Assimilation of dolomite by Bushveld Magmas in the Flatreef : implications for the origin of Ni-Cu-PGE mineralization and the Precambrian atmosphere

The formation of the Platreef in the northern limb of the Bushveld Complex remains unresolved, as does the stratigraphic correlation of the northern limb to that in the wider Bushveld Complex. This is largely due to intense contamination by country rocks modifying the stratigraphy. However, the Flatreef, located down-dip of the Platreef, is relatively less contaminated and offers an opportunity to evaluate the potential correlation between the different limbs of the Bushveld Complex. The present study, focussing on 2 drill cores intersecting the Flatreef (UMT081 and UMT094), has utilised geochemical, isotopic and geochronological data to characterise the Flatreef. Modelling the Flatreef using Mg-Nd-O isotopes indicates that the lower portions of the Flatreef in particular have been affected by contamination. The upper portions have been much less affected, showing important lithological and compositional similarities to the UCZ of the WBC. The present study has also provided the first high-precision U-Pb single-zircon geochronology data for the Flatreef, with ages of purported Merensky Reef overlapping with those of the WBC. In addition, the geochronological data show that the Bastard and UG2 reefs are younger than their hangingwall rocks, supporting a model of out-of-sequence sill emplacement in the Flatreef. The origin of PGE mineralization in the Flatreef remains less clear. Well mineralized sills such as the Bastard Reef imply sulphides were entrained in magmas prior to emplacement. However, elevated grade where the northern limb intrudes dolomitic floor rocks suggests contamination by calcareous sediments and mineralization are linked. In addition, it is likely that the assimilation of calcareous sediments by Flatreef magmas in addition to devolatization of calcareous sediments and dolomites in the wider metamorphic aureole of the Bushveld Complex released significant volumes of CO2 into the Precambrian atmosphere and potentially played a role in the global Shunga Event, whose age coincides with that of the Bushveld Complex.