Mineralogy and geochemistry of REE minerals in Esfordi alkali-magmatic phosphate mine, Bafgh, Central Iran

Nazary M.*, Khajo M.**

*Department of Geology, Islamic Azad University, Ashtiyan Branch, Ashtiyan, Iran; **Department of Geology, Education district No.15, Ministry of Education, Tehran, Iran

The Bafgh district of Central Iran is the most important Fe metallogenic province in the region and a significant district on a worldwide basis. Esfordi is a Kiruna-type Fe–P oxide deposit and one of the most P-rich of the Bafgh magnetite–apatite occurrences. It formed within a predominantly rhyolitic volcanic sequence that formed in a continental margin tectonic regime and is of Cambrian age. The spatial association of the Esfordi ore body with alkaline extrusive igneous rocks clearly indicates a genetic relationship between alkaline magmatism and iron-oxide ore formation. It has been suggested that alkaline magma is the ultimate source of Fe-P-rich melts, which have been derived from it by liquid immiscibility. The field relationships and the overall geological evidence at Esfordi indicate that the rhyolitic host rocks, the products of intracratonic magmatism and a tensional geological setting (rifting), were roughly synchronous and coeval with mineralization.

The Esfordi open-cut mine currently produces 103,000 tones of apatite concentrate a year, from a resource of 17 Mt with a grade of 13.5 wt % P2O5. Aside from the great resource and high grade of apatite ore, the economical concentration of REE, make Esfordi the most important potential of REE in the Bafgh district too. The system is enriched in REE (~ 0.2 – 1.5 %) with an associated array of accessory REE minerals. Routine, high quality REE assay data were not available from the mine operation, but semi quantitative emission spectrographic data from isolated drill holes, together with ICPMS data in the current investigation indicates that total REE contents broadly follows P levels. The majority of the REEs are contained in apatite. However, some of the REE in apatite is probably contributed by REE-bearing microscopic solid inclusions.

More than 25 samples from the different ore types, host rocks and igneous bodies were analyzed for REE by ICPMS following microwave bomb HF/HClO4/HNO3 digestion and the major elements determined by X-ray fluorescence (Philips PW1400 XRF sequential spectrometer). The REE composition of various mineral phases from 21 polished thin sections has been investigated by electron microprobe. Other than apatite, there are six different minerals with significant REE contents at Esfordi which show systematic relationships in the paragenetic stages of Fe-oxide-P mineralization and considered in three groups; phosphates, carbonates and silicates. Monazite, allanite, xenotime, britholite, further more synchysis and bastnasite as minor phases, are minerals which have been identified by electron microprobe analyses supported by optical methods and limited XRD:

,Allanite is the most abundant REE mineral in the Esfordi deposit. Allanites in this study are allanite-(Ce), characterised by higher La/Nd and La/Y ratios and also (La/Sm) and (La/Y) ratios. Microscopic studies and BSE images of Esfordi allanites show a zonation in some grains of allanite. Electron microprobe analyses show that the core is more enriched in LREE than the rim in them. Britholite always occurs in brecciated apatite-rich zone associated with allanite and monazite. Due to similarities between britholite and allanite, recognition of britholite by optical techniques was cross-checked by electron microprobe point analyses.The britholites are enriched in HREE and Y. Monazite is one of the most abundant REE mineral at Esfordi and has a close relationship with apatite. Xenotime occurs as a minor phase, mainly in the brecciated apatite-rich ore as individual grains in the matrix. The Esfordi xenotime have traces of SiO₄^4- replacing PO₄^3- , also minor substitution of U⁴⁺, Th⁴⁺ and Si⁴⁺ for Y³⁺ and Ca²⁺ for P³⁺ is indicated. šThe fluorocarbonates synchysite and bastnaesite at Esfordi are mainly observed in the brecciated apatite-rich ore and apatite-rich veins in altered host rocks. They are always observed in close relationship to the late pervasive carbonate alteration and almost always occur in carbonate-rich matrix, replacements and veins. The Esfordi synchysites are synchysite-(Ce) with high La and Nd and bastnaesites are bastnaesite-(Ce) which is the most common form of bastnaesite.

Esfordi apatites are generally dominant in the LREE similar to that of many Kiruna type magnetite apatite systems. Based on field observations, petrographic and geothermometric studies, three different types of apatite were identified. The first two generation of apatites have been affected (dissolved, leached and replaced) by subsequent brecciation and late stage carbonate and quartz veining. Apatite 1 crystals has inclusions of monazite 1 within crystals as 5 to 50 μm. A crude zonation may be observed in some apatite 1 grains with mineral (including monazite 1). Apatite 1 contains S(La,Ce,Nd) up to 1.74 wt % with an average of 1.43 wt %. Rare earth mineral inclusions within apatite 2 are rare, however intergranular monazite 2, xenotime, and allanite 1 are common. Apatite 2 shows an average of 1.19 wt % S(La, Ce, Nd). Apatite 2 is richer in Cl than apatite 1, averaging 0.43 wt %. Granular apatites contain S(La, Ce, Nd) up to 0.51 wt % with an average of 0.22 wt %. Apatites 3 crystals are without any solid inclusions and low S(La,Ce,Nd) content (averaging 0.11 wt %). Generally, Esfordi apatites are F-rich and relatively low in Cl (< 0.91 wt %) and OH content.

The LREE content of apatites is strongly correlated with corresponding content of both Na and Si. The REE abundances in minerals are largely controlled by substituting for Ca positions (McKay 1989). The selectivity of Ca-bearing minerals for REE is largely depends on the size of the Ca positions in mineral structure relative to the REE. The REE³⁺ and Y³⁺ ions substitute for Ca²⁺ with electrostatic balance provided by substitution of Si⁴⁺ for P⁵⁺ or Na⁺ for additional Ca²⁺ (Pan and Fleet 2002);

Na⁺ + (Y + REE³⁺) = 2Ca²⁺

Si⁴⁺ + (Y + REE³⁺) = P⁵⁺ + Ca²⁺

When the REE content of Esfordi deposit is compared with that of the Kiruna ores, it displays a stronger LREE/HREE fractionation and a large Eu depletion, probably indicating a relationship to alkaline parent magma, in turn suggestive of a rifting environment. Taylor and Fryer (1983) suggest that LREEs are preferentially mobilized in Cl-rich fluids, whereas F and CO2-bearing fluids are rich in HREE. Esfordi apatites are F-rich and relatively low in Cl (< 0.91 wt %) and OH content. Thus, it is likely that HREE where selectively leached from Esfordi apatite by F and CO2-rich late fluids, giving rise to the observed high LREE/HREE ratio.

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