Polyphase deformation and the structural controls on economic gold occurences within the Bendigo gold field, central Victoria, Australia

The Bendigo Goldfield is historically the largest producing goldfield within the Lachlan Fold Belt of southeastern Australia, and is a classic example of a structurally controlled turbidite-hosted gold deposit. Since 1993, Bendigo Mining Ltd. has been re-evaluating part of the goldfield and is in the process of developing an underground mine. The current development, which accesses eight auriferous reefs to a depth of ~800 m, presents a unique opportunity to re-examine the structurally controlled reefs, which characterise the central Victorian goldfields. The identification of multiple foliations, both proximal and distal to the mineralisation, prompted a reinvestigation of the deformation chronology, which revealed that the structural evolution of the goldfield was more complex than previously thought. A five-stage deformation chronology (D1-D5) based primarily on the recognition of overprinting tectonic foliations (S1-S5) is proposed. D1 to D3 represent distinct, yet progressive phases of ENE-WSW shortening possibly during the Benambran Orogeny (ca. 439-435 Ma), with D2 corresponding to the peak of deformation and metamorphism. N-S directed shortening during D4 indicates that a change in the principal shortening axis has occurred and as such, D4 could represent the later stages of the Tabberabberan Orogeny (ca. 381-377 Ma). Deformation associated with D5 suggests a period of NNW-SSE directed shortening, which may correspond to the younger Kanimblan Orogeny (ca. 360-340 Ma). A period of deformation and fault reactivation that post-dates both D5 and Jurassic dykes has been recognised within the goldfield. However, the extent of this deformation remains unclear and no associated tectonic foliation has been observed. At Bendigo, economic occurrences of gold are intimately related to the structural evolution of the goldfield and are associated with late-stage mineral phases, which post-date the syn-D2 quartz veins such as the classic saddle reefs. On the basis of structural, microstructural and paragenetic observations it is proposed that the deposition of gold occurred during D3, and more specifically, in association with the development of D3 kink bands. The D3 kinks vary in size from small-scale kink bands (S3) on a millimetre-scale, through to large-scale kinks (F3) at a kilometre-scale. The small-scale D3 kink bands consist of four sets of conjugate extensional kinks, which exhibit an approximately orthorhombic symmetry about a sub-vertical axis, suggesting deformation in response to triaxial strain. Spatial analysis of historical production data has revealed a number of previously unrecognised high-grade trends. These trends coincide with the intersection axis between a kink band axial plane and bedding. It is proposed that these intersections control ore shoot geometry and location, because they also correspond to the orientation of historically worked ore shoots and those encountered more recently by Bendigo Mining Ltd. A new understanding of the controls on ore shoots has considerable implications for future exploration within the goldfield, and elsewhere in central Victoria.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

James Cook University

Year dc:date.issued

2005

Author dc:creator

• Raine, Matthew David

Language dc:language

en