The Fe-Mn phosphate aplite 'Silbergrube' near Waidhaus, Germany: epithermal phosphate mineralization in the Hagendorf-Pleystein pegmatite province

摘要

The Silbergrube Aplite (SA) in the Hagendorf-Pleystein Pegmatite District, near Waidhaus, Germany, is a mildly peraluminous NW-SE directed leucogranite dyke. It occurs in association with quartz dykes and aplitic metamorphic mobilizates in the NE Bavarian crystalline basement. The SA differs from other aplitic mobilizates in the region in having a less well developed strain-related mineral orientation and in containing only minor amounts of garnet and tourmaline. The aplitic metamorphic mobilizates and the SA are chemically and mineralogically almost identical and yield the same age of formation of approx 302 Ma (stage I). The age of formation of the Hagendorf pegmatites seemingly post-dates the emplacement of the SA. The SA was emplaced at the boundary between fine-grained biotite granites and metamorphic country rocks within a zone of structural weakness, favouring the formation of disseminated late magmatic to hydrothermal mineralization of Li-bearing Fe-Mn phosphates (stages II and III). Brittle deformation along this zone was conducive to the faultbound Fe-Mn-Ca phosphates. Mineral telescoping is evident from the presence of Fe~(2+), Fe~(3+) and Mn~(2+) phosphates in fissures and vugs in a texturally highly variable host-rock environment (stage IV). This intimate intergrowth of phosphate minerals reflects contrasting physical and chemical conditions prevailing in a near-surface/ shallow epithermal S-deficient phosphate system (stage IV), similar to what is known from Cu-Au epithermal systems. The most recent mineral assemblages that formed under predominantly oxidizing conditions are correlated with the subtropical weathering during the Neogene which resulted in the formation of a peneplain truncating the SA and its country rocks (stage V). The SA is the root zone of the felsic aplitic-pegmatitic mobilizates in this region and is overprinted by an epithermal phosphate system.