# Magnetite Texture and Geochemistry in the Takab Ore Deposit (NW Iran): Implications for a Complex Hydrothermal Evolution

Christiane Wagner, Omar Boudouma, Nicolas Rividi, Beate Orberger, Ghasem Nabatian, Maryam Honarmand, Iman Monsef

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Christiane Wagner, Omar Boudouma, Nicolas Rividi, Beate Orberger, Ghasem Nabatian, et al.. Magnetite Texture and Geochemistry in the Takab Ore Deposit (NW Iran): Implications for a Complex Hydrothermal Evolution. Minerals, 2025, Minerals-MDPI, 15 (2), pp.137. 10.3390/min15020137. hal-04998355

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Article

# Magnetite Texture and Geochemistry in the Takab Ore Deposit (NW Iran): Implications for a Complex Hydrothermal Evolution

Christiane Wagner <fnref n="1" />*, Omar Boudouma <fnref n="1" />, Nicolas Rividi <fnref n="2" />, Beate Orberger <fnref n="3" />, Ghasem Nabatian <fnref n="4" />, Maryam Honarmand <fnref n="5" /> and Iman Monsef <fnref n="5" />

Academic Editors: Yuichi Morishita
and Napoleon Q. Hammond

Received: 24 September 2024

Revised: 24 January 2025

Accepted: 25 January 2025

Published: 29 January 2025

**Citation:** Wagner, C.; Boudouma, O.; Rividi, N.; Orberger, B.; Nabatian, G.; Honarmand, M.; Monsef, I. Magnetite Texture and Geochemistry in the Takab Ore Deposit (NW Iran): Implications for a Complex Hydrothermal Evolution. *Minerals* **2025**, *15*, 137. https://doi.org/10.3390/min15020137

**Copyright:** © 2025 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license
(https://creativecommons.org/licenses/by/4.0/).

**Abstract:** The massive magnetite deposit from Takab (NW Iran) is hosted in amphibolite layers intercalated with the chemical and terrigenous sediments of the Takab BIF. A comprehensive textural and chemical study allowed three types of magnetite (Mt) to be distinguished. Mt1 forms large (≤1 mm) inhomogeneous grains surrounded and locally invaded by magnetite Mt2. Oscillatory zoning is present in Mt1 and Mt2. Mt3 forms bands aligned along fracture planes. Mt3 may contain hematite relicts and is porous in proximity to hematite. Mt1 shows variable and higher Si (up to 1.4 wt. %), Al, Ca, and Mg and lower Fe content (68 wt. %) than Mt2. Mt3 has the lowest Si (<0.3 wt. %) and highest Fe (71 wt. %) contents. The temperature of formation decreases from Mt1 (600 °C) to Mt2 (500–550 °C) and Mt3 (380–440 °C). Mt1 likely formed in a reducing Si-rich environment. The close spatial relationship, sharp compositional boundaries, similar crystallographic structure of Mt1 and Mt2, and porosity in Mt2 suggest a fluid-assisted coupled dissolution of Mt1 and precipitation of Mt2 (CDR process). Microfracturing allowed the penetration of oxidizing fluid and the formation of platy hematite bands. Mt3 (mushketovite) formed after hematite by interaction with a reducing lower temperature fluid through a redox transformation.

**Keywords:** magnetite; textural evolution; trace elements; CDR process; hydrothermal fluid; EMPA; SEM; EBSD; Takab; Iran

## 1. Introduction

Magnetite, a common mineral in many ore deposits, can contain minor and trace elements which have been widely used to constrain the environment of formation [1–3]. However, both the texture and trace-element composition of magnetite can be significantly modified or re-equilibrated by metasomatic fluids, e.g., [4–7]. Therefore, detailed

<footnote n="1">¹ Sorbonne Université, CNRS-INSU, Institut des Sciences de la Terre de Paris, ISTE, F-75005 Paris, France; omar.boudouma@sorbonne-universite.fr</footnote>

<footnote n="2">² Sorbonne Université, CNRS-INSU, Institut de Physique du Globe de Paris, IPGP, F-75005 Paris, France; nicolas.rividi@sorbonne-universite.fr</footnote>

<footnote n="3">³ Laboratoire Géosciences Paris-Saclay, GEOPS, Université Paris-Saclay, CNRS, F-91405 Orsay, France; beate.orberger@universite-paris-saclay.fr</footnote>

<footnote n="4">⁴ Department of Geology, Faculty of Science, University of Zanjan, University Blvd., Zanjan 45371-38791, Iran; gh.nabatian@znu.ac.ir</footnote>

<footnote n="5">⁵ Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran; m.honarmand@iasbs.ac.ir (M.H.); iman.monsef@iasbs.ac.ir (I.M.)</footnote>

<footnote>* Correspondence: christiane.wagner.raffin@sorbonne-universite.fr</footnote>
studies of texture and chemical composition must be carried out to defining the processes
of magnetite formation.

Iran is the ninth most important iron ore producer in the world with about 4 billion
tons of iron ore annually produced from more than 200 iron deposits [8–11]. Iron ore de-
posits were formed during several metallogenic stages: (1) Neoproterozoic–early Cam-
brian, (2) late Paleozoic–early Mesozoic, and (3) Cenozoic. The largest iron ore deposits
formed during stage 1, mainly as Kiruna-type (IOA, iron oxide apatite) deposits, and dur-
ing stage 3, mainly as Fe-skarn deposits [12–15]. Their spatial distribution is related to the
main suture zones of the fragmented Iranian continental blocks resulting from the com-
plex evolution of the Tethyan oceans [10,16,17]. The most important iron deposits are in
the Central Iranian Zone (CIZ), in the Sanandaj–Sirjan zone (SSZ), and in the Alborz
(AMB)–Azerbaijan belts (Figure 1; e.g., [18–20] and references therein). The SSZ extends
1500 km from northwest to southeast parallel to the Zagros belt, on the east side of the
Main Zagros Fault. It hosts numerous iron and iron–manganese deposits of various ori-
gins (Figure 1; [10,21]). For example, in the south of the SSZ, the major Gol-e-Gohar iron
deposit and the Heneshk iron–manganese deposit are related to the opening of the Neo-
Tethys Ocean and are interpreted to be of volcano–sedimentary or mixed hydrothermal
and volcano–sedimentary origin [18]. Further north, the Fe-skarn-type Hamekasi iron de-
posits (e.g., Baba Ali) are related to the subduction and closure of the Neo-Tethys Ocean
(Figure 1; [10] and references therein). In northwestern Iran, the major deposit of the Zan-
jan–Soltaniyeh area, which belongs to the near structural Central Iranian Zone, comprises
both skarn-type and volcano–sedimentary iron deposits [10,20]. A hundred km west of
Zanjan in the northwestern part of the SSZ, several iron mineralizations have been re-
ported in the Takab area. In this area, some outcrops were initially explored in the 1990s
and studied for their geology and geochemistry by a few authors: at 60 km east of Takab
town (Figure 1), the Shahrak mining complex represents reserves of a total of 60 million
tons of iron ore with an average Fe grade of 50% [22,23]. It comprises 10 ore deposits,
among them the Korkora skarn deposit [23] and the Kosaj volcano–sedimentary-type iron
deposit [24]. The iron deposit of Mianaj, near Halab (Figure 2), was also reported as a
metamorphosed and folded volcano-sedimentary-type iron deposit [25]. Different petro-
genetic models have been proposed, such as the superior-type BIF [26] and Algoma-type
BIF [27], for the Halab and Ouzijan iron mineralizations.

