Ian Campbell - Academia.edu (original) (raw)

Papers by Ian Campbell

Contributions to Mineralogy and Petrology, Jun 21, 2023

The shape of chondrite normalized zircon rare earth element (REE) patterns, including the magnitu... more The shape of chondrite normalized zircon rare earth element (REE) patterns, including the magnitude of the Cerium (Ce) and Europium (Eu) anomalies, provide valuable insights into the magmatic conditions under which a zircon formed. However, lanthanum (La) and praseodymium (Pr), which are essential for the calculation of Ce anomalies, are often present at concentrations close to or below the detection limit of most analytical methods. We propose two new methods to calculate missing REE, based on Onuma diagrams (Chondrite-Onuma) and the lattice strain theory (Chondrite-Lattice), but using chondrite normalized values instead of partition coefficients. We compiled a dataset of ~ 1500 zircons with known REE + Y concentrations and used it to test and calibrate these methods and demonstrate that they are more accurate than other previously published models, with the Chondrite-Onuma method performing better than the Chondrite-Lattice method. These methods require analyses of as few as five REEs to impute the missing REE data or to estimate La and Pr concentrations or Ce anomalies in magmatic zircons, which allows a reduction in the number of REE analysed, where desirable, or to impute missing REEs in legacy data. The imputeREE package for the R programming language was written with a set of tools to apply these methods. A companion app is available to calculate missing REE and Ce and Eu anomalies.

Journal of Petrology, 2016

Komatiites are products of decompression melting of mantle so hot that they are almost exclusivel... more Komatiites are products of decompression melting of mantle so hot that they are almost exclusively restricted to the Archean. The high degree of partial melting (F) and pressure (P) required for their generation facilitates comparison between the magma composition and its mantle source. To investigate compositional variations in Archean komatiites, a global selection of 38 Archean komatiites spanning five cratons (Kaapvaal, Zimbabwe, Yilgarn, Pilbara, Superior) were analysed for their major and trace element contents. Included are the Aluminium-Depleted (ADK, Barberton-Type) and Aluminium-Undepleted (AUK, Munro-Type) petrogenetic types that have been equated with high P/moderate F and moderate P/high F, respectively, on the basis of their Al/Ti and Gd/Yb ratios. Following calculation of the primary magma composition of each suite, we show that the absolute Al content at a specified MgO proves a more sensitive indicator of P than either of the above two ratios and hence we introduce a new classification using Al. The Mg# is a reliable proxy for F, independent of the two endmember melting styles, fractional and batch. We demonstrate that most komatiites form by batch melting, ceding to fractional melting with decreasing pressure as the density contrast between the liquid and solid grows. The Munro AUKs are the only suite to show evidence of fractional melting, with melt extraction occurring at the lowest F and P, %25% melting at 5 GPa (mantle potential temperature, T P ¼ 1750 C) whereas the ADKs of Barberton segregated at the highest F and P (%40%, 9 GPa, T P ¼ 1950 C). The petrogenetic type is a combination of P and F, where, at a given pressure, higher F will produce AUKs over ADKs as majorite is consumed in the source. Through numerical simulations, it is shown that both types can occur within the same mantle plume, with ADKs forming in its cooler, distal fringes whereas AUKs occur along its axis. Furthermore, and contrary to previous views, there is no temporal distinction between the two komatiite types, with both AUKs and ADKs occurring throughout the Archean. By contrast, younger, 2Á7 Ga komatiites tend to have sources that are more depleted than those of older, 3Á5 Ga komatiites. Komatiites are invaluable records of the mantle's chemical and physical evolution during the Archean.

Earth and Planetary Science Letters, 2016

It has been proposed that the spatial variations recorded in the geochemistry of hotspot lavas, s... more It has been proposed that the spatial variations recorded in the geochemistry of hotspot lavas, such as the bilateral asymmetry recorded at Hawaii, can be directly mapped as the heterogeneous structure and composition of their deep-mantle source. This would imply that source-region heterogeneities are transported into, and preserved within, a plume conduit, as the plume rises from the deep-mantle to Earth's surface. Previous laboratory and numerical studies, which neglect density and rheological variations between different chemical components, support this view. However, in this paper, we demonstrate that this interpretation cannot be extended to distinct chemical domains that differ from surrounding mantle in their density and viscosity. By numerically simulating thermo-chemical mantle plumes across a broad parameter space, in 2-D and 3-D, we identify two conduit structures: (i) bilaterally asymmetric conduits, which occur exclusively for cases where the chemical effect on buoyancy is negligible, in which the spatial distribution of deep-mantle heterogeneities is preserved during plume ascent; and (ii) concentric conduits, which occur for all other cases, with dense material preferentially sampled within the conduit's centre. In the latter regime, the spatial distribution of geochemical domains in the lowermost mantle is not preserved during plume ascent. Our results imply that the heterogeneous structure and composition of Earth's lowermost mantle can only be mapped from geochemical observations at Earth's surface if chemical heterogeneity is a passive component of lowermost mantle dynamics (i.e. its effect on density is outweighed by, or is secondary to, the effect of temperature). The implications of our results for: (i) why oceanic crust should be the prevalent component of ocean island basalts; and (ii) how we interpret the geochemical evolution of Earth's deep-mantle are also discussed.

