The role of evaporites in Kiruna-type iron oxide-apatite deposits in the Northern Norrbotten region, Sweden: Evidence from in situ halogen analyses of scapolite

Hydrosaline liquids or salt melts have recently been suggested to play a key role in forming Kiruna-type iron oxide-apatite (IOA) deposits, with sedimentary evaporites proposed as contributors to ore-forming processes. However, the timing and role of evaporites in ore formation are poorly understood. Scapolite minerals, typical products of sodic alteration resulting from the interaction between hydrosaline liquids (also called highly saline fluids) and country rocks, may provide key insights into ore genesis. Here, we measured in situ Cl/Br ratios of scapolite from two representative IOA deposits (Mertainen and Gruvberget-Fe) and the Aitik porphyry Cu system from the Northern Norrbotten region of Sweden to trace the sources and evolution of ore-related liquids. Based on mineral assemblages, scapolite grains are classified into Scp I and Scp II. Scp I, formed during pre-ore or syn-ore alteration, exhibits distinct molar Cl/Br ratios (510–2000) across domains and grains. This variation implies multiple, though dominantly magmatic, sources for the associated liquids. Zoned Scp I crystals show relatively low molar Cl/Br ratios of 510–790 in their cores, increasing to 730–1640 in the mantles and/or rims. This trend suggests that the grains initially equilibrated with dominantly pore waters (bittern brines and/or seawater) but were later primarily buffered by liquids of magmatic origin. Scp II, associated with hematitization, have much higher molar Cl/Br ratios (2300–3500) and likely precipitated from halite-bearing evaporite-dissolution fluids derived from the Kiruna Greenstone Group. Our findings suggest that evaporite components were introduced into the studied IOA systems primarily through fluid circulation triggered by magmatic intrusion. However, later-stage evaporite dissolution fluids overprinting the ore system may create the misleading impression that evaporite-derived fluids played a key role in ore genesis.