A tale of gold transport and precipitation. The genesis of a giant mesothermal Au deposit from a multidisciplinary study (Sigma deposit - Canada).

The Sigma deposit is a large network of Au-mineralized, quartz-tourmaline veins of the Archean Abitibi greenstone belt. It produced 290 t of Au, and is a proxy example of metamorphogenic, mesothermal gold deposits. Here, we combine new constraints from a multidisciplinary study of this network to propose a new genetic model. This study considers a) a 3-D geometric reconstruction of network veins; b) a mass transfer study of the tourmaline- and albite-rich hydrothermal alteration haloes; c) a petrographic, microthermometric, Raman spectroscopy, and Laser Ablation-ICP-Mass Spectrometry study of co-genetic groups of fluid inclusions; and d) a fluid-mineral equilibria study. \nAt Sigma, the backbone of the vein network is a set of conjugated, steeply dipping, oblique-reverse shear zones with a vertical continuity of the order of kilometers. Steep veins are hosted by these shear zones, but are continuous for few hundreds of meters at best. They interconnect with vertically short splays and with small, subhorizontal extension veins. These interconnections are all gold rich (grade >25 g/t), showing that the mechanisms that controlled shear development and vein propagation controlled gold precipitation as well. About 95% of all veins are filled with quartz and tourmaline. Gold, found mostly within the veins, precipitated at all stages of vein filling and fluid-rock interaction. \nCo-genetic groups of fluid inclusions trapped within vein quartz host either an H2O-NaCl (aqueous) fluid or an H2O-NaCl-CO2 (aquo-carbonic) fluid. In both fluid types, liquid-rich and vapor-rich inclusions have low salinity (Liq: 4-8 wt% NaCleq; Vap:0.5-4 wt% NaCleq) and homogenization occurs mostly into the liquid and vapor phases. The distribution of Thtot within individual cogenetic groups show modes between 330 and 385 °C, but the total range is much larger, being ca. 200-420 °C. Microthermometric data and tests for equilibrium coexistence indicate that an “implosion” re-equilibration regime affected the co-genetic groups. Accordingly, we identify the properties of the high-Thtot inclusions as those approaching the pristine fluid. These properties are consistent with unmixing of a parent H2O-NaCl-CO2 fluid at about 350-400 °C and P<500 bars. Thus, in contrast with previous models, we identify aqueous and aquo-carbonic fluids as end-member fluids generated by unmixing. \nThe ranges of Na, K, B, and Au concentrations in the two fluid types have been measured by Laser Ablation-ICP-Mass Spectrometry of single fluid inclusion, and subsequently used to estimate pH, logaH2S and logaO2 during Au precipitation. In particular, concentration of B is in the 80-2200 ppm range for the two fluid types, and Au in the 0.5-5 ppm range. Fluid-mineral equilibria at 400 °C 300 bar and 400 °C 3 kbar have been used to test the hypothesis of low-pressure gold deposition. Results show that this hypothesis is valid, and that fluid pressure fluctuations of few tens of bars around 300 bar might have affected gold precipitation significantly. Thus, pressure fluctuations of the parent fluid caused by interconnection of distinct vein segments may have caused fluid unmixing and massive gold precipitation at Sigma. This interpretation implies that poorly connected vein networks are also relatively inefficient in precipitating gold, and their exploration should account for these features. \nCombining the estimated 70000 t of B deposited within the vein network with the measured range of B concentration within the fluid inclusions, we calculate that 0.1-2.3 km3 of hydrothermal fluid deposited all the tourmaline and gold at Sigma. This suggests that only small volumes of hydrothermal fluid were probably involved in the generation of this mineralised vein network. Simple mass-balance calculations show that metamorphic devolatilization of any dehydrating pelitic or igneous rock could not be the only process generating this volume of hydrothermal fluid.

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