In a recent study on the Takab magnetite mineralizations near Halab, Honarmand et
al. [8] investigated the banded, nodular, and disseminated magnetite layers in the folded
succession of micaschist and quartzite (the studied outcrops are represented by yellow
circles in Figure 2). Based on geochemical, Nd isotopic, and geochronological data, they
proposed that this iron ore corresponds to a Rapitan-type banded iron formation. These
data indicate that it originated in a back-arc basin through iron precipitation and the in-
corporation of ca. 20% of the terrigenous materials in the chemical precipitates. The Neo-
proterozoic felsic–intermediate crystalline basement was the main sediment source [8].
Furthermore, trace-element and Fe and O isotopic investigations of the banded, nodular,
and disseminated magnetite [28] support their genesis from a mixture of seawater and
hydrothermal fluid environments. Isotopic data point out the complex hydrothermal his-
tory from seafloor high-T° fluid to later re-equilibration with lower-T° fluid produced
during the decarbonatization of the intercalated carbonate-rich schists.

Close to the nodular and disseminated magnetite mineralization, massive magnetite
layers are intercalated in amphibolite. These layers were never studied in detail for pe-
trography, iron oxide chemistry, or textural features.

This study presents for the first time major- and trace-element electron microprobe
data and EBSD analyses of the massive magnetite layers in the vicinity of banded, nodu-
lar, and disseminated magnetite layers that were previously studied.
<figure><img src="image_2.png" /></figure>

**Figure 1.** Tectonic and structural map of Iran (after [10]). The red star indicates the location of the Takab study area (Takab town GPS coordinates are 36°24'2" E and 47°6'42" E). The black boxes correspond to the locations of different types of iron ore deposits in Iran. Zagros, Zagros ranges; KRSZ, Kermanshah Radiolarites subzone; SSZ, Sanandaj-Sirjan zone; UD, Urumieh-Dokhtar magmatic arc; the Central Iranian microcontinent includes Yadz, Posht-e-Badam block (PB), Tabas, and Lut blocks; AMB, Alborz magmatic arc; KTZ, Khazar-Talesh-Ziveh structural zone; CIZ, Central Iranian zone; Sistan, East Iran ranges; Makran, Makran zone; KD, Kopeh-Dagh ranges; Zabol, Zabol area; CMR, Cenozoic magmatic rocks [10].
<figure><img src="image_3.png" /></figure>

**Figure 2.** Simplified geological map of the Takab area modified after [8]. The marked area (47°23′51″ E to 47°25′39″ E longitude and 36°29′13″ N to 36°30′03″ N latitude) limits the location of the Takab iron ore samples from previous studies (yellow circles: banded, nodular, and disseminated iron ore, [8,28]) and from this study (yellow stars: massive iron ore).

## 2. Materials and Methods

### 2.1. *Geological Settings*

The Takab area is located in the northwestern part of the Sanandaj–Sirjan magmatic-metamorphic structural zone (Figure 1). The Takab area consists of metamorphic rocks including gneisses, amphibolites, mica- and calc-schists, quartzite, jaspilite, marble, and rare granulite. These rocks were intruded by granitoids, diorite, gabbros, and rare rhyolite [8,29,30]. The area experienced several metamorphic and deformation events during the Precambrian and the Tertiary. The latter event is related to the Alpine orogeny, leading to the faulting and folding of the Precambrian metamorphic units. The Tertiary metamorphic event produced the migmatites, at ca. 25 Ma [31], and the granitoids [10,29,31].

The metamorphic units of the late Neoproterozoic–early Cambrian (548–568 Ma) age [29,31] were overlain unconformably by Tertiary sedimentary units. The lack of Paleozoic
and Mesozoic rocks indicates that this region was exhumated at that time [32]. Exhuma-
tion started during the Early to Middle Jurassic back-arc extension and Tertiary crustal
extension related to compressional tectonics [29]. The different rock units experienced up-
per greenschist to amphibolite facies metamorphism (450° to 700 °C and 0.6–1 GPa) in the
Middle Jurassic. After exhumation and weathering, the ore was covered by Quaternary
sediments [18].

2.2. *Ore Deposit and Sample Material*

The Takab iron ore outcrops are located west of the village of Halab (marked area in
Figure 2). Recently, Honarmand et al. [8] and Wagner et al. [28] studied outcrops of
banded and nodular magnetite layers up to 2 m in thickness in the folded succession of
micaschist and quartzite from the Takab BIF, 3 to 6 km west of Halab (47°23′51″ E to
47°25′39″ E longitude and 36°29′13″ N to 36°30′03″ N latitude, Figure 2, yellow circles). In
this study, we investigate the massive magnetite layers intercalated with micaschist and
amphibolite (former meta-basalt) in the Takab BIF. The samples were collected ca. 3 km
west of Halab along an exposure 200 m in length situated at 47°25′28.1″ E and 36°29′25.5″
N (Figure 2, yellow stars).

The iron ore is hosted in a metamorphosed volcano-sedimentary sequence com-
posed of mica- and calc-schists, quartzite, and jaspilite. Some marble and amphibolite
(meta-basalt) bands are intercalated with the micaschists (Figure 3). U-Pb dating on zir-
cons from micaschist lower and upper layers of the ore body indicates a depositional age
of ca. 560 Ma [8]. The banded, nodular, and disseminated iron ore consist of magnetite,
which may be partially hematized and surrounded by goethite in some samples [8,28].
The massive magnetite layers are hosted in amphibolite interlayered with the micaschists
and quartz sequence. The host amphibolite is a massive, dark green rock with schistose
texture (Figures 4a and 5). At the microscale, transparent patches of albite (Ab92, Supple-
mentary Table S1) and microquartz occur in a light to dark green matrix mainly composed
of amphibole and minor biotite (Figure 4a). The amphibole forms elongate crystals of ac-
tinolite and a few acicular crystals of Mg-hornblende (Table 1). Actinolite hosts relicts of
a more Mg-rich amphibole (Figure 4b and Table 1). Euhedral crystals of titanite, interstitial
calcite, and large patches of epidote and chlorite are also present (Figure 4b-e). Chlorite
is mostly developed along amphibole and mica cleavages (Figure 4d) or as a few rosette-
shaped interstitial crystals. Accessories are barite (Figure 4f), rare Cl-free apatite and al-
lanite, and late pyrite. (Figure 4g,h).

