Geochemical Fingerprints of Magnetite in Yechangping Super-Large Mo-W Deposit, Western Henan, China: Constraints on Ore-Forming Evolution and Prospecting Implications

Abstract

The Yechangping super-large porphyry–skarn deposit is a key component of the East Qinling molybdenum metallogenic belt, central China. Magnetite is widely developed across all mineralization stages of this deposit, yet its systematic geochemical evolution and prospecting significance remain poorly constrained. This study presents in situ major- and trace-element analyses of magnetite via electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and elemental mapping, to unravel the ore-forming hydrothermal evolution and establish reliable prospecting indicators. Four magnetite generations are identified based on petrography and paragenetic relationships: late skarn stage (Mt1), oxide stage (Mt2 and Mt3), and polymetallic sulfide stage (Mt4). Magnetite has total iron contents (TFeO, total Fe calculated as FeO) of 82.72–95.46 wt.% (values above the 93 wt.% stoichiometric limit of pure magnetite stem from minor oxidation), with dominant isovalent Fe³⁺ and Al³⁺ lattice substitution supported by a significant negative Fe–Al correlation. Systematic stage-dependent geochemical variations are observed: Mt1 has the highest Ti (mostly >1500 ppm), V and Cr, while Mt2–Mt4 show progressive Ti depletion (mostly <100 ppm), recording continuous cooling of the hydro-thermal system. V and Cr contents decrease markedly from Mt1 to Mt3, with secondary enrichment in Mt4; Mo concentrations peak in Mt2 (average 5.06 ppm), coupled with elevated chalcophile metalloid Te, As, Pb and Bi. Elemental mapping results show that K occurs as discrete hotspots, which may be mainly derived from feldspar microinclusions, rather than lattice substitution in magnetite. These geochemical fingerprints record a transition from high-temperature magmatic–hydrothermal fluids to late contact-metasomatic fluids, with evolving fluid–rock interaction and oxygen fugacity. Our results demonstrate that magnetite chemistry is a reliable tool for discriminating mineralization stages and vectoring prospecting targets in porphyry–skarn Mo–W systems.

1. Introduction

Magnetite (Fe₃O₄), owing to its flexible crystal chemistry and strong sensitivity to physicochemical conditions, has become a powerful archive for tracing ore-forming processes and reconstructing metallogenic evolution in magmatic–hydrothermal systems [1,2,3,4]. Its major- and trace-element compositions respond sensitively to variations in temperature, oxygen fugacity (fO₂), sulfur fugacity (fS₂), fluid composition, and the intensity of fluid–rock interaction, making magnetite an effective petrogenetic and metallogenic indicator mineral [5,6]. Consequently, magnetite has been widely applied to discriminate mineralization styles, constrain ore-forming conditions, and guide mineral exploration across a broad range of deposit types, including porphyry, skarn, volcanogenic massive sulfide (VMS), and iron oxide–copper–gold (IOCG) systems [3,7].

In porphyry–skarn systems, magnetite commonly coexists with economically significant Mo and W mineralization and forms over multiple stages of hydrothermal evolution. Previous studies have shown that systematic variations in magnetite chemistry can faithfully record the transition from magmatic to hydrothermal regimes, reflecting progressive changes in fluid sources, redox conditions, and temperature. The incorporation of trace elements such as Ti, V, Cr, Co, Ni, and Mo into magnetite is strongly controlled by the nature of the parental melt or fluid, co-crystallizing mineral assemblages, and evolving physicochemical parameters. As a result, magnetite crystallized from silicate melts typically exhibits enrichment in lithophile elements and depletion in chalcophile elements, whereas magnetite precipitated from magmatic–hydrothermal fluids display more complex and deposit-specific geochemical signatures linked to fluid evolution and metal transport processes [8,9,10,11,12]. Despite significant advances in magnetite geochemistry over the past decade, several fundamental issues remain unresolved. In particular, the compositional controls on magnetite formed in systems where silicate melts, magmatic–hydrothermal fluids, and contact-metasomatic fluids coexist and interact are still poorly constrained. Most previous studies have focused on fluid source discrimination and broad metallogenic affinity [12,13], whereas the detailed temporal evolution of magnetite chemistry across successive mineralization stages—and its response to changing intensities of fluid–rock interaction—has received comparatively limited attention. This knowledge gap restricts the quantitative use of magnetite as a recorder of ore-forming evolution and as a predictive tool for exploration targeting.

The Yechangping Mo–W deposit, located in the eastern segment of the Qinling Orogen (Figure 1a,b), represents a super-large porphyry–skarn system characterized by extensive magnetite development and a close spatial association with molybdenite and scheelite mineralization. This deposit therefore provides an exceptional natural laboratory for investigating the genetic relationships between magnetite formation and multistage molybdenite–scheelite mineralization [14,15]. Although previous studies have established the regional metallogenic framework and broadly recognized the genetic link between magnetite and ore formation at Yechangping, a refined subdivision of magnetite generations corresponding to discrete mineralization stages, together with a systematic evaluation of their in situ geochemical variability, has not yet been achieved. In particular, the lack of high-resolution elemental mapping and stage-specific trace-element datasets has hindered a comprehensive understanding of magnetite-forming mechanisms and fluid evolution in this deposit.

