A geochronological framework for orogenic gold mineralisation in central Victoria, Australia

New ⁴⁰Ar/³⁹Ar geochronological data support, and significantly expand upon, preliminary age data that were interpreted to suggest an episodic and diachronous emplacement of gold across the western Lachlan fold belt, Australia. These geochronological data indicate that mineralisation in the central Victorian gold province occurred in response to episodic, eastward progressing deformation, metamorphism and exhumation associated with the formation of the western Lachlan fold belt. Initial gold formation throughout the Stawell and the Bendigo structural zones can be constrained to a broad interval of time between 455 and 435 Ma, with remobilisation of metals into new structures and/or new pulses of mineralisation occurring between 420 and 400 Ma, and again between 380 and 370 Ma, linked to episodic variations in the regional stress-field and during intrusion of felsic dykes and plutons. This separation of ages is incompatible with the view that gold emplacement in the western Lachlan fold belt was the result of a single, orogen-wide event during the Devonian. A distinct phase of gold mineralisation, characterised by elevated Cu, Mo, Sb or W, is associated with both Late Silurian to Early Devonian (~420 to 400 Ma) and Middle to Late Devonian (~380 to 370 Ma) magmatism, when crustal thickening and shortening during the ongoing consolidation of the western Lachlan Fold Belt led to extensive melt development in the lower crust and resulted in widespread magmatism throughout central Victoria. These ~420 to 400 Ma and ~380 to 370 Ma occurrences, best exemplified by the Wonga deposit in the Stawell structural zone and many of the Woods Point deposits in the Melbourne structural zone, but also evidenced by occurrences at Fosterville and Maldon in the Bendigo structural zone, clearly formed synchronous with, or post-date, the emplacement of plutons and dykes, and thus are spatially (if not genetically) related to melt generation at depth. This later, magmatic-associated and polymetallic type of gold mineralisation is economically subordinate to the earlier, metamorphic-associated type of gold deposition in the Stawell and Bendigo structural zones, but tends to be the dominant style in the Melbourne Zone. These new geochronological constraints, together with zircon U-Pb data from felsic intrusive rocks of known relationship to gold mineralisation, demonstrate that initial hydrothermal alteration associated with gold emplacement in the western Lachlan fold belt was metamorphic-related, predating the emplacement of granite plutons by as much as 80 million years. This timing differs from other important orogenic gold districts where gold deposition is closely associated spatially with felsic magmatism. The early introduction of metamorphically derived fluids well before magmatism may reflect variations in the timing of peak metamorphic conditions at different crustal levels in an accretionary prism undergoing simultaneous deformation and erosion. Consequently, no genetic link exists between the main phase(s) of gold mineralisation and magmatism in the central Victorian gold province. With the exception of formation of a minor magmatism-related and geochemically-distinct mineralisation style at about 420 to 400 Ma, and again at about 380 to 370 Ma, the apparent spatial relationship between gold mineralisation and felsic intrusions is merely the result of melts and fluids being channelised along the same structures.