Thrust-fracture network and hydrothermal gold mineralization: Witwatersrand Basin, South Africa

Abstract

This paper examines the geometry and genesis of a mineralized thrust-related fracture system, developed at a major rheological interface between Witwatersrand sediments and Ventersdorp volcanics in the Witwatersrand Basin of South Africa. At the Elandsrand Gold Mine, the conglomeratic Ventersdorp Contact Reef, which separates these successions, is imbricated by kilometre-wide zones of minor thrusts containing ultracataclastic fault rocks associated with hydrothermal fluid flow. Several sets of fractures are intimately associated with the imbricates, which link to underlying kilometre-scale thrusts. The fractures provided a linked fluid flow pathway and structural sites for precipitation of gold within the Ventersdorp Contact Reef, carrying >90% of observed gold, with the remaining 10% occupying post-diagenetic matrix corrosion spaces within a few 100#m of fractures. The relatively rigid volcanics deformed into low-amplitude kilometre-wavelength flexures, centred on the imbricate zones and underlying kilometre-scale thrusts. These major structures controlled the thrust-fracture system formation along the Ventersdorp Contact Reef-lava contact. Three progressive stages of structural evolution were identified: (1) layerparallel shear across the contact, with stratigraphy-focused shallow fracture formation;(2) propagation of a kilometre-scale thrust towards the contact, leading to kilometre-scale flexure of the volcanics, with layer-parallel shearing in the limbs producing steep fractures and minor thrusts; (3) propagation of the major thrust along the contact, imbricating the sediments with shallow fracture formation in ramps.

Hydrothermal alteration in the Witwatersrand goldfields

“…5), the wedging between Lower Wits shale-rich package and the overlying Ventersdorp metabasalt, unconformities, high strain zones in the heterogeneous lithological packages for 1-5 m above unconformities, shear zones (Mullins, 1989) and many small quartz veinlets (Phillips and Law, 2000). Newer data and more detailed interpretations support the model of fluid migration through fracture networks and thrust structures (Jolley et al, 1999(Jolley et al, , 2004, and the additional seismic interpretations confirm a network of structures on the scale of mines and goldfields (Beach and Smith, 2007;Dankert and Hein, 2010). Many of these structures are inferred to have been generated and active during the late Central Rand (Upper Wits) times, and others during post-Klipriviersberg Group-pre Platberg Group thrusting events.…”

“…The Reno framework included a complex post-burial history of the Witwatersrand Basin including syn-sedimentary and post-sedimentary deformation of the Witwatersrand Supergroup with bedding parallel deformation (Berlenbach, 1995;Coward et al, 1995;Hilliard and McCourt, 1995;Jolley et al, 1999;McCarthy, 1994;Myers et al, 1990;Roering and Smit, 1987), and overturning of some Witwatersrand reefs was identified (Tweedie, 1986). Interpretation of seismic surveys led to an interpretation of large thrust faults in the footwall to goldfields (Coward et al, 1995;Fig.…”

“…The evidence for fluid focusing in the reef packages includes the concentration of quartz veins, the development of planar fabrics and alteration including pyrophyllite (e.g. Jolley et al, 1999). The quartz vein distribution and geometry suggests high fluid pressures in the reef package, fracturing of low tensile strength siliceous rocks, and probably some capping by relatively impermeable reef-package shale units.…”

Deformational controls on the dynamics of fluid flow in mesothermal gold systems

“…Jolley et al 1999). For example, at the Revenge mine, mineralized shears exhibit evidence for early brittle slip events and brecciation, followed by ductile shearing during progressive potassic and carbonate alteration, gold precipitation and vein formation (Nguyen et al 1998).…”

Structural geometry and development of the Witwatersrand Basin, South Africa

“…In detailed studies of the relationship between gold mineralization and small-scale structure, Jolley et al (1999Jolley et al ( , 2004 presented data and interpretations that overwhelmingly support a model for the structural control of gold mineralization in fracture networks and thrust structures generated during the late Central Rand Group and post Klipriviersberg Group-pre Platberg Group thrusting events. Those workers interpreted the fracture networks, which provide a linked fluid flow pathway through the rocks, as well as sites for gold precipitation, as being generated in the tip zones of larger thrust faults deeper in the section.…”