Abstract The Bohemian Massif hosts significant hydrothermal U-deposits associated with shear zones in the high-grade metamorphic basement. But there is a lack of evidence of a genetic link between mineralization and U-fertile igneous rocks. This contribution provides constraints on the major U source of the vein-type U-deposits, the timing of ore formation and the metallogenetic model. The anomalous trace element signatures of the low-temperature hydrothermal deposits (high Zr, Y, Nb, Ti, ∑REE) and their close spatial relation with ultrapotassic rocks of the durbachite series point to a HFSE and REE enriched source rock. The durbachites have high U content (13.4–21.5 ppm) mainly stored in magmatic uraninite and other refractory minerals (e.g., thorite, zircon, allanite) that became metamict over a time interval sufficient to release U from their crystal structure, as suggested by the time gap between emplacement of the durbachites (EMP uraninite U–Pb age ~ 338 Ma) and hydrothermal activity (SIMS uranium ore U–Pb age ~ 270 Ma). Airborne radiometric data show highly variable Th/U ratios (1.5–6.0), likely reflecting a combination between (1) crystallization of magmatic uraninite, (2) hydrothermal alteration, and (3) leaching and mobilization of U along NW–SE-trending fault zones, manifested by elevated Th/U values in the radiometric map. The presence of rare magmatic uraninite in durbachites suggests almost complete uraninite dissolution; EMP imaging coupled with LA-ICP-MS analyses of refractory accessory phases revealed extensive mobilization of U together with HFSE and REE, providing direct evidence for metal leaching via fluid-driven alteration of radiation-damaged U-rich minerals. The large-scale HFSE and REE mobilization, demonstrated by the unusual trace element signatures of the U-deposits, was likely caused by low-temperature (270–300 °C), highly alkaline aqueous solutions containing F-, P-, and K-dominated complexing ligands. The first SIMS U–Pb age of 270.8 ± 7.5 Ma obtained so far for U-mineralization from the Bohemian Massif revealed a main Permian U mineralizing event, related to crustal extension, exhumation of the crystalline basement, and basin formation, as recorded by U–Pb apatite dates (280–290 Ma) and AFT thermal history models of the durbachites. The Permo-Carboniferous sedimentary cover probably represented a source of oxidized basinal brines infiltrating the basement-hosted durbachite plutons and triggering massive metal leaching. The interaction between basin-derived brines and durbachites resulted in significant modification of the chemical composition of the hydrothermal system (K and F release during biotite chloritization, P liberation through monazite alteration), leading to the formation of ore-bearing fluids responsible for the metallogenesis of the basement-hosted unconformity-related U-deposits in shear zones in the Bohemian Massif.
A methodology for the determination of the rare earth elements in uranium oxides by ion microprobe has been set up on a Cameca ims‐3f instrument. An uranium oxide reference material from a syn‐metamorphic uranium deposit related to albitisation has also been developed for this type of analysis. Applications of the methodology are presented for a series of uranium oxides selected from some major uranium deposit types: from the world's highest grade unconformity‐related uranium deposit from the Athabasca Basin (Saskatchewan, Canada; the Shea Creek and the McArthur River examples), a perigranitic vein‐type deposit (Pen Ar Ran, Vendée, France) and a volcanic caldera‐related deposit (Streltsovkoye, Transbaikalia, Russia). Each type of uranium deposit appears to have a specific REE signature. All REE patterns from the Shea Creek and the McArthur deposits are characterised by bell‐shaped patterns centred on Tb‐Dy and similar to those already published for uranium oxides from unconformity‐related deposits from Australia. Such bell‐shaped REE patterns centred on Tb‐Dy may therefore be considered as a typical signature of uranium oxides from Mesoproterozoic unconformity‐related deposits. A smoother bell shape pattern centred on Eu characterises the syn‐metamorphic albitisation related deposit of Mistamisk selected for the reference material. The REE patterns from the Pen Ar Ran deposit show a fractionation from LREE to HREE with anomalously high abundances of Sm, Eu and Gd with respect to the other REEs, similar to the REE patterns of uranium oxides from the volcanic‐related deposits of Streltsovkoye.
