Two-tiered magmatic-hydrothermal and skarn origin of magnetite from Gol-Gohar iron ore deposit of SE Iran: In-situ LA-ICP-MS analyses

Genesis of magnetite from Gol-Gohar iron deposit (GGID) of southeastern Iran is studied with reference to elemental composition. More than 1.2 billion tonnes of magnetite ore at 58% total Fe is hosted by greenschist- to amphibolite-facies metapelites and magnesian marbles. The GGID occurs in the mainly-metamorphic terrain of Sanandaj-Sirjan geological province, with continental-margin tectonic setting. The magnetite-bearing ore is divided into two distinct types of massive and banded; the latter hosted mainly by magnesian marbles. Both subtypes of the ore bodies are associated with mineral assemblages consisting of calcite, dolomite, forsterite, tremolite-actinolite, talc, serpentine, and chlorite. The ore minerals in both subtypes are magnetite, hematite with minor amounts of pyrite, pyrrhotite, and chalcopyrite. Concentrations of temperature-dependent elements, Ti, Al, and Ga in the two subtypes would suggest that temperature of formation was higher for the massive magnetite (with Ti = 626 ppm, Al = 69 ppm and Ga = 56 ppm, on average) and relatively lower for the banded magnetite (with Ti = 60 ppm, Al = 62 ppm and Ga = 4 ppm, on average). The multi-element diagrams, for massive magnetite, show patterns closer to those of magmatic-hydrothermel iron deposits (Kiruna-type), whereas, patterns of the banded magnetite match better with those of skarn deposits, particularly the magnesian skarns. Average concentrations of V ( > 1000 ppm) and Cr (similar to 4 ppm) of the massive subtype fall within the range of those of Kiruna-type deposits, and the in average lower concentrations of V (similar to 10 ppm) and Cr (similar to 1 ppm) of the banded magnetite match the values of skarn deposits. The average V content of the massive magnetite (1342 ppm) is consistent with the global range of magmatic-hydrothermal magnetite (up to 3880 ppm), but different from global values of purely-magmatic magnetites (up to 6600 ppm). The banded magnetites contain 0.21-25 ppm V, which is only consistent with the lowest values of hydrothermal magnetites. We contend that the massive magnetite was formed by magmatic-hydrothermal processes (i.e., Kiruna type) and the banded magnetites were generated via metasomatic reactions between iron-rich residual magmatic fluids and the magnesian carbonate host rocks (i.e., skarn type).