Journal of Earth Science  2018, Vol. 29 Issue (2): 295-306   PDF    
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In situ Analysis of Major Elements, Trace Elements and Sr Isotopic Compositions of Apatite from the Granite in the Chengchao Skarn-Type Fe Deposit, Edong Ore District: Implications for Petrogenesis and Mineralization
Zhenghan Li1,2, Dengfei Duan2,3, Shaoyong Jiang2,3,4, Ying Ma2,3, Hongwei Yuan5    
1. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;
2. Collaborative Innovation Center for Exploration of Strategic Mineral Resources, China University of Geosciences, Wuhan 430074, China;
3. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;
4. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;
5. The First Geological Brigade of Hubei Geological Bureau, Huangshi 435100, China
Abstract: Major elements, trace elements and Sr isotopic compositions of apatite from the granite in the Chengchao skarn-type Fe deposit of Edong ore district of Middle–Lower Yangtze River metallogenic belt were measured using EMPA (electron microprobe), LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) and LA-MC (multicollector)-ICP-MS methods in order to reveal the petrogenetic and metallogenic significance of the skarn-type iron deposits. The results show that the apatite in Chengchao granite is fluorapatite, which displays slight variation in major elements. The REE distribution pattern of the apatite is similar to that of the whole rocks, with strong negative Eu anomaly and low Sr/Y ratio. The concentration of Mn in apatite is low (140 ppm–591 ppm) and the Sr isotopic composition shows a limited variation from 0.706 9 to 0.708 2. The high oxygen fugacity of the Chengchao granite, implied by the low Mn content in apatite, is possibly attributed to contamination of the gypsum from sedimentary rock strata, which has long been thought to be an important factor that controls the Fe mineralization in the Middle–Lower Yangtze River metallogenic belt. This study also proves that the Eu/Eu* value and Sr/Y ratio in apatite can be effectively used to identify the adakitic affinity. The in situ Sr isotope analysis of apatite is in consistent with the bulk rock analysis, which indicates that the apatite Sr isotope can represent the initial Sr isotopic compositions of the magma. The Sr isotope and negative Eu anomaly in apatite imply that the Chengchao granite is likely sourced from crust-mantle mixed materials.
Keywords: apatite    in situ analysis    Sr isotopes    trace elements    adakite identification    oxygen fugacity    
0 INTRODUCTION

Apatite is a common accessory mineral in a variety of rock types including mafic and granitic rocks, carbonatites, and metamorphic rocks (Chen and Simonetti, 2014; Chen et al., 2013; Roeder et al., 1987; Watson and Green, 1981). Apatite can accommodate a wide variety of trace elements since it has special crystal structure (Hughes and Rakovan, 2015). Therefore, it can serve as an important tool to study the geochemistry and petrogenesis of the parental magma (Webster and Piccoli, 2015). The experiments performed at different temperatures, pressures and melt compositions demonstrate that the solubility of apatite in magma decreases as the temperature decreases and the polymerization of the melt increases (London, 1999; Pichavant et al., 1992; Harrison and Watson, 1984; Watson, 1980, 1979). Thus, apatite may generally occur as an early crystallized mineral facies in most magmatic rocks except peraluminous rocks (Creaser and Gray, 1992; Harrison and Watson, 1984) and can therefore represent the early geochemical information of the parent magma when it crystallized.

Apatite is often used as a useful probe to trace the contents and evolution of F, Cl and H2O in melts or fluids from which it crystallized (Schisa et al., 2015; Boyce and Hervig, 2008; Boudreau and Kruger, 1990; Boudreau and McCallum, 1989). Moreover, some specific elements such as Mn, Sr, LREE, Th, Y, Eu and Ce in apatite can also be used to trace the melt compositions as well as the redox state of the parental magma (Miles et al., 2014; Piccoli and Candela, 2002; Sha and Chappell, 1999). The Sr isotopic composition of apatite can represent the initial Sr isotopic composition of the magma due to the high Sr but low Rb concentration in apatite, which makes it powerful to trace the origin of the magma (Tsuboi, 2005). In addition, apatite can be also used for U-Th-Pb dating (Chen and Simonetti, 2014; Chen et al., 2013).

