Magnetite geochemistry of the Heijianshan Fe–Cu (–Au) deposit in Eastern Tianshan: Metallogenic implications for submarine volcanic-hosted Fe–Cu deposits in NW China

    Magnetite geochemistry of the Heijianshan Fe–Cu (–Au) deposit in Eastern Tianshan: Metallogenic implications for submarine volcanic-hosted Fe–Cu deposits in NW China Liandang Zhao, Huayong Chen, Li Zhang, Dengfeng Li, Weifeng Zhang, Chengming Wang, Juntao Yang, Xuelu Yan PII: DOI: Reference:

S0169-1368(16)30134-2 doi: 10.1016/j.oregeorev.2016.07.022 OREGEO 1886

To appear in:

Ore Geology Reviews

Received date: Revised date: Accepted date:

17 March 2016 23 June 2016 28 July 2016

Please cite this article as: Zhao, Liandang, Chen, Huayong, Zhang, Li, Li, Dengfeng, Zhang, Weifeng, Wang, Chengming, Yang, Juntao, Yan, Xuelu, Magnetite geochemistry of the Heijianshan Fe–Cu (–Au) deposit in Eastern Tianshan: Metallogenic implications for submarine volcanic-hosted Fe–Cu deposits in NW China, Ore Geology Reviews (2016), doi: 10.1016/j.oregeorev.2016.07.022

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ACCEPTED MANUSCRIPT Magnetite geochemistry of the Heijianshan Fe–Cu (–Au) deposit in Eastern Tianshan: Metallogenic implications for

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submarine volcanic-hosted Fe–Cu deposits in NW China

Liandang Zhao1, 2, Huayong Chen1,3＊, Li Zhang1, Dengfeng Li1, 2, Weifeng Zhang1, 2, Chengming Wang1, 2, Juntao Yang4, Xuelu Yan4

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1 Key Laboratory of Mineralogy and Metallogeny, Chinese Academy of Sciences, Guangzhou

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510640, China

2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China

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3 Guangdong Provincial Key Laboratory of Mineral Physics and Materials, 511 Kehua Street,

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Guangzhou 510640, China

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China

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4 No. 1 Geological Party Xinjiang Bureau of Geology and Mineral Exploration, Changji, 831100,

Corresponding author: [email protected]

Address: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, P.O. Box 1131, Tianhe District, Guangzhou 510640, Guangdong PRC

ACCEPTED MANUSCRIPT Abstract: The Heijianshan Fe–Cu (–Au) deposit is located in the Aqishan-Yamansu belt in Eastern Tianshan, NW China. As a typical Fe–Cu deposit in the region, Heijianshan is hosted in

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the Upper Carboniferous Matoutan Formation submarine volcanic / volcaniclastic rocks.

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Alteration styles, mineral assemblages and vein crosscutting relationships divide the hydrothermal alteration and mineralization processes into seven stages, namely the chromite (Stage I), epidote (Stage II), magnetite (Stage III), pyrite (Stage IV), Cu (–Au) (Stage V), late veins (Stage VI) and

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supergene (Stage VII) alteration / mineralization stages. Magnetite mineralization comprises the

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hematite (Stage III–A) and main magnetite (Stage III–B) mineralization sub-stages. The Heijianshan magnetite ores consist of massive (with mushketovite (MOM) or sulfides

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(MOS)), disseminated (DO) and magnetite clasts (with chromite (MWC) or without chromite

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(MNC)) ores. Magnetite in massive- and disseminated ores is featured by (1) Depletion in Zr, Nb

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and Ta; (2) Low Ti (< 2 wt.%) and Al (< 1 wt.%); (3) Ni/Cr ≥ 1, which all reflect a hydrothermal origin. Moreover, magnetite in massive- and disseminated ores has lower Cr (MOM: 0 – 13.2 ppm;

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MOS: 0 – 12.9 ppm; DO: 3.57 – 133 ppm) than magnetite clasts ores (MNC: 849 – 2,544 ppm; MWC: 835 – 44,132 ppm). However, the high Cr in the magnetite clasts ores may have been inherited from the chromite they replaced. From the magnetite clasts to disseminated / massive ores, formation temperature decreased and fO2 increased, which may represent the major controls on the formation of the various magnetite ore types. Compositions of the ore fluids and host rocks, formation of coexisting minerals and other physicochemical parameters (such as T and fO2) may have variably influenced the magnetite geochemistry in the different Heijianshan ore types, with fluid compositions probably playing the most important role. Discrimination diagrams, for instance, Cr vs. Co/Ni, Cr vs. Ti, V vs. Cr and Ni vs. Cr, may be

ACCEPTED MANUSCRIPT useful for magnetite mineralization type differentiation in other submarine volcanic-hosted Fe / Fe–Cu deposits in the Aqishan-Yamansu belt. Geochemical discrimination diagrams, alteration

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and mineralization paragenesis indicate that the Heijianshan Fe–Cu (–Au) deposit is best classified

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as an IOCG-like deposit, which offers a new insight for classifying and characterizing ore genetic types for similar Fe and Fe–Cu deposits in Eastern Tianshan.

Keywords: Magnetite, LA–ICP–MS, Fe–Cu (–Au) deposit, Aqishan-Yamansu belt, Eastern

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Tianshan, NW China

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1. Introduction

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The Eastern Tianshan of the Central Asian Orogenic Belt (CAOB) has been an important

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target area for iron (Fe) exploration in NW China (Fig. 1a and b; Cheng et al., 2008; Wang et al.,

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2006). A large number of Fe and Fe–Cu deposits, including the Hongyuntan, Bailingshan, Heijianshan, Yamansu, Heifengshan, Shuangfengshan and Shaquanzi deposits were discovered in

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Eastern Tianshan in the past decades (Fig. 1c; Fang et al., 2006a, b; Huang et al., 2013b; Jiang et al., 2002; Wang et al., 2005, 2007; Xiao, 2003; Zhang et al., 2006). Previous studies focused

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mainly on the petrology (Hou et al., 2014a, b; Lei et al., 2013; Li et al., 2011; Xu et al., 2014;

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Zhang et al., 2012, 2014) and ore deposit geology (Han and Zhao, 2003; Mao et al., 2005; Qin et

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al., 2005) of these Fe and Fe–Cu deposits, yet the genetic type(s) of these deposits are still controversial. For instance, the Bailingshan Fe deposit was variably argued to be of submarine

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volcanic-hosted type (Li and Li, 1999; Xu et al., 2011), ―hydrothermal metasomatic‖ type (W.F. Zhang et al., 2014) or skarn type (Mao et al., 2005). The Yamansu Fe deposit was also debated to

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be of skarn type (Hou et al., 2014b; Mao et al., 2005; Zeng et al., 2014) or submarine volcanic-hosted type (Li et al., 2014). Similarly, the Shaquanzi Fe–Cu deposit was variably proposed to be of skarn type (Mao et al., 2005), submarine volcanic-hosted type (Hou et al., 2014b) or IOCG type (Huang et al., 2013c). As a typical and important Fe–Cu mineralization system in Eastern Tianshan, the Heijianshan Fe–Cu (–Au) deposit, characterized by its abundant magnetite and less copper and gold resources, is still inadequately understood. Previous studies classified the Heijianshan Fe–Cu (–Au) deposit to be volcanic type (Han et al., 2002; Liu, 2008; Pan et al., 2005; Wang et al., 2008), ―deformed sedimentary‖ type (Cui et al., 2008), volcanic-sedimentary type (Ma and Chen, 2011), ―oxidation‖ type (Zhang, 2012), volcanic eruption-sedimentary type (Zhang,

ACCEPTED MANUSCRIPT 2000) or skarn type (Mao et al., 2005; Pirajno, 2012), with most types termed in Chinese Fe deposit classification and lack of detailed studies on the ore deposit. Furthermore, the regional

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Fe–Cu mineralization mechanism is still unclear and controversial, making detailed study of the

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Heijianshan Fe–Cu (–Au) deposit highly meaningful.

