5: Late Variscan mineralizing systems related to orogenic processes: The French Massif Central (original) (raw)
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Mineralium Deposita, 1999
The French Massif Central (FMC) represents the whole West European Variscan (WEV) belt, in terms of both the geodynamic evolution and the metallic content. Thus, a study of the metallogenic evolution of the FMC may elucidate the conditions that allow the mineralisation of a collision belt, since recent collision belts, e.g. the Himalayas or the Alps show that mineralisation does not necessarily result from the collision process. The Palaeozoic history of the FMC is divided into three geodynamic stages unevenly involved from the metallogenic view point. The Eo-Variscan stage (Cambrian to Silurian) was not important; the Meso-Variscan stage (Devonian-Early Carboniferous) was of limited importance; and most of the mineralisations formed during the Neo-Variscan stage (Late Carboniferous-Early Permian). In addition, some more mineralisation was produced during the Mesozoic because of the thermal reactivation linked with the Alpine orogenies.
International Journal of Earth Sciences, 2011
Geochronological, mineralogical, and geochemical analyses have been focussed on the Mont-Lozère-Borne plutonic complex and surrounding rocks (Cévennes, French Massif Central) in which B-W-Sn and As-Au-Sb mineralization is encountered. Two main results are highlighted: (1) the existence of a 301-306 Ma magmatohydrothermal event unrelated to the emplacement of the Pont-de-Montvert-Borne plutonic body at 316 Ma; (2) the magmatic and hydrothermal features are strongly associated, both in time and in space, thus demonstrating an intimate connection between mineralizing processes and magmatism in this part of the French Massif Central. We also show that mineralization and associated hydrothermal occurrences do not correspond to a simple and single geochemical signature and that a contamination model must be invoked in order to account for the complexity of isotopic results. This study demonstrates that the application of the O and H isotopic signatures as tracers of the source and nature of fluids in an orogenic context requires some specific care. Finally, a model of the tectonic-magmatic-hydrothermal evolution of the study area is suggested in which we discuss two alternative scenarios. The first one implies the existence of two different hydrothermal/ mineralizing events (Bo-W-Sn and As-Au-Sb ones). The second one suggests the same source for all hydrothermal and mineralized structures.
Mineralium Deposita, 2004
The Moulin de Che´ni orogenic gold deposit is the only granite-hosted deposit of the Saint-Yrieix district, French Massif Central. It occurs in 338±1.5 Maold peraluminous leucogranites and is characterized by intense microfracturing and bleaching of the granite in relation to pervasive sulfide crystallization. Formation of quartz veins and gold deposition occurred in two successive stages: an early ''mesozonal'' stage of quartzsulfide (Fe-As-S) deposition, usually devoid of gold and a late ''epizonal'' stage of base metal and gold deposition. Both stages postdate peak metamorphism and granite intrusion. The genesis of the deposit is the result of four successive fluid events: (1) Percolation of aqueous-carbonic metamorphic fluids under an assumed lithostatic regime of 400-450°C, at a maximum depth of 13 km; (2) Formation of the main quartz lodes with coeval K-alteration and introduction of As and S from aqueous-carbonic fluids percolating along regional faults. Arsenopyrite and pyrite deposition was linked to the alteration of Fe-silicates into K-feldspar and phengite at near-constant iron content in the bulk granite. Temperature was similar to that of the preceding stage, but pressure decreased to 100-50 MPa, sug-gesting rapid uplift of the basement up to 7.5 km depth;
Mineralium Deposita, 2004
The Moulin de Chéni orogenic gold deposit is the only granite-hosted deposit of the Saint-Yrieix district, French Massif Central. It occurs in 338±1.5 Ma-old peraluminous leucogranites and is characterized by intense microfracturing and bleaching of the granite in relation to pervasive sulfide crystallization. Formation of quartz veins and gold deposition occurred in two successive stages: an early “mesozonal” stage of quartz-sulfide (Fe-As-S) deposition, usually devoid of gold and a late “epizonal” stage of base metal and gold deposition. Both stages postdate peak metamorphism and granite intrusion. The genesis of the deposit is the result of four successive fluid events: (1) Percolation of aqueous-carbonic metamorphic fluids under an assumed lithostatic regime of 400–450 °C, at a maximum depth of 13 km; (2) Formation of the main quartz lodes with coeval K-alteration and introduction of As and S from aqueous-carbonic fluids percolating along regional faults. Arsenopyrite and pyrite deposition was linked to the alteration of Fe-silicates into K-feldspar and phengite at near-constant iron content in the bulk granite. Temperature was similar to that of the preceding stage, but pressure decreased to 100–50 MPa, suggesting rapid uplift of the basement up to 7.5 km depth; (3) The resulting extensional tectonic leads to the deposition of gold, boulangerite, galena and sphalerite in brecciated arsenopyrite and pyrite from aqueous fluids during a mixing process. Temperature and salinity decrease from 280 to 140 °C and 8.1 wt% eq. NaCl to 1.6 wt% eq. NaCl, respectively; (4) Sealing of the late fault system by barren comb quartz which precipitated from dilute meteoric aqueous fluids (1.6 wt% eq. NaCl to 0.9 wt% eq. NaCl) under hydrostatic conditions at 200–150 °C.
