The geology and gold deposits of the Victorian gold province

Abstract

The Palaeozoic succession of Victoria represents a major world gold province with a total production of 2500 t of gold (i.e. 78 million oz). On a global scale, central Victoria represents one of the most gold mineralized areas outside the Witwatersrand of South Africa, and remains the prime example of a ‘slate belt’ gold province (also known as ‘turbidite-hosted’, or ‘shale-greywacke’ gold province). Production came from quartz veins (1000 t), modern placers (1200 t) and palaeoplacers (300 t). Up to 7000 mines produced gold from quartz veins; however, the 168 mines of this type which produced over 1 t Au contributed 68% of the primary gold production. Twelve goldfields have each produced 30 t Au (approximately one million ounces) from all sources, with Bendigo (697 t), Ballarat (408 t) and Castlemaine (173 t) being the largest: these twelve goldfields have contributed 71% of the combined primary and secondary production. Gold mining commenced in 1851, declined dramatically around 1914, and has not increased significantly since. The Palaeozoic succession of Victoria is part of an extensive fold belt along much of eastern Australia (i.e. the Tasman Orogenic Belt). In Victoria, the succession is dominated by Cambrian to Early Devonian clastic metasedimentary rocks that have undergone major upright deformation and low-grade metamorphism, culminating in the Middle Devonian (Tabberabberan) Orogeny. North-south inliers of Cambrian metavolcanic- and metasedimentary rocks associated with regional fault zones divide the province into several geologically distinct zones. In central Victoria, there are particularly well-developed acid volcanic complexes of Late Devonian age, and synchronous peraluminous granites which represent high-temperature crustal melting. Tertiary to Quaternary basalts have covered some of the Palaeozoic gold deposits, and have also covered several rich gold placer deposits of Cainozoic age. Larger goldfields, including Bendigo and Ballarat, occur in Ordovician slate and greywacke sequences, but a wide range of metamorphosed sedimentary and igneous rock types are host to goldfields. Most, if not all, primary deposits demonstrate structural control, although the controlling structural features differ between deposits. Wallrock alteration is strongly influenced by host rock: in metasedimentary rocks, it is limited and subtle, but it is more pervasive in mafic and felsic igneous rocks. Carbonates, muscovite and pyrite are the most widespread alteration minerals in these rocks, representing CO 2, K and S addition. Enrichment of As within gold deposits is common, with Sb important in some small deposits, whereas base metals are only abundant locally, and Bi, W, Mo and Te show strong spatial association with granites and are rarely associated with economic gold deposits. Granites are an important component of the Palaeozoic succession in Victoria. No large deposits are within granites, but a few goldfields, including Maldon (65 t Au), are within contact aureoles of either S or I type granites of differing degrees of fractionation and silica content. A single-stage, but protracted and possibly diachronous, period of gold introduction near the time of the Middle Devonian (Tabberabberan) deformation can explain geological relationships at many gold deposits. The possibility that this event was slightly older (i.e. Silurian or Early Devonian) in the west of the state cannot be precluded from the available evidence. The mineralizing event appears linked to the same thermal event as diorite (‘lamprophyre’) dykes and graite intrusions, acidic and basaltic volcanism, deformation and regional metamorphism. The limited amount of fluid chemistry data from Victorian gold deposits suggests low-salinity fluids which contained CO 2, and variable methane, with homogenization temperatures from 150 to 300°C. The sulphur isotopic compositions of pyrite are mostly +1 to +5‰, with some values slightly above + 10‰ Mineralizing temperatures are inferred to be near 300°C, and pressure around 1–2 kbar. Sulphide-rich gold deposits in the east of Victoria, many of which contain significant base metals, contrast with the rest of the province (which contains mostly gold deposits with lower sulphide content), and this eastern area should possibly be viewed as separate from the Victorian gold province, per se. The benefits to Victorian gold mining coming from the 1980's global gold boom have been insignificant compared to the historical importance of the Victorian gold province. There seems little justification for assuming that all the large deposits have been discovered, and there is reason to believe new deposit styles may emerge to complement the limited current exploration activity which is focussed on the search for extensions and repeats of known deposit types. A major advance in scientific understanding of these oft neglected deposits could kindle a major exploration and production revival.