Origin and K/Ar age of cryptomelane-bearing Sn placers on silcretes, SE Germany

Abstract

During the early Miocene (Aquitanian–Burdigalian) an alluvial–fluvial drainage system evolved along the boundary fault of the Münchberg Gneiss Complex in SE Germany. The resultant NE–SW trending channel system collected debris from the enclosing Paleozoic country rocks and from gossans of cassiterite-bearing veins exposed to denudation in the roof rocks of the adjacent Late Variscan Sparnberg-Pottiga Granite. Three different types of placer minerals occur: (1) isolated grains of cassiterite, (2) armored relics of cassiterite and barite, (3) remobilized and redeposited silica aggregates. Silica is present in a zone of silcretes, in mechanically redeposited pebbles within this depression and in cobbles of quartz gangue derived from the nearby vein-type deposits. These silcretes developed from alkaline fluids and were redeposited under semiarid conditions with dryer (period of solution) and wetter seasons (period of redeposition) at ≥23Ma. Cryptomelane I formed under dry conditions around 23Ma when the pore space of the channel lag deposits was incompletely filled with water, whereas goethite points to more humid climatic conditions between 23 and 17Ma as the fluids were more acidic. Cryptomelane II filled the pore space of the clastic sediments to completeness on increasing humidity at ≤17Ma. The mineral association under study pertains to the group of supergene Fe–Mn deposits (Hunsrück-type) whose ore-forming processes span a wide range from pedological/ground water-related Fe–Mn concentration in hydromorphic soils to alluvial–fluvial pebble or placer-type concentrations. The current investigations involved geological mapping and geoelectrical DC deep sounding in the field and optical microscopy, SEM-EDX, and K/Ar age dating of cryptomelane in the laboratory.