Magmatic-hydrothermal volatile exsolution and mineralisation in Tasmanian Sn granites

Large volumes of granites were emplaced across Tasmania in the mid-Palaeozoic. Many of them have distinctive magmatic ‚Äö- hydrothermal features, and some of them produced world class Sn-W deposits. In eastern Tasmania, the weakly metaluminous George River Granodiorite, Grant Point Granite and Mt Pearson Granite have zircon U-Pb ages ranging from 405 to 396 Ma, and intruded prior to the Tabberabberan Orogeny. The peraluminous Coles Bay Granite has a U-Pb age of 388 ¬¨¬± 7 Ma, emplaced simultaneously with the Tabberabberan Orogeny in Tasmania at ~390 Ma. Granites in western Tasmania are moderately to strongly fractionated, including the Housetop, Meredith, Pine Hill, Heemskirk and Pieman Heads. They intruded from 374 to 360 Ma, after the Tabberabberan Orogeny. Initial Pb isotopic compositions of K-feldspars from the Palaeozoic granites vary from 17.638 to 20.658 (\\(^{206}\\)Pb/\\(^{204}\\)Pb), 15.549 to 15.739 (\\(^{207}\\)Pb/\\(^{204}\\)Pb) and 37.903 to 38.940 (\\(^{208}\\)Pb/\\(^{204}\\)Pb), defining a narrower range than the corresponding whole-rock Pb isotopes. These isotopic results, combined with previous studies, suggest that differentiated granites in Tasmania were strongly contaminated by crustal rocks, and that western Tasmanian granites had a crustal source with different isotopic characteristics to that of eastern Tasmania. The tin-mineralised granites in Tasmania (e.g., the Housetop, Meredith, Pine Hill and Heemskirk granites) formed in a post-collisional extensional setting, a favourable environment for the production of Sn-rich melts from the lower crust. Prolonged fractional crystallisation, low oxygen fugacity, and enrichments of volatiles were crucial factors that promoted Sn enrichment in magmatic ‚Äö- hydrothermal fluids exsolved from these felsic magmas. Distinctive tourmaline- and quartz-rich magmatic ‚Äö- hydrothermal features characterise the Heemskirk and Pieman Heads granites of western Tasmania. They include tourmaline-rich patches, orbicules, miarolitic cavities, veins, and unidirectional solidification textures (USTs). Tourmaline cavities and USTs were only observed in the Heemskirk Batholith, and not in the Pieman Heads Granite. These textural features occur in discrete layers in the roof zone of granitic sills within the Heemskirk and Pieman Heads granites. Tourmaline patches in the Heemskirk Granite occur below a tourmaline orbicule-rich granitic sill. Tourmaline-filled cavities have typically developed above the tourmaline orbicules in the White phase of the Heemskirk Granite. Both the tourmaline orbicules and cavities commonly occur below the UST layers. Tourmaline-quartz veins were developed throughout both granites, locally cutting tourmaline orbicules, cavities or USTs. Scanning electron microscope-backscattered electron images recognise three types of compositional zones in tourmalines from the western Tasmanian granites: (1) oscillatory zoning, (2) concentric zoning, and (3) radial zoning. The tourmalines are mostly schorl (Fe-rich) and foitite, with an average end-member component of schorl\\(_{45}\\) dravite\\(_6\\) tsilaisite\\(_1\\) uvite\\(_0\\) Fe-uvite\\(_3\\) foitite\\(_{31}\\) Mg-foitite\\(_4\\) olenite\\(_{10}\\). Element substitutions within tourmaline were controlled by FeMg\\(_{-1}\\), \\(^Y\\)Al\\(^X\\)‚Äöv±¬8(R\\(^{2+}\\)Na)\\(_{-1}\\), and minor \\(^Y\\)AlO(R\\(^{2+}\\)OH)\\(_{-1}\\) (where R\\(^{2+}\\) = Fe\\(^{2+}\\) + Mg\\(^{2+}\\) + Mn\\(^{2+}\\)) exchange vectors. Tourmalines from the Heemskirk Granite are enriched in Fe, Na, Li, Be, Sn, Ta, Nb, Zr, Hf, Th, and rare earth elements relative to the tourmalines from the Pieman Heads Granite, but depleted in Mg, Mn, Sc, V, Co, Ni, Pb, Sr, and most transition elements. These results imply that bulk compositions of the host granites exerted a major control on the chemical variations of tourmalines. There is a progressive decrease of most transition and large ion lithophile elements, and a gradual increase of most high field strength elements in tourmaline grouped from tourmaline patches, through orbicules and cavities, to veins. Trace element ratios (e.g., Zn/Nb, Co/Nb, Sr/Ta and Co/La) and Sn concentrations in tourmaline can distinguish the productive Heemskirk Granite from the barren Pieman Heads Granite. Scanning electron microscope-cathodoluminescence analyses reveal that different types of CL textures developed in quartz from the tourmaline-rich features, USTs and Pb-Zn veins in the Heemskirk and Pieman Heads granites. CL-bright quartz cores are typically cut by dark to gray CL patches, CL-dark streaks and healed fractures, offset by cobweb-like networks and jigsaw puzzle pieces, and/or overprinted by gray to bright CL growth zones. Al, Li, Ti, Na, K, Fe, Ge, and Rb are the most abundant trace elements in quartz. Trace element