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The distribution, mineralogy, and geochemistry of precious metals in the north-end orebody, Rosebery mine, Tasmania. Final report. (CODES publication 3)

Huston, DL and Large, RR 1986 , The distribution, mineralogy, and geochemistry of precious metals in the north-end orebody, Rosebery mine, Tasmania. Final report. (CODES publication 3).

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Study of the spatial and mineralogical distribution of precious metals in the Rosebery north-end orebody indicates that both silver and gold concentrate in the upper parts of massive sphalerite-galena-pyrite ore and
barite mineralization. Both metals tend to decrease in grade towards the fringes of the sphalerite-galena-pyrite ore, with highest concentrations above pyrite-chalcopyrite rich lenses which probably represent localized vents. However, both metals may concentrate at the fringes of
mineralization in areas of low base metals, and gold has several other occurrences. Remobilization of precious metals during or after the Tabberabberan orogeny produces interesting mineralogies but no economic concentrations of precious metals in the north-end orebody. Remobilization may be more important in the south end. Mineralogically, silver occurs in fahlore minerals and argentiferous galena. Microprobe analyses of fahlore minerals indicate an increase of silver content with antimony and that remobilized fahlore minerals are nearly pure tetrahedrite with high silver contents up to 26 percent. The abundance of silver in galena was not investigated although previous workers (Henley and Steveson, 1978) indicated that it is about 800 ppm. Other silver-bearing minerals are unimportant. Six styles of gold mineralization were recognized in this study: (1) in massive zinc-lead ore (the dominant occurrence), (2) in massive barite mineralization, (3) in the upper parts of pyrite-chalcopyrite pods (in general pyrite-chalcopyritepods are poor in gold), (4) in distal pyrite mineralization (at the edges of sphalerite-galena-pyrite mineralization and the overlying host rocks), (5) in footwall mineralization (poorly sampled and little understood), and (6) in remobilized quartz-carbonate veins. Gold in distal pyrite mineralization may be an exploration target in Rosebery and in other massive sulfide systems. Gold occurs as electrum or as free gold dominantly associated with pyrite in sphalerite-galena-pyrite ore and distal pyrite mineralization. In the sphalerite-galena-pyrite ore, gold may also occur in tetrahedrite pools, while in the barite mineralization, gold may be associated with chalcopyrite, galena or by itself. The gold has been mobilized locally into cracks and inclusions in pyrite grains; most of the gold grains have average dimensions of less than 25 microns. This explains the poor mill recoveries and the high concentrations of gold in pyrite from the tails.
Statistical treatment of diamond drill hole assays and additional trace element analyses indicates two dominant metal assemblages: zinc-lead-silver and copper-iron. Cadmium has a strong association with the zinc-lead-silver assemblage, bismuth correlates well with copper, and arsenic correlates with iron. A subsidiary metal assembalage that probably relates to barite mineralization is barium-silver-antimony. Graphical analysis of data from sphalerite-galena-pyrite ore suggests two trends of gold mineralization in this ore: a relatively low gold-moderate silver-high base metal trend and a high gold-low copper trend. The two trends may be related to the mechanisms of gold transport and deposition.
The transport of gold at Rosebery is predominantly as a bisulfide complex. Upon mixing with seawater, gold is precipitated due to dropping temperature and the oxidation of the hydrothermal fluids. Mixing with seawater may also lead to partial oxidation of sulfide in the hydrothermal fluids to produce polysulfide ions, which may complex with gold to produce the second high gold-low copper trend and other styles of gold mineralization (e.g. in distal pyrite and massive barite).

Item Type: Report (Technical Report)
Authors/Creators:Huston, DL and Large, RR
Publisher: University of Tasmania
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© University of Tasmania 1986

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