Sarah-Jane Barnes

The origin of the fractionation of platinum-group elements in terrestrial magmas

Abstract The platinum-group elements (PGEs), when chondrite normalized, have been found to be fractionated in order of descending melting point (Os, Ir, Ru, Rh, Pt, Pd and Au). Mantle-derived material (garnet lherzolite and spinel lherzolite xenoliths and alphine peridotites) have essentially unfractionated PGE patterns. Periotitic komatiites have mildly fractionated patterns ( Pd Ir = 10 ), pyroxenitic komatiites are slightly more fractionated ( Pd Ir = 30 ). Both continental and ocean-floor basalts are highly fractionated ( Pd Ir = 100 ). Data from intrusive rocks show a large range in PGE fractionation from Pd-depleted chromities of ophiolites ( Pd Ir = 0.1 ) to the extreme Pd enrichment in the JM Reef of the Stillwater Complex ( Pd Ir = 865 ). Some possible mechanisms for the origin of this fractionation are: alteration, partial melting and crystal fractionation. Carbonate alteration affects Au and Pt and hydrothermal alteration mobilizes Pd. Solid substitution of Ir (and associated Os and Ru) into olivine and chromite, during crystal fractionation or partial melting is rejected as a mechanism of fractionating the PGEs. It is suggested; that the major factor in PGE fractionation is the differences in solubility of the PGEs in a silicate magma, that Pd, Pt and Rh are more soluble than Os and Ir, which form an alloy and Ru which forms laurite. These differences in PGE solubility could fractionate the PGEs during partial melting or crystal fractionation. During crystal fractionation prior to Fe-Ni-Cu sulphur saturation the low solubility of Os, Ir and Ru leads to the formation of Os-Ir alloys and RuS2 in the magma. These may then be settled out of the magma by whatever phase is crystallizing and the remaining magma becomes fractionated in PGEs.

The use of mantle normalization and metal ratios in discriminating between the effects of partial melting, crystal fractionation and sulphide segregation on Platinum-Group Elements, gold, nickel and copper : examples from Norway

The distribution of noble metals, Ni and Cu in mafic and ultramafic rocks is thought to be controlled by sulphides, chromite, olivine and platinum-group minerals (PGM). One method for presenting noble metal, Ni and Cu data focuses on the sulphide control by recalculating the data to 100% sulphides and presenting the data chondrite normalized. The relative importance of the influence of sulphides, chromite, olivine and PGM on the noble metals, Ni and Cu is examined here using two alternative methods.

Partitioning of nickel, copper, iridium, rhenium, platinum, and palladium between monosulfide solid solution and sulfide liquid: Effects of composition and temperature

Abstract Partitioning of Ni, Cu, and Pt-group elements (Ir, Rh, Pt, Pd) between monosulfide solid solution (Mss) and sulfide liquid has been investigated in the Fe-Ni-Cu-S system at 1000 and 1100°C and one atmosphere pressure. The Nernst partition coefficients (D = wt% in Mss/wt% in sulfide liquid) for Ni vary significantly from 0.19 to 1.17, while the values of DCu show a limited range of 0.17–0.27. The partition coefficients for Ir range from 1.06 to 13. Rhodium has a partition coefficient slightly lower than that of Ir under the same conditions, ranging from 0.37 to 8.23. The partition coefficients for Pt and Pd vary from 0.05 to 0.16, and from 0.08 to 0.27, respectively. The partition coefficients depend strongly on the bulk S contents of the system. They increase with increasing S contents in both Mss and liquid. Platinum, Pd, and Cu behave incompatibly during Mss crystallization, strongly partitioning into sulfide liquid. Nickel is incompatible in S-undersaturated systems and S-saturated systems. It becomes compatible when the system is S-oversaturated. Rhodium is compatible in S-saturated and S-oversaturated systems, but incompatible in S-undersaturated systems. Iridium changes from highly compatible through moderately compatible to slightly compatible when the system changes from S-oversaturated through S-saturated to S-undersaturated. The effect of temperature on metal partitioning is observed only in S-oversaturated systems, in which the partition coefficients for Ni and Rh increase with decrease of temperature. The compatible behavior of Ir and Rh, and incompatible behavior of Pt and Pd and Cu under S-saturated conditions appears to support the hypothesis that the observed metal zonation in many sulfide ore deposits such as Sudbury, Ontario and Norilsk, Siberia resulted from sulfide liquid fractionation.

