Multiple Stage Ore Formation in the Chadormalu Iron Deposit, Bafq Metallogenic Province, Central Iran: Evidence from BSE Imaging and Apatite EPMA and LA-ICP-MS U-Pb Geochronology

The Chadormalu magnetite-apatite deposit in Bafq metallogenic province, Central Iran, is hosted in the late Precambrian-lower Cambrian volcano-sedimentary rocks with sodic, calcic, and potassic alterations characteristic of iron oxide copper-gold (IOCG) and iron oxide-apatite (IOA) ore systems. Apatite occurs as scattered irregular veinlets and disseminated grains, respectively, within and in the marginal parts of the main ore-body, as well as apatite-magnetite veins in altered wall rocks. Textural evidence (SEM-BSE images) of these apatites shows primary bright, and secondary dark areas with inclusions of monazite/xenotime. The primary, monazite-free fluorapatite contains higher concentrations of Na, Si, S, and light rare earth elements (LREE). The apatite was altered by hydrothermal events that led to leaching of Na, Si, and REE + Y, and development of the dark apatite. The bright apatite yielded two U-Pb age populations, an older dominant age of 490 ± 21 Ma, similar to other iron deposits in the Bafq district and associated intrusions, and a younger age of 246 ± 17 Ma. The dark apatite yielded a U-Pb age of 437 ± 12 Ma. Our data suggest that hydrothermal magmatic fluids contributed to formation of the primary fluorapatite, and sodic and calcic alterations. The primary apatite reequilibrated with basinal brines in at least two regional extensions and basin developments in Silurian and Triassic in Central Iran.


Introduction
The formation of apatite-bearing iron deposits is a current topic of debate. The deposits are characterized by large masses of Ti-rich to Ti-poor Fe oxides (essentially magnetite) and subordinate phosphates, in particular apatite, and are known as Kiruna-type magnetite-apatite (e.g., [1]) or iron oxide-apatite (IOA) deposits [2]. Some researchers (e.g., [3][4][5][6] included the Kiruna-type apatitebearing Fe-oxide deposits in the wide range of the iron oxide copper-gold (IOCG) deposits. However, the deposits are considered by other researchers (e.g., [7,8]) as a distinct type of iron deposits, known as iron oxide-apatite (IOA) ores.
Apatite is a common accessory mineral in a wide range of magmatic, sedimentary, and metamorphic rocks. Apatites can serve as a useful indicator mineral for IOCG-and IOA-type  [19]); (c) Geological map of the Posht-e-Badam area (modified after [24,28,33]).

Previous Geochronology
The Bafq district iron oxide ores are locally associated with REE-rich apatite ± silicate ± carbonate assemblages [34]. The timing of ore formation at Choghart has been constrained to be 515 ± 21 Ma and 529 ± 21 Ma (monazite Th-U-Pb total method; [35]). Using isotopic U-Pb dating, Stosch et al. (2011) [34] reported a range of 527 to 539 Ma for apatite samples from five iron deposits in the Bafq district. These ages fall in the age range for felsic plutonic and volcanic rocks in the district (525-545 Ma, zircon U-Pb; [25]). However, Bonyadi et al. (2011) [36] reported a U-Pb LA-ICP-MS age of 510 ± 8 Ma for REE-rich fluorapatite and the associated semi-massive magnetite from the Se-Chahun complex in the Bafq district. The authors reported a slightly older U-Pb age (525 ± 7 Ma) for sodic alteration from the same deposit.

