PGE

The North Atlantic Igneous Province (NAIP) is a large igneous province (LIP) that includes a series of lava suites erupted from the earliest manifestations of the (proto)-Icelandic plume, through continental rifting and ultimate ocean opening. The lavas of one of these sub-provinces, the British Palaeogene Igneous Province (BPIP), were some of the first lavas to be erupted in the NAIP and overlie a thick crustal basement and sedimentary succession with abundant S-rich mudrocks. We present the first platinum-group element (PGE) and Au analyses of BPIP flood basalts from the main lava fields of the Isle of Mull and Morvern and the Isle of Skye, in addition to a suite of shallow crustal dolerite volcanic plugs on Mull, and other minor lavas suites. BPIP lavas display both S-saturated and S-undersaturated trends which, coupled with elevated PGE abundances (N MORB), suggest that the BPIP is one of the most prospective areas of the NAIP to host Ni–Cu–PGE–(Au) mineralisation in conduit systems. Platinum-group element, Au and chalcophile element abundances in lavas from West and East Greenland, and Iceland, are directly comparable to BPIP lavas, but the relative abundances of Pt and Pd vary systematically between lavas suites of different ages. The oldest lavas (BPIP and West Greenland) have a broadly chondritic Pt/Pd ratio (~ 1.9). Lavas from East Greenland have a lower Pt/Pd ratio (~ 0.8) and the youngest lavas from Iceland have the lowest Pt/Pd ratio of the NAIP (~0.4). Hence, Pt/Pd ratio of otherwise equivalent flood basalt lavas varies temporally across the NAIP and appears to be coincident with the changing geodynamic environment of the (proto)-Icelandic plume through time. We assess the possible causes for such systematic Pt/Pd variation in light of mantle plume and lithospheric controls, and suggest that this reflects a change in the availability of lithospheric mantle Pt-rich sulphides for entrainment in ascending plume magmas. Hence the precious metal systematics and potential prospectivity of a LIP may be affected by contamination of plume-derived magmas by subcontinental lithospheric mantle at the margins of cratons that have been enriched by Palaeoproterozoic orogenesis.

Diamond drill core traverses across the Platreef were carried out at Tweefontein, Sandsloot, and Overysel in order to establish the relationship between crustal contamination and platinum group element (PGE) mineralization. The footwall rocks are significantly different at each of these sites and consist of banded iron formation and sulfidic shales at Tweefontein, of carbonates at Sandsloot, and of granites and granite gneisses at Overysel. As demonstrated in this study, Platreef rocks are characterized by two stages of crustal contamination. The first contamination event occurred prior to emplacement of the magma and is present in Platreef rocks at all three sites, as well as in the Merensky Reef. This event is readily identified on trace element spidergrams and trace element ratio scattergrams. The second contamination event was induced by interaction of the Platreef magma with the local footwall rocks. It is most easily identified at Tweefontein, where there is a large increase in the FeO content of the Platreef rocks, and at Sandsloot, where there is a large increase in their CaO and MgO contents, relative to Bushveld rocks that are uncontaminated by the local footwall rocks. At Overysel, the second contamination event did not result in pronounced changes in the major element composition of the Platreef rocks, but can be detected in their trace element chemistry. A strong inverse relationship between PGE tenors and S/Se ratios is interpreted to suggest that the PGE-rich sulfides were formed prior to emplacement of the Platreef magmas through assimilation of crustal S and became progressively enriched in the PGE during transport. Rather than promoting S-saturation, interaction of the Platreef magma with the footwall rocks diluted the metal tenors of the sulfides. Although both the Platreef and the Merensky Reef magmas were contaminated by the same crustal contaminant and were probably PGE-rich, they have radically different Pd/Pt ratios. Their Pd/Pt ratios suggest that whereas the Merensky Reef magma became PGE-rich due to dissolution of PGE-rich sulfides segregated from a pre-Merensky magma that had undergone relatively little fractionation prior to reaching S-saturation, the pre-Platreef magma had undergone greater fractionation prior to the sulfide saturation event, thereby increasing its Pd/Pt ratio. We suggest that the magmas that formed the Platreef and Merensky Reef may have simply been carrier magmas for sulfides that had formed elsewhere in the plumbing system of the Bushveld Complex by the interaction of earlier generations of magmas with the crustal rocks that underlie the Complex.

