Deformation and mineralisation in the Scotty Creek Basin, Agnew district, Eastern Goldfields, Western Australia: evidence for D1- and D3-related gold mineralisation

Abstract

Gold deposits in the Agnew district display markedly different structural styles. The Waroonga and Songvang deposits are hosted in layer-parallel extensional shears formed under highly ductile conditions. In contrast, the New Holland–Genesis deposits are shallow-dipping quartz-filled brittle fractures and breccia zones that cut across the tightly folded bedding and formed during east–west compression. It is difficult to attribute their formation to a single compressive event. The Waroonga and Songvang deposits formed during D1 extension, uplift and exhumation of the Agnew granitic complex and formation of the Scotty Creek Basin at ca 2670–2660 Ma. The New Holland–Genesis deposits formed during east–west D3 compression at about ca 2650–2630 Ma. An S1 foliation wraps around the Agnew granitic complex and L1 stretching lineations form a radial pattern around the granite, consistent with formation during D1 uplift of the composite granite body. Uplift and erosion of granite bodies in the surrounding area provide a source for the granite clasts in the upper parts of the Scotty Creek Basin. As clasts in the basin are undeformed, no significant deformation occurred prior to the uplift and erosion of the source granites in this area. Syn-tectonic emplacement of the Lawlers Tonalite during formation of the Scotty Creek Basin at ca 2665 Ma may have provided a good heat/fluid source for the mineralising systems during the first gold event. The distribution of the large deposits along the western edge of the Agnew granitic complex indicates that the extensional shear along the granite contact is a first-order control on gold deposition by providing a conduit for rising hydrothermal fluids. The northerly trend of high-grade shoots in the Waroonga deposit coincides with early north-trending growth faults, which are also likely fluid conduits.