Mineralogy, geochemistry and genesis of the post-Gondwana supergene manganese deposit of the Carletonville-Ventersdorp area, North West Province, South Africa

摘要

The post-Gondwana period is characterized by some of the unique geologic events which shaped the present geomorphological landscape in various parts of the world. One of the most important and unique features associated with this geologic period is the formation of high valence state (Mn4+) manganese oxides in weathering crusts of uplifted landscapes, underlain by manganiferous protores. The study of these manganese oxide minerals has proven not only to be industrially beneficial but they also provide insights into the evolution of the paleo surfaces, especially in tropical regions. Here we present new results on mineralogy, geochemistry and genetic model of the North West Manganese Deposit in the General Nice lease area. The General Nice ore deposit represents a near surface accumulation of supergene manganese wad and nodule deposit in a weathering crust underlain by manganiferous dolostones of the Transvaal Malmani Subgroup. The ore formation is attributed to in situ dissolution, leaching and surficial weathering of the underlying manganiferous dolostones of the Malmani Subgroup and carbonaceous protore of the Waterval Saprolite in a typically oxidic lacustrine setting. The combined processes led to the release and mobilization of Mn rich colloidal particles (sols) into the overlying water column of the depository, which later precipitated as manganese nodules upon interaction with the alluvial sediments which acted as substrates and centers of accretion. Mineralogical studies conducted on both scanning electron microscope (SEM -EDS) and X-ray diffraction (XRD) indicated the predominance of the manganese oxides; romanechite, cryptomelane and galaxite as major manganese oxide phases and pyrolusite, vernadite occurring in minute quantities. Diagenetic features indicate alteration of pyrolusite and its replacement by romanechite. This is evident from the romanechite veinlets cross-cutting the fractured groundmass of pyrolusite. Galaxite appears to represent the latest stage of authigenic mineral precipitation, and occurs mostly around the pore-spaces. Other mineral components include hematite/goethite, clays, muscovite, silica and inclusions of ilmenite and detrital zircon. These accessory minerals occur either in the form of nuclei grains or inclusions in manganese oxide cement.