U–Pb Dating and Trace Element Composition of Zircons from the Gujiao Ore-Bearing Intrusion, Shanxi, China: Implications for Timing and Mineralization of the Guojialiang Iron Skarn Deposit

: The Gujiao ore ﬁeld, located in the middle segment of the Lüliang Mountain in central North China Craton (NCC), is one of iron skarn deposits of western iron belt in China. The U–Pb dating results of zircon by LA-ICP-MS suggest that the ore-related monzonite from the Guojialiang deposit was formed at 129.7 ± 1.7 Ma, early Cretaceous, which is consistent with the timing of iron skarn deposits in the Handan–Xingtai district of western iron belt. The zircons of monzonite present notable positive Ce anomalies (Ce / Ce* = 23.38–45.85), high Ce 4 + / Ce 3 + values (154–385) and relatively high oxygen fugacity ( f O 2 = − 13.09 to − 15.36), and yield relatively low Ti-in-zircon temperatures. The physico-chemical conditions of the Guojialiang deposit were quite similar to these of ore-bearing plutons in the Handan-Xingtai district. The ore-bearing magmas are derived from the enriched lithospheric mantle with crustal material contribution, which played key role in oxidation state of the magma and the iron mineralization in the western iron belt.


Introduction
The North China Craton (NCC) has experienced reactivation or decratonization during the Late Mesozoic [1][2][3], accompanied by large-scale magmatism and endogenic mineralization [4][5][6][7][8][9]. The skarn-type iron deposit is one such mineralization type that has provided a crucial source of high-grade iron ores, especially in China [10]. Despite a majority of skarn-type iron deposits being associated with arc setting [11], the Mesozoic skarn Fe deposits in the NCC were generated within an intraplate extensional setting [8]. The iron skarn deposits are mainly distributed in the central-eastern NCC and roughly form two belts, eastern iron skarn deposit belt and western iron skarn deposit belt (Figure 1a) [12]. The western iron skarn deposit belt consists of the Southern Taihang district ore deposit cluster, Gujiao (or Taiyuan) ore field, and Linfen ore field (Figure 1a). All these iron deposits in the western belt have similar ore-related intrusive rocks, monzonites, and/or diorites, and similar country rocks: Middle Ordovician marine carbonate rocks. The age, geological features, and metallogenesis of iron skarn deposits in the Southern Taihang district have been well studied [7,8,13]. In contrast, there are few researches about the iron skarn deposits in the Gujiao ore field. These studies reported zircon U-Pb age and geochemistry of the Huyanshan complex in the Gujiao ore field [14,15]. However, the geology and mineralization of iron skarn deposits have not been discussed in the previous papers. Furthermore, the ore-bearing monzonite implication to iron skarn mineralization is still not clear in the Gujiao ore field. Mantle, continental crust materials, and oxidation state of magma play key role in formation of magmatic hydrothermal deposits such as porphyry copper, iron skarn, and porphyry molybdenum deposits, see, e.g., in [8,[16][17][18][19][20][21][22][23][24][25][26][27]. Study of physico-chemical conditions of ore-bearing rocks is crucial to understand mineralization and to make metallogenic prognosis. The accessory mineral zircon is not only a conventional geochronological host for uranium and thorium; it is also widely used in revealing physico-chemical conditions of host rocks [18,22,25,[28][29][30]. For example, the Ti concentration of zircon correlates with crystallization temperature [31], and Ce anomalies and Ce/Nd ratio reflect redox state of magma [29]. Sun et al. performed a contrastive study of zircons from orebearing and ore-barren plutons in the Handan-Xingtai district [8]. They concluded that the orebearing plutons had higher oxygen fugacity and lower temperature than ore-barren plutons. However, oxygen fugacity and temperature of the Gujiao ore-bearing monzonite are not studied yet. In this paper, the geological characteristics of the Guojialiang iron skarn deposit in the Gujiao ore field, and in situ zircon U-Pb dating results, and trace element composition of zircons from orebearing monzonites are presented. Geochemistry of ore-bearing rocks as well as ore mineralization age and the petrogenesis-metallogenic significance of intrusive rocks in the Gujiao ore field are discussed.