There are a large number of skarn and iron oxide-apatite deposits in the Middle–Lower Yangtze River metallogenic belt. Previous study mainly focused on the geochemical characteristics of the apatite in the iron oxide-apatite deposits, but little in the skarn-type deposits. Since these two deposit types have same ore-forming geodynamic setting, it is important to study the affinity of magma related to the skarn type mineralization and the possible difference between these two types. In this study, the in situ major elements, trace elements and Sr isotopes in apatite from the granite in the Chengchao skarn-type Fe deposit were studied. The results show that the trace elements of apatite can be used to effectively determine whether the magmatic rock has an adakitic affinity. The in situ Sr isotope analysis in apatite has high reliability and can be used to track the sources of the magmatic rocks.

1 GEOLOGICAL SETTING AND DEPOSIT GEOLOGY

The Middle–Lower Yangtze River metallogenic belt is located on the northern margin of the Yangtze Craton and the southern margin of North China Craton and Dabie orogenic belt (Chang et al., 1991) (Fig. 1a). This metallogenic belt is bordered by a series of large-scale faults and strike-slip fault systems, namely, the Xiangfan-Guangji fault to the northwest, the Tanlu fault to the northeast and the Yangxin-Changzhou fault to the south. This area has experienced three stages of tectonic evolution, including basement formation stage in Pre-Sinian, sedimentary cover rock formation stage from Sinian to Early Triassic and collision orogeny formation and post-orogenic deformation stage after Middle Triassic (Chang et al., 1991). This metallogenic belt can be divided into seven ore districts, including Edong, Jiurui, Anqing-Guichi, Luzong, Tongling, Ningwu and Ningzhen from west to east (Fig. 1a).

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Figure 1. (a) Schematic illustration of the seven magmatic and metallogenic districts from the Middle–Lower Yangtze River metallogenic belt in northeastern Yangtze Craton; (b) the granitoid batholiths in the Edong district of southeastern Hubei Province (modified from Xie et al., 2008). TLF. Tanlu fault; YCF. Yangxin-Changzhou fault.

The Edong ore district (southeastern Hubei Province) is located in the western margin of this metallogenic belt. The Ordovician to Middle Triassic marine sedimentary rocks (which are > 10 km thick), Late Triassic to Cenozoic continental deposits, and Cretaceous volcanic assemblages comprise the stratigraphy of the Edong district (Shu et al., 1992; Chang et al., 1991). Among them, the limestone and dolomitic limestone of Triassic Daye Formation have the closest relation with mineralization (Pan and Dong, 1999). There were intensive magmatic activity and the associated mineralization in this district during Mesozoic. Six large plutons occur from north to south, including Echeng (granite, and minor quartz monzonite and quartz diorite, 85 km2), Tieshan (quartz diorite, diorite, and minor quartz monzonite and gabbro, 140 km2), Jinshandian (quartz monzonite, monzonite granite and minor quartz diorite, 16 km2), Lingxiang (diorite, 54 km2), Yangxin (quartz diorite and minor diorite, 215 km2) and Yinzu (quartz monzonite and minor diorite and granodiorite, 90 km2) (Li et al., 2009). In addition, there are more than 100 small stocks, including granodiorite porphyry, porphyritic diorite and granite porphyry (Liu et al., 2009) (Fig. 1b). SHRIMP and LA-ICP-MS zircon U-Pb dating results show that there are two important magmatic events in the Edong district, namely: (1) gabbro+diorite+quartz diorite+granodiorite porphyry (about 152–134 Ma); (2) granite+ quartz monzonite+quartz diorite and volcanic rocks (about 134– 124 Ma) (Xie et al., 2013). This district is rich in mineral resources and has various types of ore deposits, including Fe, Cu, W, Mo, Zn, Pb, Au, Ag, S, Co, Ni, Ta, Ti, and U (Shu et al., 1992).

