Platinum-group Element Mineralization Research Articles

Platinum-group minerals in the LG and MG chromitites of the eastern Bushveld Complex, South Africa

The chromitites of the Bushveld Complex in South Africa contain vast resources of platinum-group elements (PGE); however, except for the economic upper group (UG)-2 chromitite seam, information on the distribution of the PGE in the ores and on the mineralogical nature, assemblages, and proportions of platinum-group minerals (PGM) is essentially missing. In the present geochemical and mineralogical study, PGE concentrates originating from the lower group (LG)-6 and middle group (MG)-1/2 chromitites were investigated with the intention to fill this gap of knowledge. Chondrite-normalized PGE patterns of bulk rock and concentrates are characterized by a positive slope from Os to Rh, a slight drop to Pt, and an increase to Pd again. The pronounced similarities of the PGE patterns indicate similar primary processes of PGE concentration in the chromitites, namely “sulfide control” of the PGE mineralization, i.e., co-precipitation of chromite and sulfide. Further, the primary control of PGE concentration in chromitites appears to be dual in character: (i) base-level concentrations of IPGE (up to ∼500 ppb) hosted within chromite and (ii) co-precipitation of chromite and sulfide, the latter containing virtually the entire remaining PGE budget. Sulfides (chalcopyrite, pentlandite, and pyrite; pyrrhotite is largely missing) are scarce within the chromitites and occur mainly interstitial to chromite grains. Pd and Rh contents in pentlandite are low and erratic. Essentially, the whole PGE inventory of the ores occurs in the form of discrete PGM. The PGM are almost always associated with sulfides. The dominant PGM are various Pt-Pd-Rh sulfides (cooperite/braggite [(Pt,Pd)S] and malanite/cuprorhodsite [CuPt2S4]/[CuRh2S4]), laurite [RuS2], the main carrier of the IPGE (Os, Ir, Ru), sulfarsenides [(Rh,Pt,Ir)AsS], sperrylite [PtAs2], Pt-Fe alloys, and a large variety of mainly Pd-rich PGM. The LG and MG chromitites have many characteristics in common and define a general, “typical” PGM spectrum of Bushveld chromitites. This PGM assemblage is characterized by the predominance of PGE-sulfides including elevated proportions of malanite, variable proportions of (sulf) arsenides, and Pt-Fe alloys in conjunction with a paucity of (bismutho)tellurides. The formation of this specific PGM spectrum is related to the distinct chromitite environment and its depositional and post-depositional history, whereby desulfurization reactions have probably played an important role. The LG-6 samples have higher contents of PGE-sulfides, including extraordinary high proportions of malanite but low PGE-arsenide and PGE-sulfarsenide contents compared to the MG-1/2 samples. This indicates a higher availability of arsenic either in the stratigraphically higher MG-1/2 samples (compared to the LG-6) or regionally in the chromitites south of the Steelpoort lineament.

Chapter 22 Precambrian metallic mineralization in India

Abstract Four Archaean cratons (Aravalli–Bundelkhand, Singhbhum, Bastar and Dharwar), together with the marginal Proterozoic mobile belts and sedimentary basins, constitute the geology of peninsular India. Huge resources of ferrous metals (Fe and Mn) and chromite, the lone granitoid-hosted Malanjkhand Cu–Mo deposit, a moderate occurrence of gold and promising platinum group element mineralization constitute the Archaean metal inventory of India. Additionally, the Proterozoic Aeon witnessed diverse mineralization in the mobile belts and craton–mobile belt contacts. These include vast resources of base metals in the northwestern Indian Shield and Mn in the central Indian block, apart from considerable U–Cu deposits in the Singhbhum Shear Zone. These ores formed as a consequence of an entire genetic spectrum, covering various orthomagmatic, volcanosedimentary and diverse hydrothermal processes, aided and abetted by supergene enrichment, as in the case of iron ores.

Stratigraphy, thickness, tectonic environment, and economic implications of the giant Suwar – Wadi Qutabah layered mafic complex in northwestern Yemen