Mine-Scale Geology
The Chadormalu deposit consists of two separate large massive ore bodies ( Figure 2). The main orebody, consisting of three closely spaced bodies in the northern part of the deposit, accounts for about 80% of the total ore reserve. Smaller ore bodies displaying faulted and brecciated contacts with metasomatized greenish host rocks occur in the southern part of the deposit. Apatite is a common accessory mineral. Magnetite also occurs in magnetite-apatite veins with sharp contacts against the altered host rocks. The veins locally contain fragments of host rocks and display a brecciated appearance. Minor hematite, ankerite, and siderite locally occur associated with magnetite from Chadormalu.
The field relationships in the Chadormalu deposit are documented by Heidarian et al. [31]. Intense alteration has obscured the original mineralogy and texture of the host rocks. Sodic alteration, marked by replacive albitization of the various original minerals is widespread and occurs as a distal feature in Chadormalu. Calcic or calcic-ferroan alteration, distinguished by the development of variable proportions of actinolite replacing various rocks, including the albitized rocks, occurs as a more proximal feature, increasing in intensity towards the ore bodies. A potassic alteration, marked by the development of K-feldspar, sericite, and biotite, locally occurs in the host rocks and appears to be post-dating earlier sodic and calcic alteration assemblages. Two other alteration types are silicification and carbonatization formed during and after the sodic and calcic-ferroan alteration types [19,31,37,38]. Minor chlorite locally occurs in the immediate wall rocks enclosing the vein-type ores. The various alteration types and their relationships in Chadormalu are schematically shown in Figure  2. During the waning stages of magnetite mineralization, minor sulfide minerals, mostly pyrite, developed as irregular veinlets and interstitial grains.  [39,40]). (b) Enlarged view of the square in (a) to show details of the ore types (this study). Magnetite-apatite veins in altered host rocks (MAVhost rock); Disseminated magnetite-apatite associations in the marginal parts of the main ore body (MAA-marginal ore); Magnetite-apatite veinlets in the internal parts of the main magnetite ore body (MAV-inner ore).
The ore texture/structure, wall rock alteration, and the common occurrence of apatite in the Chadormalu, and most other iron oxide deposits in the Bafq district (e.g., [19,31,41]) are consistent with their classification as IOA deposits. Apatite in Chadormalu occurs as magnetite-apatite veins cutting the altered rocks (Figure 3a,b); as disseminated magnetite-apatite associations and grain aggregates in the marginal parts of the main ore body (Figure 3c,d); and as irregular magnetite-apatite veinlets in the internal parts of the main ore body (Figure 3e

Materials and Methods
In this study, 8 samples representing various types of apatite associated with iron ores were selected for petrography, scanning electron microscopy using backscatter electron detector (SEM-BSE), electron probe microanalysis (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The samples include four (Cha2-1, Cha2-2, Cha8, Ch15) from magnetiteapatite veins in the altered host rocks (MAV-host rock type), two (Cha1-1, Cha1-2) from disseminated apatite grains and grain aggregates in the marginal parts of the main ore body (MAA-marginal type), and two (Cha24-1, Cha24-2) from magnetite-apatite veinlets in the internal parts of the main magnetite ore body (MAV-inner ore type) ( Figure 3). A schematic map illustrating the location of the three types of apatite is shown in Figure 2b. A description of the samples is presented in Table 1 and the locations of the samples are shown in Figure 4. In situ fluorapatite trace-element analyses were obtained for the same areas analyzed by EPMA with an Australian Scientific Instrument M-50 193 nm ArF (excimer) laser ablation system featuring a Laurin Technic Pty. S-155 large-format cell. The cell is coupled, via Nylon™ tubing, with an Agilent 7700x quadrupole ICP-MS at the Department of Earth Sciences, University of New Brunswick. Operating conditions for apatite U-Pb geochronology are explained in [42]. Trace-element concentrations were normalized to NIST610 as external standard; NIST612 was used as a consistency standard and an estimated Ca content of 39 wt % in apatite was used as internal standard. Minimum detection limits for Y and REE analyzed by LA-ICP-MS are <0.5 ppm and <0.05 ppm for Pb, Th, and U.
The element list was extended to include 204 Pb, 206 Pb, 207 Pb, and 238 U ( 235 U calculated assuming 238 U/ 235 U = 137.88) so that a U-Pb age could be obtained for distinct compositional domains. The MAD apatite standard [43] was used as a primary standard for U-Pb geochronology, and an in-house Phalaborwa apatite (~2030 Ma) was used to check the accuracy of calculated ages. The measured age for Phalaborwa of 2031 ± 43 Ma (n = 12; MSWD = 0.42) confirms the overall accuracy of the U-Pb apatite ages for Chadormalu. For both trace-element and U-Pb dating off-line selection and integration of background and analyzed signals, and time-drift correction and quantitative calibration for U-Pb dating and trace element concentrations were performed with Iolite 3.5 using the VizualAge_UcomPbine [44] and trace-element (Internally Standardized) data reduction schemes, respectively. Details of the error propagation are given in Paton (2010) [45]. The Tera-Wasserburg diagrams and lower intercept age calculations were obtained using Isoplot/Ex_ver3.75 [46].  [47] showing the locations of the samples representing the three main apatite types selected for this study.

