Mineral chemical and sulfur isotope data for precious metal mineralized rocks of the Rum layered intrusion, NW Scotland

Samples of sulfide-hosted platinum-group element (PGE) mineralization from the Rum layered intrusion (NW Scotland) were studied to elucidate the crystallization history of base-metal sulfide in magmatic ore deposits. A corollary aim was to examine the controls on platinum-group mineral exsolution from sulfide in these settings. Secondary ion mass spectrometry data for sulfides reveal significant sulfur isotopic heterogeneity in all of the Rum sulfide-bearing materials studied. For example, in a PGE-rich chromitite, we find a range of δ34S exceeding 10‰ (-4.3 to +5.9‰) in base-metal sulfides within an area of <1 cm2. Similar ranges of δ34S heterogeneity (~10‰), albeit shifted to values as low as -15‰, are observed in disseminated sulfides in troctolite and peridotite lithologies sampled from the intrusion margins. The relatively light δ34S end member reflects crustal contamination of the Rum parental magmas (with δ34S of +1.9‰) during construction of the intrusion. We attribute the extension to relatively heavy δ34S in all samples to loss of sulfur due to sulfide breakdown at relatively low temperatures; on the basis of Rayleigh distillation modelling we estimate <100 °C. Degradation of primary sulfide (pyrrhotite, pentlandite and chalcopyrite) is observed in the textures of all samples, as are low sulfur phases such as chalcocite and digenite that are generally interpreted to result from low temperature oxidation processes. The chromitite sulfides have S/Se values as low as 600, also signifying sulfur loss. In situ PGE abundance measurements in sulfides from all samples indicate that desulfurization affected precious metal tenors in the chromitite sulfides more than the other samples, and there is a strong spatial pattern of PGM occurring at sulfide margins and within Fe-oxide produced by sulfide breakdown. Collectively, our new 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 very low temperatures.