The Chengchao Deposit is the largest iron deposit that located in the north of the Edong district, with a proven reserve of 280 Mt Fe at an average grade of 36 wt.%–51 wt.% Fe (up to 61 wt.%). This deposit is related to granite (127±2 Ma) and quartz diorite (129±2 Ma) (Xie et al., 2012) that intruded into the lower members of the Daye Formation that consisted evaporate-bearing dolomitic limestones, limestones, and dolomites (Shu et al., 1992; Zhai et al., 1992). Hundreds of iron and gypsum orebodies comprise the Chengchao Deposit, among which Ⅰ to Ⅶ orebodies collectively accounting for 95% of the total Fe reserve of the deposit. These orebodies mainly distribute in NWW direction with the S or SSW inclination. The detailed deposit geology of Chengchao iron deposit has been described by a number of researchers (Li et al., 2014a; Zhai et al., 1992). The samples of this study were collected from the Chengchao granite related to the Fe-mineralization.

2 PETROGRAPHY

The granite is mainly composed of plagioclase (~15%), K-feldspar (~55%), quartz (~25%) and accessory biotite, titanite, zircon, apatite (total ~5%). Plagioclase is usually tablet and has hypidiomorphic granular texture (Figs. 2a, 2b) with grain size from 0.2 to 2.0 mm. K-feldspar also has hypidiomorphic granular texture, with a grain size of 0.2–2.0 or 2.0–4.0 mm (Fig. 2a). Sericitization in plagioclase and kaolinization in K-feldspar can be observed in some grains (Fig. 2a). Quartz usually has allotriomorphic granular texture with a diameter of 0.2–2.0 mm (Fig. 2a). The apatite occurs enclosed within plagioclase (Fig. 2c) and K-feldspar (Fig. 2d).

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Figure 2. (a) K-feldspar with crossed twinning and plagioclase with polysynthetic twin, quartz mainly interstices between K-feldspar and plagioclase; (b) tabular plagioclase; (c) apatite is enclosed in the K-feldspar; (d) apatite is enclosed in the plagioclase.
3 ANALYTICAL METHODS

The fresh granite samples were selected to separate the apatite crystals that are needed for in situ geochemical analysis. Firstly, the samples were preliminarily processed using magnetic separation and flotation method. Then, the clean and representative apatite grains were selected by hand under microscope to make the sample disks. Finally, the prepared and polished sample disks are used for in situ analysis as described below. The apatite grain usually has a width of ~100 μm with a length to width ratio of 2 to 3, but a small amount of grains can have larger size (> 100–200 μm).

3.1 Electron Microprobe Analysis of Major Elements

Electron microprobe analysis (EMPA) was performed in the Testing Center of China Metallurgical Geological Bureau of Shandong Province using the JEOL JXA-8230 electron microprobe equipped with four wave spectrometers. Before analysis, the sample was coated with a carbon film with a thickness of about 20 nm. The operating conditions were as follows: 15 kV acceleration voltage, 20 nA acceleration current and 5 μm spot beam diameter. The measurement time for Ca and P was 10 s, and the measurement time for Na, Mg, Si, Fe, Mn, Sr, F and Cl was 20 s. The measurement time of the upper and lower backgrounds was half of the measurement time of the peaks. All the analytical data were corrected by ZAF method. The standards used were apatite (Ca, P), jadeite (Na, Si), diopside (Mg), olivine (Fe), rhodonite (Mn), strontium sulfate (Sr), phlogopite (F) and tugtupitu (Cl). Precision for EMPA analysis was calculated from counting statistics, and was generally better than ±1% for contents > 10 wt.%, and better than ±5% for contents > 0.5 wt.%.

3.2 LA-ICP-MS Analysis of Trace Elements

The trace element concentrations of apatite were determined by LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan. Laser sampling was performed using a Resonetics-M50 193 nm UV ArF excimer laser produced by resolution series laser ablation instrument. Samples were analyzed by laser beam spot with a diameter of 33 μm. The denudation frequency was 10 Hz and the denudation duration time was 30 s. High-purity He gas was put into the mass spectrometer as a carrier gas after mixing with Ar gas and a small amount of N2 gas. The inductively coupled plasma source mass spectrometer (ICP-MS) was manufactured by the thermo electric corporation and its model is iCAP Qc.

