Phanerozoic analogues for carbonaceous matter in Witwatersrand ore deposits

Thoriferous bitumen nodules in quartz-pebble conglomerates and pebbly sandstones of Phanerozoic age in the British Isles are analogues for the uraniferous bitumen nodules (flyspeck) in the Archean Witwatersrand gold-uranium deposits. They are comparable in morphology, relationship to bedding planes, replacive behavior toward quartz, mineralogy, fragmentation, and displacement of inclusions. In each case the quartz-rich sedimentary host rocks represent regionally important transgressive deposits.The contrast in dominant radioelement (thorium or uranium) probably reflects differences in mobility of the elements as a function of variable Eh conditions. The Phanerozoic thoriferous nodules occur only in pale (low Eh) beds which lack uranium deposits, but in the Archean Witwatersrand deposits fractionation of uranium and thorium may not have been as marked.In the Phanerozoic examples there is good evidence for migration of hydrocarbons through the host rocks and the bitumen nodules can confidently be attributed a hydrocarbon origin. By analogy, this supports proponents of a hydrocarbon origin for the Witwatersrand carbon, rather than a microbial origin.