Trace Element and Sulfur Isotope Signatures of Volcanogenic Massive Sulfide (VMS) Mineralization: A Case Study from the Sunnhordland Area in SW Norway

Abstract

The Sunnhordland area in SW Norway hosts more than 100 known mineral occurrences, mostly of volcanogenic massive sulfide (VMS) and orogeny Au types. The VMS mineralization is hosted by plutonic, volcanic and sedimentary lithologies of the Lower Ordovician ophiolitic complexes. This study presents new trace element and δ34S data from VMS deposits hosted by gabbro and basalt of the Lykling Ophiolite Complex and organic-rich sediments of the Langevåg Group. The Alsvågen gabbro-hosted VMS mineralization exhibits a significant Cu content (1.2 to >10 wt.%), with chalcopyrite and cubanite being the main Cu-bearing minerals. The enrichment of pyrite in Co, Se, and Te and the high Se/As and Se/Tl ratios indicate elevated formation temperatures, while the high Se/S ratio indicates a contribution of magmatic volatiles. The δ34S values of the sulfide phases also support a substantial influx of magmatic sulfur. Chalcopyrite from the Alsvågen VMS mineralization shows significant enrichment in Se, Ag, Zn, Cd and In, while pyrrhotite concentrates Ni and Co. The Lindøya basalt-hosted VMS mineralization consists mainly of pyrite and pyrrhotite. Pyrite is enriched in As, Mn, Pb, Sb, V, and Tl. The δ34S values of sulfides and the Se/S ratio in pyrite suggest that sulfur was predominantly sourced from the host basalt. The Litlabø sediment-hosted VMS mineralization is also dominated by pyrite and pyrrhotite. Pyrite is enriched in As, Mn, Pb, Sb, V and Tl. The δ34S values, which range from −19.7 to −15.7 ‰ VCDT, point to the bacterial reduction of marine sulfate as the main source of sulfur. Trace element characteristics of pyrite, especially the Tl, Sb, Se, As, Co and Ni concentrations, together with their mutual ratios, provide a solid basis for distinguishing gabbro-hosted VMS mineralization from basalt- and sediment-hosted types of VMS mineralization in the study area. The distinctive trace element features of pyrite, in conjunction with its sulfur isotope signature, have been identified as a robust tool for the discrimination of gabbro-, basalt- and sediment-hosted VMS mineralization.

Keywords

• volcanogenic massive sulfide (VMS) deposits
• pyrite
• trace element geochemistry
• sulfur isotope (δ³⁴S) systematics
• the Upper Allochthone of the Scandinavian Caledonides