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Fluid-rock sulfidation reactions control Au-Ag-Te-Bi precipitation in the Val-d’Or orogenic gold vein field (Abitibi subprovince, Canada)

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Abstract

The Val-d’Or vein field (VVF), located in the southern Abitibi subprovince (Québec, Canada), is host to ~ 47 Moz gold and is therefore an example of a greenstone-hosted orogenic gold district. Gold is contained in quartz-tourmaline-carbonate veins that cut As-poor intermediate to mafic volcanic and intrusive rocks, including dioritic, granodioritic and gabbroic sills, dikes, stocks, and plutons. Five investigated orebodies (Goldex, Triangle, Plug #4, Pascalis Gold Trend, Beaufor) host gold in vein- and wallrock-hosted pyrite-rich sulfide aggregates (> 95 vol%) that show a porous core domain (Py1), with abundant inclusions of carbonate, silicate, and Fe-oxides up to several tens of µm in size. A homogeneous pyrite rim domain (Py2) surrounds Py1 and contains most of the gold as native gold and polymetallic (Au-Ag-Te-Bi) inclusions, primarily calaverite and petzite. The two pyrites show different Au and As contents (Py1 = Au ≤ 30 ppm; As ≤ 67 ppm; Py2 = Au ≤ 1250 ppm; As ≤ 550 ppm). Pyrite shows a ubiquitous shift in δ34S values of up to + 3.0‰ from Py1 (δ34S =  − 0.4‰ to 5.8‰, n = 32) to Py2 (δ34S = 0.0‰ to 6.3‰, n = 59) and records a small, slightly negative Δ33S signature between – 0.20‰ and 0.01‰. The δ34S shift suggests that removal of reduced sulfur species from auriferous hydrothermal fluids causes the formation of inclusion-hosted gold in Py2 by a decrease in the fluid sulfur fugacity (fS2) through wallrock sulfidation of Fe-oxides. The shift also correlates with locally enriched Co and Ni concentrations in Py1 (< 1 wt%), compared to lower, oscillatory zoned concentrations (< 0.1 wt%) in Py2, respectively, indicating an overall decrease in fluid oxygen fugacity (fO2). Contemporaneously, a decrease in fluid tellurium fugacity (fTe2) drives polymetallic inclusion-hosted gold formation in Py2, initially as calaverite followed by increasingly Ag-bearing petzite and hessite. The multiple sulfur isotopes and trace element compositions recorded in pyrite in the VVF indicate that a homogeneous fluid reservoir introduced gold-sulfide complexes. Even if considered a localized process at the ore-shoot scale, fluid-wallrock sulfidation reactions can lead to a coupled decrease in fS2, fO2, and fTe2 of auriferous hydrothermal fluids in a greenstone-hosted As-poor gold district.

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Modified from Reich et al. (2005) and Pokrovski et al. (2019). b A logfTe2-logfS2 diagram showing the formation of increasingly Ag-rich tellurides during fS2 decrease and Py2 growth, at conditions of T = 300 °C, P = 1 bar. Modified from Afifi et al. (1988a). Bn = bornite; ccp = chalcopyrite; po = pyrrhotite; py = pyrite; VVF = Val-d’Or Vein Field

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Acknowledgements

We appreciate financial support for this study by the Canada First Research Excellence Fund Metal Earth project (MERC-ME-2023-37). C.L. acknowledges support from the Canada Research Chair Program. The authors greatly appreciate feedback from K. Kelley, M. Fayek and in particular P. Mercier-Langevin, whose comments helped to significantly improve the state of this manuscript. M.H. greatly thanks at Université Laval M. Choquette, S. Coté, E. Rousseau, at Canadian Malartic Corp. N. Houle, M. Bilodeau, at Agnico Eagle Mines Ltd. R. Morel, J.A. Marcotte, D. Yergeau, at Eldorado Gold Corp. J. Thelland, É. Gagnon, M. Le Bacq, B. Gagnon and at Probe Metals Inc. M. Gagnon and B. Beh to accommodate the project, micro-analytical work and for logistical support. B. Wing is thanked for bulk multiple sulfur isotope analyses, and M. Aleshin is thanked for his analytical support during the acquisition of in situ sulfur isotope analyses by SIMS. The authors acknowledge the facilities and the scientific and technical assistance of Microscopy Australia at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State, and Commonwealth Governments.

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Correspondence to Michael Herzog.

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Herzog, M., LaFlamme, C., Beaudoin, G. et al. Fluid-rock sulfidation reactions control Au-Ag-Te-Bi precipitation in the Val-d’Or orogenic gold vein field (Abitibi subprovince, Canada). Miner Deposita (2024). https://doi.org/10.1007/s00126-024-01247-6

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