Shear zone hosted gold mineralization in the Moodies Group at Sheba Gold Mine, Baberton Greenstone Belt

Abstract

The Barberton Sequence of Archaean age rocks exposed at Sheba Gold Mine, Barberton, are amongst the oldest auriferous rocks in the world. Rocks of the Onverwacht, Figtree and Moodies Groups outcrop with the mine claims area, while the study area consists entirely of meta-arkoses of the Clutha Formation, Moodies Group. Gold mineralization within this group of rocks is restricted to narrow shear zones, where it is hosted by both arsenopyrite and pyrite mineralization. Several ages of both pyrite and arsenopyrite mineralization exist within each shear zone. Individual shear zones are characterized by a strongly developed sericitic cleavage, which strikes parallel to bedding within the meta-arkose, but dips steeply to the north while bedding dips steeply south. Several ages of quartz and quartz-carbonate veins occur within the shear zone. These . veins are highly sheared and boudinaged, indicative of post emplacement shearing and extension along the shear zone. Kinematic indicators from the Mamba Shear Zone, which include bedding rotation due to shearing, and lineation plunges on shear planes, identify the deformation to be dominated by a dip-slip movement component, with a reverse sense of motion. The deformation style 1s consistent with that of a brittle-ductile shear environment. The Mamba Shear Zone has undergone hydrothermal alteration which has been dominated by the breakdown of feldspar to sericite, and an enrichment in silica, arsenic and gold within the shear zones. Elevated gold values and sulphide concentrations in association. with tourmaline mineralization, suggests a granitic fluid influence in the shear zones. These zones of tourmaline occurrence exhibit minor deformation only, suggesting emplacement at a late stage into the shear zones. The Eureka and Cat's Cave Shear Zones exhibit similar deformation environments, movement directions, mineralization styles and hydrothermal alteration to that seen in the Mamba Shear Zone. From this evidence it is concluded that the Eureka, Mamba and Cat's Cave Shear Zones form a north dipping thrust imbricate stack. This thrust imbricate is bounded to the south by the south dipping Sheba fault, which exhibits a north-westerly directed thrust sense of movement, and is clearly the dominant structure within the study area. To accommodate the Eureka, Mamba and Cat's Cave thrust imbricate pile within the north-westerly directed thrust environment, they are interpreted as back thrusts in the footwall of the Sheba Fault Zone.