The amphibolite hosts massive magnetite layers of several tens to about one meter in thickness, which are slightly folded and/or boudinized (Figure 3g,h). The contact between the magnetite layers and the amphibolite is either abrupt or gradual (Figure 5b,c). In the magnetite layers, albite, amphibole, and minor biotite are interstitial to the magnetite crystals (Figure 5) and have the same composition as in the amphibolite (Table 1). Amphibole and mica can be locally altered to epidote, calcite, and chlorite. Rare interstitial calcite and rosette-like chlorite crystals are also observed (Figure 5f).
<figure><img src="image_4.png" /><figcaption>**Figure 3.** (a–f) Field photographs of the outcrops of the Takab iron ore deposit. (g,h) Close-up view of the amphibolite hosting massive magnetite ore.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Amphibolite mineralogy: optical photomicrographs (a) in plane-polarized light and (c,e) in cross-polarized light; backscattered images (b,d,f–h). (a) Actinolite and minor biotite alternating with quartz and feldspar transparent patches; (b) actinolite with a Mg-rich core; interstitial calcite crystal also shown in (c) and accessory apatite. (d) Epidote and chlorite developed in biotite. (e–h) Accessory titanite, barite, allanite, and pyrite.</figcaption></figure>

<table><caption>**Table 1.** Composition (in wt. %) of amphibole from Takab host amphibolite and iron ore.</caption><thead><tr><th rowspan="3"></th><th colspan="10">Host</th><th colspan="2">Iron Ore</th></tr><tr><th colspan="5">Actinolite</th><th colspan="2">Tr</th><th colspan="3">Mhb</th><th colspan="2">Actinolite</th></tr><tr><th>core</th><th>core</th><th>acicular crystals</th><th>core</th><th>core</th><th>core</th><th>core</th><th>core</th><th>core</th><th>core</th><th>core</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>54.44</td><td>51.83</td><td>50.8</td><td>53.9</td><td>55.97</td><td>55.97</td><td>57.32</td><td>49.36</td><td>47.34</td><td>48.45</td><td>52.55</td><td>51.75</td><td>55.93</td></tr><tr><td>TiO<sub>2</sub></td><td>0.12</td><td>0.11</td><td>0.08</td><td>0.1</td><td>0</td><td>0</td><td>0.05</td><td>0.37</td><td>0.41</td><td>0.43</td><td>0.05</td><td>0.06</td><td>0.03</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>1.51</td><td>2.97</td><td>3.61</td><td>1.27</td><td>2.85</td><td>2.85</td><td>3.02</td><td>7.42</td><td>7.63</td><td>8.05</td><td>2.1</td><td>2.83</td><td>3.81</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.04</td><td>0.11</td><td>0.05</td><td>0.06</td><td>0.03</td><td>0.03</td><td>0.01</td><td>0.13</td><td>0.13</td><td>0.14</td><td>0.03</td><td>0</td><td>0.02</td></tr><tr><td>FeO</td><td>15.96</td><td>18.01</td><td>18.34</td><td>15.6</td><td>5.9</td><td>5.9</td><td>1.97</td><td>13.93</td><td>16.36</td><td>13.9</td><td>16.81</td><td>17.02</td><td>3.82</td></tr><tr><td>MnO</td><td>0.19</td><td>0.12</td><td>0.23</td><td>0.16</td><td>0.1</td><td>0.1</td><td>0.1</td><td>0.5</td><td>0.38</td><td>0.57</td><td>0.12</td><td>0.12</td><td>0</td></tr><tr><td>MgO</td><td>13.95</td><td>12.53</td><td>12.31</td><td>14.04</td><td>20.21</td><td>20.21</td><td>22.53</td><td>13.79</td><td>12.4</td><td>13.49</td><td>13.46</td><td>12.97</td><td>20.83</td></tr><tr><td>CaO</td><td>12.02</td><td>11.65</td><td>11.87</td><td>12.33</td><td>12</td><td>12</td><td>12.63</td><td>11.28</td><td>11.57</td><td>11.33</td><td>12.11</td><td>12.36</td><td>12.46</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.45</td><td>0.67</td><td>0.84</td><td>0.38</td><td>1.18</td><td>1.18</td><td>1.13</td><td>1.5</td><td>1.28</td><td>1.48</td><td>0.49</td><td>0.64</td><td>1.38</td></tr><tr><td>K<sub>2</sub>O</td><td>0.08</td><td>0.17</td><td>0.16</td><td>0.1</td><td>0.26</td><td>0.26</td><td>0.08</td><td>0.16</td><td>0.34</td><td>0.15</td><td>0.15</td><td>0.13</td><td>0.17</td></tr><tr><td>Total</td><td>98.76</td><td>98.16</td><td>98.3</td><td>97.95</td><td>98.5</td><td>98.5</td><td>98.85</td><td>98.44</td><td>97.84</td><td>98</td><td>97.87</td><td>97.88</td><td>98.45</td></tr></tbody></table>
<table><caption>Tr = tremolite, Mhb = magnesio hornblende. mg# = 100*Mg/(Mg+Fe).*</caption><tr><td>mg#</td><td>61.1</td><td>55.6</td><td>54.7</td><td>61.8</td><td>86.1</td><td>86.1</td><td>95.4</td><td>64.1</td><td>57.7</td><td>63.6</td><td>59</td><td>57.8</td><td>90.7</td></tr></table>

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Hand specimen (a) and enlarged views of the massive magnetite ore from Takab. (b) Thin section (3.8 cm x 2.5 cm), (c) close-up view of the contact between the amphibolite and the magnetite ore, and (d) BSE image of the area marked in (c) showing the amphibole Mg-rich core. (e,f) Optical photomicrographs in cross-polarized light of prismatic, fibrous, and acicular amphibole crystals. (f) Rosette-shaped interstitial chlorite.</figcaption></figure>

## 2.3. Analytical Methods

Three representative samples of the massive iron ore were studied. The samples were prepared as standard polished thin sections for study by optical microscopy, high-resolution scanning electron microscopy (HR-SEM), electron backscatter diffraction (EBSD), and electron microprobe analysis (EMPA).

### 2.3.1. Electron Microprobe Analysis

EMPA was performed with a Cameca SX100 electron microprobe equipped with five wavelength dispersive spectrometers at CAMPARIS, Sorbonne-Université, Paris, France. Fe and O were analyzed under the following operating conditions: monochromators LLIF for the measure of Fe and LPC1 for oxygen; accelerating voltage 15 kV; beam current 10 nA; counting times, 10 s over the peak and 10 s for background measured on each side of the peak; Fe₂O₃ as standard. WDS multi-element X-ray mapping was obtained from the same EMP.

For trace elements, monochromators were LTAP (Si, Al, and Mg) and PET (for Ca). To decrease the detection limits Ti, V, Mn, Ni, Cr, and Cu were simultaneously analyzed with LIF and LLIF monochromators on three different spectrometers. The operating conditions were 25 keV, 300 nA, 40 s to 70 s, and a beam size of 1 µm. Any damage effects on the samples were checked during analyses. The concentration of V using the Kα lines was not affected by the adjacent Ti Kβ line at the observed low Ti concentration. Standards
were synthetic metal oxides and natural minerals: Ti, Mn: MnTiO₃; Fe, O: Fe₂O₃; Al; orthoclase; Cr: Cr₂O₃; Ni: NiO; Mg, Si, Ca: diopside; V: vanadinite; Cu: Cu native. Carbon coating was performed simultaneously for standards and samples to ensure the same coating thickness. These settings yielded minimum detection limits, with average concentrations as low as ~3–5 for Ca, Ti, and Mn and ~11–19 ppm for Mg, Al, and Si. The concentrations in V, Ni, Cr, and Cu were below or near the detection limit (~8–15 ppm) and could not be accurately determined.

