Microbial control on the precipitation of amorphous iron oxide and carbonate modern deposits from the C zallier hydrothermal springs, Massif Central, France

Mineral deposits related to thermal springs are generally of the carbonate or silicate precipitation types. Numerous thermal waters have been studied worldwide for their extensive deposits (Italy, Iceland, New Zealand, USA). The occurrence of other deposit types, such as oxides, associated with hydrothennal activity are few. However, the almost pure manganese oxide deposit in Japan was recently studied to demonstrate the microbial mediation in the Mn02 precipitation through Mn 2+ oxidation of Mn-rich waters [1]. The C02-rich thermal waters from the Massif-Central (France) are of special interest because both carbonate deposit and oxide precipitation generally occur [2]. As the pressure is released at the surface (bubbles of CO2) small red Ferich particles precipitate in the spring. With slower kinetics a layered carbonate travertine is formed farther downstream.