Regional structural controls of gold mineralisation, Bendigo and Castlemaine goldfields, Central Victoria, Australia

Abstract

The Bendigo and Castlemaine goldfields are classic examples of structurally controlled orogenic gold deposits in the Bendigo Zone of central Victoria, SE Australia. Detailed mapping and biostratigraphic interpretation has led to a better understanding of the regional structural controls of this type of gold-quartz mineralisation. Mineralised quartz veins are hosted by the Castlemaine Group, an Early-to-Middle Ordovician turbidite succession at least 3,000 m thick. Gold deposits are controlled by low-displacement faults that are clustered into several belts (the goldfields) indicating a regional structural control. The timing of mineralisation overlapped with that of the major period of deformation including folding, cleavage development and regional faulting. The Bendigo and Castlemaine goldfields are located in an area termed the Whitelaw thrust sheet bounded by two unmineralised, high-displacement, regional-scale faults. Mapping has revealed an interrelationship between the regional-scale faults, regional structural style and goldfield location. The goldfields lie immediately west of the boundary between the upper and lower portions of the thrust sheet and are characterised by symmetric folds with sub-horizontal to synclinal enveloping surfaces, relatively low co-axial strains and moderate cleavage development. The non-gold-bearing areas immediately east of each goldfield correspond with the lower part of the Whitelaw thrust sheet and are characterised by higher non-coaxial strains, stronger cleavage and folds with wide west-dipping limbs giving rise to easterly vergent sections and steeply west-dipping enveloping surfaces. That mineralisation was an integral part of the thin-skinned style of deformation in the central Bendigo Zone is indicated by timing relationships and the interrelationship between local-scale mineralised structures and regional-scale features such as large-displacement unmineralised faults, regional variations in fold style and overall thrust sheet geometry. The work supports previous models that suggest mineralised fluids were focussed along a linked system of deep-seated faults. The primary conduits may have been major regional-scale ‘intrazone’ faults, which are inferred to sole into detachments near the base of the Castlemaine Group. It is proposed that these structures linked with minor intrazone faults and then with networks of low-displacement mineralised faults that were strongly controlled by folds. The location of minor intrazone faults was probably controlled by internal thrust sheet geometry. The distribution of gold deposits and of gold production suggests that maximum fluid flow was concentrated along the eastern margins of networks of low-displacement faults.