Composition and mineralogy of historic gold processing residues, east Otago, New Zealand

Abstract

Small quantities (<1 m3) of arsenic-rich ore processing residues were piled outside the Golden Point battery, east Otago, by historic gold miners immediately before mining ceased. The residues originally contained concentrates (c. 10-fold) of gold-bearing pyrite and arsenopyrite, with abundant scheelite, and contain up to 10 wt% arsenic and 20 wt% iron. These concentrates were roasted to oxidise the sulfide minerals and release their contained gold. The gold was extracted by mercury amalgamation, and the residues are now enriched in mercury (up to 0.1 wt%) compared to original ore. Oxidation of the material has resulted in formation of crystalline scorodite (FeAsO4.2H2O) and bukovskyite (Fe2[AsO4][SO4][OH].7H2O), and amorphous iron oxyhydroxide. These three oxidised minerals have been locally (millimetre scale) dissolved and reprecipitated in the surficial environment, causing cementation of some residues. Minor relict pyrite remains in the residues, and oxidation of this maintains acid conditions (pH 2.2–3) in the residues. The scorodite and bukovskyite have been remarkably chemically stable in this acid oxidised environment over the >60 yr since the residues were abandoned. As long as the acidification continues, scorodite will remain stable. © 2005 Taylor & Francis Group, LLC.