Mineralogy, Geochemistry, and Fluid Inclusion Microthermometry of Apatite and Rare Earth Element Minerals in the Esfordi Deposit, NE of Bafq, Yazd Province

REE should be evaluated in rocks and minerals due to their behavior in complex geochemical processes leading to their use as tracers in geochemical environments. In addition, the lack of economic concentration of these elements is related to their contribution in rock forming minerals. Thus, high levels of technology and costs are essential for their extraction. However, REE minerals can be formed under special geological conditions. In this regard, the Esfordi iron-phosphate deposit is interesting both economically and scientifically. The present study is aimed at determination of rare earth element mineral types, along with their occurrence in this deposit by using mineralogy, geochemistry, and fluid inclusion microthermometry methods.

Method of study:

A total of 42 apatite samples were taken from different lithological units and ore-bearing veins. Following petrographic observations, 10 and 6 representative samples were analyzed using SEM and XRD methods in the Iran Minerals Processing Research Center, respectively. Geochemical properties of apatite and rare earth element minerals were determined on 8 samples using LA-ICP-Ms at the University of Tasmania, Australia. Moreover, the same was done for 6 samples using EPMA at the Geo Forschungs Zentrum Telegrafenberg of Potsdam University, Germany. In addition, 12 samples of apatite were considered for evaluating petrography of fluid inclusions. Microthermometry of the fluid inclusions was conducted on two second generation apatite samples associated with massive phosphate mineralization zone and magnetite mineralization zone in the laboratory of the Geological Survey and Mineral Explorations of Iran. Phases changes in fluid inclusions in heating and freezing tests under a Linkam THM600 microscope with TP94 Thermal Controller and LNP Type Cooler mounted on Zeiss microscope, with an accuracy of ±0.5 ˚C was performed. Given that no phase changes were produced in some inclusions (melt inclusions) up to the temperature of 600°C, two samples of apatite were studied using the Linkam TS1400XY microscope in Lithosphere Fluid Research Lab of the Department of Petrology and Geochemistry at Eötvös Loránd University, Budapest, Hungary.

Mineralogical studies of apatite in Esfordi revealed the extensive presence of monazite and a lesser amount of xenotime. The results indicate two generations of monazite in this deposit. The first generation is observed as inclusions within the apatites, while the second generation occurs along the fractures of apatites. Monazite inclusions are abundant in the dark phase of host apatites. Based on geochemical data, the second generation of monazite is enriched in La, La/Ce, Nd, and Pr compared to the first generation. Furthermore, strong negative correlation coefficients were observed between Ca, P, and ΣREE, while a positive correlation was reported between Si and P in apatite and monazite. Chondrite normalized spider diagrams indicate a negative slope (LREE/HREE>1) in all of the samples. Moreover, a strong negative correlation was observed between Sr and Y in the studied apatite and monazites. Fluid inclusions within the apatites were classified into eight groups: a) one-phase gaseous inclusions, b) one-phase liquid inclusions, c) two-phase liquid rich inclusions (L+V), d) two-phase gas-rich inclusions (V+L), e) two-phase liquid- solid inclusions (L+V), f) three-phase inclusions (V-L-S), g) three-phase CO2 bearing inclusions associated with formation of clathrate, and h) melt inclusions. The composition of the fluid inclusions is plotted in magmatic and hydrothermal fields. The salinity of most of the inclusions is low to medium (5-21 wt.% NaCl) and homogenization temperature ranges from 250 to 350˚C. Also, a limited number of fluid inclusions were homogenized in the range of 378-486 ˚C, indicating high salinity (43 to 54 wt.% NaCl). The fluid trapping depths were measured to be in the range 100-1700 m.

The Esfordi iron-apatite deposit is located NE of Bafq, Yazd province and it hosts three types of apatite mineralization in massive, vein, and disseminated forms, as well as REE-bearing minerals. Periodic variations in mineralizing fluid is evidenced by changes in REE content of the studied minerals. The presence of monazite in dark phases of the host apatite mineral indicates leaching of REE from the host apatite and redeposition during the nucleation of monazite grains (Heidarian et al., 2017). Mineralogical data indicated that the apatites are of the fluorapatite type with minor contents of chloride (Rajabzadeh et al., 2013). The quantities of Sr and Y in the studied minerals indicate a strong negative correlation, consistent with magmatic differentiation. In addition, the concentrations of Mn, Sr, and Y support the granitoid origin of the Esfordi deposit (Belousova et al., 2002). Microthermometric data plotted on magmatic and hydrothermal fields indicated that mixing fluids and boiling are the important factors in mineralization. Upon the obtained data of the present study, main parts of the Esfordi iron phosphate deposit have been formed at temperatures ranging from 146 to 486˚C and depths of 100 to 1700 m.