Gold enrichment at the crustal or mantle source has been proposed as a key ingredient in the production of giant gold deposits and districts. However, the lithospheric-scale processes controlling ...gold endowment in a given metallogenic province remain unclear. Here we provide the first direct evidence of native gold in the mantle beneath the Deseado Massif in Patagonia that links an enriched mantle source to the occurrence of a large auriferous province in the overlying crust. A precursor stage of mantle refertilisation by plume-derived melts generated a gold-rich mantle source during the Early Jurassic. The interplay of this enriched mantle domain and subduction-related fluids released during the Middle-Late Jurassic resulted in optimal conditions to produce the ore-forming magmas that generated the gold deposits. Our study highlights that refertilisation of the subcontinental lithospheric mantle is a key factor in forming large metallogenic provinces in the Earth's crust, thus providing an alternative view to current crust-related enrichment models.The lithospheric controls on giant gold deposits remain unclear. Here, the authors show evidence for native gold in the mantle from the Deseado Massif in Patagonia demonstrating that refertilisation of the lithospheric mantle is key in forming metallogenic provinces.
•We use isotope geochemistry, geochronology and modelling of REE diffusion timescale.•Cabo Ortegal pyroxenites record Cadomian arc magmatism off a pre-Gondwanan block.•Melt–rock interaction occurred ...during the reworking of a rifted continental margin.•Subduction metasomatism and metamorphism obscure the age of orogenic mantle rocks.•2nd-stage Nd model ages may provide robust time constraints on pyroxenite formation.
Pyroxenites exposed in ophiolites and orogenic peridotite massifs may record petrogenetic processes occurring in mantle domains generated and/or transferred in supra-subduction environments. However, the timing of their formation and the geochemical characteristics of their source region commonly are obscured by metamorphic and metasomatic overprints. This is especially critical in arc-related environments, where pyroxenites may be formed during the differentiation of primitive magmas. Our approach combines Sr- and Nd-isotope geochemistry and geochronology, and modelling of REE diffusion, to further constrain the origin of a well-characterized set of pyroxenites from the arc-related Cabo Ortegal Complex, Spain. In the light of petrological constraints, Sr- and Nd-isotope systematics consistently indicate that cpx and amphibole have acquired disequilibrium during two main episodes: (1) a magmatic/metasomatic episode that led to the formation of the pyroxenites, coeval with that of Cabo Ortegal granulites and corresponding to the incipient stage of a potential Cadomian arc (459–762 Ma; isochron and second-stage Nd model ages); (2) an episode of metamorphic amphibolitization upon the percolation of relatively unradiogenic and LREE-enriched hydrous fluids, subsequent to the delamination of the pyroxenites from their arc-root settings during Devonian subduction. Calculations of diffusional timescale for the re-equilibration of REE are consistent with this scenario but provide only poor additional constraints due to the sensitivity of this method to grain size and sub-solidus temperature. We thus emphasize the necessity to combine isochron ages and Nd model ages corrected for radiogenic ingrowth to put time constraints on the formation of subduction- and/or collision-related pyroxenites, along with petrological and geochemical constraints. Homogeneous age-corrected 143Nd/144Nd of 0.5121–0.5125 (εNd between 0 and +7.5) and 87Sr/86Sr of 0.7037–0.7048 provide information on the sources of the metasomatic agents involved (and potentially the parental melts) and notably indicate the contributions from enriched mantle components (EM I and/or II). This suggests the involvement of an old crustal component, which is consistent with the derivation of the pyroxenites and granulites from an ensialic island arc, potentially built on the northern margin of either Gondwana or a pre-Gondwanan continental block. This case study thus documents the role of melt–rock reactions as major pyroxenite-forming processes in the sub-arc mantle, providing further constraints on their sources and timing in the Cabo Ortegal Complex.