In this study, we present an integrated petrographic and in situ geochemical investigation of magnetite from the Yechangping porphyry–skarn Mo–W deposit. Based on detailed paragenetic constraints, magnetite is classified into distinct generations corresponding to successive mineralization stages. Major- and trace-element compositions are determined using electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and elemental mapping techniques. By comparing the compositional characteristics of magnetite formed at different stages, this study aims to (1) elucidate the controls exerted by evolving melts and fluids on magnetite chemistry, (2) reconstruct the temporal evolution of temperature, oxygen fugacity, and fluid–rock interaction during ore formation, and (3) evaluate the effectiveness of magnetite geochemical fingerprints as indicators of mineralization stages and exploration targeting in porphyry–skarn Mo–W systems.

2. Regional Metallogenic Background

The study area lies in the East Qinling segment of the Qinling composite orogenic belt, a key tectonic transition between the southern North China Craton and northern Yangtze Craton (Figure 1c). Bounded by the Sanmenxia–Baofeng Fault to the north and Shangdan Suture Zone to the south, this region forms a major tectonic node in central China, and hosts a world-class Yanshanian Mo metallogenic province [16,18,19,20,21].

The near E-W-trending Zhuxia Fault Zone, a lithosphere-scale ductile-brittle shear system, is the first-order structure controlling regional magmatism and mineralization [13,19]. It records Mesozoic reactivation of the Paleozoic Shangdan suture, with an evolutionary history from early subduction-related compression, through strike-slip deformation, to late Yanshanian extension. This compressional-to-extensional transition drove lithospheric delamination, asthenospheric upwelling, and hybrid crust-mantle magma generation. Evolved intermediate-felsic magmas ascended along Riedel shear fractures of the Zhuxia Fault Zone, emplaced as structurally aligned porphyritic intrusions and cryptoexplosive breccia bodies, and supplied the heat, metals, sulfur, and volatiles for regional Mo-dominated mineralization [17,21].

Regionally, the Mesoproterozoic–Neoproterozoic Luanchuan and Guandaokou groups are the primary formations affected by skarn alteration. These carbonate-dominated successions (dolomite, marble, and limestone) are widespread in the hanging wall of the Zhuxia Fault Zone, and their high chemical reactivity and well-developed interlayer detachment horizons make them the most favorable host rocks for skarn formation. Other regional stratigraphic units include: the Archean Taihua Group, rigid crystalline basement that restricted vertical magma ascent and promoted lateral intrusion emplacement [16]; the Mesoproterozoic Xionger Group, K- and volatile-rich volcanic-sedimentary rocks that may have contributed supplementary ore-forming components; and the Lower Paleozoic Erlangping Group, which locally acted as a physical-chemical barrier at fault intersections to drive fluid mixing and metal precipitation [22].

The intrusions responsible for regional skarn formation and coeval Mo-W mineralization are Mesozoic intermediate-felsic porphyritic stocks, dominated by granite porphyry, K-feldspar porphyry, and granodiorite porphyry. These are typically small, structurally controlled bodies elongated parallel to regional structures. Geochemically, they are enriched in Si and K, depleted in Ca, Mg, and Fe, and show mixed I-type and S-type affinities, reflecting crustal melting with variable mantle input during intracontinental tectonic reorganization [23,24].

Geochronological data confirm that molybdenite Re–Os ages are broadly coeval with zircon U–Pb ages of associated granitoids, constraining both magmatism and molybdenite-scheelite mineralization to the Mesozoic [18,20,25,26,27,28,29]. Ore-forming porphyries occur as bead-like bodies along the Zhuxia Fault Zone, preferentially emplaced at the contact between the Luanchuan and Erlangping groups. High oxidation states (high Fe³⁺/ΣFe) of these Yanshanian porphyries are a key control on Mo-W mineralization [13,29,30,31,32,33]. Their contact zones are marked by garnet–diopside–wollastonite skarn assemblages, overprinted outward by chlorite–sericite–fluorite and iron-manganese carbonate alteration, with fine-grained, patchy, or vein-like textures. Regional Cu-Mo-Pb-Zn-Ag mineralization is spatially and genetically linked to these porphyry systems, recording multistage hydrothermal overprinting.

3. Deposit Geology

3.1. Geological Features of the Mining Area

The Yechangping Mo-W deposit is located within the Luonan–Luanchuan thrust belt on the southern North China Craton, at the intersection of near EW and NE trending structural zones, and forms part of the Luanchuan Mo-W-Pb-Zn-Ag polymetallic metallogenic belt. Faults are well-developed in the mining area and exert a first-order control on intrusion emplacement, skarn alteration, and orebody localization. Major WNW- to near E-W-trending faults, subparallel to the regional Luanchuan–Heigou and Panhe–Machaoying faults (Figure 2), dominate the structural framework and control stratigraphic exposure with local repetition or omission. Secondary NE-trending faults crosscut the EW structures to form a grid-like network, and the intersections of these two fault sets host the concealed ore-forming intrusion and associated Mo-W mineralization. EW trending faults are commonly associated with intense iron-manganese carbonate alteration, while NE-trending structures are smaller with only weak mineralization.

