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

K-bearing Mn oxides may potentially constitute useful objects for isotopic dating of ore-forming events. A comprehensive Ar-40/Ar-39 study performed o n supergene K-Mn oxides sampled from different sub-alpine mountain terrains in Germany and France has been undertaken. The objective of these investig ations was to provide new insight into how and when these secondary Mn accu mulations may have formed. Developed in supergene environments at the expen se of Mn2+/Mn3+-bearing precursor minerals, the Mn4+ oxides occur either as pseudomorphic ores or as cavity-fillings and linings. The isotopic ages range from 25 to 1 Ma, indicating intense chemical weathe ring, especially during the Miocene and Pliocene. It is yet too early to de cide whether the age range represents a more or less continuous process or distinct weathering episodes. Formation of supergene Mn oxides may result f rom combined climatic and tectonic factors: local uplift, exhumation, and a ssociated fracturing of rocks provided fresh mineral surfaces for percolati ng meteoric fluids that induced subsequent weathering under warm-temperate to subtropical conditions. (C) 2000 Elsevier Science B.V. All rights reserv ed.