Petrogenesis and ⁴⁰Ar/³⁹Ar dating of hydrothermal romanechite from the sub-aerial fault-related Romanèche Mn deposit (France)

This study discusses the hydrothermal formation of romanechite (Ba_0.7Mn_4.5O_10.1.1H₂O) in the Romanèche Mn deposit and the significance of its age by combining ore petrogenesis and ⁴⁰Ar/³⁹Ar ages. Romanechite occurs as a minor phase in various manganese deposit types and remains poorly studied in comparison with other K-bearing Mn oxides (i.e., coronadite group) because of its scarcity, small grain size, intermixing and poor crystallinity. The fault-controlled Romanèche Mn deposit (Massif Central, Eastern France) investigated herein exhibits a well-crystallized and late stage massive romanechite ore occurring in a quartz-barite-fluorite gangue. Petrographical and geochemical data support manganese ore formation under sub-aerial conditions with temperatures close to fluorite precipitation (100–200 °C) and characterized by a Mn-Ba-W-As association. Five out of the eight studied samples have produced ⁴⁰Ar/³⁹Ar plateau ages ranging from 25.2 ± 0.3 to 18.2 ± 0.5 Ma. Despite a low K content (0.1–0.6 wt% K), the good crystallinity of romanechite due to its epithermal formation likely favours a better production and/or retention of the radiogenic argon than poorly crystallized phases of supergene origin. Therefore, the dataset identifies a period of at least c. 7.6 My of discontinuous hydrothermal activity, which is consistent with graben-related fault reactivation and likely volcanic activity in the Massif Central. Ultimately, this study confirms the feasibility of romanechite ⁴⁰Ar/³⁹Ar dating.