Tourmaline geochemistry and cassiterite geochronology of highly evolved tin granites and their hydrothermal systems in eastern Australia [article]

Patrick Carr, University, The Australian National, University, The Australian National
2019
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
more » ... nd 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 composi [...]
doi:10.25911/5d11f878dddff fatcat:zhw4awg5wbdk3j5u5ba24a2jiy