MODELLING THE ROLE OF CARBONATE ASSIMILATION ON THE STABILITY OF SPINELS: A FOCUS ON FLATREEF CHROMITES, BUSHVELD COMPLEX, SOUTH AFRICA

The Critical Zone-correlative Flatreef of the northern limb of the Bushveld Complex is emplaced in direct contact with footwall quartzite, shales, dolomite and BIF of the Transvaal Supergroup. There has been recorded petrological and geochemical evidence for magma-footwall interaction in the footwall assimilation zone (FAZ) but its influence on ore formation remains Assimilation of carbonate material can have a significant effect on the crystallization path and mineral assemblages of contaminated magmas and could potentially result in an increase in oxygen fugacity thus triggering crystallization of spinels^1,2. While many researchers have examined the effect of wall-rock contamination on spinels of the Bushveld Complex, no work has attempted to measure the recorded fO₂ and how their stability and composition varies with contamination.

Assimilation-fractional crystallization modelling of Critical Zone (CZ) melt with ~30 % dolomite predicts a shift in the liquid line of descent towards assemblages dominated by spinel+olivine+clinopyroxene±plagioclase. The results correspond to assemblages found in the FAZ, which comprise high proportions of disseminated spinels compared to least-contaminated units. The Cr-spinels of the FAZ have low contents of Cr (~20 wt.%) and high V (~1300 ppm), compared to Cr spinels in least contaminated units that have, on average, higher Cr (~40 wt.%) and lower V (~1100 ppm). The V/Cr ratio in spinels from the FAZ correspond to a shift in fO₂ from NN0-0.3 to NNO+1.

Application of a simple mixing model of CZ parental melt with 30 % dolomite, predicts a drop of Cr in spinels from 44 to ~20 wt.% and an increase in V from 1100 to 1300 ppm. Therefore, a key finding of this study is that local dolomite assimilation has a potential to induce fO₂ (up to NNO+1.5) thus triggering abundant spinel crystallization. However, the solubility of Cr in the melt will increase with contamination in the melt, thus producing Cr-poor spinels.

1. Ganino, C.et al. (2008).Miner. Depos. 43, 677–694
2. Hill, R. & Roeder, P. (1974).J. Geol. 82, 709–729