TRANSPORT OF CASSITERITE IN A MALAYSIAN STREAM - IMPLICATIONS FOR GEOCHEMICAL-EXPLORATION

It is generally supposed that accumulations of heavy minerals develop on stream beds because heavy minerals are transported more slowly than the sediment as a whole. However, insofar as relative transport rates of heavy minerals and sediments have not been measured in streams, de tails of the underlying sedimentological processes remain poorly under stood. To address this problem, pit traps have been used to measure tr ansport rates of cassiterite and sediments in the Sungai Petal, a cass iterite-rich stream in Perak, Malaysia. Under base level flow conditio ns (0.5 m(3) per sec) no bedload sediment is transported, However, aft er heavy rain storms flows rise rapidly to discharges that exceed 3.5 m(3) per sec. Bedload transport starts at a discharge of approximately 1.0 m(3) per sec and thereafter increases exponentially for all size fractions up to discharge of 2.2 m(3) per sec when the rate of sedimen t accumulation in the trap exceeds 5 kg per minute, Concentrations of Sn in sediments caught in the trap also increase as discharge and sedi ment transport rates increase. This increase results from the preferen tial retention of cassiterite in the bedload as more sediment is trans ported in suspension with increasing discharge, Because very fine sedi ment is swept away in suspension at the onset of bed movement, the sma llest differences between Sn concentrations at low and high flows are found in the fine sand and silt fractions. In contrast, because larger particles of sediment and cassiterite are both initially transported as bedload, the coarse fractions of the stream sediments only become e nriched in Sn under higher flow conditions. This has important implica tions for geochemical surveys insofar as enrichment of very fine cassi terite will occur under even relatively low flow conditions and hence have relatively uniform concentrations on the stream bed. Conversely, because coarse cassiterite will only be enriched under higher flow con ditions, Sn concentrations in these fractions can be expected to exhib it greater variability on the bed of the stream. It is concluded that the very fine sand and silt fractions will provide the best anomaly co ntrast and longest dispersion trains for elements dispersed in drainag e sediments as the principal constituents of heavy minerals.