The Geochemical Characteristics of Trace Elements in the Magnetite and Fe Isotope Geochemistry of the Makeng Iron Deposit in Southwest Fujian and Their Significance in Ore Genesis

Abstract

The Makeng iron deposit in southwest Fujian is a significant iron polymetallic deposit containing various types of iron ore, including garnet magnetite, diopside magnetite, and quartz magnetite. The metallogenetic type of the deposit has been a subject of debate, particularly in relation to the genesis of magnetite and the source of iron. In situ microanalysis of trace elements in magnetite from different ores shows relatively low levels of V, Ti, Cu, and Zn, with higher concentrations of Ca and Si, indicating the characteristics of a skarn type deposit. The δ⁵⁷Fe values of the magnetite range from −0.091‰ to 0.317‰. Combining these data, whole-rock iron isotope analyses, including Juzhou and Dayang granites, diabase, and the Lower Carboniferous Lindi Formation sandstone, suggest that Fe in the magnetite primarily originates from granitic pluton, with potential contributions from diabase and the Lower Carboniferous Lindi Formation sandstone. Combined with field work, these results indicate that Makeng iron deposit is a skarn-type magnetite deposit associated with Yanshanian granitic intrusions. Therefore, the initial ore-forming fluid is postulated to be a high-temperature magmatic hydrothermal fluid with high oxygen fugacity. This fluid infiltrates spaces such as interlayer fracture zones between the Upper Carboniferous Jingshe Formation–Middle Permian Qixia Formation carbonate rocks and the Lower Carboniferous Lindi Formation sandstone, resulting in diverse magnetite ores due to metasomatism. The mineralization process of the Makeng iron deposit is basically the same, as it is composed of typical skarn deposits. Magnetite was mainly formed during calcic skarn formation stage, and this process persisted until the initial phase of the retrograde alteration of skarns. In contrast, sulfide minerals, including molybdenite, sphalerite, and galena, precipitated during the quartz–sulfide stage.

1. Introduction

The Makeng iron deposit is the largest iron deposit in southeastern China, hosting 477 million tons of iron at a grade of 38.5%–61.6% and over 80,000 tons of molybdenum at a grade of 0.028%–0.11%. Since its discovery in the 1960s, it has attracted much attention due to its unique metallogenic characteristics. However, three distinct theories exist regarding its genesis: (1) terrigenous sedimentation–hydrothermal transformation genesis, which holds that this deposit has the characteristics of marine sedimentary mineralization, and its metallogenic materials are mainly derived from the pre-Devonian basement [1]; (2) marine volcanic sediment–hydrothermal transformation genesis, which holds that the deposit has the characteristics of marine volcanic sedimentary mineralization, and its metallogenic materials mainly come from the marine volcanic activity during the Early and Middle Carboniferous epoch [2,3,4]; and (3) skarn genesis, which holds that the deposit has the characteristics of contact metasomatic mineralization, and its metallogenic materials mainly come from the Yanshanian granites [5,6,7,8].

Some studies have been conducted on magnetite in Makeng, such as the phase analyses conducted by Wang et al. [2] and Liu [3], the Sr-Nd-Pb isotopic geochemical analysis conducted by Zhang and Zuo [9], and the geochemical analysis of trace elements conducted by Yang et al. [10]. These studies have greatly improved our understanding of the genesis and metallogenic mechanisms of the Makeng iron deposit.

In recent years, with the extensive use of laser denudation–inductively coupled plasma mass spectrometry (LA-ICP-MS) and multi-receive inductively coupled plasma mass spectrometry (MC-ICP-MS) in mineral deposit research, the in situ microanalysis of trace elements and tracing of non-traditional stable isotopes (such as Fe, Cu, Zn, Mg, etc.) have been rapidly developed in mineral deposit research. In view of this, this paper used LA-ICP-MS to analyze the composition of trace elements of magnetite in different types of ores and MC-ICP-MS technology to trace the source of Fe, providing new evidence to aid our understanding of the genesis of the Makeng iron deposit.

2. Geological Background

The southwest Fujian fault basin, located in the southeast edge of the Cathaysian block (Figure 1a), is an important part of the tectonic–magmatic activity belt around the Pacific Ocean in Eastern China. In addition to the absence of Silurian and Lower Middle Devonian strata, the Paleo-Proterozoic Mayuan Group and the Cretaceous Chishi Group are exposed in this area. During its long basin evolution from the Late Paleozoic to Early Mesozoic eras, a set of alternating carbonate–clastic sedimentary formations were widely deposited in the area. The strata in this area are categorized into pre-Devonian basement rocks, the Late Paleogene–Middle Triassic sedimentary cover, and Meso-Cenozoic continental volcanic–clastic rocks. Each of these stratigraphic units is distinctly demarcated by angular disconformities [11].

