ABSTRACT

A necessary condition for placer development is selective sorting at the grain scale by size and density. The effects of differential entrainment, suspension, and transport on an initial distribution composed of medium-size quartz and 10% fine-size magnetite were modeled by solving the Einstein bedload function for specific grain friction velocities (U^*) in the range 3-63 cm·sec^-1 and bed roughnesses of 0.55, 2, 5, and 10 mm. For any value of U^* and for both mineral densities, the transport rate for all sizes in the initial distribution decreases with increasing roughness, the decrease being greatest in the finer sizes. The concentration of magnetite transported in the flow increases with increasing U^* for each roughness and decreases with increasing roughness for each U^*. The settling velocity ratio of magnetite to quartz in transport decreases with increasing U^* for any value of roughness. For any value of U^*, the ratio first decreases then increases with increasing roughness, ranging from 1.36 at low U^* and roughness to 0.76 at high U^* and intermediate roughness. These results are due to variations in the reactive angles of grains and the extent to which grains hide in the lower, inner zone of the boundary layer. Concentrations of heavy minerals at the bed, bar, and system scales are explained using these results.