The origin and fate of antimony in the ore stream from the Globe-Progress mine, Reefton and Macraes, New Zealand

Milham, Lisa Anne.

Antimony commonly accompanies gold in mesothermal mineralization systems and is a useful indicator of mineralization in exploration programmes, similar to arsenic. Antimony is toxic in the environment, so the fate of antimony in the ore is of considerable environmental significance. Despite the importance of antimony, little is known about its distribution and environmental behaviour, in comparison to arsenic and gold, for example. Oceana Gold (NZ) Ltd's Globe-Progress mine, Reefton comprises a curvilinear shear zone which is host to mesothermal gold-bearing quartz lodes. The Main Lode in Globe-Progress Pit contains a distinct mineralized zone, the Globe-Progress shear zone, which locally hosts antimony in the form of stibnite (Sb2S3). Two discrete phases of mineralisation have resulted in the formation of five different ore types in the Globe-Progress shear zone. Unmineralised Greenland Group greywacke and associated metamorphic quartz veins have undergone an initial pulse of Au, As and S mineralisation to form low grade mineralised Greenland Group and hydrothermal quartz veins. Both ore types form the outer regions of the shear. A second mineralisation phase resulted in the addition of Sb, Au, As and S and the formation of cataclasite and quartz breccia from mineralised host rock and hydrothermal quartz veins respectively. Both ore types define the shear zone at Globe-Progress. The presence of euhedral sulfides in the cataclasite and late-stage, undeformed stibnite veins which infill the breccia imply that the second phase of mineralisation transpired both during and after cataclasis and brecciation. Antimony deposition is greatest in the central cataclasite and locally in the quartz breccia where stibnite veins are present, decreasing with distance from the shear zone. The Globe-Progress shear zone is structurally and mineralogically different from the nearby General Gordon deposit. Globe-Progress contains abundant cataclasite and elevated antimony concentrations, whilst the General Gordon mineralisation style is similar to the outer mineralised host rock at Globe-Progress, with low grade sulfide dissemination and low 111 Sb concentrations. Golden Bar, Macraes in comparison displays a more ductile style of deformation, with lesser cataclasite, in relation to the dominant brittle mechanisms operating at Globe-Progress. Since 2007, Reefton sulfide concentrate has been transported to the Macraes gold mine, east Otago for further refinement. Stibnite forms a small portion (2-3%) of the Reefton ore concentrates. Prior to Reefton introduction, the Macraes mine processing and environmental issues focused on arsenic, as antimony is present only at low levels at Macraes ( ~ 1 0 ppm Sb ). Reefton concentrate initially enters the autoclave at the Macraes processing plant, independent of Macraes ore. Stibnite becomes oxidized in the autoclave and readily dissolves into solution. Minor concentration of Sb occurs in scales in the Macraes autoclave (up to 1.2 wt %). Rapid cooling after pressure-oxidation causes Sb precipitation with concentrations up to 2. 7 wt % Sb in discharged material. Sb in Reefton ore residues are diluted by a ratio of approximately 100:1 by voluminous Macraes ore residues that have low Sb. Resultant tailings have low bulk Sb (~250 ppm). Despite minor Sb in the tailings at Macraes, some of the Sb must reside in solution, as precipitates from Macraes tailings waters are weakly elevated in Sb (up to 130 ppm). These precipitates are dominated by iron oxyhydroxide, and Sb is adsorbed to the surface of this precipitate. Sb is undetectable in Macraes discharge waters and occurs at low concentrations, <0.48 ppm Sb, in reeds.

xvii, 171 leaves : col. ill., maps 30 cm

2008Milham

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