Genesis of iron-apatite ores in Posht-e-Badam Block (Central Iran) using REE geochemistry
Mir Ali Asghar Mokhtari1,∗, Ghader Hossein Zadeh2 and Mohamad Hashem Emami3 1Faculty of Science, University of Zanjan, Zanjan, Islamic Republic of Iran. 2Faculty of Science, University of Tabriz, Tabriz, Iran. 3Geoscience Research Centre, Geological Survey of Iran, Tehran, Iran. ∗Corresponding author. e-mail: [email protected]
Rare earth elements in apatites of different ore types show characteristic patterns which are related to different modes of formation of the ores. Most of the apatite-bearing iron ores are associated with alkaline magmas with LREE/HREE fractionation varying from moderate to steep. Iron-apatite deposits in Posht-e-Badam Block (Central Iran) have a high concentration of REE (more than 1000 ppm up to 2.5%), and show a strong LREE/HREE ratio with a pronounced negative Eu anomaly. This REE pattern is typical of magmatic apatite and quiet distinct from sedimentary apatites (phosphorites) which have a low REE contents and Ce negative anomalies. On the other hand, they are comparable to the REE patterns of apatites in Kiruna-type iron ores in different parts of the world. The REE patterns of apatites, iron-apatite ores and iron ores are similar and only have different REE contents. This similarity indicates a genetic relation for these rocks. Most of the iron-apatite deposits in Central Iran have similar REE patterns too, which in turn show a genetic relation for all of these deposits. This similarity indicates a similar origin and processes in their genesis. There are some small intrusions around some of the iron-apatite deposits that are petrographically identified as syenite and gabbro. These intrusions also have REE patterns similar to that of iron-apatite ores. This demonstrates a genetic relation between these intrusions and iron-apatite ores. The REE patterns of apatites in different deposits of Posht-e-Badam Block iron-apatite ores show an affinity to alkaline to sub-alkaline magmas and rifting environment. The alkaline host rocks of Central Iran iron-apatite ores are clearly related to an extensional setting where rifting was important (SSE–NNW fault lines). A probable source for this large scale ore forming processes is relatively low partial melting of mantle rocks. The ores have originated by magmatic differentiation as a late phase in the volcanic cycle forming sub-surface injections or surface flows. These ores have formed during magmatism as immiscible liquids (silicate and Fe-P-rich magmatic liquids) which separated from strongly differentiated magmas aided by a large volatile and alkali element content. Separation of an iron oxide melt and the ensuing hydrothermal processes dominated by alkali metasomatism were both involved to different degrees in the formation of Posht-e-Badam Block iron-apatite deposits. We proposed that the separation of an iron oxide melt and the ensuing hydrothermal processes dominated by alkali metasomatism were both involved to different degrees in the formation of Posht-e-Badam Block iron-apatite deposits.
Keywords. Central Iran; iron-apatite ore; Kiruna-type; Posht-e-Badam Block; REE geochemistry.
J. Earth Syst. Sci. 122, No. 3, June 2013, pp. 795–807 c Indian Academy of Sciences 795 796 Mir Ali Asghar Mokhtari et al.
1. Introduction feature is typical for the igneous apatites such as those from the Skeargaard intrusion, which show Magnetite-apatite ore deposits can be classified high F/Cl ratios (Brown and Peckett 1977). into two major groups: The origin of the Posht-e-Badam Block magnetite–apatite deposits is a subject of long- • high-Ti, apatite-rich (commonly ∼30% apatite), standing debate and remains controversial. Dif- magnetite-ilmenite deposits (nelsonites) associ- ferent ore genesis models have been proposed for ated with anorthosites; and these ore deposits. • low-Ti, magnetite deposits with variable Most of the early authors proposed a metamor- amounts of apatite (Kiruna type ores) principally phic origin for the Posht-e-Badam Block Fe-apatite associated with volcanic rocks. deposits, although some geologists who mainly worked on the NISCO projects (1971, 1974, 1975, The origin of the first type is generally accepted to 1979a, b, 1980) proposed a metasomatic origin in- be magmatic, with liquid immiscibility between sil- volving metamorphism, volcanic and intrusive icate magma and iron-titanium-phosphorous-oxide activity. magma, small percent partial melting of Fe and Williams and Houshmand Zadeh (1966)pro- P-rich crustal rocks, or extreme crystal fractiona- posed a magmatic origin for the Choghart deposit tion the most commonly cited processes (Philpotts and compared it with the Kiruna Fe ore deposits. 