Metalliods - Antimony
Mobilisation and dispersal of antimony from antimony-gold mineral deposits in circumneutral pH waters: environmental consequences in Australasia
This antimony section of the Otago Geology website is taken directly from a poster presented at the International Sb Workshop inHeidelberg, Germany, May 2005. The authors are P.M. Ashley 1, D. Craw2 & M. Tighe1, the title is given above.
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- Earth Sciences, University of New England, Armidale, NSW 2351, Australia
- Department of Geology, University of Otago, PO Box 56, Dunedin, New Zealand
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Introduction
Historic mining practices at several Sb-rich and Sb-bearing orogenic gold deposits in eastern Australia and the South Island of New Zealand have led to significant environmental contamination of streams. The mineral deposits contain stibnite and Sb-bearing arsenopyrite as primary minerals, with small amounts of Sb oxides, Fe oxyhydroxides and scorodite in the oxidised zones. Although natural weathering and erosion processes have led to Sb anomalies in stream water and sediments, unconstrained disposal of mine wastes has led to extensive contamination of water and sediments, in some cases for distances of tens to hundreds of kilometres downstream. Antimony dissolves readily, along with As, in near-neutral stream waters, most likely as SbO3-. Detrital stibnite is gradually dissolved and subject to attrition, with transfer of Sb into solution and precipitated into hydrated iron oxides (HFO), clays, organic material and neoformed sulphides. In the fluvial environment, Sb can be mobilised and cycled through aquatic ecosystems and riparian vegetation. In the most contaminated streams, water and sediment quality are severely compromised and greatly exceed national guideline values.
Study Sites
Studies have been performed on the environmental geochemical effects of mining and processing of Sb-bearing ores on stream systems at Hillgrove and Mungay Creek (NSW, Australia) and Reefton, Endeavour Inlet and in Central and Eastern Otago (South Island, New Zealand). At most of these sites, the host rocks to mineralisation are low grade metamorphosed sediments, although granitoids also occur at Hillgrove. Mineralisation is typified by quartz veins and breccia containing low total sulphide concentration, with stibnite, arsenopyrite and pyrite dominating, accompanied by carbonate minerals and gold. Oxidation of ores does not yield acid mine drainage, as incipient acid production is buffered by carbonates; drainage water typically has pH values of 6.0-8.5. At each location, the climate is temperate, although rainfall varies from 2000 mm/year to 600 mm/year and evaporation varies approximately inversely. Surface stream water flow is commonly perennial.
The Hillgrove site has been the major focus as it contains the largest Sb-mining region in Australia. Historic mine waste disposal practices have caused extensive Sb and As contamination of water and sediment in the Bakers Creek-Macleay River system, for 300 km downstream to the Pacific Ocean.