2017 | 70/1 | 53–69 | 13 Figs. | 6 Tabs. | www.geologia-croatica.hr Journal of the Croatian Geological Survey and the Croatian Geological Society Alkali Metasomatism and Th-REE Mineralization in the Choghart deposit, Bafq district, Central Iran Khalegh Khoshnoodi1,2,*, Mehrdad Behzadi1, Mohammad Gannadi-Maragheh2 and Mohammad Yazdi1 1 University of Shahid Beheshti, Faculty of Earth Science, Department of Geology, Tehran, Islamic Republic of Iran; (*corresponding author: khoshnoodi78@gmail.com) 2 Atomic Energy Organization of Iran, Nuclear Science and Technology Research Institute, Material and Nuclear Fnel Research School, Tehran, Islamic Republic of Iran doi: 10.4154/gc.2017.03 Abstract The Choghart iron oxide-apatite (IOA) deposit is located 124 km southeast of Yazd, in the Bafq district within the Central Iranian microcontinent. The Choghart deposit is hosted by the rhyo- litic rocks of the Early Cambrian volcano-sedimentary sequence (the Esfordi formation). Both host rocks and the orebodies are crosscut by diabase dykes. Tectonically, the Choghart rhyolites represent the continental margin setting and the Choghart diabase dykes formed in the back-arc basin environment, respectively, indicating that the evolution of the Bafq district is associated with subduction of Palaeotethys oceanic crust beneath the Central Iranian microcontinent fol- lowed by formation of continental arc related granitoids and rhyolites and then formation of back- arc basin diabase dykes. Similar to the other subduction-related rhyolites, the Choghart rhyolite is enriched in Th and LREE compared to Ta, Nb, and HREE. The main host minerals of Th and REE in the Th-REE mineralization zone are thorite and sphene. Albitization is the most important alteration aspect related to Th-REE mineralization (mainly Th, La, Ce, Nd, and Y). In addition to albite, Th-REE mineralization is associated with actinolite, au- gite, diopside, minor microcline and orthoclase, plus magnetite, calcite, pyrite, rutile, and minor amounts of chalcopyrite. The negative Eu anomaly in Th mineralization zone, as well as the paragenetic occurrence of magnetite, pyrite and chalcopyrite with thorite suggest that Th-REE mineralization formed in relatively reduced condition. The presence of paragenetic calcite ac- companied by thorite and sphene in the Th-REE mineralization zone indicates that Th and REE were likely transported by the carbonate complexes in the mineralizing fluids. The similarity be- tween the chondrite-normalized REE patterns of the host rhyolite and the Th-REE mineraliza- tion zone suggests that post-magmatic driven fluids of continental margin rhyolitic magma played an important role in Th-REE mineralization. 1. INTRODUCTION The Choghart iron oxide-apatite (IOA) Kiruna type deposit is situated in the central lithotectonic domain of the Posht-e-Badam Block, within the Central Iranian microcontinent and contains the oldest (Late Neoproterozoic) basement of Iran. The Bafq dis- trict as one of the main metallogenic provinces is in the southern section of this lithotectonic domain (Figure 1a) and hosts many mineral deposits, including the iron oxide-apatite (IOA), Fe-Mn exhalative, and Zn-Pb sedimentary-exhalative (SEDEX) depos- its. The Bafq district is also recognized as the Zarigan-Chahmir basin, and many researchers have been studying its ore deposits (DALIRAN, 2002; DALIRAN et al., 2009; FOSTER & JAFAR- ZADEH, 1994; MOHAMMAD TORAB, 2008, NADIMI, 2007; RAJABI, 2008, 2012; RAJABI et al., 2012, 2015; RAMEZANI & TUCKER, 2003; STOSCH et al., 2011; YAGHUBPOUR & MEHRABI, 1997). Some of the iron oxide-apatite (IOA) ore deposits of the Bafq district contain REE-U-Th mineralization. The Bafq iron ore de- posits display a range of mineralization styles including massive ore bodies, veins, stockworks and metasomatic replacements, of which their distribution is considerably different within the indi- vidual deposits (MOHAMMAD TORAB, 2008). The signifi- cance in the Bafq district is the occurrence of Th-REE (in the Choghart mine) and, REE-U-Th mineralization (anomaly 5 in the Saghand ore field) in peripheral alteration zones around the iron ore deposits and also the correlation between the areas compris- ing such metals and the distribution of the alkaline metasomatic rocks. U-Mo mineralization in the Narigan ore deposit is pos- sesses, genetically, a granite related origin as an exception (IAEA, 2014) and it is not related to metasomatic processes (DAHLKAMP, 2009). Metasomatic processes are proposed as performing geo- chemical roles in the concentration of REE-U-Th bearing mine- rals in some areas of the world (e.g. the Kirovograd- Krivoi Rog district of the Ukraine, Lagoa Real in Brazil and the Beaverlodge district of Canada, WILDE (2013). Na-metasomatism, among other types of metasomatic processes, are particularly effective in the concentration of such elements. This characteristic of meta- somatism is marked by Na-bearing minerals, which are clearly in replacement relationships with primary magmatic minerals (PIRAJNO, 2009). The metasomatic and hydrothermal processes that occur with aqueous solutions, are the only endogenic pro- cesses in which the content of radioactive elements and primarily uranium reach economic grades TITAYEVA (1994). The rocks affected by this type of metasomatism were first described as Na- granites by TANATAR (1925) or as syenites by FELDMAN (1926). There has been a continuous interest in the association of Th, U and REE mineralization, and this type of metasomatism is related to the importance of albitization in the mineralization of radioactive elements. According to KINNAIRD (1985), Na- enrichment is accompanied by concentrations of Fe, U, Th, Zr, Nb, Sn, Zn and HREE. Na-metasomatised rocks also tend to be Article history: Manuscript received May 23, 2016 Revised manuscript accepted February 09, 2017 Available online February 25, 2017 Keywords: Alkali Metasomatism, Th-REE Mineralization, Choghart (IOA) deposit G eo lo gi a C ro at ic a Geologia Croatica 70/154 enriched in Rb, Th, Nb, La, Ce, Hf, Zr, and Y with respect to K- metasomatised rocks. The mineralization of natural radioactive elements controlled by the occurrence and development of the albitites is well studied in some localities such as the central Ukraine (CUNEY et al., 2012) and the Uranium City region in Canada (DAHLKAMP, 1993). High-temperature hydrothermal Th deposits are characterized by metasomatic replacements, ac- companied by amphibolitization, aegirinization, biotitization, greisenization and filled open cavities (TITAYEVA, 1994). Ob- tained apatite ages confirm that IOA and the apatite-rich rocks (apatites) of the Bafq district formed coevally with the Early Cam- brian magmatic (-metasomatic) events STOSCH et al. (2011). Al- kali-metasomatism in the Chogart iron oxide-apatite (IOA) de- posit occurred in peripheral alteration zones of the iron orebody. It is observed by ground radiometric surveys using an RS-230 spectrometer that the radioactive anomaly of Th occurs at the margin of the Fe-orebody. Since no previous study has worked on Th-REE mineralization, the aim of this paper is to define the mineralogy of Th-REE bearing mineral assemblages and the role of alteration/s and metasomatic processes in their genesis. 