Mercury distribution amongst co-existing silicates within the Bushveld Complex

Thirty separates of plagioclase, orthopyroxene and clinopyroxene from the lower Main Zone of the Northern Limb of the Bushveld Complex were analysed for their mercury contents using combustion atomic absorption spectroscopy with gold amalgamation pre-concentration. The average mercury contents of plagioclase, orthopyroxene and clinopyroxene were found to be 0.9 ppb, 1.2 ppb and 1.1 ppb, respectively. Mercury within the separates does not vary systematically with any of the major element oxides present in the minerals. Based on a positive 1:1 correlation between mercury in orthopyroxene and clinopyroxene, we estimate DOpxHg ≈ DCpxHg, and on this basis, can exclude the presence of significant Hg2+ within the melts from which these minerals crystallised. The lack of correlation between mercury in plagioclase and that in the mafic silicates may suggest diffusional loss of the element from the former during slow cooling under magmatic conditions and better retention of mercury by the mafic silicates under the same conditions. Alternatively and more likely, this lack of correlation may support earlier arguments based on distinct Sr-isotopic disequilibrium between co-existing plagioclase and mafic silicates, that plagioclase and the mafic silicates in the Northern Limb of the Bushveld Complex may have crystallised from different melts within a variably contaminated, sub-Bushveld staging chamber.