ALBERT

Description: 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.

Description: The late Proterozoic Ntaka Ultramafic Complex is a body of dominantly pyroxenitic cumulate rocks containing cyclic alternations of olivine–orthopyroxene cumulates. Chemical zoning in the pyroxenes has been imaged at 25–40 µm resolution using desktop microbeam X-ray fluorescence mapping followed up with laser ablation–inductively coupled plasma mass spectrometry analysis for minor and trace elements on selected samples. Poikilitic and granular harzburgites are finely intermingled, in some cases on a centimetre scale in the same thin section. Poikilitic varieties display spectacular textures, ranging from isolated equant orthopyroxene oikocrysts within olivine-rich heteradcumulate harzburgites to rocks composed entirely of interlocking centimetre-sized anhedral orthopyroxene oikocrysts containing sharply bounded idiomorphic Cr-enriched cores. The poikilitic harzburgites are interlayered with cumulate pyroxenites in which orthopyroxene grains show a variety of zoning patterns: Cr-rich cores similar to those in the oikocrysts; sharply bounded oscillatory zoned cores; and reverse zoning with Cr-poor cores and Cr-enriched rims. A further variation is the presence of a mingled harzburgite lithology in which dunite or poikilitic harzburgite is invaded on a centimetre scale by diffuse vein networks or patches of coarse orthopyroxenite. This range of textures and lithologies attests to a more complex set of processes than implied by the standard cumulus theory model in which oikocrysts are considered to have crystallized from intercumulus liquid within a permeable crystal mush. A range of hypotheses is proposed, including infiltration metasomatism of original olivine cumulates by migrating orthopyroxene-saturated pore fluid; however, the textural relationships, whole-rock chemistry and Cr zoning within the grains can best be explained by a model in which the orthopyroxene oikocrysts form in part or whole as mechanically accumulated cumulus grains. The complexity of zoning patterns is attributed to stirring of entrained olivine and orthopyroxene crystals within a heterogeneous flowing crystal mush, where the transporting magma has a wide range of silica contents owing to poorly stirred incorporation of siliceous country-rock material. The Cr-rich orthopyroxenite component grew from Si-enriched chromite-saturated magma. Mingled lithologies developed after accumulation as a result of percolation and infiltration metasomatism by Si-enriched liquid derived by melting of xenoliths within the crystal pile. The model may be more generally applicable: dunite–harzburgite cycles, common in many layered intrusions, may reflect variable degrees of contamination rather than cycles of fractional crystallization and replenishment.

Description: Mount Keith is a large (up to 500 m thick, and 〉2 km long) serpentinized and talc-carbonate-altered dunite body. The geologic features of Mount Keith are similar to many of the ultramafic bodies in the Agnew-Wiluna belt and other greenstone belts within the eastern Yilgarn craton. Mount Keith hosts a large disseminated nickel sulfide orebody that is presently exploited by open pit. There are currently two hypotheses that explain the site of formation of these dunite bodies: either kilometer-scale komatiite lava channels, or subvolcanic sills intruded into the greenstone sequence are responsible. The intrusive model is essentially based on two pieces of geologic evidence: the presence of a single 20-m-scale and single 20-cm-scale ultramafic apophysis in overlying dacitic rocks; and the presence of dacite inclusions within the contact zone of the upper part of the Mount Keith ultramafic unit. New geologic data show that the western part of the Mount Keith ultramafic unit and the overlying dacite sequence are allochthonous and that the contact zone between the two comprises multiple anastomosing layer-parallel fault slices of both lithologies at a scale of meters to tens of meters. Within some of the fault blocks, primary igneous contacts are present between the top of the Mount Keith ultramafic unit and the overlying dacite. New evidence shows that both pyroxene and minor olivine spinifex-textured rocks are present along much of the upper preserved part of the Mount Keith ultramafic unit and pseudomorphs after pyroxene grains exist on a scale of 20 to 200 μ m that formed during emplacement and survived the cooling of the 500-m-thick Mount Keith ultramafic unit. Moreover, the evidence for a large-scale ultramafic intrusive apophysis is unsubstantiated, as this apophysis is shown to be a slice within a brittle/ductile fault. The reinterpretation of the small-scale apophysis is consistent with this being an ultramafic enclave within a younger dacite flow. Finally, the dacite exposed within centimeters to meters of the western ultramafic contact commonly retains delicate igneous textures and shows no evidence of thermal effects from the Mount Keith ultramafic unit. Modeling of the heat budget assuming the dimensions of the Mount Keith ultramafic unit shows that an intrusion of this size should have produced wholesale melting of roof rocks. Our findings indicate that the Western Dacite sequence was not present above the Mount Keith ultramafic unit during its emplacement and cooling. A model is favored for the extrusive origin of the Mount Keith ultramafic unit with construction of the olivine cumulate pile at the floor of an approximately 2-km-wide, intermittently sulfide-bearing, turbulent lava pathway. At the terminal stages of eruption magma flow decreased and finally ceased, allowing the lava to drain and cause the upper crust to collapse onto the top of the crystal pile, producing in places sharp juxtaposition of chilled margin rocks with coarse-grained olivine cumulates. In most places rapid flow and turbulent convection resulted in resorption of the crust, but local formation of stagnant ponds allowed survival of an olivine spinifex-textural profile. More extensive ponding of the magma allowed in situ fractionation and formation of gabbroic and pyroxenitic rocks derived from highly fractionated komatiite magma. The Mount Keith ultramafic unit is believed to be the product of extrusive magmatism from which an olivine cumulate pile developed by upward accretion at the floor of a major lava pathway; these rocks were then extensively modified by deformation.