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Samples of sulfide mineral-hosted platinum-group element (PGE) mineralization from the Rum layered intrusion (NW Scotland) were studied to elucidate the crystallization history of base-metal sulfide minerals in magmatic ore deposits. A corollary aim was to examine the controls on platinum-group mineral exsolution from sulfide minerals in these settings. Secondary ion mass spectrometry data for sulfide minerals reveal significant sulfur isotopic heterogeneity in all the Rum sulfide mineral-bearing materials studied. For example, in a PGE-rich chromitite, we find a range of δ 34 S exceeding 10‰ (−4.3 to +5.9‰) in base-metal sulfide minerals within an area of &lt;1 cm 2 . Similar ranges of δ 34 S heterogeneity (~10‰), albeit shifted to values as low as −15‰, are observed in disseminated sulfide minerals in troctolite and peridotite lithologies sampled from the intrusion margins. The relatively light δ 34 S end member reflects crustal contamination of the Rum parental magmas (with δ 34 S of +1.9‰) during construction of the intrusion. We attribute the extension to relatively heavy δ 34 S in all samples to be at least partly a consequence of sulfur loss due to sulfide mineral breakdown at relatively low temperatures; on the basis of Rayleigh distillation modeling we estimate &lt;100°C. Degradation of primary sulfide minerals (pyrrhotite, pentlandite and chalcopyrite) is observed in the textures of all samples, as are low sulfur minerals such as chalcocite and digenite that are generally interpreted to result from low temperature oxidation processes. The chromitite sulfide minerals have S/Se values as low as 600, also signifying sulfur loss.  In situ  PGE abundance measurements in sulfide minerals from all samples indicate that desulfurization affected precious metal tenors in the chromitite sulfide minerals more than the other samples, and there is a strong spatial pattern of PGM occurring at sulfide grain margins and within Fe oxides produced by sulfide mineral breakdown. Collectively, our results suggest that low temperature sulfur loss, possibly as aqueous sulfate during alteration at low fluid-rock ratios, was capable of significantly modifying, but not completely erasing, primary isotopic heterogeneity in the Rum system. Aside from the effects of sulfur mobility in precious metal ore deposits, these findings highlight the micron-scale distances over which sulfur isotope heterogeneity occurs in sub-volcanic basaltic systems and show that sulfur mobility in such settings may continue to low temperatures.

Origin of Sulfur Isotope Heterogeneity in Platinum-Group Element Mineralized Rocks of the Rum Layered Suite, NW Scotland