Tl-rich pyrite and Sb-rich melnikovite in Montlouis pyrite veins (central armorican domain, Janze, Ille-et-Vilaine; France)

Lower Ordovician sandstones of the “Grès Armoricain” formation in Montlouis quarry near Janzé (Ille-et-Vilaine, France) display rare Fe-sulfide veins filled with a low-T (<300°C) assemblage pyrite, marcasite, “melnikovite”, trace amounts galena and sphalerite. Pyrite occur as contorted cm-scale columnar pyrite invading larger idiomorphic crystals coating vugs. It is usually overgrown by fibroradiated marcasite outer layer. Concentrically-layered spheroids “melnikovite” separate from marcasite. This mineral sequence consistent crystallization near-neutral, moderately reducing, H 2 S-rich hydrothermal fluids evolving toward acidic conditions. Vug-hosted cm-sized cubes may be partly covered millimeter-thick fine-grained film. Unlike other sulfide ores Central Armorican domain that carry Sb Pb indices, element analyses identify As, Tl, anomalies all distributed between Fe-sulfides, i.e. vein (up to 1.0 wt% As), 5 Sb), 7600 ppm Pb) fibrous overgrowths 2060 Tl). first time Tl anomaly reported massif. A potential reservoir for could altered dolerite body observed quarry.