In the Proterozoïc of granulite facies in S-E Madagascar the urano-thorianite bearing rocks are part of a clinopyroxene-rich calc-magnesian complex (the so-called "pyroxenites"). The rocks are produced by the metamorphic/metasomatic alteration (ca. 500-600 M.y. ?) of limestones and marls. By metasomatism K, Na, Fe and in some cases Ca are lost by the rocks ; Mg and Si are gained. The parageneses exhibited by the surrounding gneisses, leptynites and marbles as well as the Fe-Mg distribution in the pairs cordierite-garnet and cordierite-spinel constrain as follows the conditions for the main phase of regional metamorphism : T = 700-750°C ; PF ⋍ 5 kbar ; CO2 concentrations often high in the fluid phase. Similar temperatures are deduced from the scapolite-plagioclase equilibrium in the pyroxenites. Evidences of an older metamorphic phase of higher temperature (T ≥ 780°C) are locally observed. The clinopyroxenes compositions are near the diopside-Ca-tschermak (CaTs) join and show very high Al-content (up to 15.8 % AI2O3). The CaTs-content of the clinopyroxene varies according to the parageneses : below 2 % in the unusual quartz-bearing rocks (no urano-thorianite) ; 2-6 % with scapolite/plagioclase ; more than 6 % in amphibole (fluor-pargasite-rich)-bearing rocks ; from 10-11% to near 30% in spinel-bearing rocks with amphibole or scapolite/plagioclase. The CaTs-content of the clinopyroxene is clearly dependent on the silica activity in the fluid phase. It is shown that this one may have been as low as below 0.10 the silica activity for quartz-saturation in the same P-T-XCO2 conditions. The results are coherent with the required conditions for the stability of a Th-rich (Th/U > 1) urano-thorianite as they are shown by recent experimental work in the system UO2-ThO2-SiO2.
Dans la chaine hercynienne europeenne, la majorite des
mineralisations uraniferes (filons ou episyenites) est representee
par des gisements hydrothermaux spatialement associes a
des leucogranites peralumineux d’âge Carbonifere. Ainsi, dans le
Massif Armoricain, 20000 t d’uranium ont ete extraites principalement
des trois districts uraniferes associes aux leucogranites
syntectoniques de Mortagne, Guerande et Pontivy. Les leucogranites
de Pontivy et de Guerande, qui font l’objet de ce travail, se
sont mis en place, respectivement, a ca. 315 et 310 Ma ; le
premier en contexte decrochant le long du Cisaillement Sud Armoricain
et le second en contexte extensif dans la zone sud
Armoricaine (Ballouard et al., 2015). Dans le district de Pontivy,
la mineralisation est localisee dans le leucogranite ou au
contact avec son encaissant metasedimentaire. Dans le district
de Guerande, la mineralisation s’est mise en place principalement
au contact entre des schistes noirs et des metavolcanites situes
structuralement au-dessus de la zone apicale du leucogranite avec
quelques mineralisations intragranitiques moins importantes. Les
âges U-Pb et les signatures isotopiques en Hf obtenus sur les zircons
herites des leucogranites du Massif Armoricain suggerent une
contribution d’unites du Carbonifere Inferieur dans la source de ces
intrusions fertiles en uranium. Ensuite, l’etude petro-geochimique
et geochronologique des leucogranites et de leur mineralisation
associee permet de proposer un modele metallogenique. Ainsi
dans le district de Guerande, la differentiation du leucogranite
vers 310 Ma a induit la cristallisation d’oxyde d’uranium a l’apex
de l’intrusion. Vers 300 Ma des circulations de fluides hydrothermaux
oxydants d’origine meteorique dans les facies deformes de
l’apex ont induit la mise en solution de ces oxydes. Enfin, les
fluides ont pu precipiter leur uranium dans les failles au contact
entre les metavolcanites et les schistes noirs environnants qui ont
joue le role de piege reducteur. Ces circulations hydrothermales
ont pu se produire, probablement par pulses, dans la region jusqu’
a 275 Ma (Ballouard et al., in revision).
Ballouard et al., 2015. Lithos 220–223, 1–22.
Ballouard (accepted) Ore Geology Reviews.
Elucidation of time-space relationships between a given wolframite deposit and the associated granites, the nature of the latter, and their alterations, is a prerequisite to establishing a genetic model. In the case of the world-class Panasqueira deposit, the problem is complicated because the associated granites are concealed and until now poorly known. The study of samples from a recent drill hole and a new gallery allowed a new approach of the Panasqueira granite system. Detailed petrographic, mineralogical, and geochemical studies were conducted, involving bulk major and trace analyses, BSE and CL imaging, EPMA, and SEM-EDS analyses of minerals. The apical part of the Pansqueira pluton consisted of a layered sequence of separate granite pulses, strongly affected by polyphase alteration. The use of pertinent geochemical diagrams (major and trace elements) facilitated the discrimination of magmatic and alteration trends. The studied samples were representative of a magmatic suite of the high-phosphorus peraluminous rare-metal granite type. The less fractionated members were porphyritic protolithionite granites (G1), the more evolved member was an albite-Li-muscovite rare metal granite (G4). Granites showed three types of alteration processes. Early muscovitisation (Ms0) affected the protolithionite in G1. Intense silicification affected the upper G4 cupola. Late muscovitisation (Fe–Li–Ms1) was pervasive in all facies, more intense in the G4 cupola, where quartz replacement yielded quartz-muscovite (pseudo-greisen) and muscovite only (episyenite) rocks. These alterations were prone to yield rare metals to the coeval quartz-wolframite veins.
