Original paper

Morphological studies of PGM grains in alluvial-fluvial placer deposits from the Bayerischer Wald, SE Germany: hollingworthite and ferroan platinum

Dill, Harald G.; Weber, Berthold; Steyer, Gustav


In the Bayerischer Wald/ NE Bavaria, Germany, alluvial-fluvial Au-PGE placer deposits containing native gold, gold amalgam, gold-silver-copper alloys (argentocuproaurite), iridium-rhodium-arsenic sulfide (irarsite-hollingworthite s.s.s.), sperrylite, platinum-iron-, iridium-osmium- and ruthenium-iridium-osmium alloys were investigated for their grain morphology. Judging by the PGE heavy mineral spectrum, ophiolitic rocks are supposed to have delivered the PGE minerals of the placer deposits in the Bayerischer Wald. The morphology of cubic crystals of irarsite-hollingworthite s.s.s is characterized by “chaotic forms”. The crystals are dominated by well-developed faces of high Miller indices which is rather uncommon to growth forms in the cubic crystallographic system. Grains of ferroan platinum show distorted octahedral crystals twinned according to the spinel law, resulting in aggregates used to be called “Gonderbach Plates” and assumed to represent PGM from a primary source. PGE chalcogenides accommodating S and As in their structure formed at rather moderate temperature, not far away from the depocentre of the alluvial-fluvial placer, whereas the Pt-bearing alloys underwent slight reworking on transport. The PGE placer mineralization has been derived from meta(ultra) basic igneous rocks bounded by shear-zones with strong hydrothermal alteration which are found squeezed into the paragneisses of the Moldanubian Zone.