Ore Geology Reviews 64 (2015) 200–214

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Ore Geology Reviews

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Mineral chemistry of high-Mg and skarn in the Han-Xing Iron deposits of South Taihang Mountains, China: Constraints on mineralization process

Ju-Quan Zhang a,c, Sheng-Rong Li a,b,⁎,M.Santoshb, Ji-Zhong Wang c,QingLia,b a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, 29 Xueyuan Road, Beijing 100083, China b School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China c School of Gemology and Materials Technology, Shijiazhuang University of Economics, 136 Huaiandong Road, Shijiazhuang 050031, Hebei Province, China article info abstract

Article history: The Han-Xing region is located in the south Taihang Mountains (TM) in the central part of the North China Received 25 March 2014 Craton, and is an important iron producing area. The iron deposits in this region are of skarn type, related to an Received in revised form 19 June 2014 Early Cretaceous high-Mg complex, including diorite, diorite, diorite, diorite Accepted 7 July 2014 porphyrite, and monzonite. In this study we report the detailed mineral chemistry of the high-Mg diorites and Available online 15 July 2014 skarn rocks. The olivine in the gabbro diorite shows chemical composition similar to that in mantle peridotite xenoliths. Clinopyroxene in the gabbro diorite is dominantly augite, with only minor diopside, whereas the Keywords: Mineral chemistry clinopyroxenes in the diorite and monzonite are diopside. in the high-Mg diorites show composi- High-Mg diorite tional range from magnesiohornblende to magnesiohastingsite, with minor and . Most thermobarometer in the high-Mg diorite is andesine and oligoclase. The magnesio- in gabbro diorites shows Skarn iron deposit chemical characteristics of re-equilibrated primary and those in calc-alkaline rocks. In the diorite and diorite porphyrite, plagioclase shows complex chemical zoning. Clinopyroxene and garnet in skarn rocks show varying FeO contents, the former containing low FeO (b9 wt.%) and occurring as the major skarn mineral in large-scale iron deposits, and the latter within small-scale iron deposits with high FeO (mostly N25 wt.%) content. We computed the pressure, temperature, oxygen fugacity and water contents based on the mineral chemistry of amphibole and biotite. Based on the results, the magma crystallization can be divided into two stages, one within the deep magma chamber, forming clinopyroxene, amphibole and plagioclase phenocrysts; the other after emplacement, forming the rim of phenocrysts and matrix minerals. The magma during the

early stage shows high temperature (~900 °C–950 °C), pressure (~300 MPa–500 MPa), relatively high logfO2

(NNO–NNO +2), and H2O content in melt (4%–8%). During the late stage, the magma temperature dropped to

about 750 °C, and pressure came down to less than 100 MPa, with the logfO2 rising to NNO +1–NNO +2. The zoning of amphibole and plagioclase records the process of magma mixing and crystallization, with injection

of mafic magma into the felsic magma chamber. The relatively high logfO2 and H2O content inhibited partitioning of iron into mafic minerals and favored concentration of Fe in the melt. Iron ore precipitation occurred when the magma was emplaced at shallow level, and was principally controlled by the chemical composition of carbonate 3+ wall rocks. The high logfO2,Fe rich ore-forming fluid generated andradite and clinopyroxene when it reacted with limestone and dolomitic limestone respectively. © 2014 Elsevier B.V. All rights reserved.

1. Introduction mining has lasted for about 60 years, and a series of iron deposits has been explored and mined such as the Xishimen, Beiminghe, Kuangshan, The Han-Xing region is located in the southern part of the Taihang Qichun, and Fushan iron deposits, with a cumulative reserve of over Mountains (TM) in the central North China Craton, and is a well- 1000 Mt (Zheng et al. 2007a). known iron province containing numerous skarn iron deposits related Several earlier studies (e.g. Li et al., 2013; Niu et al., 1994; Shen et al., to Early Cretaceous mafic-intermediate magmatic systems. Large-scale 1977, 1979, 1981; Shen et al., 2013a, 2013b; Xu, 1986, 1987; Zhang et al., 2013; Zheng et al., 2007a, b, c) addressed the genesis of these iron deposits. Shen et al. (1977) and Xu (1986, 1987) examined the ⁎ Corresponding author at: State Key Laboratory of Geological Processes and Mineral chemical composition and mineralogy of sodic metasomatites and Resources, China University of Geosciences, 29 Xueyuan Road, Beijing 100083, China. fl Tel.: +86 10 8232 1732; fax: +86 10 8232 2176. other altered rocks and considered that magmatic uids extracted the E-mail address: [email protected] (S.-R. Li). iron, potassium, calcium and from the diorite, and formed

