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

Sonja Aulbach,Andrea Giuliani,Marco L Fiorentini,Raphael J Baumgartner,Dany Savard,Vadim S Kamenetsky,Stefano Caruso,Leonid V Danyushevky,Will Powell,William L Griffin

doi:10.1016/j.lithos.2020.105880

Abstract

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

Highlights

Due to its longevity, ancient continental lithosphere is the sole archive of Earth’s early compositional and dynamic evolution, but it is host to economically important diamond, platinum-group elements (PGE) and Au ore deposits (e.g., De Wit and Thiart, 2005; Groves et al, 1987; Hawkesworth et al, 2017)
Combined with evidence for the depth of metal saturation in the Earth’s mantle at ~250 km (Ballhaus, 1995; Rohrbach et al, 2007), which roughly corresponds to the lithosphere-asthenosphere boundary beneath intact cratons, the highly chalcophile and siderophile character of PGE suggests that they should largely reside in sulphide or platinum-group minerals (PGM; e.g., Luguet et al, 2007; Lorand and Luguet, 2016) in the lithospheric mantle
This study shows that the resulphidation of the Kaapvaal lithospheric mantle after its initial depletion led to a dilution of PGE, perhaps formerly hosted in PGM or rare sulphide inclusions, into larger volumes of sulphide

Summary

Introduction

Ancient (cratonic) continental lithosphere is the sole archive of Earth’s early compositional and dynamic evolution, but it is host to economically important diamond, PGE and Au ore deposits (e.g., De Wit and Thiart, 2005; Groves et al, 1987; Hawkesworth et al, 2017). Magmas and related fluids episodically ascending through the mantle lithosphere are the main agents of its metasomatic modification (e.g., Menzies and Hawkesworth, 1987; O'Reilly and Griffin, 2013; Pearson and Wittig, 2014). Such changes include the stabilisation or destabilisation of metal alloys and sulphides, depending on the mineral saturation or undersaturation state in any given metasomatising melt as a function of pressure, temperature, composition, and fO2 (e.g., Chowdhury and Dasgupta, 2020; Mavrogenes and O'Neill, 1999; Mungall and Brenan, 2014; Woodland et al, 2019; Wykes et al, 2015). Since kimberlite-hosted mantle xenoliths commonly show features of partial desulphurisation owing to serpentinisation (e.g., Giuliani et al, 2016a; Lorand and Grégoire, 2006), their study may provide important natural analogues for the behaviour of siderophile elements during the onset of serpentinisation

Results

Discussion

Conclusion