Ore Field Geology
The Gujiao iron skarn ore field, as one of the Mesozoic intraplate iron skarn fields in the western iron skarn deposit belt of NCC, is located in the middle segment of the Lüliang Mountain in central NCC (Figure 1a). The lithologies exposed in the Gujiao region are Precambrian metamorphic rocks, Cambrian sedimentary rocks, Ordovician carbonate rocks, Carboniferous-Permian coal-bearing sedimentary rocks, and Quaternary unconsolidated sediments (Figure 1b). The magmatic rocks are dominated by the Yanshanian period intrusive complex, Huyanshan complex, comprising monzonite, aegirine monzonite porphyry, and syenite. The complex outcropped ~50 km 2 and mainly intruded into the Ordovician carbonate sedimentary rocks (Figure 1b). The igneous rocks of the Huyanshan complex have intermediate SiO2 contents, with high Al2O3, total alkali, Sr, and light rare earth elements (LREE) contents and low Y and heavy rare earth elements (HREE) contents [14]. These geochemical features of the Huyanshan complex are similar to those in the igneous complexes in the Mantle, continental crust materials, and oxidation state of magma play key role in formation of magmatic hydrothermal deposits such as porphyry copper, iron skarn, and porphyry molybdenum deposits, see, e.g., in [8,[16][17][18][19][20][21][22][23][24][25][26][27]. Study of physico-chemical conditions of ore-bearing rocks is crucial to understand mineralization and to make metallogenic prognosis. The accessory mineral zircon is not only a conventional geochronological host for uranium and thorium; it is also widely used in revealing physico-chemical conditions of host rocks [18,22,25,[28][29][30]. For example, the Ti concentration of zircon correlates with crystallization temperature [31], and Ce anomalies and Ce/Nd ratio reflect redox state of magma [29]. Sun et al. performed a contrastive study of zircons from ore-bearing and ore-barren plutons in the Handan-Xingtai district [8]. They concluded that the ore-bearing plutons had higher oxygen fugacity and lower temperature than ore-barren plutons. However, oxygen fugacity and temperature of the Gujiao ore-bearing monzonite are not studied yet. In this paper, the geological characteristics of the Guojialiang iron skarn deposit in the Gujiao ore field, and in situ zircon U-Pb dating results, and trace element composition of zircons from ore-bearing monzonites are presented. Geochemistry of ore-bearing rocks as well as ore mineralization age and the petrogenesis-metallogenic significance of intrusive rocks in the Gujiao ore field are discussed.

Ore Field Geology
The Gujiao iron skarn ore field, as one of the Mesozoic intraplate iron skarn fields in the western iron skarn deposit belt of NCC, is located in the middle segment of the Lüliang Mountain in central NCC (Figure 1a). The lithologies exposed in the Gujiao region are Precambrian metamorphic rocks, Cambrian sedimentary rocks, Ordovician carbonate rocks, Carboniferous-Permian coal-bearing sedimentary rocks, and Quaternary unconsolidated sediments (Figure 1b). The magmatic rocks are dominated by the Yanshanian period intrusive complex, Huyanshan complex, comprising monzonite, aegirine monzonite porphyry, and syenite. The complex outcropped~50 km 2 and mainly intruded into the Ordovician carbonate sedimentary rocks (Figure 1b). The igneous rocks of the Huyanshan complex have intermediate SiO 2 contents, with high Al 2 O 3 , total alkali, Sr, and light rare earth elements (LREE) contents and low Y and heavy rare earth elements (HREE) contents [14]. These geochemical features of the Huyanshan complex are similar to those in the igneous complexes in the Handan-Xingtai district [32][33][34]. Skarns are distributed in the contact zone between monzonite and carbonate rocks. The iron skarn deposits are only associated with the monzonite in the Gujiao ore field. The zircon U-Pb dating of the Gujiao monzonites yielded 129.9 ± 2.3 Ma and 130 ± 3 Ma [14,15], which are consistent with the emplacement ages (136 to 129 Ma) of complexes in the Handan-Xingtai district [7,8,33,35,36].

