Tourmaline geochemistry and cassiterite geochronology of highly evolved tin granites and their hydrothermal systems in eastern Australia
Creators: name: Carr, Patrick
Publisher: name: The Australian National University
Year: 2018
Subjects: cassiterite
tourmaline
granite
tin
magmatic-hydrothermal
economic geology

description: Three models have been proposed for cassiterite (SnO2) mineralisation in magmatic–hydrothermal environments: (1) magmatic crystallisation from a granitic melt, (2) late-stage magmatic partition of Sn into a fluid or vapour phase and subsequent cassiterite deposition, and (3) hydrothermal leaching of Sn from granite and/or country rocks and subsequent deposition. The complex chemistry of the ‘tin’ granites, and the large and pervasive hydrothermal systems which can overprint and destroy primary features make understanding the processes responsible for Sn enrichment difficult. Two new analytical methods were developed. Firstly, a method for the determination of Rb–Sr and Sm–Nd isotopic compositions of magmatic and hydrothermal tourmalines, which can record the compositional evolution of magmas and their hydrothermal fluids. Secondly, cassiterite U–Pb geochronology to constrain the absolute age and duration of magmatic–hydrothermal Sn systems. These data, together with major and trace element compositions of tourmaline, whole-rock geochemistry, quartz δ18O values and zircon U–Pb geochronology are applied to two Sn deposits associated with the Ardlethan and Mole granites of eastern Australia. The geochemical and isotope data of tourmaline show large compositional changes across the magmatic–hydrothermal transition. In the Ardlethan Granite, tourmaline 87Rb–86Sr isotope compositions, which provide robust estimates of 87Sr/86Sr(i) because of their low 87Rb/86Sr, are used to model the assimilation and fractional crystallisation processes that lead to a 30-times enrichment of Sn in residual melts relative to the source rocks. However, caution must be taken with interpreting 87Sr/86Sr(i) tourmaline data as high 87Rb/86Sr of parental melts and fluids can lead to significant in-situ decay of 87Rb prior to tourmaline precipitation. This phenomon is hypothesised for the parental melts of the Mole Granite which due toextreme fractional crystallisation have extreme 87Rb/86Sr of ~900. Subsequently 87Sr/86Sr(i) tourmaline compositions are more evolved the the whole rock composition. The Sn concentration of tourmaline increases from magmatic to hydrothermal settings within the Ardlethan and Mole granites, recording the exsolution of a fluid from a silicate melt. The enrichment of Sn during fluid fractionation, recorded by tourmaline, agrees with experimentally determined melt–fluid partitioning coefficients. Fluid fractionation is the dominant enrichment process for greisen deposits of the Ardlethan Granite, and all deposits of the Mole Granite. Fluid leaching of host rocks is evidenced by convergence of Fe/(Fe+Mg), Sr, 87Sr/86Sr(i) and εNd(i) in hydrothermal tourmaline from the original source rock composition to the host rock composition. At Ardlethan, the host rock of mineralised breccia pipes is enriched in Sn (~50 ppm) and fluid leaching results in an increase of Sn in the mineralising fluids. Although fluid leaching occurs around the Mole Granite, the low Sn concentrations in the host rocks limits Sn enrichment. Melt/fluid-mineral partitioning is a major uncertainty in the interpretation of tourmaline trace element geochemistry. Natural studies performed here provide some constraints, however, more targeted experimental work is required. A new method for U–Pb characterisation of cassiterite by ID-TIMS has provided a matrix-matched reference material for in-situ techniques. However, common-Pb corrections of in-situ techniques remain a large uncertainty in cassiterite geochronology. At Ardlethan, the common-Pb compositions are appropriately estimated by terrestrial Earth models and are more precise than isochron ages. Conversely, the common-Pb associated with the Mole Granite appears variable between a terrestrial Earth composition and a highly evolved composition. Cassiterite U–Pb geochronology of both the Ardlethan and Mole granite mineralisation systems indicate precipitation synchronous with emplacement. The method does not have sufficient precision (~4 % absolute) to distinguish the age of cassiterite precipitation from that of zircon, however, the magmatic–hydrothermal systems of the Ardlethan and Mole granites persisted for a maximum of 4.2 Ma.