SHEAR AGGREGATION (CONVECTIVE SCAVENGING) AND CASCADE ENRICHMENT OF PGES AND CHROMITE IN MINERALIZED LAYERS OF LARGE LAYERED INTRUSIONS

Shear driven coagulation/aggregation is a common method to concentrate and separate suspended particulate matter from fluids. Convective bou ndary 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 boun dary 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, sa y, hosting chromitite layers. In the environment of double diffusive c onvection, precipitates of immiscible sulphide liquids in the cooler u pper sections of the magma will be transported downward through unders aturated, hotter layers of melt, to be resorbed and to enrich the lowe r layers, similar in operation to a chemical fractionation cascade. Fu rther cooling secures 1) ore grade levels of concentrates in zones of higher shear at the bottom and 2) supersaturation, assisting in situ s olidification there.