Mineralogy, geochemistry, and genesis of the Chahgaz (XIVA Anomaly) Kiruna-type iron oxide-apatite (IOA) deposit, Bafq district, Central Iran

Abstract

The Chahgaz iron oxide-apatite (IOA) deposit is one of the main IOA deposits in the Bafq metallogenic province, Central Iran. The Chahgaz mineral deposit is hosted by Early Cambrian felsic to intermediate, altered subvolcanic to effusive rocks that range compositionally from granite to diorite. Geochemical, geochronologic and tectonomagmatic investigations of various host rock types in the Bafq province indicate that mineralization was the product of Early Cambrian active continental margin processes that evolved calc-alkaline felsic igneous rocks followed by formation of diabase dykes in a back-arc basin environment. Magnetite is present in massive magnetite-rich ore bodies and veinlets that cut the massive ore bodies. Detailed macro- and micro-scopic characterization of mineralized samples and host rocks reveals a paragenetic sequence containing three generations of magnetite that are distinguished from one another compositionally and texturally. The massive ores contain apatite in trace amounts, consistent with IOA deposits globally, and locally exhibit textures that are visually similar to lava flow structures, as described for the El Laco IOA deposit, Chile. The ore bodies contain miarolitic cavities that are filled by calcite, hematite and quartz. The host rocks for the Chahgaz deposit have undergone widespread hydrothermal metasomatism including Na-Ca, K-, Mg-, Si-, sericitic, argillic and carbonatization alteration. The compositions of two generations of magnetite, referred to as Mag1 and Mag2, in massive ore overlap compositions reported for igneous and high-temperature magmatic-hydrothermal magnetite. The third generation of magnetite, which is referred to as Mag3 and is present in veinlets cross-cutting the massive magnetite ore bodies, overlaps compositions reported for low to moderate temperature magmatic-hydrothermal magnetite. Pyrite is present as disseminated grains coeval with Mag1 and micro-fracture filling in the massive magnetite-rich ore bodies.The δ18O values obtained for magnetite from representative samples of massive magnetite Mag1 ore range between 2.18 and 6.32‰ and are consistent with δ18O values reported for igneous and magmatic-hydrothermal magnetite from other deposits in the Bafq district and globally. The δ34S values for pyrite range from 22.54 to 24.94‰ and are consistent with an evaporitic sulfur source; plausibly by magma contamination with evaporitic rocks of the Early Cambrian Volcano-Sedimentary Sequence (ECVSS). The data presented here are consistent with formation of the massive magnetite-rich ore bodies in the Chahgaz IOA deposit by an iron-rich magmatic-hydrothermal fluid.