Abstract The lithium cycling in the supra-subduction mantle wedge is crucial for understanding the generation of Li-rich magmas that may potentially source ore deposition in continental arcs. Here, ...we look from the mantle source perspective at the geological processes controlling the Li mobility in convergent margins, by characterizing a set of sub-arc mantle xenoliths from the southern Andes (Coyhaique, western Patagonia). The mineral trace element signatures and oxygen fugacity estimates (FMQ > + 3) in some of these peridotite xenoliths record the interaction with arc magmas enriched in fluid-mobile elements originally scavenged by slab dehydration. This subduction-related metasomatism was poorly effective on enhancing the Li inventory of the sub-arc lithospheric mantle, underpinning the inefficiency of slab-derived fluids on mobilizing Li through the mantle wedge. However, major and trace element compositions of mantle minerals in other xenoliths also record transient thermal and chemical anomalies associated with the percolation of slab window-related magmas, which exhibit an “adakite”-type geochemical fingerprint inherited by slab-derived melts produced during ridge subduction and slab window opening event. As these melts percolated through the shallow (7.2–16.8 kbar) and hot (952–1054 °C) lithospheric mantle wedge, they promoted the crystallization of metasomatic clinopyroxene having exceptionally high Li abundances (6–15 ppm). Numerical modeling shows that low degrees (< 10%) of partial melting of this Li-rich and fertile sub-arc lithospheric mantle generates primitive melts having two-fold Li enrichment (~13 ppm) compared with average subduction-zone basalts. Prolonged fractional crystallization of these melts produces extremely Li-enriched silicic rocks, which may stoke the Li inventory of mineralizing fluids in the shallow crust.
The Bahla massif exposes the lower crustal section of the Oman ophiolite located close to the thrust front of the Semail nappe. It is affected by intense faulting previously attributed to tectonic ...events that dismembered a classical ophiolitic sequence during or after the obduction. Here we show that most of this complexity is primary, inherited from syn-accretion tectonics. The crustal section is exposed in a 15 by 8 km tectonic enclave surrounded by mantle peridotite. Its northern boundary corresponds to a major, steeply dipping normal fault striking WNW-ESE, at low angle to the paleo-ridge axis. Movement along this fault was accommodated by intense plastic deformation of the crustal cumulates and adjacent mantle peridotites at temperature conditions ≥900 °C. The thickness of the deformed zone reaches several hundred meters. The flattening of the cumulate layering away from the fault is correlated to a decrease in the deformation intensity. Undeformed olivine-gabbro dykes cross-cut this “tectonic Moho” indicating that the tilting occurred before the end of the igneous activity. To the southwest, the crustal enclave is bounded by a NW-SE trending transtentional shear zone that was active in the amphibolite to greenschist facies and was intensely injected by syn- to post-kinematic gabbronorite and tonalite/trondhjemite dykes and plugs. The age of one felsic sample (95.214 ± 0.032 Ma, high-precision UPb zircon dating) is within error of the age of intrusive felsic intrusions into the mantle and lowermost axial crust from the length of the Oman ophiolite, which slightly post-dates the mean crystallization age of the Semail crust (V1 magmatism; 96.1–95.6 Ma). Other contacts are low temperature features including cataclastic faults, serpentine‑carbonate breccias and flat-lying décollements.
Parent melts of the Bahla crustal cumulates were more siliceous and hydrous, i.e. more andesitic, than typical mid-ocean ridge basalt (MORB) as deduced from the frequent occurrence of early crystallizing orthopyroxene (opx) and late crystallizing amphibole. Some facies such as cumulate harzburgite and opx-troctolite have not been documented elsewhere in the Oman ophiolite and may be specific to the tectonic context in which the frontal massifs accreted. The chemical composition of the lower crustal cumulates can be accounted for by the hybridization in various proportions between MORB and a primitive andesite from a depleted source whose origin can be looked for in melts from a nascent subduction zone or from high temperature hydrothermal processes.
The structure of the Bahla lower crustal section is reminiscent of the plutonic growth faults documented along present-day slow-spreading centres in both mid-ocean ridge and back arc settings. The distinctive characteristics of the Moho and lower crustal section in the Bahla massif are tentatively related to their position at the leading edge of the ophiolite, i.e. closer to the Arabian continental margin at the time of accretion than the massifs from the internal part of the ophiolite that have a more continuous and less deformed lower crust. It indicates that the style of crustal accretion may have changed during the opening of the oceanic basin from which the Oman ophiolite issued.
