Magmatic and hydrothermal behavior of uranium in syntectonic leucogranites: The uranium mineralization associated with the Hercynian Guérande granite (Armorican Massif, France)

Most of the hydrothermal uranium (U) deposits from the European Hercynian belt (EHB) are spatially associated with Carboniferous peraluminous leucogranites. In the southern part of the Armorican Massif (French part of the EHB), the Guérande peraluminous leucogranite was emplaced in an extensional deformation zone at ca. 310 Ma and is spatially associated with several U deposits and occurrences. The apical zone of the intrusion is structurally located below the Pen Ar Ran U deposit, a perigranitic vein-type deposit where mineralization occurs at the contact between black shales and Ordovician acid metavolcanics. In the Métairie-Neuve intragranitic deposit, uranium oxide-quartz veins crosscut the granite and a metasedimentary enclave. Airborne radiometric data and published trace element analyses on the Guérande leucogranite suggest significant uranium leaching at the apical zone of the intrusion. The primary U enrichment in the apical zone of the granite likely occurred during both fractional crystallization and the interaction with magmatic fluids. The low Th/U values (18Owhole rock = 9.7–11.6‰ for deformed samples and δ18Owhole rock = 12.2–13.6‰ for other samples) indicate that the deformed facies of the apical zone underwent sub-solidus alteration at depth with oxidizing meteoric fluids. Fluid inclusion analyses on a quartz comb from a uranium oxide-quartz vein of the Pen Ar Ran deposit show evidence of low-salinity fluids (1–6 wt.% NaCl eq.), in good agreement with the contribution of meteoric fluids. Fluid trapping temperatures in the range of 250–350 °C suggest an elevated geothermal gradient, probably related to regional extension and the occurrence of magmatic activity in the environment close to the deposit at the time of its formation. U-Pb dating on uranium oxides from the Pen Ar Ran and Métairie-Neuve deposits reveals three different mineralizing events. The first event at 296.6 ± 2.6 Ma (Pen Ar Ran) is sub-synchronous with hydrothermal circulations and the emplacement of late leucogranitic dykes in the Guérande leucogranite. The two last mineralizing events occur at 286.6 ± 1.0 Ma (Métairie-Neuve) and 274.6 ± 0.9 Ma (Pen Ar Ran), respectively. Backscattered uranium oxide imaging combined with major elements and REE geochemistry suggest similar conditions of mineralization during the two Pen Ar Ran mineralizing events at ca. 300 Ma and ca. 275 Ma, arguing for different hydrothermal circulation phases in the granite and deposits. Apatite fission track dating reveals that the Guérande granite was still at depth and above 120 °C when these mineralizing events occurred, in agreement with the results obtained on fluid inclusions at Pen Ar Ran. Based on this comprehensive data set, we propose that the Guérande leucogranite is the main source for uranium in the Pen Ar Ran and Métairie-Neuve deposits. Sub-solidus alteration via surface-derived low-salinity oxidizing fluids likely promoted uranium leaching from magmatic uranium oxides within the leucogranite. The leached out uranium may then have been precipitated in the reducing environment represented by the surrounding black shales or graphitic quartzites. As similar mineralizing events occurred subsequently until ca. 275 Ma, meteoric oxidizing fluids likely percolated during the time when the Guérande leucogranite was still at depth. The age of the U mineralizing events in the Guérande region (300–275 Ma) is consistent with that obtained on other U deposits in the EHB and could suggest a similar mineralization condition, with long-term upper to middle crustal infiltration of meteoric fluids likely to have mobilized U from fertile peraluminous leucogranites during the Late Carboniferous to Permian crustal extension events

