Phosphate precipitation, phosphatisation and detrital filling dynamics in karstic contexts: The example of the Dams ‘phosphatière’, Quercy (SW France)

Abstract

Phosphate rocks are the most important natural source of phosphorus. While a large literature discusses the conditions of their formation in marine environments, few document their formation in a terrestrial context. The Quercy phosphate mines in south-west France, known as phosphatières, are notable for their exceptional Cenozoic palaeontological content and represent a rare example of phosphate precipitation in a non-marine environment. Using the Dams ‘phosphatière’ as an example, this paper reconstructs the conditions under which these terrestrial phosphorites formed during the Eocene-Oligocene interval by analysing karstic sediments (sedimentology, grain size, mineralogy, and geochemistry). It discusses the origin of the karst infills and the processes involved in the genesis 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 detrital input results from the mass transport of Eocene-Oligocene “siderolithic” sediments (formation of reddish clays with ferruginous concretions) derived from the reworking of lateritic paleosols. This siderolithic material was formed at the surface directly from fresh detrital sediments resulting from the mechanical alteration of the crystalline parent rock, located to the east in the French Massif Central.