Results
In this study, we have focused on the characteristics of the three main apatite types, including aspects of texture, mineralogy, geochemistry, and geochronology with the aim to investigate and better understand the source and evolution of the ore fluids and the timing of ore formation and any modifications to the ores after original deposition.

MAV-Host Rock Type
Apatites occurring in magnetite-apatite veins in the greenish altered wall rocks display typical comb-like textures with elongated crystals growing from the walls inward (Figure 3a,b). Similar textures have been reported from several other IOA type deposits in the Bafq district, including Se-Chahun (cf. [36]) and Gazestan [48], from Chilean IOA deposits [49], and from Yangtze Valley Mesozoic IOCG-IOA deposits, China [42]. Apatite also occurs as euhedral crystals enclosed in magnetite in the central parts of the veins (Figure 5b). Minor hematite, pyrite, quartz, and actinolite occur in the apatite-magnetite veins.
High-resolution SEM images on this apatite type reflect common dark patches overprinting the SEM-bright apatite (Figure 5a,c). No preferred orientation or pattern is displayed by the dark areas. The dark areas locally contain inclusions of monazite and less commonly xenotime (Figure 5b,c). The bright areas in the sample Ch15 were found to contain inclusions of allanite typically occurring along fractures (Figure 5d).

MAA-Marginal Type
This type of apatite consists of euhedral to anhedral apatite grains and grain aggregates in the marginal parts of the main ore body. The apatite and the associated magnetite are locally brecciated and cemented by calcite (Figure 6a,b). Minor hematite, quartz, and pyrite can also be distinguished. On SEM images, this apatite type has dark patches that encroach on the bright parts, similar to the apatite veins in the altered host rocks (Figure 6c,d). Scattered monazite inclusions occur in the dark areas ( Figure 6d). Monazite also occurs along apatite grain boundaries (Figure 6c).

MAV-Inner Type
This apatite type occurs as irregular veins and veinlets, as well as scattered apatite grains in the internal parts of the main ore body. The apatite crystals locally show a banded structure marked by alternating apatite-and magnetite-rich bands with relatively constant thickness (Figure 7a). Magnetite has also been formed as euhedral small grains in the fractures and enveloping the primary apatites ( Figure 7b). Minor quartz, calcite, hematite, and sulfide occur in this type of apatite-bearing iron ore. On high resolution SEM images, abundant fine-grained monazite inclusions are evident as well as along fractures (Figure 7c

Electron Probe Microanalysis (EPMA)
Eight samples from three different apatite types discussed above were selected for EPMA and SEM studies (Tables 2-4). Figure 8 shows comparative box-plot diagrams for selected oxides for various apatite types. The OH values were calculated via charge balance on the halogen site, following Piccoli and Candela [50]. Figure 9 shows a ternary diagram for the composition of the SEM bright and dark areas in the various apatite types in terms of F-OH-Cl contents. The diagram reveals that Chadormalu apatites are mostly fluorapatite. Similar apatite compositions have been reported for several other magnetite-apatite deposits from the Bafq district, such as Esfordi [18,35,36,51,52]. The significantly higher F contents, compared to Cl, as documented for Chadormalu, seems to be a characteristic feature of other IOA type deposits like the Kirunavaara deposit in north Sweden [53].
According to the SEM images and EPMA from different apatite types, the bright apatites contain more Cl compared to the dark apatites. The Na2O, SiO2, FeO, and SO3 in bright parts are significantly higher than those in the dark areas. Of the REE, only Ce was systematically detected in both dark and bright areas, where Ce in dark spots was found to be generally lower than that in the bright spots.     In the MAV-host rock apatites, the Na2O and SiO2 contents vary, respectively, between <0.09 wt %, and 0.02-0.14 wt % (mean. 0.11 wt %) in the dark areas and 0.11-0.45 wt % (mean. 0.25 wt %) and 0.04-0. 28  In the MAA-marginal ore apatite, the Na2O and SiO2 contents vary between <0.10 wt % and 0.015-0.21 wt % (mean. 0.17 wt %), respectively, in the dark areas and 0.12-0.37 wt % (mean. 0.23 wt %) and 0.14-0. 35 Figure 10 shows the relationships in a binary Na2O + SiO2 + Y2O3 versus CaO + P2O5 diagram for the three apatite types. For each apatite type, the bright areas show higher Na2O, SiO2, and Y2O3 contents compared to the dark areas. Geochemical characteristics of Sr and Y can define apatite compositional fields for different rock type compositions [54]. The Sr and Y values in the apatites range between <372 ppm and <1310 ppm, respectively, and mostly plot in the granitoids and iron ore fields (Figure 11a). On the Mn vs. Sr diagram, the apatites plot in the granitoid, granite pegmatite, and iron ore fields (Figure 11b). Our EPMA data are consistent with those reported from apatites from other Kiruna-type iron deposits [13,35,36,53]. The concentrations of V, Mn, and Mg have been used to discriminate IOCG-and Kiruna-type deposits [55]. The Mn contents from the Chadormalu apatites are lower than those from typical IOCG-type deposits and are similar to Kiruna-type systems ( Figure 12).

