Geology, sulfide mineralogy and petrogenesis of the Angstberg Ni-Cu-(PGE) sulfide mineralization (Lausitz Block, Bohemian Massif, Germany): A potential Ni-Cu exploration target in Central Europe?

Abstract

Germany’s largest known magmatic Ni-Cu-(PGE) sulfide enrichment is associated with the gabbroic Angstberg intrusion that is one of several small-scaled, Ni-Cu sulfide-bearing intrusions in the northern Bohemian Massif (Central European Variscan Belt). The intrusions are part of a Middle Devonian to Early Carboniferous, mafic-(ultramafic) magmatic system that emplaced within Cadomian granodiorites of the Lausitz Block. The Angstberg intrusion represents an exposed sub-vertical, dike-like body (320 × 60 m in plan view) and comprises olivine-gabbronorite, olivine-gabbro, gabbro, olivine-hornblende-gabbro, and hornblende-pyroxene-gabbro. The hornblende-bearing and hornblende-free lithologies originate from at least two variably fractionated basaltic parental magma batches which differ in their Ti, LREE, HREE and volatile contents. Fractionation probably took place in deeper-seated sections of the magmatic system, possibly within staging magma chamber(s). Overall enriched incompatible trace-element patterns suggest that the Angstberg lithologies are linked with intra-plate magmatism, likely associated with a transtensional setting in the course of the Variscan orogeny. The previously suggested Devonian age of the intrusion is confirmed by an 40Ar-39Ar biotite age of 373.4 ± 0.9 Ma for the olivine-gabbro. The magmatic sulfide mineralization is predominated by disseminated sulfides that are hosted in the olivine-richest parts of the olivine-gabbronorite and olivine-hornblende-gabbro lithologies. In addition, some sulfide-matrix breccias (up to 70 modal % sulfides) occur directly on the footwall contact of the intrusion to granodiorite. Both mineralization types are characterized by a predominance of pyrrhotite over pentlandite and chalcopyrite with minor secondary violarite and pyrite, and accessory PGE, Au, and Ag phases. Nickel-rich bismuthotellurides of the melonite–merenskyite solid-solution series and froodite are the most common platinum-group minerals. Disseminated sulfides are characterized by high Ni, moderate Cu, and low Pt and Pd tenors (∼4–9 wt% Ni, ∼2–6 wt% Cu, ∼200–800 ppb Pt, ∼200–1000 ppb Pd). Sulfide-matrix breccias feature comparable Ni, but lower Cu, Pt and Pd tenors (∼4–6 wt% Ni, ∼0.3–1 wt% Cu, ∼10–100 ppb Pt, ∼40–200 ppb Pd). Fractional crystallization and wall-rock assimilation of granodiorite are regarded as key factors for sulfide saturation in the magmas associated with the Angstberg intrusion. Sulfur isotope compositions of pyrrhotite-pentlandite-chalcopyrite assemblages (−1.4 to −0.2‰ VCDT) propose a mantle-origin of S, whereas addition of crustal S cannot be completely ruled out as the S isotope values of the sulfides overlap with those of the granodiorite (−7 to +4‰ VCDT). Olivine- and sulfide-textures suggest that disseminated sulfides likely originate from sulfide melts that were entrained in an olivine-rich crystal mush during magma rise to upper levels. The sulfide-matrix breccias also resulted from entrained sulfide melts, or from gravity-driven downward percolation of sulfide melts from stratigraphically higher, now-eroded levels. In both cases, the formation of sulfide-matrix breccias most likely was amplified by in-situ thermomechanical erosion of the granodioritic wall-rock, caused by flash-boiling of thermally dehydrating biotite. Despite spatial limitations of the mineralized zones and limited information about the architecture of the intrusions in the northern Bohemian Massif, the Angstberg intrusion, as well as other neighboring magmatic sulfide occurrences, may represent a new exploration target for Ni and Cu in Central Europe.