1967;Kolker1982; Barton and Johnson 1996; Investigators supporting an immiscible liquid mag- Naslund et al. 2000). The origin of the second type, matic model of formation included F¨orster and however, is a subject of controversy. Borumandi (1971)andF¨orster and Jafarzadeh The Kiruna-type ores are the most important. (1984) who reported magnetite lava flows, sills, They have attracted a wide genetic interest but, dykes and magnetite-bearing pyroclastic rocks at one of the most important genetic models is that Chador Malu and suggested a carbonatitic affin- they are considered as magmatic (segregation) ity for the Fe-oxide melt (a theme amplified in the deposits (Frietsch and Perdahl 1995). The mag- account of the Posht-e-Badam Block magnetite– matic genesis based on field observations is sup- apatite deposits by F¨orster and Jafarzadeh 1984). ported by the geochemical features of the main F¨orster and Jafarzadeh (1994) considered that components of the ores. More recent studies of magmatic materials in the Posht-e-Badam Block to Kiruna-type ores (Broman et al. 1999; Naslund be the product of differentiation and liquid immis- et al. 2000; Bergman et al. 2001) suggest that they cibility of a mela-nephelinitic melt. Jafarzadeh are likely to have been deposited from a Fe-oxide (1981) interpreted the Chador Malu deposit as an melt that was rich in volatiles and P, followed explosive vent filled with magnetite melt. Samani by separation of water-rich fluids and extensive (1988) classified mineralization at the Choghart hydrothermal overprint. mine and at anomalies 20 and 26, as being of The REE content of igneous minerals is magnetite-carbonatite type. controlled by the mineral-melt distribution coef- Darvish Zadeh (1982) and Darvish Zadeh and ficients and REE patterns depend on the bulk Al-e-Taha (1996) investigated the mineralogical REE composition of the parent magma (Taylor and geochemical characteristics of the Esfordi Fe–P and McLennan 1985). Of crucial importance is the deposit. Based on the mineralogical association of similarity between the REE patterns in apatite, magnetite, apatite and carbonate it was concluded magnetite and the igneous host rock, pointing to that there was a relationship between carbonatitic consanguinity between them. For example, in the magmatism and mineralization, although no in-situ Skaergaard gabbro, the apatite, pyroxene, ilmenite carbonatites were found at Esfordi. and magnetite have REE patterns similar to each Haghipour and Pelissier (1977a, b) wrote a geo- other and similar to that of the host rock (Paster logical and petrological report on the stratigraphic et al. 1974). correlation of the Posht-e-Badam Block host rocks. Apatites show a relatively wide range of chemical He had previously suggested a close relationship variations depending on the environment in which between Fe ore deposits and metasomatic meta- they have formed. Fluorine, chlorine, hydroxyl and morphism in some deposits but a volcanogenic carbonate mutually replace each other. Apatites or syngenetic origin in other deposits (Haghipour contain REE in small amounts replacing Ca2+ and 1964). Similarly, Momen Zadeh (1978, 1982, 1986) play an important role for the REE distribution proposed a syngenetic submarine volcanogenic ori- in ores and rocks (Hughes et al. 1991). Apatites gin for Posht-e-Badam Block deposits based on from iron ores of the Posht-e-Badam Block are their massive, lenticular, banded and lens-like mor- commonly fluor-apatite (Borumandi 1973; Darvish phology, their development within a specific lithos- Zadeh 1982). A similar feature is observed in the tratigraphic unit and relation to nearby stratified apatites from Kiruna-type ores (Parak 1985). This Mn and Pb deposits. Daliran (1990, 1999, 2002) Genesis of iron-apatite ores in Posht-e-Badam Block 797 also concluded that hydrothermal fluids played Rifting took place in the late Precambrian and is a prominent role in the evolution of the Bafq characterized by sequences of non-metamorphic magnetite–apatite deposits. volcanic and volcano-sedimentary rocks, local Moore and Modaberi (2003) suggested that the evaporates, bimodal volcanism, large subvolcanic separation of an iron oxide melt and the ensuing rhyolitic masses, syenite and later diabase. Iron ore hydrothermal processes dominated by alkali meta- deposits, apatite-bearing magmatic rocks which somatism were both involved to different degrees are known as apatites (Daliran 1999), Pb-Zn ores in the formation of Choghart and other similar and Th-U mineralization are related to this rifto- deposits in Central Iran. genic event (Daliran 1990; Samani 1993). Jami (2005) believed that the Esfordi Fe-P min- The Posht-e-Badam Block is a metallogenic/ eralized system displays substantial similarities tectonic province of Infracambrian age, with the with other Kiruna-type Fe-oxide deposits world- most important iron ore resources of Iran located wide. He proposed that the Esfordi deposit was between Bafq city in the south and Posht-e-Badam formed in an evolving hydrothermal system, in village in the north (figures 1 and 2). This block is a shallow marine volcanic setting, with fluids located between the western Yazd Block and the involved from diverse sources, rather than the Central Tabas Block that, together with the Lut deposit being the direct result of quenching of a Block further to the east, constitute the crustal Fe-oxide + P-rich magmatic melt. domain of the Central Iranian Terrain. The Posht- Based on oxygen isotope studies in Chador Malu e-Badam Block comprises a basement of Neo- deposit, Shamsi Pour et al. (2008) proposed that proterozoic metamorphic rocks, Early Cambrian magnetite has a magmatic origin, and hematite was diorites to tonalites and an unmetamorphosed formed by martitization of magnetite. Early Cambrian volcano-sedimentary unit with Torab (2010) proposed an extent hydrothermal minor evaporites (CVSU, Ramazani and Tucker system for the Posht-e-Badam Block iron-apatite 2003) and individual volcanic rocks. The volcan- ores based on geochemistry of apatite, REE and ism has a predominantly felsic character (rhy- Sm–Nd isotopes. olitic to rhyodacitic) with subordinate amounts of Bonyadi et al. (2010) considered that Se- andesites and is associated with spilitic basalts, Chahoun iron-apatite deposit to be an example of rare undersaturated volcanic rocks of nephelinitic a Kiruna-type deposit. Based on their EMPA anal- to basanitic composition, small mafic intrusive ysis on apatite crystals, this deposit was evolved bodies and late diabase dike swarms, both with by the hydrothermal processes. alkalic character (Ramazani and Tucker 2003). The Stosch et al. (2011) have done some U–Pb ages Early Cambrian magmatism has been related to on the apatites from the iron oxide deposits of an arc setting by Ramazani and Tucker (2003) Posht-e-Badam Block and concluded that they whereas the predominantly bimodal volcanism and have been formed 539 and 527 Ma. Obtained its alkalic character led Samani (1998), Daliran apatite ages fall entirely within the age range of (2002) and Daliran et al. (2009, 2010)tofavour the felsic magmatic rocks dated by Ramazani an extensional episode during or shortly after and Tucker (2003) [525–547 Ma]. This confirms arc volcanism. On the latest Neoproterozoic to field evidence that the ore formation was closely Early Cambrian metamorphic basement (550– related to the Early Cambrian magmatic event 540 Ma; Ramazani and Tucker 2003), an Early (Daliran 2002). Cambrian magmatic arc developed. A granodi- As mentioned above, iron-apatite ores of Posht- orite cropping out approximately 10 km NW of e-Badam Block are considered as Kiruna-type Zarigan – of the granite-tonalite suite gave a U–Pb deposit. None of these studies didn’t consider the zircon age at 533 ± 1 Ma; a dacite-porphyry REE geochemistry of the iron-apatite deposits and from Chahgaz and a rhyodacite ∼25 km north of alkaline intrusion near them. This paper presents Chahgaz each gave a U–Pb zircon age of 528 ± an investigation on the geochemistry of REE in the 1 Ma (Ramazani and Tucker 2003). In the Zari- iron-apatite ores and syenitic and gabbroic intru- gan area, the rhyolites are intruded by 525 ± 7 sions in the Posht-e-Badam Block (Central Iran) and 529 ± 16 Ma old shallow-level leucogranites of and their genetic relation. the ‘Narigan–Zarigan type’. The CVSU is uncon- formably overlain by the Lower Cambrian Zaigun– Lalun Formation and the Middle Cambrian 2. Geology trilobite-bearing Mila Formations (F¨orster and Jafarzadeh 1994). The Posht-e-Badam Block is a narrow N–S trend- The Posht-e-Badam Block constitutes the most ing rift zone which is part of the Central Ira- significant Fe-oxide and phosphate deposits in the nian Structural Zone (St¨ocklin 1968) situated in world. It contains reserves of over 2 billion tonnes its eastern part, west of the Lut Block (figure 1). of Fe (NISCO 1980) within more than 34 major 798 Mir Ali Asghar Mokhtari et al.
Figure 1. (a) Structural scheme of the Posht-e-Badam Metallogenic Block is limited by crustal meridional faults (modified after NISCO 1980). (b) Simple geological map of southern part of Posht-e-Badam Block based on Haghipour (1977a) showing the location of the iron-apatite deposits in this area.
