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

Abstract

The 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.