Structural control of greenstone-hosted Gold mineralization by transcurrent shearing: A new interpretation of the Kalgoorlie mining district, Western Australia

Abstract

The Kalgoorlie mining district encloses the Golden Mile and Mt. Charlotte mining areas, and has produced 1200 metric tons of gold. It is classed as one of the largest Archaean gold deposits in the world. The district is located in the Norseman-Wiluna greenstone belt of the Yilgarn craton, Western Australia. This greenstone belt is characterized by NNW-trending transcurrent shear zones, some of which are traceable over more than 500 km strike length. Mineralization in the district is controlled by second-, third- and fourth-order structures located between principal shear zones of regional extent. An early deformation phase of regional folding (D1), associated with prograde regional metamorphism of low grade, preceded two phases of wrenching. The geometry of mineralized shear zones in both mining areas, as well as field and microstructural studies, allow us to distinguish an early phase of transpressional sinistral shearing (D2) from a later phase of transcurrent dextral shearing (D3). The earlier sinistral shear system in the Golden Mile area was in part reactivated, but its gross original geometry was preserved. These observations are consistent with clay model experiments carried out by the authors. Hydrothermal carbonate-sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event. Mineralization within individual shear zones forms tabular orebodies, whereas pipe-shaped ore shoots occur at the intersection of shear zones. The ore shoots contain high-grade telluride mineralization locally. Relative timing relationships show that mineralization postdates the peak of regional metamorphism. Hydrothermal activity in the district was preceded by the intrusion of calc-alkaline porphyry dykes of granodioritic to tonalitic composition, and accompanied by the intrusion of lamprophyre dykes.