substitutions in quartz were mainly controlled by the [AlO\\(_4\\)/M\\(^+\\)][Si\\(^{4+}\\)]\\(_{-1}\\) (M\\(^+\\) = H\\(^+\\), Li\\(^+\\), Na\\(^+\\), K\\(^+\\), Rb\\(^+\\)) vector. Trace elements in UST quartz are consistent with those in tourmaline orbicules and cavities, whereas aplitic quartz intergrown with UST quartz has similar trace element contents to quartz in tourmaline patches from the Heemskirk Granite. Ge/Ti and Al/Ti ratios of quartz in the Heemskirk Granite (0.01‚Äö-1.0 and 3.0‚Äö-100) define wider ranges than those from the Pieman Heads Granite (0.01‚Äö-0.2 and 2.0‚Äö-306), suggesting a higher extent of fractional crystallisation of the Heemskirk Granite. High Sb concentrations (up to 100 ppm) in quartz may be an indicator of low temperature base metal mineralisation related to granitic intrusions. Melt and fluid inclusions observed in tourmaline patches, orbicules, cavities and/or veins provide unambiguous evidence for their precipitation from magmatic ‚Äö- hydrothermal fluids. Liquid-rich (type I), vapour-rich (type II) and halite-bearing (type III) fluid inclusions have been identified in quartz from these tourmaline-rich textural features. Microthermometric measurements show that fluid inclusions have homogenisation temperatures and salinities ranging from 156¬¨‚àû to 460¬¨‚àûC and 2 to 15 wt % NaCl equiv (type I), 334¬¨‚àû to 550¬¨‚àûC and 6 to 8 wt % NaCl equiv (type II), and 170¬¨‚àû to 530¬¨‚àûC and 31 to 56 wt % NaCl equiv (type III). Combined microthermometry and Ti-in-quartz geothermometry demonstrate that tourmaline patches, orbicules and cavities formed at temperatures of 500¬¨‚àû to 565¬¨‚àûC and lithostatic pressures of 0.6 to 1.3 kbars (depth of 2.8 to ‚Äöv¢‚Ä¢ 5 km). Tourmaline veins formed at 310 ¬¨¬± 20¬¨‚àûC and 400¬¨‚àû to 410¬¨‚àûC for the Heemskirk and Pieman Heads granites, respectively, at hydrostatic pressures of 0.1 to 0.3 kbars (depth of ca. 1 km). Pb-Zn quartz veins from the Heemskirk Granite precipitated under lower temperature-pressure conditions (280 ¬¨¬± 40¬¨‚àûC; hydrostatic pressure of 80 bars). In situ SIMS analyses show that boron isotopic compositions (˜í¬•\\(^{11}\\)B\\(_{Tur}\\)) of tourmaline range broadly from ‚Äö-21.7 ‚ÄövÑ‚àû to +4.1 ‚ÄövÑ‚àû. Oxygen isotopes (˜í¬•\\(^{18}\\)O\\(_{Tur}\\)) of tourmaline vary between +6.5 ‚ÄövÑ‚àû and +14.9 ‚ÄövÑ‚àû, similar to the ˜í¬•\\(^{18}\\)O range of quartz (˜í¬•\\(^{18}\\)O\\(_{Qtz}\\) = +5.0 ‚ÄövÑ‚àû to +16.1 ‚ÄövÑ‚àû) intergrown with tourmaline. Temperature-corrected boron and oxygen isotopic fluid compositions indicate that tourmaline-rich assemblages were precipitated from magmatic ‚Äö- hydrothermal fluids derived locally from their host intrusions. In combination with fluid inclusions data, the boron and oxygen isotopic variations illustrate that decompression, decreasing temperature, and/or mixing with external water could all have played positive roles in the generation and precipitation of the tourmaline-quartz-rich features. Six initial \\(^{87}\\)Sr/\\(^{86}\\)Sr compositions of tourmaline from the Heemskirk and Pieman Heads granites range from 0.719525 to 0.750317, indicating that their parental magmas were strongly contaminated by continental crust. Both boron and oxygen isotopic compositions of tourmaline increase sequentially from patches to orbicules and cavities in the Heemskirk and Pieman Heads granites. These isotopic and trace element variations in tourmaline and quartz among different tourmaline-rich textures are interpreted to have been caused by volatile exsolution and fluxing of aqueous boron-rich fluids that separated from the granitic melts during the emplacement of S-type magmas into the shallow crust (4 to 5.5 km). Volatile-rich hypersaline fluids separated from the crystallising aluminosilicate magmas due to liquid immiscibility, ascending and coalescing between grain boundaries of the crystallising melts via tube plumes and/or spanning clusters. Rayleigh fractionation modelling indicates that approximately 60 % to 77 % of the boron was removed from the initial felsic melt to produce tourmaline patches, whereas higher degrees of fractional crystallisation of the initial granitic melt (up to 90 %) led to exsolution of boron-rich hypersaline fluids from which the tourmaline orbicules and miarolitic cavities precipitated. The tin-mineralised Heemskirk and barren Pieman Heads granites from western Tasmania were emplaced simultaneously around 365 Ma. They have similar geochemical signatures and tourmaline-rich textural features. More abundant evidence for volatile exsolution (e.g., tourmaline-filled miarolitic cavities and USTs), a higher degree of fractional crystallisation, a lower redox state and higher volatile element contents characterised the Heemskirk Granite, and are considered to have made the Heemskirk Batholith to be highly fertile, producing world class Sn deposits and numerous Pb-Zn-Ag veins.