http://dx.doi.org/10.1016/j.oregeorev.2014.07.007 0169-1368/© 2014 Elsevier B.V. All rights reserved. J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214 201 the skarn and when it reacted with the Ordovician carbonate. In this paper, we present results from a systematic study of the Liu et al. (1982) proposed that NaCl-rich fluids extracted and chemistry of minerals from high-Mg diorites and skarn, in an attempt transported the iron from diorite. Some other researchers (e.g. Niu to reconstruct the physico-chemical conditions of the mineralization et al., 1994; Zhen et al., 1984) believed that the metamorphic crystalline system. Based on regional geology, ore geology, and mineral chemistry, basement contributed considerable iron to the ore formation. Recently, we propose a new genetic model of skarn iron deposits in the southern Li et al. (2013) and Shen et al. (2013a, 2013b) provided new data on He TM. This model not only provides insights into the chemical character- and Ar isotopes, and suggested that significant crustal material was in- istics of the diorite-related mineralization, but also evaluates the pro- volved in the mineralization process. Since iron is a major element in cess of evolution of the ore fluids from the magmatic to mineralization the various spheres of the earth, and occurs as a common constituent stages. in several types of igneous, metamorphic and sedimentary rocks, it is often difficult to precisely identify the source of iron. However, all the 2. Regional geology previous studies in this region correlated the iron mineralization as a byproduct of Mesozoic magmatism (e.g., Shen et al., 2013a). This im- Our study area is located in the central part of the North China plies that the magmatic processes had an important relationship with Craton (NCC) (Fig. 1a). The NCC is one of the fundamental Precambrian the iron mineralization. nuclei in Asia (Li et al., 2013; Zhai and Santosh, 2011, 2013). The late

Fig. 1. (a) Spatial distribution of Mesozoic igneous rocks in the eastern North China Craton (modified from Yang et al., 2008); (b) detailed geological map of the Han-Xing region. 202 J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214

Archean–early Paleoproterozoic Fuping and Zanhuang Groups compose main channels of magmatic emplacement (Ding, 1986; Xu and Lin, the basement of the central NCC in the TM (Li et al., 2013). The latter 1989). is the crystalline basement in the southern TM, composed of TTG Diorite–monzonite–syenite series are the major magmatic rock (–trondhjemite–) gneisses, and monzonitic and po- types in the area among which the diorite and monzonite series are tassic , with minor supracrustal rocks (Trapa et al., 2009; Xiao closely associated with ore formation. In addition, some gabbroic et al., 2011; Yang et al., 2011, 2013). The Mesoproterozoic Changcheng rocks have also been reported in the diorite complex (Wang et al, Group sandstone overlies the Zanhuang Group with an angular uncon- 2006). The intrusive rocks show high MgO, Cr, Ni, Sr and Ba contents, formity, and is in turn covered by Cambrian to Ordovician carbonate for- highly fractionated rare earth elements (REE), and strong Nb and Ta mation. Middle Ordovician limestone and dolomitic limestone comprise depletions (Qian and Hermann, 2010; Xu and Gao, 1990), and can be the wall rocks of the ore bodies (Niu et al., 1994; Shen et al., 1977, 1979, divided into three belts (Li, 1986; Zheng et al., 2007a): the eastern 1981; Xu, 1986; Zheng et al., 2007a, b, c; Zhang et al, 2013). The eastern belt (Hongshan pluton), central belt (Wuan pluton, Kuangshan pluton, sedimentary basins are covered by Carboniferous, Permian, and Triassic Qichun pluton), and the western belt (Fushan pluton). Most of the sandstone, mudstone, shale, and siltstone, with coal seams. The eastern important deposits occur in the central belt (Fig. 1b). plain is covered by Tertiary sediments (Fig. 1b). The K–Ar dating of whole rock and single mineral (K-feldspar, horn- Mesozoic magmatism and regional structures (faults and folds) blende and biotite) from the different intrusions shows ages between display NNE or NNW-trends. According to aeromagnetic data and 150 and 64 Ma (Yang, 1982). Recent zircon U–Pb dating has yielded field observations, there are also four NWW-trending hidden faults ages around 120–135 Ma where intrusive rocks of diverse compositions which developed on the basement in this region and adjacent regions show similar ages (e.g. Chen et al., 2005, 2007, 2008; Li et al., 2013; Sun (Li, 1986; Zeng, 1987). The faults of basement and cover compose the et al., 2014; Zhou and Chen, 2006).

Fig. 2. (a) Cross section of the 28th exploration line of Baijian iron deposit; (b) cross section of the 5th exploration line of Xishimen iron deposit. J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214 203