Magnetite is a key ore mineral in Fe-dominated deposits, including banded iron formation (BIF), Kiruna-type, magmatic Fe–Ti oxide, and Fe skarn deposits (Dupuis and Beaudoin, 2011;

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Huberty et al., 2012; Nadoll et al., 2012), as well as in other types of hydrothermal deposits such

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as iron oxide–copper–gold (IOCG) and porphyry Cu–Au systems (Liang et al., 2009; Williams et al., 2005), mostly being sub-economic. Magnetite contains trace amounts of Al, Ti, V, Si, Ca, Mn

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and Mg (Dupuis and Beaudoin, 2011; Nadoll et al, 2012), which can shed light on the various

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types of mineralization and ore-forming processes (e.g., distinguishing magnetite from fertile and

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barren mineralization systems; Beaudoin and Dupuis, 2009; Carew, 2004; Dare et al., 2012, Dupuis and Beaudoin, 2011; Müller et al., 2003; Nadoll et al., 2012; Rusk et al., 2009; Singoyi et

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al., 2006), and has been a focus of many research of magnetite-bearing mineralization systems (e.g., IOCG, skarn, poyphyry, BIF; Acosta-Góngora et al., 2014; Canil et al., 2016; Chen et al., 2015a, b; Chung et al., 2015; Dare et al., 2014a,b; Hu et al., 2015; Huang et al., 2015a, b; Makvandi et al., 2015; Zhao and Zhou, 2015), especially to fingerprint different deposit types in recent years (Beaudoin et al., 2007; Boutroy et al., 2014, Carew, 2004; Chen et al., 2015a, b; Chung et al., 2015; Dare et al., 2012; Duan et al., 2014; Gosselin et al., 2006; Hu et al., 2014a, b, 2015; Huang et al., 2014, 2015a, b; Liu et al., 2015; Nadoll et al., 2012, 2014a, b; Rusk et al., 2009; Singoyi et al., 2006; Zhao and Zhou, 2015). Magnetite geochemistry is influenced by the physicochemical parameters of the magmatic–hydrothermal system, including fluid compositions,

ACCEPTED MANUSCRIPT temperature and pressure, oxygen and sulfur fugacity (Nadoll et al., 2012), making it highly useful to constrain the physicochemical conditions of mineralization and to differentiate the genetic type

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of the deposits (Dupuis and Beaudoin, 2011). In Eastern Tianshan, LA–ICP–MS trace element

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geochemical data of magnetite from some Fe-dominated deposits were reported in recent years: Huang et al. (2013a) studied the magnetite composition from the Cihai Fe deposit and proposed that the magnetite may have been derived from magmatic–hydrothermal fluids related to mafic

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magmatism in a rift environment. Huang et al. (2013b, 2014) analyzed the magnetite from the

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Heifengshan, Shuangfengshan and Shaquanzi Fe–Cu deposits and argued that the magnetite there were formed from hydrothermal process rather than from magmatic fractionation. For the Tianhu

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Fe deposit, Huang et al. (2015a) proposed that its magnetite was derived from sedimentary process

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overprinted by later hydrothermal activities.

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In this paper, we present new information and data from the ore deposit geology and magnetite major- and trace element geochemistry, with the aims to characterize the magnetite from

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different ore types, and constrain the magnetite genesis and ore deposit type of the Heijianshan Fe–Cu (–Au) deposit. We suggest that the Heijianshan Fe–Cu (–Au) deposit would serve as an important example to offer new insights for other similar submarine volcanic-hosted Fe–Cu mineral occurrences in NW China.

2. Regional geology

The Central Asian Orogenic Belt (CAOB; Fig. 1a) is a tectonic collage of ophiolite suites, magmatic arc remnants, Precambrian massifs, and accretionary terranes (Sengör et al., 1993, 1996; Windley et al., 1990) formed from the Carboniferous to Early Triassic collisions among the Siberia and Tarim-Sino-Korean plates along the Solonker suture (Chen et al., 2007, 2009, 2012;

ACCEPTED MANUSCRIPT Xiao et al., 2010). The Eastern Tianshan, situated to the north of the Tarim Basin in North Xinjiang, is an important part of the CAOB (Fig. 1a and b) and has a complex tectonic framework

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(Fig. 1b; Charvet et al., 2007; Ma et al., 1997; Qin et al., 2002; Xiao et al., 2008).

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The Eastern Tianshan is divided into the Dananhu-Tousuquan Island Arc Belt, Kangguer Shear Zone, Aqishan-Yamansu Island Arc Belt and Central Tianshan Terrane (from north to south) by, respectively, the E–W-trending Kangguer, Yamansu and Aqikekuduke deep faults (Fig. 1c).

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The Dananhu-Tousuquan Island Arc Belt, bounded by the Dacaotan Fault in the north from

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the Haerlike Island Arc Belt, contains mainly Devonian to Carboniferous volcanic rocks and some plutons (Mao et al., 2005). This belt is metallogenically featured by hosting some important

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porphyry copper deposits, such as the Tuwu-Yandong and Linglong deposits (Shen et al., 2014;

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Wu et al., 2006). The Kangguer Shear Zone is an important regional structure that separates the

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Junggar Block from the Tarim Block (Mao et al., 2005), and mainly comprises ductile deformed and greenschist facies-metamorphosed Carboniferous volcaniclastic rocks (Xiao et al., 2004), as

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well as Permian mafic–ultramafic intrusions. The shear zone has hosted some Au deposits (e.g., Shiyingtan and Kangguer; Rui et al., 2002; Zhang et al., 2002) and Cu–Ni deposits (e.g., Huangshan and Huangshandong; Deng et al., 2014; Qin et al., 2009) in its western- and eastern parts, respectively. The Aqishan-Yamansu Island Arc Belt (Fig. 1c) contains mainly Carboniferous volcanic-, volcaniclastic- and clastic rocks overlain by the Permian clastic- and volcanic rocks with local carbonate interbeds (Mao et al., 2005). Late Carboniferous to the Early Triassic felsic intrusions were reported to have intruded the Carboniferous Aqishan-Yamansu strata (Li et al., 2002; Zhou et al., 2010), and the belt has hosted many important Fe- and Fe–Cu deposits, including Hongyuntan, Bailingshan, Chilongfeng, Heijianshan, Yamansu and Shaquanzi. The

ACCEPTED MANUSCRIPT Central Tianshan Terrane is composed of a Precambrian metamorphic basement (with gneiss, quartz schist, migmatite and marble) overlain by Paleozoic calc-alkaline basaltic andesite,

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volcaniclastic rocks, minor I-type granite and granodiorite (Huang et al., 2014; Liu et al., 2004;

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Mao et al., 2005). The terrane hosts many Pb–Zn (e.g., Caixiashan and Hongyuan), Ag-polymetallic (e.g., Jiyuan) and Fe (e.g., Alatage and Tianhu) deposits (Fig. 1c).

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3. Deposit geology

The Heijianshan Fe–Cu (–Au) deposit (ca. 150 km south of Hami, Xinjiang) (Fig. 1c) is a

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medium-sized (Chinese definition: 0.1 to 0.5 Mt Cu metal; XUARGS, 2003) Cu deposit. Recently,

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this deposit is mined as a Fe deposit, yet detailed metal resource data are still unknown due to lack

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of continuous exploration. Outcropping stratigraphy at Heijianshan comprises mainly the Upper Carboniferous Matoutan Formation with three members: The lowermost member contains

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(sedimentary) tuff and basalt, the middle member contains (brecciated) tuff and basalt, whilst the uppermost member contains mainly basaltic andesite porphyry (Fig. 2). The Heijianshan orebodies

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are mainly hosted in the middle member and the mineralized host rocks were commonly epidote, amphibole, chlorite and carbonate altered. The Heijianshan orebodies are commonly stratiform, with the largest one (30 – 50 m long and 8 – 10 m wide) striking 103° (NE-dipping) with dip angle of 45°. Ore types include sulfide, oxide and oxide-sulfide mix, with the latter two being the majority. Metallic minerals at Heijianshan are dominated by magnetite, hematite, and minor electrum and sulfides (including pyrite, pyrrhotite and chalcopyrite). Non-metallic minerals include mainly epidote, calcite, amphibole, chlorite, and minor K-feldspar, quartz, sphere, albite, barite, tourmaline and sericite. Magnetite ores commonly occur as massive, disseminated, magnetite clasts, and occasionally as

ACCEPTED MANUSCRIPT veinlets. Meanwhile, pyrite and chalcopyrite (–electrum) ores appear as disseminated and veinlets, occasionally as massive. Most minerals at Heijianshan are euhedral to subhedral with metasomatic

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textures commonly displayed, indicating a possible hydrothermal origin.

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Intermediate to felsic intrusive rocks at Heijianshan include granodiorite, quartz diorite, diorite porphyry, quartz syenite porphyry, porphyritic diabase dykes and minor monzonitic granite (Fig. 2). Based on the intrusive relationships, the diorite porphyry and quartz diorite postdate

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represent the youngest magmatic phase.

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quartz syenite porphyry, granodiorite and monzonitic granite, and the diabase porphyry dykes

Three fault systems and a small syncline were developed at Heijianshan. The oldest

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NW–NNW-trending faults crosscut the Matoutan Formation (second member) tuff, basalt and

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brecciated tuff. The younger NNE–NE-trending faults locally cut quartz diorite, while the

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youngest EW–NE-trending crosscut the diorite porphyry, quartz diorite, quartz syenite porphyry and monzonitic granite (Fig. 2).

