Textures and Chemical Compositions of Magnetite from Zhibo Submarine Volcanic Iron Oxide Deposit, Xinjiang, China: Implications for Re-Equilibration Processes

Abstract

The Awulale Iron Metallogenic Belt (AIMB) has developed many medium–large iron deposits, of which the Zhibo iron deposit is selected as the research object in this paper. The Zhibo deposit’s ore primarily consists of magnetite as the main mineral, accompanied by extensive epidotization. The mineral assemblage includes diopside, albite, actinolite, epidote, chlorite, K-feldspar, quartz, calcite, chalcopyrite, and pyrite. Magnetite is classified into two groups based on sulfide content and mineral assemblage (MagI for sulfide-free and MagII for sulfide-rich ores). Two-stage mineralization of magnetite has been identified based on mineral assemblages and paragenesis, including the magmatic stage MagI and hydrothermal stage MagII. Mag I shows inhomogeneous backscattered electron (BSE) textures and consists of BSE-light and -dark domains (Mag I-L and MagI-D). Seven subtypes of magnetite have been recognized in this deposit. MagI-L and MagI-D have formed in the magmatic stage and show BSE images in light and dark colors, respectively. MagI-L is anhedral to subhedral and is inclusion-free. MagI-D has mainly replaced MagI-L along fractures and contains inclusions and pores. MagII has formed in the hydrothermal stage and is characterized by coupled dissolution–reprecipitation (DRP) textures. It can be divided into five sub-generations, that is, MagII-1, MagII-2, MagII-3, MagII-L, and MagII-D. MagII-1, MagII-2, and MagII-3 comprise the core–mantle–rim texture, while MagII-L and MagII-D comprise the core–rim texture. MagII-1 is BSE-light and is enriched with inclusions and pores. MagII-2 has partly replaced MagII-1 and exhibits oscillatory zoning under BSE imaging. It also contains inclusions. BSE-light MagII-3 occurs as overgrowth along MagII-2 margins and is inclusion-free. MagI magnetite is enriched with V, Cr, and Ni, whereas MagII is enriched with W, Ta, Nb, Sr, Sb, Sn, Y, Zr, Mg, Al, and Ti, indicating a decreased temperature of magnetite formation. MagI-L crystallizes from the original magma, while MagI-D is formed from the residual magma enriched with incompatible elements. MagII crystallizes from later multiple hydrothermal activities through the dissolution of early magnetite and the re-precipitation of later magnetite or from MagI-D which has later undergone a hydrothermal overprinting process. According to the texture and chemical composition of magnetite from the Zhibo deposit, we suggest that the Zhibo iron deposit was formed from the initial magmatic origin and then underwent a hydrothermal overprinting process.

1. Introduction

The western Tianshan metallogenic belt in Xinjiang is an important part of the Central Asia Orogenic Belt (CAOB) and hosts many iron, copper, gold, and other metal–mineral resources related to the tectonic evolution of multiple accretionary orogens [1,2,3,4,5,6]. Submarine volcanogenic iron oxide (SVIO) deposits constitute the primary deposit type within the Awulale Iron Metallogenic Belt (AIMB) located in the western Tianshan metallogenic belt (Figure 1a,b). These deposits are characterized by medium to large-sized occurrences, including Shikebutai, Songhu, Chagangnuoer, Zhibo, Dunde, and Beizhan deposits, arranged from west to east (Figure 1b) [7].

The iron deposits within the AIMB are mainly hosted in Late Palaeozoic submarine calc-alkaline volcanic–subvolcanic and volcano-sedimentary rocks [10,11,12,13,14,15]. The formation of these deposits is related to a transition in genesis from magmatic and hydrothermal to sedimentary mineralization, depending on the distance from the volcanic center or core [1,5,16].

