Investigation on the genesis of the iron oxide- apatite ± REE deposits of the Bafgh-Saghand district (Central Iran), based on oxygen isotope studies

The metallogenic zone of Bafgh-Saghand in central Iran hosts huge low-titanium iron oxide-apatite (IOA) deposits (also called Kiruna type iron deposits) with more than 1500 Million tons grading 55% iron. The genesis of these deposits including Chadormalu, Choghart, She-Chahun, and Esfordi has long been a subject of debate. In this regard, several hypotheses such as magmatic, hydrothermal, carbonatititc, BIF, and sedimentary-exhalative have been proposed so far. In this study, 20samples of the magnetite ore from the deposits of Chadormalu, Choghart, She-Chahun, and Esfordi were selected and analyzed for their oxygen isotope values. Based on the analyses results, the variations of δ18O values in the deposits are meaningful and result from the characteristics of the ore forming processes involved. The δ18O values of the analyzed magnetite samples range from -0.1 to +2.2‰ and indicate the role of both orthomagmatic (>0.9‰) and hydrothermal (<0.9‰) processes in the formation of these deposits. On the other hand, the values lower than +0.3‰ can be attributed to secondary oxidation or hydrothermal processes and/or a combination of both. The oxygen isotope data of the investigated samples are identical to the deposits such as El Laco of Chile, Kiruna and Grängesberg of Sweden, and Zhibo and Chagangnuoer of China with a magmatic-hydrothermal genesis. According to the geological and analytical evidence obtained from the iron oxide-apatite deposits of the Bafgh-Saghand area, first a tonalite-trondhjemite-granodiorite, diorite, and granite magmatism related to a continental margin subduction at 533 to 525Ma has caused a magmatic mineralization of iron in the area, while a later hydrothermal process related to an alkaline intrusion (syenite and monzosyentie) has caused a hydrothermal mineralization. Therefore a magmatic-hydrothermal source can be suggested for the formation of the low-titanium iron oxide-apatite deposits of the Bafgh-Saghand area.