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Based on the mineral assemblages and crosscutting relationships, seven alteration and mineralization stages are present at Heijianshan (Fig. 3). Stage I chromite (regarded to be magmatic and associated with ultramafic to mafic rocks; Thayer, 1964; Tian, 2015) occurs as euhedral–subhedral grains in the magnetite core and is commonly replaced or crosscut by later stages magnetite (Fig. 4a). Stage II epidote is widely distributed and occurs as hydrothermal veins or aggregates, and is replaced by Stage III magnetite and amphibole (Fig. 4b). Stage III (Fe mineralization stage) comprises two sub-stages, i.e., the hematite (Stage III–A) and the main magnetite mineralization (Stage III–B) sub-stages. Stage III–A is marked by mushketovite coexisting with quartz, whilst Stage III–B contains magnetite, amphibole, quartz, sphene, apatite

ACCEPTED MANUSCRIPT and minor K-feldspar and pyrite. The Stage III–B amphibole intergrew with magnetite (Fig. 4b and c) and coexisted with quartz. The Stage III magnetite is commonly cut by later stages pyrite

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(Fig. 4d) and chalcopyrite (Fig. 4e). Stage IV is dominated by pyrite and quartz, with which the

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pyrite intergrew with pyrrhotite and was replaced or crosscut by the Stage V disseminated / vein chalcopyrite. Stage V chalcopyrite veins (chalcopyrite – electrum – chlorite or chalcopyrite – quartz – hematite) locally cut Stage IV pyrite and Stage III massive magnetite. Stage VI veins

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contain epidote, calcite, hematite, quartz, chlorite, albite, specularite and minor tourmaline and

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barite, and commonly crosscut early stages pyrite and quartz (Fig. 4f). Stage VII supergene alteration minerals include hematite, malachite, atacamite, chrysocolla, digenite, bornite,

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chalcocite and limonite (Fig. 3).

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4. Sampling and analytical method

4.1. Sample description

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As mentioned above, the Heijianshan magnetite ores are composed of massive, disseminated, magnetite clasts and veinlets (relatively rare and thus not discussed here; Fig. 5a–d) ores, all formed at Stage III-B. The massive ores are divided into those with mushketovite (abbrev. MOM: Magnetite pseudomorphs after hematite; Fig. 5e) and those with sulfides (abbrev. MOS; Fig. 5f). In the MOM, some mushketovite are dendritic and coexist with quartz ± amphibole (Fig. 5e). For MOS, sulfides (pyrite and / or chalcopyrite) commonly coexist with quartz to replace or crosscut massive magnetite (Fig. 5f). In disseminated ores (DO), fine-grained magnetite associated with amphibole replaced the Stage II epidote (Fig. 5g). The magnetite clasts ores (Fig. 5c) are divided into those with chromite (MWC) and those without it (MNC). In these two magnetite clasts ore

ACCEPTED MANUSCRIPT sub-types, fine-grained magnetite aggregates into different sized clasts and replaced the host rocks (Fig. 5h) and / or pre-existing chromite and epidote. The magnetite clasts in MNC and MWC ores

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are 0.3 – 1.2 cm and 1.0 – 10 cm in size, commonly 0.6 – 0.8 cm and 1.8 – 2.5 cm, respectively.

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Commonly in MWC ores, chromite (Fig. 4a) is rimmed by magnetite, suggesting that magnetite was formed after chromite. Nine representative magnetite samples from the different ore types were analyzed for their magnetite chemistry by means of Electron Microprobe Analysis (EMPA)

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and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA–ICP–MS), with the aims

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to explore the mineralization characteristics in the different magnetite ore types.

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4.2. EMPA

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Magnetite major element geochemistry (Table 1) was analyzed at the Testing Center of China

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Metallurgical & Geological Bureau (Shandong office) and the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS) with a JEOL JXA-8230 Superprobe and

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a JEOL JXA-8100 Superprobe, respectively. The operating conditions are as follows: 15 kV accelerating voltage, 20 nA beam current, 1–5 µm beam diameter, 10 s counting time and ZAF correction procedure for the data reduction. For the same mineral type, analytical results from the two laboratories were consistent. Back-scattered Electron (BSE), Energy Dispersive Spectrometer (EDS) and multi-element imaging with a Field Emission Gun Scanning Electron Microscope (FEG-SEM) were also performed at the Testing Center of China Metallurgical & Geological Bureau (Shandong office), with an acceleration voltage of 20 keV and a beam current of 2.0 nA.

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Magnetite trace element geochemistry was analyzed by a pulsed Resonetic 193 nm laser

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ablation system coupled with an Agilent 7500a ICP–MS at the GIGCAS. Detailed operating

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conditions and data reduction methods were described in Liu et al. (2008). Helium was applied as a carrier gas and argon used as the makeup gas and mixed with the carrier gas via a Tconnector

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before entering the ICP. Each analysis includes a background acquisition of approximately 20 s (gas blank) followed by 40 s data acquisition from the sample. Analytical spots (23 μm) were

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ablated by 160 successive laser pulses (4 Hz). Element contents were calibrated against multiple reference materials (KL2-G and ML3B-G) using

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Fe as the internal standard (Liu et al., 2008).

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Every four–five samples analyses were followed by one analysis of KL2-G as quality control to correct the time-dependent drift of sensitivity and mass discrimination. Off-line selection and

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integration of background and analytical signals, and time drift correction and quantitative calibration were performed by ICPMSDataCal (Liu et al., 2008).

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5. Results

5.1. EMPA

Magnetite from massive, disseminated and magnetite clasts ores have the FeOT of 88.80 – 95.02 wt.% (Table 1). Based on 32 ions of oxygen atoms, magnetite in the Heijianshan deposit has an average ratio of 0.60 for Fe2+/Fe3+ in atoms per formula unit (apfu), with the FeO and Fe2O3 contents being 30.10 – 32.95 wt.% and 65.00 – 69.93 wt.%, respectively, consistent with the standard magnetite composition (i.e., 31.03 wt.% FeO and 68.97 wt.% Fe2O3). The different magnetite ore types have a positive Fe2O3 vs. FeOT correlation (Fig. 6a).

ACCEPTED MANUSCRIPT In MOM ores, the magnetite contains low V2O5 (0.02 – 0.04 wt.%, average: 0.02 wt.%) and Cr2O3 (0.02 – 0.05 wt.%, average: 0.03 wt.%) and high SiO2 (0.80 – 1.08 wt.%, average: 0.85

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wt.%; Fig. 6). Contents of Na2O, CaO, MgO, Al2O3, MnO and TiO2 are of 0.02 – 0.05 wt.%, 0.10

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– 0.22 wt.%, 0.01 – 0.06 wt.%, 0.17 – 0.47 wt.%, 0.06 – 0.11 wt.% and 0.04 – 0.07 wt.%, with the corresponding average values of 0.03 wt.%, 0.15 wt.%, 0.04 wt.%, 0.26 wt.%, 0.08 wt.% and 0.06 wt.% (Table 1), respectively. For MOS ores, the magnetite contains low V2O5 (0.01 – 0.04 wt.%,

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average: 0.03 wt.%) and Cr2O3 (0.01 – 0.07 wt.%, average: 0.04 wt.%) and moderately high SiO2

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(0.16 – 1.10 wt.%, average: 0.58 wt.%; Fig. 6). Contents of Na2O, CaO, MgO, Al2O3, MnO and TiO2 are of 0.02 – 0.09 wt.%, 0.03 – 0.14 wt.%, 0.02 – 0.05 wt.%, 0.02 – 0.16 wt.%, 0.03 – 0.07

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wt.% and 0.01 – 0.05 wt.%, with the corresponding average values of 0.05 wt.%, 0.07 wt.%, 0.04

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wt.%, 0.10 wt.%, 0.04 wt.% and 0.03 wt.% (Table 1), respectively.

