Mineral chemistry from the Dragset volcanogenic massive sulphide deposit (Norwegian Caledonides, Trondheim Nappe Complex): Genetical implications

• Pyrite and chalcopyrite chemistry in metamorphosed VMS is used to reconstruct formation conditions. • Chemistry of remobilised sphalerite reveals physicochemical parameters of metamorphic fluids. • Critical metal content of pyrite and other sulphides highlights the economic potential of Caledonian VMS deposits. The Dragset deposit, central Norway, is a mafic (former Cyprus-type) volcanogenic massive sulphide (VMS) deposit hosted within oceanic back-arc affinity metavolcanic rocks of the Early Ordovician Løkken ophiolite. During the Caledonian orogeny, the Dragset deposit was deformed and metamorphosed in lower-mid greenschist facies, which lead to a local segmentation of the deposit. To study the genesis of Dragset and assess its critical metal potential, field observations, optical and electron microscopical petrography, in situ mineral chemistry (EPMA and LA-ICP-MS) and whole-rock geochemical analyses were performed. This study reveals that the chemistry of pyrite, pyrrhotite and chalcopyrite can be used to determine the temperature, oxygen- and sulphur fugacity conditions at which the sulphide ore formed, despite the low-mid greenschist metamorphic overprint. High formation temperatures (>320 °C in feeder zone and ∼320 °C in massive sulphide lens) facilitated Co-enrichment in pyrite from the massive sulphide ore body and led to the formation of cobaltite. Percolating fluids became progressively more oxidised towards the seafloor due to mixing with oxygenated seawater. This process led to strong Te-enrichment of pyrite and the appearance of Te-mineral inclusions in pyrite. The metamorphic overprint did not cause Au-leaching from pyrite but rather led to high temperature (320–480 °C) Zn remobilisation and precipitation of sphalerite-bearing ore in permeable zones of the deposit. Our results highlight the occurrence of critical metals enrichments and detail their distribution within the VMS system of Dragset.