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Economic Geology Research Institute-Hugh Allsopp Laboratory, School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050, Johannesburg, South Africa, E-mail: 065cra{at}cosmos.wits.ac.za
The southwestern portion of the Barberton greenstone belt is noteable for its varied and complex geology spanning at least 1360 Ma of earth history. A sequence dominated by metamorphosed Archaean komatiitic basalts and komatiites makes up most of the area occupied by the Nelshoogte Schist Belt. These rocks have, in the past, been correlated with the c. 3500 Ma lithologies in the lower division of the Barberton Supergroup (viz., the Tjakastad Subgroup of the Onverwacht Group in the Komati River type locality). Recent studies elsewhere in the Barberton region (including stratigraphic and geochronological investigations) have, however, introduced an alternative viewpoint suggesting that the Nelshoogte succession might be significantly younger (c. 3250 Ma). Views comparing the mafic and ultramafic rocks of the area to Phanerozoic-style ophiolites are discussed and found to be untenable. The term ‘Jamestown Ophiolite Complex’, which has been used to describe the rocks in the area, is therefore rejected. No direct ages are available for the Nelshoogte metavolcanics, but U-Pb dating of zircons from the adjacent trondhjemite-tonalite gneiss plutons (Kaap Valley and Nelshoogte plutons) have shown these intrusive bodies to be approximately 3213 to 3236 Ma. Also adjacent to the metavolcanic rocks of the Nelshoogte Schist Belt is a sequence of metamorphosed ultramafic-mafic cumulate rocks that make up the Stolzburg Layered Ultramafic Complex. This Complex, which is fault bounded, has been subdivided into a Lower Division consisting of serpentinized dunites and orthopyroxenites and an Upper Division comprising altered dunites, harzburgites, lherzolites, wehrlites, websterites, gabbro-norites and anorthositic gabbros. A zone of Ca-metasomatised gabbros and associated rodingite dykes separates the rocks of the Upper and Lower Divisions. Estimates of the bulk composition of the Stolzburg Complex have shown that the layered body formed from magma of komatiitic parent composition. By contrast, another igneous body intruded into the Nelshoogte Schist Belt in close proximity to the Stolzburg Complex (Sterkspruit Intrusion) consists almost exclusively of altered gabbroic-to-dioritic rocks and has a tholeiitic magma parentage. The Nelshoogte Schist Belt has an unusual, triangular, funnel-shaped structural form, which is considered to have developed as a result of the gravitational collapse of the metavolcanic succession between upwardly emplaced diapiric granitoid plutons (Nelshoogte pluton in the west and Kaap Valley pluton in the north). The granitoid rocks in the Nelshoogte area display several intrusive and structural phases as well as augen-gneiss development adjacent to the granite-greenstone contact. A small, homogeneous, tonalitic-to-dioritic granitoid body containing numerous amphibolite inclusions (Goedehoop pluton), is embayed into the Nelshoogte Schist Belt near the intersection of the three fold axial traces of the triangular structure and may represent an anatectic product of the infolded metavolcanic rocks. Diabase dykes, believed to range in age from c. 3000 to 1876 Ma, appear to represent the youngest magmatic events in the Nelshoogte region. They crosscut all the rocks in the area, are themselves unaffected either structually or metamorphically, but do show deuteric alteration effects. Three chrysotile asbestos deposits were mined in the Stolzburg Complex, the mineralization being controlled by faulting and folding of the dunite host rocks and their associated, structurally more competent orthopyroxenite interlayers. Minor gold and copper-nickel occurrences are also present in the Nelshoogte Schist Belt and adjacent areas. Potential for platinum group mineralization is deemed favourable, particularly in the layered ultramafic complexes of the Stolzburg type, a number of which are present elsewhere in the Barberton region.
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