Layered mafic-ultramafic intrusions of Fennoscandia: Europe's treasure chest of magmatic metal deposits

Northeastern Fennoscandia hosts a rich diversity of mafic-ultramafic intrusions of variable shape and size, emplaced in different tectonic regimes over a period spanning ca. 600 million years (from 1.88 – 2.5 Ga). Several of the bodies contain world-class ore deposits, notably the Kemi Cr deposit and the Pechenga Ni deposits. Other deposits include Ni and Cu at Kevitsa, Kotalahti and Sakatti, V at Koillismaa, and platinum-group elements at Portimo and Penikat. These deposits constitute important resources to shield Europe from potential future supply shortages of key industrial metals

Empirical constraints on partitioning of platinum group elements between Cr-spinel and primitive terrestrial magmas

Recent experimental studies and in situ LA-ICP-MS analysis on natural Cr-spinel have shown that Rh and IPGEs (Ir-group platinum group elements: Ru, Ir, Os) are enriched in the lattice of Cr-spinel. However, the factors controlling the partitioning behaviour of these elements are not well constrained. In this study, we report the Rh, IPGE, and trace element contents in primitive Cr-spinel, measured by LA-ICP-MS, from nine volcanic suites covering various tectonic settings including island arc picrites, boninites, large igneous province picrites and mid-ocean ridge basalts. The aim is to understand the factors controlling the enrichment of Rh and IPGEs in Cr-spinels, to estimate empirical partition coefficients between Cr-spinel and silicate melts, and to investigate the role of Cr-spinel fractional crystallization on the PGE geochemistry of primitive magmas during the early stages of fractional crystallization.There are systematic differences in trace elements, Rh and IPGEs in Cr-spinels from arc-related magmas (Arc Group Cr-spinel), intraplate magmas (Intraplate Group Cr-spinel), and mid-ocean ridge magmas (MORB Group Cr-spinel). Arc Group Cr-spinels are systematically enriched in Sc, Co and Mn and depleted in Ni compared to the MORB Group Cr-spinels. Intraplate Group Cr-spinels are distinguished from the Arc Group Cr-spinels by their high Ni contents. Both the Arc and Intraplate Group Cr-spinels have total Rh and IPGE contents of 22-689. ppb whereas the MORB Group Cr-spinels are depleted in Rh and IPGE (total. <. 20. ppb). Palladium and Pt contents are below detection limit for all of the studied Cr-spinels (<1-5. ppb). The time-resolved spectra of LA-ICP-MS data for Cr-spinels mostly show constant count rates for trace element and Rh and IPGEs, suggesting homogeneous distribution of these elements in Cr-spinels. The PGE spikes observed in several Cr-spinels were interpreted to be PGE-bearing mineral inclusions and excluded from calculating the PGE contents of the Cr-spinels.On primitive mantle normalized diagrams the Arc Group Cr-spinels are characterized by a fractionated pattern with high Rh and low Os. The Intraplate Group Cr-spinels show flat patterns with positive Ru anomalies. Our results, together with the experimental and empirical data from previous studies, show that PGE patterns of Cr-spinel largely mimic that of the rock in which they are found, and that Rh, Ir and Os contents increase with increasing Fe3+ contents (i.e. magnetite component) in Cr-spinel, although Ru does not. These observations suggest that the enrichment of Rh and IPGEs in Cr-spinel is controlled by a combination of the Rh and IPGE contents in parental melts and the magnetite component of the spinel. Empirical partition coefficients (D) for Rh and IPGEs between Cr-spinels and silicate melts were calculated using the Rh and IPGE contents of the Cr-spinel and their host volcanic rocks after subtracting the accumulation effect of Cr-spinel. The D values for the Intraplate and MORB Group Cr-spinels increase with increasing magnetite component in Cr-spinel and range from 6 to 512, which is consistent with previously reported experimental and empirical values. In contrast, the Arc Group Cr-spinels have significantly higher D values (e.g. up to ∼3700 for Ru) than those of the Intraplate and MORB Group at the same magnetite concentration in the Cr-spinel, suggesting Rh and IPGEs dissolved in silicate melt have stronger affinity for Cr spinel under arc magma conditions than in intraplate magmas. This may be partly attributed to the low temperature of arc magmas relative to intraplate magmas, which leads to the Arc Group Cr-spinels having more octahedral sites at the same magnetite components than the Intraplate Group Cr-spinels. Because of significantly higher D values for the Arc Group Cr-spinels, compared with the Intraplate Group and MORB Group spinels, fractional crystallization of Cr-spinel will more efficiently fractionate Rh and IPGE from Pd and Pt in arc systems than in intraplate and MORB systems, which accounts for the highly fractionated PGE patterns in arc basaltsThis study was funded by the Korea government Ministry of Science, ICT and Future Planning (NRF-2015R1C1A1A01054101) to J.-W. Park. The Russian Science Foundation (Grant #16-17-10145) provided funding to V. Kamenetsky. E. Hanski acknowledges support from Academy of Finland Grant #281859. The mineralogy and geochemistry of the Urals ankaramite was supported by RFBR Grant N 16-05- 00508 to E. Pushkarev

