TRANSPORT AND DEPOSITION OF CASSITERITE BY A MALAYSIAN STREAM

Low abundances of ''heavy'' heavy minerals, such as cassiterite and go ld, in natural sediments make it difficult to observe and monitor the sedimentological processes that lead to the development of their place r deposits in fluvial channels, We have overcome this problem by insta llation of a pit trap in a Malaysian stream that contains abnormally h igh concentrations of cassiterite. Rates of accumulation of cassiterit e grains up to 500 mu m and sediment up to 32 mm have been measured un der changing discharges during a flood event, Transport-equivalent siz es of sediment and cassiterite are obtained, and the critical shear st resses to mobilize different sizes of sediment and cassiterite are est imated from their rating curves. Rates of accumulation of both cassite rite and sediment vary systematically with grain size and changing dis charge. Mobilization of fine (< 180 mu m) cassiterite requires higher shear stresses than like-sized sediment, and once mobilized this cassi terite is transported at velocities similar to those of sediment appro ximately three times coarser-roughly 50% greater difference in size th an would be expected on the basis of hydraulic equivalence, For larger grains the critical shear stresses required to mobilize sediments and cassiterite increase, but also converge, so that near the upper limit of grain hiding, both the critical shear stresses and transport-equiv alent sizes of cassiterite and sediment are most similar, This converg ence Likely results from increased exposure to stream flow facilitatin g grain entrainment and motion, regardless of grain density, as the gr ains become too large to hide in the bed, Results emphasize the import ance of grain size and bed roughness, as well as differences in densit y, in developing fluvial concentrations of heavy minerals, In particul ar, repeated grain settling, entrainment, and motion is most effective in concentrating heavy minerals that are sufficiently fine to be full y hidden in the bed.