AbstractThe epicontinental sedimentary, epi- to mesozonal rocks of the Middle Palaeozoic of the Central Pyrenees host several economic Pb-Zn concentrations and numerous occurrences of these metals. The authors have endeavoured to recognize the original mineralogical nature of the epizonal host rocks in the concession of Baren (Haute-Garonne). Seventy-two whole-rock analyses were used for this purpose and interpreted by means of chemico-mineralogical diagrams.These diagrams use parameters chosen so as to include in the projection planes the essential minerals forming the rocks under discussion. In this way, the nature and the possible initial mineralogical composition of the rocks, which are usually obliterated during metamorphism, are made visible. The chemical compositions of the different rock units are sufficiently typical to compensate for the absence of fossils and to allow stratigraphic identifications.No volcanic contribution could be recognized in the pelites. The latter demonstrate the existence of a continent, undergoing a kaolinite-producing alteration, especially during the Silurian. The analysed trace elements (Ba, Cr, Cu, Ni, Pb, Rb, Sr, Th, U, V and Zn) are not enriched with respect to the clarkes for these rocks, except slightly in the case of U and Th and more strongly for Cr and V. The carbonate rocks contain predominantly calcite, with subordinate dolomite. They show a strong enrichment in Ba, whereas the content of the other trace elements is normal.