The Causses basin in the south of the Massif Central, France, extends over an area of 30 km from east to west and 70 km from north to south. It comprises mainly Jurassic calcarenites lying on Triassic sandstones and clays, all of which were deposited unconformably on a pre-Stephanian basement or on granite. Many mineral occurrences including deposits of Ba, Zn, Pb and Cu have been found on the border of the basin in the Liassic and/or Triassic rocks. Whatever form these mineral occurrences may take (veins, lenses, impregnations), synsedimentary or early diagenetic models involving mixing of meteoric fluids with marine or pore waters have long been suggested for their genesis. The role of hydrothermal fluids channeled through faults, stressed in recent works, still remains controversial among supporters of either hypothesis. Recent petrographic and geochemical studies on organic constituents in the Trèves Zn, Pb (Ba) deposit showed that deposition occurred in a mixed fresh/hydrothermal system (summary and references in Disnar and Gauthier, 1988). This conclusion refutes previous hypotheses favoring synsedimentary deposition of ore minerals under low-temperature conditions. The new conclusions are mainly supported by data on the maturity of organic constituents that provide evidence for a paleothermal event. The effects of this event were strictly limited to the orebodies and to their dolomitized Liassic host rocks. These conclusions and the fact that the thermal anomaly decreases in intensity away from the old basement and disappears westward in the dolomitization front, strongly suggest transformation of the original calcarenites by mineralizing fluids channelled through the bordering faults. The upward circulation of these fluids was very likely stopped by weakly permeable marls at the hanging wall of the deposit. The mixing of hydrothermal fluids with cold pore and/or meteoric waters caused sulfate reduction by specialized micro-organisms and produced ore de-debated for carbonate-hosted deposits, such as Mississippi Valley-type deposits. We also wish to emphasize the use of organic geochemical techniques in the exploration of hydrothermal deposits. The greatest exploration possibilities for revealing concealed orebodies are offered by hydrocarbon gases that can migrate through overburden rocks unaffected by hot mineralizing fluids (Disnar and Gauthier, 1988).