2. GEOLOGICAL SETTING OF BAFQ DISTRICT The geological setting of the Bafq district is described by RA- JABI et al. (2015). The crustal domain referred to as the Central Iranian microcontinent is a composite of three major structural zones, from E to W (ALAVI, 1991), the Lut, Tabas, and Yazd blocks (Figure 1a, b). The boundaries are defined by regional- scale faults (Figure 1). In addition, there is the Posht-e-Badam Block (ALAVI, 1991), a fault-bound, variably deformed and metamorphosed complex of supracrustal rocks that separates the Tabas and Yazd blocks (Figure 1). The Posht-e-Badam Block re- presents the area surrounded by three major fault systems (ALAVI, 1991) including the Chapedony, Kalmard and Kuhbanan faults. A Precambrian crystalline basement and Early Cambrian to Ter- tiary sedimentary cover are exposed in the block (FOSTER & JAFARZADEH, 1994). The Precambrian basement consists of medium- to high- grade metamorphic rocks, Late Neoproterozoic in age, grouped in the Boneh-Shurow and Posht-e-Badam complexes (Figure 2; RAJABI, 2008; RAMEZANI & TUCKER, 2003). The bulk of the Posht-e-Badam complex consists of a variable association of greenstones, schists, meta-greywackes, marbles, gneisses, am- phibolites, pyroxenites, serpentinites, meta-basalts and conglo- merates (HAGHIPOUR & PELISSIER, 1968). This complex is ex posed west of the Posht-e-Badam Fault (Figure 2) and is se- verely disrupted by granitic plutons of Triassic age (RAMEZANI & TUCKER, 2003). The Boneh-Shurow complex, which is the most widely exposed metamorphic unit, crops out east of the Posht- e-Badam Fault (Figure 2; HAGHIPOUR & PELISSIER, 1968). This complex is approximately 2000 m thick and is composed of a variety of metamorphic rocks, including quartz-feldspathic gneisses, green mica-schists and amphibolites (AGHANABATI, 2008). Zircon U/Pb dating of this complex indicates an age of 602–617 Ma (RAMEZANI & TUCKER, 2003). The Boneh- Shurow complex is covered by a 2000-m-thick sequence of grey- wackes, quartzites and quartzitic schists, marbles, amphibolites, gneiss, slaty-shales, basaltic lavas, sandstones and arkosic aren- ites, argillites, tuffaceous rocks and limestones belonging to the Tashk Formation, of Late Neoproterozoic to Early Cambrian age, Figure 1. (A) Simplified structural map of Iran (AGHANABATI, 1998) and location of the Bafq district in the Posht-e-Badam Block; (B) Eastern part of the Terranes map of the western Tethysides (RAJABI et al., 2012; modified by after STAMPFLI, 2009; according to BAGHERI & STAMPFLI, 2008). Note the location of the Iran Plate (Iranian Cimmerian terranes) between the Variscan domain in the north and the Arabian Plate in the southwest (ZTZ: Zagros Thrust Zone). G eologia C roatica Khoshnoodi et al.: Alkali Metasomatism and Th-REE Mineralization in the Choghart deposit, Bafq district, Central Iran 55 (RAMEZANI & TUCKER, 2003) that partly metamorphosed to greenschist facies. During the Early Cambrian, granitic plutons intruded the Precambrian sequence of the Tashk Formation, and felsic to in- termediate volcanic and volcano-sedimentary rocks of the Early Cambrian volcano-sedimentary sequence (ECVSS) were depo- sited. The 2000-2500 m thick ECVSS unconformably overlies the clastic sedimentary and tuffaceous rocks of the Tashk Formation. This contact is well exposed to the W and SW of the Narigan area. The ECVSS has also been termed the Cambrian Volcano-Sedi- Figure 2. Simplified geological map of the Bafq district in the Posht-e-Badam Block, (modified after HAGHIPOUR, 1977; KARGARANBAFGHI et al., 2012; RAMEZANI & TUCKER, 2003; RAJABI et al., 2015). Geochronological data from RAMEZANI & TUCKER (2003) and KARGARANBAFGHI et al. (2012). G eo lo gi a C ro at ic a Geologia Croatica 70/156 mentary Unit (CVSU), Rizu-Desu series and Esfordi Formation, in different places between the Posht-e-Badam and Kerman areas (HAGHIPOUR, 1977; HUCKRIEDE et al., 1962; RAMEZANI & TUCKER, 2003; SAMANI, 1993). The ECVSS consists of an unmetamorphosed sequence of interlayered micro-conglome- rates, sandstones, mafic to felsic volcanic rocks, pyritic black silt- stones and shales, volcanoclastic beds and tuffaceous shales, do- lomites and dolomitic limestones (HUCKRIEDE et al., 1962; RAJABI, 2012; RAMEZANI & TUCKER, 2003). This sequence is the host of the most important iron oxide-apatite (IOA) (DA- LIRAN, 2002), Fe-Mn and SEDEX Zn-Pb deposits of Iran (Fig- ure 2). Iron oxide-apatite (IOA) deposits, often referred to as Figure 3. (A) Simplified geological map of the ore body of the Choghart deposit (pit face, 2011; modified after DEHGHAN, 2011); (B) Simplified geological cross section of the Choghart deposit (modified after FOSTER & JAFARZADEH, 1994). Khoshnoodi et al.: Alkali Metasomatism and Th-REE Mineralization in the Choghart deposit, Bafq district, Central Iran 57 G eologia C roatica Kiruna-type iron ore deposits, are known to have formed from the Proterozoic to the Tertiary. They are commonly associated with calc-alkaline volcanic rocks and regional- to deposit-scale metasomatic alteration (STOSCH et al., 2011). 3. GEOLOGY OF THE CHOGHART DEPOSIT The main roughly vertical, discordant, pipe-shaped magnetite- apatite orebody at Choghart, plunging 73°NNW, has been ex- plored to a depth of 600 m, where it seems to interfinger with in- trusive metasomatized and fragmented wall-rock. The thickness of the metasomatic zone differs widely. The orebody is hosted by volcanic members (rhyolitic to rhyodacitic in composition, as well as so-called quartz albitophyre) of the Infracambrian Esfordi For- mation (MOORE & MODABBERI, 2003). It is enveloped by metasomatic rocks with variable thickness. The ore body is crosscut by both diabase dykes and N-S trending normal faults. The iron concentration is not uniform throughout the ore body: the Fe content in the centre of the deposit is greater than 65% whereas it decreases to 45–20% towards the margins (MOORE & MODABBERI, 2003). The ore body is mostly sur- rounded by sodic-calcic hydrothermally altered rocks (Figure 3a, b), with minor albitite (5–30 cm wide). The P2O5 content is very low (<0.2%) and it is