LA-ICP-MS
Trace-element compositional data on BSE bright and dark areas (114 points) was obtained for bright and dark spots in the three different apatite types to evaluate any spatial control on relative element abundances. The results are presented in Tables 5-7, and graphically shown in Figure 13. The MAV-host rock apatite type has ΣREE values ranging between 9077-39,212 ppm in the bright areas to 794-1874 ppm in the dark areas. For the MAA-marginal type apatites, the ΣREE ranges from

U-Pb Geochronology
In addition to the trace-element data described above, apatite crystals from 6 samples representing the main apatite types were analyzed for U-Th-Pb abundances and isotope ratios (122 spots) ( Figure  15, Tables 8 and 9). Several spots (n = 25/147) for which ablation was catastrophic (e.g., intersected fracture), or that encountered obvious inclusions or displayed other obvious analytical artifacts were rejected. The analyses cover the dark and bright areas, which both have low U contents (0.1-5.6 ppm). The majority of the analyses contained U-rich spikes interpreted to represent micro-scale monazite inclusions; attempts were made to exclude these spikes during offline data reduction. A small subset of spots from SEM-BSE bright areas (n = 6/80) in apatite crystals define a lower intercept age of 490 ± 27 Ma and a common-Pb intercept of 0.862 ± 0.130. Other data from the same domains show a dispersion to younger ages with a minimum age of 246 ± 17 Ma (MSWD = 0.67) calculated for a subset (n = 11/80) of the youngest spots assuming the same common-Pb 207 Pb/ 206 Pb value as the older domains. The darker REE-depleted domains define a more coherent array with a lower intercept age of 437 ± 12 Ma (n = 46/77 spots) and a common-Pb intercept of 0.849 ± 0.030.

Textural and Mineral Chemical Evidence
Textures and chemical compositions of apatite may change due to metasomatic events over a wide range of temperatures and pressures from the mantle to surficial environments, thus making apatite an important fingerprint of metasomatic processes [13]. The three main apatite types in the Chadormalu IOA deposit show similar textural features including the occurrence of bright and dark SEM-BSE apatite spots containing monazite and xenotime inclusions. Formation of larger monazite grains at the expense of smaller grains; intergrowth of monazite with magnetite, and development of allanite are due to late stage fluid-aided reaction of monazite with surrounding silicate minerals (cf. [1,13,53,54,57,58]). These features appear to be common in other Bafq district magnetite-apatite deposits, such as Esfordi, Se-Chahun, Choghart, and Mishdovan [18,20,28,35,36,52].
The EPMA and LA-ICPMS data indicate that the SEM-BSE dark spots or areas in each three types of apatites are strongly depleted in certain other elements, including Si, Na, Cl, and F and in REE. The depleted dark areas developed upon alteration and significant REE leaching from the original apatite. The alteration and associated REE leaching was not a pervasive reaction, leaving behind a mixture of original REE-rich (bright areas) and REE-poor (dark areas) zones in the apatite crystal. The alteration and element remobilization occurred via interaction of percolating fluids with the earlier generations of apatite [53]. However, leached REE were incorporated into other mineral inclusions, LREE preferentially incorporate into monazite and HREE into xenotime ( Figure 13) Similar REE-depleted, dark domains within apatite, accompanied by monazite inclusions, are reported by   [53] from the Kirunavaara deposit, northern Sweden. The occurrence of fine monazite and/or xenotime inclusions in apatite and its grain boundaries have also been reported from metamorphosed rocks (e.g., [14,59,60]). The occurrence of monazite and xenotime in REE-depleted apatites has been observed in some other Bafq district magnetite-apatite deposits [18,20,28,35,36,52].
According to experimental studies of Harlov 2015 [13], metasomatic alteration leads to the preferential removal of Na and/or Si and LREE from primary apatites. Sodium and Si deficiency in the apatite is cross-correlated with the nucleation and growth of monazite and/or xenotime inclusions. It will occur due to two general mass transfer reactions, including Si 4+ + (Y + REE) 3+ = P 5+ + Ca 2+ or Na + + (Y + REE) 3+ = 2Ca 2+ [14]. The REE contents of the monazite grains will reflect the REE concentrations of the primary apatites (e.g., [61]). The reaction, which proceeds in an open system, would lead to relative enrichment of Ca and P in apatite zones that are depleted in LREE (dark areas). According to Putnis (2009) [62] the formation and growth of monazite and xenotime inclusions or grains along rims or cracks in apatite results from coupled dissolution-reprecipitation processes due to metasomatic alteration of apatites over a wide P-T range starting at near-surface pressures and 100 °C.
However, monazite nucleation is strongly dependent on the reactivity properties of fluid and the apatite, and also the amount of REEs available [63,64]. Dissolution-reprecipitation occurs in the interconnected micro-to nano-scale fractures and porosity in the host apatites. They allow fluids to infiltrate [62,65] and rapid (hours-days) nucleation of monazite and/or xenotime inclusions [13]. Recrystallization of altered regions can impede or cease the fluid aided coupled dissolutionreprecipitation. This is supported by our data for Chadormalu apatites (see Figures 10 and 13). Dissolving portions of apatite creates space for the nucleation and growth of monazite inclusions (cf. [53]). Fewer and larger monazite inclusions in brecciated apatites (MAA-marginal ore) show that REE leaching is largely a temperature-and deformation-related process. Large allanite crystals formed in the first type of apatites could be related to incorporation of liberated LREE.

