The tin-tungsten ore system of Pilok, Thailand

Abstract The tin-tungsten mineralization in the Pilok mining area is centered on apical parts of probably Late Cretaceous alkali feldspar aplite stocks (aplogranite) which intrude earlier K-feldspar megacrystic biotite granite. The aplogranites are of transitional magmatic-hydrothermal origin and have the typical mineral assemblage of quartz-microcline-albite-muscovite-tourmaline-spessartine-fluorite-beryl. Quartz-muscovite-tourmaline alteration is dominant in and along mineralized fractures, followed locally by chloritization. Mineralization is fracture-controlled and consists of stockworks, veins and veinlets with an ore association composed mainly of the metal group AsFeZnCuSnWBi. The gangue assemblage is quartz-muscovite-tourmaline-fluorite ± K-feldspar ± beryl ± apatite. Major and trace element trends in the granitic rocks suggest a magmatic evolution controlled by fractional crystallization. The distribution pattern of Ti vs Ta defines a degree of fractionation of less than, or equal to, 0.1 and follows very closely the Erzgebirge tin-granite suite. Hydrothermal metal redistribution is widespread in the Pilok ore system. Large-scale tin depletion and local tin ore enrichment are complementary features of the same process. Stable isotope data suggest a strong magmatic input into the early fluid system, whereas the disturbed Sr isotope pattern of the Pilok aplogranites indicates external fluid overprint.