Shear aggregation (convective scavenging) and cascade enrichment of PGEs and chromite in mineralized layers of large layered intrusions

Abstract

Shear driven coagulation/aggregation is a common method to concentrate and separate suspended particulate matter from fluids. Convective boundary layers are of primary importance in securing concentration. The theoretical indication is that ore grade PGEs may be scavenged from a primary melt of Bushveld composition and aggregated in convective boundary layers well within the expected lifetime of a magma chamber. The boundary layer dynamics should also secure the observed peculiarities of Bushveld PGE concentration profiles: peaks at top and bottom of, say, hosting chromitite layers. In the environment of double diffusive convection, precipitates of immiscible sulphide liquids in the cooler upper sections of the magma will be transported downward through undersaturated, hotter layers of melt, to be resorbed and to enrich the lower layers, similar in operation to a chemical fractionation cascade. Further cooling secures 1) ore grade levels of concentrates in zones of higher shear at the bottom and 2) supersaturation, assistingin situ solidification there.