Comparison with Magnetite Alteration
Alteration and remobilization features comparable to those in apatite has recently been documented through SEM and EPMA studies on different magnetite types from the Chadormalu deposit [22]. Primary magnetite (Mag 1) formed initially from magmatic hydrothermal fluids was enriched in Si, Al, and Ca, whereas secondary magnetite (Mag 2) with higher Fe and lower Si, Al, and Ca contents were generated through interaction of the primary ore with infiltrating fluids of nonmagmatic origin.
Fluid inclusion studies on Chadormalu apatites [31] from vein and massive ores show a range of homogenization temperatures from 266° to 580 °C and 208° to 406 °C, and salinities from 0.5 to 10.7 wt % NaCl equiv., and 0.3 to 24.4 wt % NaCl equiv., respectively. From fluid inclusion data, it can be inferred that progressive evolution of a single source fluid and/or mixing of two different fluids, one with lower salinity and higher temperature and another with higher salinity and lower temperature were involved in the generation of massive-and vein-type type ores, respectively [31].
As discussed by   [53], magnetite-apatite ore deposits may have experienced some subsequent stages of fluid-rock interactions. The F − , Cl − , OH − , and SO3 contents in apatite, coupled with the possible presence of monazite and xenotime inclusions, can serve as valuable "fingerprints" for recording alteration events in rocks. Fluids with a large range of compositions can affect the apatite through REE leaching and development of monazite inclusions. Various acids, such as HCl or H2SO4 [13], KCl brines, pure H2O, and H2O + CO2 have been shown to facilitate monazite or xenotime inclusion formation, whereas NaCl and CaCl2 brines inhibit inclusion formation.
The concentration of LREE in the NaCl brines increases with XNaCl, pressure, and temperature, because complexing of REE with Cl − stops complexing with PO4 3-and F − (e.g., [63,77,78]). Also, Na and Ca can enter the apatite structure and maintain the charge balance that prevents REE complexation.   [53] noted that inclusion-absent apatites, which are documented in Chilean IOA deposits, could have formed in the presence of Na-rich fluids preventing monazite formation. Monazite and/or xenotime inclusion formation is relatively P-T insensitive and can occur at conditions from 100 °C and <100 MPa. For different IOA deposits, there are extensive sodic and calcic alteration haloes around the magnetite-apatite mineralized zones, which formed from Na-and Ca-bearing fluids [4,7,79].
The extensive sodic and calcic alterations in the Chadormalu host rocks appear to be cogenetic with the formation of the primary apatite and associated magnetite that formed from Na-and Caenriched fluids at 490 ± 21 Ma. The secondary dark apatites with monazite inclusions formed through interaction of the primary apatite with KCl-H2O fluids responsible for potassic alteration of the host rocks at 437 ± 12 Ma. The apatite-magnetite ore in Chadormalu experienced further interaction with fluids at 246 ± 17 Ma.
The common occurrence of monazite inclusions in apatites from the Bafq district IOA deposits, and the close spatial association with potassic alteration, suggest the involvement of KCl-rich fluids.   [53] proposed the introduction of residual HCl and/or H2SO4 − enriched fluids along fluorapatite grain boundaries shortly after apatite crystallization in the Kiirunavaara IOA deposit. Torab and Lehmann (2007) [35] suggested that alteration of apatite and monazite inclusions occurred due to a switch to a KCl − dominant brine composition, consistent with potassic alteration. Jami et al. (2007) [20] considered that different fluids, ranging from a hot Na-K-Ca brine to progressively cooler and more dilute brines, contributed to the formation of Esfordi IOA deposit. Taghipour et al. (2015) [52] suggested a KCl-CO2 dominated fluid for the Esfordi and Chadormalu IOA deposits and a NaCl-rich fluid for the inclusion-free apatites in the Choghart deposit.
According to various lines of evidence obtained from apatite andmagnetite, we propose three subsequent stages of fluid evolution, associated with mineralization and alteration in Chadormalu ( Figure 16). Higher temperature, low salinity magmatic fluids were responsible for the primary magnetite-apatite mineralization (Mag 1 and bright apatites) with higher Si, Na, and REE contents (Figure 16a). These fluids were also responsible for the large-scale sodic-calcic alterations of the host rocks. During the next stage, the chemistry of the fluids would have become KCl enriched. This fluid was responsible for the formation of the secondary magnetites (Mag 2) due to a dissolutionreprecipitation process, along with REE + Y, Na, and Si leaching from the primary, bright apatite. This led to the formation of dark, low REE apatite and subsequent precipitation of monazite inclusions [80] (Figure 16b). Potassic alteration of the host rocks occurred during this stage. Continued fluid introduction in later stages was associated with the growth of the fine monazite inclusions and development of large monazite grains (Figure 16c). This is consistent with the younger ages (246 ± 17 Ma) obtained from some bright apatites. The activities of Na and Ca at these later stages must have been low; otherwise the formation of REE mineral inclusions would have been retarded [15,53]. Local potassic alteration, represented by K-feldspar, sericite, and biotite in the host rocks, as well as saturation of allanite in apatites, are related to these later stages.
The negative Eu anomalies in the Chadormalu apatites are consistent with the contribution of an oxidizing hydrothermal fluid, at least partly composed of seawater or continental non-magmatic saline fluids. Hydrothermal fluids without positive Eu anomalies have been interpreted to consist mostly of seawater and/or contaminated with sedimentary detritus [81,82]. The low Y contents in dark areas of the apatites formed due to fluid-aided alteration.

