LAUSR

The origin of chromitites in the Bushveld Complex has been attributed to two principal mechanisms: (1) gravity-controlled settling of chromite onto the chamber floor from magma that was saturated in chromite, either initially or owing to some internal process; or (2) gravityand size-controlled separation of chromite from coexisting olivine and orthopyroxene within crystal-rich slurries, either formed directly within the chamber or brought into the chamber from some deep staging reservoirs. Here we present field observations from potholes, roughly circular structures in which footwall rocks were removed by magmatic erosion, that rule out both approaches. A key observation is that chromitites drape the irregular margins of potholes, even where they are vertical or overhanging. These relationships eliminate both early settling of chromite from the overlying magma and late mechanical segregation of chromite within cumulates as viable hypotheses. In addition, thick chromitites commonly consist of several texturally and compositionally distinct sublayers that are locally separated by thin partings of silicate rocks. The absence of thick sequences of intervening silicate rocks from which chromite may have been separated to form these sublayers refutes an origin from crystal slurries. Transgression of chromitite–orthopyroxenite units by hanging-wall rocks excludes the origin of chromitites from crystal slurries that intrude as late-stage sills into pre-existing cumulates. The field relationships appear to be compatible only with the emplacement of superheated, dense magma along the temporary base of the chamber that led to intense melting and dissolution of the preexisting floor cumulates, followed by the in situ crystallization of chromite directly on the irregular chamber floor. Chromitites of differing thicknesses are produced, according to the volume of the multiple replenishments. This model involving in situ crystallization of basal layers of magmas, which are saturated with chromite on cooling in the chamber, can be extended to explain the origin of chromitites in other layered intrusions.