Res_metamorphism WITS

Rob PRESTON - Gary STEVENS

West Rand Group shales from three boreholes to the the south and east of the Vredefort Dome were studied. In all samples the maximum grade was lower greenschist facies and the peak metamorphic assemblage consisted of quartz muscovite chlorite, with the phyllosilicates defining a tectonic cleavage. In many of the samples, more coarsely crystalline alteration zones, localized around minor faults and shears, crosscut both bedding and the tectonic cleavage. In these zones of fluid ingress muscovite was not observed, and the cleaved matrix assemblages have been replaced by assemblages consisting of pyrite quartz chlorite stilpnomelane calcite carbonaceous matter. Major mobility of K, Fe, S and C are indicated and gold content in these zones has also been enriched from background values to 0.5 ppm.

Chlorite thermometry indicates equilibration temperatures close to 300C for both matrix and alteration zone chlorite.

Important Consequences

Au AND U IN BITUMEN FROM THE WITWATERSRAND BASIN

Gill DRENNAN and Laurence ROBB

Radiolytic polymerization during catagenesis of the Witwatersrand Basin caused the precipitation of heavy hydrocarbons around detrital uraninite giving rise to sediment-hosted bitumen, and liberating light hydrocarbons into the basinal brines. Simultaneous dissolution of uraninite released uranyl ions (U6 ) into the circulating fluids.
These urano-organo-bearing fluids were able to scavenge gold and sulphides, and transport these components along aquifers during subsequent metagenic processes. Localized high P-T conditions experienced in late-stage fault-related quartz veining resulted in the reduction of the uranyl species and dehydrogenation of carbonaceous matter, causing the precipitation of uranium-enriched bitumen nodules together with gold and sulphides. Five bitumen nodules were individually analyzed by micro-PIXE analysis to determine the REE (rare earth element) and metal concentrations and distributions within the nodules. U, TH and As are pervasively distributes throughout individual nodules while Au (see image above), Fe and Cu occur as localized particulate concentrations. This would suggest that urano-organo-metal and thio-metal complexing occurred in the same
fluids.

The two images shown here depict the relative distributions of both gold (top left) and uranium (right) by micro-PIXE analysis

• G.R Drennan, M-C. Boiron, M. Cathelineau and L.J. Robb (1999). Characteristics of post-depositional fluids in the Witwatersrand basin. Min. & Pet., 66, 83-109.

Pb-Pb DATING OF SULPHIDE MINERALS IN THE EVANDER BASIN

Marc POUJOL

This study presents new Pb-Pb data on detrital and authigenic pyrites associated with gold mineralization in the Evander goldfield. A small proportion of the pyrite, mainly extracted from unaltered sediments in the Kimberley reef footwall, yield ages that are in excess of the minimum depositional age of the Witwatersrand Basin.

Authigenic pyrite, as well as detrital grains from highly altered portions of the Kimberley Reef, define secondary isochrons at 2370 Ma and 2020 Ma, ages that accord with the resetting signatures from pyrite analysed in previous studies. The lead isotopic signature of the 2370 Ma event is probably associated with burial of the Witwatersrand Basin by the upper portion of the Transv l Supergroup, and the circulation of highly radiogenic fluids at this time. Isotopic signatures for the 2020 Ma event are probably related to the intrusion of the Bushveld Complex and/or Vredefort catastrophism, and are associated with a fluid that was less radiogenic.

For more information:

• M. Poujol, L.J. Robb & J-P. Respaut (1999). U-Pb and Pb-Pb isotopic studies relating to the origin of gold mineralization in the Evander Goldfield, Witwatersrand Basin, South Africa. Precambrian Res., 95, 167-185.