Environmental mobility of antimony around mesothermal stibnite deposits, New South Wales, Australia and southern New Zealand

Abstract

Antimony (Sb) occurs principally in stibnite (Sb₂S₃) in mesothermal vein deposits hosted in low-grade metamorphic belts of eastern Australia and southern New Zealand. Stibnite is commonly associated with gold. Many deposits have been mined historically, with one large deposit, at Hillgrove, New South Wales, being mined recently. Natural outcrops in the relatively rugged terrains are oxidised under humid to semiarid conditions and stibnite transforms to oxides including valentinite, senarmontite, stibiconite, and rarely cervantite. Oxidation of stibnite and associated arsenopyrite and pyrite causes local acidification; however, acid is readily neutralised by carbonates in mineralised zones and host rocks, and associated waters are near neutral (pH 6-8.5). Stibnite dissolves readily in moderately oxidised waters as SbO₃-, in conjunction with antimony oxide formation. Experimental stibnite oxidation yielded up to 37 ppm dissolved Sb, and mine and tailings dam waters at Hillgrove have up to 55 ppm dissolved Sb. Natural stibnite can contain >5000 ppm As in solid solution. Dissolution of stibnite, arsenopyrite and arsenian pyrite releases arsenic, and resultant dissolved As concentrations are up to 3.6 ppm (experimental) and up to 7.2 ppm (mine and tailings dam seepages). Mine and tailings discharge waters have elevated Sb and As where they emerge, but attenuation occurs by deposition of the metals onto amorphous iron oxyhydroxides which can contain >10% each of Sb and As. Historic disposal of mineralised waste rock material into the stream system at Hillgrove has caused strong contamination of stream sediments with Sb and As. Equilibration of stream water with contaminated stream sediment, as well as additions from erosion of natural outcrops and mine and tailings dam seepages, has led to the main drainage system (Bakers Creek) containing strongly contaminated water (up to 1.8 mg/l Sb and 0.3 mg/l As) for 20 km until its junction with the Macleay River. Environmentally high values of dissolved Sb (and As), are inevitable in waters associated with mesothermal stibnite (-gold) deposits.