Ruthenium in Chromite as Indicator for Magmatic Sulfide Liquid Equilibration in Mafic-Ultramafic Systems

The platinum-group element ruthenium (Ru) is an important petrogenetic tracer of Earth\u27s accretion history, core-mantle interaction, mantle evolution and the exploration for magmatic sulfide deposits. However, its geochemical behavior in mafic-ultramafic systems is still not fully understood, which limits its usefulness in the predictive modelling of geochemical systems. To further develop the use of Ru as a petrogenetic tracer, we analyzed the Ru contents of chromites from a global sample set of komatiites, komatiitic basalts, and ferropicrites by laser ablation ICP-MS and Carius tube isotope dilution ICP-MS analysis. The Ru data are combined with full major and minor element microprobe analyses. The data show that two groups of chromite can be distinguished on the basis of their Ru contents. This bimodal distribution occurs across komatiites, komatiitic basalts and ferropicrites and corresponds to the sulfide saturation state of the magma during chromite crystallization: chromites from sulfide-undersaturated magmas contain between ∼150 and 600 ppb Ru, whereas chromites that crystallized from sulfide-bearing magmas mostly contain less than ∼150 ppb Ru. The Ru contents are independent of elements that typically document a modification of chromite, suggesting that the determined Ru concentrations reflect the primary magmatic chromite composition. The Ru contents are furthermore independent of magma type (i.e. komatiites, komatiitic basalts, ferropicrites), the magma source regions (i.e. different cratons, belt and localities), the geochemical affinity (i.e. Munro-type and Karasjok-type), and age (i.e. 2.7 Ga and 2.0 Ga) and neither local fluctuations in fO2, nor emplacement styles (i.e. intrusive vs. extrusive) can account for the bimodal Ru distribution in chromite observed during this study. As a consequence, it is argued that the Ru contents of chromites from mafic and ultramafic systems are indicative of the presence or absence of a sulfide liquid during chromite crystallization. In sulfide-saturated systems, the chalcophile Ru will dominantly partition into sulfides, whereas in the absence of sulfides, Ru preferentially partitions into chromite over all other available phases. Because (i) high Ru contents in chromites are exclusively associated with sulfide-undersaturated systems, and (ii) the Ru contents of chromites can be measured via fast and cost-effective laser ablation ICP-MS, Ru variability patterns in chromites allow the identification of magmas that have equilibrated with magmatic sulfide liquids prior to or during chromite crystallization and hence have potential to host metal sulfide orebodies

Epigenetic gold occurrence in a Paleoproterozoic meta-evaporitic sequence in the Rompas-Rajapalot Au system, Peräpohja belt, northern Finland