•The Bahla massif crops out along the thrust front of the Oman ophiolite.•Its crustal section presents distinctive structural and petrological features contrasting with the ones of most other Oman massifs.•It is affected by intense faulting inherited from syn-accretion tectonics and/or from the very early stage of intra-oceanic thrusting.•Some lithological facies from the lower crustal cumulates seem specific to the frontal massifs and are attributable to magma mixing and contamination by hydrous fluids.•Our results highlight the heterogeneity of the Oman ophiolite.•The paradigm considering the Oman ophiolite as a whole the archetype of ocean lithosphere accreted in a fast-spreading setting should be nuanced.
Mantle lithologies in orogenic massifs and xenoliths commonly display strikingly different Hf- and Nd-isotope compositions compared to oceanic basalts. While the presence of pyroxenites has long been ...suggested in the source region of mantle-derived magmas, very few studies have reported their combined HfNd isotope compositions. We here report the first LuHf data along with ReOs data and S concentrations on the Cabo Ortegal Complex, where the pyroxenite-rich Herbeira massif has been interpreted as remnants of a delaminated arc root. The pyroxenites, chromitites and their host harzburgites show a wide range of whole-rock 187Re/188Os and 187Os/188Os (0.16–1.44), indicating that Re was strongly mobilized, partly during hydrous retrograde metamorphism but mostly during supergene alteration that preferentially affected low-Mg#, low Cu/S pyroxenites. Samples that escaped this disturbance yield an isochron age of 838 ± 42 Ma, interpreted as the formation of Cabo Ortegal pyroxenites. Corresponding values of initial 187Os/188Os (0.111–0.117) are relatively unradiogenic, suggesting limited contributions of slab-derived Os to primitive arc melts such as those parental to these pyroxenites. This interpretation is consistent with radiogenic Os in arc lavas being mostly related to crustal assimilation. Paleoproterozoic to Archean Os model ages confirm that Cabo Ortegal pyroxenites record incipient volcanic arc magmatism on the continental margin of the Western African Craton, as notably documented by zircon UPb ages of 2.1 and 2.7 Ga. LuHf data collected on clinopyroxene and amphibole separates and whole-rock samples are characterized by uncorrelated 176Lu/177Hf and 176Hf/177Hf (0.2822–0.2855), decoupled from Nd-isotope compositions. This decoupling is ascribed to diffusional disequilibrium during melt-peridotite interaction, in good agreement with the results of percolation-diffusion models simulating the interaction of an arc melt with an ancient melt-depleted residue. These models notably show that HfNd isotopic decoupling such as recorded by Cabo Ortegal pyroxenites and peridotites (ΔƐHf(i) up to +97) is enhanced during melt-peridotite interaction by slow diffusional re-equilibration and can be relatively insensitive to chromatographic fractionation. Finally, we discuss the hypothesis that arc-continent interaction may provide preferential conditions for such isotopic decoupling and propose that its ubiquitous recognition in peridotites reflects the recycling of sub-arc mantle domains derived from ancient, reworked SCLM.
This is the first study on the behavior and industrial fluxes of rare earth elements (REE) in the coastal fertilizer plants of Gabes (south-eastern Tunisia), the economic losses related to their ...wastes, and their environmental and human health hazards. The concentrations of 16 REE were assessed in phosphate rock (PR), phosphogypsum (PG) and phosphogypsum foam (PGF) samples, collected from Gabes plants. REE concentrations ranged from 0.23 (for Sc in PG) to 309.33 mg kg−1 (for Ce in PGF). Ce was the most abundant in the three matrices, with concentrations ranging between 80.40 (in PG) and 309.33 mg kg−1 (in PGF). PGF was the most enriched with REE (1075.32 mg kg−1). The annual flow of REE from the fertilizer factories to the marine environment may reach 1523.67 t. The economic losses related to the discharge of phosphogypsum REE in the Gulf of Gabes (GG) was estimated at ~58 million US$ y−1. The potential hazards of discharged REE on the local environment and human health were also evaluated and discussed. These findings show the need for the development of a new industry exploiting REE from phosphogypsum wastes (short term) and phosphate ores (long term) which should lead to reduce its high environmental and human health footprint and to potential economic gains.
