Bar to bank flow convergence zones: a contribution to the origin of alluvial placers

To better understand the processes that give rise to alluvial placer deposits, magnetite-rich areas along several South African rivers were surveyed and sampled to test whether heavy-mineral concentrations form in zones of converging fluid flow. The sites selected are along the Oliphants and Steelpoort rivers near Burgersfort in the N Transvaal. The focus of each site was a sluiceway formed between a stable bank and the parallel margin of a bankward-migrating sand bar, a zone characterized by convergence of bar-top and sluiceway currents. It was established that sites formed by such conditions are likely locations for concentrating heavy minerals only if the site is mature, i.e. if converging flow is maximally constricted and bar migration has effectively ceased. Observations suggest that magnetite enrichment occurs during attainment of maximum allowable flow construction in the narrow sluiceway. Dimensions of such alluvial deposits are limited only by the scale of the system. The general characteristics of alluvial deposits formed by these processes are summarized.-J.E.S.