Siderophile and chalcophile elements in spinels, sulphides and native Ni in strongly metasomatised xenoliths from the Bultfontein kimberlite (South Africa)

• DOI: 10.1016/j.lithos.2020.105880
• Publication Date: 2020-11-13T16:51:16Z
• Authors: Sonja Aulbach; Andrea Giuliani; Marco L. Fiorentini; Raphael J. Baumga...; Dany Savard; Vadim S. Kamenetsky; Stefano Caruso; Leonid V. Danyush...; Will Powell; William L. Griffin
• Keywords: Metasomatism; Phlogopite; Peridotite; Xenolith; Ilmenite; Amphibole; Trace element; Incompatible element

The metasomatised continental mantle may play a key role in the generation of some ore deposits, particular mineral systems enriched platinum-group elements (PGE) and Au. cratonic lithosphere is longest-lived potential source for these elements, but processes that facilitate their pre-concentration later remobilisation to crust are not yet well-established. Here, we report new results on petrography, major-element, siderophile- chalcophile-element composition native Ni, base metal sulphides (BMS), spinels suite well-characterised, highly weakly serpentinised peridotite xenoliths from Bultfontein kimberlite Kaapvaal Craton, integrate data with published analyses. Pentlandite polymict breccias (failed intrusions at depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite phlogopite peridotites Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel an insignificant host all except Zn, BMS Ni account typically <25% bulk-rock High Te/S suggest PGE-bearing tellurides, which, along other compounds metasomatic origin, missing As, Ag, Cd, Sb, Te and, part, Bi unaccounted by main assemblage. close spatial relationship between minerals phlogopite, ilmenite) indicates lithospheric beneath was resulphidised metasomatism after initial melt depletion during stabilisation lithosphere. Newly-formed markedly PGE-poor, as <4.2 ppm seven samples, compared >26 craton. This represents strong dilution original abundances scale, perhaps starting precursor alloy small volumes residual BMS. latter have been which occurs unusual Ni-enriched zone harzburgite displays strongly variable, overall high (up 81 In peridotites, Au relative Pd, probably added S. A combination net introduction S, +/− asthenosphere intra-lithospheric redistribution, part sourced subducted materials, events led sulphide precipitation ~80–120 km Bultfontein. process locally enhanced metallogenic fertility this reservoir. Further mobilisation budget stored S-rich domains upwards transport into require interaction sulphide-undersaturated melts can dissolve metals they store.