Canadian Mineralogist, Dec 1, 2014

The newly discovered Bellerophon-Nelson telluride-bearing gold deposit at the St. Ives camp, West... more The newly discovered Bellerophon-Nelson telluride-bearing gold deposit at the St. Ives camp, Western Australia, is hosted by meta-sedimentary rocks of the lower Black Flag Group and alkaline intrusions. Four stages of mineralization are recognized; from oldest to youngest these are: quartz-carbonate veins (Stage I), quartz-albite-carbonate-pyrite veins and sericitepyrite seams (Stage II), quartz-pyrite veins (Stage III), and carbonate ± chlorite veins (Stage IV). Stages II and III contain economic gold mineralization, and the gold grains are strongly associated with pyrite. Intense albite and hematite alteration surround the mineralized veins, and trace amounts of gold precipitated in these altered rocks. The albite and hematite alterations are synchronous and derived from the same oxidized fluid as the auriferous veins. The occurrence and absence of hematite within the alteration zone reflects variable amounts of magnetite in the precursor rocks. Thirteen species of telluride and sulfosalt minerals have been identified in Stages II and III. The most common telluride minerals include calaverite, petzite, tellurobismuthite, and altaite, and these minerals have similar occurrences to native gold. In addition to native gold, telluride and sulfosalt minerals are also major Au carriers and account for at least 15% of the gold in this deposit. The mineral associations of PbCl(OH)-Pb 2 Cl 3 (OH)-Te-TeO 2 and BiOCl-BiO(OH,Cl)-Te-TeO 2 were formed as replacement of earlier telluride minerals as the result of reactions with Cl-bearing fluids. The intergrowth between native gold and Cl-bearing minerals + native Te/Te-oxide indicates that Au in telluride minerals was remobilized and re-deposited. The phase diagram for the telluride and sulfosalt mineral association suggests that during Stage II logƒS 2 decreased from -8 to -11, and that logƒTe 2 increased from -8 to the level required for the formation of Te-oxide. The values of logƒTe 2 and logƒS 2 in the Stage III veins were -8 to -11 and -9 to -11.5, respectively. The highly oxidized, tellurium-enriched hydrothermal fluid, which formed the Bellerophon gold telluride deposit, is consistent with the involvement of magmatic fluid, and sulfidation is the likely cause of gold precipitation.

Geochimica et Cosmochimica Acta, 2009

Detrital zircons from the Mississippi River have been analyzed for U-Th-Pb, Lu-Hf and O isotopes ... more Detrital zircons from the Mississippi River have been analyzed for U-Th-Pb, Lu-Hf and O isotopes to constrain the rate of growth of the preserved North American continental crust. One hundred and forty two concordant zircon U/Pb dates on grains mounted in epoxy, obtained by Excimer laser ablation ICP-MS method, resolved six major periods of zircon crystal-

Goldschmidt2021 abstracts, 2021

Mineralium Deposita, 2013

The largest Neoarchean gold deposits in the worldclass St Ives Goldfield, Western Australia, occu... more The largest Neoarchean gold deposits in the worldclass St Ives Goldfield, Western Australia, occur in an area known as the Argo-Junction region (e.g. Junction, Argo and Athena). Why this region is so well endowed with large deposits compared with other parts of the St Ives Goldfield is currently unclear, because gold deposits at St Ives are hosted by a variety of lithologic units and were formed during at least three different deformational events. This paper presents an investigation into the stratigraphic architecture and evolution of the Argo-Junction region to assess its implications for gold metallogenesis. The results show that the region's stratigraphy may be subdivided into five regionally correlatable packages: mafic lavas of the Paringa Basalt; contemporaneously resedimented feldspar-rich pyroclastic debris of the Early Black Flag Group; coarse polymictic volcanic debris of the Late Black Flag Group; thick piles of mafic lavas and subvolcanic sills of the Athena Basalt and Condenser Dolerite; and the voluminous quartz-rich sedimentary successions of the Early Merougil Group. In the Argo-Junction region, these units have an interpreted maximum thickness of at least 7,130 m, and thus represent an unusually thick accumulation of the Neoarchean volcano-sedimentary successions. It is postulated that major basin-forming structures that were active during deposition and emplacement of the voluminous successions later acted as important conduits during mineralisation. Therefore, a correlation exists between the location of the largest gold deposits in the St Ives Goldfield and the thickest parts of the stratigraphy. Recognition of this association has important implications for camp-scale exploration. Keywords Archean. St Ives. Gold. Zircon geochronology. Volcanology. Western Australia several world-class Neoarchean gold camps in the eastern Yilgarn Craton, it may be distinguished from the others by not being dominated by one very large deposit (e.g. the Golden Mile, Kanowna Belle, Wallaby and Sunrise Dam,

Geochimica et Cosmochimica Acta, 2017

Recent studies have shown that platinum-group elements (PGE) can be used to constrain the timing ... more Recent studies have shown that platinum-group elements (PGE) can be used to constrain the timing of sulfide saturation in evolving felsic systems. In this study, we report trace-element, PGE, Re and Au data for the barren and ore-associated suites of intermediate to felsic rocks from the Northparkes Cu-Au porphyry region, emphasizing the timing of sulfide saturation and its influence on the tenor of the associated hydrothermal mineralization. Two barren suites, the Goonumbla and Wombin Volcanics and associate intrusive rocks, are found in the region. Geochemical modelling shows that the barren suites are dominated by plagioclase-pyroxene fractionation, whereas the ore-associated Northparkes Cu-Au porphyry suite is characterized by plagioclase-amphibole fractionation, which requires the ore-bearing suite to have crystallized from a wetter magma than barren suites. The concentrations of PGE, Re and Au in the barren suites decrease continuously during fractional crystallization. This is attributed to early sulfide saturation with the fraction of immiscible sulfide precipitation required to produce the observed trend, being 0.13 and 0.16 wt. % for the Goonumbla and Wombin suites, respectively. The calculated partition coefficients for Au and Pd required to model the observed change in these elements with MgO are well below published values, indicating that R, the mass ratio of silicate to sulfide melt, played a significant role in controlling the rate of decline of these elements with fractionation. Palladium in the oreassociated suite, in contrast, first increases with fractionation then decreases abruptly at 1.2 wt. % MgO. The sharp decrease is attributed to the onset of sulfide precipitation. Platinum on the other hand shows a moderate decrease, starting from the highest MgO sample analysed, but then decreasing strongly from 1.2 wt. % MgO. The initial Pt decrease is attributed to precipitation of a platinum-group mineral (PGM), probably a Pt-Fe alloy, and the sharp decrease of both Pt and Pd at 1.2 wt. % MgO to sulfide saturation. We suggest that the Goonumbla and Wombin suites are barren because early sulfide saturation locked most of the Cu and Au in a sulfide phase in the cumulus pile of a deep parental magma chamber, well before volatile saturation, so that when the magma reached volatile saturation, it did not have access to the Cu and Au. This contrasts with the relatively late sulfide saturation in the oreassociated suite, which was followed shortly afterwards by volatile saturation. Rayleigh fractionation concentrated incompatible Cu and Au by at least a factor of five before volatile saturation. The short crystallization interval between immiscible sulfide and volatile saturation allowed some Au and Cu to be stripped from the evolving magma. Gold, with its higher partition coefficient into immiscible sulfide melts, was more affected than Cu. The result is a Cu-Au deposit. Our study also suggests that Rayleigh fractionation is as at least as important as the initial concentration of chalcophile elements in the parent magma in determining the fertility of felsic magma suites.