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Compared with other ore types, the magnetite from disseminated ores has higher V2O5 (0.13 – 0.24 wt.%, average: 0.19 wt.%) and lower Cr2O3 (0.01 – 0.06 wt.%, average: 0.03 wt.%) and

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SiO2 (0.02 – 1.00 wt.%, average: 0.33 wt.%; Fig. 6). Contents of Na2O, CaO, MgO, Al2O3, MnO and TiO2 are of 0.01 – 0.08 wt.%, 0.01 – 0.67 wt.%, 0.01 – 0.05 wt.%, 0.02 – 0.46 wt.%, 0.02 – 0.09 wt.% and 0.01 – 0.09 wt.%, with the corresponding average values of 0.03 wt.%, 0.15 wt.%, 0.03 wt.%, 0.12 wt.%, 0.06 wt.% and 0.06 wt.% (Table 1), respectively. Magnetite from the MNC ores contains moderately high V2O5 (0.10 – 0.18 wt.%, average: 0.13 wt.%), Cr2O3 (0.12 – 0.29 wt.%, average: 0.18 wt.%) and SiO2 (0.24 – 1.15 wt.%, average: 0.59 wt.%; Fig. 6). Contents of Na2O, CaO, MgO, Al2O3, MnO and TiO2 are of 0.01 – 0.03 wt.%, 0.01 – 0.31 wt.%, 0.02 – 0.29 wt.%, 0.01 – 0.16 wt.%, 0.19 – 0.24 wt.% and 0.02 – 0.09 wt.%, with the corresponding average values of 0.02 wt.%, 0.08 wt.%, 0.08 wt.%, 0.09 wt.%, 0.12 wt.%

ACCEPTED MANUSCRIPT and 0.05 wt.% (Table 1), respectively. For the MWC ores, magnetite has relatively low V2O5 (0.05 – 0.06 wt.%, average: 0.06 wt.%) and SiO2 (0.36 – 0.66 wt.%, average: 0.48 wt.%), and high

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Cr2O3 (0.20 – 1.15 wt.%, average: 0.52 wt.%; Fig. 6). Contents of Na2O, CaO, MgO, Al2O3, MnO

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and TiO2 are of 0.03 – 0.04 wt.%, 0.04 – 0.08 wt.%, 0.05 – 0.09 wt.%, 0.08 – 0.10 wt.%, 0.10 – 0.16 wt.% and 0.06 – 0.22 wt.%, with the corresponding average values of 0.03 wt.%, 0.06 wt.%,

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0.06 wt.%, 0.09 wt.%, 0.13 wt.% and 0.13 wt.% (Table 1), respectively.

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5.2. LA–ICP–MS

The LA–ICP–MS analytical results were listed in the Appendix A and illustrated in Figures

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7–9.

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5.2.1. Magnetite from massive ores

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The two massive magnetite ore (MOM and MOS) sub-types have only minor geochemical differences between them. Generally, there are positive correlations of Al vs. Mg, Mn vs. Mg, Si

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vs. Mg, Zn vs. Mg, and Ga vs. Al (Fig. 8), which may indicate that the elements substituted Fe2+ and Fe3+ in magnetite without changing the crystal texture. Magnetite from the massive ores have narrow ranges of Al (580 – 3,151 ppm), Mn (476 – 1,540 ppm), Mg (112 – 1,939 ppm), Zn (18.6 – 94.2 ppm), Ga (2.40 – 10.8 ppm), V (25.0 – 152 ppm), Cr (0 – 13.2 ppm), and wide ranges of Cu (0 – 254 ppm) and Si (1,119 – 111,599 ppm) (Appendix A). Magnetite from MOM have narrower ranges of Mg (155 – 1,939 ppm, average: 545 ppm) and Co (19.6 – 31.6 ppm, average: 28.1 ppm) than from MOS (For Mg: 112 – 1,455 ppm (average: 493 ppm); For Co: 4.02 – 31.9 ppm (average: 19.7 ppm)). However, magnetite from the massive ores has considerably lower Mg + Mn (MOM: 689 – 3,274 ppm, average: 1,260 ppm; MOS: 813 – 2,518 ppm, average: 1,318 ppm; Fig. 9a) than

ACCEPTED MANUSCRIPT other ore types (DO: 945 – 36,025 ppm, average: 9,835 ppm; MNC: 1,061 – 15,194 ppm, average: 4,247 ppm; MWC: 12,786 – 20,748 ppm, average: 16,664 ppm). The Heijianshan magnetite has

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commonly relatively low Cu (< 20 ppm), despite some high Cu (> 20 ppm; Appendix A)

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magnetite was found in MOM. Furthermore, compared to other magnetite ore types, the magnetite from massive ores has lower Cr (0 – 13.2 ppm, average: 4.74 ppm), V (25.0 – 152 ppm, average: 59.2 ppm), Ni (8.23 – 33.3 ppm, average: 18.4 ppm) and Ti (29.4 – 1,114 ppm, average: 197 ppm),

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5.2.2. Magnetite from disseminated ores

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but higher Co/Ni (0.14 – 3.47, average: 1.75) and Ni/Cr (0.90 – 10.2, average: 3.61; Fig. 9).

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Magnetite from disseminated ores (DO) has positive elemental correlations of, e.g., Al vs.

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Mg, Mn vs. Mg, Si vs. Mg, Zn vs. Mg, and Ga vs. Al (Fig. 8), similar to the massive magnetite.

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However, the distinct positive correlations of Co vs. Mg, Co vs. Ni and Co vs. Si (Fig. 8f–h) in DO are absent in the massive ores. These DO magnetite has wide ranges of Al (361 – 28,070 ppm), Mg (124 – 32,638 ppm), Si (831–38,790 ppm), Mg + Mn (945 – 36,026 ppm), and narrow ranges

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of Mn (785 – 3,387 ppm), Zn (43.1 – 335 ppm), Cu (0 – 38.8 ppm), Ni (100 – 138 ppm) and Co (14.3 – 28.9 ppm) (Appendix A). Meanwhile, magnetite in disseminated ores has low Co/Ni (0.10 – 0.25, average: 0.16) and Ti (27.2 – 890 ppm, average: 268 ppm), medium Cr (3.57 – 133 ppm, average: 58.5 ppm), high V (925 – 1,602 ppm, average: 1,300 ppm), Ni (100 – 138 ppm, average: 119 ppm) and Ni/Cr (0.91 – 36.9, average: 6.22), resembling typical hydrothermal magnetite (Ti < 2 wt.%, Ni/Cr ≥ 1; Dare et al., 2014b; Fig. 9) and similar to magnetite in the Heijianshan massive ores.

ACCEPTED MANUSCRIPT 5.2.3. Magnetite in magnetite clasts

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Magnetite from the magnetite clasts ores is mostly enriched in Mn, Zn, V and Cr and

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depleted in Nb, Mg, Ti and Co, with the magnetite from the magnetite clasts with chromite (MWC)

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unusually enriched in Zr and Sc than other ore types (Fig. 7). Magnetite from the magnetite clasts ores has commonly positive elemental correlations of Al vs. Mg, Mn vs. Mg, Si vs. Mg, Zn vs. Mg

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and Ga vs. Al (Fig. 8) as other ore types do, and share the same positive correlations with disseminated ores, e.g., Co vs. Mg, Co vs. Ni and Co vs. Si, especially for that in the magnetite

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clasts without chromite (MNC) (Fig. 8). In the magnetite, Al (242 – 72,462 ppm), Mg (144 – 18,446 ppm), Si (1,252 – 100,946 ppm), Mn (821 – 12,612 ppm), Zn (29.2 – 13,170 ppm) and Mg

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+ Mn (1,061 – 20,748 ppm) have wide ranges, whereas Ga (2.46 – 18.7 ppm), Co (13.7 – 65.2 ppm) and Ni (109 – 174 ppm) show narrow ranges (Appendix A). Meanwhile, the magnetite from

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MWC has higher Co/Ni (0.31 – 0.54, average: 0.40), Ti (1,817 – 17,073 ppm, average: 11,024 ppm) and Cr (835 – 44,132 ppm, average: 15,743 ppm) than massive- and disseminated ores. All

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magnetite from the magnetite clasts has relatively low Ni/Cr (0 – 0.19), consistent with typical magmatic magnetite (Ni/Cr = 0.09 – 1.09; Dare et al., 2014b) and lower than typical hydrothermal magnetite (Ni/Cr = 0.27 – 13.9; Dare et al., 2014b).