Abstract The Rompas-Rajapalot gold prospect is located in the northern part of the Paleoproterozoic Peräpohja belt. It covers an area of at least 10 × 10 km and comprises various styles of gold mineralization ranging from localized high-grade Au pockets in uraninite- and pyrobitumen-bearing calcsilicate-carbonate-quartz veins in mafic metavolcanic rocks (Rompas area) to disseminated gold grains in Fe-Mg-rich metasediments and quartz-tourmaline-sulfide-native gold veins (Palokas area). This study deals with the petrography and mineral chemistry of the gold mineralization at Palokas, which occurs in the eastern part of the Rompas-Rajapalot prospect. Major and trace element data and fluid inclusion characteristics of tourmaline are used to evaluate the origin and the pressure-temperature-fluid composition parameters of hydrothermal fluids. Whole-rock geochemical analyses are utilized to evaluate the nature of the protolith of the host rocks. Gold occurs in a native form in at least two different textural settings: 1) single, relatively coarse grains disseminated among the rock-forming silicates in cordierite-orthoamphibole rocks and 2) smaller grains occurring in fractures of tourmaline in quartz-sulfide-tourmaline breccias and in fractures of chloritized cordierite-orthoamphibole rocks adjacent to the tourmaline-rich breccias. Fracture-related gold is associated with Bi-Se-S-bearing tellurides, native Bi, molybdenite, chalcopyrite, and pyrrhotite. Coarser-grained disseminated gold were not found to be clearly associated with sulfides nor any fractures. Statistical correlations show that the Au concentration correlates strongly with Te, Cu, Co, Se, Bi, Mo, and Ag (ρ = 0.730–0.619) whereas Au correlates moderately with As, Fe, W (ρ = 0.523–0.511) and to a lesser extent with U, Pb, and Ni (ρ = 0.492–0.407). Gold has the strongest negative correlations with Sr and Ca. The chondrite-normalized REE patterns of tourmaline from the Au-mineralized rocks (both vein type and host-rock tourmaline) and the late- to post-orogenic granite partly overlap and show similar LREE-enriched trends, with the enrichment being lower in tourmaline from the granite. Fluid inclusion studies from tourmaline in gold-bearing quartz-tourmaline-sulfide veins indicate that the veins were formed from H₂O-Na₂O-CO₂-CH₄-(H₂S) fluids in a boiling system under pressure conditions ranging from lithostatic to hydrostatic, with the depth being ~5 km and the temperature ~300°C. The properties of the ore-forming fluids support the genetic link between the late- to post-orogenic granitoid magmatism at ~1.78 Ga and the formation of the fracture-hosted gold mineralization, suggested based on earlier studies (including Re-Os-molybdenite age and boron isotope data from tourmaline). Based on the whole-rock geochemistry, it is highly plausible that the cordierite-orthoamphibole rocks and interlayered calcsilicatealbite rocks are part of a basin-wide lacustrine, at least partly evaporitic, sequence. The protolith of the cordierite-orthoamphibole rock was most probably a lake-margin sedimentary pile with abundant Mg-rich clays

Origin of ultra-nickeliferous olivine in the Kevitsa Ni-Cu-PGE-mineralized intrusion, northern Finland