The Zhibo iron deposit is one of the largest submarine volcanic iron deposits in the AIMB. The field observations made in the Zhibo iron deposit are similar to those found in iron oxide–apatite (IOA) or Kiruna-type, suggesting that the ore deposit has a magmatic origin [12,17,18,19,20,21]. The characteristics comprise clearly defined contact between the ore and the host rock, a simplified mineral assemblage, the presence of high-grade ores, and structurally controlled ore veins [12,22]. There are two major groups of magnetite based on the ore textures, mineral associations, and elemental and isotopic composition of magnetite. The first group, enriched with Ti, V, Ni, and HFSE, resembles iron oxide–copper–gold (IOCG) ores [22]. The iron and oxygen isotope investigations of this magnetite reveal that the Zhibo iron deposit has a magmatic origin. The second group is depleted in Ti, V, Ni, and Y, with a wider range in δ⁵⁶Fe and δ¹⁸O, resembling hydrothermal Fe–skarn ores [20,21,22,23,24,25,26,27]. Therefore, the origin of the Zhibo deposit remains controversial. The mineralization process can readily alter the texture and composition of magnetite. Inadequate previous research on the texture of magnetite in the Zhibo deposit has contributed to controversies surrounding the deposit genesis [10,12,28].

Magnetite, a prevalent oxide mineral, not only occurs in sedimentary, metamorphic, and magmatic rocks as an accessory mineral [29,30,31,32,33], but also is extensively distributed as a major or trace mineral in various types of mineral deposits [34]. Magnetite encompasses numerous minor and trace elements, the concentrations of which may be affected by the diverse range of physicochemical conditions encountered during formation in different mineral deposits [35,36]. Hence, magnetite finds broad applications as a petrogenetic indicator and pathfinder mineral for mineral exploration [31,37,38]. By compiling the composition of magnetite in various deposits, a series of discriminant diagrams have been established to identify different types of mineral deposits [31,34,37,39,40,41]. Numerous detailed texture studies on magnetite have been extensively employed in the investigation of IOA, IOCG, and skarn deposits. Variations in magnetite textures and compositions at different stages are used to trace the fluid evolution process and the genesis of the deposit [18,42,43,44,45].

In this paper, we conduct detailed investigations on the textures and compositions of magnetite from the Zhibo deposit. Based on these results, we discuss the genesis of the Zhibo deposit. The findings of this study are of significant importance in understanding the origin of iron ore deposits hosted in volcanic–subvolcanic rocks.

2. Geological Setting

2.1. Geology of Western Tianshan

Located on the southwestern edge of the Central Asian Orogen Belt (CAOB), the Western Tianshan Orogen is situated between the Junggar terrane to the north and the Tarim block to the south (Figure 1a). It comprises the Northern Tianshan Accretionary (NTA), the Yili Block (YB), the Central Tianshan Arc Terrane (CTAT), and the Northern Margin of the Tarim Block (NMT). These are separated by the North Tianshan Suture Zone (NTSZ), the Nikolaev Line–North Nalati Suture Zone (NNSZ), and the South Tianshan Suture Zone (STSZ) from north to south (Figure 1a) [8,10,12,46,47,48]. Previous studies have suggested that the suture zones are linked to the subduction of the Late Palaeozoic North Tianshan, Early Palaeozoic Terskey, and the latest Early Palaeozoic–Late Palaeozoic South Tianshan oceans during Early Palaeozoic to Early Mesozoic times [5,8,12,49]. The dominant strata in the Western Tianshan Orogen consist of Precambrian metamorphic basement rocks, Palaeozoic sedimentary, and volcanic rocks [2]. The zircon U-Pb ages of the Precambrian metamorphic basement vary from 1910 Ma to 882 Ma [50].

The Awulale Iron Metallogenic Belt (AIMB) is situated to the east of the Yili Block (Figure 1b), and the strata are mainly composed of Precambrian basement and Late Paleozoic and Mesozoic rocks [51,52]. The Precambrian basement includes granitic gneisses, migmatites, schists, and amphibole monzogneiss, predominantly found in the southern AIMB (Figure 1b) [53,54]. Silurian rocks in the northeastern AIMB comprise metamorphic and clastic formations, such as siltstone, metamorphic conglomerate, tuff, sandstone, and schist. The NNW-SE trending Devonian rocks are primarily composed of pyroclastic rocks, acid volcanic rocks, and carbonates.

The Carboniferous strata consist of intermediate-acid volcanic–sedimentary clastic rocks belonging to the Dahalajunshan Formation. These rocks encompass calc-alkaline basalts, trachy andesites, trachy andesites with subordinate basalts, rhyolites, and volcaniclastic rocks. The U-Pb zircon ages of basalts, basaltic andesite, trachyte–andesite, and rhyolites range from 361Ma to 316 Ma [2].

