Accessory minerals in cassiterite: A tool for provenance and environmental analyses of colluvial–fluvial placer deposits (NE Bavaria, Germany)

Abstract

In general heavy minerals are used for analysis of the depositional environment and to identify source areas of grains. To give an idea of the lithology in the provenance area petrographic microscopy may provide sufficient first-hand information, but for a more detailed study electronmicroprobe analysis of solid solution series is necessary (e.g. garnet, amphibole or phosphate minerals). In the present study, mineral aggregates of cassiterite from colluvial to fluvial Sn–Ti placers in the NE Bavarian Basement, Germany, were investigated for their accessory minerals. Cassiterite aggregates contain a wide range of inclusions of Ti–Nb–Fe minerals which may help constrain the source rock. Fissures intersecting the mineral aggregates are filled with Ti-oxide hydroxides and aluminum phosphates. These phosphates were also found associated with kaolinite and anatase, lining cavities and vugs in the cassiterite clusters. The remaining pore space in the cassiterite aggregates was filled with an “internal sediment” similar to that known from calcareous rocks. Based on the variability of these accessory minerals six processes operative during or prior to placer evolution may be defined: (1) pegmatitic–hydrothermal granitic source rock evolution, (2) peneplanation and formation of paleoplacers, (3) chemical weathering and replacement of preexisting oxides and phosphates, (4) neomorphism, (5) internal sedimentation, (6) colluvial–fluvial placer deposition. Using the accessory minerals hosted by cassiterite, the physico-chemical conditions in the provenance area and in the depositional environment may be defined. The source rocks delivering the cassiterite minerals are of Late Paleozoic age. Placer formation lasted from the Late Cretaceous through the Recent.