Abstract

The mineralization of Jebel Stah consists almost exclusively of fluorite. It is associated to the unconformity surface which separates the palaeorelief of the Lower Liassic limestones (Oust Formation) from the condensed layers of the Carixian phosphatic limestones and the Middle-Upper Domerian marls-limestones alternance. Evidences of emergence are visible at the top of the infra-Liassic and the Carixian carbonates. These two formations have, furthermore undergone a strong epigenetic dolomitization. The mineralization is hosted in the Carixian layers essentially. We observed that the mineralization has been deposited during an epigenetic stage both replacing the dolomitic rocks and within veins and geodes. The microthermometric study of fluid inclusions in fluorite and quartz (two phase aqueous inclusions, aqueous inclusions enclosing a fine liquid hydrocarbon drop, and gazeous CO 2 -rich inclusions) shows that the mineralizing fluids are hydrothermal brines. Two stages of cristallization of the fluorite are identified. The ore s.s. (fluorite I) has deposited from a highly saline (20+ or -1 wt% NaCl eq.) under 135+ or -20 degrees C temperature conditions. Fluorite II and silica crystallized in later stages owing to the circulation of less saline (10+ or -1 and 5.5+ or -1 wt% NaCl eq., respectively) but warmer fluids (185+ or -20 degrees C and 225+ or -20 degrees C, respectively), occurring within geodes and fractures. This late heating is generated by the opening of fractures within an extensional context characterized by a high geothermal gradient. This gradient prevailed over a long period as evidence by the multitude of the crystallization phases. The association of the mineralization with the unconformity surface within carbonate host rocks that underwent a strong epigenetic dolomitization, the temperature and salinity of the fluid which deposited the fluorite ore, and the presence of liquid hydrocarbons in the late mineral phases allow to compare the Jebel Stah fluorite deposit to those of the Mississippi Valley-type group where fluorite dominates.

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