Strata exposed in the mining area are dominated by the Mesoproterozoic Guandaokou Group, with minor Quaternary cover. The Longjiayuan Formation (Pt₂l) of the Guandaokou Group is the only formation affected by skarn alteration and the sole ore-hosting horizon at Yechangping. It consists of shallow littoral to shallow-marine clastic-carbonate sequences, with dolomite as the principal wall rock for skarn formation. The conformably overlying Xunjiansi Formation (Pt₂x) is composed of siliceous banded dolomite and thick-bedded dolomite, with no significant skarn alteration or mineralization. Quaternary deposits are restricted to valleys and hillslopes.

Granite porphyry is the direct parent intrusion responsible for skarn formation and Mo-W mineralization in the mining area. Igneous rocks in the study area also include concealed quartz porphyry, monzogranite porphyry, and syenite dikes emplaced along fault zones. The ore-related granite porphyry occurs as stocks and arcuate apophyses at depths of ~200–300 m in the northeastern part of the mining area. It is light pink to grayish red with a porphyritic texture: phenocrysts are dominated by quartz (15%–25%) and K-feldspar (5%–10%), set in a fine-grained to cryptocrystalline groundmass (~75%) of quartz, feldspar, and biotite. Common hydrothermal alteration of the porphyry includes silicification, K-feldspathization, chloritization, and carbonation.

3.2. Ore Bodies and Features

Mineralization at the Yechangping deposit is jointly controlled by magmatism, structures, and host stratigraphy. Ore bodies are hosted within the Longjiayuan Formation and along both the internal and external contact zones between the Yanshanian concealed intermediate–felsic granite porphyry and the surrounding carbonate strata. In plan and section, the ore bodies display a layered domal geometry, characterized by a convex central part that dips outward and gradually tapers toward the margins. Vein-type skarn is developed along the contact between granite porphyry and carbonate wall rocks. Alteration assemblages include garnet, diopside, tremolite, pyroxene, and chlorite, and are commonly associated with molybdenite–scheelite mineralization. Layered skarn is well-developed in dolomite and marble proximal to the granite porphyry. It is characterized by alteration minerals such as diopside, tremolite, epidote, chlorite, calcite, and quartz. Planar and banded magnetite occurs within these skarns, accompanied by elevated molybdenite grades. Late-stage gypsum veins and carbonate–polymetallic sulfide veins locally cut the earlier skarn mineralization. Drill-hole grade data indicate a strong spatial and geochemical coherence between Mo and W mineralization, with significantly elevated grades on both sides of granite porphyry apophyses. Based on host rocks and modes of occurrence, two principal mineralization types are recognized. (1) Porphyry-type Mo mineralization occurs within the intrusion and is expressed as lenticular bodies, veinlets, and disseminations. Molybdenite is the dominant ore mineral, accompanied by minor magnetite and subordinate W mineralization. (2) Skarn-type mineralization is concentrated along and adjacent to the contact zone with carbonate wall rocks (Figure 3). According to occurrence, it can be subdivided into vein-type skarn mineralization (within skarn veins) and layered skarn mineralization (within dolomite and marble). Both subtypes share similar mineral assemblages and genetic mechanisms and are characterized by magnetite as a key accessory mineral, commonly associated with molybdenite and pyrite. These skarns represent the principal host for magnetite in the deposit. Late hydrothermal mineralization occurs mainly as quartz–sulfide and carbonate–sulfide veins that crosscut earlier mineralized assemblages. Ore minerals are dominated by pyrite, chalcopyrite, and sphalerite, with minor molybdenite and scheelite. Gangue minerals include quartz, calcite, and fluorite, and veins typically display veinlet and stockwork textures. Magnetite commonly occurs as fine-grained disseminations within the veins or is enclosed by sulfide minerals.

3.3. Paragenetic Sequence

Mineralization at the Yechangping Mo–W deposit can be classified into two major genetic systems: porphyry-type and skarn-type mineralization. Magnetite occurs in both systems but with markedly different abundance and timing. Within the porphyry intrusion, magnetite is only sparingly present and is associated with early potassic alteration during the initial stage of porphyry mineralization. In contrast, the majority of magnetite forms during the skarn stage, which constitutes the primary focus of this study. Skarn-type mineralization can be further subdivided into five successive stages (Figure 4): (I) early skarn, (II) late skarn, (III) oxide, (IV) quartz–sulfide, and (V) carbonate–sulfide.