Southwest Fujian has undergone a series of tectonic movements since the ancient Proterozoic era, including the formation and expansion of the Paleoproterozoic Cathaysia Paleoland, the fracturing and closure of the Neoproterozoic Cathaysia Paleoland, the Early Paleozoic Caledonian fold orogeny in South China, the Late Paleozoic intracontinental rifting, and the Middle Triassic Indochina movements. These prolonged tectonic movements ultimately shaped the prevalent structural pattern in the area, which is characterized by northeast and north–northeast tectonic structures, and supplemented by structures oriented in the NW and EW directions [12].

The area underwent multiple episodes of magmatism from the end of the Early Paleozoic to the Late Mesozoic eras. The Caledonian magmatic activity contributed to the formation of the metamorphic basement, while the Hercynian magmatic activity led to the development of small-scale intermediate-acid intrusions. Moreover, the Yanshanian magmatic activity formed large-scale granite emplacement, which was zonally distributed in the form of batholith or stock in the north–northeast direction.

Due to intense tectonic–magmatic processes, the extensively developed carbonate–clastic rock formations in the basin provide favorable geological conditions for the formation of iron polymetallic deposits, among which the Makeng iron deposit is one of the representative deposits (Figure 1b). The strata closely associated with iron–polymetallic mineralization mainly consist of the Lower Carboniferous Lindi Formation, Upper Carboniferous Jingshe Formation–Middle Permian Qixia Formation, and Middle Permian Wenbishan Formation [4]. Iron–polymetallic ore bodies mainly occur in the interior of these strata and between their interfaces [13].

3. Geological Characteristics of Deposit

The Makeng mining area is dominated by the Upper Paleozoic strata, including the Lower Carboniferous Lindi Formation, the Upper Carboniferous Jingshe Formation-Middle Permian Qixia Formation, the Middle Permian Wenbishan Formation, and the Middle Permian Tongziyan Formation. The Lindi Formation is primarily composed of quartz sandstone and siltstone, while the Jingshe–Qixia Formations primarily consist of shallow marine carbonate rocks with fine clastic or siliceous rocks. The Wenbishan and Tongziyan Formations are composed of sandstone, siltstone, argillite, mudstone, and shale. These strata are generally distributed in the northeast–southwest direction, dominated by the northwest (Figure 2).

The intrusions in the area studied mainly consist of granites, granite porphyries, granodiorites, as well as diabases. Among them, the granitic intrusions are mainly exposed on the east and west sides of the mining area, with the Juzhou intrusion in the east and the Dayang intrusion in the west. The petrological characteristics indicate that the Dayang and Juzhou intrusions exhibit a multi-stage activity, displaying a relatively intricate lithology primarily composed of granite, granodiorite, porphyritic granite, granite porphyry, and quartz porphyry [13]. A wealth of previous geochronologic data on Juzhou and Dayang intrusions suggest that the area experienced two episodes of intrusion, with the earlier episode being the Late Jurassic (145–155 Ma) and the later episode being the Early Cretaceous (125–137 Ma) [14,15,16]. In addition, diabase mainly occurs in the strata ranging from the Lindi Formation to Tongziyan Formation, characterized by irregular veins and stratiform-like lenticular bodies.

Figure 2. Detailed geological map of the Makeng mining area showing the distribution of host rocks, limonite, skarn, and granite localities (modified from [17]).

Figure 2. Detailed geological map of the Makeng mining area showing the distribution of host rocks, limonite, skarn, and granite localities (modified from [17]).

The main fold in the mining area is the Makeng anticline, which is a monoclinic structure and is composed of several secondary dorsal and synclinal folds [18]. The regional faults are roughly divided into three groups of NE, NW and nearly NS directions, among which the Tianshanao fault in the east, the Ximahe fault in the west, the F1 fault in the south, and the F3 fault in the north constitute the boundary of the mining area (Figure 2).

The main iron ore body is in the northwest flank of the Makeng Anticline, with layered or quasi-layered disposition within the interlayer fracture zones in the strata from the Jingshe–Qixia Formations to the Lindi Formation. The main ore body closely follows the orientation of the strata and folds, displaying a characteristic profile of central thickness and lateral thinness (Figure 3). Furthermore, the deposit contains numerous small-scale ore bodies with lenticular or quasi-layered shapes, which are primarily located in the Jingshe–Qixia Formations at the main ore body roof, with some scattered along the contact interface between the Wenbishan and Qixia Formations. Additionally, a limited number of small ore bodies have also been discovered in close proximity to diabase veins situated on the uppermost part of the main ore body, where magnetite frequently occurs within diabase as either banded or disseminated structures (Figure 4a). Apart from iron ore bodies, significant molybdenum mineralization is observed in this deposit. It is noteworthy that associated molybdenum ore bodies are predominantly distributed in the middle and lower sections of the main ore body, while independent molybdenum ore bodies are primarily found in skarn and altered diabase rocks in the uppermost part of the main ore body, with minor occurrence observed in Lindi Formation sandstone.

