Transport of cassiterite in a Malaysian stream: implications for geochemical exploration

Abstract

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, details of the underlying sedimentological processes remain poorly understood. To address this problem, pit traps have been used to measure transport rates of cassiterite and sediments in the Sungai Petal, a cassiterite-rich stream in Perak, Malaysia. Under base level flow conditions (0.5 m 3 per sec) no bedload sediment is transported. However, after 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 sediment accumulation in the trap exceeds 5 kg per minute. Concentrations of Sn in sediments caught in the trap also increase as discharge and sediment transport rates increase. This increase results from the preferential retention of cassiterite in the bedload as more sediment is transported in suspension with increasing discharge. Because very fine sediment is swept away in suspension at the onset of bed movement, the smallest 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 enriched in Sn under higher flow conditions. This has important implications for geochemical surveys insofar as enrichment of very fine cassiterite 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 conditions, Sn concentrations in these fractions can be expected to exhibit greater variability on the bed of the stream. It is concluded that the very fine sand and silt fractions will provide the best anomaly contrast and longest dispersion trains for elements dispersed in drainage sediments as the principal constituents of heavy minerals.