The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna greenstone belt, Western Australia

Abstract Komatiites in the western part of the Norseman-Wiluna greenstone belt in the Yilgarn Block of Western Australia display a wide variety of volcanic facies, ranging from very thin differentiated Munro Township-type flow units through to very thick olivine-rich cumulate flow units containing high proportions of adcumulate dunite. Cumulate flow units have been mapped in detail in the Agnew-Wiluna segment of the Norseman-Wiluna Greenstone Belt. In the Yakabindie and Mt. Keith areas, thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet-like sequences of finer grained olivine orthocumulates. They show fine-scale internal layering, broad-scale cryptic layering and upper fractionated sequences containing harrisites, pyroxene bearing cumulates and in some cases gabbroic derivatives. Low grade disseminated sulphide mineralisation occurs within the dunite lenses. In the stratigraphically equivalent Kathleen East area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows. A similar relationship is also seen further south, where the Perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows. These field relationships indicate an extrusive origin for the adcumulate dunite bodies. The Walter Williams Formation, in the west-central part of the Norseman-Wiluna Belt, is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent. It consists of a central sheet-like body of adcumulate dunite, underlain and overlain by olivine orthocumulates. The adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite. At its northernmost extent, the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros, dolerites and pyroxene spinifex-textured material, interpreted as a periodically replenished lava lake sequence. Both the lenticular dunite bodies of the Agnew-Wiluna Belt and the sheet-like dunite body of the Walter Williams Formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates. The adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows. Flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates. Field evidence from Perseverance, and the general geometry of the dunite lenses, suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low-melting felsic volcanic substrates. The textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion. Dunite sheets form close to the eruption sites, and dunite lenses as a result of channellisation further away from the vent, or close to the vent in the situation where the substrate is non-refractory and thermal erosion can take place readily. Kambalda-type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic, and Munro-type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions, or close to the vent of very small ones. Waning eruption rates leads to proximal facies being overridden by distal ones, a common observation in komatiite sequences. The size of thermal erosion channels requires very rapid eruption rates comparable to those in Phanerozoic flood basalt terrains.

THE BEHAVIOUR OF PLATINUM-GROUP ELEMENTS DURING PARTIAL MELTING, CRYSTAL FRACTIONATION, AND SULPHIDE SEGREGATION : AN EXAMPLE FROM THE CAPE SMITH FOLD BELT, NORTHERN QUEBEC

Abstract The behaviour of the Pt-group elements (PGE) during both normal igneous processes and during the formation of PGE deposits is poorly understood. This is in part because of the limited data set available for nonmineralized rocks. Accordingly, PGE concentrations have been determined in komatiitic basalts associated with Ni-Cu sulphide deposits rich in PGE from the Proterozoic Cape Smith Fold Belt of northern Quebec. The lavas have been divided into olivine-phyric, pyroxene-phyric, and plagioclase-phyric on the basis of the phenocrysts present. Olivine-phyric lavas have Mg#s greater than 0.66 and are thought to be primary partial melts. The bulk partition coefficients of the PGE during partial melting calculated from the average of olivine-phyric basalts are 6, 2, 0.6, and 0.2 for Ir, Rh, Pt, and Pd, respectively, which indicates a decrease in compatibility of the PGE in this order. The pyroxene and plagioclase-phyric lavas are thought to have formed from the olivine-phyric lavas by crystal fractionation of olivine and chromite in the pyroxene-phyric lavas and olivine, chromite, and pyroxene in the plagioclase-phyric lavas. Ir shows a strong positive correlation with Mg#, Cr, and Ni, and this is attributed to Ir having partitioned into olivine or chromite. If olivine, chromite, and clinopyroxene were the only phases to have crystallized, then Pd should have behaved as an incompatible element; however, it does not correlate with the lithophile incompatible elements. It does, however, correlate with Rh and Pt. This is interpreted to suggest that Rh, Pt, and Pd were controlled by sulphide segregation. The removal of a small amount of sulphide along with the olivine and chromite during crystal fractionation could have resulted in the mildly compatible behavior exhibited by Rh, Pt, and Pd. The composition of the sulphides in the sulphide deposits may be modelled by assuming that the silicate magma from which they segregated was similar in composition to the spinifex-textured komatiites containing 15% MgO.

