Microchemical and sulfur isotope constraints on the magmatic and hydrothermal evolution of the Black Swan Succession, Western Australia

Abstract

The Black Swan Succession consists of an Archean bimodal dacite-komatiite association largely dominated by ultramafic cumulates hosting a number of massive and disseminated magmatic sulfide orebodies. Although it was affected by multiple alteration events, the low degree of penetrative deformation allowed the preservation of magmatic textures and stratigraphic relationships. In this context, we apply a combined multiple sulfur isotope and microchemical approach to unravel the potential of sulfides to simultaneously retain information about their magmatic and hydrothermal history. Negative Δ 33 S signatures in both massive (−\u20090.56‰) and disseminated (−\u20090.66‰) sulfide orebodies reflect the assimilation of photochemically derived sulfur into the komatiite magma, which attained sulfide liquid supersaturation through the assimilation of crustal material analogous to the intercalated breccia dacite hosting colloform sulfides (Δ 33 S\u2009=\u2009−\u20090.91‰). The slightly different Δ 33 S signatures between disseminated and massive orebodies reflect the assimilation of different proportions of crustally derived sulfur, and imply that the respective sulfide liquids segregated from distinct komatiite magma pulses. Subsequent alteration episodes imparted to the Black Swan Succession a concentric zonation consisting of a serpentinite core and talc-carbonate margins. The Black Swan disseminated orebody extends through both alteration styles without major variations in the dominant millerite-pyrite sulfide assemblage. Pyrite in both serpentinite and talc-carbonate samples exhibits a consistent trace element signature suggesting a relative immobile behavior of Ni and Co during the percolation of the talc-carbonate fluids. Conversely, variable δ 34 S signatures between the two alteration styles reflect mobilization of sulfur during talc-carbonate alteration. Whereas the 2‰ range of δ 34 S values in serpentinites can be attributed to moderate equilibrium fractionation processes, the 10‰ range of δ 34 S values in talc-carbonate rocks suggests that highly oxidizing fluids promoted the mobilization of sulfur as SO 4 2− , which decreased the δ 34 S of the disseminated sulfides. Furthermore, sulfur isotope and microchemical signatures of euhedral pyrite from intercalated dacitic breccia indicate that this pyrite generation was deposited from a late hydrothermal event, which is characterized by variable and least negative Δ 33 S signatures (between −\u20090.20‰ and −\u20090.48‰) relative to all sulfide orebodies. These features suggest that the late fluids developed in a heterogeneous hydrothermal system with a minor component of sulfur recycled from the sulfide orebodies.