The Nam Salu horizon at Kelapa Kampit on Belitung Island, Indonesia, is the richest stratabound tin mineralization in Southeast Asia. It belongs to a sequence of Carboniferous-Permian sediments and volcanics which have been intruded by granitoids of Triassic age. The Nam Salu horizon is a steeply dipping, 35-m-thick layer of stilpnomelane-chlorite-(biotite) phyllite which is exposed over a length of 3 km. The horizon carries high-grade cassiterite ore (1-2% Sn) 100 m along strike up to a height of 100 m. This orebody, which is being mined in the Nam Salu open pit, is located symmetrically on both sides of the Khoi-Wa fault which intersects the horizon at an angle of about 45 degrees .The horizon exhibits semicontinuous banding in the 0.2- to 15-mm range which is characteristically defined by its varying iron mineral content (magnetite, pyrrhotite, pyrite, ilmenite, and siderite), averaging 8 vol percent. Eutaxitic textures are preserved in one-third of the rock indicating its derivation from tuff.In the open pit, the horizon contains disseminations of abundant fine-grained (2-200-mu m) cassiterite which is randomly distributed in the felted phyllosilicate matrix. The banded rock is intersected by veinlets with an average thickness of 0.1 mm which postdate the disseminated cassiterite mineralization and constitute approximately 1 percent of the total rock volume. The early-phase veinlets are filled with pyrite, phyllosilicates, siderite, pyrrhotite, quartz, sphalerite, chalcopyrite, magnetite, arsenopyrite, sphene, and cassiterite (in order of decreasing frequency). Late-stage veinlets filled with galena, sphalerite, carbonates, fluorite, and bismuth minerals are subordinate.The Nam Salu open-pit orebody is characterized by high concentrations of Fe (avg 39.95% Fe 2 O 3 total Fe) which vary inversely with SiO 2 , Al 2 O 3 , TiO 2 , and Zr concentrations. The contents of CaO and NaO 2 are very low (avg < 0.2%), but the concentrations of the alkali metals K 2 O (3.98%), Rb (1,270 ppm), and Cs (325 ppm) are high. The chemical composition corresponds to a mixture of iron-rich sediment (iron-formation) and basalt (texturally tuff) which has been altered by metasomatic processes. The hydrothermal fluids which interacted with the host rock produced a strong enrichment in some alkali metals by biotitization together with the introduction of Sn, Be, Bi, F, Nb, Pb, and W as well as a depletion in Na 2 O, CaO, and Sr. The hanging-wall and footwall rocks, which are composed of metasandstone, metasiltstone, and chert, have been much less reactive to hydrothermal fluids than the Nam Salu horizon and are distinguished by their low Sn and Rb concentrations.The alkali (Sn) metasomatism cannot be linked in time to the precipitation of the iron-rich sediment or the basaltic volcanism which deposited the water-laid tuff, since seawater had no or insignificant access to the operating hydrothermal system. This is evident from the high Rb and Cs concentrations as well as from the high ratios Rb/K (0.04) and Cs/K (0.01) in the Nam Salu horizon. The reaction with heated seawater would produce a depletion of Rb and Cs relative to K. Seawater has very low Rb/K and Cs/K ratios because Rb and Cs are preferentially removed from seawater through incorporation into marine sediments. Only residual fluids separating from a granite magma are known to produce rocks which have Rb and Cs concentrations as high as the Nam Salu horizon. The most likely source of the Sn-bearing fluids is granitic magmatism, though its nearest manifestations are the biotite granite outcrops 20 km to the east and 30 km to the west of Kelapa Kampit, respectively.