The Dahalajunshan Formation is the primary host rock for syngenetic submarine volcanogenic iron oxide deposits [2,28]. Multiple tectonic episodes provide the driving force for the activation and migration of endogenous mineral ore-forming materials. Additionally, geological structures serve as conduits for fluid activity and sedimentation. The WNW-trending structure and north-dipping high-angle faults in the AIMB are well developed [55]. The major iron deposits in the AIMB are controlled by the Carboniferous volcanic apparatus, with the Zhibo iron deposit situated in the center of the caldera structure [1]. Circular and radial faults play crucial roles as key ore-controlling structures and host iron orebodies.

2.2. Geology of the Zhibo Iron Ore Deposit

The Zhibo iron deposit (Figure 2a) is a significant iron deposit situated in the eastern part of the AIMB, boasting an iron ore resource exceeding 250 million tons, with an average grade exceeding 40%. The Zhibo iron mine is about 5.5 km long and 1.5 km wide (Figure 2a), and it is divided into three mining areas: eastern, middle, and western, of which the east is the principal mining area. The Zhibo deposit comprises more than 21 ore bodies which are massive, tabular, and lenticular (Figure 2b,c). The Fe8 ore bodies on the C-C ‘section line are tabular and lenticular, sub-horizontally distributed, and reach more than 80 m in length (Figure 2c).

The exposed strata in the mine are characterized by the Carboniferous Dahalajunshan Formation, which mainly consists of basalt, basaltic tuff, basaltic trachyte–andesite, trachyte–andesite, and andesite (Figure 2). The iron orebodies are primarily hosted within the basaltic andesite and andesite (Figure 2c). This basaltic andesite typically appears gray or greyish–green and contains phenocrysts of plagioclase (20~25 vol%) and clinopyroxene (10–15 vol%) (Figure 3a,b). The matrix of andesite comprises fine plagioclase, clinopyroxene, and amphibole. The plagioclase phenocrysts are euhedral–subhedral, with a particle size of around 0.2–0.5 mm. Clinopyroxene phenocrysts that have been partially replaced by epidote also display euhedral–subhedral morphology. Moreover, andesite contains magnetite which serves as an accessory mineral (Figure 3d–g).

The host andesite is exposed in the central and southern areas where it strikes NW, with a steep dip angle (50~75°) near the surface and a shallower dip angle (10~30°) at deeper levels. Previous studies have indicated that the formation age of the Dahalajunshan Formation is >361~313 Ma [2]. The intrusive rocks include granodiorite in the southwestern area and quartz diorite in the northern area (Figure 2a and Figure 3c). The diorite and granite have U-Pb ages of 318.9 ± 1.5 Ma and 304.1 ± 1.8 Ma, respectively [9,57]. The igneous zircon U-Pb ages of the Zhibo andesite and disseminated ore are 328.7 ± 2.1 Ma and 329.9 ± 1.5 Ma, respectively [28]. In situ U-Pb dating of titanite in the ore was used to constrain the time of iron mineralization and yielded three ages of 310.3 ± 1.8 Ma, 310.1 ± 1.8 Ma, and 315.3 ± 2.5 Ma [57]. In conclusion, it is generally believed that the main metallogenic age in the Zhibo deposit is not earlier than 330 Ma.

2.3. Ore Types and Paragenetic Sequence

The boundary between the orebody and wall rock exhibits primarily a gradual relationship (Figure 4a), occasionally displaying a sharp contact relationship (Figure 4b).

Several distinct ore types have been identified, characterized by their texture and mineral composition. These include massive (Figure 4c–e), disseminated (Figure 4f), breccia (Figure 4g,h), and banded types (Figure 4i), along with a minor occurrence of lava ores (Figure 4j).

