Origin of Mineralizing Fluids in the Choghart Magnetite-Apatite Deposit, NE of Bafq: Evidence from Mineralogy, Geochemistry, Microthermometry, Stable (O-H and O-C) and Unstable Isotopes (U-Pb and Nd-Sm)

Introduction The Bafq mining district (BMD) in the Kashmar-Kerman tectonic zone hosts ~34 magnetite-hematite-apatite ore deposits with about 1500 Mt of reserves in the central Iran (Torab, 2008). The mineralization has predominantly occurred in the Early Cambrian volcano-sedimentary rocks of Esfordi Formation. It consists of rhyolite, rhyodacite, spilitic basalt, and dolostone, which are intruded dominantly by the Early Cambrian granitoid intrusions (Torab, 2008). Metasomatism has not only affected vastly the ore deposits and their host rocks, but it has also controlled ore mineralization. This issue has caused ambiguity and a paradox in determining the genesis of these ore deposits. In this research study, sources of mineralization are determined in the Chogart magnetite-apatite ore deposits. The results of this research study constrain the genesis of the Choghart deposits. Moreover, they shed new light on ore-forming processes in similar ore deposits of the Bafq mining district. Methodology In this research study, 87 rock and ore samples were collected from the Choghart magnetite-apatite ore deposits. Mineralogy characteristics of 10 apatite samples and 13 magnetite-apatite ore samples were determined by using the LEO-1400 Scanning Electron Microscope, with a voltage of 19-17 kV and a beam diameter of 20 nA. Moreover, three apatite and three albite samples were examined by using X-ray powder diffraction in the Iran Mineral Processing Research Center. Seventeen magnetite-hematite ore samples were also analyzed using ICP-MS in the Zarazma Company. For microthermometry, one sample of the 1st-generation apatites was analyzed for fluid inclusions in the Geological Survey and Mineral Exploration of Iran. Four apatite samples were studied by the O-H isotope method in the Hungarian Laboratory in Hungary, seven calcite samples were studied by the O-C method in Ottawa, Canada, two apatite sample were studied by the Nd-Sm method in Queensland, Australia, and four apatite samples were studied by the Inductively Coupled Plasma Mass Spectrometry-Laser ablation method in Tasmania University in order to determine the origin of the mineralizing fluid. Results The petrography, geochemistry, and isotope studies on the host rock and magnetite-apatite ores of the Choghart deposit indicate various forms of mineralization, which is associated with extensive metasomatism. The O-H and Nd-Sm isotopes on the 1st-generation apatites and the ratios of Sr relative to Mn of these apatites demonstrate a sedimentary origin, while the presence of ilmenite exsolution texture in the 1st-generation magnetites shows a magmatic origin of the ore-forming processes. In addition, microthermometry of the fluid inclusions in the 1st-generation apatites in the magnetite-apatite ore and the evaluation of the O-C isotopes on the paragenetic calcites with the 2nd- and 3rd-generation albites of host albite-bearing metasomatites indicate a combination of magmatic and high-temperature hydrothermal processes. The results of 206Pb/238U and 206Pb/207Pb isotopic studies on the 1st-generation apatites and 1st- and 2nd-generation monazites inclusions of the 1st-apatites illustrate that monazites are younger than the apatites, which could indicate separation of U-Pb by hydrothermal fluids or formation of apatite ores during a separate phenomenon. Discussion In this research study on the Choghart magnetite-apatite ores and its albite-bearing metasomatic host rocks, three generations of albites, two generations of apatites, two generations of monazites and two generations of magnetite have been identified. The geochemical evaluation of the albites indicates lower REE-Y-Ti of the 1st-generation albites, which are sterile in terms of Th-U, compared to the 2nd- and 3rd-generation albites. The O-C isotope analysis of the paragenetic calcites demonstrated albitization due to the magmatic and high-temperature hydrothermal fluids. In addition, the O-H isotope studies on fluid inclusions of the apatites demonstrate atmospheric origin of the fluids, while the microthermometry results of the apatite fluid inclusions indicate magmatic and hydrothermal origins of these fluids. On the other hand, presence of ilmenite exsolutions in the magnetites and O-H isotope of the magnetites, conducted by Moore and Modabberi (2003) and Majidi et al. (2021) have shown the magmatic origin of the fluids. The Nd-Sm isotopic results of 1st-generation apatites, rhyolitic host rock, and iron-apatite ores indicate different initial contents of their Nd isotope ratios. The similarity of εNd in the 1st-generation apatites with phosphorite of the Soltanieh Formation, along with Sr versus Mn amounts of the apatites reinforce the idea that the apatites could have been derived from these sedimentary phosphorites, whose εNd contents increased due to the high-temperature magmatic and hydrothermal fluids, derived from the granitic intrusions, rhyolitic lavas and magnetite melts. The 206Pb/238U and 206Pb/207Pb dating demonstrate 490-561 million years for the 1st-generation apatites, 511-543 million years for the 1st-generation monazites, 503-528 million years for most of the 2nd-generation monazites, and 112-153 million years for a limited number of 2nd generation monazites. The different isotopic ages of the apatites and monazites shows their formation during long-term and extensive ore-forming processes during later effects of hydrothermal and magmatic fluids. This has resulted in enrichment of apatites in REE, formation of monazite inclusions along the fractures and on the surface of the apatites. Acknowledgements The authors would like to express their appreciation of managers and staff of the Iranian Central Iron Ore Company for their assistance during this research.

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