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 s tructures developed in a low-grade metamorphosed Ordovician quartz-ric h flysch sequence. The major mineralization is associated with a high- angle reverse fault (Wattle Gully fault) that has a maximum displaceme nt 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 h as controlled fault geometry and resulted in the localization of the j og segment where the Wattle Gully fault traverses an east-dipping fold limb to link two more steeply dipping, bedding-parallel fault segment s on adjacent west-dipping fold limbs. Cold mineralization is also ass ociated with several minor east-dipping reverse faults and a bedding-p arallel fault whose development is related to flexural slip during fol d tightening. Mineralization occurs within fault-fill veins and extens ion veins adjacent to faults. The geometry and internal structures of veins indicate that vein growth has been controlled by repeated fluctu ations in fluid pressure and shear stress associated with fault-valve behavior. Vein mineralogy is dominated by quartz, with minor carbonate s, chlorite, white mica, albite, and apatite. Arsenopyrite and pyrite are the major sulfide phases, but sphalerite, galena, chalcopyrite, an d pyrrhotite are also present. The highest Au grades tend to be locali sed in vein systems that are in proximity to carbonaceous slates. Wall -rock alteration is minor and restricted mainly to narrow zones adjace nt to major vein systems. Sulfur isotope fractionation between galena and sphalerite indicates mineralization occurred at temperatures aroun d 300 degrees C. The delta(34)S compositions of pyrite and arsenopyrit e are nearly uniform throughout the mine (1.5-2.8 parts per thousand) but are generally not in equilibrium with the delta(34)S values of ass ociated base metal sulfides (0.7-5.3 parts per thousand). The delta(18 )O values for vein quartz vary between 15.6 and 18.0 per mil and corre spond to delta(18)O values around 10 per mil in the hydrothermal fluid responsible for mineralization. Vein carbonates have delta(18)O value s that are close to equilibrium with those in quartz. The delta(13)C v alues in vein carbonates range from -2.8 to -6.8 per mil. Fluid inclus ions in vein quartz contain C-O-H fluids of variable compositions, but three main types are recognized. Type I fluid inclusions are two-phas e aqueous inclusions with low salinities, low CO2 and CH4 contents, an d quite variable CO2/CH4 ratios. Homogenization temperatures between 1 60 degrees and 240 degrees C are consistent with trapping near 300 deg rees C and fluid pressures around 130 MPa. Type II inclusions contain up to 20 mole percent CO2 and have final homogenization temperatures i n the range of 280 degrees to 310 degrees C, high but variable CO2/CH4 ratios, and variable CO2 densities. Rare, high-density carbonic inclu sions (type III) contain less than 40 mole percent H2O and could have formed by phase separation from type II fluids. The isotopic and hulk chemistry of fluids is consistent with derivation by metamorphic devol atilization or via exchange with metamorphic rocks at mid- to lower cr ustal levels late during crystal thickening and prior to widespread lo wer crustal anatexis in the Late Devonian. The timing and structural s etting of gold mineralization suggest that the regional-scale architec ture of fluid migration has been controlled by the development of a li nked system of thrusts that propagated into the Ordovician turbidite s equence late during crustal shortening. The evolution of fluid chemist ry in the Wattle Gully fault is interpreted to have been controlled by redox reactions, fluid mixing and phase separation in response to flu id pressure cycling, and fluid-rock interaction during fault-valve beh avior. A major influence on gold deposition has been mixing between de eply sourced type II fluids and more reduced CH4-bearing fluids that e volved during fluid reaction with carbonaceous pelites in the alterati on envelope. Repeated episodes of mixing of these fluids, especially a t the dilatant jog, have been governed by the cyclic changes in hydrau lic head associated with fault-valve behavior.