The Rietfontein PGE-Cu-Ni sulfide deposit, which occurs in the Eastern Limb of the Bushveld Complex, is mineralogically and geochemically similar to the economic deposits of the Platreef within the Northern Limb of the Complex. The host rocks of the mineralization are mainly norites and gabbronorites together with lesser amounts of plagioclase-bearing ultramafic rocks. The norites occur mainly at the base of the sequence and are overlain by thick sequences of gabbronorites intermixed with narrow intervals of norite and plagioclase-bearing ultramafics. The most common footwall rocks at Rietfontein are fine grained hornfelsed pelitic and quartzose sediments with lesser amounts of coarse grained massive chloritic-cordierite hornfels, and rare recrystallized calc-silicate units. Xenoliths of the footwall rocks are very common in the intrusive rocks. The norites are dominated by cumulus low Mg# orthopyroxenes that are accompanied by both cumulus and intercumulus plagioclase. The gabbronorites are dominated by high Mg# orthopyroxene, with lesser amounts of cumulus chromite and olivine and intercumulus clinopyroxene and plagioclase. The parental magma to these rocks had a similar trace element pattern to that which formed the Platreef and carries a strong crustal contamination signature. Although PGE grades at Rietfontein are similar to those of the Platreef, the mineralized intersections are much narrower. The mineralization occurs at the contact of the gabbronorites with the underlying norites; the basal norites are believed to have been emplaced before the gabbronorites as a separate pulse of magma. This suggests that the PGE-rich sulphides were introduced with the gabbronorite magmas, settling at or near the norite-gabbronorite interface once the capacity of the magma to entrain sulphides was significantly reduced. The presence of some samples with S/Se ratios > 4500 (the upper limit of the mantle S/Se range) indicates that a significant source of S in the sulphides is crustal S. As with the Platreef, there is a strong inverse correlation between PGE tenors and S/Se ratios. This suggests that the PGE-rich sulphides were formed at depth in a staging chamber and then transported by the gabbronorite magma, which may initially have been S-undersaturated. During transport, the sulphides were partially resorbed by the magma, taking Fe and S back into the magma. Those elements with high sulphide melt/silicate melt partition coefficients such as Pd, Pt, Au, Cu and Se were concentrated in the residual sulphides. The development of economic PGE mineralization is controlled by a magma's "carrying capacity", which is determined by the magma's energy, flow rate and style, viscosity, and phenocryst content. The high xenolith content of the Rietfontein rocks attests to the high energy of it's parental magma, thus enabling it to carry a significant phenocryst load and in turn transport PGE-rich sulphides View publication stats View publication stats

The Avebury Ni deposit, which has a resource of 260,000 tonnes of Ni at a grade of 0.9%, is a unique example of a significant Ni sulfide deposit associated with an ophiolite sequence; the deposit is unique because it was formed by hydrothermal processes and also because ophiolites are generally considered unprospective for Ni sulfide mineralization. The deposit is hosted by Middle Cambrian peridotite and dunite cumulate rocks that were most probably formed from S-poor boninitic magmas. The mineralization (which consists ...

A variety of seemingly unrelated processes, such as core-mantle interaction, desulfurization, and direct precipitation from a silicate melt have been proposed to explain the formation of Ru-Os-Ir alloys (here referred to as osmiridiums) found in terrestrial mantle rocks. However, no consensus has yet been reached on how these important micrometer-sized phases form. In this paper we report the results of an experimental study on the solubilities of Ru, Os and Ir in sulfide melts (or mattes) as a function of alloy composition at 1300°C. Considering the low solubilities of Ru, Os, and Ir in silicate melts, coupled with their high matte/silicate-melt partition coefficients, our results indicate that these elements concentrate initially at the ppm level in a matte phase in the mantle source region. During partial melting, the extraction of sulfur into silicate melt leads to a decrease in fS 2 that triggers the exsolution of osmiridiums from the refractory matte in the residue. The newly formed osmiridiums may persist in the terrestrial mantle for periods exceeding billions of years.

Thin (~2 mm) Cr-spinel seams are present at the bases of several of the coupled peridotite-troctolite units that comprise the Rum Eastern Layered Intrusion. In some cases, 'subsidiary' Cr-spinel seams have also developed at between 6 and 12 cm below the unit boundaries. The subsidiary seams are thinner than the normal seams (approx. 1 mm), discontinuous, and occur within and at the base of a thin (~10 cm) layer of anorthosite, which is sandwiched between peridotite and troctolite. The troctolite contains cumulus olivine, but in the Cr-spinel seam and anorthosite, olivine is intercumulus only. Cr-spinel is scarce in the troctolite, but common in the anorthosite, where it has a different composition (more Fe-and Cr-rich) and crystal size distribution (CSD) profile to the Cr-spinel in the subsidiary seam, suggesting that it represents a different crystal population. A model involving downward infiltration of hot picrite and resulting troctolitic cumulate assimilation is developed here to explain the subsidiary seams. This is based on petrographic observation, quantitative textural measurement and mineral chemical analyses. It is suggested that as the picritic magma was emplaced, downward percolation of this melt occurred into a troctolite mush. The anorthosite represents a layer of almost completely melted troctolite, formed by growth of high-anorthite zones from the contaminated picrite onto residual cumulus plagioclase. Assimilation of large amounts of the troctolite cumulate mush forced the contaminated picrite onto the olivine-spinel cotectic, leading to Cr-spinel crystallisation. Subsequently, the Cr-spinel crystals in the anorthosite have reacted with intercumulus melt over a wider temperature interval and have gained a more Fe 3+ -rich composition than the subsidiary seam Mg-and Al-rich Cr-spinels. It is suggested that the separation of a small fraction of immiscible sulphide liquid in the Cr-spinel seams is the result of locally changing SiO 2 and oxygen fugacity at the crystal-melt interface, during the peritectic reaction of some of the Cr-spinel to produce olivine and plagioclase.

Podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the Eastern Desert, Egypt. They are, in most cases, hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the Pan-African belt of Late Precambrian age. Serpentinites are the predominant components in the ophiolitic mélange, either as matrix or as variably sized blocks, and are derived from harzburgite and subordinate dunite. The central Eastern Desert (CED) chromitites have a wide compositional range from high-Cr to high-Al varieties, whereas those of the southern Eastern Desert (SED) have a very restricted compositional range. The Cr# of spinel ranges from 0.5 up to 0.8 in the former, while it is around 0.8 in the latter.

Diamond drill core traverses across the Platreef were carried out at Tweefontein, Sandsloot, and Overysel in order to establish the relationship between crustal contamination and platinum group element (PGE) mineralization. The footwall rocks are significantly different at each of these sites and consist of banded iron formation and sulfidic shales at Tweefontein, of carbonates at Sandsloot, and of granites and granite gneisses at Overysel. As demonstrated in this study, Platreef rocks are characterized by two stages of crustal contamination. The first contamination event occurred prior to emplacement of the magma and is present in Platreef rocks at all three sites, as well as in the Merensky Reef. This event is readily identified on trace element spidergrams and trace element ratio scattergrams. The second contamination event was induced by interaction of the Platreef magma with the local footwall rocks. It is most easily identified at Tweefontein, where there is a large increase in the FeO content of the Platreef rocks, and at Sandsloot, where there is a large increase in their CaO and MgO contents, relative to Bushveld rocks that are uncontaminated by the local footwall rocks. At Overysel, the second contamination event did not result in pronounced changes in the major element composition of the Platreef rocks, but can be detected in their trace element chemistry. A strong inverse relationship between PGE tenors and S/Se ratios is interpreted to suggest that the PGE-rich sulfides were formed prior to emplacement of the Platreef magmas through assimilation of crustal S and became progressively enriched in the PGE during transport. Rather than promoting S-saturation, interaction of the Platreef magma with the footwall rocks diluted the metal tenors of the sulfides. Although both the Platreef and the Merensky Reef magmas were contaminated by the same crustal contaminant and were probably PGE-rich, they have radically different Pd/Pt ratios. Their Pd/Pt ratios suggest that whereas the Merensky Reef magma became PGE-rich due to dissolution of PGE-rich sulfides segregated from a pre-Merensky magma that had undergone relatively little fractionation prior to reaching S-saturation, the pre-Platreef magma had undergone greater fractionation prior to the sulfide saturation event, thereby increasing its Pd/Pt ratio. We suggest that the magmas that formed the Platreef and Merensky Reef may have simply been carrier magmas for sulfides that had formed elsewhere in the plumbing system of the Bushveld Complex by the interaction of earlier generations of magmas with the crustal rocks that underlie the Complex.

Podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the Eastern Desert, Egypt. They are, in most cases, hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the Pan-African belt of Late Precambrian age. Serpentinites are the predominant components in the ophiolitic mélange, either as matrix or as variably sized blocks, and are derived from harzburgite and subordinate dunite. The central Eastern Desert (CED) chromitites have a wide compositional range from high-Cr to high-Al varieties, whereas those of the southern Eastern Desert (SED) have a very restricted compositional range. The Cr# of spinel ranges from 0.5 up to 0.8 in the former, while it is around 0.8 in the latter.

Exposure to pathogens induces antigen-presenting cells (APC) such as macrophages and dendritic cells (DC) to produce various endogenous mediators, including arachidonic acid (AA)-derived eicosanoids, cytokines, and nitric oxide (NO). Many secreted products of activated APC can act by themselves in an autocrine manner and modulate their function. Moreover, the cross-interaction between endogenous bioactive molecules regulates the function of professional APC with important consequences for their ability to activate and sustain immune and inflammatory responses, and to regulate immune homeostasis. Although neglected for many years when compared to their role in cardiovascular homeostasis, cancer and inflammation, the importance of eicosanoids in immunology is becoming more defined. The role of prostaglandin (PG) E2 (PGE2), one of the best known and most well studied eicosanoids, is of particular interest. It modulates the activities of professional DC by acting on their differentiatio...

