Résumé
Phosphate rocks are the most important natural source of phosphorus. While a large literature discusses theconditions of their formation in marine environments, few document their formation in a terrestrial context. TheQuercy phosphate mines in south-west France, known as phosphati`eres, are notable for their exceptional Cenozoicpalaeontological content and represent a rare example of phosphate precipitation in a non-marine environment.Using the Dams ‘phosphati`ere’ as an example, this paper reconstructs the conditions under which these terrestrialphosphorites formed during the Eocene-Oligocene interval by analysing karstic sediments (sedimentology, grainsize, mineralogy, and geochemistry). It discusses the origin of the karst infills and the processes involved in thegenesis of phosphate minerals. The site shows three phases of infill that frame the Eocene-Oligocene transition(EOT). Each deposit contains both a detrital fraction and a phosphate-bearing neo-formed fraction. The detritalinput results from the mass transport of Eocene-Oligocene“siderolithic” sediments (formation of reddish clayswith ferruginous concretions) derived from the reworking of lateritic paleosols. This siderolithic material wasformed at the surface directly from fresh detrital sediments resulting from the mechanical alteration of thecrystalline parent rock, located to the east in the French Massif Central.The climatic changes around the EOT seem to be reflected only by the groundwater dynamics recorded in thekarst deposits: channels are mainly present during the late Eocene, suggesting galleries well supplied with water,while abundant desiccation cracks indicate intense dry periods during the early Oligocene. Phosphate minerals,mainly fluorapatites and carbonate fluorapatite, are present as precipitated laminated crusts along the karstwalls, and as mineralised fossils and phosphatised karst walls showing epigenetic processes. Field data andthermodynamic modelling suggest a major phosphate precipitation phase before karst opening, filling, andsubsequent remobilisation. Phosphate rocks and phosphatised bones exhibit a marine geochemical signature,suggesting the contribution of the marine carbonate aquifer as a source of phosphorus (by dissolution of the hostrock). However, the geometric relationships of the phosphates with infills derived from reworked alterites, andthe enrichment in REE, challenge the influence of pedogenetic activity on phosphatogenesis. The laminatednature of some phosphate crusts would suggest bacterial activity in the phosphate precipitation process.