3. Mine geology The common textures of the ores are euhedral and subhedral granu- lar, xenomorphic granular, reaction rim, and lattice-like. The structures Most of the iron ore bodies, especially the large ones, are hosted by of ores include disseminated, massive, striped-banded, mottled, crystal Middle Ordovician carbonate and located in the central zone (Fig. 1b). cave, and brecciated. These ore bodies show marked diversity in morphology and size and Wall rock alteration is widely distributed in the mining area, and are controlled by the contact zones and faults within 100 m of the con- shows a close relationship with the ore bodies. Four stages of alteration tact zones. are observed according to mineral assemblages and metasomatism pro- The ore bodies range in length from tens to hundreds of meters, and ducing an albitized zone (albite + diopside + epidote + prehnite) a few even exceed 5 km. They show several meters to tens of meters close to the diorite, followed by the endoskarn zone (diopside + thickness, some exceeding 200 m. There is significant variation in thick- scapolite), the magnetite zone (magnetite + diopside + phlogopite) ness within a short distance (e.g. Fig. 2a). Generally, the ore bodies of and the exoskarn zone (garnet/diopside + + + large and medium-sized deposits are relatively simple, mostly bedded serpentine + calcite) within the marble host rock. or lenticular. The ore bodies of small deposits are more complex, with shapes ranging from ovoid to irregular (Zheng et al., 2007a), such as 4. Sample description and analytical methods in the case of the Baijian and Xishimen iron deposits. The ore bodies of Baijian iron deposit are controlled by the NS- 4.1. Sampling and petrography trending Zhongguan–Baijian anticline. The ore bodies occupy the con- tact zone and faults within 300 m of limestone in the east limb of More than 600 samples of the different rocks were collected from anticline. The form of ore bodies is controlled by the contact zone and the study area, from surface exposures, drill cores and underground the fracture in limestone, showing a NW-trend, extending for about mine workings. The samples include gabbro diorite, hornblende diorite, 1500 m with a width of 800 m (Zhang et al., 2013)(Fig. 2a). The diorite diorite, diorite porphyrite, monzonite, skarn, ore and other alteration porphyry and monzonite are the major intrusive rocks, and show in- rocks. Representative samples of the intrusions, ores and skarns were tense alteration near the contact zone. used for analytical studies. The main types of high-Mg diorite and The Xishimen iron ore is the largest deposit in this region, with iron skarn are described below. resource exceeding 120 Mt (Zheng et al., 2007b). The ore bodies in The gabbro diorite intrudes into the high-Mg diorite (Fig. 3a) and this deposit mainly occur in the contact zone of Ordovician Majiagou shows porphyritic texture and dark-green color. The essential minerals Formation limestone and Yanshanian diorite and monzonite intrusive in this rock are clinopyroxene (15–20%), amphibole (25–30%), plagio- bodies (Fig. 2b). NNE–NE trending folds and NNE trending faults are clase (40%) and biotite (5%), with minor olivine and K-feldspar (Figs. 3b the main ore-controlling structure. Twenty eight magnetite ore bodies and 4a, b, c). The accessory minerals are ilmenite, titanomagnetite, have been identified, and most of these are layered, lenticular, ovoid magnetite, , and zircon. The phenocrysts are clinopyroxene and or irregular in shape. The main ore body occurs at the contact zone, amphibole, and the matrix is plagioclase. Clinopyroxene typically with a length of about 5020 m, thickness ranging from 1.2 to 32.0 m shows a reaction rim of amphibole, and the latter is altered to biotite. with an average of 15.13 m, and maximum up to 103.42 m (Zhang Olivine is altered to serpentine, and is associated with ilmenite. et al., 2013; Zheng et al., 2007b). The hornblende diorite is gray-green in color with medium grained The grade of ore varies from 30 to 50 wt.% Fe. Ore minerals are main- granular porphyritic texture. The essential minerals are amphibole (35– ly magnetite, followed by pyrite, martite, hematite, chalcopyrite and a 45%), plagioclase (~55%) (Figs. 3cand4d, e, f), with minor K-feldspar. small amount of magnesian magnetite, pyrrhotite, bornite, chalcocite, Accessory minerals include magnetite, apatite, zircon, and sphene. goethite, etc. Gangue minerals are diopside and andradite, together Zoning is common in amphibole (Fig. 4f). with tremolite–actinolite, phlogopite, serpentine, calcite, dolomite, Diorite and diorite porphyrite show medium grained, porphyritic and a small amount of magnesian olivine, humite, chlorite, scapolite, texture, and gray-green color. The major minerals are amphibole , apatite, sphene, fluorite, zeolite etc. (20–30%), plagioclase (~65–75%) ± biotite (5%), and minor K-feldspar,

Fig. 3. Photographs of the samples of gabbro diorite (a, b), hornblende diorite (c), diorite porphyrite (d), diorite (e) and monzonite (f) in the Han-Xing region. 204 J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214

