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

Abstract

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