### 2.3.2. Scanning Electron Microscopy and Electron Backscattered Diffraction Analysis

Backscattered imaging by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) were carried out on a ZEISS Supra 55 VP at ISTeP (Sorbonne Université, Paris, France). The EBSD analysis was performed using an ARGUS™ FSE/BSE imaging system mounted on a Bruker e-Flash<sup>FS</sup> detector. The data were processed with the ESPRIT 2.2 software package from the QUANTAX EDS/EBSD system from the Bruker Company. EBSD is detailed hereafter. Carefully polished thin sections from three samples were prepared and then analyzed uncoated by SEM. Representative areas were selected. The initial conditions were as follows: angular deviation lower than 5°, minimum pixel size of 5, and spike correction of zero-solution measurements relative to the surrounding points. EBSD measurements were used to acquire mineral phases, crystal orientations, and microstructural information. Crystal orientations are described using the set of Euler angles, and each orientation is represented by a combination of RGB colors in an Euler map. Pole figure (PF) and inverse pole figure (IPF) are also used in this study. The produced pole figures are displayed in terms of points or in contoured versions indicating the strength of the clustering of poles relative to that of a random distribution. The IPF maps were generated for the three selected viewing directions parallel to the X, Y, and Z axes, quoted as IPFX, IPFY, and IPFZ.

## 3. Results

### 3.1. Morphology and Textures of Magnetite

Magnetite grains are enclosed in a matrix mainly composed of actinolite and albite (Figure 4). Based on BSE imaging and WDS X-ray mappings, three types of magnetite have been identified (Figures 6-8). The first type (Mt1) occurs as sub- to anhedral grains of an inhomogeneous dark gray magnetite in BSE images, ranging up to 1 mm in dimension. Mt1 is surrounded by a light gray type 2 magnetite (Mt2). The boundary between Mt1 and Mt2 is usually sharp (Figure 6a, bottom right, Figure 7a) but may also locally invade dark Mt1, forming lighter gray patches (Figure 6a, upper left). Three different subtypes of Mt2 have been locally recognized in sample TAK-Z1: Mt2-A and Mt2-C are light gray on BSE images, while Mt2-B is darker gray (Figure 8). Both Mt1 and Mt2-B show an oscillatory growth zoning of dark and light zones, suggesting a higher concentration of elements with a low atomic number in the dark zones (Figure 8). Mt3 appears as bands (10–80 µm width) with needle-like textures of bright magnetite aligned along microfracture planes that crosscut the Mt1–Mt2 grains (Figures 6a and 7; Supplementary Figure S1). Mt3 bands replace platy hematite, which remains as rare relict-aligned patches (Figure 6). In some bands, Mt3 appears to be porous only close to the hematite relicts. Aggregates of euhedral magnetite grains (100–200 µm) from sample TAK-Z4 also display well-developed chemical oscillatory zoning (Figure 9).
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Morphology and textures of magnetite. BSE image of magnetite grains from sample TAK-Z3. (a) Dark gray magnetite Mt1 is surrounded by lighter gray magnetite Mt2 (bottom right), which locally invaded Mt1 (upper left). Magnetite Mt3 forms bright bands, which may host relict hematite (Hem). (b) Inverse pole figure (IPF) of relict hematite phases (white arrows) in two Mt3 bands outlined by the yellow dashed lines (sample TAK-Z1). Different relict hematite patches within one single Mt3 band display similar colors, indicating that hematite patches have similar crystallographic orientation and, thus, represent a unique hematite band. (c) Reflected light. Enlarged view of an Mt3 band (outlined by the yellow dashed lines) with relict hematite (bright patches) undergoing transformation in Mt3 (red arrows).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 7.** BSE image (a) and WDS X-ray maps of selected elements in a magnetite grain from sample TAK-Z1 (b–f). Magnetite Mt1 contains higher Si, Al, Mg, and Ca than magnetite Mt2. Mt3 bands have the highest Fe and the lowest Si, Al, Mg, and Ca contents. The yellow rectangle corresponds to the zone detailed in Figure 8. The red arrow is precised in Figure 8.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Detailed BSE image (a) and WDS X-ray maps of selected elements (b–f) in a magnetite grain from sample TAK-Z1 showing dark magnetite Mt1 (D) and the different subzones of gray magnetite Mt2 (zones A, B, C). Oscillatory zoning is visible in Mt1 and Mt2-B. The total grain is shown in Figure 7. The red arrow limits the different zones of magnetite Mt2, see text for details.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 9. (a–d)** WDS X-ray maps of selected elements in an aggregate of smaller magnetite grains from sample TAK-Z4 with compositional and zoning characteristics like to those of large magnetite grains (Figure 8).</figcaption></figure>

### 3.2. Electron Backscatter Diffraction Mapping

EBSD misorientation maps of magnetite show both sudden orientation changes across grain boundaries and gradual or wavy misorientation unrelated to grain boundaries. For example, Figure 10 shows wavy misorientation in Mt1 (lower left) as well as in the Mt3 band, delimited by the dotted lines.
<figure><img src="image_11.png" /><figcaption>**Figure 10.** Inverse pole figure IPF Y (a) and pole figures (b,c) of magnetite from sample TAK-Z1. The dotted lines delimit the larger Mt3 band of Figure 6b. Abrupt changes in color reflect changes in orientation across grain boundaries. Wavy misorientation reflects intragrain deformation. In the latter case, the pole figure shows dispersed points along circle girdles (subset 2) compared to the clustered points of uniform intragrain orientation (subset 1). H: hematite.</figcaption></figure>

A small area of the different magnetite grains was selected to generate pole figures (PFs) and inverse pole figures (IPFs). Magnetite grains with a uniform color in IPF maps show discrete point clusters in PFs, reflecting uniform intragrain orientation (Figure 10, subset 1). Magnetite grains with wavy colors show less-clustered points in PFs, and the crystal orientations may disperse along small circle girdles in stereoplots of <100>, <110>, and <110> directions in individual magnetite grains (Figure 10, subset 2), which is related to the wavy intragrain misorientation. This indicates that the intragrain microstructures formed through deformation by dislocation movement without recovery or rearrangement in subgrains [33]. Magnetite thus shows at the microscale the effect of deformation, which is observed at the macroscale by the folded magnetite layers in the host amphibolite (Figure 3g,h).

### 3.3. Chemical Composition of Magnetite

WDS X-ray elemental mapping was performed to determine the distribution of minor and trace elements. Si, Al, Mg, and Ca maps clearly distinguish the different magnetite types (Figures 6–8; Supplementary Figure S1). Mn has a uniform distribution throughout the different magnetite types, whereas Ni, Cr, and Cu are below or close to the detection limits and thus give no information. Therefore, these elements are not further discussed here. Of the three texturally identified magnetite types, Mt1 is rich in Si, Al, Mg, and Ca (Figures 6–8). Mt2 shows variable enrichment or depletion of Si, Al, Mg, and Ca in the different zones Mt2-A, -B, and -C (Figure 8). Magnetite from zone Mt2-B presents oscillatory enrichment or depletion, which is also observed in magnetite Mt1 (Figure 8) and magnetite from the aggregates (Figure 9). Mt3 is characterized by the lowest contents of these elements and by the highest Fe content (Figure 7).
The average concentrations, standard deviations, and ranges of values are listed in Table 2. The complete analytical results are given in Supplementary Table S2.

