The tungsten-gold veins of Bonnac (French Massif central): new constraints for a Variscan granite-related genesis

International audienceIn the Brioude-Massiac district (French Massif Central: FMC), a network of WAs -Bi-Au quartz veins constitutes the Bonnac deposit, where tungsten is the major economic element, together with high-grade gold (up to 15 g/t Au). The evolution of this mineralization has been divided into 3 stages: (i) an early deep-seated wolframite-löllingite stage formed between 12 to 9 km, at up to 400°C; (ii) a ductile/brittle deformation stage associated with scheelite and arsenopyrite deposition, with an estimated temperature of 480-300°C; (iii) a late stage controlled by fluid-overpressure potentially triggered by fault-valve mechanism, at a depth of 7 to 5 km, and a temperature estimated between 266 to 240°C, is marked by microfracturing infilled by native bismuth, bismuthinite, hedleyite, electrum, pyrite and base-metals. Structural analysis and apatite LA-ICP-MS U/Pb dating demonstrate a spatial and temporal link between the emplacement of the peraluminous leucogranitic dykes and the Bonnac mineralization. In more details, the mineralization was deposited between 321-316 Ma, during, or just after, the emplacement of the peraluminous dykes estimated around 329-315 Ma, suggesting a magmatic-hydrothermal transition for the ore-forming process. In the proposed model, the cooling of a hidden two-mica granitic pluton could have generated a magmatic fluid, and acted as the heat source responsible for fluid flow towards inherited permeability zones. The magmatic fluid could have then re-equilibrated at high temperature by fluid-rocks interaction. The sharp changes in pressure, associated with the decrease of the temperature, and sulfidefugacity generated by a late input of meteoric fluid were responsible for the deposition of the late gold-stage. At the regional scale, the tungsten-gold event is ascribed to an early hydrothermal stage, dissociated from the formation of the antimony event in the district. The leucogranitic dykes and Bonnac quartz veins are controlled by a NW-SE stretching direction, interpreted as an expression of the Serpukhovian-Bashkirian syn-orogenic extension (D4 event of the FMC). These new data provide evidence for an early tungsten and gold metallogenic event in the FMC, prior the "Or300" event. The genetic classification of the Bonnac mineralization is equivocal. The WAs -Bi-Au-quartz veins exhibit the features of both an "orogenic gold" deposit at a relatively deep emplacement level (mesozonal), and an Intrusion-Related-Gold-Deposit (IRGD) type with a spatial-temporal link with the peraluminous intrusion emplacement. We propose that the Bonnac deposits represent an intermediate type between a typical orogenic-gold deposit and an IRGD. We argue that the presence of economic high-grade gold content in tungsten vein-type, and more generally the IRGD deposits, have been underestimated in the Variscan French Massif Central

The La Lucette Sb-Au-(W) vein-deposit (Armorican Massif, France): new time and genetic constraints to decipher the 310-295 Ma Sb-Au metallogenic peak in the Variscan Belt

In the French Armorican Massif, La Lucette is the most important late Variscan Sb deposit with an uncommon Sb-Au-(W) ore. Detailed mineralogical investigations, fluid inclusion study in gangue and ore combined with in-situ apatite LA-ICP-MS U/Pb dating were carried out to decipher the Sb-Au-W relationships, and metallogenic processes. The mineralization consists of sigmoidal quartz lenses. Four stages of ore deposition have been identified, consisting in the following mineral assemblages: i) scheelite, ii) arsenopyrite, pyrite, iii) base-metals and iv) stibnite-Au. An initial aqueous-carbonic hot-fluid (350 to 250 °C) from a metamorphic source is involved for the early scheelite to base-metals stages, and fluid/rock interaction could be responsible for the scheelite crystallization. Cooling and mixing of the initial fluid with an aqueous cold meteoric fluid (∼200 to 130 °C), are key factors for the late Sb-Au ore-deposition. P-T conditions express a shallow emplacement level (3.5 to 6 km) in a single and progressive crystallization-deformation event. No significant exhumation, and no additional magmatic source from late Variscan felsic or early Carboniferous mafic magmatic events are needed for the Sb-Au-(W) ore genesis. Structural features and new LA-ICP-MS U/Pb apatite dating associated with a regional synthesis show that all major Sb-Au mineralizations of the Armorican Massif, and most from the European Variscan Belt districts have a relatively similar genetic model. In the French Variscan massifs those deposits have been emplaced during the same mineralizing peak, which operated at around 310-295 Ma, and was controlled by the late-orogenic Variscan post-thickening tectonic event

The La Bellière gold and antimony district (French Armorican Massif): a two-stage evolution model controlled by Variscan strike-slip tectonic

