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

摘要

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 sul-fide-arsenide mineralization, dominated by pyrrhotite, arse-nopyrite, 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-1oellingite+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 hyper-saline, high temperature, CO_2-poor, and H_2S-poor.