Timing of gold mineralisation in the western Lachlan Orogen, SE Australia: A critical overview

Abstract

The western sub-province of the Paleozoic Lachlan Orogen in Victoria is dominated by thick turbidite sequences overlying Cambrian basement volcanics. The region was subjected to multiple Cambrian to Late Devonian regional deformation events, followed by extensive post-tectonic granitic magmatism. The western Lachlan Orogen is considered a typical 'orogenic' gold province and hosts a large number of goldfields, including the world-class Bendigo-Ballarat goldfields. A variety of geochronological methods (e.g. U-Pb zircon; Re-Os sulfide; 40Ar/ 39Ar whole-rock, mica) have been used to constrain the timing of gold mineralisation, and the relationship to metamorphism/deformation/ magmatism. Regional granitic magmatism is relatively well constrained from U-Pb zircon dating, with the timing of deformation/metamorphism and gold mineralisation reliant largely on 40Ar/ 39Ar dating results. Owing to inconsistencies in the available 40Ar/ 39Ar data and recent revisions to 40Ar/ 39Ar monitor ages and decay constants, we recalculate and re-evaluate all existing 40Ar/ 39Ar age results. These revisions confirm that the western Lachlan Orogen is characterised by multiple deformation/ metamorphism events, with the Stawell structural zone deformed during the ca 500 Ma Delamarian and ca 445 Ma Benambran orogenies, the Bendigo Zone deformed during the Benambran orogeny (with minor Tabberabberan overprinting), and the Melbourne Zone affected by the ca 380-370 Ma Tabberabberan orogeny. Post-tectonic granitic magmatism occurred in two main time intervals, the Early Devonian (ca 400 Ma) and the Late Devonian (ca 380-370 Ma), with the former limited to the Stawell and northwest Bendigo Zones, and the latter distributed throughout the Bendigo and Melbourne Zones and southeast Stawell Zone. Gold mineralisation occurred in two main episodes at ca 445 Ma and ca 380-370 Ma, with another possible (minor) event at ca 410-400 Ma. The ca 445 Ma event is prevalent across the Stawell and Bendigo Zones, with Late Devonian gold mineralisation restricted to the Melbourne and eastern Bendigo Zones. The timing of the two main events is supported by geological constraints, the reproducibility of 40Ar/ 39Ar results and, in the case of the Bendigo goldfield, coincidence with Re-Os data. Suggestions of a single Devonian age gold mineralisation event are not supported by the available data. The two main gold mineralisation episodes (ca 445 Ma; ca 380-370 Ma) coincide with the waning stages of the Benambran and Tabberabberan orogenies, respectively. Crustal thickening and consequent metamorphic devolatilisation during the Benambran orogeny may have been the main cause of fluid flow related to gold mineralisation at ca 445 Ma. In contrast, crustal anatexis is considered responsible for metamorphic fluid generation and Early Devonian gold mineralisation.