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.

Keywords:

• Agnew
• gold mineralisation
• deformation
• Scotty Creek Basin
• Eastern Goldfields
• Lawlers

Acknowledgements

This paper is based on extensive discussions and fieldwork with the mine, resource and exploration geology teams at Agnew. We would really like to thank the geologists and field crews at Agnew for ongoing help, advice and feedback. We are also grateful for thorough reviews of the manuscript by Kevin Cassidy, Ivan Zibra, Nicolas Thebaud and an anonymous reviewer.

Disclosure statement

No potential conflict of interest was reported by the authors.