Trace elements in magnetite as petrogenetic indicators

We have characterized the distribution of 25 trace elements in magnetite (Mg, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Mo, Sn, Hf, Ta, W, and Pb), using laser ablation ICP-MS and electron microprobe, from a variety of magmatic and hydrothermal ore-forming environments and compared them with data from the literature. We propose a new multielement diagram, normalized to bulk continental crust, designed to emphasize the partitioning behavior of trace elements between magnetite, the melt/fluid, and co-crystallizing phases. The normalized pattern of magnetite reflects the composition of the melt/fluid, which in both magmatic and hydrothermal systems varies with temperature. Thus, it is possible to distinguish magnetite formed at different degrees of crystal fractionation in both silicate and sulfide melts. The crystallization of ilmenite or sulfide before magnetite is recorded as a marked depletion in Ti or Cu, respectively. The chemical signature of hydrothermal magnetite is distinct being depleted in elements that are relatively immobile during alteration and commonly enriched in elements that are highly incompatible into magnetite (e.g., Si and Ca). Magnetite formed from low-temperature fluids has the lowest overall abundance of trace elements due to their lower solubility. Chemical zonation of magnetite is rare but occurs in some hydrothermal deposits where laser mapping reveals oscillatory zoning, which records the changing conditions and composition of the fluid during magnetite growth. This new way of plotting all 25 trace elements on 1 diagram, normalized to bulk continental crust and elements in order of compatibility into magnetite, provides a tool to help understand the processes that control partitioning of a full suit of trace elements in magnetite and aid discrimination of magnetite formed in different environments. It has applications in both petrogenetic and provenance studies, such as in the exploration of ore deposits and in sedimentology.

The Bushveld Complex, South Africa: formation of platinum–palladium, chrome- and vanadium-rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large, relatively slowly cooling, subsiding magma chamber

Platinum-group element (PGE) deposits in the Bushveld Complex and other layered intrusions form when large, incompletely solidified magma chambers undergo central subsidence in response to crustal loading, resulting in slumping of semi-consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides, oxides, olivine and pyroxene and less dense layers enriched in plagioclase. The most economic PGE, Cr and V reefs form in large, multiple-replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification. The depth of emplacement has to be relatively shallow as, otherwise, ductile crust would not be able to flex and collapse. In smaller intrusions, cooling rates are faster, subsidence is less pronounced and, where it occurs, the cumulate may be largely solidified, resulting in insignificant mush mobility and mineral sorting. Layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower, but the reefs can be relatively thicker. An additional factor controlling the PGE, Cr and V prospectivity of intrusions is their location within cratons. Intra-cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large, layered igneous bodies. Furthermore, intrusions sited within cratons are more readily preserved because cratons are underlain by thick, buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust.

Progressive mixing of meteoritic veneer into the early Earth’s deep mantle

Komatiites are ancient volcanic rocks, mostly over 2.7 billion years old (from the Archaean era), that formed through high degrees of partial melting of the mantle and therefore provide reliable information on bulk mantle compositions. In particular, the platinum group element (PGE) contents of komatiites provide a unique source of information on core formation, mantle differentiation and possibly core–mantle interaction. Most of the available PGE data on komatiites are from late Archaean (∼2.7–2.9 Gyr old) or early Proterozoic (2.0–2.5 Gyr old) samples. Here we show that most early Archaean (3.5–3.2 Gyr old) komatiites from the Barberton greenstone belt of South Africa and the Pilbara craton of Western Australia are depleted in PGE relative to late Archaean and younger komatiites. Early Archaean komatiites record a signal of PGE depletion in the lower mantle, resulting from core formation. This signal diminishes with time owing to progressive mixing-in to the deep mantle of PGE-enriched cosmic material that the Earth accreted as the ‘late veneer’ during the Early Archaean (4.5–3.8 Gyr ago) meteorite bombardment.

