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

This study discusses the hydrothermal formation of romanechite (Ba0.7Mn4.5O10.1.1H2O) in the Romane`che 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 Kbearing Mn oxides (i.e., coronadite group) because of its scarcity, small grain size, intermixing and poor crystallinity. The fault-controlled Romane`che 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 degrees 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.