|Keywords:||Earth and Planetary Sciences|
|Full text PDF:||http://digitool.library.mcgill.ca/thesisfile139101.pdf|
Hydrocarbons are a common constituent of many mineral deposits and are associated directly with high-grade mineralization. In the deposits of the Witwatersrand Basin (South Africa), the focus of this research, hydrocarbon seams are associated with over 40 % of the gold and 95 % of the uranium that has been produced. Although this association is widely known, the roles of the hydrocarbons in the ore-forming process are not fully understood.The metalliferous nature of hydrocarbons and their role(s) in the ore-forming process of the Witwatersrand are evaluated in this thesis in three studies using gold-, uranium- and pyrobitumen-bearing samples of the Carbon Leader Reef. In the first study, mineral-free pyrobitumen seams were investigated at a micro to nano scale, and yielded evidence for the in situ growth of uraninite and anatase nanocrystals that are pervasively distributed in the pyrobitumen matrix. Uraninite nanocrystals grew to complex-shaped nanocrystal aggregates due to oriented attachment, in which anatase generally provided the nuclei for the growth of uraninite. The second study examined large uraninite grains enclosed in pyrobitumen. High-resolution scanning and transmitted electron microscopic imaging showed that the large grains are in fact spongy aggregates of nanocrystals. Gold inclusions were discovered filling pores in the uraninite grains and were shown in turn to contain inclusions of small fossil oil droplets that had been thermally converted into pyrobitumen. The thin pyrobitumen layers on the pore walls of the gold grains carry in situ formed uraninite nanocrystals within their organic matrix. Lanarkite was first discovered in the Witwatersrand and is restricted to local oxidation-zones between galena and uraninite interfaces. A third study was undertaken to investigate the origin of hydrocarbons in the Carbon Leader Reef, their metalliferous nature and the timing of metal charging and discharging. Large similarities were detected in the trace element composition of pyrobitumen and that of potential source rock shales in the lower units of the Witwatersrand Supergroup. The pyrobitumen is exceptionally rich in uranium, gold and trace elements that are associated with uraninite and native gold, confirming the presence of ubiquitous and evenly distributed uraninite nanocrystals and micro-sized gold inclusions in the pyrobitumen. Based on the findings of these three studies, a genetic model for the formation of uranium and gold ore by water-oil-emulsions was developed and progressively refined. According to this model, pre-existing U-Ti minerals were dissolved in circulating oils, and metals were transported to the ore horizon(s) until thermal degradation immobilized the hydrocarbons by solidifying them as pyrobitumen, releasing uranium and titanium that are held in solution as nanocrystals. Gold-bearing hydrothermal fluids infiltrated into the reef during post-depositional regional metamorphism. In contact with immiscible oil phases, they formed a water-oil-emulsion and gold precipitated from the… Advisors/Committee Members: Anthony E Williams-Jones (Supervisor).