Iron Deposit Geology
The Guojialiang deposit is located in the eastern Gujiao ore field. The exposed rocks are middle Ordovician carbonate rocks and Carboniferous-Permian paralic deposits. The middle Ordovician limestones were intruded by monzonites, which resulted in skarnization in the contact zone between limestone and monzonite, and marbleization of limestone near the intrusion rocks ( Figure 2). The monzonite can be divided into two lithofacies by rock texture; one with equigranular texture as intrusive stock, and the other with porphyritic texture as apophysis. The transition of the textures between equigranular and porphyritic monzonite is gradual. The ore bodies mainly controlled by fold structure generally occur as lenticules in the contact zones between the limestone and monzonite apophysis as well as bedded veins in the limestone interlayers. In addition, some ore bodies occur in contact zones between limestone xenoliths and monzonite stock. Handan-Xingtai district [32][33][34]. Skarns are distributed in the contact zone between monzonite and carbonate rocks. The iron skarn deposits are only associated with the monzonite in the Gujiao ore field. The zircon U-Pb dating of the Gujiao monzonites yielded 129.9 ± 2.3 Ma and 130 ± 3 Ma [14,15], which are consistent with the emplacement ages (136 to 129 Ma) of complexes in the Handan-Xingtai district [7,8,33,35,36].

Iron Deposit Geology
The Guojialiang deposit is located in the eastern Gujiao ore field. The exposed rocks are middle Ordovician carbonate rocks and Carboniferous-Permian paralic deposits. The middle Ordovician limestones were intruded by monzonites, which resulted in skarnization in the contact zone between limestone and monzonite, and marbleization of limestone near the intrusion rocks ( Figure 2). The monzonite can be divided into two lithofacies by rock texture; one with equigranular texture as intrusive stock, and the other with porphyritic texture as apophysis. The transition of the textures between equigranular and porphyritic monzonite is gradual. The ore bodies mainly controlled by fold structure generally occur as lenticules in the contact zones between the limestone and monzonite apophysis as well as bedded veins in the limestone interlayers. In addition, some ore bodies occur in contact zones between limestone xenoliths and monzonite stock. Hydrothermal alteration is well developed in the contact zones in both pluton and the surrounding limestone. The alteration zone can be divided into altered monzonite zone, endoskarn zone, exoskarn zone, and diopside-marble zone. The altered monzonite zone commonly occurs as lenticule and the intensity of alteration increases gradually from pluton to the country rocks. In the altered monzonite zone, feldspars are replaced by sericite, clay, and epidote; amphibole is replaced by albite and garnet; and sphene and garnet occur as hydrothermal minerals (Figure 3a). The endoskarn zone is comprised of garnet, diopside, and minor phlogopite and calcite. The exoskarn zone covers larger area than the endoskarn zone and mainly consists of diopside, phlogopite, and minor calcite and quartz. The diopside-marble zone is characterized by marbleization of limestone with local diopside near the exoskarn zone. Diopside skarn is dominated. Euhedral crystals of garnet and diopside are locally found in open spaces (Figure 3b,c). Iron ore bodies commonly occur as irregular masses in the endoskarn zone and as bedded veins in the marbleized limestone layers. Hydrothermal alteration is well developed in the contact zones in both pluton and the surrounding limestone. The alteration zone can be divided into altered monzonite zone, endoskarn zone, exoskarn zone, and diopside-marble zone. The altered monzonite zone commonly occurs as lenticule and the intensity of alteration increases gradually from pluton to the country rocks. In the altered monzonite zone, feldspars are replaced by sericite, clay, and epidote; amphibole is replaced by albite and garnet; and sphene and garnet occur as hydrothermal minerals (Figure 3a). The endoskarn zone is comprised of garnet, diopside, and minor phlogopite and calcite. The exoskarn zone covers larger area than the endoskarn zone and mainly consists of diopside, phlogopite, and minor calcite and quartz. The diopside-marble zone is characterized by marbleization of limestone with local diopside near the exoskarn zone. Based on the petrographic observation, the metasomatism can be divided into four stages of skarn formation: prograde stage, retrograde stage, sulfide stage, and pneumatolytic stage. The prograde stage is dominated by garnet and diopside. The retrograde stage is characterized by phlogopite, actinolite and magnetite that replace garnet and diopside. Phlogopite veins in the diopside skarn are common (Figure 3f). A large volume of magnetite formed during the retrograde stage. The sulfide stage is weakly developed and formed calcite, quartz and pyrite, overprinting diopside and filling in pores of diopside (Figure 3g). The pneumatolytic stage is dominated by calcite, specularite, and talc. The diopside and actinolite is replaced by talc (Figure 3h). Zircon grains were separated from the monzonite sample GS01 through a series of procedures, including crushing in a jaw crusher; desliming in water; density separation in dense liquid, magnetic Based on the petrographic observation, the metasomatism can be divided into four stages of skarn formation: prograde stage, retrograde stage, sulfide stage, and pneumatolytic stage. The prograde stage is dominated by garnet and diopside. The retrograde stage is characterized by phlogopite, actinolite and magnetite that replace garnet and diopside. Phlogopite veins in the diopside skarn are common (Figure 3f). A large volume of magnetite formed during the retrograde stage. The sulfide stage is weakly developed and formed calcite, quartz and pyrite, overprinting diopside and filling in pores of diopside (Figure 3g). The pneumatolytic stage is dominated by calcite, specularite, and talc. The diopside and actinolite is replaced by talc (Figure 3h). Zircon grains were separated from the monzonite sample GS01 through a series of procedures, including crushing in a jaw crusher; desliming in water; density separation in dense liquid, magnetic separation; and, finally, handpicking under binocular microscope (United Scope LLC, Irvine, CA, USA). Zircon grains were then mounted in epoxy resin and polished. Cathodoluminescent images and transmitted /reflected microscopy were used to avoid fractures and inclusions and to select ablation locations in the zircon grains.

