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Book Chapter

The nature of hydrocarbons and related fluids in the Witwatersrand Basin, South Africa: Their role in metal redistribution

By
G.R. Drennan
G.R. Drennan
School of Geosciences, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, South Africa
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L.J. Robb
L.J. Robb
Economic Geology Research Institute, School of Geosciences, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, South Africa
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Published:
January 01, 2006

Gold and uranium mineralization within the Archean Witwatersrand Basin exhibits a close association with carbonaceous matter. In order to understand the gold and uranium mineralization associations, it is necessary to understand something about the carbonaceous matter itself as well as the fluids involved in remobilizing both hydrocarbons and mineralization within the basin.

Diagenetic maturation of primitive bacterial material released hydrocarbons that were able to migrate through the sediments of the economically important Witwatersrand Basin. Migrating hydrocarbons underwent polymerization and condensation due to the effect of ionizing radiation in the proximity of detrital uraninite during catagenesis. Isotopically heavy hydrocarbons precipitated out as bitumen seams where uraninite concentrations were high or as “fly-speck” bitumen around isolated uraninite grains, giving rise to sediment-hosted bitumen types. Lighter hydrocarbons, liberated during catagenic radiolysis, were incorporated into circulating basinal fluids. During late-catagenic/early-metagenic processes, hydrocarbon-bearing fluids migrating along aquifers (faults, bedding planes, and unconformities) precipitated insoluble carbonaceous matter, or distal/vein-related bitumen, in suitable sites. Light hydrocarbons precipitated as crosscutting veins where the vein intersected uraniferous seam bitumen, as bitumen nodules within late-stage quartz veins and as nodules and/or inclusion linings within fluid inclusions in late-stage quartz veins. Where fault zones permeated the Archean granitic basement, hydrocarbons were precipitated by radiolytic processes around high-U phases as nodular, granitoid hosted, or distal bitumen. Distal- and vein-related bitumen exhibits a higher degree of structural ordering than sediment-hosted bitumen as a result of elevated pressure-temperature conditions experienced during metagenic and fault-related processes.

Liberated light hydrocarbons in basinal fluids formed organo-urano-gold-, thio-gold-, and/or organo-urano-gold-sulphide complexes. Circulation of these hydrocarbon-rich fluids and subsequent precipitation of bitumen and mineralization provide an explanation for the apparent hydrothermal component of Witwatersrand mineralization.

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Contents

GSA Special Papers

Processes on the Early Earth

Wolf Uwe Reimold
Wolf Uwe Reimold
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Roger L. Gibson
Roger L. Gibson
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Geological Society of America
Volume
405
ISBN print:
9780813724058
Publication date:
January 01, 2006

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