The ores primarily contain metallic minerals like magnetite, followed by hematite, limonite, molybdenite, sphalerite, galena, pyrite, and chalcopyrite. Gangue minerals mainly include quartz, garnet, diopside, tremolite, actinolite, iron, magnesium amphibole, and other skarn minerals, as well as calcite, fluorite, and potassium feldspar. The magnetite ores are classified into over ten types based on the mineral combination, predominated by garnet magnetite, diopside magnetite, quartz magnetite, and tremolite magnetite (Figure 4). Spatially, these four types of magnetite ores exhibit a vertical distribution along the main orebodies from top to bottom in turn. Amphibole group minerals, such as tremolite, actinolite, hastingsite, grunerite and cummingtonite, are often found in magnetite ores, and Zhang et al. [20] argued that they mostly belong to the subcategory of calcium and iron amphibole. This paper classifies these magnetite ores, primarily consisting of amphibole group minerals, as amphibole magnetite.

Alteration in the deposit is highly complex, and is prominently characterized by skarn formation, silicification, potassic alteration, and sericitization. The mineralization process of the Makeng deposit can be broadly divided into three stages: (1) The calcic skarn formation stage, which is mainly characterized by garnet, pyroxene and other anhydrous skarn minerals, with magnetite forming within the garnet and diopside crystals during the latter part of this stage (Figure 4f,g). (2) The retrograde metasomatic stage, which is characterized by the transformation of the skarn minerals from the previous stage and the formation of hydrous skarn minerals (e.g., tremolite, actinolite, epidote) (Figure 4h,i,k,m); the formation of magnetite ceased until the early part of this stage. (3) The quartz–sulfide stage, characterized by the massive deposition of quartz, calcite, fluorite, and various metal sulfides. These minerals are often veined through the skarn minerals and magnetite formed in earlier stages (Figure 4j,l,m). A detailed description of the skarn and ore mineral paragenesis is given below, and the paragenetic sequence is listed in Figure 5.

4. Sample Collection and Analyses

4.1. Magnetite Trace Element Analysis

Twelve samples were meticulously selected from four mid-level underground mining sections for comprehensive testing. These samples represent the key ore types and mineralization stages within the mining area, as detailed in

Table 1. Description of magnetite trace elements test samples.

Sample No.	Sampling Location	Sample Name	Brief Description of Lithology
MK1-2	200 horizontal 214# transverse drift	Quartz magnetite	Disseminated structure, with quartz as the main gangue mineral, as well as garnet, diopside, etc.
MK1-4-1	200 horizontal 214# transverse drift, near the diabase vein	Mineralized diabase	Massive structure, with the actinolite and plagioclase as the main non-metallic minerals, as well as a small amount of diopside and other skarn minerals
MK2-2	175 horizontal	Amphibole (tremolite) magnetite	Disseminated structure, with tremolite as the main gangue mineral, as well as calcite, quartz, etc.
MK2-3	175 horizontal	Amphibole (actinolite) magnetite	Disseminated structure, with actinolite as the main gangue mineral, as well as garnet, etc.
MK3-2	300 horizontal 5# transverse drift	Diopside magnetite	Taxitic structure, with diopside as the main gangue mineral, as well as calcite, quartz, etc.
MK3-3	300 horizontal 5# transverse drift	Diopside magnetite	Disseminated structure, with diopside as the main gangue mineral, as well as quartz, etc.
MK4-1	420 horizontal 23# transverse drift	Garnet magnetite	Banded structure, with garnet as the main gangue mineral, as well as quartz, diopside, etc.
MK4-3	420 horizontal 23# transverse drift	Garnet magnetite	Disseminated structure, with garnet as the main gangue mineral, as well as quartz, fluorite, etc.
MK4-4	420 horizontal 23# transverse drift	Quartz magnetite	Disseminated structure, with quartz as the main gangue mineral, as well as tremolite, actinolite, etc.
MK4-5	420 horizontal 23# transverse drift	Amphibole (cummingtonite) magnetite	Dense disseminated structure, with amphibole group mineral such as cummingtonite and byssolite as the main gangue minerals
MK4-7	420 horizontal 8# transverse drift	Garnet magnetite	Taxitic structure, with garnet and diopside as the main gangue minerals
MK4-8	420 horizontal 8# transverse drift	Chlorite/epidote magnetite	Disseminated structure, with chlorite and epidote as the main gangue minerals, as well as a small amount of quartz, etc.

. The samples were polished into thin sections with a thickness of 150 μm prior to microscopic observation and analysis.

Considering that LA-ICP-MS in situ analysis is a single-point test representing the chemical composition of the entire magnetite particle, it was necessary to preliminarily analyze the crystal morphology and trace element characteristics of magnetite through microscopic observation, backscattered electron imaging (BSE), and electron probe microanalysis (EPMA) scanning, aiming to ensure the homogeneity of the tested area of magnetite crystal (Figure 6). The BSE and EPMA were conducted using a Jeol JSM5410 LV superprobe at the Advance Analytical Center of James Cook University. Under high-power microscopes, magnetite typically appears as granular with anhedral to subhedral crystals ranging from 0.01 mm to 0.1 mm in size (Figure 6a,b). Conversely, magnetite coexisting with garnet displays well-developed crystallization, primarily showing subhedral to euhedral crystals, generally exceeding 0.1 mm in size (Figure 6c). EPMA scanning reveals that the presence of metallic minerals like sphalerite, pyrrhotite, and hematite occurs within the internal areas or microcracks of the magnetite crystals (Figure 6f,g). Furthermore, some coarse magnetite crystals exhibit spinel exsolution within their interiors (Figure 6d). Additionally, Figure 6c illustrates the andradite in zonal texture replaced by magnetite, potentially indicating a latter crystallization for the magnetite than the garnet.