Differentiation of peraluminous granites 'en route' to the surface

Leucogranites display petrological and geochemical heterogeneities that can be related to either primary processes acting at the source, or to secondary processes of differentiation. In the Armorican Massif, a Hercynian domain in western France, syn-tectonic leucogranites were emplaced along major shear zones. The Lizio and Questembert massifs are derived from a similar metasedimentary source and emplaced at distinct depths, ca. 3-4 kbar and 1-2 kbar, respectively. Questembert rocks are more differentiated than Lizio rocks, as demonstrated by a lower amount of modal biotite, a higher SiO2 content and a more pronounced peraluminous character. The high-SiO2 Questembert rocks can derive from low-SiO2 Lizio samples by a 15 wt.% fractionation of the assemblage kfs + pl + bt. The magmatic evolution is well recorded by the oxygen isotope evolution of whole rocks and mineral separates, demonstrating that these granites crystallized from almost crystal-free liquids. We propose that the differences between the Questembert and the Lizio granites are related to processes acting during magmas rising. Questembert is more differentiated than Lizio because it covered a longer distance during its vertical migration, allowing more crystal-liquid segregation. Regional deformation likely enhanced this process through a filter-press type mechanism. The shallower emplacement of Questembert liquids led to the exsolution of a fluid phase, which further interacted with surrounding rocks, developing a pervasive deuteric alteration. A further influx of fluids from the surface was possible because of the very shallow depth of emplacement

Crystal-melt differential behaviour during magma ascent in dykes: insights from 2-D numerical modelling

We
 present
 the
 results
 of
 2D
 numerical
 models
 of
 flow
 in
 a
 two-phase
 medium,
 composed
 by
 crystals
 carried
 by
 a
 viscous
 melt,
 subject
 to
 a
 vertical
 pressure gradient condition.
These simulations of magma migration in a vertical
 dyke
 show
 that
 presence
 of
 crystals
 and
 their
 proportion
 perturb
 the
 flow
 velocity.
As shown in previous analytical studies,
the presence of crystals leads to
 a
 significant
 drop
 in
 vertical
 velocity.
 However
 in
 our
 case,
 this
 global
 displacement
 is
 accompanied
 neither
 by
 obvious
 crystal
 rotation
 nor
 by
 a
 significant
 horizontal
 displacement
 of
 crystals
 towards
 the
 dyke
 centre.
 Moreover,
 our
 results
 show
 that
 separation
 between
 crystal
 and
 melt
 occurs
 under
 certain
 conditions,
 which
 can
 lead
 to
 geochemical
 differentiation
 of
 magmas
 during
 their
 ascent.
 We
 provide,
 through
 a
 quantitative
 parametric
 study,
the
 range
 of 
values 
leading 
to
 crystal-melt 
segregation
 and
 compare 
the
 obtained differentiation rates to natural observations.
 Consequently, our results
 have
 fundamental
 implications
 regarding
 the
 causes
 of
 geochemical
 heterogeneities
 in
 magmatic
 suites.
 They
 provide
 constrains
 on
 differentiation
 processes
 during
 ascent,
 which
 is
 often
 neglected
 in
 comparison
 with
 the
 two
 classical
 end-members
 (i.e.,
 source-related
 processes
 vs.
 emplacement-related processes differentiation

Wagnerite in a cordierite-gedrite gneiss : Witness of a long-term fluid rock interaction in the continental crust (ile d'Yeu, Armorican Massif, france)

International audienc

Fluid infiltration along the South Armorican Shear Zone

Giant quartz veins associated with the South Armorican Shear Zone (SASZ) record huge fluid circulation during the Hercynian period. A regional-scale mapping of veins allows identifying two groups of veins on the basis of their geometric relationship with the SASZ. Veins from the fist group are parallel to the SASZ and associated with zones of intense deformation, veins from the second group developed on the southern edge of the SASZ, in an oblique meridian direction. The former likely record infiltration of fluids along permeable pathways; the latter may represent crustal-scale tension gashes associated with East-West σ3 direction in the regional context (North-South shortening and dextral working of the SASZ along a N120 direction). Microthermometry on fluid inclusions indicate that fluids were mostly aqueous with very low-salinity. Together with very low δ18O values of hydrothermal quertz, down to -2‰, these features argue for a surface origin. Scarce but significant CO2 fluid inclusions indicate a metamorphic contribution for some fluids, the origin of which probably lying in the exhumed metamorphic HT basement in the southern part of the region. Veins represent SiO2 sinks and we propose through a geochemical characterisation of deformed granites that mylonitic zones developed after syntectonic granites may represent SiO2 sources