Sample and Analytical Methods
Zircon U-Pb dating was performed using a ThermoX2 inductively coupled plasma-mass spectrometry (ICP-MS) equipped with a GeoLas Pro 193 nm ablation system at the Testing Center of Shandong Bureau of China Metallurgical Geology Bureau, Jinan, China. The output laser energy density was fixed at 10 J/cm 2 and the ablation of laser was conducted at 6 Hz for 55 s after 25 s background acquisition. A spot diameter of 30 µm was conducted for all analyses. Helium was used as the carrier gas. The zircon 91500 was used as the external standard for U-Pb isotopic element ratio dating. The NIST SRM610 glass was used as external standard and 29 Si as the internal standard to calibrate trace element concentrations. Offline data processing was conducted using the ICPMSDataCal software (version 10.8, China University of Geosciences, Wuhan, China) [37]. Uncertainties of individual analyses are reported with 2σ errors. The plotting of concordia diagrams and weighted mean age calculations were performed using Isoplot/Ex_ver3 (version 3.0, Berkeley Geochronology Center, California, CA, USA) [38]. 206 Pb/ 238 U age is accepted for zircons younger than 1000 Ma, whereas 206 Pb/ 207 Pb age is reliable for zircons older than 1000 Ma.
Zircon Ti temperature calculation following the method described by Watson et al. [39] and used by Sun et al. [8] for the complexes in the Handan-Xingtai district. Ce 4+ /Ce 3+ and ƒO 2 were calculated herein to imply oxygen state of monzonite. Zircon Ce 4+ /Ce 3+ were estimated using the method described by Ballard et al. [22]. In lattice strain model, the La and Pr are often problematic due to their low concentrations in zircon; Eu and U are also not used for their multivalence issues [28,40]. The trace element contents of melt are replaced by whole rocks from Ying et al. [14]. The ƒO 2 were calculated using the calibration method from Smythe and Brenan [28]. The detail method is described in the Supplementary Document File S1. For comparison to iron deposits in the Handan-Xingtai district, the water content in ore-bearing magma of monzonite of Huyanshan complex is considered identical to that from ore-bearing magma in the Handan-Xingtai district (8.0 wt. %) [8].

