The geochemistry of wall rock alteration in turbidite-hosted gold vein deposits, central Victoria, Australia

Abstract

Gold deposits hosted in Ordovician to Devonian turbidites in central Victoria, Australia, consist of steeply dipping quartz vein systems hosted mainly by reverse faults. Wall rock alteration of the host metasandstones, metasiltstones and shales (or slates) extends at least 20 m from the veins in the Bendigo-Ballarat zone (BBZ) and 10 m in the Melbourne zone (MZ) deposits. Alteration minerals include carbonates (ankerite, siderite and dolomite) chlorite, seriate, arsenopyrite, pyrite, chalcopyrite and sphalerite, with less common barite, albite and biotite in the BBZ and carbonates (siderite, ankerite, dolomite and ferromagnesite), sericite, chlorite, arsenopyrite, pyrite, and stibnite with less common chalcostibite in the MZ. SiO 2, Na 2O, MgO and Sr with P 2O 5 commonly decreasing during alteration while CO 2, S, As, Sb, Au, V, Al 2O 3, Ga, K 2O and Rb with Ni and Cr commonly increase. TiO 2, FeO, Fe 2O 3, MnO, Th, U, Nb, La, Ce, and Sc showed little change. Concentrations of Zn, Cu, Pb, and Ca are variable. The relatively large decrease of SiO 2 could account for most, if not all, quartz present in the ore veins. The Na 2O and MgO could have crystallized in the veins in the very minor albite and chlorite present. The addition of V in all and Ni and Cr in most deposits probably reflects a source enriched in these elements such as underlying greenstones. The source of both the volatile (S, As, Sb) and lithophile (K 2O and Rb) elements as well as Au is unknown, but they could have been derived from a magmatic source or from the metamorphism of Cambrian greenstones. CO 2, present as carbonate, was derived mainly by the reaction of graphite, originally present in the sediments, with the ore solutions. Al 2O 3, the only other major element after SiO 2, probably increased mainly due to the decrease of the latter.