University of Tasmania, Australia

Magmatic-hydrothermal volatile exsolution and mineralisation in Tasmanian Sn granites

The release of volatiles from a crystallising melt is fundamental to the formation of many magmatic-hydrothermal ore deposits. ‘Slushy’ textures indicative of the magmatic-hydrothermal transition, such as unidirectional solidification textures (USTs) and miarolitic cavities, together with other features such as tourmaline orbicules, provide evidence for fluid exsolution and concentration of volatiles in the roof zones of granitoids across the spectrum of magmatic-hydrothermal ore deposits, from Sn-W to Cu-Au systems.

Devonian granitoids in Tasmania contain many textural features indicative of phenomena associated with the magmatic-hydrothermal transition, including world-class examples of UST textures and abundant tourmaline orbicules. These granitoids are associated with world-class tin and tungsten deposits (e.g., Renison Bell, Mt Bischoff, King Island) and zoned mineral districts that contain numerous small Pb-Zn-Ag veins e.g., Dundas and Zeehan mineral fields).

Globally, research into the magmatic-hydrothermal transition is focussed primarily on oxidised magmatic systems (porphyry Cu deposits). Reduced (Sn-granite) systems have received considerably less attention in recent years, but scientifically these systems are just as significant as porphyries in terms of their potential to provide insights into the processes of fluid exsolution and mineralisation.

This PhD study will contrast processes of fluid exsolution and mineralisation in fertile granites (e.g., Heemskirk) and apparently infertile (e.g., Pieman Heads Granite), and will involve the application of the following techniques:

• Detailed mapping and sampling of key locations from Tasmanian Sn granites where magmatic-hydrothermal textures are well-exposed
• Cathodeluminescence imaging of quartz textures in USTs, orbicules and miarolitic cavities to resolve fluid pathways and the history of hydrothermal mineral deposition and dissolution during granite crystallisation
• Fluid (and melt?) inclusion analyses of primary inclusions using microthermometric, LA-ICPMS and PIXE techniques to resolve the fluid compositions and P-T-X evolution of hydrothermal fluids associated with Sn granites

The new technologies to be applied in this project have not previously been used in studies of Tasmania’s Sn granites. This project will therefore build on the previous generation of deposit-based studies, to enhance our understanding of magmatic-hydrothermal phenomena both from the Tasmanian and global context.