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. Attributing one of these models to a deposit is difficult due to the complex chemistry of the ‘tin’ granites and the large and pervasive hydrothermal systems inherent in their formation that can overprint and destroy evidence of the primary metal source, and enrichment processes. Research into the mineralisation typically focusses on either the magmatic or hydrothermal processes and a whole of system understanding is lacking.
To address the lack of a whole system models, we have developed new methods for chemical separation and determination of Rb-Sr and Sm-Nd isotopic compositions of tourmaline. Tourmaline is an ideal mineral for study in magmatic-hydrothermal Sn systems because it precipitates in both environments and tourmaline chemistry is often touted as a passive monitor of both fluid and melt compositions. However, the applicability of isotope systematics of tourmaline to deciphering fluid and melt compositional evolution is unexplored.
We applied these new techniques to two Sn deposits in eastern Australia associated with the Ardlethan and Mole granites. Granites of the Lachlan and New England orogens are an ideal location for this study as they have been subject to extensive Rb-Sr and Sm-Nd isotopic study since the inception of the I- and S-type granite classification by Chappell and White.
Tourmaline isotopic and trace element data from these deposits provides new insights into magmatic, transitional and hydrothermal enrichment processes of Sn systems. This includes a more rigorous characterisation the source rocks and tectonic settings of specialised ‘tin’ granites and a more complete understanding of processes occurring over the magmatic to hydrothermal transition.