Skarn mineralization is mainly developed along the contact zone between the granite porphyry and dolomite. During the early emplacement of the intrusion, contact metamorphism induced skarnization of dolomite and biotitization of syenite, reducing the permeability of the wall rocks and restricting the extent of hydrothermal circulation. Meanwhile, pre-existing faults and fracture zones formed along the intrusive rock interface provided effective pathways for multiphase fluid migration. The early skarn stage (Figure 3a,b) is dominated by diopside. Minor garnet, pyroxene, and disseminated pyrite occur locally along limestone contacts in the northern part of the ore district, with tremolite appearing during the later part of this stage. Most garnet grains were subsequently replaced by sericite and calcite. The late skarn stage (retrograde alteration stage; Figure 3c,d) is characterized by hydrous minerals, including tremolite, epidote, and actinolite, with minor biotite. These minerals occur as pervasive, disseminated, or veinlet-style replacements of earlier skarn assemblages. The oxide stage (Figure 3e,f) is characterized by magnetite, hematite, and quartz, with minor late-stage molybdenite, scheelite, and pyrite. Petrographic observations reveal skeletal textures formed by the replacement of diopside. The quartz–sulfide stage (Figure 3g,h) represents the principal ore-forming stage of the skarn system, with ore minerals dominated by molybdenite, pyrite, chalcopyrite, and sphalerite, accompanied by minor galena. Quartz–carbonate–sulfide veins widely crosscut earlier mineral assemblages, and mineralization is concentrated near the contact zone, weakening progressively with increasing distance from the intrusion. Chalcopyrite bleb exsolution textures are commonly observed within sphalerite. The carbonate-sulfide stage is dominated by low-temperature minerals such as calcite and fluorite, with only minor associated sulfides.

3.4. Magnetite Features

Magnetite formation in the study area is distinctly multistage. It occurs intermittently from the late skarn stage onward, becomes widespread during the oxide stage, and persists locally into the polymetallic sulfide stage, where it appears sporadically within mineralized veinlets. The paragenetic sequence of magnetite and associated minerals is strictly constrained by detailed petrographic and cross-cutting relationships, with a clear temporal framework established as: prograde skarn stage (diopside + garnet, pre-magnetite) → retrograde late skarn stage (Mt1 + tremolite) → early oxide stage (Mt2) → late oxide-ore transition stage (Mt3 + molybdenite + scheelite) → late polymetallic sulfide stage (Mt4). Late skarn-stage magnetite (Mt1) typically occurs as anhedral granular aggregates with a grayish-white appearance under reflected light. Grain sizes are generally uniform, ranging from 0.05 to 0.3 mm. Spatially, Mt1 is commonly distributed along the margins of skarn minerals, including biotite, sericite, garnet, diopside, and tremolite (Figure 5a–c), and some grains are enclosed by later-formed minerals. Early oxide-stage magnetite (Mt2) exhibits subhedral to euhedral crystal forms, with grain sizes ranging from 0.5 to 1 mm (Figure 5d). It is closely associated with hydrothermal minerals such as tremolite and tourmaline and often displays well-developed oscillatory zoning. Late oxide-stage magnetite (Mt3) occurs as dense disseminations within skarn minerals (Figure 5e). Grain sizes are variable, typically 0.2–0.5 mm, and it is closely associated with early ore minerals, including molybdenite, pyrite, and chalcopyrite. Polymetallic sulfide-stage magnetite (Mt4) occurs mainly as aggregates within composite quartz–pyrite–calcite–fluorite ± chalcopyrite veins (Figure 5f). Grain sizes range from to 0.05–0.1 mm, and the magnetite is commonly enclosed by late-stage pyrite.

4. Sampling and Analytical Methods

Representative magnetite samples were collected from typical ore bodies and alteration zones of the Yechangping porphyry–skarn-type Mo–W deposit in Henan Province. Samples were taken from different positions within the ore system, including the mineralization core, marginal zones, and altered wall rocks. Only fresh, unweathered samples without secondary oxidation were selected for analysis.

Polished thin sections were prepared for petrographic examination, and magnetite formed under different paragenetic conditions was identified by reflected-light microscopy. Major-element compositions were determined by electron probe microanalysis (EPMA) at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, using a JEOL JXA-iHP200F electron microprobe (Japan Electron Optics Laboratory Co., Ltd., Akishima, Tokyo, Japan) operated at 15 kV accelerating voltage, 10 nA beam current, and a beam diameter of 1 μm. NIST SRM 610 and 612 glass standards, together with mineral standards (e.g., magnetite and olivine), were used for calibration and matrix correction. Analytical precision for major elements is better than 5% relative error.

Trace-element concentrations were analyzed by LA-ICP-MS at Beijing Kehui Testing Technology Co., Ltd., Beijing, China, using an Analytik Jena AG (Jena, Germany) PQMS Elite ICP-MS coupled with a RESOlution 193 nm excimer laser system (Sydney, Australia). Analyses were performed with a laser spot diameter of 50 μm and 30 ablation cycles per spot. Quantification was conducted using a multi–external standard calibration approach with USGS glass standards BIR-1G and BCR-2G and NIST 612, and a natural magnetite reference material (BC28) was used for quality control [35]. Prior to analysis, samples were ultrasonically cleaned to minimize surface contamination. Elemental mapping images are presented as relative concentration maps, with color intensity proportional to elemental abundance.

