Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
6436076 | Chemical Geology | 2016 | 12 Pages |
â¢Coupled H-O-Sr isotope composition of tourmaline and quartz documentedâ¢Val-d'Or orogenic gold deposits formed by mixing of two fluidsâ¢Supracrustal fluid: T < 280 °C, δ18O < 2.8â°, δD > â 10â°, 87Sr/86Sr(i) > 0.7022â¢Deep-seated fluid: T > 490 °C, δ18O > 9.3â°, δD < â 40â°, 87Sr/86Sr(i) < 0.7007
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 to13.8â°, and from 6.5 to 9.5â°, for quartz and tourmaline, respectively. The hydrogen isotope composition of tourmaline has a range from â 63 to â 13â°. 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 2.7 Ga, 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 (< 1.5â°), high δD (>â 10â°), low temperature (< 280 °C) upper crustal fluid, and a high-δ18O (> 9.3â°), low δD (<â 40â°), 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.
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