International audienceThe La Bellière gold district, in the Ligerian domain of the Armorican Massif is the third one of France in term of production. It shows gold-bearing quartz veins neighboring with a complex network of Sb veins, hosted in the Neoproterozoic metasedimentary basement of the Mauges Nappe. New structural data, mineralogical, textural and geochemical assays have been carried out to clarify the gold and Sb veins relationships. Our results favor of two-step hydrothermal event with an early deep-seated gold event followed by a late shallower Sb event. The gold-bearing quartz veins are hosted by N60°E to N90°E sinistral strike-slip faults. These veins exhibit a four-stage paragenesis: 1) early barren quartz filling, 2) arsenopyrite-pyrite with minor scheelite assemblage, 3) gold and base metal sulfides crystallized after a ductile/brittle deformation stage, and 4) pyrite-carbonate coeval with a late brittle stage. These results support a continuous evolution model from a deep emplacement level (12 to 9 km) to a shallow one (7 to < 5 km) with a brutal change in the P/T conditions that triggered the deposition of gold (stage 2) controlled by a seismic-valve mechanism. On the contrary, Sb veins correspond to open space vein types, hosted in a N20°W to N20°E conjugated strike-slip fault network arranged in a Riedel shear model, controlled by N130-140°E regional dextral strike-slip shear zone associated to the South Armorican Shear Zone system. A two-stage evolution is recognized, the first one consists of early pyrite-arsenopyrite with microcrystalline quartz, followed by a late stibnite stage formed after geodic quartz deposition. Quartz texture argues for a shallow emplacement (< 5 km) of veins controlled by a suction-pump mechanism. Chronologically, the gold vein opening complies with a NE-SW maximum shortening direction different from the late-Variscan stress field responsible for the dextral Armorican shear zone during late Carboniferous, and constrains an indirect dating. Thus, a Visean age is proposed for the formation of gold veins. The overall features of gold deposits are close to those of mesozonal orogenic gold deposits in Europe. Conversely, Sb-bearing veins are generated by a N-S shortening during the late Carboniferous. The Sb event, distinct in time from the earlier gold one, might correspond to a different expression of the so-called “epizonal” type in the orogenic gold model. It is proposed that the Late Carboniferous “Or 300” event is not the unique metallogenic period for gold deposition in the French Variscan domain. Our study highlights the role of the sinistral strike-slip shearing of possible Visean age, in the gold metallogenesis of the Ligerian domain

The genesis of gold in Sb-As and Sb-Au ore veins, new constraints from the study of the La Belli&#232;re and La Lucette districts (Armorican Massif, France)

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Field validation of mineral prospectivity approaches, a first test of the CBA method

International audienceNumerous methods of mineral prospectivity mapping have been developed during the last decades and it is often difficult to evaluate their reliability and adequacy. Some objective a posteriori approaches allow calculating their performance such as, for instance, AUC values on ROC curves. However, we believe that field control is a necessary step, especially in the context of mineral targeting in greenfield exploration. Mineral prospectivity mapping generally relies on unequivocal associations between discrete data (e.g., known deposits) and polygons (e.g., lithology, geophysics, geochemistry, etc.). Consequently, the quality of results obtained from 'classical' methods (e.g., Weight of Evidence, Fuzzy Logic, Logistic Regression, Neural Network, etc.) strongly depends on the accuracy of input data (location of points and contours of polygons) which, in many cases, may be questionable. To address this issue of geographic inaccuracy of map elements, BRGM has developed the CBA (Cell Based Association) method. Its base principle is not to rank polygons in which known deposits are located, but to identify favorable association neighboring these deposits (i.e. in cells of a regular grid containing them). In order to assess the adequacy of CBA for mineral targeting in early exploration phases, we have done a first field validation campaign for Sb in the Vendée region (western France). 107 soil samples have been collected (auger drill) in both favorable and unfavorable cells (according to CBA results) and across know Sb-bearing veins. They have been analyzed at ultra-trace level using ICP-MS/ICP-AES (detection range of 0.05 to 10 000 ppm for Sb). In this contribution, we present an overview of the CBA method, its resulting prospectivity map for Sb in Vendée, the sampling and analyzing procedure, the preliminary results we have obtained and the first conclusion they allow

Interactions of plutons and detachments: a comparison of Aegean and Tyrrhenian granitoids

International audienceAbstract. Back-arc extension superimposed on mountain belts leads to distributed normal faults and shear zones interacting with magma emplacement within the crust. The composition of granitic magmas emplaced at this stage often involves a large component of crustal melting. The Miocene Aegean granitoids were emplaced in metamorphic core complexes (MCCs) below crustal-scale low-angle normal faults and ductile shear zones. Intrusion processes interact with extension and shear along detachments, from the hot magmatic flow within the pluton root zone to the colder ductile and brittle deformation below and along the detachment. A comparison of the Aegean plutons with the island of Elba MCC in the back-arc region of the Apennine subduction shows that these processes are characteristic of pluton–detachment interactions in general. We discuss a conceptual emplacement model, tested by numerical models. Mafic injections within the partially molten lower crust above the hot asthenosphere trigger the ascent within the core of the MCC of felsic magmas, controlled by the strain localization on persistent crustal-scale shear zones at the top that guide the ascent until the brittle ductile transition. Once the system definitely enters the brittle regime, the detachment and the upper crust are intruded, while new detachments migrate upward and in the direction of shearing