Zircon U-Pb Age
Zircon grains from monzonite (sample GS01) are commonly 100-200 µm in length and are light pink to colorless, euhedral, and prismatic. The Th and U contents in most zircons are high (from 1343 to 3937 ppm and from 1510 to 3090 ppm, respectively) except in the old inherited zircon ( Table 1). The Th/U ratios vary from 0.51 to 1.55 indicating a magmatic source. Eleven analyses with good concordance (>90%) have 206 Pb/ 238 U ages ranging from 126.2 to 135.2 Ma, with a weighted mean age of 129.7 ± 1.7 Ma (MSWD = 1.18, n = 11; Figure 4). These ages are in agreement with those from Ying et al. [14] and Huo et al. [15]. The inherited zircon analyses show 207 Pb/ 206 Pb ages ranging from 1773 to 2186 Ma, which is consistent with ages of the tectonic-magmatic event in the Lüliang district and event in the central NCC [41][42][43]. The Th/U ratios vary from 0.51 to 1.55 indicating a magmatic source. Eleven analyses with good concordance (>90%) have 206 Pb/ 238 U ages ranging from 126.2 to 135.2 Ma, with a weighted mean age of 129.7 ± 1.7 Ma (MSWD = 1.18, n = 11; Figure 4). These ages are in agreement with those from Ying et al. [14] and Huo et al. [15]. The inherited zircon analyses show 207 Pb/ 206 Pb ages ranging from 1773 to 2186 Ma, which is consistent with ages of the tectonic-magmatic event in the Lüliang district and event in the central NCC [41][42][43].

Zircon Trace Elements
The studied zircons have high total REE concentration from 3614 ppm to 4782 ppm (Table 2), and they are enriched in HREE and depleted in LREE with outstanding positive Ce and weak negative Eu anomalies. The chondrite-normalized REE patterns show a left-leaning with Ce peak (Figure 5), implying a magmatic source [44]. It has been shown that high Y (>4433 ppm) and high total REE (>~3800 ppm) content are typical for zircons originated from monzonite [45]. The La concentrations of the zircons are mostly lower than 3 ppm, whereas the Lu concentrations are mostly higher than 370 ppm. The calculated Eu/Eu* [Eu/Eu* = Eu N /(Sm N × Gd N ) 1/2 ] and Ce/Ce* [Ce/Ce* = Ce N /(La N × Pr N ) 1/2 ] ratios from the eleven zircon analyses range from 0.60 to 0.72 and 23.38 to 45.85, respectively ( Table 2). ( Figure 5), implying a magmatic source [44]. It has been shown that high Y (>4433 ppm) and high total REE (>~3800 ppm) content are typical for zircons originated from monzonite [45]. The La concentrations of the zircons are mostly lower than 3 ppm, whereas the Lu concentrations are mostly higher than 370 ppm. The calculated Eu/Eu* [Eu/Eu* = EuN/(SmN × GdN) 1/2 ] and Ce/Ce* [Ce/Ce* = CeN/(LaN × PrN) 1/2 ] ratios from the eleven zircon analyses range from 0.60 to 0.72 and 23.38 to 45.85, respectively ( Table 2).