The 2,058 ± 4 Ma mafic–ultramafic Kevitsa intrusion is located in the Central Lapland greenstone belt, northern Finland. It is hosted by a Paleoproterozoic volcano–sedimentary sequence that contains komatiitic volcanic rocks and sulfide- and graphite-rich black schists. Economic Ni–Cu–(PGE) sulfide mineralization occurs in the middle part of the ultramafic lower unit of the intrusion. Two main types of ore are distinguished, “normal” and “Ni–PGE” ores. The normal ore is characterized by ~2 to 6 vol% disseminated sulfides and average Ni and Cu grades of 0.3 and 0.42 wt %, respectively (Ni/Cu < 1). The Ni–PGE ore has broadly similar sulfide contents, but a higher Ni grade and lower Cu grade. As a result, the Ni/Cu ratio reaches 15, much higher than in the normal ore. The Ni–PGE ores occur as irregular, discontinuous, lense-like bodies in the ultramafic rocks. Notably, the olivines in the Ni–PGE ore contain extremely high Ni contents of up to 14,000 ppm, which is significantly higher than the Ni content of olivine in other mafic–ultramafic igneous rocks globally (up to ~5,000 ppm) and in harmony with the associated Ni-rich sulfide assemblage containing pentlandite, millerite and pyrite. Microprobe mapping of olivine from the Ni–PGE ore suggests relatively low and homogeneous S contents and homogeneous distribution of Ni, Mg, Fe, which is inconsistent with the presence of sulfide inclusions in the olivine grains, or diffusion of Ni from interstitial sulfides into the olivine grains. We therefore conclude that Ni substitutes for Mg in the olivine lattice. The clinopyroxenes from the Ni–PGE ore also have unusually high Ni concentrations reaching 1,500 ppm and show a positive correlation with the nickel content of the associated olivine. The Nicpx/Niolivine is ~0.1 to 0.2 corresponding to high T partitioning of Ni between clinopyroxene and olivine. K D of 20 can account for the partitioning of nickel between olivine and the sulfide phase, consistent with magmatic equilibration. These data suggest that the olivine, clinopyroxene, and sulfides all crystallized from a basaltic magma with an unexceptionally high Ni content ranging from 300 to 1,100 ppm. The Ni–PGE ores are spatially associated with ultramafic xenoliths. Olivine in these ultramafic xenoliths have relatively high Fo contents (up to 90 mol %) and high Ni contents (up to 5,200 ppm) suggesting that the xenoliths formed from a komatiitic parental magma. It is proposed that assimilation by the Kevitsa magma of massive or semi-massive sulfides associated with komatiitic rocks elevated the Ni content of the magma and resulted in the formation of Ni–PGE ores and related extremely Ni-rich olivines

A machine learning based-approach to predict the water content of mid-ocean ridge basalts

Abstract Water is critical in the evolution of the mantle due to its strong influence on the physicochemical properties of mantle rocks. Mid-ocean ridge basalts (MORBs) are commonly used to study the compositional characteristics of the convecting upper mantle. However, there remains abundant samples in the global MORB data sets without direct measurements of water contents. The commonly observed good correlation between H₂O and other incompatible trace components, such as Ce, has been applied to quantify water contents of MORBs. However, this approach assumes constant H₂O/Ce in the target samples, which is not always true as the H₂O/Ce ratios of MORBs could be rather heterogeneous even in some short ridge segments. Utilizing the present compositional data of global MORB glasses with measured water contents (n = 1,467), we construct a Random Forest Regression model based on machine learning, which can predict water concentrations of samples based on selected major and trace element data, without assuming a ratio between H₂O and other trace elements. This model allows us to precisely recover water contents for MORBs with comparable accuracy with traditional analytical methods. The predicted results of MORB glasses from this model (n = 1,931) expand the water content database of global MORBs and indicate a broad existence of high-H₂O MORBs. This new approach allows us to investigate the water content of MORBs from some ridges lacking previous water content measurements (e.g., the Chile Ridge and the Pacific-Antarctic Ridge) and infer changes in the water content of MORB sources through applying the model to transform fault samples