Fig. 4. Representative photomicrographs of mineral assemblages in the major rock types of the Han-Xing area. Gabbro diorite (a–c), hornblende diorite (d–f), diorite (g–i), and monzonite (j–l). Mineral abbreviations: Ol = olivine, Cpx = clinopyroxene, Am = Amphibole, Bt = Biotite, Pl = Plagioclase, Kfs = K-feldspar, Qtz = Quartz, Mag = Magnetite, Ilm = Ilmenite. and clinopyroxene (Figs. 3d, e, and 4g, h, i). Accessory minerals are mag- (XX-8 and XX-1), two diorite (FD-1 and CY-1), and two monzonite netite, apatite, zircon, and sphene. Plagioclase phenocrysts show com- (N803, B37) samples were analyzed for their major element composi- plex zoning texture (Fig. 4h). tion. The clinopyroxenes and garnets from skarn and ore were also The monzonite shows fine-medium grained texture, gray-green analyzed. The analyses were carried out with a JEOL JXA-8100 electron color, and the main minerals are amphibole (15–20%), plagioclase microprobe analyzer at Institute of Geology and Geophysics, Chinese (40–45%), K-feldspar (25%–30%), and quartz (5%) (Figs. 3fand4j, k, l). Academy of Sciences (IGG CAS) and a JEOL JXA-8230 electron micro- The accessory minerals are magnetite, sphene, apatite, and zircon. probe analyzer Institute of Mineral Resources, Chinese Academy of Garnet skarn occurs around the magnetite ore bodies. It shows Geological Science. Measurements were done under accelerating volt- coarse grained texture, and is associated with calcite and magnetite age of 15 kV, a specimen current of 20 nA, and a 5 μmbeamdiameter. (Fig. 5a, b). Clinopyroxene skarn is common in Han-Xing iron deposits. It shows coarse grained texture, and is associated with magnetite ore 5. Results (Fig. 5c, d). Clinopyroxene–scapolite skarn occurs in the inner contact zone, and shows coarse grained texture. Main minerals are scapolite 5.1. Minerals of high-Mg diorite (60–70%), clinopyroxene (15–25%) and magnetite (5–10%) (Fig. 5e). Clinopyroxene–hematite skarn occurs as minor veins, cutting the al- 5.1.1. Olivine tered diorite, and shows coarse grained texture (Fig. 5f). The olivines in the gabbro diorite are mostly altered into serpentine. The mineral chemical data show an FeO range of 15.56 wt.%–20.26 wt.%, 4.2. Analytical methods and a MgO range of 40.8 wt.%–44.2 wt.% (Supp. Table 1), similar to the chemical compositions of olivine in peridotite xenoliths which are Olivine, pyroxene, amphibole, biotite, plagioclase and K-feldspar enclosed in high-Mg diorites (Huang and Xue, 1990a, 1990b; Xu and from two gabbro diorite (Q-9 and XM-1), two hornblende diorite Lin, 1991; Xu et al., 2003a, 2003b). J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214 205

Fig. 5. (a) Field photo and (b) photomicrograph of garnet skarn, (c) field photo and (d), (e), (f) photomicrographs of clinopyroxene in ore and skarn. Mineral abbreviations: Mag = Magnetite, Cpx = Clinopyroxene, Grt = Garnet, Cal = Calcite, Hem = Hematite, Pl = Plagioclase, Scp = Scapolite.

5.1.2. Clinopyroxene basis of Al2O3 content, the amphiboles can be divided into two groups, The clinopyroxenes in gabbro diorite, diorite and monzonite show the first group has high Al2O3, consisting of the phenocrysts; the second variable compositions and low Al2O3 (b3.7 wt.%), TiO2 (b0.4 wt.%) group has relative low Al2O3 comprising the matrix or the rim of some and Na2O(b1.4 wt.%) contents (Supp. Table 2). The pyroxene in gabbro phenocrysts. Some of these amphiboles also occur as reaction rims of diorite (Q-9, XM-1) contains more Mg and Na than those in others, and clinopyroxene. most of them are augite, with only few showing diopside composition (Fig. 6a). However, all the pyroxenes in the diorite and monzonite are 5.1.4. Biotite diopside. Biotite is rare in the high-Mg diorite of Han-Xing region. However, the mineral is common in gabbro diorite, and substitutes amphibole 5.1.3. Amphibole (Fig. 4a, b). We analyzed 13 points in two samples. Among these, The amphiboles in high-Mg diorites are generally euhedral pheno- 7 points are on biotites coexisting with amphiboles. crysts, elongated and columnar. Some large phenocrysts show zoning The results show MgO and FeO contents of 11.27–15.85 wt.% and texture. A total of 59 points were analyzed in amphiboles from different 13.83–18.39 wt.% (Supp. Table 4), respectively. Most of the biotites high-Mg diorite samples. The results show calcic amphibole composi- belong to magnesio-biotite (Fig. 7a), and show the chemical character- tions, following the classification scheme of Leake et al. (1997), istics of re-equilibrated primary biotites (Fig. 7b) and biotites of calc- with Ca ranging from 1.6 to 1.86 a.p.f.u. (atoms per formula unit). The alkaline rocks (Fig. 7c). amphiboles are mainly magnesiohornblende and magnesiohastingsite, with a few belonging to pargasite and tschermakite (Fig. 6b, c, d; 5.1.5. Feldspar Supp. Table 3), similar to those reported by Niu and Zhang (2005) and The EPMA data show that most of the plagioclase in high-Mg diorite Qian and Hermann (2010). is andesine and oligoclase (Fig. 8a; Supp. Table 5). In diorite, plagioclase The amphibole from different high-Mg diorites shows similar char- always shows complex chemical zoning, recording the history of evo- acteristics including high MgO and relatively low FeO contents. On the lution. The EPMA analyses show that some crystals have albite or 206 J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214

Fig. 6. Classification of minerals in the intrusive suite. (a) Pyroxene in gabbro diorite, diorite and monzonite (Morimoto et al., 1988). (b, c, d) Amphibole in gabbro diorite, hornblende diorite, diorite, diorite porphyrite and monzonite (Leake et al., 1997).