Science, Jan 24, 1997

Niobium/uranium ratios in greenstone-belt basalts and gabbros indicate that parts of the Late Arc... more Niobium/uranium ratios in greenstone-belt basalts and gabbros indicate that parts of the Late Archean mantle beneath Western Australia underwent a level of melt extraction, resulting in formation of the continental crust, comparable to that seen in the present mantle. The implication is either that (i) the amount of continental crust that formed before 2.7 × 10 9 years ago was much greater than generally thought or (ii) crustal growth occurred by severe depletion of small volumes of the mantle rather than by moderate depletion of a large volume of mantle.

Goldschmidt2021 abstracts, 2021

An emerging and increasingly pervasive school of thought is that large, long-lived and largely mo... more An emerging and increasingly pervasive school of thought is that large, long-lived and largely molten magma chambers are transient to non-existent in Earth’s history1–13. These ideas attempt to supplant the classical paradigm of the ‘big magma tank’ chambers in which the melt differentiates, is replenished, and occasionally feeds the overlying volcanoes14–23. The stratiform chromitites in the Bushveld Complex – the largest magmatic body in the Earth’s crust24 – however, offers strong contest to this shifting concept. Several chromitites in this complex occur as layers up to 2 metres in thickness and more than 400 kilometres in lateral extent, implying that chromitite-forming events were chamber-wide phenomena24–27. Field relations and microtextural data, specifically the relationship of 3D coordination number and grain size, indicate that the chromitites grew as a 3D framework of touching chromite grains directly at the chamber floor from a melt saturated in chromite only28–30. Mass...

Precambrian Research, 2019

Coarsely crystalline mafic rocks, commonly referred to as dolerite and diabase, make up a substan... more Coarsely crystalline mafic rocks, commonly referred to as dolerite and diabase, make up a substantial proportion of Archean greenstone belts and are hosts to major orogenic gold deposits, yet their internal architecture and contact relationships remain poorly documented and their emplacement origins are assumed rather than supported with evidence. We present results from drill core logging of several late Archean coarsely crystalline packages from the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia, including the Golden Mile Dolerite. These large mafic bodies comprise mostly medium-to coarsely crystalline mafic rocks, but also include a variety of other products of differentiation. The bodies range from ~75 to 460 m thick and extend for tens of kilometers, are primarily hosted in mudstones and basalt and appear to be concordant to stratigraphy. As such, this study excludes mafic dykes and extremely thick and internally complex bodies such as the Bushveld Complex. From base to top, the internal stratigraphy generally consists of a lower unit of aphyric basalt, dolerite/gabbro and peridotite +/-pyroxenite, then a voluminous middle dolerite/gabbro, and an upper quartz dolerite/gabbro, and a topmost aphyric basalt. Granophyric veins and domains are common in the upper third of the mafic body. Contacts are gradational between all lithofacies, with the exception of granophric veins, which have sharp contacts. Within this overall stratigraphic framework there is variability in the relative thickness of lithofacies, number of lithofacies, as well as repetition of some lithofacies (several examples also include basalt in the interior). In well exposed examples, the key evidence for distinguishing intrusions from extrusions is provided by the top contact. However, the top contact from these examples is ambiguous. To overcome this gap, we review the character of well-studied thick ponded lavas from the literature. Most aspects of the internal character are not unique to intrusions, although the maximum groundmass grain size (excluding granophyric veins) of the Archean examples (>3 mm) is significantly coarser than that for any known ponded lavas (<<1 mm). The ability to distinguish intrusions from extrusions has important implications for reconstructing Archean greenstone stratigraphy and mineral exploration.

Journal of Petrology, 2015

Geochronological and geochemical studies, including platinum group element (PGE) analyses, were u... more Geochronological and geochemical studies, including platinum group element (PGE) analyses, were undertaken on samples from the El Abra-Pajonal igneous complex, northern Chile, to investigate the magmatic evolution of the suite. Special attention was paid to identifying the onset of sulfide saturation and to documenting how it influenced the geochemistry of the chalcophile elements and the formation of the El Abra porphyry Cu deposit. The PGE have extreme sulfide melt-silicate melt partition coefficients, making them sensitive indicators of the timing of sulfide saturation in an evolving magmatic system. In arc-related intermediate to felsic magmatic systems, which have the potential to produce porphyry deposits, the timing and extent of sulfide saturation relative to orefluid saturation may control the capacity of these systems to produce economic mineralization and, if they do, whether the deposits are Cu-only or Cu-Au. This study incorporates the first comprehensive analysis of PGE in a felsic magmatic suite associated with an economic porphyry system. The suite comprises a series of quartz monzodiorite to granite intrusions with U-Pb zircon ages between 43 and 35 Ma. Their petrography and major element chemistry, including increasing Sr/Y ratios and rare earth element patterns, suggest that crystal fractionation and crustal assimilation were the key magmatic processes governing the evolution of the El Abra-Pajonal suite. Plagioclase fractionation dominated the oldest intrusions, and their associated granites and aplites. Following the injection of a more primitive, wetter, mafic magma at 41-40 Ma, plagioclase fractionation became suppressed and amphibole became the dominant fractionating phase, leading to the formation of the El Abra porphyry intrusion and Cu deposit. Abundances of Pt and Pd in felsic rocks from the El Abra-Pajonal intrusive complex drop rapidly in samples with MgO values below 2Á5 wt %, following sulfide saturation of the magmas, which occurred slightly before ore-fluid saturation and formation of the Cu deposit. Modeling suggests that the amount of sulfide formed was very small, enough to strip the PGE and Au from the magma but not Cu, because of the lower partition coefficient of Cu relative to the precious metals, which explains why the mineralization at El Abra is a Cu-only porphyry deposit, rather than a Cu-Au deposit.