The Wadi Qutabah layered mafic intrusion in northwestern Yemen, considered part of a giant (>250 km2), Neoproterozoic (∼638.5 Ma) intrusion referred to as the Suwar – Wadi Qutabah Complex, has significant potential for economic platinum-group element and Ni–Cu–Co mineralization. A search for platinum-group element mineralization at Wadi Qutabah yielded cores from 14 drillholes along two east and west lines. Stratigraphically, they reveal ∼500 m of interlayered norites with subordinate gabbros, anorthosites, pyroxenites, and massive sulphide layers showing modal and cryptic layering that can be correlated between individual holes and between the east and west areas. Energy dispersive spectrometry major element analyses on mineral phases show gradual upward changes in plagioclase composition from An52 (lowest level drilled) to An44, and En61 to En57 in orthopyroxene, up to a distinctive, ilmenite-rich unit termed 5a (augite norite). Above 5a, minerals show a dramatic shift to more primitive An57 and En69 compositions. The gradual changes are typical of layered mafic intrusions and reflect progressive crystallization of magma upward. The dramatic changes, such as above 5a, are also common and reflect magma chamber recharge. In addition to the 500 m reported here, mineral exploration data from more-mafic rocks at Suwar suggest 400 m of layered rocks below those at Wadi Qutabah (total 900 m). A regression analysis plot of the cumulus mineral (plagioclase and orthopyroxene) compositional change versus thickness for seven well-known, giant, layered mafic intrusions suggests that the Suwar – Wadi Qutabah Complex is >2 km thick and one of the largest (thickness and area) intrusions on Earth. Comparison of the stratigraphy and mineral compositions at Wadi Qutabah with intrusions bearing PGE deposits suggests prospective areas for mineral exploration to the north and south of the study area. Recent reviews identify many similar-aged, mafic–ultramafic intrusive complexes across the Arabian–Nubian Shield. The large size of the Suwar – Wadi Qutabah Complex, its noritic composition, and subcontinental lithospheric mantle signature are characteristics typical of mafic rocks formed in extension-related, continental rift settings. Thus, the Suwar – Wadi Qutabah Complex may be part of a previously unrecognized large igneous province associated with a late Proterozoic magmatic event and preserved in what is today, the Arabian Shield.

Redox controls on Ni–Fe–PGE mineralization and Re/Os fractionation during serpentinization of abyssal peridotite

Awaruite (Ni2Fe to Ni3Fe) is often used to infer fugacity and redox gradients in hydrothermally altered peridotites. However, discrepant petrological and thermodynamic data suggest that the fO2–fS2 stability field of awaruite is not well constrained. In this study, we assess the thermodynamic properties of awaruite and re-evaluate the Fe–Ni–S systematics of hydrothermally altered peridotites. New experimental data indicate that awaruite is stable at higher fO2 than previously thought, supporting the common occurrence of awaruite in the reaction zone of modern and ancient ultramafic-hosted hydrothermal vent systems. Awaruite is known to catalyze the abiogenic synthesis of methane during active serpentinization, contributing to methanogenesis at modern oceanic hydrothermal systems and potentially on early Earth. The enhanced stability field of awaruite determined here suggests that abiogenic methanogenesis may be active at a broad range of redox conditions.Serpentinized peridotites also contain platinum-group element (PGE)–Re sulfides and metal alloys that can be derived from desulfurization of primary mantle sulfides under low fS2. Thermodynamic calculations suggest that Os will exist as metal and Re as ReS2 in the redox stability field of awaruite. Under a relatively broad range of fS2–fO2 conditions, hydrothermal alteration and desulfurization of primary mantle may produce open system behavior in the Re/Os system and variable Re–PGE ratios and contribute over time to the Os isotopic heterogeneity of the mantle, through subduction and recycling of altered oceanic lithosphere. PGE-metal alloys occur at such low abundances in peridotite that they often are nearly impossible to find. The stability of awaruite at higher fO2 suggests that it can coexist with Os–Ir–Pt metal alloys, making it an important indicator mineral for their presence in hydrothermally altered peridotites.

Metal sources for the polymetallic Ni–Mo–PGE mineralization in the black shales of the Lower Cambrian Niutitang Formation, South China

Total organic carbon content (TOC), trace element and platinum-group element (PGE) concentrations were determined in the black shales of the Lower Cambrian Niutitang Formation in the Nayong area, Guizhou Province, South China, in order to study the polymetallic Ni–Mo–PGE mineralization. The results demonstrate that numerous elements are enriched in the polymetallic ores compared to those of the nearby black shale, particularly Ni, Mo, Zn, TOC and total PGE, which can reach up to 7.03wt.%, 8.49wt.%, 11.7wt.%, 11.5wt.% and 943ppb, respectively. The elemental enrichment distribution patterns are similar to those in the Zunyi and Zhangjiajie areas except that the Nayong location is exceptionally enriched in Zn. Whereas positive correlations are observed between the ore elements of the polymetallic ores, no such correlations are observed in the black shale. These positively correlated metallic elements are classified into three groups: Co–Ni–Cu–PGE, Zn–Cd–Pb and Mo–Tl–TOC. The geological and geochemical features of these elements suggest that Proterozoic and Early Palaeozoic mafic and ultramafic rocks, dolomites and/or Pb–Zn deposits of the Neoproterozoic Dengying Formation and seawater could be the principal sources for Co–Ni–Cu–PGE, Zn–Cd–Pb, and Mo–Tl–TOC, respectively. Furthermore, the chondrite-normalized patterns of PGEs with Pd/Pt, Pd/Ir and Pt/Ir indicate that PGE enrichment of the polymetallic ores is most likely related to hydrothermal processes associated with the mafic rocks. In contrast, PGE enrichment in the black shale resembles that of the marine oil shale with terrigenous and seawater contributions. Our investigations of TOC, trace elements and PGE geochemistry suggest that multiple sources along with submarine hydrothermal and biological contributions might be responsible for the formation of the polymetallic Ni–Mo–PGE mineralization in the black shales of the Lower Cambrian Niutitang Formation across southern China.

