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

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 δ 34S 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 δ 34S 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 δ 34S 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 δ 34S 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

Geochemical characteristics of volcanogenic massive sulfide mineralizations on Bømlo and Stord islands, Sunnhordaland, SW Norway

Postponed access: the file will be accessible after 2022-08-29The results from the present study have brought forward new knowledge about geochemical, mineralogical, stable isotope and trace element data for volcanogenic massive sulfide deposits hosted within Early Ordovician Ophiolitic terrane of south western Norway. The geology of the Sunnhordaland region is comprised of rocks native to the Baltic continent and exotic rocks that formed adjacent to the Laurentian margin in the Iapetus Ocean hosting numerous deposits in the Hardanger Cu-Zn-Au Province. Though numerous, the geochemical characteristics of volcanogenic massive sulfide deposits in the region are poorly constrained. The investigated deposits are located on the islands of Bømlo and Stord and are associated with ophiolitic, island-arc and volcano-sedimentary successions. Hosted within the Lykling Ophiolite Complex the epigenetic Cu-rich Alvsvågen mineralization is spatially associated with four SW-NW trending shear zones that intersect the high-level gabbroic portion of the ophiolite complex. The Cu-rich mineral assemblage, hydrothermal alteration characteristics and the magmatic origin of sulfur suggest that the Alvsvågen mineralization was deposited within deep fluid conduits of a volcanogenic massive sulfide system. The Lindøya and Litlabø deposits feature mineral assemblages predominantly composed of Fe-sulfides with minor amounts of base metal sulfides. Both deposits are interpreted as results of exhalative venting, although vastly different depositional mechanisms have been recognized. The basalt hosted Lindøya deposit is composed of a massive layered sulfide ore body associated with sulfides hosted as veinlets and disseminations in basalt. Textural characteristics, close spatial association with the basalt and the sulfur isotopic signature indicating thermochemical reduction of seawater sulfate, suggest a proximal deposition from hydrothermal fluids. In contrast, the Litlabø mineralization is hosted by a volcano-sedimentary sequence predominantly composed of organic-rich shale, jasper and siliciclastic sediments. The mineralization is characterized by an enrichment in a suit of redox sensitive elements, including V, Mn, Mo and U, and by a strong depletion in 34S indicating bacterial reduction of seawater sulfate. Textural, mineralogical and stable isotope features suggest that the Litlabø mineralization was deposited under anoxic/euxinic conditions in a restricted ocean basin.Masteroppgave i geovitenskapGEOV399MAMN-GEO