enriched in the N-NE of the deposit. N-S, NW-SE and NE-SW trending normal faults cut each other and the ore body. The most well-known outstanding feature is the vo- luminous district-scale sodic-calcic alteration that developed in volcanic hostrocks. The Na-Ca alteration is enveloped as actino- lite, apatite, magnetite, haematite, sodic plagioclase, chlorite, with or without epidote. Within the pervasively Na-Ca altered rhyolite and volcanoclastic rocks, there are locally patches with intense Na alteration with veins and veinlets of albite and a brec- ciated texture (TAGHIPOUR et al. 2013). Apatite is the most abundant gangue at Choghart. It is a distinctive, transparent, yel- low-green fluor-apatite. Fine and coarse-grained apatite occur in Table 1. Major and trace elements (REE, HFSE, LILE) of the felsic and basic rocks of the Bafq district (major oxides in percent and minor and trace elements in ppm; Rhy: Rhyolite, Dia: Diabase, Rhyd: Rhyodacite, Dac: Dacite). Detection limit for major and minor elements <10 ppm and for trace element < 0.1 ppm.*R= RAJABI et al., 2015; R&T= RAMEZANI & TUCKER, 2003; KH= Our research; M: MIRZAEI BENI, 2014; MT= MOHAMMAD TORAB, 2008 Sample Rock Ref.* Locality SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O ES7 Rhy MT Esfordi 73.08 0.212 11.64 1.575 0.024 0.21 0.478 0.26 8.465 ES9 Rhy MT Esfordi 72.22 0.35 15.02 0.972 0.007 1.31 0.228 0.22 4.906 B3-S1 Rhy MT Esfordi 61.95 0.103 10.49 2.952 0.023 7.75 3.62 0.27 6.456 B35-S1 Rhy MT Esfordi 63.29 0.145 9.72 2.979 0.017 8.55 3.424 0.44 5.254 B43-S1 Rhy MT Esfordi 62.82 0.205 10.43 4.869 0.041 5.05 3.886 0.34 6.059 MA1 Rhy MT Chador-Malu 65.94 0.213 10.77 5.49 0.059 0.93 1.834 0.27 8.894 Ry-1 Rhy M Se-Chahun 77.17 - 10.98 2.14 - 0.7 0.73 4.76 3.32 Ry-2 Rhy M Se-Chahun 78.35 - 9.03 3.15 - 2.6 0.52 2.07 3.88 Ry-3 Rhy M Se-Chahun 74.15 - 11.81 2.7 - 0.9 1.5 5.13 3.58 Ry-4 Rhy M Se-Chahun 76.66 - 9.9 3.4 - 2.81 0.58 2.35 3.9 6 Dia R Chahmir 47.61 3.7 16.14 12.82 0.19 6.37 4.78 4.29 2.01 7 Dia R Koushk 46.75 3.29 15.75 13.89 0.3 6.4 4.88 4.1 2.29 9 Dia R Narigan 51.73 2.37 13.76 11.26 0.21 7.89 7.49 2.9 0.77 10 Dia R Narigan 52.53 2.25 13.39 11.03 0.2 7.76 7.34 3.09 0.8 16 Dia J Narigan 44 3.21 13.35 12.83 0.14 8.62 6.16 3.53 1.51 21 Rhy R&T Duzakh-Darreh 79.06 0.24 11.95 0.13 0.01 0.19 0.45 6.22 1.46 22 Dac R&T Chahgaz 67.05 0.81 13.23 6.48 0.05 0.74 1.76 6.48 1.7 26 Rhyd R Narigan 72.62 0.24 13.66 1.59 0.12 0.16 1.7 3.07 6.62 27 Rhyd R Narigan 73.97 0.21 12.68 1.43 0.11 0.19 1.48 3.38 6.34 28 Rhy R Koushk 74.81 0.24 11.51 2.55 0.03 0.54 1.16 0.21 8.64 29 Rhy R Koushk 70.59 0.27 13.2 3.84 0.08 0.51 1.32 0.3 9.34 31 Rhy R Chahmir 71.21 0.21 12.51 3.2 0.51 0.5 1.12 0.31 9.16 32 Rhy R Chahmir 73.8 0.2 13.14 1.83 0.04 0.56 1.01 0.23 8.59 38 Rhy R Chahmir 78.38 0.16 10.9 1.2 0.03 0.52 1.44 4.63 2.37 45 Dia R Zarigan KH-001 Dia KH Choghart 54.35 1.43 9.04 9.50 0.03 7.56 1.12 0.76 1.98 KH-008 Dia KH Choghart 53.5 2.15 14.26 14.85 0.12 6.63 8.39 2.52 2.00 KH-009 Rhy KH Choghart 78.37 0.11 13.32 4.31 0.012 3.81 1.95 4.82 1.55 KH-010 Rhy KH Choghart 83.06 0.22 6.39 1.69 0.006 0.08 0.42 6.30 0.91 KH-011 Rhy KH Choghart 83.45 0.22 7.10 2.84 0.004 0.44 0.66 6.67 0.90 KH-059 Rhy KH Choghart 81.08 0.21 12.72 1.93 0.008 0.86 1.25 6.55 1.51 KH-071 Dia KH Choghart 55.81 4.36 15.58 13.96 0.1 7.58 10.12 4.28 1.83 KH-072 Rhy KH Choghart 82.5 0.21 14.52 2.55 0.019 0.90 1.88 7.86 0.41 KH-096 Rhy KH Choghart 83.43 0.16 9.83 1.09 0.015 0.46 0.74 7.06 0.97 KH-097 Rhy KH Choghart 83.92 0.22 12.56 1.54 0.007 0.20 0.57 7.51 0.81 KH-098 Rhy KH Choghart 80.61 0.33 13.45 2.21 0.005 0.45 0.61 7.25 1.20 KH-099 Rhy KH Choghart 82.63 0.21 12.32 1.36 0.014 0.08 2.39 7.65 0.16 KH-100 Rhy KH Choghart 83.22 0.25 12.01 1.48 0.004 0.40 0.40 6.60 1.27 KH-101 Dia KH Choghart 48.53 4.84 15.39 13.69 0.18 6.56 9.90 3.44 2.20 KH-102 Dia KH Choghart 54.96 3.08 17.13 13.18 0.18 4.42 7.35 5.84 1.85 G eo lo gi a C ro at ic a Geologia Croatica 70/158 varying proportions with magnetite, and apatite-bearing magne- tite is the most abundant ore type at Choghart. At least two gene- rations of apatite are discernible. The first, which is contempo- raneous with the main phase of iron oxide formation, displays euhedral crystals ranging in size from a few millimetres to a few centimetres in diameter. It is intimately intergrown with magne- tite. The second generation occurs as subhedral to anhedral crys- tals in lenses, dykes, and veinlets of varying size and thickness, which cut the magnetite-apatite ore (MOORE & MODABBERI, 2003). Pyrite and quartz are distributed quite evenly in the orebody, either as solid inclusions in primary magnetite grains or as well- developed crystals in the orebody and the adjacent country rock. Some magnetite crystals are cemented by quartz and vice versa, indicating alternation of magnetite and quartz mineralization. Calcite, dolomite, secondary haematite and talc occur throughout the orebody as veinlets and cementing material of oxidized ore. Rutile and goethite are probably the result of the total transfor-the total transfor-total transfor- mation of the earlier formed martite (MOORE & MODABBERI, 2003). The Th-REE enriched zone occurs within the south-eastern margin of the Fe-orebody between the F5 and F6 faults and it also cuts the sodic-calcic metasomatism/alteration as well as the host rhyolite (Figure 3a). The radioactive anomaly of thorium is ob- served in the margin of the Fe-orebody by ground radiometric and spectrometric surveys using an RS-230 spectrometer. Chem- an RS-230 spectrometer. Chem-RS-230 spectrometer. Chem- ical analyses of samples of this zone show Th-REE mineraliza- tion.The hydrothermal breccia and albitization (pink in colour), are both associated with Th-REE mineralization shown in Figure 4a, b. Carbonate-silica veins and veinlets cross cut the host rock, the Fe-orebody and Th-REE mineralization zone which indicates that they are formed after the Th-REE mineralization. 