جستجوی مقالات مرتبط با کلیدواژه "مونازیت" در نشریات گروه "زمین شناسی"

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.

The Esfordi deposit is located at northeastern Bafq and is one of the well-known magnetite-apatite deposits in the area which consists of ore minerals hosted REE. Among the various ore minerals, apatite is one of the unique minerals because of its significant properties such as providing a budget of elements (especially in the case of REE), stability over widespread temperature and pressure domains and exclusively ionic interchanges as a respond to re-equilibrium with new environment. All of which can fit the apatite as a main source feeding the new mineral nucleation like monazite during a hydrothermal alteration. Based on petrography and geochemistry studies, mineralization of four generation apatite accompanied with dropping of REE amount in each stages and increasing of residual Ca and P in the next generations. In contrast, culminating amount of REE beside Na, F and limited Cl reached in nucleolus monazite and led to the formation of two generation of monazites. On the BSE images of apatite, the depleted areas are associated with micro-channels and micro-pores containing monazite. Consequently dissolution of nucleolus monazites provides a well chance to form the second generation of monazite as bigger grain than the first ones.

IntroductionNowadays, exploration of rare earth element (REE) resources is considered as one of the strategic priorities, which has a special position in the advanced and intelligent industries (Castor and Hedrick, 2006). Significant resources of REEs are found in a wide range of geological settings, including primary deposits associated with igneous and hydrothermal processes (e.g. carbonatite, (per) alkaline-igneous rocks, iron-oxide breccia complexes, scarns, fluorapatite veins and pegmatites), and secondary deposits concentrated by sedimentary processes and weathering (e.g. heavy-mineral sand deposits, fluviatile sandstones, unconformity-related uranium deposits, and lignites) (Jaireth et al., 2014). Recent studies on various parts of Iran led to the identification of promising potential of these elements, including Central Iran, alkaline rocks in the Eslami Peninsula, iron and apatite in the Hormuz Island, Kahnouj titanium deposit, granitoid bodies in Yazd, Azerbaijan, and Mashhad and associated dikes, and finally placers related to the Shemshak formation in Marvast, Kharanagh, and Ardekan indicate high concentration of REE in magmatogenic iron–apatite deposits in Central Iran and placers in Marvast area in Yazd (Ghorbani, 2013).
Materials and methodsIn the present study, the geochemical behavior of rare earth elements is modeled by using multivariate statistical methods in the eastern part of the Marvast placer. Marvast is located 185 km south of the city of Yazd in central Iran between Yazd and Mehriz. This area lies within the southeastern part of the Sanandaj-Sirjan Zone (Alipour-Asll et al., 2012). The samples of 53 wells were analyzed for Whole-rock trace-element concentrations (including REE) by inductively coupled plasma-mass spectrometry (ICP-MS) (GSI, 2004).
The clustering techniques such as multivariate statistical analysis technique can be employed to find appropriate groups in data sets. One of the main objectives of data clustering is to maximize both the similarity within each cluster and the difference between clusters, and finally find the structure in the data. Nowadays, cluster analysis is applied in many disciplines: biology, botany, medicine, psychology, geography, marketing, image processing, psychiatry, archaeology, etc. (Everitt et al., 2011). To execute a partitioning algorithm, the principal components analysis (PCA) algorithm is applied for feature selection, feature extraction and dimension reduction. Hierarchical clustering can be utilized to provide a nested sequence of partitions with bottom-up or top-down methods based on similarity. The single linkage and complete linkage are the most popular hierarchical algorithms (Jain et al., 1999; Ji et al., 2007).
Results and discussionThe REE chondrite-normalized pattern for the eastern area in the Marvast placer represents a high match to the standard pattern of monazite. This pattern shows the positive anomaly of Ce and the negative anomaly of Eu. To determine the distribution of REEs concentration, 2D interpolation maps were plotted in three groups of light, middle, and heavy REEs (LREE, MREE, and HREE), which were indicated in the geochemical anomaly at the south and south-west of the area. The relative ratios of (LREE/HREE) and (Ce/Eu) exposed the high proportion of LREEs to HREEs. In the next section, the hierarchical clustering algorithm was employed to partition the data in the feature and sample levels. The elements portioning demonstrated four separated groups, which can be related to atomic and chemical structures. The studied region was divided into four zones by the clustering approach. The fourth zone confine coincided with the REE anomaly area. Finally, PCA was applied as the multivariate statistical tool to this dataset. Hence three principal components modeled over 90% of the variance. For the first component, the distribution map of load factor has a good agreement with anomaly area.