oligoclase rims and labradorite or andesine cores. We chose two 6. Discussion crystals in two different thin sections for detailed analysis. One is from diorite porphyrite (CY-1), the other is from diorite (FD-1). 6.1. The physico-chemical conditions of high-Mg dioritic magma The plagioclase phenocrysts of diorite porphyrite are composed of crystallization two oscillatory zoned domains around a core, and an outer rim. The An (anorthite mol%) content of the core ranges from 24 mol% to 6.1.1. Geothermobarometry 37 mol%, showing an increasing trend in general, but that of the oscil- It has been established that the total Al content of hornblende in in- latory zoned domains shows a decrease from the inner portion to rim, termediate calc-alkaline rocks has a good correlation with pressure, ranging from 36 mol% to 18 mol%, and 29 mol% to 20 mol% respec- based on which a number of geobarometers have been proposed to tively (Fig. 8c). The An content of rim shows an obvious change calculate the pressure of crystallization (e.g. Hammarstrom and Zen, (29 mol%–31 mol%). The feldspar in diorite shows more complex 1986; Anderson and Smith, 1995; Hollister et al., 1987; Johnson and zonings, with the An content showing rapid change across a few μm Rutherford, 1989; Schmidt, 1992; Thomas and Ernst, 1990). However, (from 29 mol% to 45 mol% in 30 μm) (Fig. 8b). the study of Ridolfi et al. (2008) showed that most of these barometers K-feldspar occurs in gabbro diorite, diorite and monzonite. The albite are inaccurate when compared to experimental results, with the content of K-feldspar in gabbro diorite (17 mol% to 26 mol%) is higher average error as high as 280 MPa. Ridolfi et al. (2010) summarized the than that in the other rocks (b10 mol%) (Fig. 8a). available experimental data, and proposed new thermobarometric for- mulations to calculate the pressure, temperature, oxygen fugacity and the H O content of magma. We calculated the physical–chemical pa- 5.2. Skarn minerals 2 rameters of amphibole by a spreadsheet provided by Ridolfi et al. (2010). 5.2.1. Clinopyroxene The calculated results show that the amphibole formed in two dif- Clinopyroxene has MgO and FeO contents of 12.88–17.27 wt.% ferent stages. Most of the amphibole phenocrysts formed in a relative and 1.38–8.28 wt.% (Supp. Table 6), respectively. They show relatively high pressure (200 MPa–600 MPa) and high temperature (concentrated less FeO and Na O, higher MgO, and CaO as compared with the 2 between 900 °C and 980 °C) stage, whereas the matrix or the rims of clinopyroxene in high-Mg diorite. In Jo–Di–Hd diagram (Meinert et al., phenocrysts formed in a low pressure (b100 MPa) and relative low tem- 2005), the composition falls in the field of clinopyroxenes from the perature (concentrated between 700 °C and 800 °C) (Fig. 10a, Supp. iron deposits (Fig. 9a, b). Table 3) environment. Assuming a crustal density of 2700 kg/m3, the crystallization depth (in km) was computed. The results show 5.2.2. Garnet that most of the amphibole phenocrysts crystallized at a depth of The EPMA analyses of garnets show that andradite is the major com- 10–20 km, and the maximum depth is estimated as 23 km. The ponent, with most grains having more than 90% (mol%) andradite. Few emplacement depth calculated by matrix amphibole or the rim of am- samples show relatively high grossular (b40.3 mol%) (Supp. Table 7). phibole phenocrysts ranges from 1.3 km to 3.3 km, with most data clus- Their composition falls in the field of garnets from iron deposits in the tered at 1.5 km to 2 km. This is consistent with the result calculated by Spess + Alm-Gross-And triangular diagram (Fig. 9c). the stratigraphic thickness overlaid on intrusions. J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214 207

Fig. 8. Feldspar compositional plots. (a) Classification of feldspar in gabbro diorite, hornblende diorite, diorite, diorite porphyrite and monzonite. (b) Plagioclase phenocryst (central panel, photomicrograph on the right) of diorite (FD-1) showing complex zoning, with abrupt An variations. (c) Plagioclase phenocryst (central panel, photomicrograph on the left) of diorite (CY-1) with sawtoothed zoning pattern.

Fig. 7. Compositional plots of biotites. The biotites from gabbro diorite are mainly the mafic chemistry. The Fe/(Fe + Mg) ratio of these plotted in the magnesio-biotite field of Mg–(AlVI +Fe3++Ti)–(Fe2++Mn) ternary silicates shows a significant negative correlation with fO2, and is inde- diagram (a) (Foster, 1960), in the re-equilibrated primary biotite field of the 10 ∗ TiO – 2 pendent of the Fe/Mg ratio of the whole rock. Several experimental FeOtotal–MgO diagram (b) (Nachit et al., 2005), and in the field of calc-alkaline magma of the MgO–FeO*–Al2O3 diagram (c) (Abdel-Rahman, 1994). studies (e.g. Bogaerts et al., 2006; Dall'Agnol et al., 1999; Martel et al., 1999; Pichavant et al., 2002; Prouteau and Scaillet, 2003; Scaillet and

Evans, 1999) also suggested that fO2 exerts a dominant control on the The biotite–amphibole P–T diagram (cited from Chen et al., 1993) Fe/(Fe + Mg) ratio of the mafic silicates and the whole-rocks, at fixed and biotite Ti versus Mg/(Mg + Fe) diagram were also used (after temperature. Therefore, the composition of amphibole and biotite in Henry et al., 2005)(Fig. 10d, e, f) to estimate the crystallization temper- high-Mg diorite can be used to evaluate the redox conditions of intru- ature and pressure. The results show that the gabbro diorite crystallized sive rocks in Han-Xing region. at about 1 kbar, 700 °C–800 °C. This is highly consistent with the results The Fe/(Fe + Mg) ratios of amphiboles in the gabbro diorite calculated from the amphiboles. (0.26–0.37), hornblende diorite (0.35–0.49), diorite (0.22–0.53) and monzonite (0.26–0.44) are relatively low. Contrasting amphibole 6.1.2. Oxygen fugacity compositions formed in the deep magma chamber and after emplace-