magnetic anomalies in a 7500 km2 area. Among The ore bodies of Posht-e-Badam Block are these targets, 14 have been defined as major hosted by a lower Cambrian volcano-sedimentary deposits with over 1 billion tonnes of high grade sequence (also known as Saghand Formation) com- ore (53–65% Fe) (Taghizadeh 1976). The most eco- posed of lavas, pyroclastic, epiclesis rocks, interca- nomic iron ores consist of magnetite with apatite lated carbonates, associated with number of mafic (such as, at Choghart, Chador Malu, Esfordi, and felsic intrusions (Jami 2005; Torab 2010). Vol- Zarigan, Lakeh Siah, Gazestan, Se-Chahoun, canic rocks are rhyolite and rhyodacite in com- Mishdovan, etc.) (figures 1 and 2). SSE–NNW fault position and the sedimentary rocks are mainly lines were important for tectonic evolution of these dolomite. These ore bodies are commonly associ- ores (Daliran 1990). ated with pervasively altered rhyolitic tuffs and Genesis of iron-apatite ores in Posht-e-Badam Block 799
Figure 2. Location of the iron-apatite deposits in the southern part of the Posht-e-Badam Block (Bafq region) on the satellite image. sandstones. Some iron-apatite ores of this block are Zadeh 1982;Abedian1983; Samani and Babakhani associated with green rocks (Daliran 1990; Samani 1990; Daliran 1990; Samani 1998, 1999; Kryvdik 1993). These rocks probably are the result of alter- and Mykhaylov 2001; Rahmani and Mokhtari 2002; ation in K-feldspars rich intrusions (Daliran 1990). Jami 2005; Torab 2010). Enrichment in REE is one In addition to magnetite-apatite ores, there are of the specialities of the iron-apatite ores in this several non-ferrous ore bodies containing Pb–Zn, region. These elements are concentrated mainly Mn and U (figures 1 and 2). in apatite crystals, but other minerals such as Pebbles of iron-oxide ore were found in the basal monazite, xenotime, bastnasite, urtite, thorite and Cambrian conglomerate of the Mila Formation in bertholite have been recognized that are rich in the area of Zarigan (Haghipour 1975), indicat- REE (Kryvdik and Mykhaylov 2001; Jami 2005; ing that mineralization preceded the deposition of Torab 2010). These minerals are secondary and these Lower to Middle Cambrian conglomerates. present as small inclusions within the apatite, car- Iron-apatite ores of Posht-e-Badam Block bonate and hematite-carbonate (as monazite) and composed of magnetite (±hematite ±martite), quartz veins and veinlets (as urtite) (Kryvdik and magnetite–apatite and apatite. In most of these Mykhaylov 2001; Jami 2005; Torab 2010). deposits (e.g., Gazestan, Esfordi, Zarigan and Lakeh Siah), euhedral apatite crystals were present in the magnetite or green rocks matrix with porphyroidic texture (figure 3). Also, in some 3. Research method deposits (e.g., Esfordi), concentration of fine grain apatite crystals formed phosphatic zones within Our research includes two parts: field work and and around the iron ore body and green rocks. laboratory investigations. Field work includes sam- All analyzed apatite samples from the Posht-e- pling from the iron-apatite ores and syenitic and Badam Block are fluorine-rich and rather low in gabbroic intrusions in the Posht-e-Badam Block chlorine (Stosch et al. 2011); this is common for (Central Iran) for analyzing REEs. In the course apatite from felsic to intermediate (as well as other) of the laboratory work, 25 whole-rock samples magmatic rocks and independent of their relation and apatite crystals separated from iron-apatite to ore systems (Piccoli and Candela 2002). deposits were analyzed for REE and other rare REE mineralization in the Posht-e-Badam Block elements by ICP-MS (table 1). The analyses is related to the iron-apatite deposits (Darvish were made in ALS Chemex Laboratory in Canada. 800 Mir Ali Asghar Mokhtari et al.
