Le gisement tungstifère de Leucamp (Cantal, France). Contrôles géologiques et minéralisations

With an output of 1700 metric tons between 1916 and 1959, the Leucamp ore deposit supplied about half the tungsten produced from the (( la Châtaigneraie district D (Cantal, France). Recent exploration by the French BRGM indicates a potential resource of 7000 metric tons WO,. Typical of the economic tungsten deposits of this district, Leucamp ore deposit is composed of quartz-wolframite veins that crosscut epimetamorphic schists of unknown age and are spatially associated with a Carboniferous granite, the Veynazes granite in this case. The control of the vein system is both lithological and structural : a pelitic wall-rock seems to be more favorable than a psammitic one; the complex outcrop pattern may be due to fluid pressure and the simultaneous reopening and filling of planar structures (cleavages, axial planes, ...) created by the polyphase folding that preceded the emplacement of the Veynazes granite. Subsequently, the veins are cut by numerous normal faults, which are sealed by various eruptive dikes. The final phase involves carbonate veining with a weak sphalerite-pyrite-galena-chalcopyrite mineralization. All the mineralized veins exhibit the saine type of filling, in which different stages may be recognized : (I) an initial stage with tourmaline and accessories apatite and muscovite; (2) following an episode of fracturing, the bulk quartz filling; (3) after a second phase of fracturing, the occurrence of principal episode of mineralization, consisting of pyrite and wolframite accompanied by minor Bi, As, Zn and Au minerals. At this stage, wolframite presents a Fe/Fe + Mn ratio of 0.80 and a Mg content as high as 1.2%. The evolution of the mineralizing fluid is reconstructed based on thermochemical considerations. These relations indicate temperatures varying from 500 to 420" C with Log fS, = - 4.5 to - 8. The principal mineralization is reworked by later veining where the gangue is essentially made up of dolomite, magnesian siderite and quartz with a poor sphalerite-pyrite-galena-chalcopyrite mineralization. At this stage, the previously formed wolframite is corroded, transforming into a complex assemblage of a more manganesiferous and less magnesiferous wolframite, ferberite and some scheelite.