6. Discussion

6.1. Genesis of magnetite at Heijianshan

In this study, it is found that some elements (e.g., Cr, Co, Ni, Ti and V) can effectively discriminate the different magnetite ore types without much compositional overlaps (Fig. 9b–e). Among the various magnetite ore types at Heijianshan, both magmatic- and hydrothermal

ACCEPTED MANUSCRIPT magnetite were distinguished from the Ni vs. Cr and Ti vs. Ni/Cr diagrams (Fig. 9e and f), which were mainly summarized by Dare et al. (2012, 2014b) from different ore deposit types. The high

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Ni/Cr (≥ 1) in the magnetite from the massive- and disseminated ores was also reported in many

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hydrothermal systems (e.g., El Laco massive magnetite: Ni/Cr > 10; Dare et al., 2014a), in which the magnetite ore fluids have higher solubility of Ni than Cr (Dare et al., 2014a). The depletions of Ti (< 2 wt.%), Al (< 1 wt.%), Zr (0 – 13.4 ppm), Nb (0 – 0.71 ppm) and Ta (0 – 0.06 ppm) in most

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magnetite from the massive- and disseminated ores also suggest hydrothermal magnetite affinities

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(Ray and Webster, 2007; Dupuis and Beaudoin, 2011; Nadoll et al., 2012, 2014a; Dare et al., 2014b). However, although the high Cr (835 – 44,132 ppm) and Ni (109 – 174 ppm) in the

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magnetite from the magnetite clasts ores can have a magmatic origin (Dare et al., 2012), Cr can

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also be introduced into the hydrothermal system during chromite replacement (Fig. 4a) and

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substitution of Fe3+ / Fe2+ in the magnetite crystal lattice (Dupuis and Beaudoin, 2011). The metasomatic texture of magnetite in the magnetite clasts ores (Fig. 4a) also supports a

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hydrothermal origin. All the Heijianshan magnetite is Ni, V, Co, Zn and Mn enriched (Fig. 7), indicating that the magnetite may have formed from high-temperature hydrothermal fluids (ca. 500 – 700 °C) associated with a magmatic–hydrothermal source (e.g., porphyry and IOCG deposits; Dare et al., 2014b). Moreover, the Heijianshan magnetite shares similar bulk continental crust-normalized (Rudnick and Gao, 2003) geochemical features, including enrichments in Mn and Zn, and depletions in high field strength elements (HFSEs; e.g., Zr, Nb and Ta) (Fig. 7), and the latter mimics typical hydrothermal magnetite (Dare et al., 2014b). Therefore, we propose that the Heijianshan magnetite (massive-, disseminated- and most magnetite clasts ores) are hydrothermal.

ACCEPTED MANUSCRIPT 6.2. Factors controlling magnetite compositions

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Magnetite from different deposit types commonly contains characteristic trace element

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signatures, as influenced by the ore-forming fluids / melts compositions and the magnetite genetic

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mechanisms (Dare et al., 2014b; Frietsch and Perdahl, 1995; Nadoll et al., 2014a, b; Nystroem and Henriquez, 1994; Toplis and Corgne, 2002). Magmatic magnetite geochemistry is primarily

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controlled by: (1) Magma composition, (2) Temperature (T), (3) Pressure (P), (4) Cooling rate, (5) Oxygen and / or sulfur fugacity (fO2 and / or fS2), and (6) Silica activity (Chen et al., 2015b;

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Nadoll et al., 2014a). While the composition of hydrothermal magnetite is mainly controlled by: (1) Fluid compositions, (2) Host rock compositions, (3) Coexisting minerals, (4) Temperature, (5)

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Pressure, and (6) Oxygen fugacity (fO2) (Carew, 2004; Chen et al., 2015b; Dare et al., 2014b; Dupuis and Beaudoin, 2011; Frietsch and Perdahl, 1995; Nadoll et al., 2014a; Nystroem and

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Henriquez, 1994; Toplis and Corgne, 2002), which may have been the case for many (if not most) hydrothermal magnetite at Heijianshan.

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Before discussing controlling factors for trace element concentrations in magnetite, we conducted some BSE imaging work to clarify the texture relationships and detect the hydrothermal alteration in magnetite grains. Magnetite is commonly homogeneous under the BSE imaging, and only minor magnetite grains have apparently different intensities under BSE, showing different magnetite generations or compositions (Fig. 10). Although those magnetite grains with obviously different intensities have been observed, e.g., generation-1 magnetite (Mag-1) was replaced by generation-2 magnetite (Mag-2) under BSE, this type of magnetite was only detected in one sample from MOS ores (Fig. 10a and b). Magnetite grains with different dark- and light rims under BSE are locally observed, and they sometimes were replaced by later hematite (Fig. 10c and

ACCEPTED MANUSCRIPT d). Those magnetite grains may suggest different concentrations of trace elements (e.g., Si, Ca, Mg, Ti; Dare et al., 2014a, b; Hu et al., 2014; Knipping et al., 2015). However, due to small size

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(commonly < 10 μm as the Fig. 10 illustrated), they are impossible for further analysis by

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LA–ICP–MS and therefore we cannot explore their differences between the different intensity rims, which could result the inconsistent analytical results from the same sample and also the local differences between EMPA and LA–ICP–MS analyses. Nevertheless, only few texture differences

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were observed and most of data are from the homogeneous magnetite grains (Fig. 10e and f), we

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consider that the contribution from inconsistent analysis is minor. Therefore, we suggest that the EMPA and LA–ICP–MS analytical results of magnetite can be assumed to be obtained from the

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same magnetite generation (mainly by homogeneous magnetite) in this paper.

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6.2.1. Factor I: Fluid and host rock compositions

Fluid-rock interactions and primary mineral replacement can inherit hydrothermal magnetite

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with some geochemical features of the altered host rocks or minerals (Carew, 2004; Nadoll et al., 2014a), which also can be illustrated by BSE images of magnetite (Fig. 10). The Heijianshan Stage III magnetite was intergrowth with amphibole and replaced Stage II epidote (Fig. 4b and Fig. 5g), and is in a mineral assemblage of magnetite – amphibole – quartz – sphene – apatite – pyrite ± K-feldspar (Fig. 3). Early epidote alteration may contribute Si, Ca and Mg to the magnetite. The more variable Mg + Mn contents in the magnetite from the disseminated (945 – 36,026 ppm) and magnetite clasts (1,061 – 20,748 ppm) ores than massive ores (689 – 3,274 ppm) may indicate the role of fluid-rock interaction is more easily influenced by the surrounding fluids in disseminated ores and magnetite clasts ores to substitute Fe for Mg, Mn, Si and Al from altered host rocks (Fig.

ACCEPTED MANUSCRIPT 9a). Fluid inclusion study showed that the Stage III magnetite ore fluids are high in Na, Ca, Mg and Fe, high temperatures (> 300 °C) and medium–high salinities (up to 56.0 wt.% NaCl eqv.,

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consistent also with the magnetite trace elements characters.

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unpublished data). The high Ca and Mg may have been parts of the Stage II epidote inheritance,

The high Ni (109 – 174 ppm) and Cr (835 – 44,132 ppm) of the magnetite from the magnetite clasts ores may have been contributed by the Stage I chromite replacement. The deceasing Cr2O3

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and increasing of FeOT in residual chromite from core to rim (Table 1) also support that the Cr and

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Fe exchanged between chromite and magnetite during replacement, leading to variable Cr2O3 and FeOT contents in chromite and high Cr in magnetite.

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6.2.2. Factor II: Precipitation of coexisting minerals

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Micro-inclusions (Fig. 11) in magnetite can influence the bulk magnetite geochemistry detected. The few anomalously high Mn, Si and Zn (Figs. 8b–d and 11) magnetite grains may

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point to the presence of Mn-bearing phases, silicate and / or sphalerite inclusions. Magnetite geochemistry may also be affected by co-precipitating minerals due to the competition for elements among them (Dare et al., 2014b; Nadoll et al., 2014a). For instance, Co, Ni, Cu (relative to Nb and Mo) and Zn are preferentially partitioned into co-precipitating sulfides (e.g., pyrite) rather than into magnetite (Dare et al., 2014b). At Heijianshan, the intergrowth of minor Stage III pyrite with magnetite may have led to the depletion of Cu (relative to Mo) in the latter (Cu: 0.01 – 5.09; Mo: 0.35 – 28.77; bulk continental crust-normalized; Rudnick and Gao, 2003), which also indicates sulfide saturation in the ore-forming fluids. However, some higher Cu content (> 20 ppm) in the Heijianshan magnetite ores than many other deposits, e.g., the Lala

ACCEPTED MANUSCRIPT IOCG deposit (oxide stage: 0 – 2.57 ppm; sulfide stage: 0 – 18.5 ppm; Chen et al., 2015a) and Yinachang IOCG deposit in SW China (0.01 – 3.04 ppm; Chen et al., 2015a), Sokoman Iron

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Formation in Canada (0 – 12.4 ppm; Chung et al., 2015), Tengtie skarn Fe deposit in south China

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(0 – 1.28 ppm; Zhao and Zhou, 2015), Baima Fe–Ti deposit in SW China (0 – 21.3 ppm; Liu et al., 2015), indicate sulfide saturation was not significant during the Heijianshan magnetite

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mineralization.

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6.2.3. Factor III: Temperature and fO2

The effects of temperature (T), pressure (P) and oxygen fugacity (fO2) on the hydrothermal

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magnetite trace element geochemistry are yet to be well constrained (Huang et al., 2015a; Ilton

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and Eugster, 1989; Nadoll et al., 2014a; Toplis and Corgne, 2002). Ilton and Eugster (1989)

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demonstrated experimentally that in a system containing magnetite and supercritical chloride-rich fluids (600 – 800 °C at 2 kb), Cu, Zn, and Mn are preferentially partitioned into magnetite at lower

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temperatures. Although showing some higher Cu contents, most Heijianshan magnetite has Cu < 20 ppm, which suggests that the magnetite may have formed from high temperature fluids, as also indicated by the oxygen isotope geothermometry (ca. 590 °C; unpublished data). Two magnetite grains have high Cu (137 ppm and 254 ppm), which may be explained by the presence of fine Cu-bearing inclusions. Titanium in Fe oxides is regarded to be positively correlated with temperature (Dare et al., 2012; Huang et al., 2013a; Nadoll et al., 2012). In the (Al + Mn) vs. (Ti + V) diagram (Fig. 12a), magnetite in the different Heijianshan ore types shows decreasing temperature from magnetite clasts ores, through disseminated ores to massive ores, a trend corresponds well with the decreasing depth of occurrence of these ore types. The relatively lower

ACCEPTED MANUSCRIPT temperature may provide higher precipitating rates for magnetite formation and be inclined to aggregating to massive ores.