Zircon Thermometer and Oxygen Meter
Zircon is widely used as thermometer and oxygen fugacity gauge in recent years [8,18,22,28,29,39,46,47]. Ti-in-zircon thermometer is a useful method to constrain magma temperature condition [31,[48][49][50]. The Ti-in-zircon temperatures for eleven spots of zircon that has good concordance age from GS01 are in the range of 641 to 718 • C. The zircon oxygen-meter reflects the oxidation state of the source magma, where zircon crystallization is affected by several parameters such as Ce/Nd, Ce 4+ /Ce 3+ , and oxygen fugacity (f O 2 ) [8,21,28,29,47]. The calculated results of zircon Ce 4+ /Ce 3+ ratios for sample GS01 varies from 154 to 385. The f O 2 ranges from −13.09 to −15.36 with an average f O 2 of ∆FMQ +3.6 ( Table 2). 40 Ar/ 39 Ar dating of phlogopite associated with skarn and U-Pb dating of hydrothermal allanite with ages of 137 ± 2 Ma and 136 ± 1 Ma, respectively, are coeval with zircon U-Pb ages of 136 ± 2 Ma from ore-bearing intrusions in the Handan-Xingtai district [13,51]. In addition, Deng et al. reported U-Pb ages of hydrothermal zircons from five major iron skarn deposits ranging from 133.6 to 128. 5 Ma, which are consistent with U-Pb ages (134.1 to 128.5) of zircons from ore-bearing intrusions in each deposit in the Handan-Xingtai district [7]. These dates provide a robust correlation between the timing of the magmatism and mineralization, showing that the ages of ore deposits were identical to the ore-bearing intrusions. Our new age of 129.7 ± 1.7 Ma from zircon U-Pb dating is consistent with the zircon U-Pb age of 129.9 ± 2.3 Ma conducted by Ying et al. [14]. This shows that the Guojialiang iron skarn deposit formed around 129.7 Ma. The geochronology data suggests that the iron skarn deposits in western ore belt formed in the Early Cretaceous.

Geochronology of the Ore Deposits
The inherited zircons may provide useful information on the genesis of magmas [52,53]. Inherited zircons,~1800-2500 Ma in age, are commonly found in the ore-bearing intrusions from the Taiyuan ore field, Linfen ore field, and Handan-Xingtai district [8,[13][14][15]35,51]. Not incidentally,~1800-2500 Ma is a critical period of magmatic event and crustal growth in NCC [42,43,54]. The abundant zircons with Paleoproterozoic age in the ore-related plutons suggest that the ancient lower crustal materials played a key role in the magma genesis. This inference is further supported by Sr-Nd isotope data ( Figure 6).

The Condition of Magma
Trace element concentration in igneous zircons is shown to be sensitive to source rock type and crystallization environment [63]. Ti-in-zircon temperature may provide important evidence for the magmatic processes of crystallization. The monzonite in the Guojialiang ore deposit shows low Ti- Figure 6. Sr-Nd isotope diagram for the syenitic-monzonitic intrusions from the western ore belt. The early Cretaceous enriched lithospheric mantle is constrained by uncontaminated basalt and Ordovician kimberlite in the eastern NCC [55][56][57][58][59]. The lower crust of NCC is from work in [60]. The early Cretaceous gabbros and lamprophyres in Southern Taihang area are given for comparison [9,61,62]. The monzonite and syenite from Gujiao after the work in [14] and Handan-Xingtai after the work in [34].

The Condition of Magma
Trace element concentration in igneous zircons is shown to be sensitive to source rock type and crystallization environment [63]. Ti-in-zircon temperature may provide important evidence for the magmatic processes of crystallization. The monzonite in the Guojialiang ore deposit shows low Ti-in-zircon temperatures of 641-718 • C, which is slightly lower than the crystallization temperature of complexes in the Handan-Xingtai district. In addition, crystallization temperature of ore-bearing rocks was significantly lower than that of ore-barren plutons (Figure 7a).
The zircons from monzonite in the Guojialiang ore deposit have high Ce/Ce*, Ce 4+ /Ce 3+ , and logf O 2 values, which is similar to these from the ore-bearing rocks of the Handan-Xingtai district (Figure 7b,c) and also consistent with relatively high average oxygen fugacity of ∆FMQ+2.78. In contrast, the oxygen fugacity of ore-barren syenite ranges from ∆FMQ-3.34 to ∆FMQ-1.51 which is much lower than ore-bearing rocks (Figure 7d). Therefore, the ore-bearing magma had lower temperature and higher oxidation state compared to ore-barren magma. It is likely that the ore-bearing magmas were derived from an enriched lithospheric mantle with more crustal material contribution during ascend than that of the ore-barren magmas [8,32,34]. (Figure 7b,c) and also consistent with relatively high average oxygen fugacity of ΔFMQ+2.78. In contrast, the oxygen fugacity of ore-barren syenite ranges from ΔFMQ-3.34 to ΔFMQ-1.51 which is much lower than ore-bearing rocks (Figure 7d). Therefore, the ore-bearing magma had lower temperature and higher oxidation state compared to ore-barren magma. It is likely that the orebearing magmas were derived from an enriched lithospheric mantle with more crustal material contribution during ascend than that of the ore-barren magmas [8,32,34].

