Evidence of magmatic degassing in Archean komatiites : insights from the Wannaway nickel-sulfide deposit, Western Australia

Magmatic degassing from komatiite lava flows potentially influenced the geochemical evolution of the Archean atmosphere and hydrosphere. We argue that the escape of SO2-rich volatiles from komatiites impacted on the mineralogical, geochemical and isotopic composition of associated nickel-sulfide mineralization leaving behind detectable and measurable footprints that can be best observed where the polarity of the magmatic sequence is clearly recognizable. Here we focus on the ore-bearing sequence of the Archean komatiite-hosted N01 nickel-sulfide orebody at Wannaway, Yilgarn Craton, Western Australia. This deposit displays a volcanic sequence with a well-defined succession of stratigraphically-correlated facies comprising a massive sulfide horizon at the base of the channelized komatiite flow, overlain by matrix and disseminated sulfide mineralization. Pyrrhotite is the dominant sulfide phase in the lower part of the ore profile. The amount of troilite gradually increases from the base of the matrix ore over several meters up-sequence, eventually becoming dominant at the expense of pyrrhotite. In the upper portion of the mineralized sequence troilite is associated with accessory Mn sulfide alabandite (MnS), which is usually reported in reduced terrestrial and extra-terrestrial environments. Such mineralogical and volcanological features are consistent with upwards decreasing in fS2 and fO2 away from the basal contact of the komatiite flow. After evaluating the possible role of metamorphism, the pyrrhotite-troilite-alabandite assemblage and the progressive up-sequence decrease of the pyrrhotite/troilite ratio across the upper part of the mineralized sequence are interpreted as magmatic and indicative of progressive loss of sulfur with concomitant establishment of reducing conditions within the sulfide melt ponding at the base of the komatiite lava. In this context, the investigation of spatially constrained sulfur isotopic signatures allows to isolate the multiple sulfur fractionation processes that impacted on sulfide mineralization and ultimately permits the identification of the isotopic shift associated with magmatic degassing. Following this approach we recognize two distinct sulfur isotope exchanges processes triggered by 1) assimilation of sulfidic shales during emplacement of the komatiite flow, and 2) equilibration between the sulfide melt and the sulfur dissolved in the silicate melt. We finally correlate the remaining δ34S depletion up-stratigraphy with the loss of heavy sulfur isotopes through magmatic degassing of SO2-rich volatiles from the ultramafic flow. The emission of SO2 upon emplacement and cooling of the magma flow would also explain the progressive reducing fO2 and fS2 conditions indicated by variations in mineral assemblages from the base of the komatiite upwards