A comparison of mineralization histories for two MVT deposits, Trèves and Les Malines (Causses basin, France), based on the geochemistry of associated organic matter

Abstract

The potential offered by organic geochemistry for reconstruction of ore genesis and mineral exploration is illustrated by results obtained on the Trèves and Les Malines ZnPbBa deposits (Gard, France) and on other mineral occurrences of the eastern border of the Causses basin. The work conducted on the Trèves deposit led to the conclusion that ore deposition probably occurred early during sediment burial diagenesis in a mixed fresh water/hydrothermal system. The circulation of fluids was probably also responsible for the dolomitization of the original limestones and for ore deposition through microbial sulfate reduction. The invoked hydrothermal event could have been regional in extent, as suggested by the coincidence of hydrothermal alteration, dolomitization and mineralization in eight widely-spaced sites along the margin of the basin. Limited studies carried out on the Les Malines deposits reveal that the bitumens frequently observed in the mine are certainly allochthonous and probably played no significant role in ore genesis, contrary to conclusions from previous work (Connan and Orgeval, 1977). There is insufficient information available to unambiguously identify the role of the autochthonous organic matter in the process of ore deposition. The contrasting features of organic matter between Trèves and Les Malines agree with the general concept that MVT deposits can be formed from a variety of mechanisms. The results presented illustrate the possibilities of using organic matter to study burial diagenesis as well as the occurrence and the intensity of various epigenetic processes due to circulation of surficial and/or hydrothermal fluids. Various approaches including organic geochemistry must be combined to appraise ore genesis in sedimentary environments.