Abstract of the North Pilbara 'Virtual' Structural Field Trip

The Archean Pilbara Craton is composed of the 3.6-2.8 Ga North Pilbara granitoid-greenstone terrain and the unconformably overlying volcano-sedimentary sequences (Mount Bruce Supergroup) of the 2.77-2.3 Ga Hamersley Basin. This relatively well exposed province has been the focus of numerous studies on Archean geology, especially the classic dome-and-basin area around Marble Bar.

The long geological history of the craton has resulted in the development of a number of generally synformal greenstone packages (11) that mostly envelop domal composite granitoid complexes (> 12 pulses). The granitoid complexes make up about 45 per cent of the upper crust of the Pilbara Craton, at least to the mid-to-late Archean surface now preserved. The synformal greenstone belts consist of a succession of greenstone packages which are separated from each other by regional to local unconformities. These packages were developed during extensional periods (collapse) following far-field driven compressional deformation events. Up to 11 deformation events have been defined, many of Pilbara-wide significance. The dome-and-basin geometry characteristic of the east Pilbara was established relatively early in the geological history and was successively modified by repeated orthogonal compressive events. The result has been a locally complex development of polyphase structural elements with consistent overprinting relationships which can be correlated across much of the Pilbara. Diapirism is not believed to be the cause of the deformation elements; rather, it may have modified the deformation elements.

The Pilbara Craton has a strong northeast and northwest oriented (almost conjugate) anisotropy, best visible in regional gravity data. The curvilinear nature of the main shear zones of the northwest Pilbara (the Sholl and Wallaringa Shear Zones) has exerted a major influence on the development of the structural geology of the west and central Pilbara Craton. The central Pilbara is a geophysically, geochemically and lithologically distinct tectono-stratigraphic block between the west and east Pilbara. The regional geophysical data provide information on the steep and deep (to 14km) nature of the granite-greenstone contacts, and identifies what lineaments are significant in terms of transposition of greenstone stratigraphy (i.e., what are domain boundaries).

Meso- to epithermal Au was deposited in at least three and possibly four periods, and was mostly associated with the development of shear zones during regional shortening and later extension events. Four VMS mineralisation events were associated with extension and greenstone development. Sn-Ta mineralisation occurred with felsic granitoids during the last stages of cratonisation.