Natural and mining-related mercury in an orogenic greywacke terrane, South Island, New Zealand

Abstract

Mercury (Hg) is naturally present in warm springs and mesothermal (orogenic) gold-bearing quartz vein systems in the South Island of New Zealand. Mercury amalgamation was used historically in ore processing at gold (Au) mines, resulting in composite natural and anthropogenic Hg signatures at these sites. This study compares natural Hg enrichment of the Au vein systems, residual anthropogenic Hg added for amalgamation, and enrichment of naturally present Hg during ore processing. Mercury concentration data are presented for solids and water at historic mine sites, the modern Macraes mine, fault-related warm springs, and zones of naturally occurring cinnabar and Hg-bearing Au. Arsenic (As) concentrations are also presented, as As is the most environmentally significant element in this tectonic setting. Tailings and processing residues at historic mine sites (Blackwater mine, West Coast; Golden Point and Golden Bar, Hyde-Macraes shear zone) contain up to 1000 mg/kg Hg, and in adjacent surface waters Hg is at or slightly above background from 0.6 to 0.8 ng/L. Relative to South Island Hg, As is more environmentally significant: solid wastes at some historic mine and mineral processing sites contain up to 30.5 wt% As due to enrichment of natural As in mineralised rocks. Shallow groundwater and processing waters at the modern Macraes mine are up to 0.01 mg/L Hg due to natural Hg in mineralised rocks, and no significant Hg elevation is evident in nearby surface waters, which are <1 ng/L Hg. Mercury concentration in historic mine residues is 103 to 104 times higher than primary ore, and Hg is disproportionally increased relative to As, indicating that much of the Hg was added during the amalgamation process. Natural cinnabar deposition from warm springs results in localised, strongly elevated Hg, equal to or less than the Hg contents in historic mine processing residues. Warm spring precipitates are up to 111 mg/kg Hg and waters are 0.3 µg/L Hg, comparable to data reported for active North Island geothermal (epithermal-style) systems. Natural Hg content in the Au-bearing vein systems results in minor elevation of dissolved Hg in some catchments. South Island Au occurs as an Au-Ag-Hg alloy, up to 8.4 wt% Hg. Cinnabar (HgS) occurs in mesothermal-style veins and as detrital grains in Quaternary gravels hosting Au-Ag-Hg alloy. The Nevis River catchment water, which locally flows through gravels containing cinnabar and Hg-bearing alluvial gold, contains up to 7 ng/L dissolved Hg. This is rapidly diluted to near 3 ng/L by low-Hg waters, and no effects of anthropogenic Hg addition near alluvial mining areas were detected; the dissolved Hg is likely derived from Hg-bearing gold. Mobilisation of Hg from study sites to downstream environments is at least partially controlled by solubility of cinnabar and elemental mercury.