(a) (b)
(c) (d)
Figure 3. Coarse-grained euhedral apatite crystals within (a) the magnetite ore at the Gazestan deposit, (b) the green rocks at the Esfordi deposit, (c) the Zarigan phosphate deposit and (d) fine to coarse-grained euhedral apatite crystals within the magnetite at the Lakeh-Siah deposit.
The detection limits for analyzing these elements (Frietsch and Perdahl 1995). REE patterns in were shown in table 1. iron-apatite ores of Posht-e-Badam Block are sim- ilar to that of Kiruna in Sweden (figure 5; Frietsch and Perdahl 1995; Harlov et al. 2002), Avnik in 4. Geochemistry Turkey (Helvaci 1984;Aral1986), El Laco in Chile (Frutos et al. 1990), Abovjan in Russia Apatites from iron-apatite ores of Posht-e-Badam (Frietsch and Perdahl 1995) and Iron Spring in Block (e.g., Esfordi, Gazestan, Zarigan and Lakeh USA (Frietsch and Perdahl 1995). All the above Siah) have a high concentration of REE (1000 ppm iron ores have a magmatic origin. up to 2.5%) (Rahmani and Mokhtari 2002; Jami Some authors such as Parak (1975) considered 2005; Mokhtari and Emami 2008; Torab 2010; that the apatites of Kiruna-type iron ores to be Stosch et al. 2011), and show a common pattern, sedimentary. However, the REE content and pat- a strong LREE/HREE fractionation and negative terns show that the apatites of Kiruna-type ores Eu anomaly (figure 4). Strong LREE/HREE frac- are different from sedimentary apatites. The con- tionation is a common characteristic of magmatic tent of REE in the apatites of Kiruna iron ores is apatites connected with alkaline rocks. Eu deple- about 2000–7000 ppm (Frietsch and Perdahl 1995) tion indicates that parent magma has undergone and in the apatites of Central Iran iron ores is more feldspar crystal fractionation. than 1% (up to 2.5%) (Rahmani and Mokhtari According to Taylor and McLennan (1985), REE 2002; Mokhtari and Emami 2008), whereas the con- distribution patterns depend on the bulk REE tent of REE in apatites from phosphorites is less composition in the parent magma. Most of the than 1000 ppm and show a Ce depletion (figure 6). apatite-bearing iron ores are associated with alka- Ce deficiency, characterizing marine apatites is not line to sub-alkaline magmas with a LREE/HREE seen in igneous apatites (Laajoki 1975; Altschuler fractionation varying from moderate to steep 1980). Genesis of iron-apatite ores in Posht-e-Badam Block 801 1000 38901000 556 4010 54.6 529 566 62.5 55.6 295 540 60.4 62.9 163.5 2961000 20.51000 57.5 110 3600 167 3960 13.6 437 19.2 10664.6 534 106 38.2 415 41.9 12.7 498 11092.4 43.6 192.5 56.4 253 35.8 106.5 48.2 11.2 138 62.3 15.2 7.7 81 18489.8 10.2 9865.9 > > > > 10000 10000 10000 > > > Analysis of the samples from Central Iran iron-apatite ores (results are in ppm). Table 1. NS.16NS.20NS.21 118.5NS.42 141NS.52 144NS.55 155.5 210 5200 S.29 1010S.31 240S.40 230 257 500S.41 20 459S.44 2420 2130S.46 22.1 2180S.48 20.3 1195 2160 22.8S.49 65.4 1075 1305 5250 262S.62 67.9 1485 5370S.63 60.1 2240 67.7 5340 138.5La.1 3630 9.3 3300 122La.3 10 959 2230 613 3660La.5 1905 8.2 630 495 9.6 5460 2.2La.14 629 7990La.23 87.2 1.5 468 152.5 2330 392 648 4230 5120 1La.84a 1.6 8.6 2420 425 4420La.117 77.9 14.8 2380 4230 9 626 75.2Za.7 177.5 1475 1570 388 880 8.8 82.6 1.1 7.9 151.5Za.9 1460 9310 1555 390 8.2 564Za.10 1340 2300 397 540 1.2 10.4 20 158.5Za.12 1.1 32.3 5.3 1.1 3110 244 72 888 25.7 2050Ga.1 3580 33.2 2030 245 85.2 363 2050 170Ga.2 6 33 103.5 1890 358 5.4 1 371 5.2 3390Ga.3 1100 21 19.7 462 107.5 9.3 13.2Ga.4 3120 23 2100 20.7 374 4770 46.9 297 433 1Ga.5 1.1 34.3 541 1 230 4630 46.5 263 521 47.1 2.6 