Structure and Distribution of the Gold-Related Quartz Vein Systems in the Southwestern Part of the Barberton Greenstone Belt (South Africa, Eswatini)

This study investigates the structural control of the numerous gold occurrences in the southern part of the Barberton Greenstone Belt in the Malolotja and Steynsdorp areas. The gold-bearing event distribution is studied using field structural geology associated with a petrological and microstructural analysis. Three major tectonic events have been identified in quartz veins and direct country rocks. The first event (De) created a regional schistosity (Se), probably associated with an early thrusting event. The second event (Df) is related to a large-scale folding, which formed the anticline at Steynsdorp and synform at Malolotja. It resulted from a main E–W direction of shortening and is responsible for the folding of the Se schistosity. It formed a N–S-striking axial planar cleavage (Sf), observed and associated with the emplacement of the main gold-bearing veins. The mineralised system exhibits a complex vein network, in which shallow dipping veins have developed coevally with steep west- and east-dipping veins. A third deformation event (Dl) produced by NE–SW shortening is related to the late barren hydrothermal quartz vein formation. Mineralogically, the veins are filled with abundant quartz, and scarce tourmaline and feldspars. Alteration halos composed of neoformed tourmaline and plagioclase within a talc and white mica matrix developed specifically during the late Dl event. A model of regional deformation giving rise to large-scale folds and quartz vein formation formed in response to E–W and NE–SW directions of shortening globally is proposed and discussed in this paper.