THE SOUTHEAST-ASIAN TIN BELT

The Southeast Asian Tin Belt is a north-south elongate zone 2800 km lo ng and 400 km wide, extending from Burma (Myanmar) and Thailand to Pen insular Malaysia and the Indonesian Tin Islands. Altogether 9.6 millio n tonnes of tin, equivalent to 54% of the world's tin production is de rived from this region. Most of the granitoids in the region can be gr ouped geographically into elongate provinces or belts, based on petrog raphic and geochronological features. - The Main Range Granitoid Provi nce in western Peninsular Malaysia, southern Peninsular Thailand and c entral Thailand is almost entirely made up of biotite granite (184-230 Ma). Tin deposits associated with these granites contributed 55% of t he historic tin production of Southeast Asia. - The Northern Granitoid Province in northern Thailand (0.1% of tin production) also has domin ant biotite granite (200-269 Ma) but it is distinguished by abundant p ost-intrusion deformation. - The Eastern Granitoid Province extends fr om eastern Peninsular Malaysia to eastern Thailand. The Malaysian part is subdivided into the East Coast Belt (220-263 Ma), Boundary Range B elt (197-257 Ma) and Central Belt (79-219 Ma). The granitoids cover a wide compositional range from biotite granite to hornblende-biotite gr anite/granodiorite and diorite-gabbro. Tin deposits are associated wit h biotite granite in the East Coast Belt (3% of tin production). The g ranitoids in the other areas of the Eastern Granitoid Province are bar ren. - The Western Granitoid Province (22-149 Ma) in northern Peninsul ar Thailand, western Thailand and Burma has biotite granite and hornbl ende-biotite granite/granodiorite. Tin deposits are associated with bi otite granite, which probably is the dominant phase (14% of tin produc tion). The granitoids of the Indonesian Tin Islands (193-251 Ma) do no t permit grouping into geographically distinct units. Main Range-type and Eastern Province-type plutons occur next to each other. Most of th e tin deposits are associated with Main Range-type plutons (28% of tin production). Tin-mineralized plutons are characterized by high concen trations of SiO2, K2O, Rb, Sn, Th and U, whereas the concentrations of Fe2O3, MgO, CaO, Na2O, Ba and Sr as well as the Fe2O3/FeO ratios are low. Tin-mineralized plutons are also distinguished by high initial Sr -87/Sr-86 ratios and low magnetic susceptibilities. Simple veins are t he most abundant type of hydrothermal ore deposit, constituting 52% of all known primary tin-tungsten deposits. Complex infilling-dominated deposits of the sheeted-vein or stockwork type are less abundant (10% of primary deposits). Other types are replacement-dominated deposits ( 24%), tin-tantalum mineralized pegmatites (12%) and breccia deposits ( 1%). Placer tin deposits derived from the primary tin mineralization i n the pre-Miocene basement are the major source of tin. The oldest ter restrial sediments deposited on the extensively peneplaned basement ha ve a Miocene-Pliocene to Lower Pleistocene age. These sediments, which are up to 60 m thick, host the most important placers. They are overl ain by younger alluvial strata or marine sediments, which are up to 25 m thick and usually do not contain minable tin mineralization. Fluvia l allochthonous bottom placers are of major economic importance. Fluvi al residual bottom placers, fluvial above-bottom placers and alluvial- fan placers are also significant. Eluvial, colluvial and littoral plac ers are of minor importance.