5. Results

Magnetite from the Yechangping deposit is dominated by TFeO, with contents ranging from 82.72 to 95.46 wt.%, and contains variable amounts of MnO, TiO₂, Al₂O₃, MgO, SiO₂, Na₂O, K₂O, and CaO (Table S1). Mt1 magnetite shows TFeO contents of 84.71–95.46 wt.% (average 90.79 wt.%), MnO contents of 0.37–1.95 wt.% (average 1.06 wt.%), and TiO₂ contents of 0.10–4.13 wt.% (average 0.89 wt.%). Mt2 magnetite has TFeO contents of 82.72–94.86 wt.% (average 90.86 wt.%), slightly higher MnO contents than Mt1 (0.59–2.29 wt.%, average 1.71 wt.%), and markedly lower TiO₂ contents (0.01–0.38 wt.%, average 0.10 wt.%). Mt3 magnetite exhibits slightly higher TFeO contents (89.33–93.68 wt.%, average 91.50 wt.%), elevated MnO contents (1.69–3.55 wt.%, average 2.40 wt.%), and very low TiO₂ contents (<0.06 wt.%, average 0.01 wt.%). Mt4 magnetite shows TFeO contents of 87.41–92.75 wt.% (average 89.24 wt.%), the highest MnO contents among the four types (2.32–4.43 wt.%, average 3.48 wt.%), and TiO₂ contents generally below 0.01 wt.%. Overall, from Mt1 to Mt4, TFeO contents remain relatively constant, whereas Ti contents decrease systematically and Mn contents increase progressively. Magnetite generally has high contents of Si, Mg, and Mn, and low contents of Ti, Al, and Ca.

LA-ICP-MS analyses indicate that magnetite contains measurable concentrations of Co, Ni, Cr, V, and Ti (Table S2). Mt1 magnetite is characterized by the highest V and Cr contents, with average values of 970.49 ppm and 519.75 ppm, respectively. It also contains relatively high Co and Ni contents (average 39.48 ppm and 40.35 ppm), whereas the average Mo content is low (2.51 ppm). Mt2 magnetite shows markedly lower V and Cr contents (average 249.50 ppm and 3.67 ppm), little change in Co contents (average 32.16 ppm), and reduced Ni contents (average 14.94 ppm). The average Mo content increases to 5.06 ppm. Mt3 magnetite displays further decreases in V and Cr contents (average 46.27 ppm and 2.21 ppm), whereas Co and Ni contents remain comparable to those of Mt2 (average 49.81 ppm and 16.70 ppm). The average Mo content is 3.35 ppm. Mt4 magnetite shows an increase in V and Cr contents relative to Mt2 and Mt3 (average 263.58 ppm and 237.15 ppm), while Co and Ni contents remain relatively constant (average 49.04 ppm and 29.22 ppm). The average Mo content is 4.02 ppm.

Elemental mapping results reveal distinct compositional features between early and late magnetite generations (Figure 6). Early high-temperature magnetite (Mt2) shows heterogeneous, patchy distributions of lithophile elements such as Al, Si, and K. These elements locally form irregular enrichment domains. “enrichment” in this study refers to a statistically significant increase in element concentration from early to late magnetite generations, corresponding to a specific hydrothermal evolution process. Ca and Ce display spatial associations with these lithophile elements, suggesting incorporation through isomorphic substitution and/or contributions from fluid-related components. Ore-forming elements (W, Mo, and Nb) show only weak and localized enrichment in Mt2 magnetite. In contrast, late low-temperature magnetite (Mt3) exhibits relatively homogeneous distributions of V, Mn, and Co, indicating efficient lattice-controlled incorporation under lower-temperature hydrothermal conditions. Sulfide-related elements (As, Pb, and Bi) contents are relatively higher and are spatially associated with microfractures and inclusions within magnetite, suggesting the involvement of late-stage sulfide-rich fluids. Ore-forming elements such as Mo, Te, and W show broader enrichment zones and higher intensities than in early magnetite, indicating enhanced metal input during late-stage hydrothermal evolution.

6. Discussion

6.1. Genesis of Magnetites

Magnetite from the Yechangping Mo-W deposit exhibits a clear multi-stage evolutionary history with contributions from multiple material sources, closely linked to the evolution of a magmatic–hydrothermal system and prolonged fluid–rock interaction. Magnetite first appeared during the late skarn stage, became extensively developed in the oxide stage, and continued to precipitate during the polymetallic sulfide stage. The systematic variation in its occurrence and mineral associations records a continuous and evolving mineralization process. Early magnetite formed during the late skarn stage (Mt1) is generally anhedral to subhedral. It occurs in close association with skarn minerals such as biotite and garnet. This occurrence indicates crystallization under high-temperature skarn-forming conditions. During the oxide stage, early magnetite (Mt2) shows relatively well-developed crystal forms. This feature suggests more stable physicochemical conditions during its formation. In contrast, late oxide-stage magnetite (Mt3) occurs as dense disseminations. It is closely associated with molybdenite and pyrite, reflecting a significant evolution of the ore-forming fluids. Magnetite formed during the polymetallic sulfide stage (Mt4) mainly occurs as veins. It is commonly enclosed by pyrite, indicating precipitation from relatively low-temperature, sulfur-rich hydrothermal fluids. Overall, these systematic changes in occurrence and mineral assemblages record a progressive evolution of the ore-forming fluids, from high-temperature magmatic–hydrothermal to lower-temperature hydrothermal conditions. Major-element compositions show that Fe contents in magnetite generally range from 82 to 93 wt.%. A few magnetite analyses yielded TFeO contents slightly above the theoretical maximum (up to 95.46 wt.%), which we interpret as the result of minor surface oxidation during sample preparation and electron probe microanalysis (EPMA), leading to a slight overestimation of total iron. Elements such as Mg, Ca, and Mn display clear negative correlations with Fe, indicating their incorporation into the magnetite lattice via isomorphic substitution. Early-stage magnetite is characterized by relatively high Ti contents and low Mg, Mn, and Cr concentrations, which are typical geochemical features of magmatic magnetite [36]. This suggests that magmatic input played a dominant role during the early stages of magnetite formation.