Petrogenesis of the Ni–Cu–PGE sulfide-bearing Tamarack Intrusive Complex, Midcontinent Rift System, Minnesota

Abstract The Tamarack Intrusive Complex (TIC, 1105.6 ± 1.2 Ma) in NE Minnesota, was emplaced during the early stages of the development of the Midcontinent Rift System (MRS, “Early Stage”: 1110–1106 Ma). Country rocks of the TIC are those of the Paleoproterozoic Thomson Formation, part of the Animikie Group including sulfide-bearing metasedimentary black shale. The magmatic system is composed of at least two principal mafic–ultramafic intrusive sequences: the sulfide-barren Bowl Intrusion in the south and the “dike” area intrusions in the north which host Ni–Cu–Platinum Group Elements (PGE) mineralization with up to 2.33% Ni, 1.24% Cu, 0.34 g/t Pt, 0.23 g/t Pd and 0.18 g/t Au. Two distinct intrusive units in the “dike” area are the CGO (coarse-grained olivine-bearing) Intrusion, a sub-vertical dike-like body, and the overlying sub-horizontal FGO (fine-grained olivine-bearing) Intrusion. Both intrusions comprise peridotite, feldspathic peridotite, feldspathic pyroxenite, melatroctolite and melagabbro. Massive sulfides are volumetrically minor and mainly occur as lenses emplaced into the country rocks associated with both intrusions. Semi-massive (net-textured) sulfides are distributed at the core of the CGO Intrusion, surrounded by a halo of the disseminated sulfides. Disseminated sulfides also occur in lenses along the base of the FGO Intrusion. Olivine compositions in the CGO Intrusion are between Fo89 and Fo82 and in the FGO Intrusion from Fo84 to Fo82. TIC intrusions have more primitive olivine compositions than that of olivine in the sheet-like intrusions in the Duluth Complex (below Fo70), as well as olivine from the smaller, conduit-related, Eagle and East Eagle Intrusions in Northern Michigan (Fo86 to Fo75). The FeO/MgO ratios of the CGO and FGO Intrusion parental magmas, inferred from olivine compositions, are similar to those of picritic basalts erupted during the early stages of the MRS formation. Trace element ratios differ slightly from other intrusions in the MRS, and are indicative of significant crustal contamination. Differences in textures, whole-rock and mineral compositions, and sulfide distribution are consistent with the emplacement of at least two distinct sulfide saturated magmatic pulses. Ni-enrichment in the TIC indicates that sulfide saturation was attained prior to the sequestration of major proportions of Ni by olivine, possibly at a deeper chamber in the magmatic system. The addition of crustal S from the Thomson Formation sulfidic country rocks is thought to have been the principal process which drove the early attainment of sulfide saturation in the magmas. The CGO Intrusion carried the greater abundance of sulfide liquid, but both the CGO and FGO intrusive sequences represent the accumulation of dense silicate minerals and sulfide liquid in a conduit system. The genetic processes that were operative in the formation of Ni–Cu–PGE mineralization in the Tamarack Intrusive Complex appear to be typical of conduit-style magmatic sulfide deposits associated with large continental basaltic provinces.

Geochemical exploration for platinum-group element deposits in Miyi County, Sichuan Province, Southwestern China

A large Pt–Pd geochemical province within the Emeishan basalt province has been identified previously and was considered as an exploration target for platinum-group element (PGE) mineralization. To evaluate thoroughly the prospect of economic Pt–Pd mineralization in this region, a series of geochemical surveys was conducted in a selected area, with an exploration strategy involving the progressive reduction of the target area and stepwise increase in the sampling density. A regional Pt and Pd geochemical anomaly was delineated by a stream sediment survey at a sampling density of 1 sample km −2 over an area of mafic intrusive bodies and basalts in Miyi County, Sichuan Province. In a follow-up phase, three local Pt and Pd geochemical anomalies were defined by data from a stream sediment survey with a density of 4 samples km −2 . Finally, PGE mineralization within a gabbroic rock was discovered in the geochemical anomalies delineated by the following more detailed survey, namely a soil survey with a 100 m × 20 m sampling grid. The level of Pt+Pd mineralization was 0.33 μg g −1 . A new concealed mafic–ultramafic intrusive body containing PGE mineralization was predicted.The success of this case has provided an excellent example of how to identify possible PGE target areas in the basalt environment using geochemical surveys at various scales.

