ABSTRACT

Low abundances of "heavy" heavy minerals, such as cassiterite and gold, in natural sediments make it difficult to observe and monitor the sedimentological processes that lead to the development of their placer deposits in fluvial channels. We have overcome this problem by installation of a pit trap in a Malaysian stream that contains abnormally high concentrations of cassiterite. Rates of accumulation of cassiterite grains up to 500 µm and sediment up to 32 mm have been measured under changing discharges during a flood event. Transport-equivalent sizes of sediment and cassiterite are obtained, and the critical shear stresses to mobilize different sizes of sediment and cassiterite are estimated from their rating curves.

Rates of accumulation of both cassiterite and sediment vary systematically with grain size and changing discharge. Mobilization of fine (< 180 µm) cassiterite requires higher shear stresses than like-sized sediment, and once mobilized this cassiterite is transported at velocities similar to those of sediment approximately three times coarser--roughly a 50% greater difference in size than 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-equivalent sizes of cassiterite and sediment are most similar. This convergence likely results from increased exposure to s ream flow facilitating grain entrainment and motion, regardless of grain density, as the grains become too large to hide in the bed. Results emphasize the importance of grain size and bed roughness, as well as differences in density, in developing fluvial concentrations of heavy minerals. In particular, repeated grain settling, entrainment, and motion is most effective in concentrating heavy minerals that are sufficiently fine to be fully hidden in the bed.