40Ar/39Ar dating of central European K–Mn oxides — a chronological framework of supergene alteration processes during the Neogene

Abstract K-bearing Mn oxides may potentially constitute useful objects for isotopic dating of ore-forming events. A comprehensive 40 Ar/ 39 Ar study performed on supergene K–Mn oxides sampled from different sub-alpine mountain terrains in Germany and France has been undertaken. The objective of these investigations was to provide new insight into how and when these secondary Mn accumulations may have formed. Developed in supergene environments at the expense of Mn 2+ /Mn 3+ -bearing precursor minerals, the Mn 4+ oxides occur either as pseudomorphic ores or as cavity-fillings and linings. The isotopic ages range from 25 to 1 Ma, indicating intense chemical weathering, especially during the Miocene and Pliocene. It is yet too early to decide whether the age range represents a more or less continuous process or distinct weathering episodes. Formation of supergene Mn oxides may result from combined climatic and tectonic factors: local uplift, exhumation, and associated fracturing of rocks provided fresh mineral surfaces for percolating meteoric fluids that induced subsequent weathering under warm–temperate to subtropical conditions.