Abstract

The Rosebery pyritic zinc-lead-copper-silver-gold orebody occurs in dominantly felsic volcanic rocks of Cambrian age in western Tasmania. Ore formation occurred in a marine environment following eruption of a thick pyroclastic unit consisting mainly of welded, ashflow tuffs, and subsequent subsidence. The main orebody consists of a discontinuous massive sulfide horizon and in the southern part of the mine there is a generalized metal zonation with a lower and central Cu-Fe-rich zone surrounded and overlain by Zn-Pb-Ag-rich ore. Barite-sulfide ore occurs as separate lenses higher in the sequence separated from the sulfide orebody by unmineralized, cleaved siltstone.The orebody is underlain by an extensive zone of alteration marked by depletion of Na and Sr and enrichment in Rb, K, Mg, Mn, and H 2 O. The pyrite and chalcopyrite content of the altered footwall tuff and the Co content of the pyrite is highest beneath the Fe- and Cu-rich zones in the sulfide orebody and a feeder is inferred in this area.The delta 34 S values for sulfides in the sulfide orebody are higher in the richest Zn-Pb-Ag ore (>14.1ppm) than the Fe-Cu ore (>7.8ppm), a pattern paralleled by a decrease in the Fe/Fe + Mg + Mn ratio of chlorite and a decrease in the FeS content of sphalerite. The delta 34 S values for sulfides in the barite ore are even higher (14.5 to 19.8ppm) and the FeS content of sphalerite lower. Barite throughout the barite and sulfide orebodies is fairly constant between 34.6 and 41.2 per mil, but that in the sulfide ore appears to be epigenetic. The delta 34 S values for barite-sulfide pairs indicate a temperature range of 255 degrees to 298 degrees C for the barite orebody, but the temperature of barite ore formation was probably about 250 degrees C.The use of delta 34 S values for sulfides as stratigraphic markers indicates that the sulfide orebody is diachronous. It is possible to account for the metal and delta 34 S distribution by assuming that the ore solutions are buoyant on reaching the sea floor during the Fe- and Cu-rich phases but show reversing buoyancy during the Pb-Zn-rich phase.The coexisting arsenopyrite-pyrite-chlorite-quartz assemblage in the sulfide orebody defines a range of aO (sub 2 (super -) ) T conditions using the six-component solid solution chlorite model of Walshe and Solomon (1981). The maximum indicated temperature of 300 degrees C is consistent with the minimum temperature required by iron and copper solubilities. However, the chlorite compositions may be postdepositional, as seems to be the case for the FeS contents of sphalerite. The delta 34 S (sub Sigma S) for the sulfide orebody is probably derived by almost complete reduction of Cambrian seawater sulfate mixed with sulfur leached from underlying rocks. The delta 34 S values for the barite orebody may represent partial reduction of seawater sulfate.The ore fluid is supposed to have been generated by convective circulation of seawater in a rock pile heated by a Cambrian granitoid pluton.

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