THE PHUKET SUPERSUITE, SOUTHWEST THAILAND - FRACTIONATED I-TYPE GRANITES ASSOCIATED WITH TIN-TANTALUM MINERALIZATION

The Phuket Supersuite of southwestern Thailand displays a transition f rom an I- to S-type character that may result from extensive crystal f ractionation of an I-type melt, rather than implying a metasedimentary protolith for the latter. The most evolved granites form large, low-g rade tin-tantalum deposits and are associated with the development of extensive, mineralized pegmatite fields.The less evolved members of th e Phuket Supersuite, including the Khao Prathiu Suite, consist of meta luminous granites (ASI = 0.95-1.00) that contain homblende, allanite, titanite, and magnetite. The Kata Beach Suite granites (ASI = 0.97-1.1 3) contain homblende + 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 serie s biotite +/- muscovite granites (ASI = 1.03+/-1.15) that contain ilme nite, monazite, and tourmaline among the minor mineral phases. The Lam Pi Suite is associated with the development of tin-tantalum pegmatite s. 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 Tos ae Suite consists of biotite-muscovite granites (ASI = 1.05-1.17) that contain garnet, ilmenite, monazite, tourmaline, and cassiterite and i s associated with extensive tin-tantalum pegmatites that range to a ve ry high ASI value (> 1.5). Muscovite-tourmaline and tourmaline-muscovi te granites in the Nok Hook area (ASI = 1.13-1.26) contain topaz, mona zite, rutile, cassiterite, and columbite and form large, low-grade tin -tantalum deposits that are mined by hydraulic techniques. Considerati on of existing isotopic data suggests that the Phuket Supersuite may h ave originated via partial melting of a mafic to intermediate meta-ign eous source of probable Proterozoic age. These source rocks had evolve d to relatively high Sr-87/Sr-86 by the Cretaceous and were capable of yielding partial melts with Sr-87/Sr-86 initial ratios > 0.71. The hi gh initial ratios interpreted for some of the mineralized phases of th e Phuket Supersuite (e.g., Sr-87/Sr-86 initial ratio > 0.14 for the Kh ao Tosae Suite) probably originated through high-level processes invol ving crystal fractionation, hydrothermal alteration, and/or exchange o f Sr with the host metasediments. The conclusions reached in this stud y appear to have application to tin-mineralized granite batholiths in other parts of southeast Asia, particularly to other parts of the Sino burmalaya block. These include especially the Main Range province of M alaysia and the central granite belt of Burma which, in common with th e western belt of Thailand, appear to consist dominantly of fractionat ed and variably reduced I-type granites.