Microbial control on the precipitation of modern ferrihydrite and carbonate deposits from the Cezallier hydrothermal springs (Massif Central, France)

Actively precipitating hydrous iron oxides and carbonates an commonly assoc iated with cool CO2-rich spring waters in the Massif Central (France). Chem ical, isotopic and mineralogical analyses, and thermodynamic calculations w ere undertaken to elucidate the processes involved in the formation of thes e deposits. Hydrous iron oxide particles rich in Al, Si, Ca and As are the first to form in the springs. Accompanying the iron oxides are layered carb onate deposits, composed mainly of well-crystallised calcite. SEM examinati on of both oxides and carbonates sampled at the outlet of the springs shows that they basically consist of grape-shaped agglomerates of small (300 nm to 1 mu m) anhedral crystals. Light acid etching of these agglomerates reve als colonies of spherical nannobacteria. Water trace partitioning between o xide and carbonate favours the hydrous oxide with As, Pb, Zn, Th concentrat ion. The microscopic features of both types of deposit indicate that microb ial mediation probably contributes to their formation. (C) 1999 Elsevier Sc ience B.V. All rights reserved.