Differences in composition of chromites from low-Ti and high-Ti picrites of the Emeishan Large Igneous Province and comparison with chromites of the UG-2 platinum-deposit of the Bushveld complex

Abstract

Chromite is among the first minerals to crystallize from mantle derived magmas and one of the last to be consumed during partial melting of the mantle. Chromite is also an important mineral in major ore deposits of Cr and Pt. The composition of chromite could, therefore be of use in interpreting the petrogenetic conditions during partial melting of the mantle, crystallization of primitive magmas and formation of Pt and Cr ore deposits. However, most mafic rocks contain very little chromite and post-crystallization processes such as re-equilibration during cooling, metamorphism and weathering could change the composition. The composition of chromites from high- and low-Ti picrites from the Emeishan large igneous province have been determined to assess the degree to which the chromite compositions reflect the melt compositions. Aluminium, Sc, Ti, Ga, Nb, Sn, Hf and Ta concentrations in the chromites do appear to reflect the melt compositions in that they correlate with the whole rock compositions and have empirical partition coefficients similar to those determined in experiments. The V contents of both types of picrite are similar, but concentrations of chromites from high-Ti picrites are higher than those of low-Ti picrites. This can be explained if, in the high-Ti picrite more of the V was in the V3+ state (which can more readily substitute into chromite than V4+ or V5+) than in the low-Ti picrite. This implies that fO2 was lower for high-Ti picrites than low-Ti picrites. Concentrations of elements with a 2+ charge, Mg, Mn, Fe, Co, Ni and Zn are different in chromites included in olivine and chromites in the matrix and appear to have re-equilbrated. The compositions of the UG-2 chromites from the World's largest Pt deposit show some similarities with the chromites from the high-Ti picrite, but are depleted in Sc and Ti and enriched in Ga and Al.