Homologues de la lillianite (gustavite, vikingite, heyrovskyite riche en Ag et Bi...) de l'indice à W-As-(Pb,Bi,Ag) de La Roche-Balue (Loire Atlantique, France)

Abstract

In the gneiss quarry of La Roche-Balue (West of Nantes, France), a quartz vein is principally mineralized with scheelite, arsenopyrite and galena, with numerous Pb-Bi-Ag sulfosalts. Three stages of deposition have been inferred : — stage 1 : pneumatolytic, with quartz and orthoclase ; — stage 2 : hypothermal (350-400 °C), with quartz, scheelite, arsenopyrite (+ pyrrhotite and molybdenite), gustavite I + vikingite + Ag-Bi rich heyrovskyite + eskimoite (+ gold), galena ; — late stage 3 : epithermal, with pyrite, marcasite, chalcopyrite, argentiferous cosalite, gustavite II, bismuth and tetrahedrite. Between crossed polars, gustavite I and II show ubiquitous complex twinning. In gustavite I, numerous exsolution lamellae of an Ag-Bi poor member of the gustavite-lillianite series are always present. On the contrary, the smaller crystals of gustavite II are pure members of the series. Vikingite is polysynthetically twinned, with another twinning, analogous to that of gustavite, visible on a smaller scale. Ag-Bi rich heyrovskyite is untwinned. These three sulfosalts frequently form epitactic associations. Weak anisotropy of galena reveals polysynthetic twinning of mechanical origin. Microprobe analyses show 3.1 % Bi and 1.6 % Ag in anisotropic galena. Pb sulfosalts contain 0.02 - 0.05 % Se, 0.04 - 0.14 % Sb and 0.10 - 0.80 % Cd. In the lillianite homologues, Cd increases with Ag-Bi content. Cosalite contains 1 % Cu and 2.6 % Ag. In stage 2, the sulfosalts show a composition trend subparallel to the Bi2S3-Pb2S2 join in the Pb2S2-Bi2S3-Ag2S system. An X-ray single crystal study of lillianite homologues permits to identify the five sulfosalts included in galena, mainly the two members of the gustative-lillianite series, and the minor eskimoite in epitactic association with heyrovskyite. During the final epithermal stage, the primary Pb sulfosalts dissolved to give (gustavite II + cosalite + galena) association. Decomposition of the lillianite homologues may be either due to their instability at low temperature, or to the presence of copper which favours the crystallization of cosalite. Cd content of Pb-Bi-Ag sulfosalts is similar to Cd content frequently observed in the andorite series. Apparently, the Cd enrichment is typical for ore solutions mobilized in post-magmatic processes generated by granitic intrusions.