4. MATERIAL AND METHODS Thirty-five samples were collected for petrographic, ore micro- scopic and geochemical studies from the Th-REE mineralization Table 1. continued Sample P2O5 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm ES7 0.022 30.3 57 <50 17 2.5 0.7 <0.5 ES9 0.072 1.4 5 <50 <5 0.4 <0.2 <0.5 B3-S1 0.626 129 221 <50 106 16.1 2.1 2.3 B35-S1 1.071 147 324 57 108 20.9 3.1 2.8 B43-S1 0.487 71 138 <50 60 11.3 1.4 1.7 MA1 0.056 34.2 105 <50 43 12.9 2 1.8 Ry-1 0.03 40.5 73.65 9.8 34.2 7.1 1.3 5.3 0.8 4.3 0.8 2.1 0.3 Ry-2 0.06 38.4 67.14 8.7 29.7 6 1.1 4.4 0.9 4.1 0.8 2 0.3 Ry-3 0.05 39.5 70.54 9.16 31.59 6.58 1.21 4.84 0.88 4.63 0.88 2.25 0.32 Ry-4 0.05 36.2 64.64 8.39 28.93 6.03 1.11 4.44 0.81 4.26 0.81 2.07 0.29 6 0.61 33 65.2 7.7 32.2 7.8 2.6 7.8 1.1 5.6 1 2.5 0.3 7 0.8 30.1 63 7.5 30 7.7 2.5 7.7 1 5.4 0.9 2.3 0.3 9 0.34 22.7 49.9 6.6 27.3 6.5 2.1 6.4 1 5.3 1 2.5 0.4 10 0.37 24.1 52 6.7 29 7 2.2 6.4 1 6 1.1 3 0.4 21 0.08 30.2 57.2 6.3 22.4 4.1 0.9 0.7 22 0.21 57.8 135.2 16.7 67 17.8 3.9 3.3 26 0.05 9.2 19.1 2.3 8.5 1.7 0.3 1.8 0.3 2.1 0.5 1.6 0.3 27 0.05 9 17 2.1 7 1.5 0.2 1.5 0.3 1.8 0.4 1.3 0.2 28 0.03 20.9 43 4.9 17.5 3.4 0.7 4.9 1 7.5 1.8 5.8 0.9 29 0.1 17 38 4.8 15 3.1 0.6 4.5 0.9 7.3 1.7 5.3 0.9 31 0.88 14 31 4.3 13 3 0.6 4.1 0.8 5.5 1.5 5.1 0.7 32 0.4 11 27 3.9 11 2.8 0.5 3.9 0.7 5 1.3 4.9 0.6 45 56.3 113 13.3 48.1 8.9 2.8 6.9 1 5 0.9 2.3 0.3 KH-001 0.32 31.25 65.52 8.06 37.30 5.04 2.02 5.04 0.91 3.02 0.50 2.02 0.30 KH-008 0.45 27.86 65.02 7.22 37.15 7.22 2.06 7.22 0.93 4.13 0.83 2.06 0.31 KH-009 0.02 8.92 30.72 2.97 1119.8 1.98 0.30 1.98 0.30 1.98 0.59 1.98 0.30 KH-010 0.03 0.71 2.04 0.31 1.02 0.20 0.08 0.31 0.10 0.51 0.10 0.20 0.08 KH-011 0.02 1.07 3.20 0.32 2.13 0.53 0.11 3.20 0.08 0.43 0.08 0.53 0.11 KH-059 0.01 23.23 62.62 5.05 27.27 1.01 0.40 4.04 0.20 2.02 0.51 1.01 0.10 KH-071 0.54 5.90 10.82 0.98 5.90 0.98 0.39 1.97 0.20 0.98 0.20 0.98 0.10 KH-072 0.01 71.86 159.68 18.96 99.80 14.97 4.99 19.96 2.00 7.98 2.00 3.99 0.50 KH-096 0.01 6.97 10.95 1.99 5.97 1.00 0.40 1.00 0.10 0.40 0.10 0.50 0.10 KH-097 0.01 5.95 9.92 0.99 4.96 0.99 0.40 0.69 0.10 0.50 0.10 0.30 0.10 KH-098 0.00 2.94 4.91 0.98 4.91 0.98 0.39 1.96 0.20 1.96 0.29 0.98 0.10 KH-099 0.00 2.90 4.84 0.58 3.87 0.97 0.39 1.93 0.19 0.97 0.19 0.87 0.19 KH-100 0.01 2.96 7.88 0.99 3.94 0.39 0.10 0.59 0.10 0.49 0.10 0.39 0.10 KH-101 0.40 52.31 110.66 12.07 51.31 8.05 3.02 10.06 1.01 8.05 1.01 4.02 0.50 KH-102 1.03 79.70 174.96 15.55 71.93 12.64 4.86 14.58 0.97 6.80 0.97 2.92 0.10 G eologia C roatica Khoshnoodi et al.: Alkali Metasomatism and Th-REE Mineralization in the Choghart deposit, Bafq district, Central Iran 59 Table 1. continued Sample Yb Lu Nb Ta Zr Hf Nb Ta Pb Y Zr Th U ES7 2 0.44 4 <0.5 188 5 .375 <0.5 <3 9 188 19.5 6.8 ES9 0.9 0.25 6 <0.5 227 5 .375 <0.5 <3 <3 227 2.5 1.5 B3-S1 5.8 0.86 <2 <0.5 181 6 .375 <0.5 <3 47 181 14.9 1.2 B35-S1 7.7 1.16 2 <0.5 195 4 .375 <0.5 <3 124 195 14.9 1.9 B43-S1 6.1 0.91 <2 <0.5 175 5 .375 <0.5 3 54 175 18.8 4.7 MA1 3.2 0.41 <2 0.6 159 5 .6 0.6 11 38 159 6 1.5 Ry-1 2 0.4 3.07 0.4 58.6 2.35 .4 0.4 1.43 21.2 58.6 12.3 2.1 Ry-2 2.1 0.3 5.18 0.4 62.8 2.27 .4 0.4 2.2 24.3 62.8 10.2 2.1 Ry-3 2 0.3 4.1 0.4 60.2 2.29 .4 0.4 1.8 22.3 60.2 11.8 2.1 Ry-4 2 0.4 4.9 0.4 59.1 2.32 .4 0.4 2.1 23.9 59.1 10.7 2.1 6 1.9 0.2 52.2 3.3 252 6.4 3.3 3.3 45 27.1 252 4.3 1.3 7 1.9 0.2 47 3.1 245 6.1 3.1 3.1 40 25 245 4.2 1.2 9 2 0.3 26 1.8 211 6 1.8 1.8 25.1 211 3.2 0.8 10 2.1 0.4 29 1.9 215 7 1.9 1.9 26 215 3.5 0.9 21 2.5 0.4 2.4 0.4 125 3.5 .4 0.4 26 125 5.8 2.3 22 13.7 2.1 22.7 1.6 850 23 1.6 1.6 114 850 8.4 2.4 26 1.8 0.3 5 0.5 116 4 .5 0.5 13.3 116 8.4 2.1 27 1.7 0.2 4.8 0.5 109 3 .5 0.5 13 109 8.2 2 28 5.8 0.9 5 0.6 172 5 .6 0.6 54.8 172 11.6 2.4 29 5.5 0.8 4.8 0.6 165 4.5 .6 0.6 51 165 11.3 2.4 31 4.5 0.7 3 0.5 158 4.2 .5 0.5 49 158 11 2.3 32 3.7 0.6 3 0.5 160 3.9 .5 0.5 47 160 11.1 2.3 45 1.9 0.3 125 8.1 262 6 8.1 8.1 21.1 262 9.7 3.7 KH-001 2.02 0.30 36.29 2.02 141.12 3.02 2.02 2.02 3.02 17.14 141.12 16.13 82.66 KH-008 2.06 0.21 39.22 2.06 144.48 2.06 2.06 2.06 4.13 20.64 144.48 3.10 5.16 KH-009 1.98 0.40 1.98 0.69 138.74 2.97 .69 0.69 1.98 15.86 138.74 21.80 2.97 KH-010 0.41 0.10 3.06 0.61 122.28 2.04 .61 0.61 2.04 1.02 122.28 4.08 2.04 KH-011 0.43 0.11 2.13 0.21 117.37 2.13 .21 0.21 2.13 2.13 117.37 4.27 1.07 KH-059 2.02 0.30 6.06 0.51 161.60 4.04 .51 0.51 5.05 12.12 161.60 40.40 11.11 KH-071 1.97 0.20 3.94 0.30 118.08 2.95 30 0.30 2.95 8.86 118.08 17.71 2.95 KH-072 2.00 0.30 109.8 2.99 199.60 2.99 2.99 2.99 3.99 33.93 199.60 7.98 2.00 KH-096 0.50 0.10 0.50 0.10 139.30 0.90 .10 0.10 1.00 2.99 139.30 3.98 1.00 KH-097 0.50 0.10 1.98 0.20 119.04 1.98 .20 0.20 2.98 3.97 119.04 10.91 1.98 KH-098 1.96 0.10 1.96 0.29 147.15 1.96 .29 0.29 3.92 6.87 147.15 12.75 1.96 KH-099 0.97 0.10 1.93 0.10 116.04 1.93 .10 0.10 1.93 6.77 116.04 12.57 1.93 KH-100 0.59 0.10 1.97 0.20 137.90 1.97 .20 0.20 1.97 1.97 137.90 9.85 0.99 KH-101 3.02 0.20 45.27 0.91 140.84 2.01 .91 0.91 3.02 130. 8 140.84 4.02 1.01 KH-102 0.97 0.10 54.43 0.97 320.76 2.92 .97 0.97 4.86 25.27 320.76 4.86 1.94 Figure 4. (A) Th-REE mineralization within hydrothermal breccia; (B) Hand specimen of pinkish albitite including Th-REE mineralization. G eo lo gi a C ro at ic a Geologia Croatica 70/160 zone (20 samples), volcanic host rock (10 samples) and diabase dykes (5 samples) at the Choghart deposit. The whole-rock con- centrations of major oxides and trace elements were determined by inductively coupled plasma (ICP) optical emission spectros- copy (OES) and mass spectrometry (MS) at the Jaber-ebne- hayyan laboratory of Nuclear Science and Technology Research Institute in Tehran, Iran. The detection limit for the main oxides and also the minor elements by ICP-OES is <10 ppm, and for the trace elements by ICP-MS is < 0.1 ppm. In addition, the data obtained from previous studies in the Bafq district were also used to strengthen the discussion of results obtained from the present study (Table 1; RAMEZANI & TUCKER, 2003; MOHAMMAD TORAB, 2008; MIRZAEI BENI, 2014; RAJABI, et al., 2015). Petrographic and ore micro- scopic studies (including general recognition of mineral assem- blages, alteration, metasomatic replacements and textures) were performed using an Olympus BX60 microscope at the petrogra- phy laboratory at the Shahid Beheshti University of Tehran. Based on optical microscope observations, 12 Th-REE minera- lized thin-polished sections were selected for electron probe mi- cro-analysis (EPMA). The detailed study of mineral assemblages, alteration, metasomatic replacements and textures was carried out at the mineralogy division of the Iranian Mineral Processing Research Centre (IMPRC) using a Cameca SX-100 electron mi- croprobe equipped with 5 wavelength-dispersive crystal spec- trometers. Measurements were performed using an accelerating voltage of 15 kV, a beam current of 20 nA, beam size of ~5 µm and 15s counting time for major elements and an accelerating voltage of 25 kV, a beam current of 20 nA, beam size of ~3 µm and 30s counting time for trace elements. Natural oxides and sili- cates were used as standards and for calibration. Raw data were corrected for matrix effects with the ZAF correction procedure implemented by CAMECA. 