No mass-independent sulfur isotope fractionation in auriferous fluids supports a magmatic origin for Archean gold deposits

The source of the auriferous fluids that formed Archean greenstone gold deposits remains controversial; metamorphic fluids, fluids derived from felsic magmas, mantle fluids, and meteoric water have all been suggested. We report the results of ion microprobe (sensitive high-resolution ion microprobe stable isotopes) multiple sulfur isotope analyses (³²S, ³³S, ³⁴S) conducted to constrain the source of sulfur in four Archean gold deposits. The analyses are of pyrite samples that are genetically related to the gold mineralization from the Bellerophon, Victory, and Wallaby deposits in the Eastern Goldfi eld superterrane of the Yilgarn craton, Western Australia, together with sulfi de from black shale from the Golden Mile deposit and St. Ives gold camp. Ore-associated pyrite from the Beattie gold deposit of the Abitibi greenstone belt of the Superior craton in Canada was measured to test whether the results obtained from the Yilgarn craton apply to other Archean gold deposits. Sulfur analyses from sulfi de from shale show mass-independent fractionation (MIF) with δ³³S as high as +1.8%, whereas those from the 4 Archean gold deposits are dominated by mass-dependent fractionation, with only a minor contribution of MIF sulfur. In this respect, Archean gold deposits contrast with Archean NiS deposits, which are dominated by MIF sulfur. The absence of MIF in the pyrite from the four Archean gold deposits analyzed in this study is consistent with the auriferous fluid originating from a felsic magma or mantle source, but inconsistent with the metamorphic and meteoritic fluid hypotheses.