Economic Geology, 2015

We report sensitive high mass resolution ion microprobe, stable isotopes (SHRIMP SI) multiple sul... more We report sensitive high mass resolution ion microprobe, stable isotopes (SHRIMP SI) multiple sulfur isotope analyses (32S, 33S, 34S) to constrain the sources of sulfur in three Archean VMS deposits—Teutonic Bore, Bentley, and Jaguar—from the Teutonic Bore volcanic complex of the Yilgarn Craton, Western Australia, together with sedimentary pyrites from associated black shales and interpillow pyrites. The pyrites from VMS mineralization are dominated by mantle sulfur but include a small amount of slightly negative mass-independent fractionation (MIF) anomalies, whereas sulfur from the pyrites in the sedimentary rocks has pronounced positive MIF, with ∆33S values that lie between 0.19 and 6.20‰ (with one outlier at −1.62‰). The wall rocks to the mineralization include sedimentary rocks that have contributed no detectable positive MIF sulfur to the VMS deposits, which is difficult to reconcile with the leaching model for the formation of these deposits. The sulfur isotope data are best explained by mixing between sulfur derived from a magmatic-hydrothermal fluid and seawater sulfur as represented by the interpillow pyrites. The massive sulfide lens pyrites have a weighted mean ∆33S value of −0.27 ± 0.05‰ (MSWD = 1.6) nearly identical with −0.31 ± 0.08‰ (MSWD = 2.4) for pyrites from the stringer zone, which requires mixing to have occurred below the sea floor. We employed a two-component mixing model to estimate the contribution of seawater sulfur to the total sulfur budget of the two Teutonic Bore volcanic complex VMS deposits. The results are 15 to 18% for both Teutonic Bore and Bentley, much higher than the 3% obtained by Jamieson et al. (2013) for the giant Kidd Creek deposit. Similar calculations, carried out for other Neoarchean VMS deposits give value between 2% and 30%, which are similar to modern hydrothermal VMS deposits. We suggest that multiple sulfur isotope analyses may be used to predict the size of Archean VMS deposits and to provide a vector to ore deposit but further studies are needed to test these suggestions.

Journal of Petrology, 1996

The 230 km 2 Proterozoic Bjerkreim-Sokndal layered intrusion has a monzonoritic bulk composition ... more The 230 km 2 Proterozoic Bjerkreim-Sokndal layered intrusion has a monzonoritic bulk composition and comprises a 6 km thick, broadly leuconoritic Layered Series (LS) overlain by unlayered mangerite and quartz mangerite. In the Bjerkreim lobe the LS comprises six megacydic units (MCU 0, IA, IB, II, HI and IV) in a syncline. This lobe is surrounded by migmatitic gneisses; the roof to the intrusion is missing. The mgnumber opx, An % and whole-rock initial Sr/Sr isotope ratios (Sr 0) display broadly parallel trends through MCUs II and III, and into MCU IV, with decreasing mg-number (75-58) and An % (50-40) accompanying increasing Sro (0-7050-0-7085). This correlation breaks down in the upper part of MCU IVand mangerite; the quartz mangerite has Sro values of ~0 7085. Abrupt reversals occur across the MCU boundaries. The LS crystallized on the floor of a periodically replenished magma chamber that was continually assimilating country rock gneisses. Strong compositional zoning of the magma developed as a result of repeated replenishments with relatively dense, primitive ferrobasaltic magma (Sr 0-0-7049) along the floor and the development of extensive buoyant roof melts. Assimilation took place on a massive scale (up to ~50%) in the upper parts of the chamber.

Geochimica et Cosmochimica Acta, 2012

Experimental studies, performed under oxidized conditions (fO 2 > QFM + 2, where QFM is quartz-fa... more Experimental studies, performed under oxidized conditions (fO 2 > QFM + 2, where QFM is quartz-fayalite-magnetite oxygen buffer), have shown that Rh, Ru, Ir and Os are strongly compatible with Cr spinel, whereas empirical studies of Cr spinels from ultramafic-mafic rocks suggest that the experimental results may overestimate the partition coefficients. We report laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of platinum-group elements (PGE), Au and Re abundances in Cr spinels from the Ambae volcano, Vanuatu (fO 2 = QFM + 2.5), the Jimberlana layered intrusion, western Australia, and the Bushveld complex, South Africa (fO 2 $ QFM). The results show that Rh and IPGEs (Iridium-group PGE; Ru, Ir, Os) partition strongly into the Cr spinels that crystallized from the oxidized Ambae lavas whereas most of the Cr spinels from the more reduced Jimberlana layered intrusion and the Bushveld complex contain no detectable PGE, Au or Re, with exception of $10 ppb of Ir in some Jimberlana Cr spinels. In the Ambae Cr spinels, Rh, Ru and, to lesser extent Os, are positively correlated with Fe 3+ , Ni and V. The homogeneous distribution of Rh and IPGEs in LA-ICP-MS time-resolved spectra indicates that these elements are in solid solution in Cr spinels. Pt-Fe alloys occur as inclusions within the Ambae Cr spinels, which indicate that the Ambae melt was saturated with Pt. Our results show that partitioning of Rh, Ru and Ir into Cr spinels increases with increasing oxygen fugacity, which suggests that the high concentrations of these elements in the Ambae Cr spinels are due to the high oxygen fugacity of the host magma. Therefore, Cr spinels may play an important role in controlling the concentrations of Rh and IPGEs during fractional crystallization of oxidized ultramafic-mafic magmas and during partial melting of oxidized arc mantle.

Geochimica et Cosmochimica Acta, 2012

Platinum group element (PGE) abundances in the upper continental crust (UCC) are poorly constrain... more 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 lower than published values. We suggest that the high Ru data and Ru N /Ir N 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 effect of eolian fractionation on PGE abundances in loess appears to be limited because Chinese loess from different locations shows similar PGE patterns and concentrations. This conclusion is supported by strong positive correlations between the PGE (except for Pt) and other compatible elements such as Fe 2 O 3 , Ni, Cr, Co. Using a compilation of PGE data for loess from China, Argentina and Europe, including our data but excluding one sample with an anomalously high Pt content, we propose average PGE abundances for global loess of Ir = 0.022 ppb (ng/g), Ru = 0.030 ppb, Rh = 0.018 ppb, Pt = 0.599 ppb, and Pd = 0.526 ppb, and suggest that these are the best current estimates for the PGE abundances of the UCC.

