulfide concentrate was transported 700 km to the Macraes orogenic gold mine (active from 1990) for processing through a pressure oxidation autoclave at 225 degrees C. The Macraes ore has low Sb contents, so the temporary introduction of a Sb-rich component produced a short-term Sb signal in the autoclave system and tailings waters. Oxidation of stibnite occurred rapidly in the autoclave, in parallel with the As in the arsenopyrite, producing ferric antimonate (tentatively identified as As-bearing tripuhyite) and ferric arsenate (FeAsO4). Dissolved Sb in the Macraes tailings waters remained < 0.1 mg/L throughout the period of Reefton concentrate addition. The formation of tripuhyite in the high-temperature autoclave stabilised Sb in the Macraes tailings, so that dissolved Sb < As, in contrast to the low-temperature processes at Reefton where dissolved Sb > As.