In the Zhibo deposits, high-grade massive ores can be classified into two types based on their mineral compositions. One type consists mainly of magnetite as the ore mineral, along with diopside, albite, and actinolite (Figure 4c). In this type of ore, magnetite predominates with subhedral to euhedral fine-grained aggregates (Figure 5a,b). The other type of ore not only contains massive aggregates of magnetite but also shows dendritic aggregates of magnetite, along with pyrite, chalcopyrite, and epidote (Figure 4d,e). Magnetite in this type of ore exhibits subhedral to euhedral crystals (Figure 5c), and platelet magnetite can be observed within dendritic magnetite aggregates (Figure 5d). Pyrite mainly displays an anhedral to subhedral structure (Figure 5c,d). Brecciated magnetite ores come in two types. One variety of volcanic rock breccias is identified by the presence of magnetite as the cementing matrix (Figure 4g). The other type is characterized by epidote cement and magnetite fragments (Figure 4h). The banded magnetite ores consist of a layer of magnetite, K-feldspar, and epidote, which are intersected by minerals resulting from late alterations (Figure 4i and Figure 5e). Disseminated magnetite ores are characterized by magnetite distributed within an epidote–chlorite–K-feldspar–chlorite matrix (Figure 5f).

The hydrothermal alteration of wall rocks encompasses sodic(–calcic) alteration (actinolite and albite), epidote± sulfide ±chlorite, and K-feldspar alteration.

Based on paragenesis, texture, and crosscutting relationships, the alteration and mineralization processes at Zhibo could be divided into four stages: (1) Na–Ca alteration stage, (2) K–Ca alteration stage, (3) epidote–sulfide stage, and (4) calcite–quartz stage (Figure 6). The Na–Ca alteration stage (Stage 1) comprises volcanic rocks and tuff diopside–albite alteration assemblages. The K-Ca alteration stage (Stage 2) is characterized by magnetite, actinote, and epidote. The epidote–sulfide alteration stage (Stage 3) is marked by the prevalence of magnetite, pyrite, epidote–potassium feldspar, (pyrrhotite), and (chalcopyrite). The calcite–quartz stage (Stage 4) exhibits a combination of quartz–calcite–chlorite–(hematite) and (pyrrhotite).

3. Sample and Method

In this study, magnetite samples exhibiting various types and mineral assemblages were carefully selected to conduct comprehensive investigations into their textural and mineral chemical characteristics. Specifically, five samples were collected from platform 3580 and drill ZK3602 located in the east mining area. Subsequently, three distinct types were subjected to further study, encompassing massive (18ZB45, 18ZB53-1, ZK3602-626), densely disseminated or semi-massive (ZK3602-629), and complex breccia (ZK3602-631) magnetite ores displaying different hydrothermal alteration signatures.

Backscattered electron (BSE) images and electron microprobe analysis (EMPA) were performed at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). All samples were examined on carbon-coated thin sections within an evacuated chamber.

Japan JEOL Series Electronic Probe (JXA-8100 EMP) is the experimental facility used to analyze the components of major and trace elements of magnetite at an accelerating voltage of 15 kV, a beam current of 20 nA, a spot size of 2 μm, and a 10–30 s peak counting time. The following standards are used for calibration: diopside (Ca and Si), rutile (Ti), jadeite (Na and Al), garnet (Fe), bustamite (Mn), K-feldspar (K), pyrope (Mg), NiO (Ni), Cr₂O₃ (Cr), and V₂O₅ (V). The spectral line adopted by each element is Na (129.393 nm), Mn (146.186 nm), K (120.157 nm), Mg (107.399 nm), Si (77.371 nm), Fe (134.625 nm), Al (90.551 nm), Ca (107.907 nm), Ti (88.751 nm), Cr (159.287 nm), V (174.129 nm), and Ni (115.165 nm), respectively. A program based on the ZAF procedure is used for data correction.

We chose the flat and clean magnetite under the microscope as the research object to reduce the impact of inclusions on the obtained experimental results. Magnetite trace element analyses were conducted by using LA-ICP-MS at the MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS. Laser sampling was performed using RESOlution S-155. A Bruker M90 ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as a carrier gas. Each analysis incorporated a background acquisition of approximately 15–20 s (gas blank) followed by 40–45 s of data acquisition from the sample. Element contents were calibrated against multiple reference materials (BCR-2G, BIR-1G, and GSE-1G) without applying internal standardization [58]. The preferred values of element concentrations for the USGS reference glasses are from the GeoReM database. Off-line selection and integration of background and analyte signals, time drift correction, and quantitative calibration were performed by ICPMSDataCal-10.2 [58,59].