Implication for Mineralization
Zircon, as a common accessory mineral in natural rocks, is a robust indicator for many geological processes, such as crustal assimilation [64], crustal recycling [65], and metal concentrations [11]. Previous studies indicate that involvement of mantle materials can elevate iron contents of orerelated magma for formation of Fe skarn deposits [11]. Oxidation state of magmas could also be originally inherited from magma source and could be changed by magma processes such as crystallization, assimilation and mixing [66][67][68]. The inherited zircons and Sr-Nd isotope data suggest that the ore-bearing monzonite plutons likely formed by magma mixing from metasomatized mantle and crust, whereas ore-barren syenite plutons are derived from the metasomatized mantle with insignificant crustal contamination in the western Fe skarn ore belt ( Figure 6) [8,14,34]. The zircon trace elements show that the oxygen fugacity of ore-bearing monzonite was much higher than ore-barren syenite (Figure 7). Thus, the high oxygen fugacity of monzonite magma can be probably attributed to ancient crustal materials [8]. Fe 3+ is more incompatible than Fe 2+ for most early

Implication for Mineralization
Zircon, as a common accessory mineral in natural rocks, is a robust indicator for many geological processes, such as crustal assimilation [64], crustal recycling [65], and metal concentrations [11]. Previous studies indicate that involvement of mantle materials can elevate iron contents of ore-related magma for formation of Fe skarn deposits [11]. Oxidation state of magmas could also be originally inherited from magma source and could be changed by magma processes such as crystallization, assimilation and mixing [66][67][68]. The inherited zircons and Sr-Nd isotope data suggest that the ore-bearing monzonite plutons likely formed by magma mixing from metasomatized mantle and crust, whereas ore-barren syenite plutons are derived from the metasomatized mantle with insignificant crustal contamination in the western Fe skarn ore belt ( Figure 6) [8,14,34]. The zircon trace elements show that the oxygen fugacity of ore-bearing monzonite was much higher than ore-barren syenite (Figure 7). Thus, the high oxygen fugacity of monzonite magma can be probably attributed to ancient crustal materials [8]. Fe 3+ is more incompatible than Fe 2+ for most early crystalized minerals in magma (e.g., olivine and pyroxene). Under high f O 2 condition, such as ∆FMQ+1.2 to ∆FMQ+3, the Fe 3+ /Fe 2+ ratio is high and sulfur in the magma occurs as sulfate, such as SO 3 2− and SO 4 2− rather than S 2− [69]. Such conditions are needed to prevent Fe loss; otherwise, the Fe 2+ would combine with S 2− to form insoluble FeS. Metasomatic reactions in igneous-related hydrothermal fluids, such as diffusion and infiltration, are commonly major mechanisms for skarn formation [70,71]. Dissolution-assimilation is proposed as a mechanism for the formation of skarn deposits at Guojialiang. Euhedral diopside and garnet have been found in geodes from the diopside and garnet endoskarn zones. Assimilation of the country limestrone by post-magmatic hydrothermal fluid effectively promoted skarn formation. The Fe-enriched hydrothermal fluid caused metasomatic alteration and deposited ore along contact zones when the temperature decreased.