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Formation of the Trimouns talc‐chlorite deposit (Pyrenees) from persistent hydrothermal activity between 112 and 97 Ma

For many economically important deposits, precise duration and age of hydrothermal ore‐formation are poorly known. We tackle these questions for the large Trimouns talc‐chlorite deposit by U–Pb analyses of xenotime and monazite, which crystallized within cm‐size geodes in the hanging wall dolostones. Formation of geodes and REE‐minerals occurred in relation to 300–350°C/0.2–0.3 GPa metasomatism along a host shear zone. Sixteen xenotime fragments, broken off from four single euhedral grains, define an age‐range from 112 to 97 Myr, and a monazite age of 99 Myr. Some ages of individual fragments are grain‐specific, whereas others show the same range of dates. Given the analytical uncertainties on individual fragments (about ± 1 Myr), the absence of inherited components, and the fact that no thermal event has affected these crystals after their formation, these ages are interpreted to record continuous crystallization between 112 and 97 Ma. This means that the Trimouns deposit was produced by hydrothermal activity lasting for at least 16 million years, occurring along a shear zone through which fluids continuously (or episodically) circulated to allow ion transport from the country gneisses to the deposit locality. Persistent deformation and high heat flow, related to the transtensional regime that opened the Bay of Biscay, seems to be the most likely tectonic scenario to explain this activity.