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The pyrite-gold association in dolomites and carbonated metavolcanics from the Turk Mine, Zimbabwe: the link between late shearing and gold introduction

Economic Geology Research Institute, University of the Witwatersrand, Private Bag 3, PO WITS, 2050, South Africa, e-mail: 065liza{at}cosmos.wits.ac.za

Gary Stevens

Department of Geology, University of Stellenbosch, Private Bag X1, 7602 Matieland, South Africa, e-mail: gs{at}land.sun.ac.za

Wojciech J. Przybylowicz

Van de Graaff Group, National Accelerator Centre, PO Box 72, Faure 7131, South Africa, e-mail: przybylowicz{at}nac.ac.za

In this study, micro-PIXE analyses were used to investigate pyrite geochemistry, and in particular the gold-pyrite association in the vicinity of Turk Mine, Bubi greenstone belt, Zimbabwe. The sample material consisted of pyrite from sulphidised, Fe-rich portions of a carbonate sequence developed to the east of Turk Mine, and from the quartz + carbonate + pyrite altered metabasalt that constitutes the main gold deposit at Turk Mine. In the carbonate sequence, pyrite is developed in sedimentary breccia horizons and occurs as poikiloblastic euhedral crystals, pseudomorphic replacements of Fe-rich sedimentary features, and as broken or abraded clasts. In contrast, a single euhedral pyrite generation was identified in the metavolcanic samples that have been locally cataclastically deformed in shear bands. Pyrite ³⁴S values from the carbonate sequence vary as a function of the dominant pyrite textural variety present in the samples and are interpreted to indicate a mixed source of sulfur. Most samples have ³⁴S values close to 0 , and this is interpreted to reflect a magmatic sulfur source. However, a small proportion of the samples have ³⁴S values that range from +4 to +16. These values are interpreted to reflect a component of sulfur derived from dissolved seawater sulfate. The samples with the highest ³⁴S values contained a high proportion of poikiloblastic euhedral pyrite (> 40 modal prop.). Elemental maps of all pyrite generations in both rock types were obtained via PIXE analysis. These were complemented by nuclear microprobe point analyses in selected areas of the mapped pyrite crystals. The distribution of As and other elements confirmed the petrographic interpretation of several different pyrite generations in the carbonate sequence. Gold concentrations in the pyrites analysed from the carbonate sequence were too low for gold introduction in this rock type to be positively correlated with a particular pyrite generation, although a tentative case can be made for gold enrichment in early-developed As-rich zones within pyrite. In the altered metabasalts of the Turk ore deposit, gold introduction can be shown to be the product of late fluid influx along micro-shears that post-date the formation of the quartz-carbonate-pyrite alteration assemblage and that were responsible for the brecciation of the originally euhedral pyrite crystals.