5. RESULTS AND DISCUSSION 5.1. Geochemistry of host rocks and mineralization The rare earth elements (REE), high-field-strength elements (HFSE), Th and transition elements are usually immobile dur- ing intense hydrothermal alterations. In addition, the hydro- thermal fluids do not have a great capacity to transport some of the major elements such as Ti and P (e.g. ZHOU, 1999). In contrast, Mg, Ca, Mg, Na, K, and some of the large ion litho- phile elements (LILE; e.g. Sr, Ba and Rb) are mobile under hydrothermal conditions (e.g., mobility SMITH & SMITH, 1976). Immobile trace element geochemistry (including REE) is utilized here to deduce the tectonic setting of the igneous rocks. Figure 5. Log (Nb/Y) versus Log (Zr/TiO2) plot of WINCHESTER & FLOYD (1977) (A) for pyroclastic and effusive rocks; (B) for diabase dykes of the Choghart- Bafq dis- trict; (C) Diagram of Th/Hf vs. Ta/Hf ratios for felsic rocks; (D) The Log (Ce/Nb) versus Log (Th/Nb) plot of SAUNDERS & TARNEY (1991) for diabase rocks. G eologia C roatica Khoshnoodi et al.: Alkali Metasomatism and Th-REE Mineralization in the Choghart deposit, Bafq district, Central Iran 61 Figure 6. shows the Chondrite- and primitive-mantle-norma- lized REE patterns. The patterns for basaltic rocks (Figure 6a, b) display significant variation from LREE to HREE, similar to oce- anic-island basalts (OIB). Most REE patterns of felsic rocks dis- play enrichment in the LREE, with relatively flat HREE patterns (Figure 6c). Rhyolitic rocks of the Bafq district are noticeably en- riched in Th and depleted in Ta, Nb and Ti (Figure 6d). Because the mantle was progressively modified by subducted oceanic crust, felsic volcanic rocks became enriched in Th and LREE com- pared to Ta, Nb, and HREE (SCHANDL & GORTON, 2002). The Primitive-mantle-normalized geochemical patterns, ar- ranged in order of incompatibly elements, indicate that the basal- tic rocks of the Bafq district are more LREE- and HFSE (Th, Nb, Ta, Zr, Hf)-enriched rather than E-MORB or N-MORB (Figure 6d). Considering the afore mentioned context, the Choghart rhy- olite is similar to other felsic volcanic rocks and ECVSS granites formed in a continental margin setting, and like other continental margin rocks are noticeably enriched in Th and LREE compared to Ta, Nb, and the heavy rare earth elements. Likewise, the dia- base dykes of Choghart have alkaline compositions similar to other diabase dykes and mafic rocks of the early Cambrian vol- cano-sedimentary sequence and were formed in a back-arc basin environment. Based on the results obtained during the study, the evolution of the Bafq district is genetically, related to the subduc- tion of Palaeotethys oceanic crust beneath the Central Iranian microcontinent followed by formation of continental arc grani- toids and rhyolites and then formation of back-arc basin diabase dykes (RAJABI et al., 2015). Diabase dykes were formed in a back-arc extensional setting after magnetite-apatite and Th-REE Since the volcanic host rocks of the Choghart deposit are af- fected by hydrothermal alteration, the immobile elements are em- ployed for petrogenetic interpretations of the rhyolites in the Bafq district (Chahmir, Koushk, se-Chahun, Choghart, etc.). On the log (Zr/TiO2) vs. log (Nb/Y) plot of WINCHESTER & FLOYD (1977), effusive rocks range from rhyolite to dacite in composition (two samples of Choghart lie within trachy-andesite field), and the samples of diabase dykes from the Bafq district plot within the alkaline basalt field (two samples of Choghart plot within the basalt field) (Figure 5a, b). HFSE ratios in felsic volcanic rocks reflect the regional tectonomagmatic evolution. Th/Ta, Th/Hf, Ta/Hf, Th/Yb, and Ta/Yb ratios help to define the tectonic setting of volcanism, sepa- rating oceanic arcs, active continental margins, and within-plate volcanic zones (SCHANDL & GORTON, 2002). The rhyolites of the Bafq district particularly Choghart are enriched in Th and the light rare earth elements compared to Ta, Nb, and the heavy rare earth elements that are related to subduction zones. Variable Th/Ta ratios within individual deposits show fractionation trends (Figure 5c; SCHANDL & GORTON, 2002). The Ce/Nb vs. Th/Nb plot of SAUNDERS & TARNEY (1991) is useful in discriminating the tectonic environment of mafic rocks, because these elements are immobile during altera- tion processes. The diabase dyke samples of the Bafq district plot in the field of within-plate basalts (WPB); except for two samples of Choghart that plot out of this field in the field of continental margin volcanic arc basalt. This is possibly because of high con-. This is possibly because of high con- This is possibly because of high con- centrations of Th (Figure 5d). Furthermore, Th and Ta behave coherently in non-subduction-related basalts, decoupling only in the subduction environment (WOOD, 1980). Figure 6. Chondrite and primitive mantle normalized rare earth elements and incompatible elements distribution patterns (Spider diagrams) (SUN and MC  DONOUGH, 1989) for the Bafq district (A, B) diabase dykes and (C, D) rhyolitic rocks G eo lo gi a C ro at ic a Geologia Croatica 70/162 mineralization in the Choghart deposit. The K2O vs. Na2O dia- gram (Figure 7a) illustrates that all Th-REE mineralized samples lie within the high-Na field. This diagram indicates that in the Choghart Th-REE mineralization zone the sodic alteration oc- curred pervasively. The Chondrite-normalized REE patterns (Fig. 7b) of Th-mineralization zone samples show a weak LREE/HREE frac- tionation and pronounced negative Eu anomaly. In addition, the similarity between the Chondrite-normalized REE patterns of the rhyolites and Th-mineralization zone in the Choghart mine is obvious. The negative Eu anomaly in the Th-mineralization zone of Choghart could be inherited from the fluid source. This implies that the fluid originated from rocks in which plagioclase has removed Eu2+ or equilibrated with them, because Eu2+ sub- stitutes for Ca2+ in plagioclase, early crystallizing plagioclase in magma will remove Eu2+ from the system and thus a later exsolv- ing fluid will be depleted in Eu. Additionally, if a fluid equili- brates with a plagioclase-bearing rock and doesn’t dissolve pla- Figure 7. (A) The K2O vs. Na2O diagram for Th-REE mineralized samples; (B) Comparison of chondrite-normalized REE patterns of the Choghart rhyolite and the Th-REE mineralization zone. Figure 8. (A) Photomicrograph (transmitted-light) of thorite associated with actinolite and calcite; (B) Photomicrograph (reflected-light) of thorite accompanied by magnetite, pyrite and chalcopyrite; (C) BSE image; Conversion of ilmenite to sphene and rutile and also to rutile and titanomagnetite (Th: Thorite, Act: Actino- lite, Ilm: Ilmenite, Ab: Albite, Py: Pyrite, Tim: Titanomagnetite, Ccp: Chalcopyrite, Cal: Calcite, Mag: Magnetite, Sph: Sphene, Rut: Rutile, Kfd: K-feldspar). G eologia C roatica Khoshnoodi et al.: Alkali Metasomatism and Th-REE Mineralization in the Choghart deposit, Bafq district, Central Iran 63 gioclase, then the depletion in Eu will be inherited by the fluid. The negative Eu anomaly may reflect reducing conditions of the mineralizing fluid and depositional environment. Beside the RE- DOX conditions, the Eu2+/Eu3+ ratio can be controlled by tem- perature (BAU & MÖLLER 1992). Since paragenetic magnetite, pyrite and chalcopyrite in mineral assemblages of the Th-mine- ralization zone were observed, the negative Eu anomaly is most likely due to the reducing conditions of the mineralizing fluid and depositional environment. According to Figure 7b, the chondrite- normalized REE patterns of the Choghart rhyolite and Th-REE mineralization zone are similar, suggesting that Th and REE origi nated from continental margin Choghart rhyolitic magma. 