Trace element geochemistry of Magnetite from Iron oxide-apatite (Kiruna-type) and magnetite skarn deposits: A discriminant of deposit type and genesis?

Iron oxide apatite (IOA) deposits and the related iron oxide-copper-gold (IOCG) deposits, are major repositories of base metals (Fe, Cu). However, the genesis of IOA deposits remains a topic of debate, with both magmatic and hydrothermal models. Close parallels exist between IOA deposits and some skarns, which exhibit sodic alteration in silicic host rocks, but are unequivocally metasomatic in origin. In this study we compared the trace element chemistry of magnetite from IOA deposits in the Kiruna District, Sweden, with magnetite skarns from the Turgai district, Kazakhstan. Comparison with published discrimination diagrams for deposit types shows poor correspondence with defined fields. The two districts correspond closely in terms of Sn and Ga contents, with close correspondence to previous analyses of porphyry and skarn deposits. When estimates of temperature (T) from Mg in magnetite are considered Sn and Ga show little correlation with T, whereas Ni increases and Mn decreases with decreasing T. Rare earth element patterns correspond to local igneous rocks, albeit at lower absolute concentration. Tin and Ga, as high valence ions in tetrahedral sites in magnetite are potentially more resistant to re-equilibration and preserve a high temperature magmatic-hydrothermal signature comparable to Fe skarns and the early magmatic stages of some IOA deposits in the Kiruna district. Overall, the data are consistent with an early high temperature mineralisation stage, potentially resulting from hypersaline brines or salt melts interacting with volcanic rocks (Kiruna district) or limestone and volcanic rocks (Turgai district), followed by subsequent hydrothermal magnetite mineralisation to relative low T. The high temperature stage is better represented in the Turgai skarns compared to the Kiruna district IOAs. Overprint of sulphide mineralisation on magnetite results in an increase in Ni content which may be an effective tracer for IOCG mineralisation related to IOA deposits, or sulphide mineralisation in skarns, whilst metamorphism may homogenise and reduce trace element concentrations.