Formation of the Chah-Gaz iron oxide-apatite ore (IOA) deposit, Bafq District, Iran: Constraints from halogens, trace element concentrations, and Sr-Nd isotopes of fluorapatite

Abstract

The textures, chemistry and Sr-Nd isotopic compositions of apatite from the Chah-Gaz iron-oxide apatite (IOA) deposit in the Bafq metallogenic belt, central Iran, were studied to investigate the formation of this ore deposit. Two generations of apatite were recognized based on Cl/F and Cl/OH ratios. Primary fluorapatite, which is coeval with magnetite in the massive ore bodies, is chemically homogeneous and characterized by Cl/F < 0.05 and Cl/OH in the range of 0.01–0.07. By contrast, F-depleted apatite rims are present in apatite hosted in veinlets that crosscut the massive ore bodies and are disseminated in the igneous host rocks, and have Cl/F and Cl/OH-apatite ratios of 0.08–0.12 and 0.15–0.79, respectively. The F-depleted rims are also depleted in LREEs, Th and U, consistent with the presence of secondary monazite, xenotime and thorite that formed by coupled dissolution-reprecipitation. The whole-rock Nd–Sr isotopic data ((87Sr/86Sr)(I) = 0.7052 to 0.7064 and ɛNd(t) = +1.3 to + 2.7) of gabbro-diorite indicate an mantle source, while the high -K, calc-alkaline-shoshonitic volcanic host rocks have εNd(t) = − 5.5 to − 7.6, clearly reflecting mixing between mantle-derived mafic magmas and assimilated Proterozoic basement. The initial 87Sr/86Sr ratios and ɛNd values of both F-rich and -depleted apatites (0.7038 to 0.7050 and −0.3 to + 6.5, respectively) are similar to gabbroic rocks and support a magmatic source for primary fluorapatite, with minimal or no crustal contribution, and indicate that the magmatic event with negative ɛNd values did not affect the whole-rock Sm-Nd signature of the ore. Petrography plus geochemical and Nd-Sr isotopic data of both studied fluorapatite, which come from iron oxide stage, are consistent with a combined igneous/magmatic-hydrothermal genesis for the Chah-Gaz IOA deposit, with low degrees of hydrothermal overprint, as evidenced by the formation of F-depleted (Cl-rich) apatite rims. The relatively constant Sr-Nd data in apatite are consistent with superimposed, episodic hydrothermal fluids from the same, evolving, magmatic-hydrothermal system.