<table><caption>**Table 2.** Fe and O (wt. %) minor- and trace-element (ppm) contents of magnetite from Takab.</caption><thead><tr><th>Sample</th><th></th><th>TAK-Z1</th><th>TAK-Z1</th><th>TAK-Z1</th><th>TAK-Z4</th><th>TAK-Z4</th></tr><tr><th>Mt type</th><th></th><th>Mt1</th><th>Mt2</th><th>Mt3</th><th>Mt1</th><th>Mt2</th></tr></thead><tbody><tr><td>n analyses</td><td></td><td>12</td><td>4</td><td>10</td><td>8</td><td>6</td></tr><tr><td rowspan="2">Fe (wt. %)</td><td>Average</td><td>68.0</td><td>69.8</td><td>70.8</td><td>67.6</td><td>69.6</td></tr><tr><td>Range</td><td>66.1-69.4</td><td>69.3-70.4</td><td>69.9-71.3</td><td>67.2-68.7</td><td>69.0-70.1</td></tr><tr><td rowspan="2">O (wt. %)</td><td>Average</td><td>29.4</td><td>28.6</td><td>28.5</td><td>29.1</td><td>28.7</td></tr><tr><td>Range</td><td>29.0-29.7</td><td>28.2-28.8</td><td>28.0-29.1</td><td>28.0-29.6</td><td>27.8-29.1</td></tr><tr><td rowspan="3">Si</td><td>Average</td><td>11,017</td><td>4336</td><td>214</td><td>9942</td><td>3550</td></tr><tr><td>sd</td><td>1107</td><td>409</td><td>195</td><td>1444</td><td>397</td></tr><tr><td>Range</td><td>8899-12779</td><td>3846-4840</td><td>11-426</td><td>8274-11,941</td><td>3149-4045</td></tr><tr><td rowspan="3">Ti</td><td>Average</td><td>130</td><td>86</td><td>26</td><td>293</td><td>104</td></tr><tr><td>sd</td><td>61</td><td>35</td><td>20</td><td>120</td><td>18</td></tr><tr><td>Range</td><td>51-276</td><td>37-126</td><td>BDL-63 (2)</td><td>149-474</td><td>73-106</td></tr><tr><td rowspan="3">Al</td><td>Average</td><td>2489</td><td>1031</td><td>275</td><td>2321</td><td>985</td></tr><tr><td>sd</td><td>479</td><td>76</td><td>94</td><td>364</td><td>173</td></tr><tr><td>Range</td><td>1428-3103</td><td>947-1120</td><td>187-434</td><td>1617-2773</td><td>818-1301</td></tr><tr><td rowspan="3">Mn</td><td>Average</td><td>262</td><td>227</td><td>261</td><td>279</td><td>223</td></tr><tr><td>sd</td><td>26</td><td>59</td><td>107</td><td>54</td><td>27</td></tr><tr><td>Range</td><td>198-289</td><td>161-305</td><td>166-519</td><td>198-368</td><td>193-273</td></tr><tr><td rowspan="3">Mg</td><td>Average</td><td>1263</td><td>410</td><td>34</td><td>1225</td><td>382</td></tr><tr><td>sd</td><td>320</td><td>134</td><td>26</td><td>378</td><td>131</td></tr><tr><td>Range</td><td>807-1882</td><td>234-560</td><td>BDL-71 (4)</td><td>825-1794</td><td>205-553</td></tr><tr><td rowspan="3">Ca</td><td>Average</td><td>2677</td><td>1289</td><td>313</td><td>2799</td><td>1192</td></tr><tr><td>sd</td><td>585</td><td>109</td><td>95</td><td>720</td><td>294</td></tr><tr><td>Range</td><td>1606-3529</td><td>1127-1349</td><td>194-484</td><td>1760-3821</td><td>1018-1155</td></tr><tr><td rowspan="3">V</td><td>Average</td><td>22</td><td>19</td><td>34</td><td>nd</td><td>nd</td></tr><tr><td>sd</td><td>10</td><td>5</td><td>6</td><td></td><td></td></tr><tr><td>Range</td><td>13-43</td><td>12-24</td><td>22-42</td><td></td><td></td></tr><tr><th>Sample</th><th></th><th>TAK-Z3</th><th>TAK-Z3</th><th>TAK-Z3</th><th>TAK-Z3</th><th>TAK-Z3</th></tr><tr><th>Mt type</th><th></th><th>Mt1</th><th>Mt2</th><th>Mt3 porous</th><th>Mt3</th><th>Mt3</th></tr><tr><td>n analyses</td><td></td><td>23</td><td>10</td><td>8</td><td>13</td><td>13</td></tr><tr><td rowspan="2">Fe (wt. %)</td><td>Average</td><td>68.2</td><td>69.7</td><td>71</td><td>71.1</td><td>71.1</td></tr><tr><td>Range</td><td>67.5-69.2</td><td>69.5-70.2</td><td>70.1-71.5</td><td>70.4-71.8</td><td>70.4-71.8</td></tr><tr><td rowspan="2">O (wt. %)</td><td>Average</td><td>29.4</td><td>29.0</td><td>28.7</td><td>28.8</td><td>28.8</td></tr><tr><td>Range</td><td>28.9-29.9</td><td>28.6-29.2</td><td>28.3-29.7</td><td>28.4-29.5</td><td>28.4-29.5</td></tr><tr><td rowspan="3">Si</td><td>Average</td><td>11806</td><td>5430</td><td>587</td><td>432</td><td>432</td></tr><tr><td>sd</td><td>1231</td><td>936</td><td>445</td><td>274</td><td>274</td></tr><tr><td>Range</td><td>9769-13,785</td><td>3949-7170</td><td>12-870</td><td>55-884</td><td>55-884</td></tr><tr><td rowspan="3">Ti</td><td>Average</td><td>143</td><td>56</td><td>9</td><td>38</td><td>38</td></tr><tr><td>sd</td><td>61</td><td>13</td><td>5</td><td>22</td><td>22</td></tr><tr><td>Range</td><td>38-246</td><td>37-82</td><td>BDL-17 (2)</td><td>8-65</td><td>8-65</td></tr><tr><td rowspan="3">Al</td><td>Average</td><td>2781</td><td>1190</td><td>278</td><td>239</td><td>239</td></tr><tr><td>sd</td><td>500</td><td>252</td><td>95</td><td>104</td><td>104</td></tr><tr><td>Range</td><td>1745-3480</td><td>759-1621</td><td>175-468</td><td>105-519</td><td>105-519</td></tr><tr><td rowspan="3">Mn</td><td>Average</td><td>222</td><td>240</td><td>198</td><td>181</td><td>181</td></tr><tr><td>sd</td><td>23</td><td>32</td><td>45</td><td>45</td><td>45</td></tr><tr><td>Range</td><td>181-262</td><td>196-276</td><td>148-292</td><td>101-261</td><td>101-261</td></tr><tr><td>Mg</td><td>Average</td><td>1272</td><td>775</td><td>42</td><td>62</td><td>62</td></tr></tbody></table>
<table><thead><tr><th></th><th>sd</th><th>308</th><th>220</th><th>24</th><th>28</th></tr><tr><th>DL = 19</th><th>Range</th><th>614–1738</th><th>342–1064</th><th>BDL-70 (2)</th><th>18–89</th></tr></thead><tbody><tr><td rowspan="2">Ca</td><td>Average</td><td>2866</td><td>1661</td><td>473</td><td>416</td></tr><tr><td>sd</td><td>521</td><td>353</td><td>213</td><td>179</td></tr><tr><td>DL = 3</td><td>Range</td><td>1966–3575</td><td>1150–2226</td><td>249–824</td><td>228–797</td></tr><tr><td rowspan="2">V</td><td>Average</td><td>24</td><td>21</td><td>nd</td><td>27</td></tr><tr><td>sd</td><td>10</td><td>8</td><td></td><td>7</td></tr><tr><td>DL = 9</td><td>Range</td><td>11–39</td><td>11–30</td><td></td><td>22–42</td></tr></tbody></table>

Ni and Cu are not reported as their contents are below or close to the detection limits (DL = 12 and 8 ppm, respectively). BDL = below detection limit. (n) = number of analyses below the detection limit. nd = not determined. sd = standard deviation.

