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

Much gold in the Witwatersrand conglomerates is directly associated with se ams of carbon that are generally parallel with bed ding. Some seams locally crosscut bedding and form anastomosing fracture patterns, showing that emp lacement of liquid hydrocarbons in those seams was postdepositional and inf illed 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 pe bbles in mineralized conglomerates. Planes of fluid inclusions parallel to the microfractures in pebbles show further evidence for the importance of b edding parallel fluid flow, The occurrence of carbon seams on sedimentary d iscontinuity surfaces may reflect preferential emplacement of carbon along planes of weakness, Brittle extensional fracturing in pebbles indicates hig h fluid pressures, which may relate to hydrocarbon generation. Bedding-para llel mineralization by quartz, clays, metamorphic chlorite/pyrophyllite, an d gold, which reflect a complex multistage paragenesis, indicate repeated r eactivation of bedding-plane permeability. Previous arguments against a hydrocarbon origin for the carbon seams have b een based upon the low intergranular permeability of the rocks; however, th e present study shows that this condition actually caused focusing of hydro carbon fluid Bow along discontinuity surfaces and facies boundaries. Intera ction with uranium could have caused fixation of fluid hydrocarbon by radio lytic polymerization. The columnar nature of carbon in many of the seams, i nterpreted by others as a biogenic structure, could instead reflect polymer swelling normal to bedding in the direction of least confining stress.