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Compared with the rain forests of the old shield areas of West Africa and the Amazon Basin, the rain forests of SE Asia are characterized by high relief, active tectonism and volcanism. These features, together with heavy tropical rains, result in the major rivers having some of the highest sediment yields in the world. Smaller streams and rivers, even in areas of only moderate relief, have coarse sand and gravel beds. Frequent tropical rain storms mobilize these sediments and rapidly flush very fine sand, silt and clay from the stream bed in suspension. During this flushing process fine-grained heavy minerals, for example gold and cassiterite, lag behind and accumulate on the bed of the stream. Because the finer ( Logging of tropical rain forests and conversion of land to agricultural use greatly increases rates of soil erosion. Studies of Au anomalies in Thailand indicate that, depending on the extent of the soil anomaly within a catchment basin, this can dilute Au concentrations to a point where the Au anomaly in sediments can go undetected. However, if soil erosion is minimized by re-establishing a cover of ground vegetation, as in mature rubber plantations, fine sediments are again flushed from the stream bed and geochemical anomalies return to more natural conditions with accumulations of heavy minerals on the stream bed. |
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