Environmental geochemistry of mine waste and waters at Macraes gold mine, Otago
The mineralogy and geochemistry of mine waste can determine the mobility of elements into the wider environment. This research covers four phases of mine waste at Macraes, an active orogenic gold mine in Otago, New Zealand. It encompasses the generation of mine waste in pressure-oxidation autoclave processing, the evolution of tailings in storage, how water-rock interactions in waste-rock influence water chemistry, and treatment methods for high sulphate water affected by tailings and waste rock.Arsenopyrite (FeAsS), pyrite (FeS2), and for a time stibnite (Sb2S3), were processed by a pressure oxidation autoclave at Macraes, which changes the nature of the waste to oxidised material. Handheld XRF and SEM-EDS analyses were used to characterise the scales formed in the autoclave, focusing on the mineralogy of arsenic (As) and antimony (Sb). Results show that Sb may form the insoluble oxide tripuhyite (FeSbO4) with co-precipitated As in the autoclave. This may explain why Sb concentrations did not increase in waters at Macraes when the mine was processing externally mined Sb rich ore.The geochemistry and mineralogy of the tailings is studied to assess how the autoclave influences tailings mineralogy, attenuation and leaching processes over 10-20 years of burial. Most sulphides (arsenopyrite and pyrite) in 77 m deep tailings appear unaltered since deposition. More recent tailings (44 m depth and surface) have had most sulphides oxidised by the autoclave during processing. At these shallower depths arsenic was primarily associated with ferric oxides and some ferric arsenate but some arsenopyrite and pyrite was present, unoxidized by the autoclave.Extensive water quality data from 20 years of water analysis has been used to assess the water-rock interactions occurring in the waste rock stacks and tailings. In waste rock stacks pyrite weathering produces sulphuric acid, which is neutralised by chlorite and calcite in the waste rock stacks, leaving high concentrations of sulphate (~2800 g/m3), magnesium (~800 g/m3) and bicarbonate (~700 g/m3) in solution.Calcium:bicarbonate and magnesium:sulphate ratios have been identified as markers for identifying tailings influenced water from waste-rock influenced water at Macraes.A proposal to manage high sulphate waters at the site through irrigation to force precipitation of sulphate minerals such as gypsum is investigated. The composition of the precipitates formed in a waste-rock stack affected waterway were found to be primarily aragonite, but geochemical models showed that with enough evaporation, gypsum would precipitate. A laboratory irrigation trial found that gypsum did precipitate in soil, but sulphate concentrations regained near-original levels in drainage after 8 weeks of irrigation.The arsenic and antimony in the tailings appear geochemically stable, with any aqueous components being adsorbed to the abundant ferric oxyhydroxides produced in the autoclave, but high sulphate water from waste rock and tailings is a remaining issue at the site.
Advisor: Craw, Dave
Degree Name: Master of Science
Degree Discipline: Geology
Publisher: University of Otago
Keywords: Macraes; sulfate; arsenic; antimony; gold; environmental geochemistry; mining
Research Type: Thesis