Abstract
Chromites from the LG-3, LG-4, and LG-6 chromitite layers at the Zwartkop mine are investigated and subsolidus behavior of chromite in olivine and orthopyroxene-hosted environments is compared. Chromitite layers LG-3 and LG-6 are orthopyroxene hosted, but layer LG-4 is olivine hosted, with abundant orthopyroxene formed by a reaction relation between olivine and liquid. Chromite has reequilibrated with host silicate to a degree dependent on the nature of the host silicate and the modal amounts of chromite present. In an olivine environment, reequilibration between chromite and silicate proceeds to lower temperatures with consequently more iron-rich chromite than in an orthopyroxene-hosted chromite. In a given environment the closure temperature for silicate-chromite reequilibration decreases systematically as the modal amount of chromite decreases. From this systematic relation the solidus temperature can be estimated and some inferences about the original chromite composition can be made. In the LG-6 layer, chromite compositions can be projected back to solidus compositions along a trajectory of constant activity of the MgAl 2 O 4 component, and at the solidus temperature, chromite composition within and adjacent to the chromitite layer is essentially the same. The activity of MgAl 2 O 4 remained constant because chromite was in equilibrium with the host orthopyroxene throughout the temperature range in which reequilibration took place. In layer LG-4 the activity of MgAl 2 O 4 increased during cooling because the solidus chromite was not in equilibrium with orthopyroxene formed by reaction between olivine and liquid. Because the liquid which reacted with olivine was more aluminous than the liquid from which the olivine originally crystallized, subsolidus reequilibration between orthopyroxene and chromite led to an increase in the concentration of Al 2 O 3 in chromite. Consequently the decrease in Mg content of chromite during cooling is accompanied by an increase in Al content, in contrast to the more common decrease in Al content observed in layer LG-6, which results from the higher stability of the MgAl 2 O 4 and FeCr 2 O 4 components relative to their reciprocal counterparts, MgCr 2 O 4 and FeAl 2 O 4 . In layer LG-3, chromites follow a trend similar to layer LG-6, although the trend is displaced slightly in the direction of the layer LG-4 trend. The presence of a thin olivine-bearing layer above the LG-3 chromitite layer attests to the possibility of some degree of disequilibrium between orthopyroxene and chromite.Estimated solidus temperatures increase upward from layer LG-3 to LG-4 then decrease sharply to layer LG-6. The increase in solidus temperature at layer LG-4 is attributed to the injection of ultramafic liquid into the pyroxenitic liquid already in the chamber. Partial mixing between the ultramafic and pyroxenitic liquid was accompanied by crystallization of olivine and chromite, but unmixed pyroxenitic liquid, with a lower solidus temperature remained in place below the olivine-chromite cumulate horizon. Sinking of olivine-chromite clots into the underlying liquid resulted in the formation of orthopyroxene by reaction between olivine and the pyroxenitic liquid. Chromites in layer LG-6 probably formed by mixing between liquids, with the injected liquid possibly of gabbroic composition.Reequilibration with respect to oxygen fugacity appears to have been comprehensive, for Fe (super +2) /Fe (super +3) ratios within and adjacent to a chromitite layer are essentially constant for all the layers examined.
