The geogene and anthropogenetic impact on the formation of per descensum vivianite-goethite-siderite mineralization in Mesozoic and Cenozoic siliciclastic sediments in SE Germany

摘要

Different types of Fe mineralization developed during the post-glacial period by per descensum processes (mineralization deposited near-surface by the descending meteoric waters) in the topmost parts of the NE Bavarian basement and its foreland sediments. These ferricretes sensu lato contain mainly vivianite, some goethite and siderite. They were investigated by means of SEM-EDX, XRD, and XRF and examined under the petrographic and ore microscope. The age of formation of these ferricretes was determined by radiocarbon and optical stimulated luminescence dating. Based upon these age data, morphological differences and their mineralogical and chemical compositions, two different types of ferricretes were established in the study area. Type-Ⅰ in the topmost part of the alluvial fan deposits is older than 6000 BC and of geogene origin only. Type Ⅱ was formed between 645 and 875 AD in the regolith on the crystalline basement rocks. It was caused by geogene and anthropogenic processes, when the first settlers in this area began smelting iron ore. Vivianite is also a common mineral of the ironstone deposits in the Cretaceous foreland sediments and of vein mineralization in crystalline basement rocks near the ferricretes under study. Post-glacial per descensum vivianite mineralization in ferricretes may be distinguished from these older per ascensum (mineralization formed at shallow depth or deeper from circulating hydrothermal fluids mainly along fissures and fault zones) vivianite mineralization by their micromorphology (stellar and acicular aggregates). All ferricretes under study developed under temperate humid climatic conditions in a narrow physico-chemical setting. This interpretation is based on (1) age dating, (2) correlation with data from the literature, (3) calculation of the stability fields of authigenic minerals involved in the development of these ferricretes. The pH values of the mineralizing fluids fluctuated between pH 5 and 9, with an Eh ≤0. These post-glacial ferricretes abundant in vivianite represent the initial and proximal facies of continental ironstones which are widespread in Mesozoic and Cenozoic sedimentary series in Central Europe.