Timing of Apatite Alteration and Its Implication
Alteration of apatite, as documented for Chadormalu, appears to be a common feature in IOA type ore deposits (e.g., Kiirunavaara magnetite-apatite ore [53], Los Colorados iron oxide-apatite [83], and Se-Chahun magnetite-apatite deposit [36]). Such reequilibration and modification in Chadormalu ore was already documented for magnetite by Heidarian et al. [21]. The relative timing of apatite alteration in the IOA type deposits in the Bafq district and elsewhere, however, is controversial. While some studies show that the alteration occurred shortly after the primary apatite formation [35,36,42,53], monazite-bearing apatite grains have yielded U-Pb ages indicative of the occurrence of the alteration long after (on the scale of geological periods) the formation of the primary apatite [34,84]. Stosch et al. (2011) [34] reported U-Pb ages of 539-527 Ma for monazite-free apatites from Lakkeh Siah, Esfordi, Mishdovan, and Zarigan IOA type deposits in the Bafq district. The authors reported U-Pb ages ranging between 440 and 130 Ma for monazite inclusions in apatite. The latter age is similar to the 150-160 Ma ( 39 Ar/ 40 Ar) measured for potassic alteration in the host rocks [39]. However, using U-Pb data, Bonyadi et al. (2011) [36] determined an age of 510 ± 8 Ma for the Se-Chahun IOA deposit in the Bafq district. The Th-U-Pb data of monazite from Choghart deposit resulted in 515 ± 21 to 529 ± 21Ma [35]. Therefore, they [35] discussed that monazite inclusions formed during the later metasomatic events after Fe-P mineralization. , and P, which are Na-and Ca-deficient, in alteration process and related dissolution-reprecipitation stage, in the formation of secondary magnetite (Mag2). These fluids also interacted with the primary bright apatites, leading to remobilization and developing of Si-, Na-, and Y + REE-depleted dark areas in the apatites, as well as formation of monazite and rarely allanite crystals due to dissolution-reprecipitation process. This reaction stage probably occurred at 437 ± 12 Ma; (c) Further alteration caused growth of fine monazite inclusions.
The maximum age of 490 ± 21 Ma calculated for BSE-bright apatite in this study overlaps, within error, the lower end of previous age estimates for primary apatite in the Bafq district. On the other hand, the maximum age of 437 ± 12 Ma obtained from BSE-dark apatite provides an important evidence of a later process that led to development of dark domains and monazite inclusions; this appears to have occurred synchronous with the overprint potassic alteration in Chadormalu altered host rocks. This event was consistent with a regional extension and associated basin development and magmatism in Silurian, the evidence for which is provided by syenite intrusions and alkali basalts in Central Iran [85].
However, some spots from bright apatite type exhibit evidence for resetting of the U-Pb system defining an array with a lower intercept of 246 ± 17 Ma. Younger ages are generally assumed to reflect resetting of the U-Pb system due to later thermal or alteration events [86]. This might have occurred due to redistribution of radiogenic Pb from some parts of the apatite domains analyzed. These inconsistent ages reflect some resetting processes that caused Pb loss, which might be related to fluid flow associated with Triassic granite-tonalite magmatism, including the Saghand granitoid intrusions [25]. Several diabasic dykes in Chadormalu deposit, which are post mineralization, could have resulted in the younger ages of apatites.

