Mercury in the Bushveld Complex, South Africa, and the Skaergaard Intrusion, Greenland

Abstract

We present mercury concentrations for layered mafic rocks of the Bushveld Complex, the Skaergaard Intrusion, MORB and a Karoo dolerite sill as determined by combustion atomic absorption spectroscopy (combustion-AAS) with gold amalgamation pre-concentration. Mercury in the Bushveld Complex and the Skaergaard Intrusion is generally low, with about 1.1 and 0.8ppb Hg on average, respectively. The Karoo dolerite sill (chill margins) and MORB have average Hg concentrations of 2.5 and 6.4ppb, respectively. The reason for the low whole-rock Hg concentrations in layered mafic intrusions is that the element is incompatible with respect to the cumulus mineral assemblages. Our results show far lower Hg concentrations for the Skaergaard cumulates than previous measurements, and we find no evidence for preferential enrichment in plagioclase-rich rocks as previously reported. We attribute this difference to inaccurate analytical methodology of the early measurements. There are Hg enrichments in distinct levels in both intrusions. Mercury enrichments in the Bushveld Complex are observed near the top of the Lower Zone (2.4ppb), in the Merensky Reef (at 8.6ppb), the Platreef (at 3ppb average), and in the Upper Zone, near the Main Magnetitite Layer (4.7ppb). These enrichments are due to the concentration of Hg by sulfides as suggested by a positive correlation between Cu and Hg. There is no upward increase in Hg with stratigraphic height or positive correlation between Hg and Zr in the Bushveld Complex as would be expected if Hg acted as an immobile, incompatible element. The lack of correlation is explained by periodic degassing of the Bushveld magma chamber, in which volatile elements including Hg and S escaped. In contrast, the Skaergaard Intrusion probably did not degas, and there is Hg enrichment in the Sandwich Horizon (53ppb) because of the combination of the incompatibility and the presence of sulfides.