Genesis of a florencite-bearing kaolin deposit on ordovician schists at Saint-Aubin-des-Châteaux, Armorican Massif, France

International audienceSeveral kaolinite occurrences of economic interest have been reported in Europe and are usually interpreted as resulting from weathering during the Paleogene tropical climate. However, a hydrothermal contribution is also recognized in some kaolinite occurrences. Such occurrences are common throughout the Hercynian Armorican Massif, where they are developed mostly on granitoids. The kaolin deposit of the Tertre Rouge pit, South Brittany (France), has been mined for the past 70 years. It is the only deposit developed from carbon-rich (~1% C) Ordovician schists affected by a low-grade metamorphic event during the late Hercynian orogeny. Herein, we report the results of a detailed field and petrological study combining optical microscopy with SEM investigation, XRD analyses, and ICP-OES and ICPMS geochemical data. A typical weathering profile was identified in the pit. It is marked from the bottom to the top by an increasing kaolinite volume (~45%) at the expense of muscovite/illite, a decreasing volume of chlorite and graphite, and numerous occurrences of Fe oxides/oxyhydroxides that replaced former pyrites or sulfates in veins. Despite this intense weathering, the analyzed Tertre Rouge schists preserved a black shale bulk chemical composition characterized by lithophile trace elements contents at 2× to 5× the Upper Continental Crust composition. This was made possible by accessory minerals inherited from the parent rock, especially Al phosphates, namely florencite (Ce), which were systematically identified as the main concentrators of LREE and Sr. Their occurrence in the Tertre Rouge kaolinized schist confirms the regional Sr and P geochemical anomalies previously reported in the Paleozoic sedimentary formations in this area of the Armorican Massif. High temperature clay minerals, pyrophyllite (>270 °C) and nacrite/dickite (generally considered to be high-T kaolinite polymorphs), were also identified inside the weathered zone, filling millimeter-thick veinlets cutting across or lying parallel to the cleavage plane S of the schist. We ascribe this high-temperature clay mineral assemblage to a hydrothermal event related to the numerous documented evidences of Late Hercynian fluid circulations in Central/South Armorican tectonometamorphic domains. Hence, we strongly favor a two-stage formation for the Tertre Rouge kaolin deposit, including a minor hydrothermal contribution to weathering-related kaolinite