Platinum-group elemental chemistry of the Baima and Taihe Fe–Ti oxide bearing gabbroic intrusions of the Emeishan large igneous province, SW China

Nickel-, copper-, and platinum group element (PGE)-enriched sulphide mineralization in large igneous provinces has attracted numerous PGE studies. However, the distribution and behavior of PGEs as well as the history of sulphide saturation are less clear in oxide-dominated mineralization. Platinum group elements of oxide-bearing layered mafic intrusions from the Emeishan large igneous province are examined in this study. Samples collected from the Baima and Taihe oxide-bearing layered gabbroic intrusions reveal contrasting results. The samples from Baima gabbroic rocks have low total PGE abundances (ΣPGE<4ppb) whereas the Taihe gabbroic rocks, on average, have more than double the concentration but are variable ranging from ΣPGE<2ppb to ΣPGE∼300ppb. The Baima gabbro is platinum-subgroup PGE (PPGE=Rh, Pt and Pd) enriched and iridium-subgroup PGE (IPGE=Os, Ir and Ru) depleted, with a distinct positive Ru anomaly on a primitive mantle normalized multi-element plot. The Taihe gabbros are also PPGE enriched but with negative Ru and Pd anomalies on a primitive mantle normalized multi-element plot. The PGE concentrations of Baima rocks are indicative of fractionation of a relatively evolved, mafic, S-undersaturated parental magma that was affected by earlier sulphide segregation. In contrast, the Taihe rocks record evidence of both S-saturated and S-undersaturated conditions and that the parental magma was likely emplaced very close to S-saturation. Comparisons of the platinum group element contents in the Emeishan flood basalts and the Emeishan oxide-bearing intrusions suggest that the PGE budget in a magma is not controlled by magma series (high-Ti vs. low-Ti), but very much by crustal contamination. The unlikelihood of substantial crustal contamination in the Taihe magma allowed the magma to remain S-undersaturated for a longer duration. PGE and sulphide mineralization was not identified in the Taihe intrusion but the presence of one PGE-enriched sample (Pt+Pd=∼300ppb) suggests that the parental magma likely did not experience sulphide segregation and is a potential target for further prospecting.

Injection of additional batches of magma into the formed magma chamber: The case study of Lukkulaisvaara intrusion, North Karelia

The probable implications of the injection of additional batches of melt into the magma chamber and the correlation of ore formation to these processes are considered. The assumed model of hydraulic impact (stamp) explains a number of the structural features of layered basite-hyperbasite intrusions and, in particular, formation of microgranular rocks, whose structure indicates a high rate of crystallization, and, probably, platinum group element (PGE) mineralization in the Lukkulaisvaara layered intrusion, North Karelia. It is shown that intrusion of additional batches of magma can lead to thermodiffusion, which is most effective in the vicinity of the contact between the chamber and the portion of magma. This, in turn, should result in the redistribution of chemical components in this zone and, probably, to anomalous concentrations of productive components at early penetration stages.

Rheological conditions for emplacement of Ural–Alaskan-type ultramafic complexes

Ural–Alaskan- (or Alaskan-) type complexes correspond to a particular class of ultramafic intrusions that attract particular attention due to their deep mantle origin and their platinum-group element (PGE) mineralization. When defined as massifs of dunite–clinopyroxenite, only forty-six complexes are reported in the literature. These large-scale dunite pipe-like structures are rarely isolated and they even can appear in clusters. To better understand genesis of these relatively young (<460Ma) complexes, a worldwide compilation has been built, and three categories have been defined: single circular or elliptical bodies, twin bodies with similar shapes, and dismembered dunite bodies. PGE enrichment in Alaskan-type complexes is highest for the second category, where twin bodies are interpreted as horizontal sections of Y-shaped dunite pipes. To constrain mechanical properties of the lithosphere allowing emplacement of the Alaskan-type complexes, the forceful diapiric ascent hypothesis is investigated through numerical thermo-mechanical models. One hundred high resolution experiments accounting for realistic phase changes and softening mechanisms have been performed. The experiments show that with no rheological softening of the host rock and in case of a relatively weak ductile lower crust, the uprising magma tends to spread laterally without reaching the surface. To account for the forceful ascent of deep magmas, it is hence necessary to assume a strong lower crust rheology and strong local softening mechanisms. Besides reproducing the clustered distribution of the weakness zones representing magma pathways, these latter experiments reproduce large-scale pipe-like (cylindrical) structures, Y-shaped and funnel-shaped bodies, and laterally-shifted structures. Interestingly, zones of highest strain rates are located at the bottom parts of the inclined edges of Y-shaped and funnel-shaped bodies. The restricted age range of Alaskan-type complexes (<460Ma) would mean that prior to this time, the lower crust was less resistant due to the hotter geotherm, prohibiting the possibility of “Alaskan-type magmatism”.

