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

The Val-d'Or vein field (VVF), located in the southern Abitibi subprovince (Qu & eacute;bec, Canada), is host to similar 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 mu m 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 delta S-34 values of up to + 3.0 parts per thousand from Py1 (delta S-34 = - 0.4 parts per thousand to 5.8 parts per thousand, n = 32) to Py2 (delta S-34 = 0.0 parts per thousand to 6.3 parts per thousand, n = 59) and records a small, slightly negative Delta S-33 signature between - 0.20 parts per thousand and 0.01 parts per thousand. The delta S-34 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 (fS(2)) 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 (fO(2)). Contemporaneously, a decrease in fluid tellurium fugacity (fTe(2)) 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 fS(2), fO(2), and fTe(2) of auriferous hydrothermal fluids in a greenstone-hosted As-poor gold district.