Fluid mixing in orogenic gold deposits: Evidence from the H-O-Sr isotope composition of the Val-d'Or vein field (Abitibi, Canada)

Abstract

Quartz and tourmaline from the Val-d'Or, Québec (Canada) orogenic gold vein field have oxygen isotope fractionation indicating equilibrium over a range of temperature (280–492°C). The range of δ18O values is from 9.2 to13.8‰, and from 6.5 to 9.5‰, for quartz and tourmaline, respectively. The hydrogen isotope composition of tourmaline has a range from −63 to −13‰. The initial Sr isotope composition (87Sr/86Sr(i)) of vein tourmaline at the time of mineralization ranged from 0.700710 to 0.702246. Vein carbonates have a similar range in Sr isotope composition, form 0.701243 to 0.703641. A series of samples from the Sigma deposit displays no systematic variation with depth for δ18O values of quartz or tourmaline, δD values of tourmaline, temperature of equilibrium, or 87Sr/86Sr(i). The 87Sr/86Sr(i) of local komatiite, basalt, andesite, grauwacke and granodiorite, at 2.7Ga, ranged from 0.681971 to 0.7128706. Country rocks with low 87Sr/86Sr(i) are likely a consequence of hydrothermal resetting of the Rb/Sr system in these samples.Covariation of the calculated equilibrium δDH2O and δ18OH2O with quartz-tourmaline equilibrium temperatures indicates mixing between a low-δ18O (<1.5‰), high δD (>−10‰), low temperature (<280°C) upper crustal fluid, and a high-δ18O (>9.3‰), low δD (<−40‰), high temperature (>490°C) deep-seated metamorphic fluid. At temperatures below the critical point for low-salinity hydrothermal fluids, δDH2O are affected by liquid-vapour phase separation, yielding the high δD values characteristic of the upper crustal fluid. A broad covariation between δ18OH2O and the 87Sr/86Sr(i) of tourmaline is consistent with mixing of two fluids from two reservoirs with different Sr concentrations and δ18OH2O values. The low 87Sr/86Sr(i) (<0.7007) inferred for the deep-seated metamorphic fluid end-member is consistent with Archean prograde metamorphic dewatering of typical volcanic and sedimentary country rocks of the Val-d'Or area. The higher 87Sr/86Sr(i) (>0.7022) of the upper crustal fluid end-member most likely resulted from a long history of water-rock exchange between Archean seawater and carbonate and radiogenic plutonic rocks of the Abitibi sub-province.