The magnetite composition is presented in Figure 11 for sample TAK-Z3, as it is representative of magnetite from the three studied samples. In the three samples of magnetite, Mt1 has the most variable composition and the lowest Fe content (66–68 wt. %), while both Mt2 and Mt3 show a restricted range of composition in minor and trace elements (Figure 11a, b) and higher Fe contents (70 and 71 wt. %, respectively). In the three samples, Mt1 magnetite has high contents of Si (0.83–1.38 wt. %) and moderately high Al (0.14–0.35 wt. %), Ca (0.16–0.38 wt. %), and Mg (0.06–0.19 wt. %). Mt2 magnetite has lower Si (0.32–0.72 wt. %), Ca (0.10–0.22 wt. %), Al (0.08–0.16 wt. %), and Mg (0.02–0.11 wt. %). Mt3 (porous and non-porous) magnetite has the lowest minor- and trace-element contents, with Si (<<0.01–0.08 wt. %), Al (0.02–0.05 wt. %), Ca (0.02–0.08 wt. %), and Mg (<<0.01 wt. %). The three types of magnetite contain similar low Mn (0.02–0.05 wt. %), while Ti is <<0.01 wt. % in Mt2 and Mt3 but can reach 0.04 wt. % in Mt1. Mt1 and Mt2 have low similar V contents (19–24 ppm), while Mt3 is slightly richer in V (27–34) ppm. The composition of the thin oscillatory dark and lighter gray bands in Mt1 and Mt2-B magnetite grains could not be resolved with the microprobe.

<figure><img src="image_12.png" /><figcaption>**Figure 11.** Binary plots of selected elements in magnetite from Takab, sample TAK-Z3. (a) [Si + Al + Mg + Ca + Mn] vs. Fe (wt. %) and (b) Ca vs. [Si + Al + Mg] (ppm), showing high trace-element and</figcaption></figure>
low Fe contents in magnetite Mt1 compared to magnetite Mt2. Magnetite Mt3 has the lowest trace-element and the highest Fe contents. (c) Si and Al vs. Fe³⁺ and (d) [Ca + Mg + Mn] vs. Si diagrams (in apfu) show the negative correlation of Fe³⁺ with Si and Al, possibly indicating the incorporation of Si in the magnetite structure by substitution for Fe³⁺, while Si is positively correlated with Ca, Mg, and Mn as valence-state species related to the substitution. Blue circles, Mt1; yellow squares, Mt2; filled red triangles, non-porous Mt3; and red open triangles, porous Mt3. Calculation of Fe²⁺/Fe³⁺ partitioning explained in Supplementary Table S2.

# 4. Discussion

## 4.1. Magnetite Formation Conditions

The minor- and trace-element composition of magnetite has been used in numerous studies to fingerprint its environment of formation based on a series of discrimination diagrams, e.g., [1,3,34]. However, coexisting phases may affect the composition of magnetite, for example, the co-precipitation of amphibole and epidote could decrease the incorporation of Mg and Al, respectively, in magnetite. In this study, as the different magnetite types Mt1, Mt2, and Mt3 co-crystallized with the same coexisting assemblage, any major control of the co-precipitated phases on compositional differences between the three magnetite types is unlikely.

The contents of Mg, Mn, Ti, V, Ni, and Cr in magnetite, for example, can be used to discriminate between formation in a hydrothermal or a magmatic environment. In this study, the different types of magnetite plot in the field of hydrothermal magnetite characterized by low Ti content on a Fe/(Mg + Al + Mn) discrimination diagram (Figure 12). On the V vs. Ti diagram, most of the different types of magnetite plot outside the hydrothermal field due to a very low V content (Supplementary Figure S2), which does not give here any reliable information. On the other hand, the extremely low Ni and Cr contents of all three types of magnetite (Table 2) further support a hydrothermal origin. This agrees with a previous study of the layered, nodular, and disseminated magnetite from the Takab BIF, in which it was shown that both the trace-element and Fe and O isotopic compositions provided evidence for a complex hydrothermal history [8,28].

<figure><img src="image_13.png" /><figcaption>**Figure 12.** Discrimination diagram of Fe/(Mg + Mn + Al) vs. Ti. The different magnetite types from samples TAK-Z1 and TAK-Z3 plot in the hydrothermal domain. The boxes outline the domains of banded (B), nodular (N), and disseminated (D) magnetite previously studied [28]. Hydrothermal and magmatic domains are after [34].</figcaption></figure>
Element partition coefficients are temperature-dependent and can therefore be used for constraining the temperature of formation of magnetite. The incorporation of Ti increases with temperature in magmatic and in high-temperature hydrothermal environments, whereas it is negligible in low-temperature hydrothermal fluids [35,36]. Discriminating diagrams, such as (Al + Mn)/(Ti + V), have been proposed to outline different fields of temperature [3,37]. As can be seen in Table 2 and Figure 11, magnetite Mt1 has a variable composition with up to 1.4 wt. % Si and up to 0.4 wt. % Al. This raises the question of whether Si is really incorporated in the magnetite structure or present as Si-bearing nanoparticles [37–39]. In the absence of a definitive argument (as discussed below in Section 4.1), we tentatively estimated the possible contamination of the analyses of magnetite by nanoinclusions of amphibole at 5–6% (Supplementary Figure 3). The appropriate correction was applied before using the (Al + Mn)/(Ti + V) diagram and temperature calculations.

The decreasing trend shown in the (Al + Mn)/(Ti + V) diagram (Figure 13) indicates a decreasing temperature from Mt1 to M2 and Mt3. Overall, the different magnetite types formed at medium temperature ca. 300 °C or 200–300 °C. These temperature ranges are similar although slightly lower than that those recorded by the banded, nodular, and disseminated magnetite previously studied [28], shown by the rectangles in Figure 13.

<figure><img src="image_14.png" /></figure>

**Figure 13.** Approximate temperatures of formation of magnetite. Temperature reference fields are after [3,38]. The boxes outline the domains of banded (B), nodular (N), and disseminated (D) iron ore samples previously studied [28].

Canil and Lacourse [40] proposed a T°<sub>Mg-mag</sub> empirical geothermometer, T°<sub>Mg-mag</sub> = -8344 (±320)/[lnX<sub>Mg</sub> - 4.1 (±0.28)] - 273, that was applied to magnetite from various settings: igneous rocks, hydrothermally altered rocks, and ore deposits. Figure 14 presents the results of the T°<sub>Mg-mag</sub> calculations of the three magnetite types excluding the analytical points from the areas showing oscillatory zoning in magnetite Mt1 and Mt2b. The data are presented in Supplementary Table S3.
<figure><img src="image_15.png" /></figure>

**Figure 14.** Calculated formation temperatures for Takab magnetite show a decreasing trend from magnetite Mt1 and Mt2 to magnetite Mt3. The temperatures were calculated using the T°$_{Mg-mag}$ thermometer of [40]. The calculated T° values have an uncertainty of about ±60 °C. In sample TAK-Z3, Mt3p corresponds to porous Mt3 adjacent to relict hematite. Mt3 was not observed in sample TAK-Z4. The points correspond to the total analyses in Supplementary Table S2 and calculated temperatures in Supplementary Table S3. (X): mean values.

