Evolving metalloid signatures in waters draining from a mined orogenic gold deposit, New Zealand

The Globe-Progress gold mine has recently (since 2007) been developed as an open pit on the site of historic underground mine workings on an orogenic-type Au deposit. Historic mines followed auriferous quartz veins with arsenopyrite and low Sb content. Ore in the modern mine includes later-stage cataclasite with abundant stibnite. Hence, rock exposed in the area has evolved from Sb/As ratio <1 to Sb/As ratio near to or greater than 1, with locally high Sb contents. Water that has interacted with these rocks in the mine and processing system developed Sb/As ratios that were similar to those of the rocks, as indicated by compositions of pit waters and leachates from static leach tests. Mine drainage waters are circumneutral (pH 6–8) with high HCO 3 - and SO 4 - (both up to 300 mg/L). Dissolved As in drainage waters was typically near 0.5 mg/L or below, but reached ∼1 mg/L at times, and Sb reached ∼3 mg/L. The Sb/As ratio of mine drainage waters has evolved from <1 in historic mines, to ∼30 below the modern mine. The rise in Sb/As was enhanced by a ferric chloride treatment system for process waters that preferentially removed more As than Sb. Arsenic behaved conservatively during downstream dilution, and the As flux from the modern mine, ∼30 mg/s, is similar to As fluxes from historic mines. In contrast, the Sb flux has increased from ∼1 mg/s from historic mines to nearly 100 mg/s below the modern mine. Excavation of the modern open pit, combined with the addition of a water treatment system, removed Fe from the mine waters that historically produced abundant Fe oxyhydroxide precipitates. Hence attenuation of metalloids by adsorption, which was widespread in the historic mines, has ceased. Changing mining methods and contrasting ore types can have a dramatic effect on the metalloid contents of circumneutral mine drainage waters, and these metalloid contents evolve with time through the mining process.