Following a selection process, magnetite crystals for measurement were meticulously chosen, and specific tested areas were precisely delineated. To reduce potential interference from fine internal inclusions and exsolutions, the tested areas were carefully delineated to avoid these minerals as much as possible (Figure 6a,b).

Furthermore, Yang et al. [10] identified two distinct magnetite generation stages in the Makeng iron deposit: the diopside–magnetite stage and the amphibole–chlorite–epidote stage, with the former being the primary period for magnetite formation. In this study, both stages of magnetite were also observed in the MK4-8 samples, with late magnetite (Mt2) passing through early magnetite (Mt1) in a fine vein (Figure 6e,f).

The measurement was performed at James Cook University’s Advanced Analytical Center in Australia. The laser wavelength was 193 nm, operating at a frequency of 10 Hz with an energy density of 6 J/cm². Each measurement point was tested for 65 s, comprising 30 s for background value measurement and 35 s for signal acquisition. The silicate glass sample NIST SRM 610 served as the external standard, while Fe was used as the internal standard. Raw data were processed using Glitter 4.4 software (GEMOC LA-ICP-MS Total Trace Element Reduction software package), according to the methodology outlined in [21].

4.2. Fe Isotope Analysis

The samples for Fe isotope analysis included whole-rock and magnetite (

Table 2. Fe isotope results of Makeng iron deposit (‰) (Data No. 11–16 are from [22]).

SN	Sample No.	Sample Name	Sampling Location	Test Type	Results
					δ56Fe	2se	δ57Fe
1	ZK9501-14b1	Granite from Juzhou intrusion	571.9 m down the ZK9501 borehole	Whole rock	0.013	0.022	0.019
2	ZK7924-14b1	Granite from Dayang intrusion	1001 m down the ZK7924 borehole	Whole rock	0.260	0.022	0.383
3	Mkjx-b30	Diabase	100 horizontal 106# transverse drift underground in mining area	Whole rock	0.185	0.022	0.273
4	Mkjx-b31	Mineralized diabase	100 horizontal 106# transverse drift underground in mining area	Whole rock	0.081	0.022	0.119
5	Mu-14-2	Lindi Formation sandstone	100 horizontal 106# transverse drift underground in mining area	Whole rock	0.019	0.022	0.028
6	Mu-14-2	Lindi Formation sandstone	100 horizontal 106# transverse drift underground in mining area	Magnetite	0.216	0.022	0.319
7	ZK7922-14b2	Magnetite ore	819.1 m down the ZK7922 borehole	Magnetite	0.215	0.022	0.317
8	202 transverse drift -14b4	Magnetite ore	200 horizontal 202# transverse drift underground in mining area	Magnetite	0.125	0.022	0.184
9	Mu2	Magnetite ore	100 horizontal 106# transverse drift underground in mining area	Magnetite	−0.061	0.022	−0.090
10	Mu-13-2	Magnetite ore	100 horizontal 106# transverse drift underground in mining area	Magnetite	−0.062	0.022	−0.091
11	MKO-2 Mt	Magnetite ore	200 horizontal 283# transverse drift underground in mining area	Magnetite	0.086	0.022	0.127
12	MKO-3 Mt	Magnetite ore	200 horizontal 283# transverse drift underground in mining area	Magnetite	0.059	0.022	0.087
13	MKO-4 Mt	Magnetite ore	100 horizontal 106# transverse drift underground in mining area	Magnetite	0.096	0.022	0.142
14	MKO-6 Mt	Magnetite ore	100 horizontal 106# transverse drift underground in mining area	Magnetite	0.178	0.022	0.263
15	MKO-10 Mt	Magnetite ore	200 horizontal 214# transverse drift underground in mining area	Magnetite	0.028	0.022	0.041
16	MKO-11 Mt	Magnetite ore	200 horizontal 200# transverse drift underground in mining area	Magnetite	0.155	0.022	0.229

). The process began by crushing the samples into powder. For single-mineral testing, pure magnetite crystals were carefully selected under a binocular microscope and grounded into a 200-mesh powder. In the subsequent procedure, an appropriate amount of sample powder was dissolved in a mixed-acid solution (HF + HNO₃ + HCl + HClO₄). The powder was allowed to completely dissolve and it was subsequently evaporated. Then, Fe was separated from the solution using anion exchange resin. Matrix elements were eliminated with 6 mol/L HCl, and the Fe was leached using 0.4 mol/L HCl.

