Polyphase deformation of mylonites from the Renco gold mine (Zimbabwe): identified by crystallographic preferred orientation of quartz

Abstract

Gold mineralization at Renco mine in the Northern Marginal Zone of the Limpopo Belt, Zimbabwe, is hosted by a set of intersecting steep and shallow dipping shear zones, locally termed ‘reefs’. Both steep and shallow reefs are closely associated with quartz-mylonites. The crystallographic preferred orientation of quartz from samples of these quartz-mylonites indicate that deformation in steep reefs occurred under amphibolite-facies conditions. Pole figures and microstructures of samples from shallow reefs, in contrast, show evidence for an origin of the auriferous mylonites at amphibolite-facies conditions that were subsequently overprinted by deformation under lower-greenschist-facies conditions. Since mineralization is hosted by both reef geometries, gold mineralization is inferred to have occurred during deformation and associated fluid infiltration along mylonites at amphibolite-facies grades, coeval with Late-Archaean thrusting of rocks of the Northern Marginal Zone onto rocks of the Zimbabwe Craton. The crystallographic preferred orientation patterns of quartz point to a normal sense of movement during reactivation of the shallow reefs, probably related to the Meso-Proterozoic tectonometamorphic event that has affected the central parts of the Limpopo Belt at ca. 2.0 Ga. Normal faulting along the boundary between the Northern Marginal Zone and Zimbabwe Craton has, to date, not yet been documented, and may provide some important constraints for the understanding of the kinematics of the Meso-Proterozoic tectonics in the Limpopo Belt.