Age and petrogenesis of Ni-Cu-(PGE) sulfide-bearing gabbroic intrusions in the Lausitz Block, northern Bohemian Massif (Germany/Czech Republic)

The Lausitz Block, located in the northernmost part of the Bohemian Massif, hosts a large number of dike- to stock-shaped gabbroic intrusions that mainly comprise brown hornblende-poor (Group I; i.e. olivine gabbronorite, olivine gabbro, gabbro and diorite) and subordinately brown hornblende-rich lithologies (Group II; i.e. olivine-hornblende gabbro and hornblende gabbro). Several of these intrusions host small-scaled magmatic Ni-Cu-(PGE) sulfide accumulations. The intrusions are part of an interconnected mafic–(ultramafic) plumbing system that intruded Cadomian granodiorites of Lausitz Block in the Middle to Late Devonian during the early stages of the Variscan Orogeny. The previously inferred Devonian age of the intrusions is refined by biotite Ar-Ar dating that yield ages between 372.2 ± 3.7 Ma and 389.1 ± 3.9 Ma (2σ). Group I and Group II lithologies differ in their mineralogical and geochemical composition. Compared to the Group I lithologies those of Group II are characterized by higher modal contents of primary brown hornblende, Fe-Ti oxides and apatite, by Ti- and Al-enriched clinopyroxene and by lower contents of SiO 2 and increased contents of TiO 2 , P 2 O 5 , LILE, HFSE and LREE. The differences suggest at least two different magmatic series where Group I rocks are linked to tholeiitic basaltic magmas with low to moderate Ti and volatile contents, whereas Group II rocks are derived from Ti- and volatile-enriched moderate-alkaline basaltic magmas. The magmas experienced clinopyroxene fractionation during their crustal ascent and storage, but were only minor affected by crustal contamination (< 5 %) according to Sr-Nd-Pb isotope systematics. Clinopyroxene and whole-rock trace element compositions suggest that primary magmas of both series are linked to an intraplate setting. REE systematics suggest primary magma contributions from both garnet and spinel peridotite sources. Group II samples bear evidence for higher proportions of garnet peridotite-derived melts, and trace element modelling indicates melting degrees between ~5–20 % for both groups. The proposed intraplate magmatism is might been related to a subduction slab retreat within the framework of the Variscan orogeny, which leads to lithosphere extension and enhanced decompression melting of the mantle beneath the Lausitz Block. Cu/Zr ratios < 1 of gabbroic rocks from several intrusions suggest a previous segregation of magmatic sulfides in other sections of the magmatic plumbing system and give rise for a vertical and lateral Ni-Cu exploration potential