Evolution of ore forming fluids at the St. Jonsfjorden Au-Cu prospect, Svalbard

Abstract

The southern part of St. Jonsfjorden hosts Au and Cu mineralization in epigenetic quartz carbonate veins that cross-cut Ordovician-Silurian sediments and are localized along an E-W orientated thrust fault. The mineralization can be traced along the thrust fault, and it is enriched in Cu at Copper Camp (78°29'53.891'' N, 12°44'9.366'' E, 40 m a.s.l) and in Au at Holmesletfjella (78°28'49.091'' N, 12°52'40.109'' E, 663 m a.s.l). Adjacent to the mineralization, lenses of listwanite were discovered. The St. Jonsfjorden deposit shares characteristics similar to those of orogenic Au and Carlin-type deposits. Textural features, ore grade analysis, trace element composition of individual sulfide phases (in-situ LA-ICP-MS analysis), stable isotope analyses (δ13C, δ18O, δ34S), and fluid inclusion study have been combined to gain insight into the geochemical characteristics of the mineralization, P-T-X conditions of metal deposition, and the ore deposit genesis. The mineralization reflects formation during the late stages of the Caledonian orogeny, where compression and local extensional forces could have operated. Despite widespread re-equilibration, preserved fluid inclusions together with abundant anhydrite and halite solid inclusions revealed that oreforming fluids had a high salinity and an oxidative character, suggesting that chloride complexes initially transported metals. The δ34S values of sulfides (up to 20.4‰) suggest that sulfur, and presumably fluid salinity, were gained by leaching of evaporite lithologies. The Cu mineralization at Copper Camp was deposited due to the reduction of oxidized metal-bearing fluids during their interaction with organic-rich sediments and is supported by high δ34S and co-existing CO2 and CH4 in inclusions. The reduction resulted in the conversion of sulfate to sulfide species, promoting the stabilization of Au-bisulfide over Au-chloride complexes, causing Au to be mobilized further up in the system. The sulfidation of sediment-hosted Fe phases in the Holmesletfjella formation has been recognized as the prevailing mechanism for depositing Au-bearing pyrite. Adiabatic boiling caused by a decrease in confining pressure during the fluid ascent may have contributed to the deposition of minor amounts of native Au. Carbon isotopes revealed that the majority of CO2 was sourced from marine carbonates. The ultramafic protolith of listwanite is suggested as the most suitable source for Cu, Au, and As, while organic-rich sediments could have contributed with trace elements, including Cd, Hg, Mo, and V.