Ore Geology Reviews, 2018
Numerous Sb-Au deposits are known in the European Variscan belt and are often associated with the late Variscan hydrothermal events linked to postorogenic extension and strike-slip faults, through the whole belt. The central part of the Armorican Massif (Central Brittany) hosts several Sb-Au deposits, such as the Le Semnon deposit. However, little is known about its formation, its structural control or hydrothermal history. In order to provide new constraints on the Sb-Au mineralization framework of the Variscan Armorican Massif, structural, geochronological and chemical analyses have been performed on the Le Semnon Sb-Au deposit. First, we highlight that Sb-Au mineralization consists of fault-fill and flat extensional vein systems resulting from small-scale fault-valve mechanism controlled by fluid overpressure at shallow depth. Indeed, this deposit is characterized by the presence of dolerite dykes which acted as the plumbing system for fluids within impermeable slates. The low permeability of the slates permitted the establishment of local supralithostatic fluid pressure in a predominantly hydrostatic regime. Coupled with new (micro-) textural evidences, a reappraisal of the para-genetic evolution and new physico-chemical data, 40 Ar/ 39 Ar and U-Pb analyses show that the Le Semnon Sb-Au mineralization was emplaced around 360 Ma and therefore indicates an early hydrothermal event in the Variscan history of the Armorican Massif. Moreover, absolute dating of mafic magmatism coupled with hy-drothermal alteration associated with the deposition of Sb-Au mineralization strongly suggest that the empla-cement of the Sb-Au mineralization was coeval with the emplacement of the host rock doleritic dyke. Furthermore, the high temperature recorded in fluid inclusions and the paragenetic evolution suggests an elevated thermal anomaly in Central Brittany. Thus, we propose that the emplacement of the widespread mafic magmatism at ca. 360 Ma played a major role in the mobilization of fluid flows and the redistribution of metals in the Armorican crust. A Silurian N sandstone and black shale Upper Ordovician metapelite and shale Middle Ordovician sandstone Lower to Middle Ordovician 78
BSGF - Earth Sciences Bulletin
The Puy-les-Vignes W deposit, located in the northwestern French Massif Central (FMC), is a rare occurrence of a wolframite-mineralized hydrothermal breccia pipe hosted in high-grade metamorphic gneisses. We present an integrated study of this deposit aiming to characterize the ore-forming hydrothermal system in link with the Variscan late-orogenic evolution of the FMC. Based on a set of representative samples from the host rocks and mineralization, we describe a detailed paragenetic sequence and we provide the major and trace element geochemistry of the granitic rocks and W–Nb–Ta–Sn–Ti oxide minerals, in situ U/Pb and 40Ar/39Ar geochronology, and a fluid inclusion study of quartz and wolframite. We demonstrate that the formation of this W-mineralized breccia pipe results from a multistage development related to four major episodes during the late Carboniferous. The first episode corresponds to the emplacement of an unexposed peraluminous granite at ca. 324 Ma, which generated micro...
Tectonophysics, 2001
The relative chronology of¯uid migration, quartz and Au-deposition in a silici®ed fault from the main Au-district (Laurieras, St Yrieix district) from northern French Massif Central has been determined from microstructural,¯uid inclusion, isotopic and ore mineral evidences. Three main stages of¯uid circulation, microfracturing and quartz crystallization, and ore deposition were distinguished on the basis of textural relationships and the pressure, temperature and composition of the palaeo-¯uids: (1) a series of early¯uid events was responsible for the localized drainage of retrograde metamorphic¯uids along the main fault and the subsequent sealing by milky and microcrystalline quartz preceeded the main Au-ore stages. Early¯uids were aqueous± carbonic, trapped under lithostatic to sublithostatic pressures at temperatures in the range 350±5008C. Subsequently, several types of microstructures were developed in the early quartz matrix. (2) NS microfractures ®lled by clear quartz, arsenopyrite and boulangerite (I) contain signi®cant refractory gold concentrations. Clear quartz formed from aqueous±carbonic¯uids of lower densities than those of the earlier¯uids. Signi®cant pressure drops, down to pressures around 55 MPa were responsible for a local immiscibility of the aqueous±carbonic¯uids at temperatures of 340^208C. (3) The main ore stage is characterized by the formation of dense sets of sub-vertical (EW) microfractures, healed¯uid inclusion planes in quartz, and ®lled by ore minerals (native gold, galena and boulangerite II) when they crosscut earlier sul®des. The¯uids are aqueous with low and decreasing salinity, and probable trapping temperatures around 2308C. Isotopic data, obtained on micro®ssured quartz, indicate these dilute aqueous¯uids may be considered as meteoric waters that deeply in®ltrated the crust.
Avant-propos Hydrothermalisme en domaine continental
U-Pb and Th-Pb dating of monazite from hydrothermal quartz veins ("Alpine veins") from the Lauzière massif (North Belledonne) together with Ar/Ar ages of adularias from the same veins constrain the age of the last tectono-metamorphic events that affected the External Crystalline Massifs (ECM). Ages obtained are surprisingly young. The study of the structural context of the veins combined with our chronological data, allow us to propose a tectonic scenario of the northern ECM for the 15-5 Ma period, which was poorly documented so far.