5.2. Mineralogy The microscope studies indicated that the main Th mineral in Th-REE mineralization zone is thorite (Figure 8a, b) which oc- curs in veinlets, either disseminated or as inclusions in other mine- rals such as albite and clinopyroxene. Minor amounts of thorite are associated with the sodic-calcic altered magnetite ore body. Sphene is the other host mineral of REE but is quantitatively less important than thorite. Two generations of sphene are recognized; the first, primary sphene which occurs as individual crystals and was likely precipitated directly from the mineralizing fluids (Figu re 9a), and the second generation of sphene that formed by alteration of ilmenite (Figure 8c). Calcium- and silica-rich mineralizing fluids cause the alteration of ilmenite that is con- verted to sphene and rutile. Iron released from ilmenite alteration precipitated as magnetite (Figure 8c). In some microscope sec- tions, ilmenite is observed to be converted to rutile and titano- magnetite (Figure 8c). Formation of magnetite, sphene and rutile suggests that probably the temperature of mineralizing fluids was relatively high. The occurrence of magnetite as a fracture filling in thorite indicates that it precipitated after thorite formation. The silicate minerals accompanied by Th-REE mineraliza- tion include albite, amphibole and pyroxene. Following the no- menclature of LEAKE et al. (1997) and EPMA analyses (Table 2), Figure 9. (A) BSE image; primary sphene as an individual crystal; (B) Photomicrograph (reflected-light) of the first generation of paragenetic galena with Th-REE mineralization; (C) BSE image; the second generation of galena formed after Th-REE mineralization as fracture filling of thorite (Th: Thorite, Ccp: Chalcopyrite, Mag: Magnetite, Sph: Sphene, Rut: Rutile, Aug: Augite, Gn: Galena). Figure 10. Classification of the calcic amphiboles in the Th-REE mineralization zone of Choghart. G eo lo gi a C ro at ic a Geologia Croatica 70/164 the amphibole belongs to the calcic group and is actinolite (except for 2 points which fall in the tremolite field, Figure 10). According to the nomenclature of MORIMOTO et al. (1989) and EPMA analy- ses (Table 3), all pyroxenes belong to Ca-rich pyroxene and fall mainly in the augite field and three samples have the composition of diopside (Figure 11). Also calcite, magnetite, pyrite, chalcopy- rite and trace amounts of galena are associated with this type of mineralization. Two generations of galena are discernible; the first is contemporaneous with Th-REE mineralization (Figure 9b) and the second generation of galena formed after Th-REE mineraliza- Table 3. Representative electron microprobe analyses of pyroxene in some studied samples, Detection limit < 0.01%. PX1 PX2 PX3 PX4 PX5 PX6 PX7 PX8 PX9 PX10 PX11 PX12 PX13 PX14 PX15 SiO2 56.83 54.26 54.73 57.12 56.41 54.23 55.65 58.14 55.62 57.23 56.74 58.38 57.26 58.01 58.09 TiO2 0.00 0.13 0.13 0.00 0.02 0.05 0.00 0.01 0.13 0.02 0.04 0.17 0.21 0.29 0.24 Al2O3 0.47 0.3 0.36 0.24 0.15 0.2 0.77 0.66 0.39 0.27 0.52 1.05 1.01 1.16 1.09 Fe2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Cr2O3 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.01 0.02 0.00 0.00 0.00 0.00 0.00 FeO 11.79 6.77 6.92 6.01 5.87 7.19 19.3 9.72 6.53 12.74 13.58 7.18 8.06 6.22 7.53 MnO 0.13 0.1 0.13 0.12 0.11 0.11 0.59 0.31 0.12 0.28 1.6 0.12 0.08 0.07 0.1 MgO 17.5 14.48 14.69 15.27 15.1 13.97 10.63 18.06 15.91 16.08 14.43 20.6 19.59 21.82 20.43 CaO 11.64 22.05 21.9 21.41 22.77 22.76 8.69 12.72 20.9 12.89 11.87 12.54 12.37 11.42 12.28 Na2O 0.24 1.07 1.14 0.58 0.31 1.01 2.43 0.14 1.04 0.07 0.68 0.6 0.28 1.44 0.66 K2O 0.02 0.02 0.00 0.00 0.00 0.01 0.09 0.03 0.00 0.00 0.06 0.17 0.19 0.36 0.18 Total 98.62 99.18 100.0 100.8 100.8 99.53 98.15 99.79 100.7 99.6 99.52 100.8 99.05 100.8 100.6 Formulae based on 4 cations and 6 oxygens Si 2.136 2.014 2.014 2.089 2.068 2.012 2.157 2.150 2.025 2.152 2.147 2.097 2.108 2.056 2.092 Ti 0.00 0.004 0.004 0.00 0.001 0.001 0.00 0.00 0.004 0.001 0.001 0.005 0.006 0.008 0.007 Al 0.021 0.013 0.016 0.01 0.006 0.009 0.035 0.029 0.017 0.012 0.023 0.044 0.044 0.048 0.046 Fe3+ 0.00 0.03 0.031 0.00 0.00 0.037 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Cr3+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.001 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.371 0.18 0.181 0.184 0.18 0.186 0.626 0.301 0.199 0.401 0.43 0.216 0.248 0.184 0.227 Mn 0.004 0.003 0.004 0.004 0.003 0.003 0.019 0.01 0.004 0.009 0.051 0.004 0.002 0.002 0.003 Mg 0.981 0.801 0.806 0.833 0.825 0.773 0.614 0.996 0.863 0.901 0.814 1.103 1.075 1.153 1.097 Ca 0.469 0.877 0.863 0.839 0.894 0.905 0.361 0.504 0.815 0.519 0.481 0.483 0.488 0.434 0.474 Na 0.017 0.077 0.081 0.041 0.022 0.073 0.183 0.01 0.073 0.005 0.050 0.042 0.02 0.099 0.046 K 0.001 0.001 0.00 0.00 0.00 0.00 0.004 0.001 0.00 0.00 0.003 0.008 0.009 0.016 0.008 Sum 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 Table 2. Representative electron microprobe analyses of amphibole in some studied samples, Detection limit < 0.01%. AM1 AM2 AM3 AM4 AM5 AM6 AM7 AM8 AM9 AM10 AM11 AM12 AM13 SiO2 56.64 56.7 56.83 57.35 56.85 58.19 57.65 55.65 57.09 55.93 55.87 62.63 55.71 TiO2 0.06 0.06 0.00 0.02 0.03 0.04 0.03 0.00 0.04 0.18 0.15 0.03 0.03 Al2O3 0.46 1.08 0.47 0.42 0.57 0.66 1.41 0.77 0.75 0.47 0.48 0.26 0.38 Fe2O3 7.58 8.08 5.46 0.03 0.00 0.00 0.00 1.78 0.00 2.97 16.16 0.00 1.32 FeO 10.84 9.89 11.79 9.45 9.65 6.14 6.09 17.70 11.54 13.00 0.00 5.77 12.62 MnO 0.07 0.09 0.13 0.21 0.21 0.03 0.02 0.59 0.25 0.68 0.39 0.02 0.27 MgO 18.67 19.11 17.5 18.28 17.66 19.51 19.58 10.63 16.07 13.81 16.24 27.75 14.64 CaO 11.61 11.5 11.64 12.86 13.12 13.5 13.45 8.69 12.90 10.66 8.28 0.08 11.69 Na2O 0.19 0.19 0.24 0.27 0.2 0.00 0.05 2.43 0.17 1.02 0.39 0.03 0.08 K2O 0.03 0.06 0.02 0.04 0.05 0.04 0.00 0.09 0.06 0.07 0.14 0.07 0.05 H2O* 2.17 2.19 2.16 2.15 2.13 2.17 2.17 2.05 