Wagnerite in a cordierite-gedrite gneiss : Witness of a long-term fluid rock interaction in the continental crust (ile d'Yeu, Armorican Massif, france)

International audienc

Carbonate alteration associated with talc-chlorite mineralization in the eastern Pyrenees, with emphasis on the St. Barthelemy Massif

International audienceThe eastern Pyrenees host a large number of talc-chlorite mineralizations of Albian age (112­97?Ma), the largest of which occur in the St. Barthelemy massif. There talc develops by hydrothermal replacement of dolostones, which were formed by alteration of calcite marbles. This alteration is progressive. Unaltered calcite marbles have oxygen isotope composition of about 25 (V-SMOW). The d18O values decrease down to values of 12 towards the contact with dolostones. This 18O depletion is accompanied by Mg enrichment, LREE fractionation and systematic shifts in the Sr isotope compositions, which vary from 87Sr/86Sr = 0.7087­0.7092 in unaltered calcite marbles to slightly more radiogenic compositions with 87Sr/86Sr = 0.7094 near dolomitization fronts. Dolostones have d18O values (about 9) lower than calcitic marbles, higher REE content and more radiogenic Sr isotope composition (87Sr/86Sr = 0.7109 to 0.7130). Hydrothermal calcites have d18O values close to dolostones but substantially lower d13C values, down to -6.5, which is indicative of the contribution of organic matter. The REE content of hydrothermal calcite is one order of magnitude higher than that of calcitic marbles. Its highly radiogenic Sr composition with 87Sr/86Sr = 0.7091 to 0.7132 suggests that these elements were derived from silicate rocks, which experienced intense chlorite alteration during mineralization. The chemical and isotopic compositions of the calcite marbles, the dolostones and the hydrothermal calcites are interpreted as products of successive stages of fluid-rock interaction with increasing fluid-rock ratios. The hydrothermal quartz, calcite, talc and chlorite are in global mutual isotopic equilibrium. This allows the calculation of the O isotope composition of the infiltrating water at 300?°C, which is in the d18O = 2­4.5 range. Hydrogen isotope compositions of talc and chlorite indicate a dD = 0 to -20. This water probably derived from seawater, with minor contribution of evolved continental wate

Element distribution in the root zone of ultramafic-hosted black smoker-like systems: Constraints from an Alpine analog

Fluid-rock interactions at Mid-Oceanic Ridges lead to metal deposition in the so-called seafloor massive sulfides at ultramafic-hosted systems. Due to restricted access to the seafloor and scarcity of 3D exposures, these systems are poorly understood at-depth. A way to access the vertical dimension is to focus on fossil analogs preserved on-land such as the one preserved in serpentinites from the Platta nappe (SE Switzerland). For this example, we document the element distribution in the mineralized rocks at three distinct levels in the rock column using both whole rock chemical analyses and LA-ICPMS analysis of sulfide (chalcopyrite and pyrrhotite) and magnetite. We bring, for the first time, chemical signatures of the mineralization in the root zone of ultramafic-hosted black smokers. At any given depth, the Co/Ni ratio is maximum in the most mineralized samples indicating that this ratio is linked to the intensity of hydrothermal alteration. Additionally, the Co/Ni ratio decreases in mineralized rocks towards the paleosurface, whereas the Se content increases. An episode of carbonation recorded in the highest structural level of the system was responsible for a slight remobilization of the former Cu stock. We propose a model in which the uprising mineralizing fluid mixed with seawater within the host serpentinites, before venting at the paleoseafloor

Fluid circulation at the origin of the trimouns talc deposit (pyrenees, France).

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