4. Results

4.1. Petrography and Texture of Magnetite

Two main stages of magnetite can be identified based on mineral assemblage and paragenesis. The first-stage magnetite, MagI, is magmatic in origin and occurs in sulfide-free ores, whereas the second-stage magnetite, MagII, is associated with hydrothermal sulfide-rich ores. The latter is further subdivided into five subtypes on the basis of ore types and BSE textures.

The massive ores are distinguished into sulfide-free ores (18ZB-45/18ZB-53) and sulfide-rich ores (ZK3602-626). Within the sulfide-free ores (18ZB-45/18ZB-53), two varieties of magnetite are observed, characterized as light and dark magnetite. MagI-L magnetite appears subhedral or euhedral and lacks inclusions (Figure 7a,b). In contrast, MagI-D magnetite, which replaces MagI-L magnetite along mineral fissures, typically contains inclusions and voids ranging from the faintly distributed micro- to nanometer scale (Figure 7a,b). The sulfide-rich ores (ZK3602-626) exhibit enriched subhedral sulfide (pyrite) and feature two types of magnetite. MagII-D, found near the contact point with pyrite, is dark and rich in inclusions, while MagII-L is relatively pure, bright magnetite without inclusions in the inner part of the magnetite along the cracks (Figure 7c). In terms of texture, some 120° triple junction textures are observed in the massive magnetite (MagII), indicating balanced crystallization (Figure 7d).

In samples ZK3602-629 and ZK3602-631, the magnetite manifests as platy magnetite. The magnetite in disseminated and breccia ores exhibits a more complex nature, with multiple generations of magnetite. MagII in samples (ZK3602-631/ZK3602-629) displays distinct characteristics of multi-stage formation. In sample ZK3602-629, magnetite with oscillatory zoning displays two types: MagII-L and MagII-D. MagII-D, characterized by its dark magnetite rich in inclusions and voids, has undergone replacement by MagII-L that grows in the boundary contact of MagII-D magnetite (Figure 7e).

The magnetite in dendritic ores (ZK3602-631) exhibits extremely complex core–mantle–rim textures. The core (MagII-1) consists of bright-colored magnetite rich in inclusions and voids. This core magnetite is subsequently replaced by the second-generation magnetite (MagII-2), characterized by alternating dark and light bands filled with irregular inclusions and voids (Figure 7f). The rim magnetite (MagII-3) develops along the second-generation light magnetite (MagII-2) and appears as pure magnetite with minimal inclusions and voids.

4.2. Major Element Composition of Magnetite

The EMPA results of different types of magnetite are presented in Supplementary Table S1. The major elements are reported as oxides, including FeO and CaO (Figure 8). All magnetite samples exhibit relatively high SiO₂, MgO, Al₂O₃, and TiO₂ contents. Additionally, the MgO content is notably high across all samples. Other elements such as Na₂O, Cr₂O₃, K₂O, MnO, and V₂O₃ are either lower or slightly above their detection limits.

Throughout all stages of magnetite ore formation, dark magnetite generally displays higher and more variable SiO₂, CaO, MgO, and Al₂O₃ contents compared to light magnetite, whereas light magnetite tends to have higher FeO contents.

The main feature of MagI is its high content of V₂O₃ (0.13–0.34 wt%), in which MagI-D (V₂O₃, 0.22–0.34 wt%) shows higher contents of V₂O₃ compared to MagI-L (V₂O₃, 0.13–0.321 wt%). MagI-L has fewer impurities and a higher FeO content. However, the contents of impurities in MagI-D fluctuate greatly; for example, the contents of SiO₂ (0.09–3.10 wt%), MgO (0.009–0.533 wt%), and CaO (0.03–0.89 wt%) have a large variance.

The composition variations among different subtypes of MagII are significant. The BSE-light magnetite (MagII-2) shows higher FeO contents than the BSE-dark magnetite (MagII-1 and MagII-3), which is similar to MagI. From MagII-1 to MagII-3, the contents of MnO, Al₂O₃, Si₂O, and CaO change in an inverted V shape. The composition change of MagII-D and MagII-L is similar to that of MagII-2 and MagII-3. Among all magnetite ores, MagII-2 has the lowest FeO content (85.06–91.45 wt%) and the highest SiO₂ content (1.03–4.43 wt%). The content of V₂O₃ is similar across MagII-1 (0–0.03 wt%), MagII-2 (0–0.04 wt%), and MagII-3 (0.01–0.04 wt%).