2.13 2.10 2.15 2.24 2.07 Total 101.5 101.68 101.32 101.06 100.48 100.28 100.45 100.38 101.0 100.88 100.25 98.9 98.9 Formulae based on 16 cations and 23 oxygens Si 7.818 7.771 7.794 7.999 7.991 8.046 7.959 8.125 8.035 8.000 7.791 8.368 8.064 Al 0.075 0.174 0.125 0.069 0.086 0.108 0.189 0.132 0.124 0.079 0.000 0.041 0.065 Ti 0.006 0.006 0.006 0.002 0.003 0.004 0.003 0.000 0.004 0.019 0.016 0.003 0.003 Fe3+ 0.787 0.833 0.810 0.003 0.195 0.319 1.696 0.144 Mg 3.842 3.904 3.873 3.801 3.701 4.022 4.030 2.314 3.372 2.945 3.376 5.527 3.159 Fe2+ 0.464 0.301 0.382 1.100 1.134 0.710 0.703 2.161 1.358 1.555 0.000 0.645 1.528 Mn 0.008 0.010 0.009 0.025 0.025 0.004 0.002 0.073 0.030 0.082 0.046 0.002 0.033 Ca 1.717 1.689 1.703 1.922 1.976 2.00 1.989 1.359 1.945 1.634 1.237 0.011 1.813 Na 0.051 0.050 0.051 0.073 0.055 0.00 0.013 0.688 0.046 0.283 0.105 0.008 0.022 K 0.005 0.010 0.008 0.007 0.009 0.007 0.00 0.017 0.011 0.013 0.025 0.012 0.009 OH* 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 Sum 16.773 16.750 16.803 17.002 16.990 16.900 16.930 17.064 16.927 16.929 16.371 16.619 16.845 G eologia C roatica Khoshnoodi et al.: Alkali Metasomatism and Th-REE Mineralization in the Choghart deposit, Bafq district, Central Iran 65 tion as a fracture filling of thorite (Figure 9c). Pb released from thorite (the decay product of Th) probably forms the second gene- ration of galena, provided the activity of S is sufficient. The pre- sence of magnetite, pyrite and chalcopyrite is likely due to rela- tively reduced conditions in the thorite depositional environment. The paragenetic mineral assemblage of Th-REE mineralization of the Choghart iron oxide apatite deposit is shown in Figure 12. The results of bulk chemical analyses show a high correla- tion of Ce, La, Y and Nd concentrations with the Th content. The EMPA analyses show that thorite contains mean values of 9460 ppm Ce, 1950 ppm La, 1810 ppm Y and 110 ppm Nd (Table 4). The EMPA analyses of sphene shows mean of 8050 ppm Ce, 3650 ppm Y while the contents of La and Y are below the detection limit (Table 5). 5.3. Alkali Metasomatism Alkali metasomatism implies the transfer of the alkali ions, Na and K, from a fluid phase, generated during the cooling of an ig- neous body, to the adjoining rocks. Albitite, microclinite, fenites, and the core zones of potassic alteration of porphyry systems are Figure 11. Classification of the Ca-rich pyroxene in the Th- REE mineralization zone of Choghart. Minerals Stages Pre-mineralization Thorium mineraliztion Last alteration O re m in er al s Magnetite Type I Type II Thorite Type I Type II Titano-magnetite Type I Type II Ilmenite Pyrite Chalcopyrite Galena Type I Type II Hematite Sphene Type I Type II Rutile Al te ra tio n m in er al s Albite Type I Type II Diopside Type I Type II Augite & Aegirine Actinolite Type I Type II Calcite Type I Type II Type III Microcline & Orthoclase Quartz Apatite Type I Type II Chlorite, Talc & Serpentine Figure 12. The paragenetic mineral assemblage of Th- REE mineralization of the Choghart deposit. G eo lo gi a C ro at ic a Geologia Croatica 70/166 blage (PIRAJNO, 2013). Perthite formation in the Choghart Th-REE mineralization zone clearly shows alkali metasomatism (Figure 13a). The replacement of K by Na in feldspars, probably by ion exchange, is shown by the presence of albite inclusions in the K-feldspar host. These inclusions can occur either as veinlets or irregular shapes, and do not follow crystallographic directions, as would be the case for melt-fluid unmixing (PIRAJNO, 2013). commonly formed by alkali metasomatism. Also in some cases, perthitic textures in feldspar may indicate alkali metasomatism, rather than the separation during cooling of a two-phase assem- Table 4. Electron microprobe analyses of thorite in some studied samples, Detection limit < 0.01%. sample /element Y La Ce Nd P Ca Ch-001 0.35 0.01 0.92 nd 0.18 0.96 Ch-001 0.17 0.04 0.96 nd 0.19 1.07 Ch-002 0.26 0.1 0.91 0.49 0.13 0.88 Ch-002 0.2 0.01 0.97 0.46 0.19 1.2 Ch-003 0.36 0.07 0.94 0.18 0.13 1.16 Ch-003 0.14 nd 0.91 nd 0.12 1.18 Ch-004 0.22 nd 0.88 0.05 0.09 0.88 Ch-004 0.1 0.02 0.91 0.69 0.07 1.23 Ch-005 0.16 0.04 0.9 0.12 0.16 1 Ch-005 nd nd 1 0.01 0.15 0.24 Ch-006 0.27 0.01 0.94 nd 0.16 1.01 Ch-006 nd 0.03 0.98 nd 0.18 1.05 Ch-007 0.1 nd 0.94 nd 0.14 0.98 Ch-007 nd 0.04 0.93 0.02 0.17 1.12 Ch-008 0.11 nd 1.03 nd 0.17 1.07 Ch-008 0.03 0.01 1.01 nd 0.1 1.2 Ch-009 0.54 nd 0.95 0.02 0.11 0.96 Ch-009 nd nd 1 nd 0.09 1.15 Ch-010 0.37 0.01 0.93 0.1 0.09 1.01 Ch-010 0.24 nd 0.91 nd 0.18 0.78 Table 5. Electron microprobe analyses of sphene in some studied samples, Detection limit < 0.01%. Sample /element CaO MnO FeO Al2O3 SiO2 TiO2 P2O5 Y Ce Ch-002 28.71 nd 1.26 0.74 30.36 35.04 0.03 0.23 1.08 Ch-005 28.69 0.02 0.95 0.42 30.52 36.65 0.02 0.08 1 Ch-007 28.93 0.03 1.37 0.11 30.55 36.35 0.03 nd 1.06 Ch-008 28.55 0.01 1.36 0.62 30.35 36.83 0.01 0.15 1.08 Table 6. Electron microprobe analyses of Thorite in some studied samples for major oxides (in percent), Detection limit < 0.01%. sample /element MgO CaO MnO FeO Al2O3 SiO2 TiO2 P2O5 Ch-001 0.05 2.06 0.08 0.37 0.04 18.21 0.12 0.28 Ch-002 0.1 2.14 0.14 0.52 0.05 19.04 0.11 0.16 Ch-005 nd 2.35 0.08 0.35 0.05 18.11 nd 0.58 Ch-006 nd 2.46 0.05 0.3 0.06 18.4 nd 0.45 Ch-009 0.01 1.29 0.08 0.47 0.06 19.19 0.01 0.6 Ch-010 nd 1.3 0.1 0.43 0.02 18.32 0.04 0.43 Figure 13. (A) BSE image; Perthite formation due to alkali metasomatism; (B) BSE image; Formation of albite inclusions in K-feldspar due to alkali metasomatism; (C) BSE image; Replacement of K-feldspar by albite (Th: Thorite, Ab: Albite, Py: Pyrite, Di: Diopside, Kfd: k-feldspar). G eologia C roatica Khoshnoodi et al.: Alkali Metasomatism and Th-REE Mineralization in the Choghart deposit, Bafq district, Central Iran 67 This event is clearly associated with Th-REE mineralization at Choghart (Figure 13b). The replacement of alkali feldspar by albite (albitization or Na-feldspathization) shows the more common form of sodic metasomatism. This replacement may proceed from either pre- existing perthites, or direct replacement of K-feldspar with late formed albites (POLLARD, 1983). The replacement of K-feldspar by albite occurs pervasively accompanied with the Th-REE mineralization zone at Choghart (Figure 13c, 4b). Albitization of K-feldspar can be written as fol- lows: KAlSi3O8+Na+=NaAlSi3O8+K+ The development of albite-rich rocks (albitites) is typically accompanied by rare element mineralization (Nb, Ta, Sn, W, Li, Be). Sodium-enrichment is associated with elevated concentra- tions of Fe, U, Th, Zr, Nb, Sn, Zn, and HREE. Na-metasomatised rocks are also more prone to be enriched in Rb, Th, Nb, La, Ce, Hf, Zr, and Y with regard to