International standard of NIST SRM 612 was used to correct the signal drift. The international basalt glass standards of BCR-2G, BHVO-2G and BIR-1G were used as external standards and Ca content in EMPA analysis was used as the internal standard to calibrate trace elements. The analytical data were processed off-line by ICPMSDataCal software (Liu et al., 2010a, b) and the software version was 9.9.

3.3 LA-MC-ICP-MS Analysis of Sr Isotopic Compositions

The in situ Sr isotope analysis of apatite was performed in the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan. Laser sampling was performed using a Resonetics-M50 193 nm UV ArF excimer laser produced by resolution series laser ablation instrument. The measurements involved correction of critical spectral interferences that include Kr, Rb and doubly charged rare earth elements (REEs, e.g., Chen and Simonetti, 2014; Chen et al., 2013; Paton et al., 2010; Ramos et al., 2004). A modern-day coral (Qingdao) was used as an external, in-house standard, which has been well characterized for its 87Sr/86Sr value by ID-TIMS. The coral standard and apatite grains were analyzed using a 130 μm spot size, 10 Hz repetition rate, and an energy density ~11 J/cm2. The average 87Sr/86Sr ratio obtained for the coral standard was 0.709 23±0.000 02 based on five measurements, which is within analytical error compared to the corresponding TIMS value of 0.709 25±0.000 02.

4 RESULTS

Little variation is observed in the chemical compositions of Chengchao apatite, and the analytical results are listed in Table 1. The contents of CaO, P2O5, SiO2, Na2O, F and Cl in apatite are 54.18 wt.%–54.77 wt.%, 41.16 wt.%–43.08 wt.%, 0.26 wt.%–0.51 wt.%, 0.11 wt.%–0.31 wt.%, 2.14 wt.%–2.96 wt.% and 0.12 wt.%–0.41 wt.%, respectively. The contents of FeO, MgO and MnO are low, some of which are even lower than the detection limit (Fig. 3, Table 1).

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Figure 3. Variation of CaO with the other major elements in apatite from the Chengchao granite.
Table 1 Major element compositions of apatite in the Chengchao granite

The chemical formula calculation of apatite was based on 8 oxygens, and the calculation of formulas for end-members F-, Cl-, and OH-apatite was based on the method of Piccoli and Candela (2002). The mole fraction of fluorapatite end member is XFApAp=CFAp/3.767 where CFAp is the concentration (wt.%) of F in apatite. The mole fraction of chlorapatite is XCl ApAp=CClAp/6.809 where CClAp is the concentration (wt.%) of Cl in apatite. The mole fraction of hydroxyl apatite is XH ApAp=1-XCl ApAp-XF ApAp. According to the calculation, the apatite in Chengchao granite is mainly fluorapatite (Fig. 4).

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Figure 4. Plots of the halogen contents in apatite from the Chengchao granite. The data of apatites in Ningwu and Luzong are from Zeng et al. (2016) and Yue (1983).

The trace elements of the apatite show a large variation among different grains, and the results are listed in Table 2. Apatite has a high REE and Y concentrations (ΣREE=6 055 ppm– 16 994 ppm, Y=473 ppm–1 746 ppm). The contents of Na, Si, Fe, Sr, Mn and Mg are high, which are 818 ppm–2 387 ppm, 1 941 ppm–6 093 ppm, 973 ppm–1 461 ppm, 277 ppm–547 ppm, 127 ppm–577 ppm and 30 ppm–182 ppm, respectively. In contrast, the contents of Ti, V, Ga, Ge, Th and U are low, which are 47 ppm–51 ppm, 10.2 ppm–37 ppm, 25 ppm–82 ppm, 22 ppm–82 ppm, 32 ppm–104 ppm and 7.8 ppm–33 ppm, respectively. The concentration of Rb is extremely low (0.01 ppm–0.51 ppm) compared with Sr (Table 2). The chondrite normalized rare earth elements patterns show a slightly LREE enrichment than HREE and an obvious negative Eu anomaly (Fig. 5).