The study of Anderson and Smith (1995) shows that fO2 with fH2O ment respectively; our data show that the former has a greater range and total pressure is more important than temperature in controlling of variation (0.22–0.53), and the latter shows a relatively narrow 208 J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214

Fig. 9. (a) Classification of pyroxene from the Han-Xing skarn. (b) Jo–Di–Hd ternary showing the composition of pyroxenes. The compositional fields for pyroxene in Fe skarn deposits (Meinert et al., 2005) are shown in both diagrams for comparison. (c) Composition of garnet from the Han-Xing skarn. The compositional field for garnet in Fe skarn deposits is shown for comparison (Meinert et al., 2005).

range (0.26–0.37). Based on amphibole Fe/(Fe + Mg) vs. AlIV diagrams variations (Fig. 11b). Most of the plots fall in the field between the

(Fig. 11a) (Anderson and Smith, 1995), it can be interpreted that amphi- Ni–NiO (NNO) and Fe2O3–Fe3O4 (HM) buffers, but two samples show boles in the high-Mg diorite crystallized under high fO2 conditions, and slightly lower fO2, falling in the field between the Fe2SiO4–Fe3O4 the fO2 after emplacement is higher than the value in the magma (FMQ) and Ni–NiO (NNO) buffer. Four samples show even higher fO2, chamber. beyond the stable field of magnetite. We carefully examined the thin

Amphibole compositions have also been used to calculate fO2 sections, and found that the samples with lower fO2 are associated following the method of Ridolfi et al. (2010). The results show that with olivine xenocrysts and ilmenites. The olivine xenocrysts may logfO2 range of NNO −0.35 to NNO +2.66, and most values lie between have acted as a buffer when they broke down. NNO +0.50 and NNO +2.30. Without exception, logfO2 of gabbro diorite (NNO +0.67–NNO +2.66), hornblende diorite (NNO −0.35– 6.1.3. H2Ocontents NNO +0.99), diorite (NNO −0.21–NNO +2.28) and monzonite The high content of in hornblende diorite (N40%) and high-Mg dio- (NNO −0.05–NNO +2.34) are in the stable range of magnetite. From rite (10%–40%) indicates that the magma contained a high percentage of VI magma chamber to the emplacement depth, logfO2 of magma shows water. Ridolfi et al. (2010) identified that the Al in amphibole is mainly an obvious increase (from average NNO +0.6 to NNO +2.1) (Fig. 10b, sensitive to water content in the melt, and offered a formulation to Supp. Table 3). calculate the water content. Following this method, we calculated the Fe3+/Fe2+ ratio of minerals is a robust indicator of the oxygen fu- water content of melt at the time of amphibole crystallization, and gacity of magma. Fe3+/Fe2+ ratio of amphiboles in our study is mark- the results show about 4 wt.%–7 wt.% water content in the melt in the edly high, with most values in the range of 0.5–2.0. This confirms deep magma chamber, and about 2 wt.%–4.5 wt.% in the melt after em- a high fO2 for high-Mg diorite magma. The chemical compositions of placement (see Fig. 10c; Supp. Table 3). The gabbro diorite shows little biotite in the gabbro diorite are plotted in the Fe3+–Fe2+–Mg diagram variations (3 wt.%–5 wt.%) before and after the emplacement, but the (Wones and Eugster, 1965), and the results show a large range of high-Mg diorite shows relatively large variations. J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214 209

Fig. 10. P–T(a),logfO2–T (b) and T–H2Omelt (c) diagrams for the calcic amphiboles in Han-Xing high-Mg diorite calculated after Ridolfi et al. (2010). The curves and error bars in (a)–(c) are from Ridolfi et al. (2010). Error bars represent the expected uncertainty (σest) (22 °C) and maximum logfO2 errors (0.4 log unit). (a) and (c) display representative error bars indicating the variation in accuracy with P and H2Omelt. In the maximum relative P errors range from 11% (at the maximum thermal stability curve; black dotted line) to 25% (at the upper limit of consistent amphiboles; black dashed line). (b) shows the NNO, NNO +2 (from O'Neill and Pownceby, 1993) and MH curves (from Eugster and Wones, 1962). The maximum thermal stability (black dotted line) and the (lower) limit (black dashed line) of consistent amphiboles are also reported in (c) where the black and red error bars show the maximum relative error (15%) and rest (0.4 wt.%), respectively. (d) Plot of amphibole Ti/(Mg + Fe + Ti + Mn) vs biotite Ti/(Mg + Fe + Ti + Mn) diagram, (e) amphibole Al/(Al + Mg + Fe + Ti + Mn + Si) vs biotite Al/(Al + Mg + Fe + Ti + Mn + Si) diagram (cited from Chen et al., 1993)and(f)TiversusMg/(Mg+Fe)forbiotite(afterHenry et al., 2005).