74.7Ga.6 229 2030 44.9 59.9 20.1 230 478 75.4 2290 333Ga.7 46.8 220 28.4 3 312 1505 5920 434 15.2 270Ga.8 2.8 9.4 30.1 18 0.3 698 2720 2.8 64.5 227 43.1 7.3 288 2.3 29.6 1150 658 2290 13.8 1670 42.6 153 4420 41.6 43.9 28 124.5 247 340 52.5 0.4 305 2.2 0.4 242 146.5 0.4 41.2 1010 17.5 2640 209 6 41.8 119 857 35.5 622 5560 2 2470 257 5.6 30.4 14.8 262 4970 27.9 163 2.2 120.5 0.3 28 2.6 171.5 3.6 2.2 5.2 2.2 1230 22.7 622 40.7 60.5 160 13.8 412 3250 32.7 88.5 82.2 11.9 634 50 81.1 231 14.2 368 34.1 15.8 21.4 5227.6 129.5 10.3 121 0.3 748 446.8 0.3 92.4 0.3 77.1 605 6.2 2360 216 728 10.8 10 11.6 22.8 101 249 4.2 1.6 144.5 78 9.6 23.8 538 30.6 2.9 22.1 1.8 536.2 506.1 14.3 484.5 152 26.8 362 2780 377 60.6 9.5 93 11656.8 2491.7 16.5 16.8 105 155.5 53.4 2810 7.4 422 70 333 11.8 30 2608.3 41.1 82.5 9.7 11921.7 4.6 213 5 18.8 7.6 45.5 92.5 30.6 433 65.9 543 6.7 162 65.6 11851 1.3 358 28.2 40.3 10.2 9.15 506 7.6 102 0.5 241 7348 11.4 26.8 87.9 14.8 454 217 7977.8 11871.6 98.2 8.2 31.1 5.75 3.4 66.7 25.8 40.9 17175.6 144 31 2.9 16.5 38.1 10.4 63.9 10985.4 424 1.6 69.2 11.8 354 58.6 35.4 83.4 0.4 5.1 120 53.4 502 58.6 0.4 23.9 5.6 60.8 328 114 8.4 9.35 54.2 10.7 53.7 9587.9 2.3 6.94 15.3 76.4 7.4 69.2 53.2 464.5 59.8 307 171 9.4 62.3 52.7 14.7 1.1 40 0.3 327 88.8 7964.7 9.93 171 9.36 8.63 29.8 48.7 85.2 3013.9 21.8 7.47 10.7 59.2 53.4 29 375.2 49.6 159 4.76 121 7786.47 10.2 3.79 22.8 11529.2 160 4834 6.73 11047.9 8.97 22.3 130 20.6 3.98 13.6 5752.46 30.1 20.4 114 24 15070.9 2.5 14.1 116 4 11113.4 2791.74 12.3 2.76 12.1 13411.9 24.5 1659.87 12581.9 2.69 1661.72 SampleD.L. LaNS.2NS.7NS.9 1080 0.5 Ce 450 4910 2490 0.1 1075 Pr 265 0.03 120.5 Nd 981 446 0.1 Sm 149.5 0.03 71.8 Eu 14.2 0.03 7.7 148.5 Gd 0.05 67.2 19.4 Tb 0.01 100.5 8.6 Dy 0.05 21 42.6 0.01 Ho 59.6 8.9 0.03 Er 7.7 23.6 0.01 Tm 41.9 3.1 0.03 Yb 5.5 16.6 0.01 5383.8 Lu 2.2 – ΣREE 2343.8 802 Mir Ali Asghar Mokhtari et al.
(a) (b)
(c) (d)
Figure 4. Chondrite-normalized REE pattern for apatites, apatite-bearing iron ores and iron ores from Posht-e-Badam Block iron-apatite ores (a: Esfordi, b: North Esfordi, c: Lakeh Siah, d: Gazestan).
Figure 5. Chondrite-normalized REE patterns of apatites from Kiruna-type iron ores. Figure 6. Chondrite-normalized REE pattern for apatites from phosphorites (Frietsch and Perdahl 1995). Chondrite normalized REE patterns of apatites (apatite crystals and phosphte ores), apatite- bearing iron ores and magnetite ores with neg- Iron ores of Lakeh Siah deposit indicate flat ligible amount of apatite or without apatite are REE patterns with a weak LREE/HREE frac- similar in each iron-apatite ores of Posht-e-Badam tionation (figure 4). This can be related to presence Block (figure 4), however with a varying content of of pyroxene minerals (altered to tremolite-actinolite) REE (very high in apatite crystals, high in apatite- together with magnetite. Moreover, a strong bearing iron ores and low in iron ores). Commonly, LREE/HREE fractionation in apatites can lead to all phases are LREE-enriched and show negative HREE enrichment in the iron ore. Eu anomalies (iron ores in Lakeh Siah deposit indi- Most of the iron-apatite ores of Pasht-e-Badam cate nearly flat REE patterns). These similar REE Bolck (Central Iran) have similar patterns too patterns indicate that these units have a common (figure 4), which in turn show a genetic relation origin. for these deposits. Samani (1999) believed that Genesis of iron-apatite ores in Posht-e-Badam Block 803
(a) (b)
Figure 7. Chondrite-normalized REE pattern for apatites, apatite-bearing iron ores, iron ores and syenitic and gabbroic intrusions adjacent to them from Posht-e-Badam Block iron-apatite ores (a: north Esfordi, b: Lakeh-Siah).
