Stable isotopes (H, C, O) and 3D fluid flow modeling constraints on gold endowment along the Augmitto-Bouzan orogenic gold deposit (Abitibi subprovince, Quebec)

The Augmitto-Bouzan deposit is a 12 km long segment of the Larder Lake-Cadillac Deformation Zone (LLCDz) south of Rouyn-Noranda (Québec, Canada) that is characterized by an uneven gold distribution hosted in quartz-carbonate ± tourmaline veins within Piché Group ultramafic rocks. This study compares the fluid flow conditions between the variable gold-endowed sectors to identify deposit-scale processes responsible for gold endowment. Stable isotopes indicate that quartz and tourmaline have equilibrium temperatures (228–⁠420 °C) that likely define a high vertical thermal gradient (~ 30 °C/100 m) along the LLCDz. Covariation between temperature and computed δ¹⁸O_H2O and δD_H2O is interpreted to result from mixing between a high temperature (> 420 °C), high δ¹⁸O (> 10.8‰), and low δD (< –29‰) deep-seated metamorphic fluid, and a low temperature (< 230 °C), low δ¹⁸O (< 4‰) and high δD (~ 0‰) upper crustal pore fluid. Local upwelling of auriferous deep-seated fluid, shown by interpolation of δ¹⁸O_H2O in the gold-endowed Augmitto-Cinderella and Astoria segments, was likely focused along higher permeability deformation-related pathways. Sectors of low gold endowment have lower δ¹⁸O_H2O and fluid/rock ratios, likely reflecting a larger proportion of upper crustal fluid and differences in fluid-flow behavior. Modeling of fluid flow shows that this is due to 1) weaker metamorphic fluid flux in the thinner band of Piché Group rocks and 2) more porous volcanic rocks north of the LLCDz, drawing more pore fluid into the fault. We suggest that most of the variation of gold endowment is related to variations in advection of auriferous metamorphic fluid along the segment, whereby a weaker metamorphic fluid flux or increased admixture of upper crustal fluids decrease the gold potential along the LLCDz.