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Journal of the Geological Society

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Indonesian fluvial cassiterite placers and their genetic environment

International Institute for Aerospace and Earth Sciences (ITC), Kanaalweg 3, 2628 EB Delft, The Netherlands

Placer valley systems in the areas of Belitung and Singkep islands and in the Tujuh archipelago, Indonesia, are described with the use of geological and contour maps, longitudinal sections and grade distribution charts. The main cassiterite concentration (>95% of all minable cassiterite) in these deposits rests directly on weathered bedrock. Cover-layer (Miencan) cassiterite concentrations occur at various levels, indicating continued upstream erosion and downstream sedimentation of cassiterite. The valley bottoms currently reach depths below mean sea level of ~50 m; greater depths, down to –100 m, are recorded on sonic sections and can be deduced from wildcat drill-holes downstream from the mined areas. All offshore and large parts of the onshore placers are palaeoplacers, which have been protected against erosion by marine, littoral and paludal sediments. The stratigraphical succession of this cover has been established in the Tujuh area, but a definite dating has not yet been achieved. The collected data indicate a concentration process on the bottoms of fluvial valleys, which at that stage contained only a thin sediment layer that was composed mainly of coarse, angular clastics and heavy minerals. The distribution of cassiterite on the valley floors is locally determined by morphology—at the lowest parts, in local depressions and behind obstacles—whereas on a larger scale it indicates a largely autochthonous or residual provenance, with maximum fluvial transportation distances of ~500 m. In contrast, many of the W Malaysian cassiterite placers are alluvial concentrations and found at various levels within the relatively fine-grained sedimentary valley fill.

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