high LREE content is the result of mantle meta- Posht-e-Badam Block deposits had been consid- somatism and enrichment of these elements in flu- ered as result of liquid immiscibility and injection ids originating from mantle that caused regional or extrusion of ore magmas originated from alka- metasomatism. line/carbonatitic magmas, for many years (Samani The REE geochemistry of the apatites from the 1988;F¨orster and Jafarzadeh 1994; Darvish Zadeh Posht-e-Badam Block is not only similar to other and Al-e-Taha 1996; Daliran and Stosch 2005). global apatite-magnetite ores, including those of Samani (1988) reported field observations of car- Kiruna in Sweden, but also in part to both bonatite (?) dykes in some locations at the Bafq nelsonites and to the carbonatite association. How- and Saghand areas. ever, they differ fundamentally from the sedimen- Different tectonomagmatic settings have been tary phosphorites, thus excluding a marine source. proposed for the magmatic events associated with Some of the researchers believed that iron- the iron-apatite mineralization in Central Iran. apatite ores of Posht-e-Badam Block have genetic Extensional tectonic setting with the formation relation with alkaline intrusions that took place of an aborted rift that was originally introduced nearby (Williams and Houshmand Zadeh 1966; by Haghipour (1975), was further supported by Daliran 1990;F¨orster and Jafarzadeh 1994; Darvish Zadeh and Al-e-Taha (1996), Samani Darvish Zadeh and Al-e-Taha 1996; Samani 1999; (1999) and Daliran (1990, 2002). Darvish Zadeh Moore and Modaberi 2003; Jami 2005; Daliran and and Al-e-Taha (1996) believed that mineraliza- Stosch 2005). In this part of the paper, we are going tion in this area is related with a high volcanism to compare the REE patterns of syenitic and gab- rifting structure (HVRS). Ramazani and Tucker broic intrusions located near the Esfordi and Lakeh (2003) describe the magmatic event of Bafq region Siah deposits with patterns of the iron-apatite ores. to the arc-related calc alkaline magmatism of a Comparison of chondrite-normalized REE pat- Neoprotrozoic-Early Cambrian orogeny along terns of apatites, apatite-bearing iron ores and iron the Proto-Tethyan margin of the Gondwana ores in the Esfordi and Lakeh-Siah deposits with super-continent. those of syenitic and gabbroic intrusion located Daliran and Stosch (2005) believe that Posht-e- near them indicate very similar patterns (enrich- Badam Block phosphate-REE deposits most likely ment in LREE, a strong LREE/HREE fractiona- were formed from sulphur-poor fluid source that tion and negative Eu anomalies) (figure 7). These was enriched in Fe, P and REE. The source of similar patterns demonstrate that the mentioned these fluids may be linked to mantle degassing or syenitic and gabbroic intrusions and iron-apatite to carbonatite magmas as is suggested by Samani ores originated from a common magmatic source. (1998). Hauck (1990) states that alkaline magma is the ultimate source of iron-rich melts, which he suggests to have been derived from it by liquid 5. Ore genesis immiscibility. Moore and Modaberi (2003) describe at least Origin of the iron-apatite deposits of the Posht- two generations of apatite within the Choghart e-Badam Block, similar to their counterparts of deposit. The first, which is contemporaneous with the Kiruna-type systems, is controversial. The the main phase of iron oxide formation, displays 804 Mir Ali Asghar Mokhtari et al. euhedral crystals ranging in size from a few mil- constituents such as CO2 and F is also important limeters to a few centimeters in diameter. It is in this process (Weidner 1982). The presence of intimately intergrown with magnetite. The sec- fluor-apatite and hydrous minerals (amphiboles) ond generation occurs as subhedral to anhedral also suggests high F and H2O content. Both experi- crystals in lenses, dikes, and veinlets of varying mental and theoretical analyses of the behaviour of size and thickness, which cut the magnetite–apatite volatile-rich magma systems stress the important ore. Moreover, Jami (2005) reported four gener- role liquid immiscibility may play (Kolker 1982). ations for apatite crystals based on mesoscopic In some places, immicible portions of phosphates, and microscopic observations. The early stage is oxides and silicates can be seen within another por- a LREE-rich apatite that occurs within the mas- tion. It means that segregation of the magma were sive magnetite ore. The other generations are asso- done within the crustal depth and wasn’t complete. ciated with hematite, actinolite, quartz-carbonate Silicate portion formed syenitic and gabbroic intru- ± REE and carbonate-quartz-actinolite-chlorite- sions and oxide-phosphate melt ascent to upper epidote ± bastnaesite ± synchesite vein and vein- parts. lets extending into the host rocks and magnetite The breakdown of this late immiscible phase ore body. has resulted in the formation of magnetite–apatite The field relationships and the overall geological melt, which behaved intrusively at Choghart and evidence at the Posht-e-Badam Block iron-apatite similar deposits, and probably extrusively in occur- deposits indicate that the rhyolitic to syenitic and rences such as Chador Malu and Lakeh Siah in gabbroic host rocks are the products of intra- the Posht-e-Badam Metalogenic Block. The intru- cratonic plutonism and a tensional geological set- sion or extrusion of the separated iron-oxide melt ting (rifting) were roughly synchronous and coeval depends upon depth of separation, volatile content with mineralization. of iron-rich melt, and finally presence or absence of Presence of vesicular magnetite lava flows, favourable tectonic and structural conditions. sills, dikes and magnetite-bearing