Sedimentological evaluation of Pb-Zn exploration potential of the Precambrian Griquatown Fault Zone in the Northern Cape Province, South Africa

The Griquatown Fault Zone (GFZ) is a major target for Pb-Zn exploration in South Africa. The sedimentary, structural and thermal history of the fault zone are evaluated. The fault zone experienced a synsedimentary period of activity between 2550 and 2500 Ma and a major post-Postmasburg Group (less than 2223 Ma) episode of mainly vertical movements. Possible source rocks for generation of metalliferous brines are abundant along the southwestern margin of the Kaapvaal craton and shales southwest of the GFZ are time correlative to a thick peritidal stromatolitic carbonate sequence in the northeast. Fluids driven by compaction and orogenic pressure migrated across the GFZ , via the carbonates, towards the east. Metamorphic overprint south of the GFZ, based on illite crystallinity, and fluid inclusions north of the fault zone are above the oil window. Metamorphic peaks south of the fault zone are at 1750 Ma and 1213 Ma and the intensity of metamorphic overprint decreases from west to east. Because of high temperatures of metamorphic and orogenic overprint and possible remobilisation of fluids, Mississippi-Valley-Type (MVT) ore deposits are unlikely to be found within the Griquatown Fault Zone, but are expected, rather, to the northeast of it. Higher temperature, remobilised vein-related deposits could, however, occur in the GFZ itself. Received: 11 June 1996 / Accepted: 7 January 1997     

An overview of the geology of the Transvaal Supergroup dolomites (South Africa)

 In the Neoarchaean intracratonic basin of the Kaapvaal craton, between approximately 2640 Ma and 2516 Ma, two successive stromatolitic carbonate platforms developed. Deposition started with the Schmidtsdrif Subgroup, which is probably oldest in the southwestern part of the basin, and which contains stromatolitic carbonates, siliciclastic sediments and minor lava flows. Subsequently, the Nauga formation carbonates were deposited on peritidal flats located to the southwest and were drowned during a transgression of the Transvaal Supergroup epeiric sea, around 2550 Ma ago. This transgression led to the development of a carbonate platform in the areas of the preserved Transvaal and Griqualand West basins, which persisted for 30–50 Ma. During this time, shales were deposited over the Nauga Formation carbonates in the southwestern portion of the epeiric sea. A subsequent period of basin subsidence led to drowning of the stromatolitic platform and to sedimentation of chemical, iron-rich silica precipitates of the banded iron formations (BIF) over the entire basin. Carbonate precipitation in the Archaean was largely due to chemical and lesser biogenic processes, with stromatolites and ocean water composition playing an important role. The stromatolitic carbonates in the preserved Griqualand West and Transvaal basins are subdivided into several formations, based on the depositional facies, reflected by stromatolite morphology, and on intraformational unconformities; interbedded tuffs and available radiometric age data do not yet permit detailed correlation of units from the two basins. Thorough dolomitisation of most formations took place at different post-depositional stages, but mainly during early diagenesis. Partial silicification was the result of diagenetic and weathering processes. Karstification of the carbonate rocks was related to periods of exposure to subaerial conditions and to percolation of groundwater. Such periods occurred locally at the time of carbonate and BIF deposition. Main karstification, however, probably took place during an erosional period between approximately 2430 Ma and 2320 Ma. Received: 15 September 1996 · Accepted: 12 May 1998     

Aerosol concentration measurement with multiple light scattering system and laser aerosol spectrometer

The performance of two different optical concentration-measuring techniques was investigated over a concentration range starting with about 10² cm⁻³ and extending over more than four decades. Both instruments are capable of real-time counting, however due to their particular design-single particle counter and ensemble particle-measuring system—they operate in overlapping, but different concentration ranges. The upper, coincidence-free counting limit for the single particle counter used in this study was established to be in the order of 10⁴ cm⁻³. The ensemble technique was found to be functional and stable for concentrations of about 10³ cm⁻³ and limited by the onset of multiple scattering at concentrations nearby 2×10⁶ cm⁻³. Within the determined boundaries, both techniques proved to provide reliable aerosol concentration data.     

Mass occurrence of benthic coccoid cyanobacteria and their role in the production of Neoarchean carbonates of South Africa

The sparse Archean fossil record is based almost entirely on carbonaceous remnants of microorganisms cellularly preserved due to their early post-mortem silicification. Hitherto as an exception, sedimentary carbonate rocks from the Neoarchean Nauga Formation of South Africa contain calcified microbial mats composed of microbiota closely resembling modern benthic colonial cyanobacteria (Chroococcales and Pleurocapsales). Their remains, visible under the scanning electron microscope (SEM) after etching of polished rock samples, comprise capsular envelopes, mucilage sheaths, and groups of cells mineralized by calcium carbonate with an admixture of Al–K–Mg–Fe silicates. The capsular organization of the mucilaginous sheaths surrounding individual cells and cell clusters forming colonies and the mode of mineralization are the characteristic common features of the Neoarchean microbiota described and their modern analogues. The new findings indicate massive production of calcium carbonates by benthic coccoid cyanobacteria in the Neoarchean, and offer a solution to the problem of the origin of Archean carbonate platforms, stromatolites and microbial reefs.