Conclusions
Several lines of evidence including field observations, textural and chemical data from apatites from the Chadormalu IOA deposit in this study demonstrate that: The Chadormalu IOA deposit is hosted within late Precambrian-Cambrian intrusive and extrusive rocks, which have undergone widespread sodic (albitization), calcic and ferroan (actinolitization), and potassic (K-feldspar and sericitization) alteration, as well as carbonatization. Apatite occurs as irregular veins within the internal parts of the main iron ore body, disseminated assemblages in the marginal part of the main ore body, and apatite veins in the altered host rocks. The SEM-BSE images of the apatites are indicative of primary bright areas and secondary dark areas with monazite/xenotime inclusions. The EPMA and LA-ICP-MS data reflect that the primary REErich fluorapatite was altered through a coupled dissolution and reprecipitation process during subsequent hydrothermal activity and (or) paleoweathering. NaCl-rich fluids containing F, SO3, and P were responsible to large-scale sodic alteration in the host rocks and development of the primary fluorapatites enriched in REE + Y, Na, and Si. A KCl-enriched brine was most likely responsible for the dissolution and leaching of Na, Si, and REE + Y leaching from primary apatites (bright areas) and development of REE-depleted apatites (dark zones) containing monazite/xenotime inclusions. These KCl-bearing fluids caused local potassic alteration in upflow zones.
The U-Pb dating of different apatite types in the Chadormalu deposit yielded a minimum age of at least 490 ± 21 Ma for the primary apatite, which is similar to the ages reported from other Bafq iron deposits and also the Zarigan-type granite intrusions. From different lines of evidence, it can be concluded that the magmatic hydrothermal fluids associated with Zarigan-type intrusions were responsible for mineralization and for the large-scale sodic alteration. During later stages, KClenriched fluids were responsible for potassic hydrothermal alteration and lower T dissolutionreprecipitation that affected the primary magnetite and apatite assemblages and produced dark apatite patches. The U-Pb ages of these apatites yield 437 ± 12 Ma. These younger ages correspond to a regional extensional episode in the Posht-e-Badam block and elsewhere in Central Iran. This is supported by Silurian alkali basalts and syenitic intrusions. Extensional basins and related brines caused hydrothermal K-rich fluid flow and related dissolution-reprecipitation processes within the IOA ores. The younger Triassic ages (246 ± 17 Ma) also correspond to a prevailing extensional regime and associated high heat flow, fluid circulation, and recurring reequilibration of the original magnetite-apatite ore in Chadormalu.

Conflicts of Interest:
The authors declare no conflict of interest.