Nature and origin of ‘carbon’ in the Archaean Witwatersrand Basin, South Africa

摘要: The nature and origin of the carbonaceous matter (‘carbon’) in gold-bearing reefs Archaean Witwatersrand basin South Africa has long been a subject controversy debate. importance carbon results from its direct association with gold grade. studies reported here were directed at resolving carbon. Techniques used include stope-scale (macroscopic) mapping, mesoscopic (hand specimen) examination, microscopic incorporating use optical reflectance measurement. Macroscopic mapping demonstrated braided habit bodies over tens square metres, close parallel to sedimentology. Mesoscopic observations, however, show vein-like seams are often cross-cutting sedimentary features, occupying an anastomosing set bed-parallel fractures. Microscopic observations globular form for inclusions uraninite particles. In seam globules generally modified into elongated ‘spindles’ oriented perpendicular bedding. Optical properties dominated by high anisotropy ‘swirling’ liquid crystal structure, classifying as ‘carbonaceous mesophase’. It is this fine mesophase texture that previously wrongly attributed plant (algal) morphology. Highly significantly, bed (rather than normal burial) indicating formation during fracture opening. Both bireflectance range values all scales. No stratigraphic or depth trends apparent, though geographic variation points localization maximum temperature event(s). Given anisotropies, 4–8% R o minima 1–2% give arithmetic mean about 4.0% . Kinetic modelling equates these short term (50 000 years) events c. 400°C burial temperatures 280°C. Lower 2.0% obtained shales suggests thermal detachment permeable impermeable beds, characteristic pulsed hydrothermal flux. included grains influence orientation surrounding anisotropic demonstrating predated event. Regional palaeo-stress ‘frozen’ indicates NW–SE deviatoric stress. combination hydrocarbon, radiation fluid flow marks Basin pyrobitumens out being different conventional oil. Putative source rocks oil identified intra-basinal shale units. Lithological control explained poro-perm mechanical unconformity sequence footwall, reef conglomerates hanging wall quartzites favouring along surfaces. An overpressured oxidizing created network hosted also introduced uranium uranyl ion. On meeting reducing agent (oil), precipitated stabilizing polymerization. Further hotter flux then ‘coked’ bitumen fibrous (spindle-shaped) mesophase. Continued deposited available pore spaces, most notably those formed lateral contraction between spindles. speculative role methane reductive precipitation indicated presence any co-eval porosity largely independent mineral matrix.