Browsing by Author "Jones, Caitlin"
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- ItemControls of hydrothermal fluid flow and gold mineralization in the Sheba-Fairview Complex, Barberton Greenstone Belt, South Africa(Stellenbosch : Stellenbosch University, 2021-12) Jones, Caitlin; Kisters, Alexander; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: The Barberton Greenstone Belt hosts high-grade gold-quartz-sulphide-carbonate orebodies where more than 350 ton of gold has been mined over 130 years. Some of the top gold-producing mines are located within the Sheba-Fairview Complex (SFC). On a deposit scale, the localized fluid conduits and subsequent sinks (reefs) are characteristically low-displacement faults or shear zones. Orebodies are located throughout the greenstone sequence, from the Weltevreden Formation in the Sheba Anticlinorium, into the well-bedded metasedimentary rocks of the Fig Tree and Moodies Groups on both the isoclinally folded northern limb of the Ulundi Syncline and southern, overturned limb of the Eureka Syncline. This study forms the link between the regional fluid flow and local fluid sinks underlain by the crescent-shaped Eureka and Ulundi synclines. This is accomplished by examining two bedding- discordant orebodies, namely the high-grade ‘flat structure’ hosted within the green fuchsite-quartz schist of the Weltevreden Formation, and the steeply-dipping Thomas fracture hosted in the subarkose of the Moodies Group. Structural, petrographic and geochemical analyses of the wall-rocks and auriferous reefs within the arcuate hinge zone of the SFC, reveal that the gold mineralization can be subdivided into three main structural associations. These include (1) reefs related to the central, regional-scale Sheba Fault characterized by quartz-carbonate stockworks, (2) orebodies associated with the anticlinal structures of Weltevreden rocks that are commonly shoot-like, particularly along the contacts of the anticlines with the overlying Fig Tree Group metaturbidites, and (3) smaller reefs within the arenaceous Moodies Group but also in the Fig Tree Group that crosscut bedding at high-angles. Mineralized fractures in the SFC are confined to certain lithological packages (greywacke, quartzite or green fuchsite-quartz schist) within the formations. The orebodies are better developed in competent lithologies with low tensile strengths that promote fracturing, especially in the presence of high fluid pressures. The competent gold-hosting units include the quartzitic units of the Moodies Group, the greywacke-dominated packages of the Fig Tree Group, but also the green fuchsite-quartz schist of the Weltevreden Formation. The competent horizons are related to either original compositions (e.g., greywacke and quartzite) or to an earlier alteration event during deformation that resulted in the increased competency (e.g., green schist). Bedding-parallel veins, sulphide mineralization and alteration are abundant within the well-bedded metasediments of the Ulundi and Eureka synclines and attest to the widespread bedding-parallel fluid flow during flexural-slip folding, where bedding planes acted as low-displacement slip surfaces. The concentration of orebodies in the hinge zone testifies to the regional fluid flow paths with fluid flow, following hydraulic gradients, from the fold limbs towards the hinge during the refolding of the synclines. Gold mineralization occurs in the later stages of the deformational history and is related to the refolding of the synclines during NW-SE directed shortening against the competent buttresses in the northwestern portion of the belt.