Entrapment and re-equilibration of gold-bearing fluid inclusions (Sigma deposit - Abitibi belt - Canada). A multidisciplinary study.

Sigma is a large network of steeply dipping, Au-mineralized quartz-tourmaline-carbonate shear veins and subhorizontal extension veins. We have used 1) petrographic identification of eight fluid inclusions assemblages (FIAs) trapped during quartz, tourmaline, and gold precipitation within the veins; 2) microthermometry; 3) Raman spectroscopy; and 4) Laser Ablation-ICP-MS of single fluid inclusions to determine PTX estimates of the vein fluid. Either an H2O-NaCl-B\ub1CH4-N2 (aqueous) fluid or an H2O-NaCl-CO2-B (aquo-carbonic) fluid is trapped within single FIAs, which are formed by liquid- and vapor-rich inclusions. Within all FIAs, inclusions show specific morphologies. Relatively large inclusions are dendritic, hook-like, and annular, while smaller inclusions are relatively more regular and show sharp tips. In both aqueous and aquo-carbonic fluid types, liquid-rich and vapor-rich inclusions have salinities of 4-8 wt% NaCleq and 0.5-4 wt% NaCleq, respectively. Homogenization occurs into the liquid and vapor phases. In the aquo-carbonic fluid, the carbonic phase melts between -56.5 and -57 \ub0C. Clathrate melts between about 6.0 C and 8.0 \ub0C, and ThCO2 ranges from -12 to 31 \ub0C. The distribution of Thtot within individual FIAs show modes between 330 and 385 \ub0C, but low-temperature outliers are common (ranges: 100-180 \ub0C). These data, inclusion morphologies, and tests for equilibrium coexistence of liquid- and vapor-rich inclusions show that post-entrapment modifications affected the FIAs. We interpret these features as indication of an \u201cimplosion\u201d re-equilibration regime, and propose a general method to interpret microthermometric data of heterogeneously trapped fluids in terms of distinct regimes. Following this method, the properties of the high-Thtot inclusions of Sigma are those approaching the pristine fluid. Notably, the selection of these inclusions contrasts with that commonly used for the estimation of the PT trapping conditions of heterogeneously trapped fluids at equilibrium. However, the properties estimated with the high-Thtot inclusions predict gold and tourmaline solubilities that agree with the bulk concentrations of B and Au measured with Laser Ablation-ICP-MS in single fluid inclusions. They also predict a quartz solubility that agrees with that estimated independently. Thus, our data are consistent with gold precipitation within the quartz-tourmaline veins after unmixing of a parent H2O-NaCl-CO2 fluid at about 350-400 \ub0C and 300-500 bars