HELVITE FROM THE FRENCH PYRENEES AS EVIDENCE FOR GRANITE-RELATED HYDROTHERMAL ACTIVITY

The first example of helvite (s.s.) to be documented in France has been found in a zone of manganiferous mineralization associated with lower Carboniferous layers of jasper at Loudervielle and Vielle Aure in the Pyrenees. Mineralized areas are localized near granitic cupolas found in rocks regionally folded and metamorphosed at the end of the Variscan orogeny. There are four superimposed associations: 1) a banded sedimentary siliceous and carbonate-rich manganiferous rock with radiolaria, 2) metamorphic Mn-rich silicates, 3) veinlets of Mn-rich minerals cutting across the earlier foliation and stratification, related to hydrothermal circulation around granitic cupolas, and 4) Mn oxides and hydroxides produced by supergene leaching during post-Permian weathering. Rhodochrosite, rhodonite, quartz, tephroite, spessartine, friedelite, braunite, stilpnomelane, chlorite, alleghanyite, sonolite, hubnerite, alabandite, sphalerite, and welinite are associated with the helvite; the crystals of helvite are greenish lemon-yellow, attain 1 cm, and fluoresce red in cathodoluminescence. Electron-microprobe analyses gave SiO2 32.94, BeO 13.27, MnO 49.32, ZnO 0.37, FeO 0.48, S 5.68, total 99.34 wt.%, close to the end-member composition. The presence of helvite provides evidence for granite-related hydrothermal activity.