K-metasomatised rocks (KINNAIRD, 1985). The Choghart Th-REE mineralization zone is enriched in Th, La, Ce, Nd, and Y. The high-temperature post-magmatic processes can promote the mobilization and concentration of thorium. In these processes thorium is usually associated with rare-earth elements (the lan- thanides). Most post-magmatic thorium-containing deposits are connected with pneumatic-hydrothermal or high-temperature hy- drothermal processes. At present we know that Th-REE deposits have post-magmatic genesis (TITAYEVA, 1994). Most high-temperature Th-REE deposits (of pneumato-hy- drothermal genesis) were formed as a result of the metasomatic replacement of country rocks. They are characterized by early albitization, K-feldspathization, aegirinization, amphibolitiza- tion, and the formation of skarn. Ore minerals are niobates, tan- talo- and titano-niobates (euxenite, pyrochlore, priorite, fergu- sonite, and others), and the silicates of the rare earths, thorium, and zirconium. High-temperature hydrothermal deposits are characterized by metasomatic replacements, associated with am- phibolitization, aegirinization, biotitization, greisenization, and filled open cavities. Niobates, titano- and tantalo-niobates are represented here in small quantities. Rare-earth phosphates (monazite and xenotime) dominate with some rare-earth silicates (orthite, cerite, britholite, etc.). The medium- and low-tempera- ture deposits are usually veined with a characteristic change in country rocks (feldspathization, carbonatization, and silicifica- tion). Ore minerals are represented by haematite, carbonates and fluorocarbonates of the rare earths (bastnaesite, parisite, synchy- site, etc.), and the sulfides of Fe, Cu, Pb, and Zn (TITAYEVA, 1994). The main paragenetic associations of thorium are the lantha- nides. In high-temperature deposits Th is usually associated with Та, Nb, Ti, and occasionally with Mo, W, Sn, Be. In moderate- temperature deposits Th is paragenetically associated with Pb, Zn, Cu. The anion components such as Cl–, HCO3 –, CO3 2–, HS–, S2– and F- are important and control the mobility of metals in post-magmatic fluids (TITAYEVA, 1994). As mentioned above the Th-REE mineralization zone (tho- rite and sphene) in Choghart is associated with albitization, am- phibolitization (actinolite), pyroxenitization (augite and diopside), carbonatization and minor K-feldespatization (microcline), as well as the occurrence of pyrite and minor amounts of chalcopy- rite, considered to be a moderate-temperature post-magmatic Th- REE mineralization type. 5.4. Mineral formation conditions Thermodynamic calculations have indicated that fluoride or mixed carbonate-fluoride complexes can dominate REE specia- tion (e.g., WOOD, 1990a, b). The scarcity of the F-bearing REE minerals (f lourapatite) in the Th-REE mineralized zone of Choghart, however, implies that a high concentration of fluorine was probably, only locally, achieved and is not necessarily a com- ponent of the composite ore-forming fluids. The presence of calcite along with calcic– amphibole and – pyroxene (actinolite and augite, respectively), the high content of Ca in thorite (Table 6; up to 2.3% CaO) and the occurrence of sphene in the Choghart Th-REE mineralization zone imply that thorite crystallized from a Ca2+- and CO3 2–- bearing aqueous so- lution under conditions of low activity of F–. Considering that dolomitic xenolites of the Esfordi formation are observed in the volcanic host rocks of the Choghart deposit, the portion of CO2 in the mineralizing fluids is supplied by diges- tion of these xenolites. In postmagmatic mineralization, thorium migrates in alka- line solutions, particularly in the form of complex carbonate com- pounds with the formula Th(CO3)n –(2n–4) (TITAYEVA, 1994). According to the presence of paragenetic calcite accompa- nied with thorite and sphene in Th-REE mineralization, Th and REE transport was likely because of the presence of the carbo- nate complexes in mineralizing fluids. According to TITAYEVA (1994) and PIRAJNO (2009) at the end of albitization processes, the thorium and uranium con- centrations increase. Part of REE and U-Th is retained in micro- fissures filled with haematite and chlorite. The reddish colour is attributed to the presence of micro-haematite in micro-fractures. Although, WILDE (2013) has portrayed that the red or pink colo- ration is owing to the presence of fine-grained disseminated haematite or hydrothermal apatite. Our data revealed that albites from the Choghart Th-REE mineralization zone contain K-feld- spar and minor Th silicate mineral inclusions (i.e. thorite) that give albite a more reddish colour (Figure 4B). 6. CONCLUSIONS MOORE & MODABBERI (2003) suggest that the separation of an iron oxide melt and the following hydrothermal processes dominated by alkali metasomatism have both been involved to in different degrees in the formation of the Choghart deposit. Alter- natively, the Choghart rhyolite is enriched in Th and LREE simi- lar to other continental margin felsic volcanic rocks. Chondrite- normalized REE patterns of the Choghart rhyolite are similar to the Th-REE mineralization zone, indicating that Th and REE originated from a continental margin Choghart rhyolitic magma. We suggest that in the Early Cambrian (about 533 Ma) calc- alkaline granitoid magmatism associated with a subduction zone in the Choghart deposit initially formed a magnetite-apatite ore body and sodic-calcic alteration followed by penetration of the remaining magmatic fluids into the rhyolitic host rocks and also sodic-calcic alteration related to iron mineralization and partially brecciation. These fluids maintain elevated concentrations of metals such as Fe, Th and REE that were originally scavenged from the rhyolitic silicate melt due to the magmatic temperatures and high salinity of the fluid (REED et al., 2000; SIMON et al., 2004, 2005, 2006; ZAJACZ et al., 2012; FRANK et al., 2011; MIGDISOV et al., 2014; HURTING & WILLIAMS-JONES, 2014). These fluids caused sodic metasomatism (albitization), car- bonatization, actinolitization and pyroxenitization, minor K-feld- G eo lo gi a C ro at ic a Geologia Croatica 70/168 spatization (microcline) as well as generation of the minerals such as thorite and sphene. Alternatively, the Th-REE mineralization zone may have formed from entirely different fluids related to a later intrusion such as the resurgent central plutons. The high CO2 content of the fluid facilitates formation of metal-carbonate complexes. The presence of paragenetic calcite with thorite suggests that carbonate complexes most likely trans- ferred the Th and REE. 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