Hydrothermal and unexpected diagenetic alteration in Permian shales of the Lodève epigenetic U-deposit of southern France, traced by K-Ar illite and K-feldspar dating

- Pyroclastic materials are altered directly into illite without a smectite precursor.
- The illite yields Permian K-Ar ages of about 260 Ma close to the sedimentation age.
- Authigenic K-feldspars of Triassic to Cretaceous K-Ar ages surround the U-deposit.
- The multi-episodic evolution of the U-deposit lasted nearly 100 Ma.

Mineralogical and chemical analyses combined with K-Ar dating of illite and K-feldspars were carried out on bulk samples and different fractions from Permian shales around the Lodève U-deposit to identify diagenetic and hydrothermal alteration episodes and their timing. Authigenic 2M1 and 1M illite with K-Ar ages between 270 and 245 Ma are the predominant mineral types in the shales. They are of early diagenetic origin relative to deposition and formed apparently directly from pyroclastic materials in a lagoonal to deltaic environment under semi-arid conditions. Albite and a first generation of K-feldspar developed by a replacement of analcime, the K-feldspar growing only in the former lagoonal environment that favored also early diagenetic dolomitization at the expense of calcite.The hydrothermally altered shales next to the U-deposit contain authigenic quartz, Fe-chlorite, neoformed illite, and a second-generation K-feldspar with K-Ar ages beginning at about 220 Ma (Triassic), although Jurassic and Cretaceous ages were also obtained. This younger age spectrum (Jurassic/Cretaceous) is close to previous U-Pb isotope data from the U-deposit, probably reflecting several episodes of U-mobilization. Therefore, the hydrothermal activity extended episodically from Triassic to Cretaceous time within the U-deposit. The hydrothermal temperatures attained locally > 220 °C as the K-Ar system of illite from early Permian diagenesis is partially reset.Triassic hot fluids ascended along faults into the northern area of the Lodève basin during an initial rifting of the nearby western Tethys Ocean. Further fluid pulses were recorded at Lodève during the opening of the North Atlantic and separation of Europe from Africa (Jurassic/Cretaceous), which also caused the peak hydrothermal activities in Central and Western Europe. As the Lodève area is located between two independently developing oceans it could have been supplied with hot fluids for a long period of time.