Genesis of a magmatic metamorphic hydrothermal system; the Sn-W polymetallic deposits at Pilok, Thailand

Cassiterite, wolframite, and base metal mineralization in the Pilok area are hosted by veins and stockworks in the apical portion of leucocratic granite stocks, close to their contacts with pelitic metasedimentary rocks. A minimum pressure of emplacement of approximately 1,500 bars is indicated by the presence of two coexisting feldspars in the granites, and a moderate depth is supported by a lack of coeval volcanism and absence of porphyritic textures that characterize hypabyssal intrusions. A contact metamorphic-metasomatic aureole in the surrounding pelitic metasediments consists of tourmaline, biotite, and muscovite zones that are superimposed on muscovite-chlorite zone regional metamorphism. Whole-rock carbon analyses demonstrate a volatilization of graphite in the metasediments as a function of distance from the intrusion, and the presence of andalusite in the aureole constrains the maximum pressure to approximately 3,800 bars. The earliest mineralization is late magmatic to early postmagmatic wolframite + cassiterite intergrown with Li- and F-rich muscovite in pegmatite pods. The bulk of the wolframite mineralization was subsequently deposited in veins with potassic alteration which could have originated either by orthomagmatic or remobilization processes. Finally, cassiterite and sulfides (chalcopyrite-sphalerite) were deposited in association with greisen alteration and formed as a result of the remobilization of tin from the host granite. Fluid inclusion and stable isotope data indicate the involvement of at least four fluid types: a moderately saline fluid of presumed magmatic origin, aqueous-carbonic metamorphic fluids, low-temperature and low-salinity meteoric fluids, and high-salinity formational or metamorphic fluids. The isotopic compositions of waters calculated from delta O ¹⁸ analyses of quartz, muscovite, and cassiterite and delta D analyses of muscovite range from 1.4 to 9.4 and -63 to -118 per mil, respectively; these are consistent with the mixing of magmatic and organic-rich metamorphic fluids. Oxygen isotope thermometry indicates temperatures of 350 degrees to 500 degrees C for cassiterite mineralization. The oldest preserved fluid inclusions are primary aqueous-carbonic inclusions from wolframite and cassiterite mineralization, Both the homogenization of the carbonic phase and the bulk...