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Vanadium in magnetite can trace the melt / liquid evolution and identify magma

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replenishment and mixing (Barnes et al., 2004; McCarthy and Cawthorn, 1983; Namur et al., 2010; Tegner et al., 2006). Among the many valence states of V (+3, +4 and +5) in nature fluids, only V3+ can be easily incorporated into the magnetite crystal lattice (Bordage et al., 2011; Nadoll et al.,

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2014a; Toplis and Corgne, 2002). Namely, incorporation of V3+ into magnetite increases with

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decreasing fO2 (Toplis and Corgne, 2002). Subsequently, the high V in the Heijianshan magnetite suggested that the ore fluids may have had low fO2 (Toplis and Corgne, 2002). The Heijianshan

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magnetite in the disseminated- and magnetite clasts ores has higher V (925 – 1,602 ppm and 505 –

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1,323 ppm, respectively) than the massive ores, indicating lower fO2 in the former two ore types

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(Fig. 12b).

In summary, during the magnetite formation, Fe-dominated magmatic–hydrothermal ore

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fluids may have reacted with altered host rocks (tuff and brecciated tuff) to form the various magnetite ore types with slightly variable temperature and fO2. The Heijianshan magnetite may have inherited some geochemical signatures from the altered host rocks (e.g., for Si, Al and Cr) and / or minerals (e.g., for Si, Ca, Mg and Al) and competed for some elements (e.g., Si, Mn, Zn and Cu) with the co-precipitating minerals during fluid-rock interactions.

6.3. Classification of the Heijianshan Fe–Cu (–Au) deposit type

Heijianshan had experienced extensive pre-mineralization alteration (Fig. 3), resembling typical IOCG deposits (Williams et al., 2005; Chen, 2013) but distinct from typical skarn Fe

ACCEPTED MANUSCRIPT deposits (which epidote alteration is syn-mineralization; Zhu et al., 2015). Although the pre-mineralization alteration styles are different, i.e., Ca–Mg alteration (epidote – calcite –

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tourmaline – sericite, also supported by our unpublished H–O isotopes) for Heijianshan and

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Na–Ca alteration for IOCG deposits, such difference can be generated by the different compositions of hydrothermal fluids and host rocks. The Heijianshan deposit also mimics some IOCG deposits (e.g., Mina Justa (Peru), Raúl-Candestable (Peru), Mantoverde (Chile) and La

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Candelaria (Chile); Chen et al., 2011) in its assemblage of magnetite, pyrite and Cu (–Au)

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mineralization (characterized by quartz + chalcopyrite + hematite and chalcopyrite + electrum + chlorite veins). The different ore mineral assemblages and their paragenesis between Heijianshan

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and typical skarn Fe deposits (Meinert, 1992; Meinert et al., 2005) and the absence of clear

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intrusion-mineralization relationship in the former further suggest that Heijianshan is not a skarn

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deposit.

Magnetite geochemistry can be used to discriminate different deposit types. At Heijianshan,

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magnetite from the massive-, disseminated- and (possibly most) magnetite clasts ores are hydrothermal and have similar geochemical signatures as IOCG and skarn deposits (Fig. 7). In the Ti/V vs. Ti discrimination diagram (Fig. 13a), the Heijianshan data are mainly plotted within or nearby the IOCG field, similar to the Sossego IOCG deposit in Brazil (Monteiro et al., 2008), implying that Heijianshan has more IOCG affinities. In the diagrams of Ca + Al + Mn vs. Ti + V (Fig. 13b), the data are mostly plotted in the fields of skarn, IOCG and porphyry deposits, which indicate it’s clearly a magmatic–hydrothermal deposit but is not distinguishable among these major deposit types. The confused discrimination phenomena also can be found in Ernest Henry IOCG deposit in Australia (Carew, 2004; Dupuis and Beaudoin, 2011), Vegas Peledas Fe skarn

ACCEPTED MANUSCRIPT deposit in Argentina (Pons et al., 2009), Tengtie Fe skarn deposit in China (Zhao and Zhou, 2015), Pipe Mine BIF deposit in Canada (Dare et al., 2014b) and Sokoman BIF deposit in Canada

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(Chung et al., 2015). As we have discussed before, many evidences show that Heijianshan is not

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likely a skarn deposit, therefore we consider that using magnetite discrimination diagram must be based on detailed description of mineralogy and textural relationships, alteration and mineralization paragenesis, and chemistry of fluids to precipitate magnetite. The current magnetite

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chemistry diagrams for ore deposit type discriminations still need refining and improvement based

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on more well-studied deposit cases (Chen and Han, 2015).

Therefore, we conclude that the Heijianshan deposit is best classified as an IOCG-like

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deposit, which may provide a new insight for classifying and characterizing similar submarine

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volcanic-hosted Fe (–Cu) deposits in the Aqishan-Yamansu belt.

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6.4. Comparison and implication for Fe mineralization in Eastern Tianshan

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In order to further explore the Fe mineralization in this belt of the Eastern Tianshan, we compare the Heijianshan data to those of the Heifengshan, Shuangfengshan and Shaquanzi Fe (–Cu) deposits in the Aqishan-Yamansu belt and the Tianhu Fe deposit in Central Tianshan. In Figure 14, these deposits have similar Ti, Co and Ga, but different Mg, Al, V, Cr, Mn, Ni and Zn, indicating that the magnetite mineralization may have occurred at similar temperature (inferred by similar Ti) but different fluid-rock interactions (inferred by different Mg, Al, Mn and Zn). Furthermore, Huang et al. (2013c) reported pyrite Re–Os ages and stable isotopes from Heifengshan, Shuangfengshan and Shaquanzi, and proposed that those Fe (–Cu) deposits were formed at ca. 296 Ma in a back-arc extensional setting. Recently, Zhang et al. (2015) reported that

ACCEPTED MANUSCRIPT the Aqishan-Yamansu belt was a back-arc or intra-arc basin during ca. 350 – 325 Ma, followed by basin inversion (ca. 325 – 300 Ma) and felsic–intermediate magmatism emplacement, which is

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similar to the Mesozoic Central Andes, indicating a Late Carboniferous – Early Permian Fe (–Cu)

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mineralization event in the Aqishan-Yamansu belt. Although those deposits may form in the similar ages and tectonic settings, variable concentrations of trace elements in magnetite, e.g., Zn, V, Ni, Mn and Al, also can be observed and used to distinguish them (Fig. 15). Of those

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discriminating diagrams, magnetite grains from massive magnetite ores at Heijianshan have little

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overlaps with magnetite ores in other deposits (Fig. 15), indicating close geochemical characteristics between the Heijianshan massive magnetite ores and magnetite ores in the

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Heifengshan, Shuangfengshan, Shaquanzi and Tianhu deposits. Those obvious discriminating

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diagrams of Zn vs. V, Zn vs. Ni, Ni vs. V, Ni vs. Mn, Ni vs. Al and V vs. Al (Fig. 15), which

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mainly show different fO2 (e.g., V), fluid-rock interaction (e.g., Al, Mn, Zn) and nature of fluids (e.g., Ni) during magnetite formation, may be applied to other Fe and Fe–Cu deposits in the

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Aqishan-Yamansu belt or even the Eastern Tianshan.

7. Conclusions

Magnetite from the massive-, disseminated- and magnetite clasts ores at Heijanshan has markedly different geochemistry, and can be clearly discriminated by the Cr vs. Co/Ni, Cr vs. Ti, V vs. Cr and Ni vs. Cr diagrams. Formation of these different magnetite ore types may have been influenced by the interactions between the Fe-dominated ore fluids and the host rocks, which precipitated magnetite with decreasing temperature under variable fO2. The ore fluids and host rock compositions, influence of the coexisting minerals and other physicochemical parameters (such as T and fO2) may have altogether controlled the magnetite geochemical variation in the

ACCEPTED MANUSCRIPT different ore types, with fluid compositions being the most important parameter. Alteration, mineralization mineral paragenesis and magnetite geochemistry altogether suggest

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that the Heijianshan Fe–Cu (–Au) deposit is best classified as an IOCG-like deposit, which

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provides a new insight for classifying and characterizing similar submarine volcanic-hosted Fe / Fe–Cu deposits along the Aqishan-Yamansu belt. Besides, the Cr vs. Co/Ni, Cr vs. Ti, V vs. Cr and Ni vs. Cr discrimination diagrams may also be applicable to differentiate the magnetite ores

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from other submarine volcanic-hosted Fe / Fe–Cu deposits in Eastern Tianshan.