4.3. Trace Element Composition of Magnetite

The LA-ICP-MS results of five types of magnetite are presented in Supplementary Table S2. MagI has relatively lower W, Ta, Nb, Zn, Sr, Sn, Sb, Ga, Y, Zr (0.01–1 ppm), Mg, Ti, and Al (100–1000 ppm), while having relatively higher V, Cr, and Ni (100–10,000 ppm) than the element contents of MagII (Figure 9; the difference in content is mainly between one and three orders of magnitude). However, MagI-D has relatively higher concentrations of Zn, Cr, Ti, and Al, but lower contents of Mg, Sn, Sb, and V than MagI-L. The concentration of Sr and Ni in MagI varies, with small fluctuations, and some elements (Y and Zr) of MagI are below the detection limit.

The concentration of MagII displays similar trends, although with different degrees of enrichment. MagII-1 and MagII-3 are purer forms of magnetite than MagII-2, and their higher Fe content leads to lower trace element content compared to MagII-2.

MagII-2 has the highest Mg, Al, Si, Ca (1000–10,000 ppm), Ni, Ga, Sr, Y, and Sb (0.1–30 ppm) among MagII. The Cr (10–50 ppm) and W (~0.1–3000 ppm) components of MagII-1 are higher than of MagII-2 and MagII-3.

MagII-1 has transitional characteristics between MagI and MagII, which has the lowest content of Mg, Al, Si, Ca, Sr, Y (one order of magnitude lower than in MagII-2), Ni, Ga, and Sb (~10 ppm). MagII-2 has the lowest Cr (10–20 ppm) and W (1–100 ppm) content. The content of W in MagII magnetite is characterized by a large variance. The contents of V and Zn (10–100 ppm) in MagII magnetite are similar.

5. Discussion

5.1. Constrains on Texture and Composition of Magnetite

Magnetite (Fe₃O₄) has an inverse spinel structure with stoichiometry AB₂O₄, where A represents a divalent cation (Fe²⁺) and B represents a trivalent cation (Fe³⁺) [60,61]. Due to its mineral crystal structure characteristics, Fe²⁺ and Fe³⁺ iron elements can have a wide range of homomorphism. The replacement of trace elements in magnetite can reflect environmental changes during mineral formation [34,38]. The composition of magmatic magnetite is mainly controlled by the following conditions: (1) whole rock, magmatic, or fluid composition; (2) temperature; (3) pressure; (4) cooling rate: (5) oxygen fugacity; (6) sulfur fugacity; (7) Si activity [62,63,64,65].

According to the magnetite mineral classification and discrimination diagram (Figure 10) [37], the magnetite from Zhibo primarily falls within the categories of Kiruna, IOCG (iron oxide–copper–gold), skarn, and BIF (banded iron formation) deposits. Consequently, our next step will involve examining the factors contributing to the occurrence of such a diverse range of magnetite types.

Firstly, it is important to note that MagI-L shares the same Kiruna-type (IOA) origin as typical IOA deposits, but it differs slightly in composition. MagI-L is characterized by being a low-Ti magnetite, which sets it apart from typical volcanic magnetite. This variation in composition could be attributed to the influence of the host rocks’ composition [34]. The low-Ti magnetite may be the result of the depletion of high-field strength elements in volcanic rocks in the deposit [9,10]. Furthermore, the composition of Zhibo magnetite could also be influenced by the low-Ti content in the initial fluid. This may result in minimal oxy-exsolution of Ti-rich oxides, which is a common feature of magnetite in igneous rocks [66].

Magnesium and vanadium are good indicators to distinguish magmatic and hydrothermal magnetite. Generally, hydrothermal magnetite contains higher Mg (>1000 ppm) and lower V (<1000 ppm), whereas magmatic magnetite is the opposite. The chemical behavior of Cr in magmatic and hydrothermal magnetite also tends to be enriched in magmatic magnetite [34,39].