The sample crushing and dissolution, Fe separation and purification, and Fe isotope analysis were performed at the Experimental Center of the Institute of Sciences, China University of Geosciences (Beijing). The testing utilized the Neptune Plus multi-collector plasma mass spectrometer, instrument number 1205926S. The processed sample solution underwent plasma treatment via a stable injection system and then medium/high resolution electric and magnetic dual focusing before measurement in a collector. Throughout the process, instrument mass fractionation was corrected using the standard-sample-bracketing method, with igneous rock reference material BHVO-2 being the standard sample. To enhance accuracy, standard-sample-bracketing tests were repeated four times, and the average result is expressed as the thousandth deviation relative to the standard sample:

δ⁵⁶Fe_{igneous rock} = [(⁵⁶Fe/⁵⁴Fe)_sample/(⁵⁶Fe/⁵⁴Fe)_{standard sample} − 1] × 1000,

(1)

To allow for comparison and discussion, the result was converted relative to the reference material IRMM-014 using the formula detailed in [23]:

δ⁵⁶Fe_IRMM-014 = δ⁵⁶Fe_{igneous rock} + 0.09‰,

(2)

Furthermore, the δ⁵⁷Fe result was calculated using mass fractionation in [24]:

δ⁵⁶Fe = 0.678 × δ⁵⁷Fe.

(3)

For detailed experimental parameters and testing processes, refer to [25].

5. Results

5.1. Magnetite Trace Element Composition

The magnetite trace element composition results are shown in Table S1. Al, Si, Mn, and Ca concentrations are notably high, mostly ranging from several hundred to over 10,000 ppm, while Mg, K, and Zn concentrations are mostly within a range of several hundred to 1000 ppm, with some Zn concentrations exceeding 2000 ppm. V, Ti, Cr, Co, Ni, Cu, Mo, Ga, and Sn concentrations are lower, mainly between 1 and 200 ppm. Meanwhile, Sc, Zr, and Nb concentrations are extremely low, usually ranging from 0.1 to 1 ppm. Box plots directly represent the concentration and dispersion of a dataset. Significant variations can be observed by comparing the box shapes of trace elements in different magnetite ores. The concentrations of Mn and Ni in magnetite from amphibole magnetite ores, Al and Ga in magnetite from diopside magnetite ores, K in magnetite from quartz magnetite ores, Al in magnetite from chlorite/epidote magnetite ores, and Ca, Ti, Cr, and Ga in magnetite from magnetized diabase exhibit a wide range. Other elements show narrower concentration ranges (Figure 7).

Table S1 and Figure 7 show no discernible patterns in vertical distribution or ore types with regard to elemental concentrations. Yet, the concentration of Zn in magnetite from amphibole magnetite ores, Si in magnetite from quartz magnetite ores and mineralized diabase, as well as V, Ti, Cr and Ni in magnetite from mineralized diabase are high, which may indicate correlations between specific elements and the prevalent gangue mineral assemblages in the ores.

Furthermore, comparing the elemental concentrations of the two stages of magnetite in the chlorite/epidote magnetite ore, Mt1 (measurement points 4-8-1 and 4-8-2) displays higher levels of Al, Si, Ca, Mn, Mg, Zn, and Ti in comparison to Mt2 (measurement points 4-8-3 and 4-8-4) (Figure 8a). Despite this difference, their standardized curves show remarkable similarities, showing a loss of elements including V, Ti, Cr, Ga, Nb, and Zr (Figure 8b), which suggests a consistent genesis and source for the two stages.

5.2. Fe Isotopic Concentration

The whole-rock δ⁵⁷Fe values of Dayang and Juzhou granites are 0.383‰ and 0.019‰, respectively, indicating a substantial difference between the two. However, both fall within the δ⁵⁷Fe variation range of upper-crustal igneous rocks [24]. The notable distinctions might arise from distinct magmatic source areas or different stages of magmatic activity. According to Zhao et al. [27], weathering sedimentation has minimal impact on Fe isotope fractionation. In this study, a single sample of the Lindi Formation sandstone was tested, displaying a whole-rock δ⁵⁷Fe of 0.028‰, nearly approaching 0, which aligns with previous assessments of the Fe isotope composition of sedimentary rocks [28,29]. Furthermore, the measured whole-rock δ⁵⁷Fe of fresh and mineralized diabase is 0.273‰ and 0.119‰, respectively, signifying a noticeable decrease in δ⁵⁷Fe for mineralized diabase. All the Fe isotope results are shown in Table 2. Four magnetite ore samples have been analyzed. Integrating these outcomes with data from six magnetite ore samples obtained by Zhang and Zuo [22], these findings demonstrate that the δ⁵⁷Fe values of magnetite in ores range from −0.091‰ to 0.317‰, with an average of 0.121‰. Furthermore, the magnetite in the Lindi Formation sandstone has a δ⁵⁷Fe value of 0.319‰.

6. Discussion

6.1. Constraint of Magnetite Trace Element Composition on Deposit Genesis

Previously, researchers have presented three models for the genesis of the Makeng iron deposit: terrigenous sediment–hydrothermal transformation, marine volcanic sediment–hydrothermal transformation, and contact metasomatic skarn. These models collectively agree that the formation of magnetite in Makeng is linked to a hydrothermal process.