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•16 REE were analyzed in phosphate rock, phosphogypsum and phosphogypsum foam.•Phosphogypsum foam was the most enriched with REE (1075.32 mg kg−1).•REE annual flow from fertilizer plants to marine environment reached 1523.67 t.•Economic losses related to phosphogypsum REE discharges was ~58 million US$ y−1.•REE have many potential environmental, ecological and human health impacts.
This paper presents and discusses new geochronological and petrological data on a suite of calc-alkaline plutons composed predominantly of diorites and tonalites from the West Massif Central. Their ...petrochemical fingerprints are compatible with partial melting of a hydrous mantle wedge followed by fractional crystallization of amphibole and plagioclase before final emplacement between 5 and 8 kbar within the continental upper plate of a subduction system. In situ U-Pb zircon dating on tonalites yields a fairly narrow age range of 365−354 Ma (including uncertainties) for igneous crystallization. These calc-alkaline plutons imply active margin magmatism near the Devonian-Carboniferous boundary and are contemporaneous with the back-arc magmatism and HP metamorphism as dated by recent studies. However, such isolated igneous bodies do not form a transcrustal magmatic arc but rather represent dispersed plutons emplaced within less than 30 Myr when all data from the Variscan belt of France are considered. In Limousin, they intrude migmatitic paragneisses and retrogressed eclogites from the Upper Gneiss Unit (UGU), suggesting that the high pressure rocks were already exhumed at 19−30 km depth before 365 Ma. Moreover, the diorites and tonalites are never found within units below the UGU. It therefore suggests that these tectono-metamorphic units of the Western French Massif Central were piled up after 354 Ma. Altogether these results support the monocyclic model for Variscan geodynamics in the French Massif Central, with the transition between oceanic subduction and continental collision taking place between Upper Devonian and Lower Carboniferous.
Cette étude apporte de nouvelles contraintes géochronologiques et pétrologiques sur une série de plutons calco-alcalins composés principalement de diorites et de tonalites et situés Massif central occidental. Leur signature pétro-géochimique implique la fusion partielle d’un coin mantellique hydraté suivie de la cristallisation fractionnée d’amphibole et de plagioclase avant la mise en place finale entre 5 et 8 kbar dans la plaque continentale supérieure d’un système de subduction. La datation U-Pb in situ des zircons provenant des tonalites donne une gamme d’âge de 365 à 354 Ma (erreurs incluses) interprétée comme représentant la cristallisation magmatique des zircons. Ces plutons calco-alcalins proviennent d’un magmatisme de marge active à la limite Dévonien-Carbonifère et sont synchrones du magmatisme d’arrière-arc et du métamorphisme de haute pression datés par des études récentes. Ces intrusions ne forment cependant pas un arc magmatique d’échelle crustale mais représentent plutôt des plutons dispersés qui se sont mis en place en moins de 30 millions d’années si l’on considère toutes les données disponibles, réévaluées et robustes pour la chaîne varisque en France. Dans le Limousin, ces plutons intrudent notamment les paragneiss migmatitiques et les éclogites rétromorphosées de l’Unité Supérieure des Gneiss, ce qui suggère que les roches de haute pression étaient déjà exhumées à une profondeur de 19-30 km avant 365 Ma. En outre, les diorites et les tonalites n’ont pas été observées dans les unités situées sous l’Unité Supérieure. Cela indique que les unités tectoniques du Massif central occidental ont été empilées après 354 Ma. Ces résultats soutiennent donc le modèle monocyclique de la géodynamique varisque dans le Massif central français où la transition subduction océanique/collision continentale se produit entre le Dévonien supérieur et le Carbonifère inférieur.
