Le complexe cénozoïque alcalin d’Ambohimirahavavy à Madagascar : origine, évolution et minéralisations en métaux rares

This work aims to study the geology of the alkaline ring complex Ambohimirahavavy, northwest of Madagascar, focusing on understanding the phenomena at the origin of the formation of rare metal mineralization (Zr , Hf, Nb, Ta, Th, U and REE)...Ce travail a pour objectif l'étude de la géologie du complexe annulaire alcalin d'Ambohimirahavavy, au nord-ouest de Madagascar, en mettant l'accent sur la compréhension des phénomènes à l'origine de la formation des minéralisations à métaux rares (Zr, Hf, Nb, Ta, Th, U et terres rares). Le complexe, mis en place à la limite oligo-miocène, serait issu de la différenciation magmatique d'un magma parent d'origine mantellique. En fin d'évolution, des phénomènes de contamination crustale expliqueraient l'association des séries magmatiques sous- et sursaturées en silice dans le complexe. Les termes les plus différenciés de ces deux séries évolueraient vers des compositions hyperalcalines dans des conditions de fugacité en oxygène opposées.La minéralisation à métaux rares est essentiellement associée aux roches des faciès sursaturés, à savoir, des filons de granite et de pegmatite hyperalcalins. Deux types essentiels de minéralisation ont été reconnus, une miaskitique caractérisée par la présence de plusieurs types de zircon et une agpaïtique riche en eudialyte. La formation de ces minéralisations serait liée à la fois à des processus magmatiques et hydrothermaux. Les granites et les pegmatites riches en métaux rares se formeraient à partir de magmas hyperalcalins enrichis à l'extrême en éléments fluants (F, Cl); la minéralisation est alors représentée par un zirconosilicate riche en métaux rares, l'eudialyte. Les fluants vont diminuer considérablement la température du solidus et augmenter la solubilité de l'eau dans le magma. En fin de cristallisation, on pourra avoir des phénomènes de démixtions de fluides orthomagmatiques riches en Na, Si, F et Cl à l'origine des pseudomorphoses de l'eudialyte en zircon et des néoformations de minéraux à métaux rares. Des datations U-Pb sont en accord avec la formation de ces zircons à partir d'un fluide, donnant des âges autour de 21 Ma, soit 3 Ma plus jeunes que la mise en place des syénites à néphéline, datées également par la méthode U-Pb sur zircon. Cette étude montre également que la composition de l'encaissant peut jouer un rôle essentiel en forçant la précipitation des minéraux à métaux rares dans des formations de type skarn, dans laquelle interviennent aussi des fluides météoriques.Ce travail met en évidence la complexité des processus d'enrichissement et de fractionnement des métaux rares depuis le magma parent d'origine mantellique aux faciès évolués pegmatitiques conduisant à la diversité des minéralisations rencontrées dans ces roches hyperalcalines. Enfin, cette étude propose également une nouvelle cible potentielle de minéralisation en relation avec les faciès sous-saturés

REE concentration processes in ion adsorption deposits: evidence from the Ambohimirahavavy alkaline complex in Madagascar

Ion adsorption deposits, in which the rare earth elements (REE) occur adsorbed onto clay mineral surfaces, currently provide the world�s dominant supply of heavy REE (Gd-Lu). Concentration of REE within ion adsorption deposits has been proposed to be a dominantly supergene process, where easily degradable REE-minerals (e.g. REE-fluorcarbonates) break down and release REE that are then adsorbed onto clay mineral surfaces in the weathered material. Here we present data from the Cenozoic Ambohimirahavavy alkaline complex in Madagascar, with the aim of further constraining controls on the formation and HREE enrichment processes in ion adsorption deposits. The laterite weathering profiles described here are developed on alkaline igneous rocks, including both SiO2-undersaturated and oversaturated lithologies. The latter group includes REE mineralised peralkaline granitic pegmatites and granitic dykelets. The weathering mineralogy includes gibbsite, clay minerals and minor Fe and Mn oxyhydroxides. X-ray diffraction and infrared spectroscopy show that the clay fraction in all sites is dominated by kaolinite and halloysite (7�à and 10�à ). Extraction with ammonium sulfate (0.5�M (NH4)2SO4, pH�=�4; removes weakly adsorbed metals retained on the solid surface and dissolves carbonates) indicates that, in the different profiles and along the same profile, the leachable REE content is heterogeneous, ranging from 5 to 2300�mg/kg total REE, with 1 to 32 heavy REE. Leaching with magnesium chloride (0.5�M MgCl2, pH�=�6; removes only weakly adsorbed REE) releases similar total REE concentrations, suggesting that most of the leachable REE are adsorbed onto mineral surfaces. In most of the laterite profiles, the amount of leachable REE continuously increases with depth up to the saprock. Recovery rates depend on the REE atomic number, generally decreasing from La to Lu except for Ce which is invariably low. In the Ambohimirahavavy complex, the nature of the protolith is the main factor controlling the amount of easily leachable REE in the laterite weathering profile. Hydrology and topography are secondary factors. The most favourable protoliths include SiO2-undersaturated volcanic lithologies and altered mudstone with granitic dykelets. The main primary REE minerals include agpaitic minerals (eudialyte) and allanite-(Ce). Locally in granitic pegmatite dykes, autometasomatism by late magmatic fluids inhibits formation of ion adsorption ore by transforming easily weathered agpaitic minerals into unweatherable zircon

