Geochemical characterisation of surface waters and precipitates in Nenthorn historic gold mine, East Otago, New Zealand

Abstract

Nenthorn is one of the historically mined gold districts in East Otago, Southern New Zealand. After mining ceased in the early 1900s, the mine battery and adits have exposed fresh schists and mineralised goldbearing quartz vein that has undergone continuous chemical weathering and alteration reactions mainly involving, arsenopyrite(AsFeS) and calcite (CaCO3); producing alkaline waters (pH range of 7.5 to 8) with elevated trace elements. Oxidation and dissolution of sulphide (ore) minerals usually results in elevated sulphate and other heavy metal concentration in mine waterways, hence creating a potential environmental concern. However, with semi-arid climates like East Otago, the occurrence of ochreous precipitates and efflorescence are common. Water samples and precipitates were collected from the mine workings and the adjacent creek to determine the major and trace element content in a one-time sampling program. The analytical data shows elevated concentration of trace elements in precipitates than the water samples, however, the study site was identified as not extremely concentrated or contaminated with heavy metals. The ochreous precipitates samples used for this study most likely formed from the precipitation of supergene alteration minerals like iron hydroxysulphates and iron oxyhydroxides, and primary minerals like arsenopyrite due to the elevated concentration of Iron, Arsenic, and Manganese. The sorption of metals like Copper, Lead, and Zinc to oxyhydroxides of Iron and Manganese sulphates could explain the absence of these metals in the water samples. The circumneutral pH of the water is caused by calcite dissolution, leading to very high Calcium (Ca) concentration in all the samples analysed, however highly undersaturated concerning halite, gypsum, and calcite. Conclusively, it was observed that in the Nenthorn area, arsenic concentration is above the maximum acceptable values for inorganic determinants of health significance in New Zealand. Contamination of surface waters quality in the abandoned Nenthorn mines is small enough that it does not affect the Deighton Creek just downstream. However the study does reflect the potential for even small mineral deposits to actively engage with the surrounding geochemical environment.