Abstract

The Boulder-Lefroy shear zone, developed in the ca. 2.7 to 2.65 Ga Archean greenstone sequence of the Kalgoorlie terrane in the Yilgarn craton, is remarkable for the number and importance of gold deposits it hosts. These include, from south to north, the following gold districts (with tons of contained Au): St Ives (253 t), Hampton-Boulder-Jubilee (123 t), Golden Mile (1,821 t) plus Mount Charlotte (219 t), and Paddington-Broad Arrow (112 t). Detailed structural studies reveal that the shear zone system developed in two main stages, the first related to a phase of north-northwest–south-southeast–trending folding and thrusting (regional D2) and the second related to sinistral shearing (regional D3) that reactivated and linked the preexisting north-north-west–south-southeast–trending network of thrust planes. Sinistral movement resulted in only limited displacement (less than 12 km) and evolved from a ductile shearing in broad zone toward semibrittle shearing along narrow planes (regional D4). The structures suggest a temporal evolution from crustal thickening during D2 through lateral escape during D3 to the development of brittle structures during D4.

The Fimiston and Oroya lodes at the Golden Mile in Kalgoorlie are also exceptional in their gold endowment and associated alteration and were most likely developed during D2. All other deposits along the shear zone system were formed within 25 m.y. of the Fimiston and Oroya lodes, late in the tectonic history of the terrane, either during D3 or D4. Intense mineralization within and around the shear zone resulted from multiscale focusing of mineralizing fluids. At a regional scale, fluids were focused into the shear zone system and its large anticlines which expose deeper mafic and ultramafic rocks. At a local scale, fluids were further focused into structural pathways resulting from strain-induced permeability in rocks with contrasting rheology.

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