Elemental mapping further constrains the genetic evolution of magnetite (Figure 6). During the early high-temperature stage, magnetite is enriched in lithophile elements (Ca, Si, and K), accompanied by low content in W and Mo. These features are consistent with fluids derived from magmatic differentiation. At this stage, silica-rich fluids facilitated the incorporation of lithophile elements, whereas ore metals had not yet undergone large-scale precipitation. Based on elemental mapping and full dataset statistics, we defined concentration thresholds for true lattice substitution in magnetite: Si (<100 ppm), Al (<2.0 wt.% with homogeneous distribution), and Nb (<2 ppm without co-located HFSE hotspots). All element hotspots are attributed to microinclusions of other phases. In-situ analysis spots were strictly selected to avoid these hotspots, and all geochemical interpretations are based on data from homogeneous, inclusion-free magnetite matrix. However, Si and K in the analyzed magnetite are likely derived primarily from submicron-scale feldspar micro inclusions, rather than from lattice substitution. In contrast, the late low-temperature stage is characterized by high content of sulfide-affinity elements (As, Pb, Bi and Te), together with decreased Si and Al contents. This compositional shift is interpreted to reflect fluid depressurization and cooling following extensive fluid–rock interaction. Decreasing sulfide solubility promoted sulfide precipitation, while high Te content further indicates low-temperature hydrothermal conditions [37,38]. These features suggest a progressive transition of the ore-forming fluids from magmatic–hydrothermal to contact metasomatic hydrothermal systems. The observed trace-element signatures indicate that enrichment in lithophile elements is diagnostic of the early high-temperature magmatic–hydrothermal stage, whereas enrichment in sulfide-related elements characterizes the late low-temperature hydrothermal stage. These geochemical contrasts provide reliable indicators for distinguishing different mineralization stages. Such a two-stage evolutionary pattern is consistent with the magmatic–hydrothermal metallogenic model of typical W–Mo deposits [2,39], confirming that the Yechangping deposit experienced a complete mineralization process from high-temperature magmatic–hydrothermal to low-temperature hydrothermal conditions. Notably, W and Mo are enriched during both stages, indicating a persistent magmatic source. The high content during the late stage is likely related to changes in fluid physicochemical conditions, such as decreasing oxygen fugacity and increasing sulfur fugacity, which promoted the precipitation of W and Mo in magnetite and associated sulfides and ultimately improved mineralization efficiency.

Geochemical discrimination diagrams indicate that magnetite from the Yechangping deposit was mainly formed during the skarn stage (Figure 7a) and is predominantly hydrothermal in origin (Figure 7b). Distinct spatial clustering of magnetite from different stages is observed, accompanied by systematic variations in formation temperature. Magnetite formed during the early late-skarn stage crystallized at relatively high temperatures and was strongly influenced by the intrusive body, whereas magnetite from the oxide and polymetallic sulfide stages formed at comparatively lower temperatures. The Fe–Si/(Na + Ca) diagram suggests that the primary material source of magnetite was silica-rich magmatic rocks (Figure 7c). In addition, Mg contents show a gradual increase from Mt1 to Mt4, and Fe-Mg values display a clear negative correlation (Figure 7d), indicating that dolomitic wall rocks participated in magnetite formation through fluid–rock interaction. The relatively large fluctuations in Fe-Mg values observed in Mt1 magnetite may reflect unstable and heterogeneous reaction conditions at lithological interfaces during the early stages of mineralization. As a member of the spinel group (general formula AB₂O₄), magnetite commonly accommodates extensive isomorphic substitutions, including Ti⁴⁺ for Fe³⁺ and Mg²⁺ for Fe²⁺ (e.g., Fe²⁺ + Ti⁴⁺ ↔ Fe³⁺). Together with variations in oxygen fugacity, temperature, and pressure across different mineralization stages, these features further indicate that magnetite formation at Yechangping was not the result of a single process, but rather the product of superimposed magmatic–hydrothermal and contact metasomatic hydrothermal processes operating throughout the evolution of the ore-forming system.

