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.
The data were generated in two labotories of the Dalhousie University in Halifax during a series of experiments to determine the solubility of chromite in komatiite mixed with different crustal contaminants. The experiments were designed to determine the solubility of the mineral chromite in silicate melt, with the dominant variable being the silica and iron content of the melt.
After equilibrating chromite with melt at 1192-1430 degrees Celcius, samples were quenched and the composition of the chromite, quenched melt (now glass), and olivine run-products were measured for major and minor elements by electron microprobe, and the chromium concentration in the glass was measured by laser ablation ICP-MS. Analytical procedures are included in the associated data description file.
The data are provided in a series of Excel worksheets containing five data tables. Table 1 is a summary of the composition of the starting materials used in experiments. Table 2 is a summary of the conditions of temperature, oxygen fugacity, experiment duration and initial sample composition. Table 3 is a summary of the major and minor element composition of the glass measured by electron microprobe and laser ablation ICP-MS. Tables 4 and 5 are summaries of the major and minor element composition of the olivine and chromite, respectively, measured by electron microprobe.
This data set contains mineral chemical analyses of chromite, orthopyroxene and plagioclase of five chromitite layers and their immediate host rocks from drill core ZK135 of the northwestern Bushveld Complex. The sampled interval of ZK135 covers the transition of the Lower and Middle Group chromitites (LG6, LG6a, MG1, MG2, MG2 II). Detailed geochemical profiles are presented by data sheets and graphically to reveal small-scale variations in mineral chemistry.Mineral chemical analyses were conducted on drill core material from borehole ZK135 from the Thaba mine, operated by Cronimet Chrome Mining SA (Pty) Ltd. Briefly, each layer is hosted in pyroxenite and comprises a main chromitite layer and in some chromitites additional stringers above or below the main layer. Chromitites are composed of chromite, which are cemented by intercumulus plagioclase and orthopyroxene oikocrysts. A detailed study of the orthopyroxene oikocrysts in MG1 was published by Kaufmann et al. (2018). Additionally, euhedral to subhedral cumulus orthopyroxene is present in the LG6a, MG1 and MG2 layers. Pyroxenitic partings, small layers of pyroxenite within the chromitite, occur throughout the LG6 to MG2 layers. The main mineral phases chromite, orthopyroxene and plagioclase were analyzed by electron microprobe in each layer and the adjacent pyroxenitic host rocks.Major-element compositions of chromite, orthopyroxene and plagioclase were analyzed by electron microprobe at the Museum für Naturkunde Berlin with a JEOL JXA-8500F EMP equipped with a field emission cathode and five wavelength-dispersive spectrometers. For more information please consult the data description file and Kaufmann et al. (2019) to which these data are supplementary material.