Magnetite from hydrothermal deposits (including skarn deposits) typically had low levels of Ti (<2300 × 10⁻⁶), V (<400 × 10⁻⁶), Ga (10 × 10⁻⁶–28 × 10⁻⁶) [30], and Sc (mean = 1.26 × 10⁻⁶) [31]. Conversely, the SiO₂ concentration could reach up to 5.40% in magnetite of skarn deposits [32]. In this paper, apart from mineralized diabase, Ti, V, and Sc concentrations in magnetite from other ores range from 10 × 10⁻⁶ to 561 × 10⁻⁶, from 1.64 × 10⁻⁶ to 38.11 × 10⁻⁶, and from 0.42 × 10⁻⁶ to 5.28 × 10⁻⁶, respectively. The average Sc concentration is 1.14 × 10⁻⁶, akin to the hydrothermal deposit magnetite Sc concentration, and SiO₂ varies from 0.06% to 3.80%. Despite the high TiO₂ values of magnetite in mineralized diabase (0.87%~1.33%), this falls outside the typical range for mafic–ultramafic rock genesis magnetite (ranging from 3.02% to 10.22%) [31], suggesting a hydrothermal rather than a mafic–ultramafic rock genesis for magnetite in mineralized diabase. These facts fully show the hydrothermal mineralization characteristics of the Makeng iron deposit.

Comparing trace element concentrations across different ore deposit types, magnetite linked to volcanic sediment genesis typically exhibits a higher Cu concentration (averaging around 50 × 10⁻⁶), whereas marine volcanic sedimentary genesis magnetite shows an elevated Zn concentration (up to 8800 × 10⁻⁶). Conversely, magnetite from skarn genesis displays lower Cu and Zn concentrations, averaging approximately 20 × 10⁻⁶ and 185 × 10⁻⁶, respectively [31]. In this study, the maximum Cu concentration in the magnetite was 17.04 × 10⁻⁶, and the Zn concentration in magnetite ranged from 18.96 × 10⁻⁶ to 2093.60 × 10⁻⁶. These values align closely with the Cu and Zn concentrations found in skarn genesis magnetite. The low Cu and Zn concentrations of the Makeng magnetite notably differentiate it from volcanic and marine volcanic sedimentary genesis deposit, possibly indicating a skarn genesis.

Multiple diagrams have been proposed to classify magnetite genesis based on its unique trace element composition [33,34,35]. Despite their limitations, these classification diagrams are highly indicative for categorizing magnetite genesis [36,37,38]. In both the (Ca + Al + Mn)−(Ti + V) and (Ti + V)−Ni/(Cr + Mn) diagrams, the magnetite samples all fall within the skarn field. This suggests that although the V and Ti concentrations of mineralized diabase differ significantly from other samples, the samples might still be associated with skarn genesis (Figure 9).

In summary, the trace element composition of diverse magnetite notably differs from that of magnetite linked to volcanic or marine volcanic sedimentary genesis, in agreement with the skarn genesis for the Makeng iron deposit.

6.2. Fe Isotopic Composition and Fe Source

The Fe isotope composition of magnetite in the iron ores in the Makeng deposit is relatively consistent, with δ⁵⁷Fe values predominantly clustering around Bulk Silicate Earth values (Figure 10). Given the association of Makeng magnetite with a hydrothermal metasomatic skarn genesis, fluid exsolution emerges as a pivotal factor in its formation. Research indicates that this exsolution triggers Fe isotope fractionation, leading to the enrichment of lighter Fe isotopes in magnetite [39].

In this study, the δ⁵⁷Fe values of magnetite from the magnetite ores and Lindi Formation sandstone are generally lower than the whole-rock δ⁵⁷Fe value of Dayang granite, and sometimes even below the whole-rock δ⁵⁷Fe value of Juzhou granite. Given Fe isotope fractionation linked to fluid exsolution, it is inferred that Fe in the magnetite mainly originates from Dayang granite, possibly with some contribution from Juzhou granite. The results of isotopic chronology show that the intrusion periods of the Dayang and Juzhou granites are all Yanshanian [14,15,16]. Consequently, we can at least affirm that metal materials (Fe) are mainly generated from the Yanshanian granitic intrusions within the mining area.

Fresh diabase has a whole-rock δ⁵⁷Fe value of 0.245‰, while δ⁵⁷Fe values of the magnetite show greater variability than the diabase. The limited samples make it difficult to determine the contribution of the diabase to Fe. However, mineralized diabase displays an enrichment of lighter Fe isotopes, compared to the fresh diabase (δ⁵⁷Fe = 0.131‰ It is indicated that metasomatism and Fe exchange occurred during the ore-forming fluid passing through the diabase causing Fe isotopic fractionation. Heavier Fe isotopes enter the ore-forming fluid, indicating that diabase contributed a certain amount of Fe to the mineralization. Fe concentration scanning of ZK18 core by Zhang [40] and ZK614 and ZK617 cores by Yi [18] indicates decreased Fe content in altered diabase compared to fresh diabase, suggesting Fe precipitation from diabase into the ore-forming fluid, suggesting diabase is a source of Fe.