6.2. Ore-Forming Conditions

The elemental composition and mineralogical characteristics of magnetite from the Yechangping deposit provide constraints on the thermal and redox evolution of the hydrothermal system. Classical constraints on temperature and oxygen fugacity in magmatic–hydrothermal systems commonly rely on equilibria among coexisting Fe–Ti oxides (e.g., magnetite–ilmenite pairs) as established by early solution models and subsequent thermodynamic formulations [2,4,41,42,43]. However, because our dataset focuses on skarn-stage magnetite and does not consistently include equilibrium ilmenite–magnetite pairs suitable for quantitative thermo-oxybarometry, we primarily evaluate relative thermal and redox evolution using magnetite trace-element systematics (e.g., Ti–V behavior) and textural evidence, following recent developments that highlight the redox sensitivity of V partitioning in magnetite.

Ti contents in magnetite show a systematic decrease from the late skarn stage (Mt1) to the polymetallic sulfide stage (Mt4) (Figure 8a). Experimental and natural studies demonstrate that Ti is preferentially incorporated into magnetite at elevated temperatures, and its concentration generally decreases with cooling in magmatic–hydrothermal systems [3,38]. The observed progressive Ti decline therefore indicates a cooling trend from high-temperature magmatic–hydrothermal conditions toward lower-temperature contact-metasomatic and late hydrothermal environments. In addition, Ti exhibits a positive correlation with V (Figure 8b), consistent with the coupled behavior of these elements under high-temperature conditions [44]. The highest Ti and V contents occur in early-stage magnetite (Mt1–Mt2), whereas later generations show significantly reduced Ti concentrations. This systematic variation supports a progressive decrease in formation temperature during fluid evolution. Similar ultra-low Ti magnetite has been widely documented in the giant Nannihu–Sandaozhuang–Shangfanggou porphyry–skarn Mo deposits, which are located in the same East Qinling Mo metallogenic belt as the Yechangping deposit. Previous studies have shown that the late-stage hydrothermal magnetite from these deposits has Ti contents as low as <5 ppm, which is consistent with our test results [45,46]. Comparable ultra-low Ti hydrothermal magnetite has also been reported in Ca-rich skarn deposits in the Middle-Lower Yangtze River Metallogenic Belt [47] and porphyry–skarn Mo deposits in the Canadian Cordillera [44,48]. The ultra-low Ti contents in late-stage magnetite of the Yechangping deposit are genetically consistent with the hydrothermal evolution of porphyry–skarn systems: Ti solubility in hydrothermal fluids decreases sharply with cooling, and extensive fluid–rock interaction with dolomitic limestone wall rocks leads to significant Ti consumption via the formation of Ti-bearing skarn minerals (e.g., titanite) in the proximal alteration zone, resulting in extremely low Ti activity in late-stage ore-forming fluids, which is ultimately recorded in hydrothermal magnetite.

Oxygen fugacity exerts a fundamental control on magnetite stability and trace-element partitioning [49]. Although absolute fO₂ values cannot be calculated without equilibrium constraints involving coexisting mineral pairs and pressure estimates, the dataset allows evaluation of relative redox evolution through multiple independent lines of evidence.

First, V partitioning in magnetite is known to be sensitive to both temperature and redox state, as V can occur in multiple valence states (V³⁺, V⁴⁺, V⁵⁺). Higher V incorporation into magnetite is commonly associated with relatively oxidized and high-temperature conditions. The elevated V contents in Mt1 magnetite, together with high Ti concentrations and association with magmatic–hydrothermal minerals, suggest relatively oxidized early conditions. Second, textural observations indicate redox fluctuations during later stages. Petrographic observations confirm that hematite in the Yechangping deposit is mainly the alteration product of magnetite via martitization. Partial replacement of magnetite by hematite, as well as magnetite being locally overgrown or replaced by sulfide minerals (pyrite and chalcopyrite), implies changes in redox and sulfur fugacity during fluid evolution [36,50]. The transition from oxide-dominated assemblages to sulfide-rich vein systems reflects increasing sulfur activity and evolving redox conditions. Third, elemental mapping reveals that late-stage magnetite (Mt3–Mt4) is enriched in sulfide-affinity elements such as As, Pb, and Bi, often associated with microfractures and inclusions. These features indicate the infiltration of sulfide-bearing fluids under conditions distinct from those of the early high-temperature stage. Such enrichment patterns are consistent with a hydrothermal system that evolved from relatively oxidized magmatic–hydrothermal conditions toward more variable redox states during the main sulfide mineralization stage [36]. Taken together, these observations support a model in which the Yechangping system evolved from an early, relatively oxidized magmatic–hydrothermal regime to a later, dynamically fluctuating hydrothermal system characterized by increasing sulfur activity and progressive fluid–rock interaction [51].

The progressive increase in Mn and Mg contents from Mt1 to Mt4 further suggests enhanced interaction with dolomitic wall rocks. Carbonate buffering during fluid–rock interaction can modify both fluid composition and redox conditions. The negative correlation between Al and Fe (Figure 8c), combined with increasing (Mg + Mn) signatures (Figure 8d), indicates that the intensity of fluid–rock interaction increased during mineralization, contributing to evolving physicochemical conditions [3,52].

