New Zealand Journal of Geology and Geophysics abstracts
Composition and mineralogy of historic gold processing residues,
east Otago, New Zealand
D. Mains
D. Craw*
Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand
dave.craw@stonebow.otago.ac.nz
*Author for correspondence.
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.
Keywords gold; arsenic; mine waste; scorodite;
bukovskyite; arsenopyrite; slag; sulfide; acid rock drainage; Otago
Schist
G05005; Received 25 January 2005; accepted 7 July 2005; Online
publication date 10 October 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48:
641–647
0028–8306/05/4804–0641 © The Royal Society of New Zealand 2005
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