Abstract

Tikus is the most important granite-hosted primary tin-tungsten deposit on the tin island, Belitung. The bulk of the ore is located in greisen zones which enclose very irregularly shaped bodies composed almost exclusively of quartz + or - cassiterite + or - wolframite. A small volume of ore is also located in albitized granite which grades into albitite.Detailed geochemical studies indicate that the greisen and albitite were formed by alteration of a K feldspar megacrystic medium-grained biotite granite. Greisenization produced an enrichment in Sn (avg 3,000 ppm), W (1,110 ppm), Fe 2 O 3 , MnO, Bi, Cs, Cu, F, Li, Pb, Rb, and Zn, as well as a depletion of Na 2 O, CaO, and Sr. Albitization is characterized by an enrichment in Na 2 O, CaO, Al 2 O 3 , Cu, F, Pb, Sr, and Zn, as well as by a depletion of K 2 O, MgO, SiO 2 , and Ba. However, Sn (2,100 ppm) and W (914 ppm) were concentrated only with moderate albitization (albitized granite) whereas extreme albitization (albitite) is characterized by low concentrations of these metals.CO 2 -H 2 O(-CH 4 -salt) fluid inclusions with highly variable CO 2 /H 2 O ratios are the most abundant type in the quartz of the greisen as well as in the coarse-grained quartz + or - cassiterite + or - wolframite bodies enclosed in it. They homogenize in the 200 degrees to 340 degrees C range and indicate minimum trapping pressures of 0.7 to 2 kbars. CO 2 -(CH 4 ) inclusions with densities of 0.53 to 0.99 g/cm 3 without a visible H 2 O phase are less common. Aqueous inclusions without a visible CO 2 phase are subordinate in greisen and enclosed quartz bodies (0.5-8 equiv wt % NaCl) but are the only type observed in albitized granite (1-18 equiv wt % NaCl). The CaCl 2 /NaCl ratios are high in the albitized granite (up to 1:1); the homogenization temperatures are in the 150 degrees to 355 degrees C range.It is assumed that an evolutionary relationship exists between greisenization and albitization. Na and Ca were released through fluid-silicate reactions during greisenization and the fluids lost CO 2 (-CH 4 ) with falling temperature. The resulting Na-Ca-enriched aqueous fluids deposited albite and fluorite by replacement reactions in the granite surrounding the greisen. The major deposition mechanism for cassiterite and wolframite was pH increase and concurrent temperature decrease in both greisen and moderately albitized granite (with secondary muscovite and topaz). Strong albitization (albitite) did not increase the fluid pH, and therefore, did not precipitate cassiterite or wolframite.

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