Deposition Mechanisms of Magmatic Sulphide Liquids: Evidence from High-Resolution X-Ray Computed Tomography and Trace Element Chemistry of Komatiite-hosted Disseminated Sulphides

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Magmatic sulphides are a widespread component in mafic and ultramafic rocks and contain variable concentrations of nickel, copper and platinum-group elements. Previous literature has been concerned with the whole-rock geochemistry of magmatic sulphide ores and their host-rocks and relatively little attention has been paid to the physical nature of magmatic sulphide transport and accumulation. Our high-resolution X-ray computed tomography study quantifies for the first time the 2D and 3D size, shape and textural relationships, and distribution of disseminated magmatic sulphides and olivine in adcumulates from komatiites. These new data are combined with analysis of trace-element concentrations within sulphides to provide important information about the mechanisms of transport, deposition and post-accumulation migration of sulphide liquid in dynamic magmatic systems. Olivine shows evidence of textural maturation, with larger crystals growing at the expense of small ones to different degrees depending on the sulphide content of the rock. The olivine texture and the presence of poikilitic chromite provide evidence of in situ nucleation of olivine and chromite at the interface between a flowing magma and a basal pile of crystals. Disseminated to strongly interconnected base-metal sulphides are located at contacts between olivine crystals or in some cases can be entirely or partially enclosed within chromite. Based on their 3D morphologies, their size distribution and their Pd concentrations, the sulphides are divided into four main categories: finely disseminated sulphides; disseminated to slightly interconnected sulphides; disseminated to globular sulphides; disseminated to strongly interconnected sulphides. All samples contain a population of sub-spherical sulphide blebs (<1000 µm equivalent sphere diameter; ESD), which are observed in the olivine–sulphide cotectic proportion and which contain the lowest Pd concentrations. These small droplets are interpreted to have formed by segregation of immiscible sulphide liquid upon cooling of a komatiitic magma flowing in a magma conduit or channel. These newly formed droplets were trapped in situ by the crystallizing framework of olivine and/or chromite. Larger sulphide blebs (up to 10 mm ESD) are present where the sulphide abundance is >3 wt % and the sulphide bleb size population is multi-modal. The Pd content of the sulphide blebs is variable and positively correlated with the sulphide bleb size. The overall sulphide abundance, sulphide bleb size and Pd concentrations indicate that these sulphides have been transported in a flowing sulphur-saturated magma over some distance and accumulated at their present site by mechanical processes. Strongly interconnected network to matrix sulphides are observed in samples containing more than 5 wt % sulphide with small variability in Pd concentrations within and between blebs. These sulphides are interpreted to reflect the accumulation and coalescence (by film drainage) of small sulphide blebs. Overall our results show that komatiite-hosted disseminated sulphides form by a mechanical accumulation process that takes place against a background of steady-state in situ nucleation of small blebs along the olivine–sulphide liquid cotectic.

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