Table 2 Trace element compositions of apatite in the Chengchao granite
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Figure 5. (a) Chondrite-normalized rare earth element distribution patterns (chondrite data are from Boynton, 1984). The gray area is the data for apatite from the Luzong volcanic rocks (Tang et al., 2012), the blue area is the data for apatite from the magnetite ores in the Ningwu region (Zeng et al., 2016). The apatite data refers to the left y-axis, whereas the bulk rock data refers to the right y-axis. (b) Plot of Eu/Eu* in apatite versus Eu/Eu* in bulk rock.

Due to the grain size limitation, we only obtained two Sr isotope analyses for two large apatite grains, which show 87Sr/86Sr ratios of 0.706 90±0.000 18 and 0.708 19±0.000 15.

5 DISCUSSION 5.1 Possible Lattice Substitution of Apatite

The ideal chemical formula of apatite is A5(XO4)3Z, where A site is mainly occupied by Ca2+, X site is mainly occupied by P5+ and Z site is occupied by F-, Cl- and OH-. Among all the trace elements, Th4+, U4+, REE3+, Sr2+, Pb2+, Mg2+, Mn2+, Fe2+, Eu2+ and Na+ preferably enter into the A site, whereas X site is preferred for Si4+, S6+ and C4+ (Pan and Fleet, 2002).

Divalent elements, such as Sr2+, Pb2+, Mg2+, Mn2+, Fe2+ and Eu2+ can directly substitute Ca2+ in apatite (Piccoli and Candela, 2002). The concentrations of these elements in apatite are mainly related to their concentrations in melt as well as the temperature, pressure, water content and oxygen fugacity of melt (Prowatke and Klemme, 2006). However, the monovalent elements, such as Na+, K+ and Li+ and trivalent elements, such as REE3+ and Y3+ as well as tetravalent elements (e.g., Th4+ and U4+) enter apatite by complex substitutions (Pan and Fleet, 2002).

For example, REE3+ and Y3+enter into apatite by the following complex substitutions (Pan and Fleet, 2002; Sha and Chappell, 1999; Ronsno, 1989)

$ {\rm{2RE}}{{\rm{E}}^{{\rm{3 + }}}}{\rm{(}}{{\rm{Y}}^{{\rm{3 + }}}}{\rm{) + }}\left[ {\rm{V}} \right]{\rm{ = 3C}}{{\rm{a}}^{{\rm{2 + }}}} $ (1)
$ {\rm{RE}}{{\rm{E}}^{{\rm{3 + }}}}{\rm{(}}{{\rm{Y}}^{{\rm{3 + }}}}{\rm{) + N}}{{\rm{a}}^{\rm{ + }}}{\rm{ = 2C}}{{\rm{a}}^{{\rm{2 + }}}} $ (2)
$ {\rm{RE}}{{\rm{E}}^{{\rm{3 + }}}}{\rm{ + S}}{{\rm{i}}^{{\rm{4 + }}}}{\rm{ = C}}{{\rm{a}}^{{\rm{2 + }}}}{\rm{ + }}{{\rm{P}}^{{\rm{5 + }}}} $ (3)

where [V] represents the vacancy. As shown in Fig. 6, the contents of REE+Y and Si+Na show a significant correlation (R2=0.77), indicating that the substitutions (2) and (3) are two possible mechanisms for REE and Y to enter into apatite. However, the slope of the regression curve in Fig. 6 is only 0.31, indicating that only about 30% REE and Y may enter into apatite through substitutions (2) and (3), while the other 70% REE and Y may enter into apatite through substitution (1).

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Figure 6. Plot of REE+Y versus Si+Na variations in apatite from the Chengchao granite.