A decreasing trend is evidenced from magnetite Mt1 to Mt2 and Mt3 in samples TAK-Z1 and TAK-Z3, whereas in sample TAK-Z4, magnetite Mt1 and Mt2 from the agglomerates formed at a similar temperature. In sample TAK-Z3, the porous magnetite Mt3 records lower temperature than the non-porous magnetite Mt3. However, the calculated temperatures are much higher than those estimated from the (Al + Mn)/(Ti + V) diagram (Figure 13): e.g., average 600 °C for Mt1, 500 °C for Mt2, and 440 °C for Mt3. This difference is difficult to explain.

Oxygen fugacity can also influence the composition of magnetite. For example, V has three valence states ($V^{3+}$, $V^{4+}$, and $V^{5+}$), and as only $V^{3+}$ can enter the magnetite structure in high abundance, its incorporation is linked to $f_{O_2}$ and increases with decreasing $f_{O_2}$. In contrast, Ti has only one valence state ($Ti^{4+}$) and a rather constant partition coefficient. Thus, Ti/V ratios decreasing in magnetite indicates a reduced environment. The Ti/V ratios are low in magnetite Mt1 (6.6), Mt2 (3.8), and Mt3 (1.2), thus suggesting a rather reduced environment of formation. Relatively constant V (Table 2) content in the different magnetite types suggests quite constant $f_{O_2}$.

In summary, magnetite from the massive magnetite ore formed in a rather reduced hydrothermal environment at medium temperature for Mt1 and Mt2, which slightly decreased for Mt3.

## 4.2. Occurrence of Si and Elemental Substitution Mechanisms in Magnetite Mt1

Si-bearing magnetite with $SiO_2 > 1$ wt. % have been reported in numerous studies in a variety of rocks from igneous rocks to mid-ocean serpentinites, banded iron formations, porphyry deposits, skarns, and others (see review by [41] and so-called "silician magnetites"). Si can be either structurally incorporated or forms mineral micro- to nanoparticles.

Si and Al can enter in the structure of magnetite due to their small ionic radii ($Si^{4+}$, 0.26 Å; $Al^{3+}$, 0.39 Å), preferentially substituting for $^{IV}Fe^{3+}$ (0.57 Å) in tetrahedral sites rather than for $^{VI}Fe^{3+}$ (0.63 Å) or $^{VI}Fe^{2+}$ (0.69 Å) in octahedral sites [42]. Their incorporation in tetrahedral coordination requires coupled substitutions with divalent cations in octahedral sites for valence state balancing, with or without octahedral vacancy [41,43]. In this case, well-defined elemental trends are observed. For example, $Fe^{3+}$ is negatively
correlated with Si and Al, while Si is positively correlated with divalent elements, Fe²⁺, Ca, and Mg. Structural Si has been evidenced by combining high-resolution analytical techniques (X-ray microdiffraction, XRMD, and TEM) [37,41]. Xu et al. [38] reported the presence of Si in magnetite lattice as discrete nanoprecipitates, in which Si replaces IVFe³⁺ with vacancies in the octahedral sites, using combined Z-contrast imaging and ab initio calculations. Deditius et al. [37] reported two types of trace-element incorporation in magnetite by both structural incorporation and the formation of mineral nanoparticles (NPs). Moreover, Ciobanu et al. [39] reported the presence of submicroscopic Si-Fe precipitates and calc-silicate inclusions in magnetite from the Olympic Dam Iron Oxide Copper Gold deposit, south Australia.

Here, EMPA spot analyses were carefully located to avoid the rare inclusions, which were detected by high-resolution SEM analysis. Magnetite Mt1 contains up to 1.4 wt. % Si and 0.4 wt. % average Al and Ca. EMPA results show the anti-correlations of Fe³⁺ with Si and Al, while Si is positively correlated with divalent elements, Fe²⁺, Ca, and Mg as well as Al (Figure 11c,d). These trends would be consistent with the structural incorporation of Si. However, in the absence of further analysis by high-resolution techniques, the presence of nanoparticles in magnetite cannot be excluded. To examine the possible presence of amphibole inclusions in magnetite, we checked whether the amphibole plots on the continuation trend of the magnetite compositions. The slopes of the compositions of magnetite and the line connecting pure magnetite and amphibole average composition are similar in terms of Ca/(Si + Al), for example (Supplementary Figure S3). This allowed the approximation of up to 5–6% of the amphibole component in the magnetite composition. However, this admixture fails to reproduce the observed Ca content of magnetite for a given (Si + Al) content. On the other hand, there is no agreement between the slopes when adding Mg to the check in terms of Ca/(Si + Al + Mg).

Nevertheless, regardless of the mechanisms involved, the presence of these elements in magnetite Mt1 suggests its precipitation in a Si (Al, Mg)-rich and reducing environment, as reported for silician magnetite from hydrothermal, volcano-sedimentary, and metamorphic environments [41,43–46].

### 4.3. *Dissolution-Reprecipitation Replacement of Mt1 by Mt2*

Magnetite Mt1 was replaced by magnetite Mt2 mainly along grain margins. Mt1 magnetite is rich in minor elements (Si, Ca, and Al) compared to magnetite Mt2 (Table 2 and Figure 11). The relations between Mt1 and Mt2 are the following: (1) A close spatial relationship between Mt1 and Mt2 with a sharp boundary between Mt1 and Mt2 visible on BSE images and WDS X-ray maps (Figures 6–8). IPF maps of composite Mt1 and Mt2 magnetite grains indicate a single-crystal domain shown by the same color, indicating the same orientation (Figure 15a,b). Similarly, the pole figures of selected areas in Mt1 and Mt2 show the coincidence of the crystallographic orientations of Mt1 and Mt2 (Figure 15c,d). This supports the inheritance of the crystallographic structure during the epitaxial growth of Mt2. (2) Porosity is locally developed in Mt2. (3) There is a sharp reaction front between Mt1 and Mt2 without diffusion, as shown in EMPA profiles (Figure 16c). Such compositional and textural characteristics suggest a fluid-induced replacement of Mt1 by Mt2 by a coupled dissolution-reprecipitation process (CDR) [5,47–50]. The CDR process allows the dissolution of early trace-element-rich Mt1 magnetite in the core and the precipitation of secondary Si-poor Mt2 magnetite at the rim. The presence of structural Si or nano-Si-rich inclusions in magnetite Mt1 would have caused a crystal lattice defect and/or deformation that would have facilitated its dissolution. Moreover, porosity would have facilitated the infiltration of hydrothermal fluid that, in turn, may have boosted the dissolution process. Coupled dissolution-reprecipitation evidenced here removed Si, Ca, and
Mg from Mt1 magnetite near the reaction front and formed secondary Mt2 magnetite with lower trace-element content (type 1-CDR process in [5]).

