Th-REE mineralization and associated hydrothermal alteration in the Chahgaz iron oxide- apatite (IOA) deposit, Bafq District, Central Iran

The mineralogical, chemical and isotopic characteristics of Th-REE mineralization from the Chahgaz Kiruna- type IOA deposit in the Bafq district were investigated to determine the origin and evolution of mineralization. This deposit formed in a continental arc setting during the Early Cambrian subduction of the Proto-Tethyan oceanic plate beneath the Central Iranian microcontinent. The Th-REE mineralization was superimposed on the peripheral parts of iron ore bodies, coeval with second-stage of Na-Ca and Mg metasomatism. Optical microscope and FE-SEM studies reveal that Th and REE host minerals are thorite, zircon, allanite, bastnaesite, xenotime, synchysite and titanite. Geochemical data reveal LREE (La-Sm) enrichment with pronounced negative Eu anomalies. The presence of magnetite and pyrite accompanied by thorite, as well as negative Eu anomaly are indicative of reduced conditions prevailing during deposition. Considering calcite paragenesis with thorite and REE minerals, Th and REE were transported together as soluble carbonate complexes in the alkaline mineralizing hydrothermal fluids. The formation temperatures of Th-REE mineralization range from ∼370-400°C for second-stage Na–Ca metasomatism, using the albite-actinolite oxygen isotope pairs. The δ D and δ 18 O compositions of actinolite are in accordance with a magmatic fluid originated from felsic magma, mixed with less meteoric water generating the Chahgaz Th-REE mineralization.