Absence of hydrothermal oxygen isotope variations in host rocks supports magmatic origin of the giant Grängesberg iron oxide–apatite (IOA) deposit, Central Sweden

Abstract The origin of Kiruna-type iron oxide–apatite ores is controversial, and debate presently centres on a ‘magmatic’ versus ‘hydrothermal’ mode formation. To complement recent investigations the Grängesberg ore deposit in northwestern part Palaeoproterozoic Bergslagen province central Sweden, we investigated oxygen isotope composition host rocks this large body. As metavolcanic metagranitoid country around body either pre-date or are coeval with formation, they would be expected to record an extensive isotopic imprint if had formed by large-scale hydrothermal processes involving externally sourced fluid. A direct magmatic formation process, turn, have produced localized alteration only, concentrated immediate vicinity Here, test these two hypotheses assessing variations main We analysed isotopes quartz from ( n = 17) 14) body, up 2 km distance along across strike Remarkably, find no significant variation δ 18 O values any deviations rock common and/or regional values. Only samples show shifts more negative than range, indicating highly overprint only. large-scale, low-temperature through voluminous fluid percolation left distinct rocks, our results consistent ortho-magmatic for ore.