Geochemistry, mineralogy and plate tectonic setting of a Late Cretaceous Sn-W Granite from Sumatra, Indonesia

The Hatapang ranite was discovered uring geological mapping and mineral exploration in northern Sumatra by the British Geological Survey in conjunction with the Indonesian Directorate of Mineral Resources. The pluton comprises a two-mica granite which shows significant greisenization a d veining around its margins associated with Sn and W mineralization. An Rb-Sr isochron derived for the pluton indicates an age of 80 Ma and an initial S7Sr/S6Sr atio of 0.7151. This together with major and trace element data show the Hatapang to be of clear S-type affinity. The greisens are quartz mica topaz rocks and are almost otally deficient in Na. Trioctahedral mica compositions progress from biotite through siderophyllite ozinnwaldite during final differentiation and greisenization f the granite. Accompanying dioctahedral micas are phengitic. Associated with late-stage differentiation of the granite is the precipitation of tourmaline and various Nb-Ta oxides. Sn and W mineralization is manifested as cassiterite in the greisens, while wolframite tends to be related to quartz veining. A later and lower temperature sulphide event produced a suite of base metal sulphides and Ag-Bi-Pb sulphosalts. The identification of a Sn-W granite of Cretaceous age in northern Sumatra provides a link with occurrences ofeconomically important Late Cretaceous Sn-W granites in Thailand and Burma and increases the potential of an area which until recently was thought o lie outside the SE Asian tin belt