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.
This dataset comprises the analyses of the intraoceanic arc rocks of the Olyutorsky terrain: major elements, minor elements, platinum-group elements, Rb-Sr, Sm-Nd, Lu-Hf and Pb-Pd isotopic systems. Samples are late Cretaceous in age and comprise picrites from the Tumrok and Valaginsky Ranges, and picrites, magnesian basalts and basalts from the Koryak Highlands. Major elements were measured by XRF, minor/trace elements by ICP-MS at the University of Tasmania (in 2019) and the Russian Geological Institute (in 2015); platinum-group elements were measured by ICP-MS using the Ni sulfide fire assay-isotope dilution method at the Seoul National University. Radiogenic (Sr-Nd-Hf-Pb) isotope compositions were determined at the University of Melbourne and the Institute of the Earth’s Crust, Irkutsk, using multi-collector ICP-MS in 2019.
A subset of these data were originally published as a supplement to Kutyrev et al. (2021), Primitive high-K intraoceanic arc magmas of Eastern Kamchatka: Implications for Paleo-Pacific tectonics and magmatism in the Cretaceous, Earth-Science Reviews 220, 103703, https://doi.org/10.1016/j.earscirev.2021.103703.
This work was funded by the Ministry of Science and Higher Education of the Russian Federation (Grant No. 075-15-2019-1883), The National Research Foundation (NRF) of Korea (Grant No. 2019R1A2C1009809A) and the Russian Science Foundation (Grant No. 21-17-00122).