Exposure to pathogens induces antigen-presenting cells (APC) such as macrophages and dendritic cells (DC) to produce various endogenous mediators, including arachidonic acid (AA)-derived eicosanoids, cytokines, and nitric oxide (NO). Many secreted products of activated APC can act by themselves in an autocrine manner and modulate their function. Moreover, the cross-interaction between endogenous bioactive molecules regulates the function of professional APC with important consequences for their ability to activate and sustain immune and inflammatory responses, and to regulate immune homeostasis. Although neglected for many years when compared to their role in cardiovascular homeostasis, cancer and inflammation, the importance of eicosanoids in immunology is becoming more defined. The role of prostaglandin (PG) E2 (PGE2), one of the best known and most well studied eicosanoids, is of particular interest. It modulates the activities of professional DC by acting on their differentiatio...

Platinum group element (PGE) abundances in the upper continental crust (UCC) are poorly constrained with published values varying by up to an order of magnitude. We evaluated the validity of using loess to estimate PGE abundances in the UCC by measuring these elements in seven Chinese loess samples using a precise method that combines NiS fire assay with isotope dilution. Major and trace elements of the Chinese loess show a typical upper crustal composition and PGE abundances are consistent with literature data on Chinese loess, except for Ru, which is a factor of 10 lowe than published values. We suggest that the high Ru data and RuN/IrN values of Chinese loess reported by Peucker-Ehrenbrink and Jahn (2001) (Geochem. Geophys. Geosys.2, 2001GC000172) are an analytical artifact, rather than a true geochemical characteristic of loess because likely sources of loess are not significantly enriched in Ru and transport and deposition processes cannot preferentially enrich Ru in loess. The...

Podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the Eastern Desert, Egypt. They are, in most cases, hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the Pan-African belt of Late Precambrian age. Serpentinites are the predominant components in the ophiolitic mélange, either as matrix or as variably sized blocks, and are derived from harzburgite and subordinate dunite. The central Eastern Desert (CED) chromitites have a wide compositional range from high-Cr to high-Al varieties, whereas those of the southern Eastern Desert (SED) have a very restricted compositional range. The Cr# of spinel ranges from 0.5 up to 0.8 in the former, while it is around 0.8 in the latter.

The Rietfontein PGE-Cu-Ni sulfide deposit, which occurs in the Eastern Limb of the Bushveld Complex, is mineralogically and geochemically similar to the economic deposits of the Platreef within the Northern Limb of the Complex. The host rocks of the mineralization are mainly norites and gabbronorites together with lesser amounts of plagioclase-bearing ultramafic rocks. The norites occur mainly at the base of the sequence and are overlain by thick sequences of gabbronorites intermixed with narrow intervals of norite and plagioclase-bearing ultramafics. The most common footwall rocks at Rietfontein are fine grained hornfelsed pelitic and quartzose sediments with lesser amounts of coarse grained massive chloritic-cordierite hornfels, and rare recrystallized calc-silicate units. Xenoliths of the footwall rocks are very common in the intrusive rocks. The norites are dominated by cumulus low Mg# orthopyroxenes that are accompanied by both cumulus and intercumulus plagioclase. The gabbronorites are dominated by high Mg# orthopyroxene, with lesser amounts of cumulus chromite and olivine and intercumulus clinopyroxene and plagioclase. The parental magma to these rocks had a similar trace element pattern to that which formed the Platreef and carries a strong crustal contamination signature. Although PGE grades at Rietfontein are similar to those of the Platreef, the mineralized intersections are much narrower. The mineralization occurs at the contact of the gabbronorites with the underlying norites; the basal norites are believed to have been emplaced before the gabbronorites as a separate pulse of magma. This suggests that the PGE-rich sulphides were introduced with the gabbronorite magmas, settling at or near the norite-gabbronorite interface once the capacity of the magma to entrain sulphides was significantly reduced. The presence of some samples with S/Se ratios > 4500 (the upper limit of the mantle S/Se range) indicates that a significant source of S in the sulphides is crustal S. As with the Platreef, there is a strong inverse correlation between PGE tenors and S/Se ratios. This suggests that the PGE-rich sulphides were formed at depth in a staging chamber and then transported by the gabbronorite magma, which may initially have been S-undersaturated. During transport, the sulphides were partially resorbed by the magma, taking Fe and S back into the magma. Those elements with high sulphide melt/silicate melt partition coefficients such as Pd, Pt, Au, Cu and Se were concentrated in the residual sulphides. The development of economic PGE mineralization is controlled by a magma's "carrying capacity", which is determined by the magma's energy, flow rate and style, viscosity, and phenocryst content. The high xenolith content of the Rietfontein rocks attests to the high energy of it's parental magma, thus enabling it to carry a significant phenocryst load and in turn transport PGE-rich sulphides