Historic gold and coal mines in the same catchment near Reefton, New Zealand allow comparison of environmental effects of the different mines in the same climate and topography. Gold mine discharge waters (neutral pH) deposit hydrated iron oxide (HFO) abundantly at mine entrances, whereas coal mine discharge waters (low pH) precipitate HFO tens to hundreds of metres downstream as pH rises. Waters leaving historic mines have up to 59 mg L−1 dissolved arsenic, and HFO at gold mines has up to 20 wt% arsenic. Coal mine discharge waters have low dissolved arsenic (typically near 0.01 mg L−1) and HFO has <0.2 wt% arsenic. Minor dissolved Cu, Cr, Ni, and Zn are being leached from background host rocks by acid solutions during sulfide oxidation, and attenuated by HFO downstream of both gold and coal mines. A net flux of 30 mg s−1 arsenic is leaving the catchment, and nearly all of this arsenic flux is from the gold mining area, but >90% of that flux is from background sources. The present study demonstrates that elevated trace metal concentrations around mines in a wet climate are principally from non-anthropogenic sources and are readily attenuated by natural processes.
Quantification and Prediction of Bulk Gold Fineness at Placer Gold Mines: A New Zealand Example