Fault-segment rupture, aftershock-zone fluid flow, and mineralization

Abstract

We propose that zones of transient high permeability around ancient fault systems can be predicted if fault segments and likely locations for paleo-rupture arrest are identified. Lode gold deposits in the Kalgoorlie terrane, Western Australia, are the products of focused fluid flow through faulted crust. Deposits in the Mount Pleasant area are clustered on small-displacement structures over ∼10 km of the >50-km-long Black Flag fault. Field relationships and net slip distribution along the fault indicate that the deposits are adjacent to, but not within, a kilometer-scale dilatant jog, where two segments of the fault are linked. On this basis we infer that the dilatant jog was a long-term rupture-arrest site. The observations are compatible with rupture on segments of the Black Flag fault changing stress in the surrounding crust and bringing specific zones closer to failure. By analogy with active seismogenic fault systems, those zones correspond to regions where aftershocks occur preferentially after failure. Stress-transfer modeling of the system helps explain the location of mineralized small-displacement structures around the Black Flag fault and indicates that gold deposits in the area are located on structures that became transiently permeable and localized fluid flow during repeated aftershock ruptures. Thus, localized through-flow, or mixing of fluids within fault systems, is likely to be controlled by the distribution of aftershocks following rupture events; this distribution is predictable.