Formation of the Ertelien and Langedalen magmatic Ni–Cu sulfide deposits in Norway: investigating the evolution of platinum-group-element-depleted systems at convergent margins

Abstract. The Ertelien and Langedalen magmatic Ni–Cu sulfide deposits are hosted within mafic intrusions of the Kongsberg Lithotectonic Unit in the southwestern Fennoscandian Shield, formed in a convergent-margin tectonic setting. The Ertelien deposit occurs within a gabbronorite intrusion of approximately 600×500 m, with the main sulfide mineralization being located at the contact with the surrounding gneiss. In contrast, the Langedalen deposit consists of 1–2 m massive sulfide lenses hosted in 10–50 m gabbronorite lenses that are extensively deformed within shear zones. Sulfide mineralization in Ertelien ranges from disseminated to net-textured and massive ores, whereas Langedalen ranges primarily comprise massive sulfide lenses. A distinctive feature of Langedalen ranges is the local presence of an Au-rich quartz vein adjacent to the sulfide lenses. In both deposits, pyrrhotite is the dominant sulfide mineral, followed by pentlandite and minor chalcopyrite. Secondary pyrite is present, particularly in altered zones. This study provides a comprehensive characterization of the Ertelien and Langedalen deposits and constrains their genesis within a convergent-margin context. We analysed S, platinum group elements (PGEs), TABS+ (Te, As, Bi, Sb, Se), and other chalcophile elements in whole rocks and sulfide minerals across different ore textures, as well as U–Pb and Hf isotopes in zircon. Whole-rock geochemistry reveals a positive correlation between S and Ni, Cu, and Co, with sulfide tenors of ca. 2.2 wt % Ni, 1.5 wt % Cu, and 1200 ppm Co. Sulfides from both deposits are notably depleted in PGEs, consistently with derivation from a PGE-depleted parental magma compositionally similar to cogenetic dykes. This depletion may result from an event of prior sulfide segregation. However, we suggest that an alternative explanation is that the parental magmas were derived from a hydrous metasomatised pyroxenitic mantle source. In addition, a zircon U–Pb crystallization age of 1559±7 Ma and εHf(i) values of +3 to +5 for the Ertelien intrusion support formation during a prolonged subduction-related magmatic episode involving oceanic crust recycling. Despite post-magmatic alteration and secondary pyrite formation, sulfide minerals largely preserve their primary magmatic signatures, comparably to other Ni–Cu magmatic systems globally. However, parts of the Langedalen sulfides exhibit hydrothermal overprint, reflected in elevated concentrations of mobile elements (Te, As, Bi, Sb), likely linked to the formation of the Au-rich quartz vein.