Considering the pressure–temperature conditions derived from am- regime in the central NCC at ~125 Ma. Xu et al. (2009) and Wang phibole chemistry, we infer that the magma emplacement process must et al. (2011) believed that the high-Mg diorite forming by reaction of have been rapid. This inference is also supported by the widespread melt derived from a delaminated lower continental crust with perido- porphyritic texture in high-Mg diorite. When the water-rich magma in- tite at mantle depth. Qian and Hermann (2010) proposed that these truded from deep magma chamber to shallow crust, the reduction in rocks were formed by the interaction of felsic magma with peridotite pressure suddenly led to water exsolution from melt. at crustal depth. Chen et al. (2005, 2008) modeled the formation of the high-Mg diorite complex through partial melting of a depleted 6.2. The genesis and evolution of high-Mg dioritic magma mantle source (asthenosphere) or an enriched mantle source (SCLM), with subsequent mixing of variable crustal components. The genesis of high-Mg diorites in this region has been the focus of The chemistry of amphibole and biotite can be used to distinguish many studies in the past (e.g. Chen et al., 2004, 2005, 2008; Luo et al., magma sources. Almost all the data of the cores of amphibole pheno- 1997, 2006; Qian and Hermann, 2010; Xu and Lin, 1989, 1991; Xu crysts fall in the mantle source field, whereas most data of the matrix et al., 2009), with different explanations. Based on studies on the and rim of amphibole phenocrysts fall in the crust-mantle mixed gabbroic rocks from this region and adjacent area, Wang et al. (2006) source field except those data from monzonite which fall in the crust proposed that these rocks were generated from a refractory hydrated source field in the TiO2 versus Al2O3 diagram (Fig. 12a). All biotite mantle, during a sudden change from a convergent to extensional compositions lie in the crust–mantle mixed source field in a plot of

Fig. 11. (a) Amphibole Fe/(Fe + Mg) vs AlIV diagrams (Anderson and Smith, 1995) showing the possible oxygen fugacity conditions during the crystallization of high-Mg diorite of Han-Xing region. (b) Biotites from gabbro diorite plotted in the Fe3+–Fe2+–Mg diagram (Wones and Eugster, 1965). 210 J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214

Fig. 12. (a) Plot of TiO2 versus Al2O3 for hornblende (after Jiang and An, 1984); (b) plot of TFeO/(TFeO + MgO) versus MgO for biotite (after Zhou, 1986); TFeO indicates total iron as FeO.

TFeO/(TFeO + MgO) versus MgO (Fig. 12b). These features imply with high An cores (67–76) surrounded by low An rims (35–38), and that the high-Mg magma was derived from mantle and mixed with considered the feature to be an evidence of magma mixing. From crustal components during the crystallization process, which is con- the above discussion, we infer that mantle-derived mafic magma mi- sistent with the explanations from Sr–Nd–Pb isotopic results (Chen grated from depth into the upper crust and invaded the felsic magma et al., 2004). chamber. The gabbroic rocks containing mantle peridotite xenoliths Magma mixing may play an important role in the process of magma always occur as small intrusions or large xenoliths in other high-Mg evolution and ore mineralization (Li and Santosh, 2014; Li et al., 2014). diorite, and their zircon U–Pb ages (~125 Ma) (Wang et al., 2006) are By studying the complex zoning in which is widespread in younger than those of the diorites and monzonites (127–132 Ma) high-Mg diorites in Han-Xing area, we found that the chemical compo- (Chen et al., 2005, 2007, 2008; Li et al., 2013; Sun et al., 2014; Zhou nents vary greatly (sample CY-1 and FD-1). The FeO contents have sim- and Chen, 2006). ilar variation with An, with an obvious correlation between increased Fractional crystallization is one of the key processes for the forma- An content and high Fe concentrations (Fig. 8b, c). This is explained as tion of high-Mg diorite and ore. Abundant amphibole phenocrysts a sign of felsic magma mixing with more calcic magmas (Ruprecht formed in the magma chamber at a depth of 10–20 km. The porphyritic Ruprecht and Wörner, 2007). Each abrupt rise of An composition texture widely displayed by high-Mg diorite also indicates that exten- might represent one pulse of magma mixing. After the mixing, the sive crystallization occurred before the magma emplacement. As the magma system changes from open to closed, leading to the crystalliza- major mafic mineral, the high MgO content of amphibole contributes tion of the plagioclase domains with smoothly decreasing An content. to the high MgO content of the diorites. Our data show that this process occurred during multiple times, From core to rim in a zoned amphibole (e.g. XX-1(1)-1 to XX-1(1)-2, resulting in the complex oscillatory zoning pattern. The zoning in amphi- XX-1(1)-3 to XX-1(1)-4, XX-1(1)-5 to XX-1(1)-6 in Fig. 13a), the values bole phenocrysts in the hornblende diorite also records this process. We of ΔNNO, Fe3+/Fe2+, and Mg/(Mg + TFe) rise with the drop of P and T computed the P–T–logfO2 from the zoned amphiboles, and the results (Fig. 13b, c, d, e, f), suggesting more iron residue in the melt under stable show an abrupt rise of pressure, temperature, and a drop of ΔNNO, crystallization process. The An content slowly declines in one of the Fe3+/Fe2+, and Mg/(Mg + TFe) between two zones (Fig. 13). We con- zoned plagioclase illustrating increasing sodium in the melt after crys- sider this as a credible evidence for hot, low-logfO2,withmaficmagma tallization of amphibole and plagioclase. After emplacement, the rapid intruding into the felsic magma chamber resulting in magma mixing. reduction of pressure and temperature led to the exsolution of water, Chen et al. (2008) also found disequilibrium texture in plagioclase, and the formation of Na–Fe-rich ore-forming fluid. Thus, the plagioclase J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214 211

in the matrix is always albite, and sodic metasomatism is widely distrib- uted inside of the contact zone.