Trace element contents of mantle-derived magmas through time

Abstract A large compilation of quality-curated major and trace element data has been assembled to investigate how trace element patterns of mafic and ultramafic magmas have varied with time through particular settings from the Archean to the Phanerozoic, the primary objective being to recognise at what times particular patterns of variation emerge, and how similar these are to baseline data sets representing tectonic settings in the modern Earth. The most informative element combinations involve Nb, Th and the REE, where REE are represented by ‘lambda’ parameters describing slope and shape of patterns. Combinations of the ratios of Th, Nb, La and lambda values from Archean and early Proterozoic basalts and komatiites reveal a distinctive pattern that is common in most well-sampled terranes, defining a roughly linear trend in multi-dimensional space from compositions intermediate between modern n-MORB and primitive mantle at one end, towards compositions approximating middle-to-upper continental crust at the other. We ascribe this ‘Variable Th/Nb’ trend in most instances to varying degrees of crustal contamination of magmas with similar compositions to modern oceanic plateau basalts. Komatiites had slightly more depleted sources than basalts, consistent with the hypothesis of derivation from plume tails and heads, respectively. The most significant difference between Precambrian and Phanerozoic plume-derived basalts is that the distinctive OIB-like enriched source component appears to be largely missing from the Archean and Proterozoic geologic record, although isolated examples of OIB-like trace element characteristics are evident in datasets from even the oldest preserved greenstones. Phanerozoic intra-cratonic LIPs, such as the 260 Ma Emeishan LIP in China, have fundamentally different geochemical characteristics to Archean and Paleoproterozoic assemblages; the oldest Proterozoic LIP we have identified that has this type of ‘modern’ signal is the Midcontinent Rift at 1100 Ma. The data are consistent with plume tail sources having changed from being dominantly depleted in the Archean Earth to dominantly enriched in the Phanerozoic Earth, while plume head sources have hardly changed at all. Trace element patterns considered to be diagnostic of subduction are locally present but rare in Archean terranes and become more prevalent through the Proterozoic, although this conclusion is tempered by the large degree of overlap in compositional space between continental arc magmas and continental flood basalts. This overlap reflects the difficulty of distinguishing the effects of supra-subduction metasomatizm and flux melting from those of crustal contamination. Additional factors must also be borne in mind, particularly that trace element partitioning systematics may have been different in all environments in a hotter planet, and large-scale asthenospheric overturns might have been predominant over modern-style plumes in the Archean Earth. Some basaltic suites in particular Archean terranes, notably the western parts of both the Yilgarn and Pilbara cratons in Western Australia and parts of the Superior Craton, have restricted, but locally predominant, suites of basalts with characteristics akin to modern oceanic arcs, suggesting that some process similar to modern subduction was preserved in these particular belts. Ferropicrite magmas with distinctive characteristics typical of modern OIBs and some continental LIPs (notably Emeishan) are rare but locally predominant in some Archean and early Proterozoic terranes, implying that plume sources were beginning to be fertilised by enriched, probably subducted, components as far back as the Mesoarchean. We see no evidence for discontinuous secular changes in mantle-derived magmatism with time that could be ascribed to major mantle reorganisation events. The Archean–Proterozoic transition appears to be entirely gradational from this standpoint. The transition from Archean-style to Phanerozoic-style plume magmatism took place somewhere between 1900 Ma (age of the Circum-Superior komatiitic basalt suites) and 1100 Ma (the age of the Midcontinent Rift LIP)

Mantle hydration and the role of water in the generation of large igneous provinces

Abstract The genesis of large igneous provinces (LIP) is controlled by multiple factors including anomalous mantle temperatures, the presence of fusible fertile components and volatiles in the mantle source, and the extent of decompression. The lack of a comprehensive examination of all these factors in one specific LIP makes the mantle plume model debatable. Here, we report estimates of the water content in picrites from the Emeishan LIP in southwestern China. Although these picrites display an island arc-like H2O content (up to 3.4 by weight percent), the trace element characteristics do not support a subduction zone setting but point to a hydrous reservoir in the deep mantle. Combining with previous studies, we propose that hydrous and hot plumes occasionally appeared in the Phanerozoic era to produce continental LIPs (e.g., Tarim, Siberian Trap, Karoo). The wide sampling of hydrous reservoirs in the deep mantle by mantle plumes thus indicates that the Earth’s interior is largely hydrated