Structural and geochemical controls on the development of turbidite- hosted gold quartz vein deposits, Wattle Gully mine, central Victoria, Australia

Gold quartz vein deposits in the Wattle Gully mine are fault-related structures developed in a low-grade metamorphosed Ordovician quartz-rich flysch sequence. The major mineralization is associated with a high-angle reverse fault (Wattle Gully fault) that has a maximum displacement of 40 m. Fold geometry has controlled fault geometry and resulted in the localization of the jog segment where the Wattle Gully fault traverses an east-dipping fold limb to link two more steeply dipping, bedding-parallel fault segments on adjacent west-dipping fold limbs. Mineralization occurs within fault-fill veins and extension veins adjacent to faults. The geometry and internal structures of veins indicate that vein growth has been controlled by repeated fluctuations in fluid pressure and shear stress associated with fault-valve behavior. Vein mineralogy is dominated by quartz, with minor carbonates, chlorite, white mica, albite, and apatite. The isotopic and bulk chemistry of fluids is consistent with derivation by metamorphic devolatilization or via exchange with metamorphic rocks at mid- to lower crustal levels late during crustal thickening and prior to widespread lower crustal anatexis in the Late Devonian. The timing and structural setting of gold mineralization suggest that the regional-scale architecture of fluid migration has been controlled by the development of a linked system of thrusts that propagated into the Ordovician turbidite sequence late during crustal shortening. The evolution of fluid chemistry in the Wattle Gully fault is interpreted to have been controlled by redox reactions, fluid mixing and phase separation in response to fluid pressure cycling, and fluid-rock interaction during fault-valve behavior. -from Authors