Geochemistry and Mineralogy of Platinum Group Element Mineralization in the River Valley Intrusion, Ontario, Canada: A Model for Early-Stage Sulfur Saturation and Multistage Emplacement and the Implications for "Contact-Type" Ni-Cu-PGE Sulfide Mineralization

The River Valley intrusion within the ~2.48 Ga East Bull Lake intrusive suite in Ontario, Canada, is an example of a mafic igneous intrusion with “contact-type” Ni-Cu-PGE sulfide mineralization along its base. Whereas many contact-type deposits are thought to form from in situ contamination of the magma by the addition of crustal S during emplacement, there are some intrusions, including the River Valley intrusion, which appear to have a much more complex history where the timing of S saturation, and thus the critical ore genesis processes, may have occurred much earlier, prior to emplacement. The River Valley intrusion is made up of a basal ~100 m of unlayered, inclusion-bearing units, overlain by layered cumulates. The basal units contain autoliths of gabbroic rocks and inclusions of footwall gneiss and amphibolites, all within a gabbroic matrix. Platinum-group element-rich magmatic sulfide mineralization occurs throughout both the inclusions and the matrix as blebby and disseminated sulfides. The matrix and inclusions can be separated into two distinct textural types: hydrothermally altered greenschist assemblages and unaltered metamorphic amphibolite assemblages. The platinum group mineral (PGM) assemblages vary only between textural types, and not between inclusions and matrix, being dominated tellurides in all rock types. The hydrothermally altered rocks, however, have fewer tellurides and an increased amount of Sb- and As-bearing PGM, indicative of minor fluid interaction, although the PGM have not been mobilized significantly away from the base metal sulfides. Precious and base metal geochemistry shows all rock types to have an excellent correlation between all the platinum group elements (PGE), indicating the presence of a single, well homogenized, PGE-rich sulfide liquid. However, Au and Cu appear to be decoupled from the PGE at low concentrations, although correlate well with each other, which is interpreted to be due to minor fluid redistribution and alteration of sulfide bleb margins. The overlying Layered units above the mineralized units are not PGE depleted. Trace element data, including (Th/Yb)PM and (Nb/Th)PM ratios, demonstrate that all River Valley rocks were formed from crustally contaminated magmas following interaction with local country rocks in a deeper subchamber; although some samples have S/Se ratios indicative of crustal S, most have S/Se ratios lower than the mantle range, indicative of S loss. We propose a multistage model for the formation of the mineralization in the River Valley intrusion with a major contamination event at depth with the addition of S from local crustal rocks, inducing sulfide saturation. Sulfide droplets were then enriched in PGE within a conduit system with possible further upgrading of sulfide metal tenors (and reduction of S/Se ratios) via partial dissolution of sulfide. The PGE-enriched sulfide liquid then settled in a staging chamber and partially crystallized before a major pulse of magma entrained sulfide liquid, eroded blocks of precrystallized and mineralized gabbro and footwall rocks, and emplaced an inclusion-bearing package as the lower 100 m or so of the River Valley intrusion. Later emplacement of main River Valley magma was from an S-undersaturated, PGE-fertile magma. The River Valley intrusion is thus an example whereby contact-type mineralization is purely a function of the earliest magma intruded containing preformed sulfide mineralization, rather than contamination triggering sulfide saturation in situ. In such cases, processes at depth determine the generation and subsequent tenor of the mineralization. In particular, dissolution of the sulfide can upgrade metal tenor, but subsequently will reduce S/Se ratios, masking the signature of crustal contamination. In addition, a multistage emplacement such as this will not necessarily preserve the characteristic increase in Cu/Pd ratios in the overlying cumulates that is often used in exploration for PGE deposits in mafic intrusions. Thus, a full understanding of all the field, geochemical, and mineralogical factors is required to construct genetic models for such deposits and especially in the interpretation of S/Se and Cu/Pd ratios as an indicator of crustal contamination and the presence of PGE mineralization.

Joint inversion of gravity, magnetic, and petrophysical data — A case study from a gabbro intrusion in Boden, Sweden

We have studied a gabbro intrusion in northern Sweden, using 3D inversion of airborne magnetic data, ground-based gravity data, and petrophysical measurements on outcrop samples. Gabbro intrusions are of interest because they are potential hosts of Cu-Ni and platinum group element mineralization. We developed a joint inversion algorithm and applied it to both potential-field data sets to obtain spatial distributions of density and magnetic susceptibility. The distributions were coupled through a nonrigidly enforced parameter relationship determined from the petrophysical samples. We managed the problem of balancing the influence of the two data sets by a novel adaptive reweighting scheme that enforced the discrepancy principle for each data set independently. We demonstrated in tests with synthetic data that neither individual nor joint inversions gave reliable estimates for the depth extension of the intrusive body, the near-surface details, or any complex geometrical features. However, the joint inversion improved the image of the interface between the intrusion and the surrounding rocks and revealed that the density and susceptibility models satisfied the observed petrophysical relationship, which, in turn, caused the structures in the models to align. The geometry of the intrusion was an intrinsic result of the inversion, based on the two distinct petrophysical trends for the gabbro and the surrounding rocks. The inferred shape was simple and concise, and was therefore a useful and testable hypothesis about the subsurface geology that was in agreement with both potential-field data sets and the petrophysical information.

