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The mining of placer deposits of tin has been the major metal mining industry of Southeast Asia and for almost a century has dominated world tin production. Economic cassiterite placers in Southeast Asia may be grouped into three broad categories: residual or 'klllil' deposits formed essentially ill.,illl without major lateral transport or sorting of the minerals. washed out or elutriated residual 'kaksa' deposits where the coarser and heavier minerals remain close to source while the finer and lighter minerals are removed by water, and transported or 'mintjan' deposits where the heavy minerals. after varying degrees of transport, are trapped and retained in a suitable sedimentary environment. The cassiterite recovered from different types of placer deposits show characteristic size analyses which reflect the interaction between the original primary form of the mineral and the processes leading to the final workable concentration. Cassiterite from k IIlit deposits accurately reflects the original sizing. Large amounts. ofcoarse (+ 12 mesh BSS) and fine ( 200 mesh BSS) cassiterite may be present and no clear peak size need be present. Kaksa deposits show a marked depletion in the finer sizes., a sharply peaked size distribution and a rapid shift downwards of the peak size in the direction of transport. Mintjan placers show a lack of both plus 36 mesh and minus 200 mesh cassiterite, a very slow downward decline in the peak size and a close correlation between the average size and the grade of ;111 alluvial. It is clear that cassiterite coarser than about 40 mesh BSS resists fluvial transport and tends to accumulate at or close to source. that medium-fine tin may be carried long distances before final entrapment. but that cassiterite finer than 200 mesh is lost from the fluvial system. Below 40 mesh cassiterite appears to undergo little mechanical breakdown during transport while (except in mass flows of material) plus 40 mesh cassiterite experiences little if any transport. The form and size of cassiterite in primary concentrations is closely related to the geological type ofthe deposit. and certain tinfields are characterized by certain types of primary mineralisation (Hosking. 1973; 1974 and 1979). It follows therefore that different tinfields may have very different ratios of kaksa:mintjan placers and that this ratio may be predicted from a consideration of the gross geology of the field and a study of its primary tin occurrences. The Phuket-Takuapa area of Thailand is characterized by high tantalum cassiterite derived largely from pegmatites and yields essentially kaksa placers with little or no transport of the cassiterite away from the source granites. The West Coast Tin Belt of the Malay Peninsula and the island of Bangka contain a wide diversity of primary ore types all of which are closely related to granite contacts. 'In well studied areas such as the Kuala Lumpur Tinfield total tin tonnage is roughly equally divided between kaksa placers located on or close to the granite contact, for example in the Puchong Valley. and mintjan placers up to 25 kilometres away. as in the Selangor Deeps. A substantial part of the primary tin mineralisation in the East Coast Tin Belt and in Belitung is exogranitic and characterized by fine cassiterite. Placers of all types are smaller and poorer. with the best kaksa placers being developed on or near the mineralised granites. as at Gambang. where mineralisation suitable for kaksa formation occurred.

Some thoughts on the development of the alluvial tinfields of the Malay-Thai Peninsula