Multiple carriers of Q noble gases in primitive meteorites

The main carrier of primordial heavy noble gases in chondrites is thought to be an organic phase, known as phase Q, whose precise characterization has resisted decades of investigation. Indirect techniques have revealed that phase Q might be composed of two subphases, one of them associated with sulfide. Here we provide experimental evidence that noble gases trapped within meteoritic sulfides present chemically- and thermally-driven behavior patterns that are similar to Q-gases. We therefore suggest that phase Q is likely composed of two subcomponents: carbonaceous phases and sulfides. In situ decay of iodine at concentrations levels consistent with those reported for meteoritic sulfides can reproduce the 129Xe excess observed for Q-gases relative to fractionated Solar Wind. We suggest that the Q-bearing sulfides formed at high temperature and could have recorded the conditions that prevailed in the chondrule-forming region(s)

Adsorption of rare earth elements in regolith-hosted clay deposits

Global resources of heavy Rare Earth Elements (REE) are dominantly sourced from Chinese regolith-hosted ion-adsorption deposits in which the REE are inferred to be weakly adsorbed onto clay minerals. Similar deposits elsewhere might provide alternative supply for these high-tech metals, but the adsorption mechanisms remain unclear and the adsorbed state of REE to clays has never been demonstrated in situ. This study compares the mineralogy and speciation of REE in economic weathering profiles from China to prospective regoliths developed on peralkaline rocks from Madagascar. We use synchrotron X-ray absorption spectroscopy to study the distribution and local bonding environment of Y and Nd, as proxies for heavy and light REE, in the deposits. Our results show that REE are truly adsorbed as easily leachable 8- to 9-coordinated outer-sphere hydrated complexes, dominantly onto kaolinite. Hence, at the atomic level, the Malagasy clays are genuine mineralogical analogues to those currently exploited in China

Ecological implications of pedogenesis and geochemistry of ultramafic soils in Kinabalu Park (Malaysia)

In Sabah, Malaysia, ultramafic rock outcrops are widespread (totalling 3500 km2, one of the main outcrops in the tropical zone), and predominantly of the peridotite type. However, strongly serpentinised peridotite is also locally common, particularly along fault lines in the Mt. Kinabalu area. This study aimed to determine the extent of chemical variation in ultramafic soils in relation to the degree of serpentinisation and the weathering intensity, and consequent potential ecological implications linked to resulting soil chemical fertility. It was hypothesized that young soils and soils derived from bedrock with a significant degree of serpentinisation strongly differ from typical Geric Ferralsols and result in soil chemistries with more adverse properties to plant life (e.g. low availability of the essential nutrients N, P, K and Ca and high concentrations of potentially phytotoxic Mg and Ni). Ultramafic soil diversity linked to the age of the soil or the degree of serpentinisation would thus be a main factor of plant diversity and distribution. The diverse topography of Kinabalu Park (ultramafic soils present between 400 and 2950 m asl) has given rise to high pedodiversity with the broad overall ultramafic soil types being: (i) deep laterite soils (Geric Ferralsols); (ii) moderately deep montane soils (Dystric Cambisols) with mor humus; (iii) shallow skeletal soils at high altitude (Eutric Cambisols Hypermagnesic); and (iv) bare serpentinite soils (Hypereutric Leptosols Hypermagnesic) at low altitude (200–700 m asl). Leptosols on serpentinite and Eutric Cambisols have the most extreme chemical properties in the whole Kinabalu Park area both with very high Mg:Ca molar quotients, with either high available Ni (Cambisols) or high pH (Leptosols). These soils host specific and adapted vegetation (high level of endemism) that tolerates geochemical peculiarities, including Ni hyperaccumulators. Geric Ferralsols present far less chemical constraints than Hypermagnesian Cambisols soils to the vegetation and host a tall and very diverse rainforest, not so different than that on non-ultramafic soils. It therefore appears that altitude, soil age and degree of bedrock serpentinisation are the main determining factors of soil properties: the qualifier “ultramafic” alone is not sufficient to define soil geochemical and ecological conditions in the Kinabalu Park area, probably more than in any other ultramafic region in the world