New mineralogical and isotopic constraints on Main Zone-hosted PGE mineralisation at Moorddrift, northern Bushveld Complex

The northern limb of the Bushveld Complex, South Africa contains a number of occurrences of platinum-group element (PGE) mineralisation within Main Zone rocks, whereas the rest of the complex has PGE-depleted Main Zone units. On the farm Moorddrift, Cu–Ni–PGE sulphide mineralisation is hosted within the Upper Main Zone in a layered package of gabbronorites, mottled anorthosites and thin pyroxenites. Our observations indicate that a 10-m-thick, ‘reef-style’ package of mineralisation has been extensively ‘disturbed’, forming a mega breccia which in some localities may distribute mineralised rocks over intersections of over 300 m. The sulphides are made up of pyrrhotite, pentlandite and chalcopyrite, heavily altered around their margins and overprinted by secondary pyrite. Platinum-group mineral assemblages typical of primary magmatic deposits, with Pt and Pd tellurides and sperrylite, are present in the ‘reef-style’ package, whereas there is a decrease in tellurides and an increase in antimonides in the ‘disturbed’ package, interpreted to be related to hydrothermal recrystallization during veining and brecciation. Sulphur isotopes show that all sulphides within the mineralised package on Moorddrift have a crustal signature consistent with local country rock sediments of the Transvaal Supergroup. We interpret the mineralisation at Moorddrift as a primary sulphide reef, likely produced as a result of the mixing of crustally contaminated magmas in the Upper Main Zone, which has been locally disrupted post-crystallisation. At present, there are no firm links between Moorddrift and the other known PGE occurrences in the Main Zone at the Aurora and Waterberg projects, although the stratigraphic position of all may be similar and thus intriguing. Nonetheless, they do demonstrate that the Main Zone of the northern limb of the Bushveld Complex, unlike the eastern and western limbs, can be considered a fertile unit for potential PGE mineralisation.

AGE OF THE BIRD RIVER SILL, SOUTHEASTERN MANITOBA, CANADA, WITH IMPLICATIONS FOR THE SECULAR VARIATION OF LAYERED INTRUSION-HOSTED STRATIFORM CHROMITE MINERALIZATION

The Archean Bird River sill, a 20-km-long and up to 800-m-thick mafic-ultramafic layered intrusion in the Bird River greenstone belt of southeastern Manitoba, Canada, contains significant resources of chromium and nickel-copper, and locally anomalous concentrations of platinum group elements. Stratiform chromitite, displaying both remarkable lateral continuity and local irregularities due to synmagmatic disruption, occurs in up to six main intervals over a thickness of ~60 m in the lower ultramafic part of the intrusion. Ni-Cu sulfide mineralization is hosted in ultramafic rocks at the base of, or just below, the sill (e.g., past-producing Maskwa-Dumbarton mines). In this study, we report a U-Pb age (chemical abrasion-thermal ionization mass spectrometry) for a leucogabbro from the Chrome property of 2743.0 ± 0.5 Ma (n = 8 single grains of zircon), which is interpreted as the age of crystallization of the sill and defines the timing of the associated Cr-Ni-Cu ± platinum group element (PGE) mineralization. Other Neoarchean mafic-ultramafic intrusions containing economic concentrations of chromite and/or Ni-Cu-(PGEs) occur in the northwestern part of the Superior province in the Canadian Shield (e.g., McFaulds Lake, Big Trout Lake, Puddy Lake, Shebandowan) and share broadly similar geologic relationships. The age of intrusion-hosted stratiform chromite mineralization worldwide, excluding ophiolite-hosted chromitites, ranges from Archean to Paleoproterozoic, which is comparable to that for komatiite-related Ni deposits. This temporal restriction is consistent with both deposit types requiring the involvement of high-MgO, Cr-rich parent magmas produced during large degrees of mantle melting early in Earth history.

Platinum group elements geochemistry of ultramafic and associated rocks from Pindar in Madawara Igneous Complex, Bundelkhand massif, central India

