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Fluid Inclusion and Stable Isotope Study of the Esfordi Apatite-Magnetite Deposit, Central Iran

Morteza Jami, Alistair C. Dunlop and David R. Cohen

School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales 2052, Australia

Corresponding author: e-mail: d.cohen{at}unsw.edu.au

The Esfordi apatite-magnetite deposit is situated in the Bafq district of central Iran. It is one of the most P rich deposits in that district and is hosted by a sequence of early Cambrian rhyolitic volcanic rocks and intercalated shallow-water sediments. The zoned and strata-bound ore has a prominent Fe-oxide–rich core and an overlying body of breccia that is rich in apatite and hematite. The P-rich ore is associated with a proximal body of amphibole alteration within a pervasively altered sequence of volcanic rocks dominated by secondary K-feldspar. There are three major paragenetic stages of mineralization, each with characteristic hydrothermal fluid compositions indicated by apatite textures, fluid inclusion, and isotopic data.

Type 1 apatite is mainly associated with the Fe-oxide–rich (magnetite-apatite) core and displays dissolution textures. The overlying, brecciated P-rich ore is dominated by vein-style apatite-hematite mineralization, dominated by type 2 apatite. Type 1 apatite has homogenization temperatures of 375° to 425°C and indicated salinities of 14.0 to 18.0 wt percent NaCl equiv. The magnetite has ¹⁸O values of –0.1 to 1.7 per mil, suggesting precipitation from fluids with a dominantly magmatic source with ¹⁸O of 7.8 to 9.6 per mil at ~400°C. Type 2 apatite contains fluid inclusions that homogenize between 195° and 295°C, with indicated salinities of 13.0 to 19.0 wt percent NaCl equiv. The hematite associated with this assemblage has ¹⁸O values of –0.2 to 2.3 per mil, which would be in equilibrium with fluids having ¹⁸O values of 10.7 to 13.2 per mil at ~250°C. These isotopically heavy fluids could have evolved through interaction of magmatic fluids with cooler saline fluids in the underlying and adjacent carbonate-rich sequences. Late-stage quartz-carbonate assemblages contain type 3 apatite and quartz with fluid inclusions that homogenize at 145° to 155°C and ¹⁸O values of 16.0 to 17.1 per mil, corresponding to fluids having ¹⁸O values of –0.7 to –2.1 per mil. These late fluids may have been derived from the introduction of a cooler, less saline and isotopically depleted fluid such as seawater.

The fluid inclusion and stable isotope data indicate an evolving hydrothermal system involving at least three different fluids, ranging from an early, moderately hot and saline fluid through progressively cooler and more dilute fluids similar to those reported from other Kiruna-type deposits in the Bafq district.