Episodic gold mineralisation correlated with discrete structural events at Ballarat East,southeast Australia

New ⁴⁰Ar/³⁹Ar geochronological data suggest orogenic gold mineralisation at the Ballarat East deposit, southeast Australia, occurred in three main episodes at ca. 445–435 Ma, ca. 420–415 Ma and ca. 380–370 Ma. The gold mineralisation is localised in muscovite-bearing quartz and quartz-carbonate veins hosted in the steep faults (70–90°), on limbs of tight and isoclinal folds in an Ordovician turbidite sequence, and within west-dipping (≤45°) faults, historically known as leather jacket lodes. Initiation of the ≤45° faults that are confined to fold culminations, begins at ca. 445 Ma, with peak metamorphic conditions at 440 Ma. The earliest vein sets (V₁), were emplaced on limb thrusts at ca. 445–435 Ma and are characterised by arsenopyrite-dominated quartz veins. These V₁ veins parallel arsenopyrite-rich shale units, historically referred to as ‘indicator beds’. Both the steep and ≤45° faults were reactivated during fold amplification with deposition of the V₂ auriferous veins at ca. 420–415 Ma. A later set of auriferous veins (V₃–V₄) with ages of 380–370 Ma, dominated by pyrite-sphalerite-galena-white-mica quartz-(V₃) or carbonate-rich (V₄) veins are predominantly associated with reactivation of the ≤45° west-dipping faults. This new geochronological data constrains the local kinematic history of the Ballarat East deposit and has regional implications. The V₁–V₂ vein development appears to be synchronous across the entire western section of the Lachlan Orogen, where previous studies have suggested that initial gold mineralisation was linked to orogenesis at ～440 Ma, as a result of metamorphic devolatilisation reactions in the lower crust. In contrast, a close spatial and temporal relationship exists between the felsic dykes and the mineralisation recognised in the V₃–V₄ veins. The deformation that accompanies V₃–V₄ vein development is attributed to small, localised events during east-west shortening, utilising pre-existing fold and fault structures. The origin of the fluids producing the V₃–V₄ veins may be metamorphic devolatilisation associated with widespread felsic magmatism that occurred at this time across central Victoria.