Na can enter into apatite through substitution (2) and/or the following two complex substitutions (Sha and Chappell, 1999; Ronsno, 1989)

$ {\rm{N}}{{\rm{a}}^{\rm{ + }}}{\rm{ + }}{{\rm{S}}^{{\rm{6 + }}}}{\rm{ = C}}{{\rm{a}}^{{\rm{2 + }}}}{\rm{ + }}{{\rm{P}}^{{\rm{5 + }}}} $ (4)
$ {\rm{2N}}{{\rm{a}}^{\rm{ + }}}{\rm{ = C}}{{\rm{a}}^{{\rm{2 + }}}}{\rm{ + }}\left[ {\rm{V}} \right] $ (5)

However, as shown in Fig. 6, Na enters into apatite mainly through substitution (2) and the other two substutions (4) and (5) are the secondary ways.

Th and U may enter into apatite mainly by the following three ways (Casillas et al., 1995)

$ {\rm{2T}}{{\rm{h}}^{{\rm{4 + }}}}{\rm{(}}{{\rm{U}}^{{\rm{4 + }}}}{\rm{) + }}\left[ {\rm{V}} \right]{\rm{ = 2C}}{{\rm{a}}^{{\rm{2 + }}}} $ (6)
$ {\rm{T}}{{\rm{h}}^{{\rm{4 + }}}}{\rm{(}}{{\rm{U}}^{{\rm{4 + }}}}{\rm{) + S}}{{\rm{i}}^{{\rm{4 + }}}}{\rm{ = RE}}{{\rm{E}}^{{\rm{3 + }}}}{\rm{ + }}{{\rm{P}}^{{\rm{5 + }}}} $ (7)
$ {\rm{T}}{{\rm{h}}^{{\rm{4 + }}}}{\rm{(}}{{\rm{U}}^{{\rm{4 + }}}}{\rm{) + C}}{{\rm{a}}^{{\rm{2 + }}}}{\rm{ = 2RE}}{{\rm{E}}^{{\rm{3 + }}}} $ (8)

As shown in Fig. 7, Th+U shows a good correlation with REE+Y and Si, with coefficient (R2) of 0.77 and 0.87, indicating Th and U enter into apatite mainly through substitutions (7) and (8). In contrast, substitution (6) is a secondary way.

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Figure 7. (a) Plot of Th+U versus Si variation; (b) plot of Th+U versus REE+Y variations in apatite from the Chengchao granite.
5.2 The Oxygen Fugacity and Volatile Content of Magma and Its Implication for Mineralization

After studying the compositional zoning in Criffell granitic pluton in southern Scotland, Miles et al. (2014) found that the concentration of Mn in apatite has a negative correlation with the oxygen fugacity. As a result, they proposed an empirical formula for the calculation of the oxygen fugacity using the Mn concentration in apatite

lg fO2 = -0.002 2(±0.000 3)Mn(ppm) -9.75(±0.46)

