The formation of the volcanic-hosted massive sulfide ore deposit at Rosebery, Tasmania

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doi: 10.2113/gsecongeo.76.2.304
Authors:Green, G. R.; Solomon, M.; Walshe, J. L.
Author Affiliations:Primary:
Tasmania Geol. Surv., Dep. Mines, Hobart, Australia
Univ. Tasmania, Australia
Volume Title:special issue on the geology and mineral deposits of Tasmania
Volume Authors:Solomon, M., editor; Eastoe, C. J.
Source:A special issue on the geology and mineral deposits of Tasmania, edited by M. Solomon, J. L. Walshe and C. J. Eastoe. Economic Geology and the Bulletin of the Society of Economic Geologists, 76(2), p.304-338. Publisher: Economic Geology Publishing Company, Lancaster, PA, United States. ISSN: 0361-0128
Publication Date:1981
Note:In English. 95 refs.; illus. incl. 72 anals., 9 tables, geol. sketch map
Summary: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 H2O. 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 δ34S 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 δ34S 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 δ34S values for barite-sulfide pairs indicate a temperature range of 255° to 298°C for the barite orebody, but the temperature of barite ore formation was probably about 250°C.The use of δ34S values for sulfides as stratigraphic markers indicates that the sulfide orebody is diachronous. It is possible to account for the metal and δ34S 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 aO2-) T conditions using the six-component solid solution chlorite model of Walshe and Solomon (1981). The maximum indicated temperature of 300°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 δ34SΣS for the sulfide orebody is probably derived by almost complete reduction of Cambrian seawater sulfate mixed with sulfur leached from underlying rocks. The δ34S 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. [P.Br.]
Subjects:Cambrian; Data; Economic geology; Exhalative processes; Geochemistry; Geologic thermometry; Host rocks; Igneous rocks; Isotopes; Major elements; Massive deposits; Metal ores; Mineral deposits, genesis; Paleozoic; Polymetallic ores; Processes; S-34/S-32; Stable isotopes; Sulfides; Sulfur; Temperature; Trace elements; Volcanic rocks; Australasia; Australia; Rosebery Australia; Tasmania Australia; Ratios
Coordinates:S414600 S414600 E1453300 E1453300
Abstract Numbers:82M/1220
Record ID:1981053274
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists
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