The kinematics, modes of assembly, and the processes governing the evolution of magmas shape plutonic intrusions. Granite bodies have been suggested to emplace incrementally, with successive magmatic batches locally solidified as dikes or sills. Yet, the complexity and longevity of large-scale plutons hinders a unified model for their emplacement and concomitant differentiation. This is especially true for highly differentiated granites which usually lack continuous outcrops limiting our understandings of the detailed assembly of these igneous complexes and of the related magmatic processes. To tackle this issue, we focus on the Beauvoir intrusion (Massif Central, France), a small pluton (~800 m thick) of high economic interest (Li-Be-Nb-Ta) whose fully recovered 900 m borehole crosscutting the entire granite bring new insights on plutonic processes. The Beauvoir granite contains early-crystallised euhedral quartz and topaz associated with albite, lepidolite (Li-mica), K-feldspar and late amblygonite (Li-phosphate). Here based on numerous high resolution petrographic data (modal composition, intrusive and layering features, mineral morphologies and textural relations, etc.), and on the variation of lepidolite composition throughout the granite, we demonstrate that the whole intrusion formed through the stacking of at least eighteen decametric crystal-poor sills. Those intrusive bodies form the Beauvoir sub-units that emplaced successively without significant magma mixing with previous injections. Based on structural and geochemical features, we constrain the first relative chronology of a highly-differentiated stacked intrusion with an overall over-accretion mechanism. Once intruded, sill differentiation occurred via fractionation of quartz and topaz producing albite-, lepidolite-, amblygonite-saturated residual liquids, notably enriched in incompatible elements such as Li, Be, F and P. Channel like forming albite-rich segregates, representing escaped residual liquids from the solidifying quartz-rich mush often pounds beneath the overlying subsequently intruded sill, indicating a protracted plutonic construction faster than the solidification of a single sill. Alternatively, such evolved residual melts locally accumulate to form weakly-viscous potentially eruptible melt lenses, which possibly fed the rhyolitic dikes intruding the surrounding host-rocks.
Twenty percent of the uranium produced in France was extracted in the Armorican Massif, in three different locations (Pontivy, Guerande et Mortagne) which belong to the High Heat Production Belt, a 100 km wide NW-SE zone characterized by elevated contents in radioactive elements present in most of the geological formations [1], including 320 to 315 Ma old peraluminous granites [2]. In the Guerande uranium district, the Pen Ar Ran deposit is located in a deformed zone, at the contact between porphyroids and quartzitic black schists. This deformed zone corresponds to an E-W shear zone that affected both the metamorphic formations (porphyroids and black schists) and the intrusive Guerande leucogranite, during its emplacement. Mineralization is under the form of massive veins carrying pechblende and sulphide that are spatially associated with the leucogranite. However, this uranium deposit is different from the other vein type known in France because of the unusual nature of its uranium oxides (between pechblende and uraninite). Furthermore, the fluids responsible for this mineralization are hotter (350-400°C) than elsewhere. The question of the source of the uranium is also complex. The uranium content of the metamorphic formations is rather low (around 3 ppm) but it is also low in the leucogranite (max. 9 ppm). It is however possible that the uranium was leached out of the granite by surface derived oxidizing fluids while the granite was still at depth [3]. Therefore, the leucogranite could represent the source of the uranium. In order to test this hypothetis, we performed a comprehensive petrological, geochemical, and geochronological study of the Guerande granite and compared these data to the available data from the porphyroid and the uranium mineralization. [1] Vigneresse, J.L., Cuney, M., Jolivet, J., Bienfait, G. (1989. Tectonophysics, 159, 47-60. [2] Tartese, R., Ruffet, G., Poujol, M., Boulvais, P., Ireland, T.R. (2011). Terra Nova, 23, 390-398. [3] Tartese, R., Boulvais, M., Poujol, M., Gloaguen, E., Cuney, M. (2013). Economic Geology, 108, 379-386.
