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

Beaudoin, Georges
Published in Chemical Geology. 2016, vol. 437, p. 7-18
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 2.7 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 ($_amp_$lt; 1.5‰), high δD ($_amp_$gt;− 10‰), low temperature ($_amp_$lt; 280 °C) upper crustal fluid, and a high-δ18O ($_amp_$gt; 9.3‰), low δD ($_amp_$lt;− 40‰), high temperature ($_amp_$gt; 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) ($_amp_$lt; 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) ($_amp_$gt; 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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Other version: http://linkinghub.elsevier.com/retrieve/pii/S0009254116302388
Research groups Isotope Geochemistry, Geochronology and Thermochronology
Mineral Resources and Geofluids
(ISO format)
BEAUDOIN, Georges, CHIARADIA, Massimo. Fluid mixing in orogenic gold deposits: Evidence from the H-O-Sr isotope composition of the Val-d'Or vein field (Abitibi, Canada). In: Chemical Geology, 2016, vol. 437, p. 7-18. https://archive-ouverte.unige.ch/unige:83687

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Deposited on : 2016-05-17

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