Abstract

Preferential accumulation of minerals with high densities forming placer deposits in longitudinal bars of braided rivers relies on the segregation of this denser material from the quartz and feldspar sands dominating sediment in transport. Flume experimentation conducted on mixtures of quartz, pyroxene, magnetite, and lead sand transported over rough cobble beds demonstrated that the sand distribution was more uniform above the bed than theory predicted. The deviation from theory was inversely proportional to the density of the mineral. Fall velocity was not a good predictor of the amount of a mineral species carried in suspension, as coarse-grained quartz dominated the suspension population in the flume runs where lead was a major constituent in the traction population, even though the quartz had a greater fall velocity. Applying these results to placer deposits indicates that during flood events, even the smaller dense minerals and especially the super-dense minerals will be transported with the gravel and cobble load, whereas the quartz and feldspar will be spatially segregated during transport and temporally and spatially segregated during deposition. This mechanism of placer formation is capable of creating the types of deposits present in orebodies and showings such as the uraniferous paleoplacers at Elliot Lake, Canada, and the gold-bearing longitudinal bars in the Witwatersrand.

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