The calculated oxygen fugacity of Chengchao granite ranges from -11.02 to -10.03, with an average value of about -10.64. Using the geochemical data in literature (Xie et al., 2008), the zircon saturation temperature of Chengchao granite is about 760 ℃ (Miller et al., 2003). Zircon and apatite usually crystallized early in granitic rocks, so we assume the crystallization temperature of apatite in Chengchao granite is 700–800 ℃. As shown in Fig. 8, the oxygen fugacity of magma is close to MH buffer line when apatite crystallized. This is higher than the oxygen fugacity of magma in the Tonglüshan Cu-Fe (Au) deposit (NNO+1) as indicated by petrographic evidence that the amphibole crystallized simultaneously with the apatite (Duan and Jiang, 2017). The high oxygen fugacity of Chengchao granite is also indicated by the biotite composition, which gives the highest oxygen fugacity estimation in the single Fe ore deposits (such as the Chengchao Deposit) (Li W et al., 2014). The high oxygen fugacity of Chengchao granite may attribute to the following three reasons: (1) High oxygen fugacity reflects the nature of the source region; (2) The high temperature promotes the decomposition of water in magma system. Since hydrogen is easier to escape from the magma system, partial oxygen pressure, i.e., the oxygen fugacity is likely to increase during this process (Tan, 1991); (3) High oxygen fugacity is the result of the assimilation of gypsum-bearing sedimentary strata (Li Y H et al., 2014, 2013; Fan et al., 1995; Cai, 1980). Compared to the other granites in the Edong district, those granites that related to single iron mineralization show the highest oxygen fugacity (Li W et al., 2014). Considering the similar source and spatial and temporal relationship (Li W et al., 2014), we suggest that the reasons (1) and (2) are unlikely to be the main reasons for the high oxygen fugacity of Chengchao granite. There are mainly two types of Fe deposits, namely, the skarn-type and the iron oxide-apatite type in the Middle–Lower Yangtze River metallogenic belt. The skarn-type Fe deposit is represented by the Chengchao and Jinshandian deposits in the Edong district (Shu et al., 1992). The iron oxide-apatite deposit is related to Cretaceous intermediate-basic volcanic-subvolcanic rocks that occur in the Mesozoic volcanic basin in Ningwu and Luzong districts (Li Y H et al., 2013). Many studies have shown that the Fe mineralization in the Middle– Lower Yangtze River metallogenic belt is closely related to the gypsum-bearing sedimentary strata (Li Y H et al., 2014, 2013; Fan et al., 1995; Cai, 1980). When the magma intruded into the gypsum-bearing sedimentary strata, the assimilation of gypsum will promote the oxygen fugacity of magma and oxidize Fe2+ to Fe3+ in the magma. Therefore, the high oxygen fugacity of Chengchao granitic magma may be resulted from assimilation of the gypsum, which implying the important role of gypsum-bearing sedimentary strata in iron mineralization in the district.

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Figure 8. Plot of oxygen fugacity versus temperature of the Chengchao granite. Red rectangle area shows the oxygen fugacity range of the Chengchao granite.

Data for apatite in the volcanic rocks of Luzong (Yue, 1983) and the iron ores in Ningwu area (Zeng et al., 2016) are also cited in this study for comparison. As shown in Fig. 4, the apatites in Luzong volcanic rocks have higher Cl concentration than that in Chengchao granite (Yue, 1983), indicating Luzong magma is richer in Cl than the Chengchao magma. Moreover, the apatite in Ningwu iron oxide-apatite deposits has the highest Cl content (Zeng et al., 2016), reflecting the highest Cl concentration in ore-forming hydrothermal fluids. Therefore, the difference in dissolved Cl concentration in ore-related igneous rocks may be one of the factors that control the mineralization type that differs in the Edong district and the Luzong and Ningwu districts.

5.3 Implication for Distinguishing the Adakitic Affinity

The granite rocks with adakitic affinity, which are closely related to Cu-Fe mineralization, are extensively exposed in the Edong district, such as the Tonglüshan quartz monzodiorite porphyry in the Tonglüshan Cu-Fe(Au) Deposit (Zhao et al., 2010) and the Tieshan quartz diorite in Tieshan Fe-(Cu) Deposit (Li et al., 2009). According to the definition of Defant et al. (2002) and Defant and Drummond (1990), adakite is characterized by high Sr/Y and La/Yb ratios as well as low Y and Yb contents. Because the partition coefficients of REE in apatite do not vary dramatically (Zhao, 2010), the REE distribution pattern of the apatite tends to be similar to the bulk rock composition of Chengchao granite (Fig. 5a). Moreover, Sr is highly compatible in apatite (Prowatke and Klemme, 2006). Therefore, the relative concentration of REE, Y and Sr in apatite should be controlled by the melt composition. Because of the strong resistance to alteration, the contents of Sr, REE and Y in apatite may provide a new possibility to identify the adakitic affinity even if the granite suffered extensive weathering or alteration. The Chengchao granite is a non-adakitic rock (Defant et al., 2002; Defant and Drummond, 1990), with low Sr/Y and (La/Yb)N ratios of 4.2 and 11.1, respectively. Furthermore, it also has a low Sr (93.2 ppm) but high Y concentration (22.1 ppm) (Xie et al., 2008).

