Chromite and PGE-enriched layers in the Bushveld are modelled on a process of the cooling of resident lower zone magma by subsequent magmas parental to the lower critical zone, upper critical zone and main zone, respectively. The hot lower zone magma is considered to be derived from a mantle diapir which halted in the lower crust. Flattening of the diapir led to melting of the lower crust and formation of the lower critical zone magma. Cumulates from this crustal melt were remelted to produce the upper critical zone magma. The main zone magma is a higher-level melt of more hydrous crust.