Ultramafic rocks comprising dunite, harzburgite, lherzolite, olivine webserite and websterite occur as intrusives in the form of small hillocks at Pindar into the granite–gneisses of Bundelkhand Gneissic Complex (BnGC). The peridotites are dominated by olivine cumulates where chromite and precious metal-bearing sulphides crystallized along with pyroxenes, subsequent to crystallization of olivine into the interstitial spaces of cumulates during cooling. Ultramafic rocks of Pindar are characterized by high MgO (up to 46.0 wt%) and FeO (up to 5.8 wt%); low SiO2 (40.8 to 48.0 wt%), TiO2 (0.2 to 0.5 wt%), Al2O3 (~3.2 wt% av.), CaO (~2.7 wt% av.) and Cu (11 to 73 μg/g). Cr and Ni values range from 2297 to 3150 μg/g and 2434 to 2767 μg/g, respectively. Distribution of Ir (up to 20 ng/g), Ru (27 to 90 ng/g), Rh (3 to 14 ng/g), Pt (18 to 72 ng/g), Pd (10 to 27 ng/g) and Au (22 to 57 ng/g) indicate platinum group element (PGE) and associated gold mineralization in these ultramafic rocks. A mineral phase representing sperrylite (PtAs2) was also identified within the sulphides in scanning electron microscopy with energy dispersive spectrometer (SEM–EDS) studies. The primitive mantle-normalized siderophile elements pattern shows platinum group element PGE (PPGE) enrichment (Rh, Pt, Pd). Discrimination diagrams of Pd/Ir vs. Ni/Cu, Pd/Pt vs. Ni/Cu, Cu/Pd vs. Pd, and Cu vs. Pd for the peridotites of Pindar attribute to affinity towards komatiite magma, derived from high degree of partial melting of prolonged depleted mantle, and the sulphur saturation condition incurred during the crystallization of chromite which was favourable for PGE mineralization.

Platinum-group element geochemistry of intraplate basalts from the Aleppo Plateau, NW Syria

AbstractEarly–Middle Miocene intraplate basalts from the Aleppo Plateau, NW Syria have been analysed for their platinum-group elements (PGEs). They contain extremely low PGE abundances, comparable with most alkali basalts, such as those from Hawaii, and mid-ocean ridge basalts. The low abundances, together with high Pd/Ir, Pt/Ir, Ni/Ir, Cu/Pd, Y/Pt and Cu/Zr are consistent with sulphide fractionation, which likely occurred during partial melting and melt extraction within the mantle. Some of the basalts are too depleted in PGEs to be explained solely by partial melting of a primitive mantle-like source. Such ultra-low PGE abundances, however, are possible if the source contains some mafic lithologies. Many of the basalts also exhibit suprachondritic Pd/Pt ratios of up to an order of magnitude higher than primitive mantle and chondrite, an increase too high to be attributable to fractionation of spinel and silicate minerals alone. The elevated Pd/Pt, associated with a decrease in Pt but not Ir and Ru, are also inconsistent with removal of Pt-bearing PGE minerals or alloys, which should have concurrently lowered Pt, Ir and Ru. In contrast, melting of a metasomatized source comprising sulphides whose Pt and to a lesser extent Rh were selectively mobilized through interaction with silicate melts, may provide an explanation.

Flotation model based on floatability component approach – PGE minerals case

This paper discusses how modelling and simulation can be used in predicting the recovery of platinum group elements (PGE) in flotation process. In platinum group ores, commonly a floatability component approach is used where each element is divided in model fitting to two or three components. This approach has the problem of breaking the natural association of platinum group element in minerals, floatability components do not have any physical meaning and as the model fitting problem is ill-posed, comparison of different tests by floatability component parameters is difficult. To avoid this modelling and simulation here is based on minerals. Additionally, the mass proportion of fast floating material was fixed on the basis of mineralogical study and results achieved in laboratory flotation tests. Finally as the rougher and cleaning flotation was performed in fixed chemical regime the model fitting was done for both rougher and cleaning stages simultaneously using cascade model. It was found in this way a number of model fitted parameters decreased giving possibility to compare tests results easier but still reaching good fit between observed, modelled and simulated data. The developed model was used in screening the best flotation conditions and to forecast the metallurgical performance in full scale process.

Shale-hosted Ni–(Cu–PGE) mineralisation: a global overview

Shale-hosted Ni–Cu–platinum group element (PGE) sulphide occurrences represent significant resources of not only Ni, Cu and the PGEs but also, in some cases, other elements such as Mo, Zn, Mn, V and U. The largest known deposit of this type that is currently being mined is the Talvivaara Zn–Ni–Cu–Mn–U shale deposit in Finland. The total resource of this deposit is ∼1·550 Bt of ore, at 0·22%Ni, 0·13%Cu, 0·49%Zn, 200 ppm Co and 17 ppm U, all of which can be extracted using a bioheapleaching mineral processing approach. Other significant examples of this deposit type are the Mo–Ni–Zn–Au–PGE deposits of the Niutitang Formation in China and the Ni–PGE–Zn–Mo–Re–Ag mineralisation of the Nick deposit in the Selwyn Basin, Yukon, Canada. However, although they provide insights into the processes responsible for mineralisation in shale-hosted Ni deposits, the Chinese and Selwyn Basin deposits are of too low a tonnage to make them high priority exploration targets. Other potential resources discussed here include the Okcheon shales in Korea, the European Alum Shale and the Chattanooga Shale of the Appalachian Basin. Possible modern analogues for the shale-hosted Ni–Cu–PGE deposits at Talvivaara, Nick, and in China include the Rainbow and Logatchev vent fields of the Mid-Atlantic Ridge, where hydrothermal fluids interact with ultramafic rocks, stripping them of metals before venting them at the seawater–ocean floor boundary, and sapropel formation in the Mediterranean, where organic carbon-rich sediments are interlayered with organic carbon-poor sediments. These organic carbon-rich layers are associated with significantly elevated base metal and sulphur concentrations when compared to the carbon-poor layers, indicating preferential deposition of base metals during sapropel sedimentation. Two genetic models for shale-hosted Ni–Cu–PGE mineralisation are currently favoured: synsedimentary mineralisation, with metals being scavenged from the overlying water column, and exhalative hydrothermal mineralisation, with metals being deposited into sedimentary basins from hydrothermal vents. This paper focuses on shale-hosted Ni mineralisation and the potential for these shales as an economic source of unconventional Ni, Cu and the PGEs.