MagI-L and MagI-D have similar characteristics as magmatic magnetites, such as low Mg (<400 ppm), high Cr, and high V (>2000 ppm) contents (Figure 9). MagI has a high V content and low Ti/V ratio (Figure 11), indicating that the magnetite formed under reducing fluid conditions. The ratio of Ti+V/Al+Mn can be used to distinguish between magmatic magnetite and hydrothermal magnetite, with values typically ranging from 0.1 to 0.5 considered characteristic of hydrothermal magnetite [34]. The mineral assemblage and the composition of the massive sulfide-free magnetite (MagI) support the origin of magmatic magnetite. In terms of texture, MagI-D differs significantly from MagI-L, and the characteristics of pores and irregular morphology in magnetite particles both indicate that the metallogenic system at that time was unstable (Figure 7a,b) [41].

Compared with MagII, Mag I exhibits more pure magmatic magnetite characteristics (Figure 12). Kiruna-type deposits can be distinguished from other deposits such as magmatic Fe–Ti–V-, porphyry-, and IOCG-type deposits by comparing V and Cr contents in the magnetite [45]. On the other hand, MagII displays the characteristics of hydrothermal magnetite. This observation further underscores that Zhibo magnetite possesses the attributes of multi-stage and multi-genesis formation. Because of the low-Ti volcanic rocks, V could act as a thermometer whose content is positively correlated with temperature [34]. From MagI-L to MagII-D, the decreasing content of V indicates a continuous decline in temperature.

Therefore, we believe that MagI-L has the characteristics of high iron and low incompatible elements during the crystallization process of the original magma. The more magnetite crystallized from the original magma, the more incompatible elements will be enriched in the residual magma, resulting in the enrichment of incompatible elements in MagI-D crystallized at this stage. It is generally believed that IOCG and skarn are the genesis of hydrothermal deposits [67]. Some studies believe that IOA and IOCG deposits are a single genetic continuum through a flotation model, wherein Mag I-D indicates its placement within the IOCG field (Figure 10) [31,68].

MagII from the ores containing sulfides coexists with similar mineral assemblages to hydrothermal magnetite [69]. Moreover, MagII has the chemical characteristics of hydrothermal magnetite (Figure 12). MagII-1 and MagII-2 have pores or voids, and the magnetite has distinct interfaces at different generations (Figure 7f). The evolving relationship between MagII-D and MagII-L is similar to that from MagII-2 to MagII-3, from skarn-type deposits to BIF. The trace element data of MagII-1 are mostly plotted in the same region as those of MagI-D in the Ca + Al + Mn vs. Ti + V diagram, indicating that it has a certain genetic inheritance relationship with MagI-D (Figure 10). At the same time, MagII exhibits the characteristics of hydrothermal magnetite (Figure 11 and Figure 12). The dissolution–reprecipitation reaction is very common in the mineral metasomatic process in the hydrothermal system, and it can be distinguished by the following characteristics: (1) the outer form of the original mineral phase is preserved; (2) there is a distinct interface between the original mineral facies and the metasomatic mineral facies; (3) metasomatic minerals have good porosity; (4) there is a consistent crystallographic orientation between the original mineral facies and the metasomatic mineral facies [70,71]. The formation of MagII can be attributed to the change in physical and chemical parameters of the fluid and the dissolution–reprecipitation (DRP) reaction caused by the interaction of Mag-I crystals with the late fluid metasomatism.

The content of trace elements in hydrothermal magnetite is controlled by many factors and it is difficult to attribute such changes to a single physicochemical parameter. The multi-stage successive alteration can further complicate the magnetite geochemical composition [34,39]. Silicon, Mg, V, Ca, Al, and Mn could act as mobile elements during these processes.

Silicon could replace the tetrahedral site of magnetite crystals, being aware that the abundant inclusions in dark Mag may bias the measured amount of Si. The high Si content of the dark magnetite reflects the acidic ore-forming fluid that is not conducive to magnetite precipitation [33].

The vanadium content of magnetite is sensitive to oxygen fugacity (fO₂) and temperature during the DRP process. The vanadium content of magnetite is negative with fO_2, but positively correlated with temperature, which is consistent with the activity of Ti and Ga in the DRP process [32,39,72,73]. From MagII-1 to MagII-3, V and Ga contents have an increasing and then decreasing trend. It is characterized by an initial increase followed by a decrease in V and Ga concentrations, indicating that the temperatures are initially increased and then decreased and fO₂ is initially decreased and then increased [34,62,72]. The formation of MagII-1 and MagII-2 may be a process of increasing temperature due to the reactivation of the new magmatic–hydrothermal system, while the temperature from MagII-2 to MagII-3 gradually drops.

