Both Recent and fossil tufa deposits from Belgium were studied by petrographical and geochemical means. Study of the Recent deposits provides information on precipitation conditions and on possible present-day biological influences of organisms on tufa precipitation. Comparative petrographical and geochemical investigations of both Recent and fossil deposits give information about diagenetic modifications. The Recent deposits form within small livers, where they display two macro-types, i,e. precipitates on organic substrates (plants, mosses, algae, cyanobacteria) and laminated carbonate crusts. The latter occur on the river bed or on different substrates (limestone rocks, snail shells, artificial glass and limestone plates). Characteristic petrographical features are, on the one hand, horizontal laminations within the carbonate crusts, which relate to seasonal growth patterns of the present cyanobacteria and on the other hand, the close association between the organic matter and carbonate precipitates in the actively forming deposits. Due to encrustation in growth position, the organic structures determine the overall structure of the precipitates. Characteristic crystal morphologies (pseudo-hexagonal crystals and penta-dodecahedral crystals on mosses and algae) and specific features (e.g. micritic cyanobacterial bushes) show possible direct influence of the organisms on calcite precipitation. The stable carbon isotope signature of the Recent deposits with a mean delta(13)C value of about -11.05 parts per thousand PDB points to a mixture of three possible CO2 sources, i.e. atmospheric-related CO2, recycled marine CO2 and biological CO2. The studied fossil deposits consist of granular detrital tufa and porous cascade tufa with precipitates on organic substrates and laminated carbonate crusts, which show similar characteristics to the macrotypes found in the Recent deposits. However, there are differences between Recent and paleo-deposits that can be attributed to diagenetic changes. Moulds and prints within the tufas are created by disappearance of the original organic matter. The fossil deposits are also characterized by the presence of sparry crusts that originate from cementation and recrystallization of initial micritic cyanobacterial bushes. Furthermore, specific calcite crystals such as euhedral, gothic arch, needle and needle fibre calcite crystals have been recognized. These are interpreted to have formed during phreatic and vadose diagenesis. The stable carbon and oxygen isotope compositions of the fossil deposits show a shift towards slightly less depleted values and smaller variation with respect to the Recent deposits. This feature is also interpreted in terms of diagenetic changes within the tufa deposits. (C) 1999 Elsevier Science B.V. All rights reserved.