The Granites gold deposits, Northern Territory, Australia: evidence for an early syn-tectonic ore genesis

The ore deposits of The Granites goldfield are shear-hosted within Palaeoproterozoic amphibolite facies metasedimentary rocks in the Tanami Region, Northern Territory, Australia. The ore bodies are located within a 5- to 35-m thick sequence of steeply dipping unit of metamorphosed iron-rich metasedimentary rocks. Deformation at The Granites was complex and is characterized by five successive deformation phases (D(1-5)). Shear veins (central and oblique) are the dominant type of vein geometry, with minor development of extensional veins and reverse-fault related veins. Four generations of syn-tectonic veins, corresponding to D(1), D(3), D(4), and D(5), have been recognized and are comprised of quartz, quartz-carbonate, calc-silicate, and calcite. In addition, two generations of disseminated sulfide-arsenide mineralization, dominated by pyrrhotite, arsenopyrite, and loellingite, with minor pyrite, chalcopyrite and rare marcasite, formed syn-D(1) and syn- to post-D(3). Textural and structural evidence indicates deposition of gold was contemporaneous with the syn-D(1) veins and sulfide-arsenide mineralization. Four hydrothermal phases are proposed for the formation of the veins and disseminated sulfide-arsenide assemblages. The first phase (D(1)) was responsible for transport and deposition of the majority of the gold. Minor remobilization and deposition of gold occurred during the D(3) and D(4) phases. Little is known about the nature of the D(1) ore fluid, although a relatively low sulfur content is indicated by the assemblage pyrrhotite-arsenopyrite-loellingite+rare pyrite. The growth of amphibolite facies metamorphic minerals andalusite and almandine garnet during D(1) indicates a high temperature for the fluid. The D(3) hydrothermal phase coincided with peak metamorphism. D(4) fluids were hypersaline, high temperature, CO(2)-poor, and H(2)S-poor.