Based on this magnetite study, we believe that MagI-L represents magmatic magnetite. With the evolution of the original magma, MagI-D enriched with incompatible elements was formed. Then, magmatic magnetite was overprinted by multiple magmatic–hydrothermal alteration events to form hydrothermal magnetite.

5.2. Magmatic or Hydrothermal Origin

The Zhibo deposit has the characteristics of a magmatic affinity similar to the IOA deposit, such as the sharp contact between the ore body and wall rocks (Figure 4b), brecciated ores (Figure 4g), and flow structures (Figure 4j), which have been interpreted as evidence of magmatic origin [21,74,75]. The Zhibo deposit exhibits extensive late-stage hydrothermal alteration, characterized by significant epidotization during the transition from massive to disseminated ores (Figure 4a). Elevated content of sulfide and alteration assemblages are observed within the ores. These ore types have massive ores (Figure 4d,e), brecciated ores (Figure 4h), and banded ores (Figure 4i), which show obvious characteristics of a hydrothermal deposit [76,77].

In addition, there is dendritic magnetite in MagII (Figure 4e and Figure 5d). This magnetite is widely developed in Kiruna and hydrothermal deposits. Previous studies have suggested that this magnetite is magmatic in origin, formed through rapid crystallization from immiscible iron–silicon melt [78] or degassed oxidative melt [79] during temperature decreases [80]. Multiple magmatic–hydrothermal fluids may have mixed with the magmatic fluid and consequently caused the undersaturation of iron in the fluids, leading to the dissolution of MagII-1 (high Fe and low Si, Al, and Ca concentrations) and re-precipitation of MagII-2 (low Fe and high Si, Al, and Ca), with significant porosity and abundant mineral inclusions. The incompatible elements in magnetite are discharged in the later re-equilibration process, which leads to the high Fe content in MagII-3 [42,64].

Along with the magmatic evolution, the magmatic fluid gradually reacted with the wall rock to form the hydrothermal magnetite and gangue [31,81]. Through a garnet U-Pb geochronology study, it has been confirmed that the AIMB has undergone at least three episodes of magmatic–hydrothermal activity [4]. Magmatic–hydrothermal activities led to multiple stages of skarn alteration, which resulted in the pre-formed minerals undergoing multi-stage alteration, thus creating a variety of ores types and magnetite textures.

Based on in situ iron isotope studies, the bimodal characteristics of Zhibo magnetite are identified [22]. It is argued that Zhibo magnetite is of magmatic and hydrothermal origin [22]. Combined with the study of magnetite in this paper, we argue that the Zhibo iron deposit was mainly formed by magmatic magnetite from an original iron-rich magma and then overprinted by hydrothermal mineralization.

6. Conclusions

Combining the field study and experimental analysis of the Zhibo iron deposit, one can observe that it has a variety of structural ores, such as massive, disseminated, banded, and brecciated. The contacts between the massive orebody and volcanic rock are sharp, while they are transitional between the disseminated orebody and epidotized wall rock. Field observations and laboratory studies revealed both magmatic and hydrothermal origin. There are two stages and seven types of magnetite in the Zhibo iron deposit. MagI-L crystallizes from the original magma, and MagI-D from the residual magma rich in incompatible elements. MagII has a core–mantle–rim texture, and MagII-1 is bright-colored magnetite rich in inclusions and pores. MagII-2 results from the metasomatism of MagII-1, forming magnetite with oscillatory zoning, but also containing inclusions. MagII-3 grows along MagII-2 and is bright-colored magnetite. MagII-L and MagII-D are similar to MagII-2 and MagII-3, but MagII-L and MagII-D do not have a core–mantle–rim texture. The chemical composition of MagII magnetite indicates its hydrothermal origin, but the difference in the chemical composition of magnetite in different generations reflects the change in temperature and fO₂. The formation of the Zhibo iron deposit has the characteristics of early magmatic genesis and late hydrothermal overprinting and has the characteristics of multi-stage mineralization.