Magmatic to hydrothermal zircons: Textural and chemical evolution

International audienceZircon is undoubtedly the most sought-after mineral forgeochemical studies, for its ability to provide information onits host rock, spanning from geochronology, tracing of sourceand processes, geothermometry, and, recently, redoxconditions [1]. However, it is crucial to ascertain its primaryorigin, and in the past two decades there has been increasingevidence of its crystallization from hydrothermal fluids [2].Two of the main characteristics that are widely used toascertain the magmatic or hydrothermal origin of zircon aretexture and trace-element chemistry. However, most of thesedata are contradictory and can be similarly attributed to aprimary and secondary origin [3], resulting in a poorunderstanding of hydrothermal zircon characteristics.We present data on a suite of zircons from theAmbohimirahavavy alkaline complex, Madagascar, thatdisplay impressive textural, morphological and compositionalvariations, strongly suggesting a span in origin from magmaticto hydrothermal (Fig. 1). Clearly magmatic zircons yield agesof 20.40 ± 0.16 and 21.21 ± 0.44 Ma. Hydrothermal zirconyields a similar age of 20.64 ± 0.48 Ma. Evidence forhydrothermal origin includes its occurrence with quartz inpseudomorphs after primary minerals, as botryoidal crystalsfilling cavities, and precipitation in exoskarn [4]. Strongvariations in the amounts and distribution of trace element alsooccur among different sectors in zoned crystals

Alkali pyroxene as monitor of concentration and fractionation processes of Rare Earth Elements in peralkaline granites

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Alkali pyroxenes and amphiboles: a window on rare earth elements and other high field strength elements behavior through the magmatic-hydrothermal transition of peralkaline granitic systems

International audiencePeralkaline granites and pegmatites are a prime repository of REE and HFSE, critical raw materials. Although it is accepted that magmatic processes are fundamental in concentrating these metals, the role of hydrothermal fluids in concentrating and fractionating these elements remains unclear. This paper investigates the global reproducibility of the magmatic-hydrothermal evolution of alkaline silica-saturated systems using alkali pyroxene and amphiboles from six alkaline complexes. These minerals contain significant amounts of REE and other HFSE, and pyroxene is stable throughout the magmatic and hydro-thermal stages. Amphibole consists of mostly unzoned arfvedsonite, leakeite, and katophorite, while pyroxene is always aegirine. Two types of aegirine were defined. In all complexes, type-I aegirine is zoned; its core is enriched in Ca, REE, Zr, Hf, Sc and Sn, and the rims in Na, Fe 3+ and contains secondary rare-metal bearing minerals and fluid inclusions. Type-II aegirine replaces amphibole and is oscillatory zoned. We interpret the amphiboles and REE-rich cores of type-I aegirine to have grown during the magmatic stage, whereas the rims of REE-poorer type-I and II aegirine are formed during the hydrothermal stage. During magmatic crystallization, REE intake into amphiboles and pyroxene as well as LREE-HREE fractionation were favored by their crystallographic properties and by competition among them and other minerals. During subsequent hydrothermal stages, REE and other HFSE were remobilized, locally reconcentrated and fractionated in mineral pseudomorphs and secondary pyroxene. These observations point out the importance of studying rock-forming minerals such as pyroxenes and amphiboles to unravel geological events controlled by common processes globally