The Golden Mile, Kalgoorlie; a giant gold deposit localized in ductile shear zones by structurally induced infiltration of an auriferous metamorphic fluid

Abstract

Mesoscopic and microscopic analysis of the structurally controlled gold mineralization of the Golden Mile, Kalgoorlie, has demonstrated that the mineralogically simple gold pyrite mineralization style (70% of production) is restricted to an anastomosing system of narrow high strain zones (ductile shear zones) and to related tension fractures in the intervening unstrained rock. Virtually all ductile shear zone lode structures occur in the Golden Mile Dolerite because it was essentially mechanically isotropic prior to deformation. It is proposed that during the near-upright regional-folding event numerous ductile shear zones were established in an approximately axial planar relationship to the folds, with synchronous conjugate shear zones developed at low angles to this direction. A dynamic analysis shows that the tensional cross lodes can be related geometrically to the three ductile shear zones. Golden Mile-style mineralization is syntectonic and was not deposited posttectonically in structurally prepared sites. Microtextural analysis demonstrates that minerals associated with gold emplacement are strongly modified during strain within the ductile shear zones by ductile deformation dominated by mass transfer processes. In particular there is abundant evidence of crack-seal-type microfracturing within and marginal to grains and in microscopic transgranular sites; this deformation style contributed prominently to the bulk plastic flow of the shear zones. It is proposed that high fluid pressure gradients were established at each crack-seal cycle leading to much enhanced permeability and to considerable focusing of fluid flow through the narrow foliated zones. This structurally induced infiltration mechanism allowed the ore fluid to pervade the deforming zones and also controlled the straining process by moving dissolved mineral components from highly stressed contacts to sites of deposition. Thus the mineralization was introduced during the ductile phase of shear zone evolution, and though mesoscopic veins are present in the ductile shear zones, they are not a necessary part of the gold emplacement event: the majority of veins formed late in the tectonic history. Gold emplacement in the ductile shear zones was controlled by microscopic deformation processes and ductile shear zones are good exploration targets even in the absence of macro- to mesoscopic dilation sites.