Furthermore, Figure 10 illustrates a significant contrast in δ⁵⁷Fe values between the magnetite in the ores and the sandstone of Lindi Formation. The limited samples complicate a definitive conclusion on the contribution of the Lindi Formation to Fe. However, based on field observations, extensive metasomatism was observed not only in the vicinity of carbonate rocks but also within the Lindi Formation. Figure 11 illustrates that the silicification belt within the Lindi Formation is accompanied by skarn, and certain skarn minerals (primarily hedenbergite and andradite) formed iron–manganese minerals after weathering and leaching. Additionally, under the ore bodies, the Lindi Formation commonly hosts quartz magnetite and thick-bedded quartzite. The Magnetite trace element test shows a notably higher Si concentration of magnetite in quartz magnetite than that in other types of ores. The Fe concentration of clastic rocks in southwest Fujian shows that the total iron concentrations of the sandstone of the Linlin Formation ranges from 1.01% to 7.25% [41]. When the iron-bearing sandstone encounters the high-temperature ore-forming fluid, some of the iron in it can be released and mobilized into the fluid [42]. These facts suggest that hydrothermal metasomatism and material exchange exist between the ore-forming fluid and the Lindi Formation. Collectively, the sandstone of the Lindi Formation might have contributed some Fe during mineralization.

The elemental geochemical analysis categorizes the Juzhou and Dayang granites as continental crustal anatectic granites. Concurrently, the zircon Lu-Hf isotope of the diabase in the Makeng district implies a mantle origin for the diabase [13]. Additionally, Sr, Nd, and Pb isotope analyses conducted by Zhang and Zuo on magnetite, granite, diabase, limestone, and sandstone in the Makeng deposit revealed that both magnetite and granite exhibit similar characteristics in terms of their Sr, Nd, and Pb composition, suggesting an origin from the Upper Crust [9]. Based on these findings, it is hypothesized that the origin of the metal materials in Makeng deposit is complex. Metal materials (Fe) are mainly sourced from crustal granites, but some Fe from mantle diabases and sedimentary rocks may be involved in the mineralizing process.

6.3. Evolution of Mineralization Fluid and Constraints of Mineralization Process

Skarn deposits commonly occur in the contact zones between intrusive rocks and carbonate rocks, or nearby. Altered mineral assemblages often show zonal patterns, particularly near these contact zones. However, in the Makeng iron deposit, the main ore body predominantly lies within interlayer fracture zones between the carbonate rocks of the Jingshe–Qixia Formations and the sandstone of the Lindi Formation. Small ore bodies mainly exist in the carbonate rocks of Jingshe–Qixia Formations above the main ore body, some positioned near the contact interface between the Wenbishan and Qixia Formations. These ore bodies are not in direct contact with the granitic intrusions. The zonation of the skarn mineral assemblage surrounding the main ore body is intricate. The upper portion is primarily composed of garnet and magnetite, while the middle section includes diopside and magnetite. The lower part includes tremolite, quartz, and magnetite, whereas the edge is composed of manganese skarn with a small amount of sulfides. Additionally, the limestone of Upper Carboniferous Jingshe Formation in the roof of the ore body is transformed into marble by strong metamorphism, while the quartz sandstone of the Lower Carboniferous Lindi Formation at the bottom is transformed into silicified sandstone by silicification alteration. Furthermore, far from the main ore body, sandstone and mudstone strata contain abundant low-temperature alteration minerals such as chlorite, sericite, and kaolin. Overall, from the mineralization center to the edge of the ore body, the mineralization–alteration zonation and mineral assemblage of Makeng iron deposit are not significantly different from most skarn iron deposits in the world. In addition, field surveys have shown that most skarn minerals in the orebody margin are rich in manganese, consistent with the characteristics of distal skarn [43].

The changes in mineral assemblage and geochemical characteristics of skarn deposits are often affected by various factors such as the types of intrusive rocks, the composition of surrounding rocks, and fluid–rock interactions [44,45]. Experiments have confirmed that certain skarn mineral assemblages can be generated through metasomatism with appropriate proportions of Ca, Si, Mg, Al, and Fe under certain conditions [46]. The trace element composition of magnetite shows a strong relationship with the element concentration in the ore-forming fluid [47]. This study reveals high Ca (291 × 10⁻⁶~4780 × 10⁻⁶) and Si (304 × 10⁻⁶~17,748 × 10⁻⁶) concentrations in Makeng magnetite, indicating that a calcareous and siliceous ore-forming fluid infiltrated the favorable mineralization space during its formation, like the interlayer fracture zones between the Jingshe–Qixia Formation carbonate rocks and the Lindi Formation sandstone. Magnetite ores with different mineral assemblages were formed through metasomatism with the surrounding rocks. For instance, garnet magnetite and diopside magnetite primarily resulted from carbonate rocks, while quartz magnetite mainly originated from metasomatism between the Lindi Formation sandstone and the ore-forming fluid.