Therefore, rather than representing a single redox state, the magnetite generations record a continuous redox and thermal evolution, reflecting the transition from high-temperature magmatic–hydrothermal conditions to lower-temperature contact-metasomatic and sulfide-rich hydrothermal environments.

6.3. Implications for Mineralization and Exploration

Previous studies have shown that trace-element compositions of hydrothermal magnetite can vary systematically with ore type, temperature, and redox conditions, providing discriminant power for exploration targeting in porphyry and skarn systems [44,48]. Magnetite at the Yechangping deposit records a systematic geochemical evolution that can be directly applied to stage discrimination and exploration targeting in porphyry–skarn Mo–W systems [53].

The four generations of magnetite recognized in this study exhibit systematic and reproducible geochemical variations that provide a robust basis for stage discrimination within the Yechangping magmatic–hydrothermal system. Early high-temperature magnetite (Mt1–Mt2) is characterized by relatively elevated Ti and V contents and enrichment in lithophile elements such as Al and Si, consistent with crystallization under high-temperature, magmatic-dominated conditions. Its close association with prograde skarn minerals, including garnet and diopside, further supports formation during the early skarn stage when melt-derived components exerted strong control.

In contrast, magnetite from the main ore stage (Mt3) records a shift toward hydrothermal-dominated conditions. Lower Ti contents reflect decreasing temperature, whereas elevated Mn and Mg suggest intensified fluid–rock interaction, particularly with carbonate wall rocks [54]. The spatial association of Mt3 with molybdenite and pyrite, together with increasing incorporation of Mo and W, directly links this generation to peak mineralization. The more homogeneous distribution of V indicates relatively stabilized physicochemical conditions during the principal ore-forming episode.

Late-stage magnetite (Mt4) is distinguished by enrichment in chalcophile elements such as As, Pb, Bi, and Te and typically occurs in quartz–carbonate–sulfide veins. These features reflect enhanced sulfur activity and redox fluctuations in structurally controlled hydrothermal conduits, representing a late overprinting event rather than primary skarn formation [39]. Collectively, these geochemical fingerprints provide a practical framework for distinguishing early magmatic–hydrothermal magnetite from ore-stage and late magnetite in exploration samples.

The progressive decrease in Ti and V, coupled with increasing Mn and combined (Mg + Mn) signatures, documents the transition from high-temperature magmatic influence to evolving hydrothermal conditions dominated by fluid–rock interaction. This systematic trend has direct vectoring significance. Magnetite with high Ti–V signatures indicates proximity to intrusive centers and early skarn development, whereas lower Ti together with elevated Mn and Mo–W contents signals evolution toward the main mineralization stage [34]. Enrichment in sulfide-affinity trace elements marks late-stage fluid pathways and structural conduits. Accordingly, spatial variations in magnetite chemistry across drill cores or alteration halos can help delineate zones of enhanced ore-forming fluid flux and potential high-grade mineralization.

From an operational perspective, magnetite is an advantageous indicator mineral in porphyry–skarn systems. It is ubiquitous in skarn assemblages and can be readily separated from drill-core or surface samples. Major- and trace-element compositions can be efficiently determined using EPMA and LA-ICP-MS, and stage-specific geochemical signatures are commonly preserved even where sulfide mineralization is weak or partially overprinted. In early exploration, magnetite chemistry can aid in distinguishing barren from mineralized skarn. In more advanced stages, systematic compositional profiling along drill sections can constrain hydrothermal evolution trends and assist in prioritizing target zones. Beyond its genetic implications, the trace-element geochemistry of magnetite at Yechangping demonstrates its value as a practical and reliable tool for stage discrimination and vectoring in porphyry–skarn Mo–W systems.

7. Conclusions

(1)

Magnetite from the Yechangping Mo–W deposit in Henan Province exhibits a complex, multi-stage genesis involving multiple material sources. Magnetite formation spans from the late skarn stage to the polymetallic sulfide stage, during which the ore-forming fluids evolved from a magmatic–hydrothermal system to a contact metasomatic hydrothermal system. The primary material sources include silica-rich intrusions and dolomitic rocks. Isomorphic substitution within the magnetite lattice, together with variations in temperature, pressure, and oxygen fugacity, collectively controlled the compositional characteristics of magnetite.

(2)

Magnetite records a systematic evolution of mineralization conditions. Mineralization temperatures decreased progressively with declining Ti contents, whereas oxygen fugacity was relatively high during the early stages and became more variable at later stages. The ore-forming fluids evolved from Si-, Al-, Na-, and K-rich magmatic–hydrothermal fluids to Mg- and Mn-rich contact metasomatic hydrothermal fluids, consistent with a porphyry–skarn system characterized by multi-stage hydrothermal overprinting.

(3)

Magnetite provides clear indications for mineralization. Its close association with polymetallic ore assemblages and genetic discrimination results confirms that the Yechangping deposit represents a porphyry–skarn-type polymetallic system. Systematic variations in magnetite composition, characterized by decreasing Ti, Al, and V contents and increasing Fe, Mo, and W contents, effectively mark the main mineralization stage, making magnetite an effective indicator mineral for exploration.