Contributions to Mineralogy and Petrology, Jun 21, 2023

The shape of chondrite normalized zircon rare earth element (REE) patterns, including the magnitu... more The shape of chondrite normalized zircon rare earth element (REE) patterns, including the magnitude of the Cerium (Ce) and Europium (Eu) anomalies, provide valuable insights into the magmatic conditions under which a zircon formed. However, lanthanum (La) and praseodymium (Pr), which are essential for the calculation of Ce anomalies, are often present at concentrations close to or below the detection limit of most analytical methods. We propose two new methods to calculate missing REE, based on Onuma diagrams (Chondrite-Onuma) and the lattice strain theory (Chondrite-Lattice), but using chondrite normalized values instead of partition coefficients. We compiled a dataset of ~ 1500 zircons with known REE + Y concentrations and used it to test and calibrate these methods and demonstrate that they are more accurate than other previously published models, with the Chondrite-Onuma method performing better than the Chondrite-Lattice method. These methods require analyses of as few as five REEs to impute the missing REE data or to estimate La and Pr concentrations or Ce anomalies in magmatic zircons, which allows a reduction in the number of REE analysed, where desirable, or to impute missing REEs in legacy data. The imputeREE package for the R programming language was written with a set of tools to apply these methods. A companion app is available to calculate missing REE and Ce and Eu anomalies.

Journal of Petrology, 2016

Komatiites are products of decompression melting of mantle so hot that they are almost exclusivel... more Komatiites are products of decompression melting of mantle so hot that they are almost exclusively restricted to the Archean. The high degree of partial melting (F) and pressure (P) required for their generation facilitates comparison between the magma composition and its mantle source. To investigate compositional variations in Archean komatiites, a global selection of 38 Archean komatiites spanning five cratons (Kaapvaal, Zimbabwe, Yilgarn, Pilbara, Superior) were analysed for their major and trace element contents. Included are the Aluminium-Depleted (ADK, Barberton-Type) and Aluminium-Undepleted (AUK, Munro-Type) petrogenetic types that have been equated with high P/moderate F and moderate P/high F, respectively, on the basis of their Al/Ti and Gd/Yb ratios. Following calculation of the primary magma composition of each suite, we show that the absolute Al content at a specified MgO proves a more sensitive indicator of P than either of the above two ratios and hence we introduce a new classification using Al. The Mg# is a reliable proxy for F, independent of the two endmember melting styles, fractional and batch. We demonstrate that most komatiites form by batch melting, ceding to fractional melting with decreasing pressure as the density contrast between the liquid and solid grows. The Munro AUKs are the only suite to show evidence of fractional melting, with melt extraction occurring at the lowest F and P, %25% melting at 5 GPa (mantle potential temperature, T P ¼ 1750 C) whereas the ADKs of Barberton segregated at the highest F and P (%40%, 9 GPa, T P ¼ 1950 C). The petrogenetic type is a combination of P and F, where, at a given pressure, higher F will produce AUKs over ADKs as majorite is consumed in the source. Through numerical simulations, it is shown that both types can occur within the same mantle plume, with ADKs forming in its cooler, distal fringes whereas AUKs occur along its axis. Furthermore, and contrary to previous views, there is no temporal distinction between the two komatiite types, with both AUKs and ADKs occurring throughout the Archean. By contrast, younger, 2Á7 Ga komatiites tend to have sources that are more depleted than those of older, 3Á5 Ga komatiites. Komatiites are invaluable records of the mantle's chemical and physical evolution during the Archean.

Earth and Planetary Science Letters, 2016

It has been proposed that the spatial variations recorded in the geochemistry of hotspot lavas, s... more It has been proposed that the spatial variations recorded in the geochemistry of hotspot lavas, such as the bilateral asymmetry recorded at Hawaii, can be directly mapped as the heterogeneous structure and composition of their deep-mantle source. This would imply that source-region heterogeneities are transported into, and preserved within, a plume conduit, as the plume rises from the deep-mantle to Earth's surface. Previous laboratory and numerical studies, which neglect density and rheological variations between different chemical components, support this view. However, in this paper, we demonstrate that this interpretation cannot be extended to distinct chemical domains that differ from surrounding mantle in their density and viscosity. By numerically simulating thermo-chemical mantle plumes across a broad parameter space, in 2-D and 3-D, we identify two conduit structures: (i) bilaterally asymmetric conduits, which occur exclusively for cases where the chemical effect on buoyancy is negligible, in which the spatial distribution of deep-mantle heterogeneities is preserved during plume ascent; and (ii) concentric conduits, which occur for all other cases, with dense material preferentially sampled within the conduit's centre. In the latter regime, the spatial distribution of geochemical domains in the lowermost mantle is not preserved during plume ascent. Our results imply that the heterogeneous structure and composition of Earth's lowermost mantle can only be mapped from geochemical observations at Earth's surface if chemical heterogeneity is a passive component of lowermost mantle dynamics (i.e. its effect on density is outweighed by, or is secondary to, the effect of temperature). The implications of our results for: (i) why oceanic crust should be the prevalent component of ocean island basalts; and (ii) how we interpret the geochemical evolution of Earth's deep-mantle are also discussed.

