17 March 2022 l Overberg Geoscientists Group talk by Alex Kisters

Alteration and the interplay with fluids, structure, & Archean gold mineralisation

The last half century has seen a massive shift in geological thinking with respect to alteration and its relationships to fluids, structure, and metamorphism. No longer is crustal fluid flow envisaged as being up and down open cracks, nor is structural geology simply the measuring of fault displacement; today, alteration is an integral part of much ore formation, and metamorphism goes well beyond the estimation of pressure—temperature. Mineralogy and whole rock geochemistry become intertwined and influence how rocks deform. Linking all these fields are heat energy, fluids, and deformation. Successful exploration and mining are the goals of the improved scientific understanding.

In the 1970s and early 1980s, pioneering works of Bill Fyfe (UK and Canada) and John Ferry (USA) drew attention to the intimate relationships between fluids and many crustal processes including partial melting, metamorphism, deformation, alteration, and ore formation. Some of this thinking was introduced to South African geoscience in the early 1980s through undergraduate and postgraduate courses, workshops, research projects at the Univ Witwatersrand and RAU, and through the TDOGS symposium (Tectonics Division of the Geological Society of South Africa GSSA). Like so many rapid advances in science, introduction of this new view of crustal fluids heralded a period in which there were virtually two geological languages depending upon one’s background and training: traditionally rocks were viewed as solid, dry, and they cracked under a stress to offset an orebody. The alternative viewpoint emphasised heat that generated fluid and new minerals, migration of that fluid through a network of structures, deformation driven by high fluid pressures and controlled by rheological contrasts, and all this crustal activity may include a major gold forming event. Much of the evidence for all this is contained with alteration mineral assemblages.

It is in this context that the Overberg Geoscientists Group (OGG) in conjunction with the GSSA, brings a fivepart series on alteration and its linkages to fluids, structures, and gold deposits. The timing takes advantage of the significant advances on Barberton gold deposits under the leadership of Professor Alex Kisters at University of Stellenbosch. The incorporation of examples from the goldfields of the Yilgarn Craton of Western Australia emphasises the opportunities arising from the global transfer of ideas for exploration and mining.