A total of 50 boreholes have been drilled in the sediments of the study area in south west of Marvast. Based on heavy minerals investingation, the amount of monazite in these sediments rangs from 50 to 525 gram per tons. Analysis of monazites from Marvast and sediment samples with ICP-MS and normalization of some data with upper earth crust and chondrite showed that Marvast monazite enriched in all of REE specially LREE. The source rock of monazite is Upper Triassic black shale which is interbedded with sandstone, limestone and conglomerate. Erosion and weathering of these rocks have released monazite and enriched them in the sediments nearby. Iso-grade maps of  monazite in different depths from 1 to 6 m showed that the strongest and the most widely anomalies are at the depth 1 m, and with increasing the depth, intensity and distribation of the anomalies decreased, as the deepest anomaly has 5 meters thickness. Also, with the simple comparison, a good correlation exists between the presence of anomalies and the thickness of sediments or the depth of bed rock.

Apatite is the most common phosphate mineral in the Esfordi ore body. Euhedral crystal (2-20 cm) of the apatite occurs as an intergrowth in magnetite and hematite, vein and dike. There are two types of apatite in the Esfordi deposit, based on Petrographic studies: primary and secendary. Fresh and altered parts of primary apatite display dark and light color respectively using BSE images. EMP analyses demonstrate that primary apatites (light area) were chlorapatite in composition and have been partially changed into hydroxyle-flourapatite (dark area) by the metasomatic process. Lighter areas represent more Cl, SiO2, Na2O and LREE+Y enriched apatites. Monazite and xenotime inclusions in apatite can be classified into two types: primary (30-100µm) and hydrothermal (5-20µm). The hydrothermal inclusions are found in the dark area, apatite crack and along grain boundaries. The monazite and xenotime inclusions in the dark areas are enriched in LREE and HREE+Y respectively. Monazite-xenotime thermometer yielded a temperature of about 150-350°C for apatite metasomatism and hydrothermal monazite-xenotime formation, coincides with greenschist facies conditions.

The S-type granitoid batholith of Shir-Kuh, which is part of central Iran Zone, is located in SW of Yazd and consists of three main granodioritic, monzogranitic and leucogranitic units. The systematic changes in the composition of plagioclase reveal that the granodiorite have been a calcic core plagioclase-rich magma, the monzogranites is a differentiated melt, and the leucogranite is a late residual melt. Totally, all biotites have high AlVI (3.2 to 6.2 apfu) which is characteristic of peraluminous granites. The high almandine component of garnet is similar to those in other peraluminous plutons and, in particular, to the magmatic garnets. Muscovite appears as both primary and secondary-looking grains. Monazite occurs as two types of chemically crystals: monazite and brabantite [CaTh (PO4)2]. The observed homogeneous grains of Th and U poor monazite and tiny microcrystals of brabantite inside the apatite indicate dissolution of apatite during anatexis of sedimentary-metamorphic rocks. Little hematite (less than 10%) composition, which included within restitic biotite ± silimanite assemblages consistent with the idea that the Shir-Kuh granite is generated from the sedimentary source materials contained graphite. Considering the mineral assemblages, presented in the batholith, the fact that some biotites may represent restite and the mean temperature of 820°C is in agreement with the saturation thermometry; such liquids may have formed at a temperature 750 to 850°C by dehydration melting of biotite.

Monazite is a member phosphate group, and a phosphate of Th and REE. Monazite is a rare mineral and occurs as an accessory mineral in granites, gneisses, aplites, pegmatites and as rolled grains in the sands derived from decomposition of such rocks.Due to the high content of turium and rare earth elements this mineral is the most important and economic mineral in placer deposits.Marvast monazite placer, has been identified through the regional geochemical exploration in Yazd – Sabzevaran zone. Detailed exploration shows that younger gravel fan and recent alluvium contain monazite grains. The grade of monazite in this placer varies from 50-525 g/ton. The reserve of placer is 8866425 t. with a grade of 150 g/ton of monazite. Monazite concentrate samples analysed by mass spectrometry and microprobe techniques. The results show that Marvast monazite enriched in light rare earth elements (Ce,La,Nd,Pr,Eu,Sm) and the content of REE2O3 is about 55.45%. In this placer deposit, Monazite accompanied by hematitized pyrite, epidotes, celestie, cerisite, rutile, magnetite, garnets, ilmenite, pyroxens, amphibols and sometime with zircon and apatite. Black shales are the host rock of monazite in the Marvast area, interbeded with calcareous sandstone, limestone and conglomerate. Monazite nodules, occurs sporadically in shales. Monazite grains are released by weathering and erosion of these rocks and are concentrated in younger gravel fan and recent alluvium.