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

Quartz and tourmaline from the Val-d'Or, Quebec (Canada) orogenic gold vein field have oxygen isotope fractionation indicating equilibrium over a range of temperature (280-492 degrees C). The range of delta O-18 values is from 9.2 to 13.8%, 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 (Sr-87/Sr-86((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 delta O-18 values of quartz or tourmaline, delta D values of tourmaline, temperature of equilibrium, or Sr-87/Sr-86((i)). The Sr-87/Sr-86((i)) of local komatiite, basalt, andesite, grauwacke and granodiorite, at 2.7 Ga, ranged from 0.681971 to 0.7128706. Country rocks with low Sr-87/Sr-86((i)) are likely a consequence of hydrothermal resetting of the Rb/Sr system in these samples. Covariation of the calculated equilibrium delta D-H2O and delta O-18(H2O) with quartz-tourmaline equilibrium temperatures indicates mixing between a low-delta O-18 (<1.5%), high delta D (>-10%), low temperature (<280 degrees C) upper crustal fluid, and a high-delta O-18 (>9.3%), low dD (<-40%), high temperature (>490 degrees C) deep-seated metamorphic fluid. At temperatures below the critical point for low-salinity hydrothermal fluids, delta D-H2O are affected by liquid-vapour phase separation, yielding the high delta D values characteristic of the upper crustal fluid. A broad covariation between delta O-18(H2O) and the Sr-87/Sr-86((i)) of tourmaline is consistent with mixing of two fluids from two reservoirs with different Sr concentrations and delta O-18(H2O) values. The low Sr-87/Sr-86((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 Sr-87/Sr-86((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. (C) 2016 Elsevier B. V. All rights reserved.