Petrography and geochemistry of the pre-Mapedi “bostonite” dykes and sills in the Kalahari Manganese Field, Northern Cape Province

M.Sc. (Geology)Abstract: Mafic dykes and sills intruding and affecting ore deposits are a well-known phenomenon around the world. The role played by magmatic hypogene fluids globally in the genesis of high-grade hematite ore is well-recognized. In the Northern Cape Province of South Africa, a series of intrusions have been emplaced in association with various sedimentary-hosted iron and manganese deposits. In the Kalahari Manganese Field (KMF) of the Northern Cape Province, the intrusions are referred to as “bostonites”.These dykes and sills are pre-Mapedi in age and contact metamorphosed the ore resulting in a general decrease of ore quality. The main aim of the study is to characterize the “bostonite” dykes and sills more extensively than previously by determining whether they are related to the same magmatic event and by evaluating the style of alteration that affected the “bostonite” chemistry. Comparison with the previously studied “bostonites” and the well-known large igneous provinces of the Kaapvaal Craton was made to determine the comagmatism. A paleoweathering profile on the mafic intrusive rocks (so-called “Bostonite”) was intersected during the exploration drilling of the Avontuur Deposit and provides an opportunity to describe the behavior of major, trace and REE in weathered rock relative to the unweathered parent rock. This weathering profile developed below the basal Gamagara/Mapedi unconformity overlying the iron formation (IF) and manganese formation (MF) of the Hotazel Formation and marks the base of Olifantshoek group. The “bostonites” of the Main Kalahari Deposit are composed predominantly of pyroxenes (diopside and augite), plagioclase (labradorite and andesite), Fe-Ti oxides (ilmenite, titanomagnetite) and minor rutile. The “bostonites” of the Avontuur Deposits, on the other hand, are composed predominantly of pyroxene (augite), plagioclase (albite) and Fe-Ti oxides (ilmenite, titanomagnetite, titanite). Based on the geochemistry, “bostonites” of both the Main Manganese and Avontuur Deposits are characterized by low Mg# indicating an evolved nature of the magma, as well as low Cr, Co and Sc signifying fractionation of the mafic minerals within the magma chamber. These mafic intrusives are characterized by negative Sr, Nb and Ta anomaly and positive K and Pb..