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Acknowledgments

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This study was funded by the National Natural Science Foundation of China (41572059),

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Chinese National Basic Research 973-Program (2014CB440802), Creative and Interdisciplinary Program, CAS (Y433131A07), CAS/SAFEA International Partnership Program for Creative

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Research Teams (20140491534) and the Xinjiang Major Basic Research Project (201330121). We thank Drs. Bing Xiao and Congying Li (Guangzhou Institute of Geochemistry, Chinese Academy

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of Sciences) for their help in the EMPA and LA–ICP–MS analyses. We are also grateful to the Xinjiang No. 1 Geological Team for their support in the field work. Two anonymous reviewers and Prof. Bo Wang (Guest Editor) have provided insightful comments for an earlier version of the manuscript. This is contribution No. IS-2281 from GIGCAS.

Appendix A. Supplementary data

Supplementary data of the full LA–ICP–MS analytical results (in ppm) for the Heijianshan magnetite is given in the Excel file.

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Figure captions

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Fig. 1. (a). Tectonic framework of the Central Asian Orogenic Belt (CAOB; modified after Sengör

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and Natal, 1996); (b). Tectonic map of North Xinjiang (simplified after Chen et al., 2012); (c).

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Geological map of the Eastern Tianshan Orogenic Belt and mineral deposit distribution (modified

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after Wang et al., 2006; Deng et al., 2014).

Fig. 2. Geological map of the Heijianshan Fe–Cu (–Au) deposit (modified after Xinjiang Uygur

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Autonomous Region Geological Survey, 2003).

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Fig. 3. Alteration and mineralization paragenesis of the Heijianshan Fe–Cu (–Au) deposit.

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Fig. 4. Representative mineral assemblages of the Heijianshan Fe–Cu (–Au) deposit. (a). Irregular chromite replaced or crosscut by magnetite. (b). Amphibole intergrew with Stage III magnetite to

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replace Stage II epidote. (c). Actinolite with Stage III magnetite. (d). Stage IV pyrite vein crosscut Stage III massive magnetite. (e) Stage IV massive pyrite crosscut by Stage V chalcopyrite and electrum vein. (f) Stage IV pyrite and quartz crosscut by Stage VI calcite veins. Abbreviations: Act: actinolite, Am: amphibole, Cal: calcite, Ccp: chalcopyrite, Chr: chromite, Elc: electrum, Ep: epidote, Hem: hematite, Mag: magnetite, Py: pyrite, Qtz, quartz, Sph, sphene.

Fig. 5. Magnetite in the different Heijianshan magnetite ore types. (a). Magnetite with sulfides in massive ores. (b). Magnetite in disseminated ores replaced epidote. (c). Magnetite in magnetite clasts ores. (d). Magnetite (with / without actinolite) veins crosscut magnetite mineralized host

ACCEPTED MANUSCRIPT rocks. (e). Magnetite (mushketovite) with quartz in MOM. (f). Magnetite replaced by chalcopyrite and quartz in MOS. (g). Magnetite (with amphibole) replaced epidote in DO. (h) Fine-grained

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magnetite accumulates replaced host rocks in magnetite clasts.

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Abbreviations: Act: actinolite, Am: amphibole, Cal: calcite, Ccp: chalcopyrite, Cv, covellite, Ep: epidote, Hem: hematite, Mag: magnetite, Pl: plagioclase, Py: pyrite, Qtz, quartz, Ser, serite.

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Fig. 6. Binary diagrams of the Heijianshan magnetite geochemistry of the massive-, disseminated-

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and magnetite clasts ores.

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Fig. 7. Multi-element diagram for the Heijianshan magnetite, normalized to bulk continental crust

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(normalization values from Rudnick and Gao, 2003). Data for IOCG from the Ernest Henry

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deposit (Dare et al., 2014b), BIF from the Sokoman Fe formation (Chung et al., 2015), skarn from the Tengtie skarn Fe (Zhao and Zhou, 2015) and Vegas Peledas calcic skarn Fe (Dare et al., 2014b)

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deposits are shown for comparison.

Fig. 8. Binary diagrams of (a) Al vs. Mg, (b) Mn vs. Mg, (c) Si vs. Mg, (d) Zn vs. Mg, (e) Ga vs. Al, (f) Co vs. Mg, (g) Co vs. Ni and (h) Co vs. Si for the Heijianshan massive-, disseminated- and magnetite clasts ores.

Fig. 9. Binary diagrams of (a) (Mg + Mn) vs. (Si + Al)/(Mg + Mn), (b) Cr vs. Co/Ni, (c) Cr vs. Ti, (d) V vs. Cr, (e) Ni vs. Cr and (f) Ti vs. Ni/Cr for the Heijianshan magnetite from the different ore types. Fields for Ni–Cu–PGE sulfide deposits, hydrothermal- and sedimentary deposits, magnetic-

ACCEPTED MANUSCRIPT and hydrothermal magnetite of Figure 9e and 9f are from Dare et al. (2012) and Dare et al.

IP

T

(2014b), respectively.

SC R

Fig. 10. BSE images showing magnetite textures. (a). Massive magnetite replaced by Stage V chalcopyrite and hematite in MOS. (b). Rarely observed generation-1 magnetite (Mag-1) was replaced by generation-2 magnetite (Mag-2) which showed apparently dark- and light color rims.

NU

(c). Few magnetite grains have obvious intensities (dark- and light rims) under BSE and were

MA

replaced by later hematite. (d). Clasts of fine-grained magnetite aggregates replaced host rocks. (e). Massive homogeneous magnetite (mushketovite) intergrew with quartz in MOM. (f). Stage II

D

epidote replaced by disseminated homogeneous magnetite, amphibole and apatite.

TE

Abbreviations: Am: amphibole, Ap: apatite, Ccp: chalcopyrite, Dg: digenite, Ep: epidote, Hem:

CE P

hematite, Mag: magnetite, Pl: plagioclase, Qtz: quartz, Ser, sericite. Notes: The analytical results of magnetite obtained by EMPA and LA–ICP–MS are assumed to be

AC

the same magnetite generation (mainly by homogeneous magnetite).

Fig. 11. Time-resolved analytical signals of a LA–ICP–MS analysis from disseminated ores. Suspected fluid inclusions are characterized by high Al, Mn, Mg, Si and Sr intensities (cps counts per second).

Fig. 12. Plots of (a) (Al + Mn) vs. (Ti + V) and (b) Ni vs. V for the Heijianshan magnetite from the different ore types (Fig. 12a modified after Nadoll et al., 2014a).

ACCEPTED MANUSCRIPT Fig. 13. Binary diagram of (a) Ti/V vs. Ti are calculated on the basis of 32 (O) for the Heijianshan magnetite from the different ore types obtained by EMPA. Plot of (b) (Ca + Al + Mn) vs. (Ti + V)

IP

T

for magnetite from different ore and deposit types. Fields for the magnetite from IOCG, Kiruna,

SC R

porphyry, VMS, BIF, and Fe–Ti deposits in Figure 13a based on Monteiro et al. (2008). Data of the Sossego IOCG deposit in Brazil and Chengchao skarn deposit in China are from Monteiro et al. (2008) and Hu et al. (2014b), respectively. Reference fields (Fig. 13b) are after Dupuis and

NU

Beaudoin (2011), and data of the Ernest Henry (Australia) (Carew, 2004; Dupuis and Beaudoin,

MA

2011), Vegas Peledas (Argentina) (Dare et al., 2014b), Tengtie (China) (Zhao and Zhou, 2015), Pipe Mine (Canada) (Dare et al., 2014b) and Sokoman Fe formation (Canada) (Chung et al., 2015)

D

are shown for comparison. Skarn: Fe–Cu skarn deposits; IOCG: iron–oxide–copper–gold deposits;

TE

Porphyry: porphyry Cu deposits; BIF: banded iron formation; Kiruna: Kiruna apatite-magnetite

CE P

deposits; Fe–Ti, V: magmatic Fe–Ti-oxide deposits. Open symbols data from EMPA and solid

AC

symbols data from LA–ICP–MS.

Fig. 14. Box and whisker plots for magnetite from the Heijianshan, Heifengshan, Shuangfengshan, Shaquanzi and Tianhu deposits in Eastern Tianshan. Data for Heifengshan, Shuangfengshan and Shaquanzi deposits are from Huang et al. (2014), and Tianhu deposit is from Huang et al. (2015a).

