Mineralogical and chemical distribution patterns of placers and ferricretes in Quaternary sediments in SE Germany: The impact of nature and man on the unroofing of pegmatites

Abstract Placer deposits mark the ultimate stage of mineral separation in clastic sediments by physical processes in an aquatic regime, whereas duricrusts are an example of extreme element separation by chemical processes in continental environments of deposition. In Quaternary sediments in SE Germany such contrasting types of sediments formed on the NE Bavarian Basement which is underlain by metamorphic and pegmatitic rocks reflecting intense primary element differentiation by themselves. The placer deposits were subdivided into trap and bed placers. Together with three types of encrustation they were put into a scheme illustrating six stages of land formation. The first-mentioned group of placer deposits formed from the Late Cretaceous to the Late Tertiary on a vast peneplain under subtropical conditions (stages I–II). Towards the Quaternary its more acidic meteoric fluids turned alkaline and linear erosion gradually replaced peneplanation as the main process of shaping the landscape (stage III). Evidence for these types of alluvial–fluvial Ti and Sn heavy mineral accumulations and the subtropical climatic conditions can only be found in the major placer mineral aggregates nigrine and cassiterite which preserved a great variety of marker minerals in fractures and vugs. Duricrust I consist of aluminum–phosphate (APS)-bearing phoscretes. Bed placer deposition started off with colluvial to alluvial placers passing eventually into fluvial placers (stages IV–V). Type II duricrusts evolved under humid temperate conditions in the aftermaths of the glacial period. Per-descensum fluids provoked the precipitation of these phosphate-bearing ferricretes. The clastic sediments were reworked in a low-sinuosity fluvial drainage system during stage VI. In addition to the ferricretes mentioned above another type of ferrous encrustations was added to the depositional system by man in form of fayalite-bearing slags from blast furnaces. These smelting artifacts were dumped on the floodplain of the modern river system and kilns which they derived from, were fed with phosphorous iron from duricrusts of type II. Age data for the latest stages of placer formation have been obtained from radio-carbon dating of drift wood and luminescence analyses of quartz and feldspar in placer deposits. These data reveal that morphological processes sparking placer deposition have obviously been influenced by the intensive clearing of settlers in the “Oberpfalzer Wald” (= Forest of Upper Palatinate). The results of the present study may be of assistance to geomorphologists dealing with continental placer deposits (Sn, Ti, Ce, Zr) and chemical sediments (ferricretes, phoscretes) which developed in-situ on ancient and modern peneplains. This is true for geomorphologists in academia and consulting companies alike. Archaeometallurgists may benefit from these results as they were investigating smelting residues scattered within alluvial and fluvial deposits of Quaternary age. Commonly they focus their attention on waste disposals and smelting sites and to a lesser extent on reworked smelting residues in drainage systems. The target group number one is among exploration geologists who are in search of continental placer deposits and want to know more about the clastic dispersion haloes around phosphate- and oxide-bearing pegmatites. Alluvial to fluvial placer deposits containing stable and ultrastable heavy minerals may well be preserved as to their mineralogical composition and chemical patterns even through long-lasting periods of shaping the landscape. These mineralogical and chemical patterns can be used to reconstruct the original morphology of a pegmatitic body which today is present only as “pegmatite ruin”.