Canadian Mineralogist, Dec 1, 2014

The newly discovered Bellerophon-Nelson telluride-bearing gold deposit at the St. Ives camp, West... more The newly discovered Bellerophon-Nelson telluride-bearing gold deposit at the St. Ives camp, Western Australia, is hosted by meta-sedimentary rocks of the lower Black Flag Group and alkaline intrusions. Four stages of mineralization are recognized; from oldest to youngest these are: quartz-carbonate veins (Stage I), quartz-albite-carbonate-pyrite veins and sericitepyrite seams (Stage II), quartz-pyrite veins (Stage III), and carbonate ± chlorite veins (Stage IV). Stages II and III contain economic gold mineralization, and the gold grains are strongly associated with pyrite. Intense albite and hematite alteration surround the mineralized veins, and trace amounts of gold precipitated in these altered rocks. The albite and hematite alterations are synchronous and derived from the same oxidized fluid as the auriferous veins. The occurrence and absence of hematite within the alteration zone reflects variable amounts of magnetite in the precursor rocks. Thirteen species of telluride and sulfosalt minerals have been identified in Stages II and III. The most common telluride minerals include calaverite, petzite, tellurobismuthite, and altaite, and these minerals have similar occurrences to native gold. In addition to native gold, telluride and sulfosalt minerals are also major Au carriers and account for at least 15% of the gold in this deposit. The mineral associations of PbCl(OH)-Pb 2 Cl 3 (OH)-Te-TeO 2 and BiOCl-BiO(OH,Cl)-Te-TeO 2 were formed as replacement of earlier telluride minerals as the result of reactions with Cl-bearing fluids. The intergrowth between native gold and Cl-bearing minerals + native Te/Te-oxide indicates that Au in telluride minerals was remobilized and re-deposited. The phase diagram for the telluride and sulfosalt mineral association suggests that during Stage II logƒS 2 decreased from -8 to -11, and that logƒTe 2 increased from -8 to the level required for the formation of Te-oxide. The values of logƒTe 2 and logƒS 2 in the Stage III veins were -8 to -11 and -9 to -11.5, respectively. The highly oxidized, tellurium-enriched hydrothermal fluid, which formed the Bellerophon gold telluride deposit, is consistent with the involvement of magmatic fluid, and sulfidation is the likely cause of gold precipitation.

Geochimica et Cosmochimica Acta, 2009

Detrital zircons from the Mississippi River have been analyzed for U-Th-Pb, Lu-Hf and O isotopes ... more Detrital zircons from the Mississippi River have been analyzed for U-Th-Pb, Lu-Hf and O isotopes to constrain the rate of growth of the preserved North American continental crust. One hundred and forty two concordant zircon U/Pb dates on grains mounted in epoxy, obtained by Excimer laser ablation ICP-MS method, resolved six major periods of zircon crystal-

Goldschmidt2021 abstracts, 2021

Mineralium Deposita, 2013

The largest Neoarchean gold deposits in the worldclass St Ives Goldfield, Western Australia, occu... more The largest Neoarchean gold deposits in the worldclass St Ives Goldfield, Western Australia, occur in an area known as the Argo-Junction region (e.g. Junction, Argo and Athena). Why this region is so well endowed with large deposits compared with other parts of the St Ives Goldfield is currently unclear, because gold deposits at St Ives are hosted by a variety of lithologic units and were formed during at least three different deformational events. This paper presents an investigation into the stratigraphic architecture and evolution of the Argo-Junction region to assess its implications for gold metallogenesis. The results show that the region's stratigraphy may be subdivided into five regionally correlatable packages: mafic lavas of the Paringa Basalt; contemporaneously resedimented feldspar-rich pyroclastic debris of the Early Black Flag Group; coarse polymictic volcanic debris of the Late Black Flag Group; thick piles of mafic lavas and subvolcanic sills of the Athena Basalt and Condenser Dolerite; and the voluminous quartz-rich sedimentary successions of the Early Merougil Group. In the Argo-Junction region, these units have an interpreted maximum thickness of at least 7,130 m, and thus represent an unusually thick accumulation of the Neoarchean volcano-sedimentary successions. It is postulated that major basin-forming structures that were active during deposition and emplacement of the voluminous successions later acted as important conduits during mineralisation. Therefore, a correlation exists between the location of the largest gold deposits in the St Ives Goldfield and the thickest parts of the stratigraphy. Recognition of this association has important implications for camp-scale exploration. Keywords Archean. St Ives. Gold. Zircon geochronology. Volcanology. Western Australia several world-class Neoarchean gold camps in the eastern Yilgarn Craton, it may be distinguished from the others by not being dominated by one very large deposit (e.g. the Golden Mile, Kanowna Belle, Wallaby and Sunrise Dam,

Geochimica et Cosmochimica Acta, 2017

Recent studies have shown that platinum-group elements (PGE) can be used to constrain the timing ... more Recent studies have shown that platinum-group elements (PGE) can be used to constrain the timing of sulfide saturation in evolving felsic systems. In this study, we report trace-element, PGE, Re and Au data for the barren and ore-associated suites of intermediate to felsic rocks from the Northparkes Cu-Au porphyry region, emphasizing the timing of sulfide saturation and its influence on the tenor of the associated hydrothermal mineralization. Two barren suites, the Goonumbla and Wombin Volcanics and associate intrusive rocks, are found in the region. Geochemical modelling shows that the barren suites are dominated by plagioclase-pyroxene fractionation, whereas the ore-associated Northparkes Cu-Au porphyry suite is characterized by plagioclase-amphibole fractionation, which requires the ore-bearing suite to have crystallized from a wetter magma than barren suites. The concentrations of PGE, Re and Au in the barren suites decrease continuously during fractional crystallization. This is attributed to early sulfide saturation with the fraction of immiscible sulfide precipitation required to produce the observed trend, being 0.13 and 0.16 wt. % for the Goonumbla and Wombin suites, respectively. The calculated partition coefficients for Au and Pd required to model the observed change in these elements with MgO are well below published values, indicating that R, the mass ratio of silicate to sulfide melt, played a significant role in controlling the rate of decline of these elements with fractionation. Palladium in the oreassociated suite, in contrast, first increases with fractionation then decreases abruptly at 1.2 wt. % MgO. The sharp decrease is attributed to the onset of sulfide precipitation. Platinum on the other hand shows a moderate decrease, starting from the highest MgO sample analysed, but then decreasing strongly from 1.2 wt. % MgO. The initial Pt decrease is attributed to precipitation of a platinum-group mineral (PGM), probably a Pt-Fe alloy, and the sharp decrease of both Pt and Pd at 1.2 wt. % MgO to sulfide saturation. We suggest that the Goonumbla and Wombin suites are barren because early sulfide saturation locked most of the Cu and Au in a sulfide phase in the cumulus pile of a deep parental magma chamber, well before volatile saturation, so that when the magma reached volatile saturation, it did not have access to the Cu and Au. This contrasts with the relatively late sulfide saturation in the oreassociated suite, which was followed shortly afterwards by volatile saturation. Rayleigh fractionation concentrated incompatible Cu and Au by at least a factor of five before volatile saturation. The short crystallization interval between immiscible sulfide and volatile saturation allowed some Au and Cu to be stripped from the evolving magma. Gold, with its higher partition coefficient into immiscible sulfide melts, was more affected than Cu. The result is a Cu-Au deposit. Our study also suggests that Rayleigh fractionation is as at least as important as the initial concentration of chalcophile elements in the parent magma in determining the fertility of felsic magma suites.