An alternative model for the Damara Mobile Belt: Ocean crust subduction and continental convergence

Abstract The Pan-African Damara Mobile Belt has previously been described as ensialic, possibly resulting from a modified aulacogen. Three features of the Damara Mobile Belt are difficult to reconcile with ensialic models. Firstly, the complex asymmetrical structural pattern of linear zones, with up to 80% shortening across the belt. Secondly, the markedly asymmetric metamorphic pattern broadly follows the structural pattern forming two distinct, parallel metamorphic belts of relatively high (northern belt) and low (southern belt) geothermal gradients, respectively. Abundant granitic intrusions occur in the high-grade metamorphic belt. Thirdly, the evolution of the Damara igneous rocks; the early (Nosib) igneous rocks are alkali; mid-Damara (Matchless Member) amphibolites resemble oceanic-floor basalts. Depleted upper-mantle material representing oceanic lithosphere was tectonically emplaced into the Damara metasediments during early tectonism. An extensive calc-alkali suite (the Salem Suite) intruded the high-grade metamorphic belt during a long period spanning most of the Damara tectonism. A model invoking the formation of alkali rocks, followed by the development of oceanic crust, initiation of northwestward subduction and ocean closure terminating in continental collision is considered to explain the major features.

Geochemistry of mineralised and barren komatiites from the Perseverance nickel deposit, Western Australia

The Perseverance (formerly known as Agnew) and the neighbouring Rockys Reward nickel deposits are associated with metamorphosed komatiite flows erupted onto a substrate of felsic crystal tuffs. The deposits are overlain by the Perseverance Ultramafic Complex, which consists of a central dunite lens, 700 m thick by 3 km wide, flanked by olivine orthocumulates and spinifex textured komatiites. A suite of samples of A-zones of komatiite flows from the area has been analysed for major and trace elements, rare earth elements and platinum-group elements. The samples fall into two distinct geochemical groups on the basis of major and rare earth elements. 1. (1) Samples from A-zones of spinifex-textured flows flanking the central dunite show well constrained, linear trends of major and trace elements typical of komatiite suites related by simple olivine fractionation. 2. (2) Samples from the Perseverance Mine (mineralised and basal flows), from Rockys Reward and from unmineralised flows at the base of the Perseverance Ultramafic Complex all show evidence for LREE enrichment relative to the first group. They are also generally higher in SiO2 and lower in FeO for a given MgO content. Geochemical features of the second group are consistent with the effects of crustal assimilation. This is in line with current models for nickel sulphide genesis, based largely on Kambalda, which emphasise the role of thermal erosion of sulphidic footwall sediments. The chalcophile element (Ni, Ir, Ru, Pt, Pd, Au and Cu) concentrations in A-zone samples from the study area are typical of komatiites. Komatiites from the Perseverance mineralised units and from Rockys Reward show evidence of depletion of the more strongly chalcophile PGE relative to Ni and Cu. This feature is also evident in massive and matrix sulphide ore samples, although less evident in the main Perseverance cloud sulphides. Quantitative geochemical computer modelling indicates that the major and rare earth element chemistry of the samples analysed is consistent with derivation by combined assimilation of felsic material and crystallisation of olivine from a primitive komatiite magma. The parental composition is represented by the spinifex textured units flanking the dunite lens. Compositions of komatiite liquids, sulphides and cumulus olivines can be modelled successfully by varying degrees of batch equilibration between komatiite magma, olivine and assimilated sulphidic sediment. Models involving combined assimilation and fractional crystallisation, in which olivine and sulphide are fractionally segregated as they form, are much less successful at matching the data. In physical terms, this suggests that the mineralisation is due to wholesale assimilation of floor rocks close to the site of sulphide accumulation. Comparison with quantitative computer model trends suggests an average ratio of assimilation to olivine fractionation in the order of 0.5 to 1 in order to generate the observed degrees of LREE enrichment in flowtops, Ni depletion in olivine and PGE depletion in sulphides and flow tops.