Structural controls of auriferous reefs at Fairview Mine, Barberton Greenstone Belt, South Africa

[摘要] ENGLISH ABSTRACT: Auriferous shear zone hosted gold mineralization at Fairview Mine, along the central-northwestern margin of the Mesoarchean Barberton Greenstone Belt (BGB), is correlated with regional-scale fluid migration and focusing during progressive F3 folding of the Ulundi Syncline during the late-stages of compressional D3 deformation. Gold-sulphide mineralization at Fairview Mine is hosted in low-grade metaturbidites of the Fig Tree Group on the NW limb of the regional-scale, doubly-plunging and distinctly arcuated or refolded (F3) Ulundi Syncline that is bounded against the structurally underlying Eureka Syncline by the ductile-brittle Sheba Fault. Exceptionally well-mineralized orebodies (10-60 g/t) are centred around geometrically contrasting brittle-ductile shear zones, termed 'reefs, with distinct orientations, host lithologies, wall-rock alterations, kinematics and timing.This study focusses on two distinctly dissimilar reef structures, namely the main ore shoot of the Main Reef Complex (MRC), that hosts the bulk of the gold mineralization with grades in excess of 40-60 g/t, and the Hope reef, one of a number of smaller, but still economically significant reefs with average grades up to 10-20 g/t, situated above the MRC, collectively referred to as 'hangingwall reefs. This study is aimed at characterizing these orebodies and their relation to host rock structures, kinematics along the controlling structures and the controls of fluid focusing, mineralization and ore shoot formation. The overarching aim is to formulate an integrated conceptual structural and kinematic framework for the controls of mineralization for both the Hope reef and the MRC.Gold-sulphide mineralization of the Hope reef is associated with quartz-carbonate veining and a chlorite-sericite-carbonate-albite wall-rock alteration that is centred around an up to 2.5 m wide, shallow southeast-dipping (045/15 SE), brittle-ductile fault zone localized within Fig Tree greywacke units, some ca. 200 m above the Sheba Fault. The formation of shear and extensional vein sets documents fault-valve behaviour along the low-angle thrust, driven by close-to-lithostatic fluid pressures and under very low differential stresses (<5 Mpa). High-grade ore shoots in the plane of the shallow reef delineate lithologically and structurally controlled interconnected fault-fracture meshes that correspond to the development of frontal-ramp duplex structures. Kinematic indicators suggest an origin of the Hope reef as an optimally orientated low-angle, top-to-the-NW thrust that likely formed as a secondary accommodation structure during continued NW-SE subhorizontal shortening after the F3a lock-up of the Ulundi Syncline.Gold-sulphide mineralization of the MRC is associated with a graphite-carbonate wall-rock alteration and is localized in the immediate hangingwall of the steep easterly dipping Sheba Fault within discrete steeply plunging ore shoots contained along a gently undulating system of spaced, low-displacement, broadly bedding-parallel faults and shear zones. Fabrics and structures in the ore shoot record combined top-to-the NW thrust sense and dextral strike-slip kinematics and strongly constrictional strains, interpreted to indicate dextral-transpression and associated steep extrusion of the rocks during progressive NW-SE (D3) shortening. The main ore shoot corresponds to dilational jog geometry that developed during dextral-transpressive, bedding-parallel shearing consistent with the flexural-slip refolding (F3b) of the Ulundi Syncline. High-grade pockets within the jog correlate with sheared, commonly dismembered, graphite-rich and sulphide-mineralized shale units, testifying to the preferential fluid focusing and strain localization into incompetent shale units during deformation.Cross-cutting relationships with other auriferous reefs from the Fairview Mine complex indicate a late timing of the MRC-type mineralization, underlining the complex fluid focusing and utilization of differently orientated structures with different kinematics during progressive deformation, and either episodic or protracted fluid flow events consistent with the main phase of upright regional folding (D2/3) during the late accretionary evolution of the BGB.The orebodies at Fairview Mine share many structural, mineralogical, and fluid geochemical similarities with Phanerozoic-aged orogenic gold deposits, highlighting the importance of the regional-scale mechanical coupling between rheological contrasting lithological packages and progressive folding and thrusting, enabling prime loci for strain localization, focused regional-scale fluid migration and efficient mineralization trap site development within the BGB.

[发布机构] Stellenbosch University