SUPERGENE AND HYPOGENE HEMATITIZATION EVENTS ASSOCIATED WITH PALEOPROTEROZOIC IRON AND MANGANESE ORES, SOUTH AFRICA

Paleoproterozoic orebodies in the Northern Cape Province of South Africa show a variety of enrichment mechanisms, many of which are related to chemical conditions of the Earth?s developing atmosphere and hydrosphere. Both the giant Sishen-type iron orebodies and Kalahari manganese deposits are associated with hematitization events, amenable to characterization through paleomagnetic study. Our recent work has documented distinct paleomagnetic directions from three stages of hematite growth within the Kalahari manganese field, during early diagenesis, late diagenesis/low-grade metamorphism, and higher-grade metasomatism. The latter event , spatially associated with a prominent set of high-angle faults, produced high-grade Wessels-type orebodies at about 1250-1100 Ma, coincident with deformation within the adjacent Namaqua belt. A multistage hypogene origin for the Kalahari manganese ores fits the paleomagnetic and petrographic data better than other proposals such as a primary volcanic-exhalative model with Mn sourced from the immediately underlying Ongeluk lavas. High-grade Kalahari manganese orebodies thus cannot be used to constrain the Paleoproterozoic atmosphere-hydrosphere system. In contrast, the Sishen and Beeshoek iron mines show a strong relationship to a paleoweathering surface and ferruginous paleosol, which has been noted as the oldest well-developed oxygenated laterite at ~2100 Ma. Our paleomagnetic conglomerate test, performed on rounded fluvial pebbles of Fe ore in the directly overlying red mudstones of the Gamagara Formation, demonstrates convincingly that hematitization preceded erosion and transport of the Fe ore, and therefore validates previous suggestions that the Sishen-type hematite enrichment is supergene and can be used to estimate minimum Paleoproterozoic atmospheric oxygen levels.