When the ore-forming fluid engages in metasomatic reactions with distinct wall rocks, certain element concentrations in the newly formed magnetite often align with those of the metasomatized wall rocks [48,49]. For example, quartz magnetite ores formed near the Lindi Formation sandstone display notably higher Si contents of magnetite compared to that of other types of ores. Likewise, magnetite formed near diabase veins exhibits elevated V and Ti contents in comparison to those near the surrounding rocks. These observations further confirm that the various magnetite types in the Makeng district result from metasomatism between ore-forming fluid and the different wall rocks.

Nadoll et al. suggested that decreasing temperature could reduce the solubility and concentration of elements like Si, Al, Mg, and Ca, along with a decrease in the distribution coefficient between magnetite and the fluids [36]. The early magnetite from chlorite/epidote samples (from points 4-8-1 and 4-8-2) showed notably higher concentrations of Si, Al, Mg, and Ca compared to late magnetite (from points 4-8-3 and 4-8-4). This difference likely indicates a drop in ore-forming fluid temperature during the late formation. The temperature discrimination diagram for magnetite confirms that the temperature during the formation of early magnetite was higher than that during the formation of late magnetite, in agreement with temperature variations observed in skarn deposits (Figure 12). The results of the fluid inclusion test show that the inclusions in sulfide cracks and quartz veins are mostly of vapor- and liquid-rich fluid inclusions with low temperature and low salinity (the homogenization temperature varies between 160 and 300 °C), while the inclusions in garnet and diopside are mostly of halite-bearing fluid inclusions with high temperature (the homogenization temperature varies between 460 and 600 °C) [6]. C, H and O isotopic geochemistry shows that the ore-forming fluids of the skarn stage were mainly derived from magmatic water, whereas the fluids of the quartz–sulfide stage were mixed with meteoric water [6]. The mixture of magmatic water with meteoric water resulted in mineralizing fluids with lower temperatures and lower salinities in the sulfide stage, which suggests that the skarn system continued to cool during the late skarn stage [44].

To a certain extent, the concentrations of V and Cr in magnetite are influenced by the oxygen fugacity of the ore-forming fluid [36]. A decrease in oxygen fugacity is favorable for the incorporation of V into magnetite [49]. When comparing early chlorite/epidote magnetite samples to late magnetite, lower V and Cr concentrations in the former suggest a gradual reduction in the oxygen fugacity of the ore-forming fluid. The europium anomaly observed in rocks, ores, and magnetite in the Makeng iron deposit signifies a continuous decrease in both temperature and oxygen fugacity over time, which transitioned the ore-forming fluid into a hydrothermal solution with lower temperature and reducing condition during the quartz–sulfide stage [50].

In summary, the mineralization process of the Makeng iron deposit closely resembles that of typical skarn deposits. During the Yanshanian period, the intrusion of Juzhou and Dayang granites triggered the entrance of magmatic hydrothermal fluids into the Makeng district. These fluids interacted with various geological structures like fold axes, faults, and fractures, initiating metasomatic reactions with the surrounding rocks. This led to the extensive formation of initial skarn minerals like garnet and diopside, extracting iron from the surrounding rocks. As the process continued, the fluid’s temperature and oxygen fugacity gradually decreased, prompting the deposition of Fe ions as magnetite at suitable locations. During the retrograde metasomatic stage, declining temperature and oxygen fugacity led to the retrogressive transformation of skarn minerals, forming minerals such as tremolite, actinolite, and epidote. The precipitation of magnetite continued until the early part of this stage. Transitioning to the quartz–sulfide stage, a significant reduction in both temperature and oxygen fugacity resulted in the generation of a hydrothermal solution characterized by lower temperature and reducing conditions. Therefore, metallic elements like Fe, Mo, Cu, Pb, and Zn crystallized in sulfide form and inserted in a vein-like manner among the pre-existing skarn minerals and magnetite. In the latter portion of this stage, there was a prominent increase in carbonate minerals, marking the gradual conclusion of the mineralization process.

7. Conclusions

(1)

The magnetite in the Makeng iron deposit exhibits significantly lower concentrations of V, Ti, Cu, and Zn and higher levels of Si than magnetite sourced from volcanic or marine volcanic sedimentary origins. It is evident that the Makeng magnetite is closely linked to skarn genesis.

(2)

The δ⁵⁷Fe values in magnetite range from −0.091‰ to 0.317‰. The Makeng iron deposit has multiple Fe sources, with Yanshanian granitic intrusions likely being the primary contributor. There might also be contributions from mantle-derived diabase, and a potential role of Lindi Formation sandstone as a source of Fe for mineralization cannot be ruled out.

(3)

The Makeng iron deposit is a skarn deposit closely linked to Yanshanian granitic intrusions. Initially, the ore-forming fluid was a high-temperature, high-oxygen fugacity magmatic hydrothermal solution. Over time, it transitioned into a hydrothermal solution with lower temperatures and reducing conditions. Magnetite formation occurred primarily during the calcareous skarnization stage, extending into the early part of the retrograde metasomatic stage. Additionally, sulfides like molybdenite, pyrite, chalcopyrite, sphalerite, and galena were generated during the quartz–sulfide stage.