Pan et al. (2016) studied apatite compositions in adakitic and non-adakitic rocks in the Sanjiang area. They found that adakitic rocks have higher Sr/Y and Eu/Eu* ratios compared to non-adakitic rocks. As shown in Fig. 9, the Sr/Y ratios and Eu/Eu* values of the apatite in Chengchao granite are low, plotting in the non-adakite region. This result shows that the Sr/Y ratios and Eu/Eu* values of apatite can be used to distinguish the adakitic rock from non-adakitic ones.

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Figure 9. The discrimination diagram of adakitic rocks using Sr/Y versus Eu/Eu* values (after Pan et al., 2016).
5.4 Source Region of Chengchao Granite

Since the Rb/Sr ratio is extremely low in apatite, the radioactive decay of Rb has negligible effect on 87Sr/86Sr composition in apatite. As a result, the in situ Sr isotopes of apatite can directly reflect the initial Sr isotopic composition of the granitic magma (Tsuboi, 2005), which have avoided the shortcomings that whole rock compositions require an age correction and is susceptible to weathering and alteration. The 87Sr/86Sr value of Chengchao apatite is 0.706 9–0.708 2 (Table 3), which is consistent with the initial value of 87Sr/86Sr value (0.707 2) of the whole rock (Xie et al., 2008), indicating that the in situ Sr isotopes of apatite can represent the initial Sr isotopic composition of the granite. Combined with the value of Nd isotope, the Sr-Nd isotopes of Chengchao granite distribute between lower crust and MORB (Fig. 10), indicating the crust-mantle mixed character. In addition, Chengchao apatite has obvious negative Eu anomaly. This negative Eu anomaly may indicate that Chengchao granite derived from a crust-mantle mixed source where plagioclase was either stable or underwent the fractional crystallization of plagioclase before apatite crystallization. Combined with the previous study, the former explanation is more reasonable (Xie et al., 2008).

Table 3 87Sr/86Sr value of apatite in the Chengchao granite
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Figure 10. Plot of εNd(t) versus initial Sr isotopic composition of the Chengchao granite. The gray area is the in situ Sr isotopes variation estimated from apatite. The data of Yangtze Craton lower crust are from Jahn et al. (1999). The data of the upper continental crust (UCC) are from Taylor and McLennan (1985). The data for Early Cretaceous mafic rocks in the Middle–Lower Yangtze River metallogenic belt (MYRB) are from Yan et al. (2008). MORB data are from Hofmann (2014). The short red line (-11.2) is the initial Nd value of the bulk rock (Xie et al., 2008).
6 CONCLUSIONS

The major elements, trace elements and Sr isotopes of the apatite in Chengchao granite show that the Chengchao granitic magma had a high oxygen fugacity, which possibly due to assimilation and contamination of surrounding gypsum-bearing sedimentary strata. The high oxygen fugacity may provide favorable conditions for the extraction of Fe from magma. The results have shown that the trace elements in apatite can be used to effectively identify adakitic affinity. The Sr isotopes of apatite show that the Chengchao granite was derived from a crust-mantle mixed source where the plagioclase was stable at source region. Moreover, the in situ Sr isotopes of apatite can represent the initial Sr isotopic composition of the magma without age correction.

ACKNOWLEDGMENTS


The field sampling was supported and assisted by Drs. Shanggang Jin and Ketao Wei from the First Geological Team in southeastern Hubei Province. Undergraduate students of China University of Geosciences (Wuhan), including Cheng Bai, Ziqi Chen and Mingyu Cao helped during the field work. Sample analysis was performed in the State Key Laboratory of Geological Processes and Mineral Resources and the Shandong Geological Testing Center. Prof. Kuidong Zhao and Dr. Yaoming Xu provided helps for data processing and interpretation. Two anonymous reviewers provided valuable comments and suggestions to improve this manuscript significantly. This study was supported by the National Key R & D Program of China (No. 2016YFC0600206) and the China Geological Survey (No. 12120114051801). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0837-x.


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