6.3. The influence of P–T–fO2 of ore-forming fluid on the mineralization process

Magmatism to hydrothermal mineralization is a continuous process. The ore-forming fluids derived from magmas (Zheng, 2007), possess

high temperatures and fO2. Homogenization temperature of melt-fluid inclusions in garnet and clinopyroxene is in the range of 580 °C– 795 °C, and the data are consistent with the temperature of magma crystallization in the late stage. The temperatures of ore-forming fluid vary from 204 °C–597 °C to 89 °C–389 °C (Zheng, 2007), obtained by homogenization of fluid inclusions in garnet, clinopyroxene and calcite. The mineralization temperature ranges between 258 °C and 560 °C, with an average of 416 °C, obtained by decrepitation temperatures of 14 magnetite samples (Zhang et al., 1996). The pressure decreases from early stage of skarn to the late stage, varying from 52–130 MPa to 12–25 MPa (Zheng, 2007).

The high fO2 of ore-forming fluid affects the skarn mineral chemis- 3+ try. The iron is mostly in the form of Fe in high fO2 condition, and therefore it is incorporated within garnet rather than clinopyroxene. The experimental studies on skarn mineral formation also show that andradite forms in a more oxidizing condition than grossular (Liang, 1994). The garnets from this study show high-content of andradite (always N90%), and the clinopyroxenes contain relative low FeO. Con- siderable iron gets into andradite leading to a decrease in the scale of the magnetite ore body. The geological scenario in our study area is con- sistent with this, and the andradite rich skarns carry only small scale iron ores. Clinopyroxene is ubiquitously associated with the iron ore body, and occurs in the large iron deposits such as the Xishimen and Baijian deposits.

6.4. Metallogenic model

The metallogeny and magmatism in the Han-Xing region took place during 120–137 Ma (e.g. Chen et al., 2005, 2009; Li et al., 2013; Peng et al., 2004; Shen et al., 2013a; Xu et al., 2009; Zheng et al., 2007c), which is consistent with the timing of the large scale Mesozoic magmat- ic event and related metallogeny in the North China Craton (e.g. Chen et al., 2007, 2009; Li et al., 2013; Mao et al., 2005, 2011), which are also correlated to the destruction of the NCC (e.g., Gao et al., 2004, 2009; Jiang et al., 2005; Wu et al., 2005; Xu et al., 2009; Zhu et al., 2012), Early Cretaceous magmatism and mineralization have also been widely documented in several studies with the proposal that the lithospheric mantle under the eastern part of the NCC was under- going major changes during this stage (Guo et al., 2013, Li and Santosh, 2014). In the geodynamic setting of the destruction of the NCC, the lithosphere beneath the south TM became unstable, with magma derived from mantle emplaced into the crust. Mg–Fe rich magmas were generated through complex magma mixing, assimilation and contamination. The continuum from magma crystallization to min- eralization is summarized in the following stages: 1) low Fe/(Fe + Mg)

ratio clinopyroxene and amphibole crystallized from high fO2 magma, and Fe was enriched in the residual melt; 2) partially crystalline water-rich magma was emplaced rapidly, and Fe-rich fluid exsolved

from the magma with a decrease in water content; 3) high fO2 ore- forming fluid interacted with limestone and dolomitic limestone, and formed high-Fe garnet and low-Fe clinopyroxene, with the coeval for- Fig. 13. The compositional zoning of amphibole. (a) Photomicrograph of a zoned mation of different iron ore bodies. The above processes are schemati- amphibole grain with the analytical domain marked by circle. (b), (c), (d), (e), cally shown in Fig. 14. (f) The pressure, temperature, ΔNNO, Fe3+/Fe2+ and Mg/(Mg + TFe) of different sites in the zoned amphibole grain. The vertical gray bars show the position of bound- 7. Conclusion ary between the different compositional domains. See analytical data XX-1(1)-1 to 6 in Supp. Table 3. (1) The crystallization process of high-Mg diorites can be divided into two stages based on the mineral chemistry of amphibole. 212 J.-Q. Zhang et al. / Ore Geology Reviews 64 (2015) 200–214

Fig. 14. (a) P–T conditions from magma crystallization stage to hydrothermal mineralization stage. The P–T conditions of magma crystallization are estimated by amphibole composition (see Fig. 6 b, c, d). The temperatures of skarn and mineralization are according to Zheng (2007) and Zhang et al. (1996), and the pressure is the average of the data obtained by fluid inclusion (Zheng, 2007). (b) Schematic model of mineralization. See text for discussion.

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