Petrographic evidence for emplacement of carbon into Witwatersrand conglomerates under high fluid pressure

Queen's University of Belfast, School of Geosciences, Belfast, United Kingdom

Much gold in the Witwatersrand conglomerates is directly associated with seams of carbon that are generally parallel with bedding. Some seams locally crosscut bedding and form anastomosing fracture patterns, showing that emplacement of liquid hydrocarbons in those seams was postdepositional and infilled fractures that are generally parallel to bedding. Seams that are not observed to crosscut bedding are nonetheless petrographically identical and are believed to have a common, postdepositional origin. Carbon also occurs along sets of bedding-parallel microfractures through individual quartz pebbles in mineralized conglomerates. Planes of fluid inclusions parallel to the microfractures in pebbles show further evidence for the importance of bedding-parallel fluid flow. The occurrence of carbon seams on sedimentary discontinuity surfaces may reflect preferential emplacement of carbon along planes of weakness. Brittle extensional fracturing in pebbles indicates high fluid pressures, which may relate to hydrocarbon generation. Bedding-parallel mineralization by quartz, clays, metamorphic chlorite/pyrophyllite, and gold, which reflect a complex multistage paragenesis, indicate repeated reactivation of bedding-plane permeability. Previous arguments against a hydrocarbon origin for the carbon seams have been based upon the low intergranular permeability of the rocks; however, the present study shows that this condition actually caused focusing of hydrocarbon fluid flow along discontinuity surfaces and facies boundaries. Interaction with uranium could have caused fixation of fluid hydrocarbon by radiolytic polymerization. The columnar nature of carbon in many of the seams, interpreted by others as a biogenic structure, could instead reflect polymer swelling normal to bedding in the direction of least confining stress.

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