<figure><img src="image_16.png" /><figcaption>**Figure 15.** Enlarged BSE image (a) and inverse pole figure IPF Y (b) of Mt1 and Mt2 adjacent zones in a magnetite grain from sample TAK-Z1, and pole figures (c,d) of selected areas corresponding to Mt1 and Mt2 showing a similar orientation of Mt1 and Mt2 in favor of a CDR process (see text).</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 16.** BSE images of the magnetite from sample TAK-Z1 (a) and enlarged view of the yellow contoured zone (b). In (b), the red arrow indicates the EMPA element profile (c) through the different magnetite zones Mt1, Mt2, and Mt3. The multi-element profiles show the difference in</figcaption></figure>
composition and the sharp contact between the different magnetite types. WDS X-ray maps of the same area are given in Figures 7 and 8. See text for explanations of the different zones A-E.

Among the different factors that may control a dissolution-reprecipitation process, such as temperature, oxygen fugacity, and fluid composition [5,51–53], a change in fluid composition played here a major role in inducing the CDR process of magnetite Mt1/Mt2. The slight temperature decrease from Mt1 to Mt2 (Figure 13) may also have influenced the process. In contrast, the V content, similarly low (19–24 ppm, Table 2) in both Mt1 and Mt2, argues against a possible role of varying oxygen fugacity in the process.

### 4.4. Origin of Oscillatory Zoning in Magnetite Mt1

Oscillatory zoning of thin dark and light gray bands (in BSE) is observed in magnetite Mt1 and Mt-B large grains (Figure 8) and in the magnetite agglomerates (Figure 9). The decrease in brightness in BSE images indicates a lower concentration of trace elements with lower atomic numbers than Fe, such as Si, Al, and Ca. Oscillatory zoning is too fine to be analyzed, and no clear compositional variations could be demonstrated in EMPA profiles (Figure 16, zones B and D). Oscillatory zoning is generally interpreted as growth zoning under cyclical variation in fluid composition, temperature, or oxygen fugacity, which changes trace-element partitioning between magnetite and fluid [2,5,53]. The oscillatory zoning is parallel to the interface between Mt1 and Mt2, which likely results from a growth zoning under fluctuating fluid composition (Figures 8 and 9). The preservation of oscillatory zoning requires crystal growth faster than the intracrystalline diffusion. Moreover, assuming that the thin dark and light gray bands may be similar in composition to Mt1 and Mt2 magnetite, respectively, we suggest that the oscillatory zoning would likely result from a fluctuating fluid composition rather than temperature or oxygen fugacity variation (see previous paragraph).

### 4.5. Origin of Magnetite Mt3 (Mushketovite)

Magnetite Mt3 has replaced the former platy hematite, now present as rare relicts, and is developed along cleavages and microfracture planes or crosscutting Mt1 and Mt2 magnetite grains (Figure 6). It is characterized by low concentrations of Si, Al, Mg, Ca, Mn, and Ti (Table 2, Figures 7, 11 and 16). The similarity between the crystal structures of magnetite and hematite controls the precipitation of magnetite and leads to the coincidence of orientation between the two minerals (Supplementary Figure S4). The pseudomorphic replacement of hematite by magnetite (mushketovitization) has been reported in many ore deposits in which mushketovite shows a platy morphology (e.g., [4,34] and references therein). It may result from two different processes: redox reactions by the reduction of $Fe^{3+}$ to $Fe^{2+}$ with the removal of oxygen and non-redox reactions by the addition of $Fe^{2+}$ ions [54,55]. In the latter case, the number of Fe atoms increases, whereas in the redox process, the replacement of $Fe^{3+}$ with $Fe^{2+}$ causes a volume decrease calculated at 1.64% [4]. Magnetite Mt3 immediately adjacent to relict hematite shows microporosity, but porosity decreases with distance from hematite or in the bands without relict hematite. Thus, we propose that magnetite Mt3 likely formed via the redox transformation mechanism in a reducing environment compared to that prevailing for the formation of hematite.

## 5. Conclusions

This first study on the texture and chemistry of massive magnetite layers in amphibolites and micaschists confirms findings from previous studies on the disseminated and layered iron ore in micaschists from the same area, i.e., a Rapitan-type BIF mineralization formed in a back-arc basin, through iron precipitation and incorporation of terrigenous materials in the chemical precipitates of the original BIF. The protolith of the amphibolites
was a basalt, typically interlayered with chemical and terrigenous marine sediments. Hydrothermal fluids formed and modified the iron ore.

Our findings support a formation in four steps (Figure 17): Step 1: The silician magnetite Mt1 likely formed in a rather reduced Si-rich environment, a hydrothermal volcano-sedimentary setting typical for BIF. The incorporation of structural or nano-Si-rich inclusions may have caused lattice defects or deformation promoting fluid circulation and alteration of the magnetite grains. Step 2: A fluid-induced coupled dissolution of Mt1 and precipitation of Mt2 (CDR process) is supported by a close spatial relationship, sharp compositional boundaries, the similar crystallographic structure of Mt1 and Mt2, and porosity in Mt2. Porosity promotes the infiltration of hydrothermal fluids, accelerating the CDR process. Step 3: Microfracturing increasing the permeability led to the penetration of a more oxidized fluid along cleavage planes and the formation of platy hematite. By increasing the contact between the fluid and the magnetite grain, microfracturing allows an acceleration of the oxidation process and dissolution of magnetite. Step 4: Finally, after progressive sealing of the fractures, mushketovite Mt3 formed after hematite from a more reduced fluid composition by a redox transformation supported by the volume decrease. Deformation can be attributed to the two latter steps.

<figure><img src="image_18.png" /><figcaption>**Figure 17.** Schematic illustration of the multiple processes involved in the formation of magnetite from the Takab iron ore deposit. CDR: coupled dissolution-reprecipitation process. Dark, gray, and bright magnetite as defined on BSE images.</figcaption></figure>

**Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15020137/s1; WDS X-ray maps of selected elements in a magnetite grain from sample TAK-Z3. Figure S2: Discrimination diagram V vs Ti for the Takab magnetite. Figure S3: Ca vs. (Si + Al) plot of magnetite Mt1 and amphibole from sample TAK-Z1. Figure S4: Pole figures of magnetite Mt3 and relict hematite. Table S1: Composition of feldspar in amphibolite host rock and iron ore from Takab. Table S2: Fe, O, and trace-element composition of magnetite from the Takab iron ore samples. Table S3: Average formation temperature for the Takab magnetite.
**Author Contributions:** Conceptualization, C.W. and B.O.; investigation, C.W., O.B., N.R. and B.O.; project administration, C.W. and B.O.; resources, C.W., B.O., G.N., M.H. and I.M.; supervision, C.W.; validation, C.W., O.B. and N.R.; visualization, C.W., G.N. and M.H.; writing—original draft, C.W.; writing—review and editing, C.W., B.O., G.N. and M.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** We thank the Institute of Advanced Sciences of Basic Science and University of Zanjan, Zanjan, for logistic support during fieldwork and the stay of BO in Zanjan. Campus France and the French embassy (Gundishapur project No. 40624TK) are thanked for financing the stay of BO at IASBS, Zanjan. The institute ISTEp of Sorbonne Université is thanked for financial support of the analyses through the French National TRIGGER project, French-Iranian cooperation.

**Data Availability Statement:** The original contributions presented in this study are included in the article and supplementary material.

**Acknowledgments:** L. Reisberg (CRPG, Vandoeuvre-lès-Nanvy, France) is kindly acknowledged for a thorough review and English revision of the manuscript. We thank three anonymous reviewers for their helpful comments.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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