pyroclastic rocks Most of the high concentration of REEs was at iron-apatite deposits of Posht-e-Badam Block probably partitioned as F-REE complexes in the is reported by some researchers (F¨orster and fluorine-dominated fluid generated during volatile Borumandi 1971; Jafarzadeh 1981;F¨orster and exsolution. The majority of the REEs are con- Jafarzadeh 1984; Daliran 1990; Jafarzadeh 1994; tained in apatite and REE-bearing microscopic Moore and Modaberi 2003; Jami 2005; Stosch et al. solid inclusions such as monazite and xenotime in 2011). These evidences are suggested to be diag- apatite (Jami 2005; Torab 2010; Stosch et al. 2011). nostic of a magmatic origin. Also, geochemical evi- Presence of volatiles led to the alteration, bre- dences in these deposits (e.g., REE patterns; figure ciation and metasomatism of country rocks. In 4) indicate magmatic origin for them. this stage, iron ores formed with some amounts Pre-requisite for the formation of the Central of apatite. Crystallization of magnetite caused Iran iron-apatite ores is an association with alka- increasing of pressure in the fluid phase rich in line to sub-alkaline magmatic rocks in combination volatiles and incompatible elements (such as P, with proximity to large fault systems and an older REE, Th, U, F and Cl). Finally, this fluid invaded basement. A probable source for this large-scale the iron ores and country rocks and formed the ore forming process was partial melting of mantle phosphatic zone or iron-apatite zones around or rocks. within the iron ores. It can be seen as phosphatic The iron-apatite ores of Posht-e-Badam Block veins and veinlets within the iron ores at Choghart, were probably formed by magmatic differentiation Esfordi, Gazestan and Lakeh Siah deposits (Moore of an alkaline magma rich in Fe and incompotible and Modaberi 2003; Jami 2005). elements such as P, REE, Th, U, F and Cl that was Based on rare earth element distribution in derived by partial melting of upper mantle. Deep apatites, liquid immiscibility is proposed as a seated faults caused transfering of parent magma process of primary ore generation in the iron- and replacement in the upper parts of the crust. apatite deposits of the Posht-e-Badam Block. In this magma reservoir, the high phosphorous However, textural evidence to support the hypothe- and alkali content probably led to the formation sis of liquid immiscibility is mostly destroyed by of a Fe–F, CO2–H2O–P–Na dominated immisci- younger metasomatic/hydrothermal modifications ble melt which separated from a silica-rich melt. and recrystallization. The high phosphorous content and the so-called alkali-iron effect (Guilbert and Park 1997) fluxed the immiscible melt and kept iron in solution at 6. Conclusion temperatures significantly lower than the melting point of magnetite at rather shallow depth (Moore The consistent spatial and temporal association of and Modaberi 2003). The role of other volatile the iron-apatite ores within the narrow zone of Genesis of iron-apatite ores in Posht-e-Badam Block 805
Posht-e-Badam Block provides conclusive evidence Norrbotten County (east of the Caledonian orogen); Sver. of a common source of these ores. Iron-apatite Geol. Unders 56 110p. ores from Posht-e-Badam Block (Central Iran) Bonyadi Z, Mehrabi B, Davidson J, Meffre S and Ghazban F 2010 Apatite chemistry and its application to the have comparable REE patterns that demonstrate hydrothermal evolution of the Se-Chahun magnetite– a similar origin and processes in their genesis. The apatite deposit; The 1st International Applied Geolog- REE patterns of apatites in Central Iran iron ores ical Congress, Islamic Azad University, Mashhad, Iran, show an affinity to alkaline to sub-alkaline magmas pp. 2103–2106. and rifting environment (SSE–NNW fault lines). A Borumandi H 1973 Petrographische und lagerstatten kundliche untersuchungen der Esfordi-formation zwischen probable source for this large-scale ore forming pro- Mishdovan und Kushk bei Bafg (Zentral Iran); Diss. cess was partial melting of mantle rocks. The ores RWTH Aachen, 174p. were formed during magmatism as immiscible liq- Broman C, Nystrom J, Henriquez F and Elfman M 1999 uids which separated from strongly differentiated Fluid inclusions in magnetite–apatite ore from a cool- magmas, aided by large volatile and alkali elements ing magmatic system at El Laco, Chile; GFF 121 253–267. content. Brown G M and Peckett A 1977 Fluor-apatite from the In summary, it may be concluded that following Skeargaard intrusion, east Greenland; Mineral. Mag. 41 the establishment of late Proterozoic extensional 227–232. tectonic regime in the Posht-e-Badam Block and Daliran F 1990 The magnetite–apatite deposit of Mishdovan, the onset of alkaline magmatism, iron oxide min- east central Iran. An alkali rhyolite hosted ‘Kiruna- eralization occurred at this region. In view of the type’ occurrence in the infracambrian Bafg metallotect; Ph.D. thesis, Univ. of Heidelberg, Geowiss. Abhandl. 37 enormous size of the ore body, the large volume 248p. of melt or high temperature hydrothermal fluid Daliran F 1999 REE geochemistry of Bafg apatites: Impli- needed for the formation of these deposits, and the cation for the genesis of Kiruna-type iron ores; In: Min- conflicting evidence for and against liquid immis- eral deposits: Processes to processing (eds) Stanley et al., cibility or hydrothermal origin, neither of the two Balkema, Rotterdam, pp. 631–634. Daliran F 2002 Kiruna-type iron oxide-apatite ores and hypotheses alone can be held responsible for the apatitites of Bafq district, Iran, with an emphasis on formation of them. 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MS received 27 May 2012; revised 31 December 2012; accepted 16 January 2013