Science, Jan 24, 1997

Niobium/uranium ratios in greenstone-belt basalts and gabbros indicate that parts of the Late Arc... more Niobium/uranium ratios in greenstone-belt basalts and gabbros indicate that parts of the Late Archean mantle beneath Western Australia underwent a level of melt extraction, resulting in formation of the continental crust, comparable to that seen in the present mantle. The implication is either that (i) the amount of continental crust that formed before 2.7 × 10 9 years ago was much greater than generally thought or (ii) crustal growth occurred by severe depletion of small volumes of the mantle rather than by moderate depletion of a large volume of mantle.

Goldschmidt2021 abstracts, 2021

An emerging and increasingly pervasive school of thought is that large, long-lived and largely mo... more An emerging and increasingly pervasive school of thought is that large, long-lived and largely molten magma chambers are transient to non-existent in Earth’s history1–13. These ideas attempt to supplant the classical paradigm of the ‘big magma tank’ chambers in which the melt differentiates, is replenished, and occasionally feeds the overlying volcanoes14–23. The stratiform chromitites in the Bushveld Complex – the largest magmatic body in the Earth’s crust24 – however, offers strong contest to this shifting concept. Several chromitites in this complex occur as layers up to 2 metres in thickness and more than 400 kilometres in lateral extent, implying that chromitite-forming events were chamber-wide phenomena24–27. Field relations and microtextural data, specifically the relationship of 3D coordination number and grain size, indicate that the chromitites grew as a 3D framework of touching chromite grains directly at the chamber floor from a melt saturated in chromite only28–30. Mass...

Precambrian Research, 2019

Coarsely crystalline mafic rocks, commonly referred to as dolerite and diabase, make up a substan... more Coarsely crystalline mafic rocks, commonly referred to as dolerite and diabase, make up a substantial proportion of Archean greenstone belts and are hosts to major orogenic gold deposits, yet their internal architecture and contact relationships remain poorly documented and their emplacement origins are assumed rather than supported with evidence. We present results from drill core logging of several late Archean coarsely crystalline packages from the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia, including the Golden Mile Dolerite. These large mafic bodies comprise mostly medium-to coarsely crystalline mafic rocks, but also include a variety of other products of differentiation. The bodies range from ~75 to 460 m thick and extend for tens of kilometers, are primarily hosted in mudstones and basalt and appear to be concordant to stratigraphy. As such, this study excludes mafic dykes and extremely thick and internally complex bodies such as the Bushveld Complex. From base to top, the internal stratigraphy generally consists of a lower unit of aphyric basalt, dolerite/gabbro and peridotite +/-pyroxenite, then a voluminous middle dolerite/gabbro, and an upper quartz dolerite/gabbro, and a topmost aphyric basalt. Granophyric veins and domains are common in the upper third of the mafic body. Contacts are gradational between all lithofacies, with the exception of granophric veins, which have sharp contacts. Within this overall stratigraphic framework there is variability in the relative thickness of lithofacies, number of lithofacies, as well as repetition of some lithofacies (several examples also include basalt in the interior). In well exposed examples, the key evidence for distinguishing intrusions from extrusions is provided by the top contact. However, the top contact from these examples is ambiguous. To overcome this gap, we review the character of well-studied thick ponded lavas from the literature. Most aspects of the internal character are not unique to intrusions, although the maximum groundmass grain size (excluding granophyric veins) of the Archean examples (>3 mm) is significantly coarser than that for any known ponded lavas (<<1 mm). The ability to distinguish intrusions from extrusions has important implications for reconstructing Archean greenstone stratigraphy and mineral exploration.

Journal of Petrology, 2015

Geochronological and geochemical studies, including platinum group element (PGE) analyses, were u... more Geochronological and geochemical studies, including platinum group element (PGE) analyses, were undertaken on samples from the El Abra-Pajonal igneous complex, northern Chile, to investigate the magmatic evolution of the suite. Special attention was paid to identifying the onset of sulfide saturation and to documenting how it influenced the geochemistry of the chalcophile elements and the formation of the El Abra porphyry Cu deposit. The PGE have extreme sulfide melt-silicate melt partition coefficients, making them sensitive indicators of the timing of sulfide saturation in an evolving magmatic system. In arc-related intermediate to felsic magmatic systems, which have the potential to produce porphyry deposits, the timing and extent of sulfide saturation relative to orefluid saturation may control the capacity of these systems to produce economic mineralization and, if they do, whether the deposits are Cu-only or Cu-Au. This study incorporates the first comprehensive analysis of PGE in a felsic magmatic suite associated with an economic porphyry system. The suite comprises a series of quartz monzodiorite to granite intrusions with U-Pb zircon ages between 43 and 35 Ma. Their petrography and major element chemistry, including increasing Sr/Y ratios and rare earth element patterns, suggest that crystal fractionation and crustal assimilation were the key magmatic processes governing the evolution of the El Abra-Pajonal suite. Plagioclase fractionation dominated the oldest intrusions, and their associated granites and aplites. Following the injection of a more primitive, wetter, mafic magma at 41-40 Ma, plagioclase fractionation became suppressed and amphibole became the dominant fractionating phase, leading to the formation of the El Abra porphyry intrusion and Cu deposit. Abundances of Pt and Pd in felsic rocks from the El Abra-Pajonal intrusive complex drop rapidly in samples with MgO values below 2Á5 wt %, following sulfide saturation of the magmas, which occurred slightly before ore-fluid saturation and formation of the Cu deposit. Modeling suggests that the amount of sulfide formed was very small, enough to strip the PGE and Au from the magma but not Cu, because of the lower partition coefficient of Cu relative to the precious metals, which explains why the mineralization at El Abra is a Cu-only porphyry deposit, rather than a Cu-Au deposit.