Mantle xenoliths from the Kerguelen Archipelago record a complex multistage history involving a high degree (15 to 25%) partial melting that created a harzburgitic mantle completely stripped of Base ...Metal Sulfides (BMS), followed by pervasive melt-rock reaction with alkaline melts above the Kerguelen mantle plume. Subsequent reaction of the highly refractory protolith with small volumes of carbonate-rich silicate melts led to a re-enrichment in BMS (up to 0.05wt.%). Two BMS precipitation mechanisms are suggested: immiscibility from the silicate-carbonate melt and sulfidation reactions from a CO2-rich supercritical fluid. In-situ analyses of chalcophile and siderophile elements (major and trace levels) in the BMS shed new light on their origin. The BMS phases that precipitated via immiscibility are metal-rich sulfide melts which progressively evolved toward Ni and Cu-rich end-members by cumulate fractionation of monosulfide solid solution (mss) during percolation inside the peridotites. Some cumulate mss have elevated and fractionated IPGE contents (200–900× C1-Chondrite abundances), indicating random digestion of preexisting Os, Ir, Ru-rich PGM by the percolating sulfide melt. The BMS that precipitated by sulfidation reactions from a CO2-rich vapour phase are subsolidus exsolution products from Cu-bearing but Ni-poorer mss. They have the highest concentrations of PGEs and show selective enrichment in S, Pd, Pt and Os over Cu, S, Ir, Ru and Rh. Their PGE compositions confirm experimental data, which demonstrate that S, Pd, Pt and Os can be efficiently transported in a CO2-rich supercritical fluid. Superchondritic (S/Se), (Os/Ir) and (Pd/Pt) in both bulk-rocks and individual sulfides are inferred to be the geochemical fingerprints of sulfide crystallisation from a CO2-rich vapour exsolved from a highly evolved carbonate-rich metasomatic melt.
► Chalcophile and siderophile elements are reported for sulfides in mantle xenoliths. ► We examine fractionations of these elements during metasomatic addition. ► Different elemental fractionations depend on the sulfide precipitation mechanisms. ► PGE are best transferred during metasomatism involving a fluid/vapour-rich phase. ► PGE fingerprints may record dissolution of preexisting PGM during metasomatism.
This work explores the potential of geochemical and petrographic characteristics of detritalzircons coming from the sedimentary record of the Centinela District in Northern Chile to identifythe ...presence of buried porphyry copper deposits under a transported gravel cover. The sampledsedimentary section was recovered from the pit of the exotic copper deposit of El Tesoro, locatedapproximately 2 and 4 km west of the Esperanza and Mirador porphyries, respectively. Thesedimentary cover comprises four units, Tesoro II, Tesoro III, Arrieros and Recent gravels,deposited since the late Cenozoic in an arid continental environment dominated by alluvial fans.Except for the Tesoro III gravels, all other units contain exotic-Cu mineralisation. In order tointerpret the geochemical footprint of the investigated zircons, the Porphyry Indicator Zircon (PIZ)concept (Pizarro et al., 2020) is used. A PIZ need to comply with each of the following geochemicalvalues: Hf >8,750 (ppm), Ce/Nd >1, Eu/Eu* >0.4, 10,000×(Eu/Eu*)/Y >1, (Ce/Nd)/Y >0.01,Dy/Yb <0.3 and 0.1 < Th/U < 1. These zircons also have Ti <9 ppm and Ce/Ce* <100 and usuallyshow euhedral morphologies characterised by prismatic forms of type {110}. The geochemical andpetrographic characteristics of the PIZs collected in the gravels are similar to zircons from thenearby Mirador and Esperanza porphyries. The highest PIZ concentration coincides with the gravelhorizons with exotic-Cu mineralisation. Therefore, the PIZs found in the sedimentary record are apotential tracer of adjacent copper porphyries and represent a promising exploration tool for thistype of hidden ore deposits in challenging sediment-covered areas.
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