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

Abstract

Gold quartz vein deposits in the Wattle Gully mine arefault-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. Over 10 6 metric tons (t) of vein quartz, averaging 10.5 g/t Au, occur within a dilatant jog in this structure. 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 sediments on adjacent west- dipping fold limbs. Gold mineralization is also associated with several minor east-clipping reverse faults and a bedding-parallel fault whose development is related to flexural slip during fold tightening. 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. Arsenopyrite and pyrite are the major sulfide phases, but sphalerite, galena, chalcopyrite, and pyrrhotite are also present. The highest Au grades tend to be localised in vein systems that are in proximity to carbonaceous slates. Wall-rock alteration is minor and restricted mainly to narrow zones adjacent to major vein systems. Sulfur isotope fractionation between galena and sphalerite indicates mineralization occurred at temperatures around 300 degrees C. The delta 34 S compositions of pyrite and arsenopyrite are nearly uniform throughout the mine (1.5-2.8%) but are generally not in equilibrium with the delta 34 S values of associated base metal sulfides (0.7- 5.3%). The delta 18 O values for vein quartz vary between 15.6 and 18.0 per mil and correspond to delta 18 O values.