B- and O-isotopic compositions of tourmaline constrain late-stage magmatic volatile exsolution in Tasmanian tin-related granite systems

The Devonian Sn-mineralized Heemskirk and barren Pieman Heads granites of western Tasmania (southeast Australia) contain abundant tourmaline-rich features, including orbicules, patches, cavities, and veins within their roof zones. The δ¹¹B and δ¹⁸O compositions of tourmaline from these texturally different features range from − 21.7 to + 4.1‰ (average − 4.7 ± 4.0‰, n = 127), and from + 6.5 to + 14.9‰ (average + 10.7 ± 1.8‰, n = 38), respectively. These data suggest that the tourmaline-rich assemblages precipitated mostly from magmatic-hydrothermal fluids derived from their host plutons, mixed with minor external components sourced from metasedimentary, meta-ultramafic rocks, and/or meteoric water. The B-isotopic values increase sequentially from tourmaline patches to orbicules and/or cavities in both granites, probably caused by progressive volatile exsolution and systematic fluxing of aqueous boron-rich fluids from the S-type magmas during emplacement into the shallow crust. High degrees of fractional crystallization of the melt may have caused the exsolution of boron-rich hypersaline fluids from which the tourmaline orbicules and cavities formed. Bimodal δ¹¹B populations (− 21.7 to − 12.7‰ and − 3.9 to + 4.0‰) imply that the granitic melts were mostly sourced from ¹⁰B-rich metapelitic rocks. The granites underwent fluid exsolution during the late magmatic stage that contributed to the widespread development of tourmaline-rich features and also to the formation of tin deposits associated with the Heemskirk Granite.