ORIGIN OF PLATINUM-GROUP MINERALS IN MAFIC AND ULTRAMAFIC ROCKS: FROM DISPERSED ELEMENTS TO NUGGETS

Based on the studies of paragenetic assemblages of platinum-group minerals (PGM) from placers and chromitite ores associated with zonal-ring dunite intrusions in the Aldan Shield, on the Siberian platform, a novel model for the PGM formation is offered, which implies separation of a Cr-rich oxide liquid with significant concentration of the platinum-group elements (PGE) from a picritic magma. The presence of PGM alloys in sulfide-poor chromitite ores from layered intrusions and zonal gabbro–dunite massifs indicates that PGE mineralization is genetically related not just to sulfide formation but to chromite formation as well. A hypothetical (MgO + FeO + CaO) – SiO2 – (Cr2O3 + Fe2O3 + Al2O3) triangular diagram shows that fractional crystallization of olivine results in the enrichment of the residual melt in Cr, PGE, a range of metals, and easily fusible and volatile elements, and in magma separation into a light silicate-rich liquid and a dense PGE–Cr-rich oxide liquid. In fluid-saturated magma, a Cr-enriched oxide melt completely separates from a silicate melt, its crystallization temperature is lower, and the solubility of PGE is higher. It is likely that the presence of PGE in the Cr-rich oxide liquid takes the form of sybotactic domains, clusters surrounded by ligands of easily fusible and volatile elements, which prevents the PGE from removal via an untimely crystallization from the melt as micronuggets. Crystallization of a PGE-, Cr-rich oxide melt can be traced in the schematic chromite – Os – Pt ternary diagram proposed, which shows a wide field of immiscibility existing between chromite and the PGE. Owing to the low solubility of PGE in chromite, crystallization of the Cr-rich melt results in accumulation of PGE in the interstitial liquid. With IPGE prevailing over PPGE, the crystallization trend does not reach the immiscibility field, ending in synchronous crystallization of chromite and small grains of Pt–Ru–Ir–Os alloys. In case of Pt dominance, the crystallization trend reaches the immiscibility field, Pt gradually accumulates in the residual melt, with final crystallization occurring at a triple eutectic point, which creates favorable conditions for the formation of large Pt nuggets from tens of grams to a few kilograms in weight, which are characteristic only of PGE–chromite deposits of the Uralian–Alaskan and Aldanian types.

The mineralogy and petrology of platinum-group element-bearing sulphide mineralisation within the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, south of Mokopane, northern Bushveld Complex, South Africa

The Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member is a 400 to 800 m thick cumulate package located in the northern limb of the Bushveld Complex, south of the town of Mokopane. On the farm Rooipoort it forms the lowermost unit of the magmatic stratigraphy, overlying Transvaal Supergroup sediments, whereas further south on the farm Grasvally it overlies Lower Zone rocks of the Bushveld Complex. The GNPA member is divided into three units; the Lower Mafic Unit (LMF), the Lower Gabbronorite Unit (LGN) and the Mottled Anorthosite Unit (MANO). Platinum-group element (PGE) mineralisation is closely associated with base metal sulphides (BMS) and is confined to the LMF and MANO where PGE grades range from 1 to 4 ppm (3PGE+Au). A number of distinct BMS assemblages are observed throughout the area and are interpreted to be the result of a combination of primary magmatic processes and low temperature alteration. In areas where the GNPA member is underlain by Lower Zone rocks, a pyrrhotite–chalcopyrite–pentlandite sulphide assemblage dominates, representing initial orthomagmatic sulphide mineralisation. Late-stage low temperature alteration has significantly altered much of the sulphide mineralogy, producing two secondary pyrite–chalcopyrite–pentlandite±pyrrhotite±millerite and pyrite–pentlandite±millerite sulphide assemblages. The primary assemblage was variably altered by crystallisation of pyrite and millerite from pyrrhotite and pentlandite at temperatures below 230°C. Sulphide replacement was associated with the precipitation of quartz and secondary silicates. This replacement of sulphides is more prevalent towards the base of the unit where the GNPA member is underlain by quartzites. These features suggest a strong footwall control over the low temperature alteration and thus the extent of the development of the secondary sulphide assemblages.