Mineralogy, geochemistry, and petrogenesis of iron oxide-apatite ores in the Bafq mining district, Central Iran: Proposed a new tectonic setting for mineralization

The Posht-e-Badam block (PBB), as a part of the Central Iran microcontinent zone, hosts many large Kiruna-type iron oxide-apatite (IOA) deposits. The Late Neoproterozoic–Early Cambrian volcano-sedimentary sequence of the Rizu series hosts intrusive rocks and IOA mineralization. The ore occurrences are observed as stratiform and epigenetic forms in the Bafq region. Stratiform FeMn oxide ores are commonly associated with brownish dolomite layers, or is intercalated with jaspilite in association with submarine basaltic lava flows and tuffaceous shale formed by direct precipitation from the ore-forming fluids during submarine exhalative/or submarine volcano-sedimentary processes on the seafloor. Epigenetic IOA ores occurred as massive, lenses, vein-veinlets, and brecciated forms in the metasomatic host rocks. Various generations of magnetite and apatite are observed in IOA ores in this region. Magmatic generation of magnetite (Mag 1) and apatite (Ap 1) comprises >95 vol% of the massive and lenses ores. Hydrothermal generations of magnetite (Mag 2 and Mag 3) and apatite (Ap 2, Ap 3, and Ap 4) are as vein-veinlets, open space-fillings and brecciated forms, and accompanied by actinolite, hematite, calcite, and quartz. Hydrothermal alterations associated with the IOA ore deposits in the Bafq region include sodic, actinolitic, sericitic, argillic, silicic, and carbonatization. All of these alteration types resulted from intrusion of alkaline magmatic bodies into the Rizu series in an extensional regime (rift setting) governing the area. Based on magnetite and apatite contents, the Bafq deposits are classified into magnetite-rich, magnetite-apatite, and apatite-rich deposits. Enrichment of LREE relative to the HREE and the strong negative Eu anomaly are the important geochemical signatures of the Bafq IOA deposits. Also, geochemical data shows that the all samples from the IOA deposits in the Bafq region fall into the field of the Kiruna-type IOA deposits. The δ¹⁸O values of most minerals (+1.1 to +7.6 ‰) represent the magmatic-hydrothermal (δ¹⁸O > +0.9 ‰) process, while δ¹⁸O values below +0.9 ‰ indicate a low-temperature hydrothermal alteration and a degree of secondary oxidation process. Comparing the ages of igneous rocks and mineralization shows the genetic association of primary iron oxide-apatite deposits (~543 to 490 Ma) with co-magmatic gabbro, diorite, quartz diorite and granites (~547 to 525 Ma), while the secondary dark apatites and monazites (~440 and 437 Ma) are correlated with syenite intrusions in the Late Ordovician-Early Silurian (~452 to 430 Ma). These events are consistent with magmatism and sedimentation in a regionally extension and basin development in the Late Neoproterozoic-Early Cambrian to Silurian in the Bafq region. Our results suggest that the Bafq IOA mineralization formed in a post collisional extensional rift setting resulting from relaxation and then extension of the central Iran microcontinent after subduction of the Proto-Tethys oceanic lithosphere under the northern margin of the Gondwana supercontinent during the Late Neoproterozoic-Early Cambrian and its continuation to the Silurian.