Fig. 15. Plots of (a) Zn vs. V, (b) Zn vs. Ni, (c) Ni vs. V, (d) Ni vs. Mn, (e) Ni vs. Al, (f) V vs. Al in magnetite from Heijianshan, Heifengshan, Shuangfengshan, Shaquanzi and Tianhu deposits in the Eastern Tianshan. Data source as in Figure 14.

ACCEPTED MANUSCRIPT

Table captions

IP

T

Table 1. EMPA data for the Heijianshan magnetite and chromite.

Massive ores with

Sa

HJ1

HJ1

3-

3-

Disseminated ores

SC R

Massive ores with sulfide

HJ

HJ

HJ

HJ

HJ

HJ

HJ

13

13

13

13

13

13

13

HJ1

-

-

-

-

-

-

-

3-

02

02

02

02

02

02

02

NU

mushketovite

HJ1

HJ1

HJ1

HJ1

019

HJ1

HJ1

090

090

3(

3(

3(

3(

3(

3(

3(

le

3-00

3-00

3-00

3-00

(kua

3-01

3-09

(kua

(kua

1)-

2)-

1)-

1)-

1)-

2)-

2)-

nu

1-

2-

2-

5-

i)-

9-

0-

i)-

i)-

1

2

3

4

4

1

1

mb

4Mt

2Mt

2Mt

4Mt

3Mt

2Mt

2Mt

2Mt

3Mt

Mt

Mt

Mt

Mt

Mt

Mt

Mt

er

-1

-1

-2

-1

-1

-1

-1

-1

-1

-1

-1

-1

-1

-2

-1

-2

Na

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.

0.

0.

0.

0.

0.

bd

2O

2

5

2

bdl

4

5

9

2

4

02

05

01

08

03

02

l

Si

0.8

1.0

0.8

0.6

0.8

1.1

0.5

0.3

0.1

0.

0.

0.

1.

0.

0.

0.

O2

3

8

0

9

4

0

0

2

6

06

47

32

00

17

16

06

Ca

0.1

0.2

0.1

0.1

0.1

0.0

0.0

0.0

0.0

0.

0.

bd

0.

0.

0.

0.

O

3

2

4

0

4

8

5

6

D

TE

CE P 0.0

P2 O5

MA

mp

bdl

bdl 0.0

O

bdl

2

M

0.0

0.0

gO

1

5

bdl

bdl

0.0

bdl

1

0.0

1

bdl

bdl

3

AC

Ni

3

0.0

0.0

0.0

0.0

0.0

5

6

5

2

bdl

3

03

08

l

17

19

13

01

0.0

0.

0.

bd

bd

bd

0.

bd

1

05

01

l

l

l

02

l

bd

0.

bd

0.

bd

bd

bd

l

04

l

05

l

l

l

0.

0.

bd

0.

0.

0.

bd

01

05

l

05

03

01

l

93

93

93

91

92

93

94

0.0 bdl bdl

3 bdl

bdl bdl

Fe

89.

94.

90.

93.

92.

92.

88.

93.

94.

.7

.3

.5

.6

.7

.6

.0

T

58

21

77

84

94

86

80

66

44

6

8

2

4

6

6

7

0.1

0.4

0.1

0.2

0.1

0.1

0.1

0.0

0.0

bd

0.

0.

0.

bd

bd

0.

7

7

8

3

6

3

5

5

2

l

09

05

46

l

l

02

M

0.0

0.0

0.1

0.0

0.0

0.0

0.0

0.0

0.0

0.

0.

0.

0.

0.

0.

0.

nO

8

6

1

8

4

7

4

5

3

03

08

07

09

04

07

07

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

bd

bd

0.

0.

bd

0.

0.

4

2

5

2

bdl

2

7

1

5

l

l

06

01

l

02

01

0.0

0.0

0.0

0.0

0.0

0.

0.

0.

0.

0.

0.

0.

O

Al 2O 3

Cr 2O 3

V2

0.0

O5

2

bdl

2

4

1

bdl

2

4

bdl

18

24

22

18

14

19

22

Ti

0.0

0.0

bdl

0.0

0.0

0.0

bdl

0.0

bdl

0.

bd

0.

0.

0.

bd

0.

ACCEPTED MANUSCRIPT O2

7

7

4

5

4

1

10

l

06

06

02

l

07

94

94

94

93

93

94

94

To

90.

96.

92.

95.

94.

94.

89.

94.

94.

.2

.4

.2

.7

.3

.2

.5

tal

94

25

16

08

28

40

73

24

76

4

8

9

8

7

7

4

0. 02

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

00

13

27

10

00

24

28

53

10

23

9

0.2

0.3

0.2

0.1

0.2

0.3

0.1

0.0

51

08

38

99

45

19

52

94

Ni

0.0

0.0

0.0

0.0

0.0

42

67

44

30

43

25

17

18

0.0

0.0

0.0

0.0

0.0

0.0

00

00

43

00

00

00

0.0

0.0

21

00

0.

0.

0.

0.

00

04

01

01

00

3

3

6

0

0

0.

0.

0.

0.

0.

0.

0.0

01

13

09

29

05

04

01

46

8

7

4

2

1

7

8

0.

0.

0.

0.

0.

0.

0.

0.0

00

02

00

05

06

04

00

09

8

4

0

2

0

1

4

0.

0.

0.

0.

0.

0.

0.

0.0

07

01

00

00

00

02

00

08

3

5

0

0

0

6

0

0.

0.

0.

0.

0.

0.

0.

NU

0.0

MA

P

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

00

01

00

01

00

00

00

00

05

01

00

00

07

00

07

00

0

0

0

2

0

0

0

0.

0.

0.

0.

0.

0.

0.

D

Ca

0.0

TE

Si

7

SC R

0.

0.

IP

Na

0.

T

Number of ions on the basis of 32 (O)

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

00

02

00

02

01

00

00

g

05

21

21

26

22

09

02

00

00

5

0

0

3

3

3

0

8.

8.

9.

9.

8.

8.

8.

Fe

9.1

9.0

2+

12

99

14.

3+

782

Al

14.

9.0

9.0

9.0

9.1

8.9

8.9

8.9

94

93

01

03

85

89

93

71

66

67

56

46

63

05

7

5

0

9

2

0

3

15

15

15

14

15

15

15

14.

AC

Fe

CE P

M

593

811

14.

14.

14.

15.

15.

15.

.2

.0

.0

.6

.2

.2

.2

875

830

694

049

176

298

35

45

93

12

65

44

56

0.

0.

0.

0.

0.

0.

0.

0.0

0.1

0.0

0.0

0.0

0.0

0.0

0.0

0.0

00

03

01

15

00

00

00

61

57

63

77

56

45

54

17

07

0

1

7

8

0

0

6

0.

0.

0.

0.

0.

0.

0.

M

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

00

02

01

02

00

01

01

n

20

15

29

19

10

18

10

11

07

8

0

7

2

9

6

8

0.

0.

0.

0.

0.

0.

0.

Cr

V

Ti

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

00

00

01

00

00

00

00

09

05

12

05

00

05

16

01

11

0

0

3

3

0

5

3

0.

0.

0.

0.

0.

0.

0.

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

04

05

05

04

03

04

05

05

00

05

08

03

00

06

09

00

1

5

1

2

3

5

2

0.

0.

0.

0.

0.

0.

0.

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

02

00

01

01

00

00

01

15

15

00

08

11

09

00

02

00

3

0

2

2

4

0

6

ACCEPTED MANUSCRIPT 24

24

24

24

24

24

24

24.

24.

24.

24.

24.

24.

24.

24.

24.

.3

.3

.3

.3

.3

.3

.3

tal

313

312

347

311

311

314

325

308

312

68

20

09

10

04

27

06

AC

CE P

TE

D

MA

NU

SC R

IP

T

To

AC

Figure 1

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

Figure 2

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

Figure 3

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Figure 4

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Figure 5

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

Figure 6

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

Figure 7

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Figure 8

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Figure 9

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Figure 10

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

Figure 11

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

Figure 12

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

Figure 13

AC

Figure 14

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Figure 15

Graphical abstract

SC R

IP

T

ACCEPTED MANUSCRIPT

NU

Highlights

MA

(1) The Heijianshan magnetite from the massive-, disseminated-and mostmagnetite clasts ores are hydrothermal. (2) The Cr vs. Co/Ni, Cr vs. Ti, V vs. Crand Ni vs. Crdiagrams can discriminate the differentmagnetite ores. (3) Temperature, fO2and other factors control the element distribution in magnetite.

AC

CE P

TE

D

(4) The Heijianshan Fe–Cu (–Au)deposit can be classified as an IOCG-like deposit.