Economic Geology, 2015

We report sensitive high mass resolution ion microprobe, stable isotopes (SHRIMP SI) multiple sul... more We report sensitive high mass resolution ion microprobe, stable isotopes (SHRIMP SI) multiple sulfur isotope analyses (32S, 33S, 34S) to constrain the sources of sulfur in three Archean VMS deposits—Teutonic Bore, Bentley, and Jaguar—from the Teutonic Bore volcanic complex of the Yilgarn Craton, Western Australia, together with sedimentary pyrites from associated black shales and interpillow pyrites. The pyrites from VMS mineralization are dominated by mantle sulfur but include a small amount of slightly negative mass-independent fractionation (MIF) anomalies, whereas sulfur from the pyrites in the sedimentary rocks has pronounced positive MIF, with ∆33S values that lie between 0.19 and 6.20‰ (with one outlier at −1.62‰). The wall rocks to the mineralization include sedimentary rocks that have contributed no detectable positive MIF sulfur to the VMS deposits, which is difficult to reconcile with the leaching model for the formation of these deposits. The sulfur isotope data are best explained by mixing between sulfur derived from a magmatic-hydrothermal fluid and seawater sulfur as represented by the interpillow pyrites. The massive sulfide lens pyrites have a weighted mean ∆33S value of −0.27 ± 0.05‰ (MSWD = 1.6) nearly identical with −0.31 ± 0.08‰ (MSWD = 2.4) for pyrites from the stringer zone, which requires mixing to have occurred below the sea floor. We employed a two-component mixing model to estimate the contribution of seawater sulfur to the total sulfur budget of the two Teutonic Bore volcanic complex VMS deposits. The results are 15 to 18% for both Teutonic Bore and Bentley, much higher than the 3% obtained by Jamieson et al. (2013) for the giant Kidd Creek deposit. Similar calculations, carried out for other Neoarchean VMS deposits give value between 2% and 30%, which are similar to modern hydrothermal VMS deposits. We suggest that multiple sulfur isotope analyses may be used to predict the size of Archean VMS deposits and to provide a vector to ore deposit but further studies are needed to test these suggestions.

Journal of Petrology, 1996

The 230 km 2 Proterozoic Bjerkreim-Sokndal layered intrusion has a monzonoritic bulk composition ... more The 230 km 2 Proterozoic Bjerkreim-Sokndal layered intrusion has a monzonoritic bulk composition and comprises a 6 km thick, broadly leuconoritic Layered Series (LS) overlain by unlayered mangerite and quartz mangerite. In the Bjerkreim lobe the LS comprises six megacydic units (MCU 0, IA, IB, II, HI and IV) in a syncline. This lobe is surrounded by migmatitic gneisses; the roof to the intrusion is missing. The mgnumber opx, An % and whole-rock initial Sr/Sr isotope ratios (Sr 0) display broadly parallel trends through MCUs II and III, and into MCU IV, with decreasing mg-number (75-58) and An % (50-40) accompanying increasing Sro (0-7050-0-7085). This correlation breaks down in the upper part of MCU IVand mangerite; the quartz mangerite has Sro values of ~0 7085. Abrupt reversals occur across the MCU boundaries. The LS crystallized on the floor of a periodically replenished magma chamber that was continually assimilating country rock gneisses. Strong compositional zoning of the magma developed as a result of repeated replenishments with relatively dense, primitive ferrobasaltic magma (Sr 0-0-7049) along the floor and the development of extensive buoyant roof melts. Assimilation took place on a massive scale (up to ~50%) in the upper parts of the chamber.

Geochimica et Cosmochimica Acta, 2012

Experimental studies, performed under oxidized conditions (fO 2 > QFM + 2, where QFM is quartz-fa... more Experimental studies, performed under oxidized conditions (fO 2 > QFM + 2, where QFM is quartz-fayalite-magnetite oxygen buffer), have shown that Rh, Ru, Ir and Os are strongly compatible with Cr spinel, whereas empirical studies of Cr spinels from ultramafic-mafic rocks suggest that the experimental results may overestimate the partition coefficients. We report laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of platinum-group elements (PGE), Au and Re abundances in Cr spinels from the Ambae volcano, Vanuatu (fO 2 = QFM + 2.5), the Jimberlana layered intrusion, western Australia, and the Bushveld complex, South Africa (fO 2 $ QFM). The results show that Rh and IPGEs (Iridium-group PGE; Ru, Ir, Os) partition strongly into the Cr spinels that crystallized from the oxidized Ambae lavas whereas most of the Cr spinels from the more reduced Jimberlana layered intrusion and the Bushveld complex contain no detectable PGE, Au or Re, with exception of $10 ppb of Ir in some Jimberlana Cr spinels. In the Ambae Cr spinels, Rh, Ru and, to lesser extent Os, are positively correlated with Fe 3+ , Ni and V. The homogeneous distribution of Rh and IPGEs in LA-ICP-MS time-resolved spectra indicates that these elements are in solid solution in Cr spinels. Pt-Fe alloys occur as inclusions within the Ambae Cr spinels, which indicate that the Ambae melt was saturated with Pt. Our results show that partitioning of Rh, Ru and Ir into Cr spinels increases with increasing oxygen fugacity, which suggests that the high concentrations of these elements in the Ambae Cr spinels are due to the high oxygen fugacity of the host magma. Therefore, Cr spinels may play an important role in controlling the concentrations of Rh and IPGEs during fractional crystallization of oxidized ultramafic-mafic magmas and during partial melting of oxidized arc mantle.

Geochimica et Cosmochimica Acta, 2012

Platinum group element (PGE) abundances in the upper continental crust (UCC) are poorly constrain... more 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 lower than published values. We suggest that the high Ru data and Ru N /Ir N 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 effect of eolian fractionation on PGE abundances in loess appears to be limited because Chinese loess from different locations shows similar PGE patterns and concentrations. This conclusion is supported by strong positive correlations between the PGE (except for Pt) and other compatible elements such as Fe 2 O 3 , Ni, Cr, Co. Using a compilation of PGE data for loess from China, Argentina and Europe, including our data but excluding one sample with an anomalously high Pt content, we propose average PGE abundances for global loess of Ir = 0.022 ppb (ng/g), Ru = 0.030 ppb, Rh = 0.018 ppb, Pt = 0.599 ppb, and Pd = 0.526 ppb, and suggest that these are the best current estimates for the PGE abundances of the UCC.