Les minéralisations formées en milieu réducteur karstique. Ore deposits in a reducer karstic environment

In the average geologist's mind the word karst does not evoke a reducing environment. For 10 years economic geologists have demonstrated the presence of sulphides in karstic morphological traps. The «Les Malines » mine has permitted to elaborate a theoretical model, from a large number of samples and of chemical facts. This model demonstrates that an old karst-hole was transformed into a reducing environment during the Triassic transgression. The working parameters of this environment are the following : (1) a polje-type karstic morphology, separated from the lagunar Triassic environment by hills ; (2) a climate with alternating dry and wet seasons favoring tropical vegetation around and in the polje ; (3) presence of old karstic cavities of various sizes connected with the polje ; (4) a strictly dolomitic host rock. The reducing karst seems to operate in the following way : (1) during the rainy season, the rain erodes the slopes of the polje. The polje is quickly filled with muddy water, containing detrital organic matter and dissolved metallic sulphates. The environment is oxidizing, identically in the polje and in the karst connected with it ; (2) during the dry season, evaporation will quickly lower the water level in the polje, isolating the karstic cavities which will develop independently (less intense evaporation). The polje behaves this time like an evaporitic depression (presence of gypsum bird's eyes and anhydrite layers in the middle of the depression). In the karstic cavities, evaporation is accompanied by bacterial fermentation which saturates the environment with carbon dioxide. A sulphato-reduction occurs after, precipitating tiny sulphide crystals (10 to 30 p m in diameter), on top of the fresh sediments. Sometimes on the upper parts of the layer, at the end of a sedimentary cycle, a sulphide crust is built up. This crust is often reworked in the upper layer by desiccation and sedimentation phenomena. The «Les Malines » mine provides a possible explanation for a kind of primitive sulphide-bearing intrakarstic sediment. This sediment occurs in a dolomitic environment and under a humid tropical with alternating rainy and dry season. The preconcentrated layers of metallic sulphides occurring on the slopes of the paleopolje seem to only influence the grade of the metal found in the intrakarst sediments.L'étude du gîte des Malines (Gard) a permis d'établir un modèle dans lequel un réceptacle karstique est devenu le siège d'un milieu réducteur à la fin de sa vie active, lors de la transgression triasique. Ce modèle défini par l'auteur est ensuite comparé avec d'autres exemples de sulfures piégés dans des réceptacles karstiques afin d'en dégager les points communs.Verraes Gérard. Les minéralisations formées en milieu réducteur karstique. Ore deposits in a reducer karstic environment. In: Sciences Géologiques. Bulletin, tome 39, n°3, 1986. Concentrations métalliques et milieux confinés. pp. 233-250