Volatile exsolution in Western Tasmanian Sn granites: insights from mineral textures and stable isotopes

Western Tasmania is endowed with several world class Sn-W deposits (e.g., Renison Bell, Mt Bischoff, King Island) that are closely associated with Devonian to Carboniferous I- and/or S-type granitic intrusions. Both fertile and barren granites are characterized by abundant tourmaline which commonly occurs in the apical regions of the granites. Field relationships indicate that a range of magmatic-hydrothermal textures occur within the Heemskirk and Pieman Heads granites, and these can be discriminated into four groups based on texture, paragenesis, mineralogy and geochemistry.

Spherical to oblate tourmaline-quartz orbicules are one of the most distinctive features of the Western Tasmanian tin granites. They are most intensely developed proximal to the contacts between different granitic phases, and near contacts with surrounding country rocks. Instead, tourmaline patches are smaller than the orbicules and comprise grey quartz phenocrysts, coarse-grained dendritic tourmaline and K-feldspar. Subvertical tourmaline-quartz veins occur within both granites and locally cross-cut tourmaline orbicules. Tourmaline-filled cavities are common in the apical region of the Heemskirk granite. The cavities contain prismatic tourmaline crystals that radiate outwards from the core of the cavities. They are typically associated with quartz and minor muscovite.

Besides, unidirectional solidification textures (USTs) comprising quartz, K-feldspar, mica, tourmaline and magnetite are well exposed in the roof zones and towards the upper margins of the well-mineralized Heemskirk granite. In contrast, there are not either tourmaline-filled miarolitic textures or USTs cropped out in the unmineralized Pieman Heads granite.

The Heemskirk granite’s magmatic-hydrothermal textures formed at discrete levels in space and time. They are concentrated in the roof zones of the red and white granite phases. In the roof of the white granite, tourmaline patches are situated in a layer below an orbicule-rich layer, which is below a roof zone rich in miarolitic textures; all of them are apparently overlaid by a UST-dominated zone. Locally, subvertical veins have transversed the orbicules, but they have not cut the USTs. The tourmaline veins therefore formed later than the patches and orbicules, although the temporal relationship among other textures is still obscure.

The boron isotope composition (δ11B) of tourmaline ranges from -21.7 ‰ to 4.1 ‰ across the different styles of tourmaline mineralization. The oxygen isotope composition (δ18OSMOW) of tourmaline varies between 6.5 and 14.9 ‰, which is similar to the range in δ18OSMOW value of quartz (5.0 to 16.1 ‰) intergrown with tourmaline. Generally, the δ11B and δ18OSMOW of tourmaline progressively shift to more positive values across the four textural groups.

The magmatic-hydrothermal textures as well as the stable isotopes provide evidence for volatile exsolution from magmas melts during the fractional crystallization of the Heemskirk and Pieman Heads granites. The full suite of textures is preserved in the Heemskirk granite, and is considered to be a favorable sign for mineralization, given the rich endowment of Sn, Cu, and Ag-Pb-Zn deposits associated within the Heemskirk granite. The lack of miarolitic cavities and USTs in the Pieman Heads granite implies lower volumes of hydrothermal fluid exsolution, and may explain the scarcity of mineralization associated with this intrusion.