Study of the Dragset and Høydal VMS deposits (Central Norwegian Caledonides): genesis, effects of metamorphic overprint, critical raw material potential

The central Norwegian Dragset and Høydal deposits are small-scale Cyprus-type volcanogenic massive sulphide (VMS) deposits 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, while Høydal is one of the very few VMS deposits in the Caledonides that experienced only minimal effects of deformation and metamorphosis. Despite their proximity to the world-known Løkken, the formation process as well as critical metal content is poorly understood. Hence, the present study aims to contribute to these aspects through field observations, optical and electron microscopical petrography, in situ mineral chemistry (EPMA and LA-ICP-MS) and whole-rock geochemical analyses.

Massive sulphide as well as stockwork mineralisation in altered greenstone are observable at both study locations. Pyrite is abundant in both deposits and in each ore type, while chalcopyrite is more common in the stockwork zones. Traces of sphalerite occur at each location and mineralisation type, as well as local sphalerite enrichment in some massive sulphide samples. These mineralogical observations support well the observed differences in whole rock geochemistry data where Cu/Zn ratios decrease towards more distal ore types.

The widespread appearance of pyrite combined with its resistance to later processes make it a perfect tool to reconstruct the formation environments. Higher formation temperatures (up to >320°C) in Dragset facilitated Co incorporation in pyrite as well as occurrence of cobaltite, resulting in overall higher Co content of the ore, compared to Høydal (up to 277 ppm vs. up to 37 ppm). Also, in both deposits the trace element content (As, Te) in pyrite indicates a change in redox conditions, i.e., that fluids became progressively more oxidised towards the seafloor due to mixing with oxygenated seawater.

Sphalerite, unlike pyrite, was affected by metamorphism in Dragset, leading to Zn remobilisation and high temperature (320-480°C) sphalerite precipitation in more permeable zones. As a contrary, submarine hydrothermal sphalerite was found in Høydal, formed at lower temperature (below 200-250°C), enriched in the distal massive sulphide samples. It formed during the waning stage of the hydrothermal process, together with late pyrite and quartz precipitation; the preliminary fluid inclusion study of quartz proves the <200°C formation temperature from an enriched, seawater originated fluid.

Besides the Cu and Zn content, the high temperature formation conditions at Dragset were favourable for the enrichment of a few critical metals (in addition to the above-mentioned Co, Se and Te also). Though some distal samples of Høydal are enriched in lower temperature sphalerite, their Ga content remain below economic grade.

This still ongoing study draws the attention to the effects of formation temperature differences as well as metamorphic overprint on the metal occurrence and distribution in the VMS deposits.