Nodular organic matter in granites: implications for the origin of "kerogen" in the Witwatersrand Basin, South Africa

Exploration & Mining Geology, Vol. 3, No. 3, 1994

L.J. ROBB, Department of Geology, University of the Witwatersrand, Republic of South Africa. P. LANDAIS CREGU et CNRS, France. P.M. MEYER, Department of Geology, University of the Witwatersrand, Republic of South Africa. D.W. DAVIS, Jack Satterly Geochronology Laboratory, Royal Ontario Museum, Toronto, Ontario, Canada

Nodules of carbonaceous matter have been observed in several Archaean peraluminous granites adjoining the gold- and uranium-bearing Witwatersrand Basin. These nodules, which resemble solid bitumen, usually replace uraninite or uranothorite and are paragenetically late. They show many similarities to kerogen seams and nodules associated with the conglomerate horizons in the Witwatersrand Basin, including composition, maturation, intimate association with uraninite, and a late paragenesis. U-Pb isotope ratios for granite-hosted and Witwatersrand kerogens define a single, albeit somewhat diffuse, Pb-loss discordia suggesting an upper intercept age of circa 2300 Ma. Carbon isotope analysis indicates lighter 513C values for the granite-hosted nodules than for the sediment-hosted kerogen seams. Rarnan spectroscopy indicates that granite-hosted kerogen is more highly ordered than Witwatersrand seam kerogen. A model is presented which links the granitoid-and sediment-hosted organic matter to an event of oil generation in the Witwatersrand Basin circa 2300 Ma ago. Light hydrocarbons migrated through the sediments and into underlying/adjacent granitic rocks, undergoing polymerization and accumulation in response to radiation accompanying the presence of high-uranium phases.