GEOCHEMICAL AND SEDIMENTOLOGICAL EVIDENCE OF THE PALAEOPROTEROZOIC OCEANIC ARC-BACKARC DEPOSITIONAL ENVIRONMENT OF THE GRANITES-TANAMI OROGEN, AUSTRALIA

The Palaeoproterozoic Granites-Tanami Orogen in central Australia is an important orogenic gold province. Gold mineralisation is hosted by structurally-controlled epigenetic quartz veins with a suggested mineralisation age of ~1805 and ~1795 Ma associated with granitic intrusions. The quartz veins are emplaced in the Tanami Group, which consists of mostly lower-greenschist metamorphic grade volcaniclastic-sedimentary rocks and basalts. The Tanami Group depositional age is between ~1900 and 1850 Ma, and was deposited in an oceanic back-arc basin setting. Due to lack of exposure and the complex structural setting, the stratigraphic evolution of this basin is poorly understood, and regional stratigraphic correlations are difficult. In this presentation, we present sedimentological interpretations together with newly developed geochemical tools for regional-scale correlation. High field strength elements (HFSE) were used to classify basalts in different study areas and are interpreted to be geochemically affiliated with either backarc basin (Supplejack area) or volcanic arc (Capstan and Bluebush areas). Massive pyroclastic flow deposits and interflow sedimentary rocks have a mafic geochemical signature close to a basaltic composition of intermittent basalt flows. In contrast, thinly bedded to laminated volcaniclastic-sedimentary rocks have an increasingly more andesitic to dacitic composition and are overlying the massive mafic-derived successions. It is interpreted that the change in composition is related to changing igneous composition in a maturing volcanic arc. Therefore, we can use the geochemical composition for the volcaniclastic-sedimentary rocks of this succession as relative age and depositional environment indicator in a deformed, partly preserved oceanic basin succession that is poorly exposed and lacks drill core data.