The Middle Triassic Tanjungpandan batholith on Belitung Island is associated with major alluvial tin ore deposits. The batholith consists of two petrogenetically different, ilmeniteseries rock suites: an areally dominating biotite granite suite and a quartz syenite suite of more restricted extent. The granite suite has a low-pressure minimum-melt composition; the syenite suite covers a large compositional field from gabbroic cumulate rocks to alkali-feldspar granite pegmatite. Both rock suites show extended fractional crystallization trends. Trace element patterns are partially disturbed by fluid interaction. Extremely low tin contents (mean Sn concentrations: 2-4 ppm) seem to be related to hydrothermal removal of tin by high-temperature, low f (sub O 2 ) fluids; the unusually low Fe (super +3) /Fe (super +2) ratios of the granitic rocks allowed an exceptional degree of hydrothermal redistribution of tin. The tin distribution pattern clearly distinguishes the Tanjungpandan situation (and that of neighboring tin granites of the "Tin Islands") from other major tin granite suites in the world.

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