The geochemistry of vanadiferous magnetite in the bushveld complex: Implications for crystallization mechanisms in layered complexes

Detailed trace-element analyses of pure magnetite from four continuous borehole intersections through the main magnetitite layer from the upper zone of the Rustenburg Layered Suite of the Bushveld Complex are presented. One section has been analysed at one centimetre intervals. Rapid depletion of Cr occurs over short, vertical sequences near the base of the layer, which is due to bottom-crystallization and the resulting chemical depletion of a thin layer of liquid. Sudden increases in Cr content of magnetite are attributed to convection cells which bring undepleted magma into the zone or crystallization. We suggest that these cells individually have lateral extents no greater than hundreds of metres, but collectively may be traced at a specific stratigraphic horizon for several tens of km. This lateral traceability of the effect of convection cells at approximately uniform stratigraphic height demonstrates the long-held implicit assumption that time-planes are in general parallel to the layering, and does not support the hypothesis that layers in the Bushveld Complex grew laterally. The activity of these cells is highly variable, with long periods of quiescence interspersed with periods of rapid, small-scale overturn. Most convection cells do not impinge upon the floor, and the abruptness of the resulting chemical reversal is largely a function of the thickness of the layer of depleted liquid trapped between the cumulates and the sole of the convection cell. Occasionally, these cells do touch the cumulate pile and may even cause erosion. This material may be redeposited elsewhere in the magnetitite layer either as mineralogically distinct fragments if erosion penetrated below the layer or, in the present instance, as a chemically chaotic pile of magnetite. The abruptness of the chemical reversals severely restricts the extent to which post-cumulus redistribution of elements or re-equilibration with percolating trapped liquid (infiltration metasomatism) may have occurred. The appearance of disseminated plagioclase in magnetite layers in variable proportions and in a non-systematic manner in the four profiles is attributed to fluctuations in pressure.