HYPOTHESIS ABOUT THE PALEOENVIRONMENTAL C ONDITIONS DURING THE DEPOSITION OF THE SULFUR ORE OF LES-CAMOINS (MARSEILLES, FRANCE) AND THE ROLE OF BACTERIA-METABOLISM IN THE PROCESS, INTO A SULFATED CONTINENTAL LAGOON

The groundwater lying under the small hills around Les Camoins (Marsei lles, South France) is sulphated and sulphurous. For more than one hun dred years, this water has been used for the hydropathic establishment of ''Les Camoins-les-Bains''. It is mineralized when passing through a rocky massive, formed during the Stampian (Oligocene times), and com posed of gypseous and carbonated laminites, in which organic beds and native sulphur were observed. The study of mineral and organic materia ls, along the outcrops and into the old sulphur mine, showed that bact erial activity was the most important factor explaining the deposit. T he comparison between original sediments and similar deposits generate d under actual bioclimatic conditions allowed to propose a naturalisti c model for this Stampian sedimentation. We hypothezise that the depos it was formed in a shallow continental lagoon of sulphated and biologi cally stratified water, which dried periodically under contrasted trop ical climate. Photosynthetic sulphur bacteria activity, resulting in a n intracelullar accumulation of elemental sulphur, was, at time interv als, ehanced by sudden, short and strong dystrophic crisis, enriching the sediment with both organic matter and sulphur. During the arid con ditions that ended each crisis, elemental sulphur was protected from a ny microbial reduction by a layer of gypsum. Such bioclimatic conditio ns were rarely encountered, and this may explain the scarcity of minin g sites for Stampian sedimentary sulphur in France: the four existing are located in the Mediterranean area.