THE GEOCHEMISTRY OF WALL ROCK ALTERATION IN TURBIDITE-HOSTED GOLD VEIN DEPOSITS, CENTRAL VICTORIA, AUSTRALIA

Gold deposits hosted in Ordovician to Devonian turbidites in central V ictoria, Australia, consist of steeply dipping quartz vein systems hos ted mainly by reverse faults. Wall rock alteration of the host metasan dstones, metasiltstones and shales (or slates) extends at least 20 m f rom the veins in the Bendigo-Ballarat zone (BBZ) and 10 m in the Melbo urne zone (MZ) deposits. Alteration minerals include carbonates (anker ite, siderite and dolomite) chlorite, sericite, arsenopyrite, pyrite, chalcopyrite and sphalerite, with less common barite, albite and bioti te in the BBZ and carbonates (siderite, ankerite, dolomite and ferroma gnesite), sericite, chlorite, arsenopyrite, pyrite, and stibnite with less common chalcostibite in the MZ. SiO2, Na2O, MgO and Sr with P2O5 commonly decreasing during alteration while CO2, S, As, Sb, Au, V, Al2 O,, Ga, K2O and Rb with Ni and Cr commonly increase. TiO2, FeO, Fe2O3, MnO, Th, U, Nb, La, Ce, and Sc showed little change. Concentrations o f 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 Na2O and MgO could have crystallized in the veins in the very min or albite and chlorite present. The addition of V in all and Ni and Cr in most deposits probably reflects a source enriched in these element s such as underlying greenstones. The source of both the volatile (S, As, Sb) and lithophile (K2O and Rb) elements as well as Au is unknown, but they could have been derived from a magmatic source or from the m etamorphism of Cambrian greenstones. CO2, present as carbonate, was de rived mainly by the reaction of graphite, originally present in the se diments, with the ore solutions. Al2O3, the only other major element a fter SiO2, probably increased mainly due to the decrease of the latter . (C) 1997 Elsevier Science B.V.