The Phuket Supersuite, Southwest Thailand; fractionated I-type granites associated with tin-tantalum mineralization

James Cook University, Department of Earth Sciences, Townsville, Queensl., Australia

The Phuket Supersuite of southwestern Thailand displays a transition from an I- to S-type character that may result from extensive crystal fractionation of an I-type melt, rather than implying a metasedimentary protolith for the latter. The most evolved granites form large, low-grade tin-tantalum deposits and are associated with the development of extensive, mineralized pegmatite fields. The less evolved members of the Phuket Supersuite, including the Khao Prathiu Suite, consist of metaluminous granites (ASI = 0.95-1.00) that contain hornblende, allanite, titanite, and magnetite. The Kata Beach Suite granites (ASI = 0.97-1.13) contain hornblende + biotite in less evolved parts, but only biotite in the more evolved parts; minor minerals include allanite, titanite, ilmenite, and tourmaline. The Lam Pi Suite consists of ilmenite series biotite + muscovite granites (ASI = 1.03-1.15) that contain ilmenite, monazite, and tourmaline among the minor mineral phases. The Lain Pi Suite is associated with the development of tin-tantalum pegmatites. The most evolved granites in the Phuket Supersuite include the Khao Tosae Suite on Phuket Island and the Nok Hook granites that form part of the Khao Kata Khwam batholith in the Takua Pa region. The Khao Tosae Suite consists of biotite-muscovite granites (ASI = 1.05-1.17) that contain garnet, ilmenite, monazite, tourmaline, and cassiterite and is associated with extensive tin-tantalum pegmatites that range to a very high ASI value (>1.5). Muscovite-tourmaline and tourmaline-muscovite granites in the Nok Hook area (ASI = 1.13-1.26) contain topaz, monazite, rutile, cassiterite, and columbite and form large, low- grade tin-tantalum deposits that are mined by hydraulic techniques. Consideration of existing isotopic data suggests that the Phuket Supersuite may have originated via partial melting of a mafic to intermediate meta-igneous source of probable Proterozoic age. These source rocks had evolved to relatively high ⁸⁷ Sr/ ⁸⁶ Sr by the Cretaceous and were capable of yielding partial melts with ⁸⁶ Sr/ ⁸⁷ Sr initial ratios > 0.71. The high initial ratios interpreted for some of the mineralized phases of the Phuket Supersuite (e.g., ⁸⁷ Sr/ ⁸⁶ Sr initial ratio > 0.74 for the Khao Tosae Suite) probably originated through high-level processes involving crystal fractionation, hydrothermal alteration, and/or exchange of Sr with the host metasediments...

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