Orogenic arc-related Ni-Cu-(PGE) mineralization of the Ransko massif, Czech Republic, and implications for the metallogeny of the European Variscan belt

Abstract

The Ransko (ultra)mafic massif, Bohemian Massif, Czech Republic, hosts several Ni–Cu–(PGE) deposits and peculiar Zn–Cu–Ba ores. Geochronology integrated with petrography, bulk-rock, and mineral compositions together with Sr–Nd–Pb–Hf–Os–O isotopic systematics of barren and variably mineralized (ultra)mafic lithologies as well as massive ores reveal a complex evolution of the Ransko massif and its mineralizations. The Sm–Nd and U–Pb ages obtained for gabbros and cross-cutting granite porphyry, respectively, overlap with Re–Os ages of Ni–Cu–(PGE) and Zn–Cu ores and limit the formation age of (ultra)mafic rocks and metal accumulations to ~ 370–345 Ma. Compositional variations indicate derivation of parental melts of the Ransko massif from metasomatized, Variscan sub-arc mantle and underscore the importance of assimilation–fractional crystallization and crystal accumulation processes. The Ni–Cu ores were emplaced through the gravity-driven percolation of dense sulfide liquids along previously weakened structures associated with the downward crystal fractionation. The orogenic and arc-related nature of the Ransko Ni–Cu–(PGE) mineralization shares some remarkable similarities with some other Ni–Cu deposits in the European Variscan Belt highlighting the significance of these deposits emplaced in arc-related crustal domains. Yet, the variable nature of these mineralizations indicates complex processes that happen during the emplacement and evolution of the parental magmas driving their favourable metal contents.