Abstract
New data on the multiple sulfur isotope signature of Archean sulfides from country rocks and magmatic mineralization at the Moran deposit (Kambalda, Western Australia) were combined with previously published geochemical data to constrain the various stages of the dynamic evolution of this magmatic system, unveiling new insights into the transport mechanisms of sulfide liquids in komatiite magmas. Sulfides in the Archean magmatic and sedimentary host rocks of the komatiites display a unique mass-independent sulfur isotope signature (Δ 33 S), which records a photochemical reaction of sulfur in an oxygen-poor atmosphere prior to the Great Oxidation Event. Sedimentary rocks that are thought to be assimilated by komatiite show a distinctly positive Δ 33 S signature (+ 0.9 to + 2.4‰). Early ore sulfides situated above these sedimentary rocks contain relatively few valuable metals and display an overlapping Δ 33 S range (+ 0.6 to + 1.0‰). Subsequent but still early ore sulfides are situated above basalt, as the sedimentary rocks were thermo-mechanically eroded by the sulfide melt, displaying more mantle-like signatures (+ 0.2 to + 0.3‰) and valuable metal content - indistinguishable from the main ore deposit. This reflects a progressive dilution of the contaminant signature by the magmatic isotope signature of the komatiite liquid. Calculated volumes of the interaction of silicate melt and sulfide melt to explain the metal tenor of the ore and its Δ 33 S signature indicate a decoupling between chemical and isotopic signatures. This can be explained by upgrading the sulfide melt with valuable metals simultaneously with the dissolution of sulfur in the komatiite melt.