Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis

Journal of Asian Earth Sciences xxx (2015) xxx–xxx

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Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis Hou-Min Li a,⇑, Jian-Hua Ding a, Zhao-Chong Zhang b, Li-Xing Li a, Jing Chen c, Tong Yao a a

MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, China c CITIC Construction Co., Ltd., Beijing 100027, China b

a r t i c l e

i n f o

Article history: Received 7 October 2014 Received in revised form 28 May 2015 Accepted 25 June 2015 Available online xxxx Keywords: Iron-rich melt Fenner trend Iron deposits Yamansu Eastern Tianshan Mountains

a b s t r a c t Volcanic rock-hosted iron deposits are among the important iron ores in China. However, the nature of primary magma and petrogenesis associated with these iron ores remains controversial. Here, we report iron-rich fragments (IRF) from the Yamansu iron deposit in Eastern Tianshan Mountains, NW China, which occurs in association with volcanic breccia, submarine volcanic breccia and ignimbrite. The IRF is composed of five types including oligoclase-iron oxide type (OIO), oligoclase-albite-iron oxide type (OAIO), albite-iron oxide type (AIO), albite-K-feldspar-iron oxide type (AKIO) and K-feldspar-iron oxide type (KIO). These fragments display typical volcanic fabric features, such as porphyritic texture, hyalopilitic texture of the groundmass and vesicles filled by minerals to form amygdales. The feldspar phenocrysts of IRF are dominantly albite. The groundmass of IRF consists of magnetite and feldspar. The magnetite is distributed in between the feldspar laths, and together display hyalopilitic texture which could be observed only in volcanic rocks. The vesicles are filled with magnetite, feldspar, chlorite and calcite from the margin to the interior. The IRF has high Si, Al, Fe, Ca, Ti, Na and K contents and low Mg content. The average total Fe is 26 wt.%. The magnetite is mostly titanium–vanadium magnetite, with the TiO2 content ranging up to 4.86 wt.% and V2O3 content up to 3.20 wt.%. The IRF probably came from iron-rich melts and represent the products of the Fenner magma evolution. The basaltic magma evolved into the Fe–Na-rich residual melts by crystallization under low oxygen fugacity condition in a closed magma chamber after intruding into the shallow crust. The Fe–Na-rich residual melts were emplaced in hypabyssal environments or erupted generating the orebodies or providing the material source for the generation of the high-grade iron ores which were subsequently enriched by the late-stage hydrothermal fluids. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Volcanic rock-hosted iron deposits are among the important types of iron ores in China (Li et al., 2012, 2015; Zhang et al., 2014a,b). The typical examples include the porphyrite iron deposits related to terrestrial volcanic rocks located in the Lower Yangtze River Valley (e.g. Chen et al., 1982; Zhai et al., 1992; Zhou et al., 2010a; Mao et al., 2012) and the iron ores associated with submarine volcanic rocks distributed in the eastern and western Tianshan Mountains (Dong et al., 2013). The submarine volcanic rock-hosted iron oxide (SVIO) deposits are different from the well-known volcanic-associated massive sulfide deposits (VMS),

⇑ Corresponding author at: No. 26 Baiwanzhuang Road, Xicheng District, Beijing 100037, China. E-mail address: [email protected] (H.-M. Li).

because ores of the former are characterized by iron oxides whereas the latter comprises sulfides of Fe, Cu, Pb and Zn. Although volcanic rock-hosted iron deposits have been reported globally (Belevtsev, 1982), only few studies have been carried out, except for the Kiruna iron deposit in Sweden (e.g. Frietsch, 1978; Hildebrand, 1986; Nyström and Henriquez, 1994), the El Laco deposit in Chile (e.g. Bookstrom, 1977; Frutos and Oyarzun, 1975; Henriquez et al., 2003) and the Marcona deposit in Peru (Chen et al., 2010a, 2011). Recent studies have led to the discovery of several such deposits in the Western Tianshan Mountains in Xinjiang, China, and are emerging as one of the most important iron reserves. However, whether an iron-rich magma served as the source for the volcanic rock-hosted iron deposits remains equivocal. Most workers suggested that these are skarn deposits related to the volcanic and intrusive rocks or formed through gaseous emanations from marine volcanos (Wang and Chen, 2001;

http://dx.doi.org/10.1016/j.jseaes.2015.06.026 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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Mao et al., 2005; Pirajno, 2010; Xu et al., 2010; Yang et al., 2010; Lei et al., 2013; Li and Wu, 2013; Chai et al., 2014; Duan et al., 2014; Hou et al., 2014a; Jiang et al., 2014; Li et al., 2014a; Zhang et al., 2014c). However, some others considered the iron-rich ores in volcanic rock-hosted iron deposits to be of magmatic origin, such as the Gushan and Meishan iron deposits in the Lower Yangtze River Valley, the Manyang iron deposit in Yunnan, the Xiertala iron deposit in Inner Mongolia, and the Wuling iron deposit in Western Tianshan Mountain (e.g. Song et al., 1981; Zhai et al., 1996; Li et al., 2014b; Yan and Niu, 2014). However, no iron-rich magma has yet been reported for the SVIO deposits in the Eastern Tianshan Mountains. The Yamansu iron deposit located in the Eastern Tianshan Mountains is a typical SVIO deposit. Abundant iron-rich fragments (IRF) are observed in the ore-bearing volcanic rocks, with total Fe content up to 26%. These fragments display typical volcanic features, such as porphyritic texture, hyalopilitic texture of the groundmass and filling of cavities of vesicles to form amygdales. In this contribution, we report the geological and geochemical characteristics of the fragments to constrain for the mineralization process. 2. Regional geology The Eastern Tianshan Mountains is composed of the following tectonic units, from north to south: the Dananhu–Tousuquan accretionary arc (Silurian-Early Carboniferous), the Xiaorequanzi-Wutongwozi intra-arc basin (Early Carboniferous), the Kanggur–Huangshan ductile shear zone, the Aqishan– Yamansu back-arc basin (Carboniferous) and the Middle– Tianshan old massif (Fig. 1) (e.g. Qin et al., 2003; Xiao et al., 2004, 2013). The Aqishan–Yamansu is an important polymetallic belt in China and is well known for hosting several Fe (–Cu) deposits in the volcanic centers (Mao et al., 2005; Zhang et al., 2008, 2014a), with the major mineralization formed during Carboniferous (Zhou et al., 2010b; Huang et al., 2013; Hou et al., 2014b; Li et al., 2014c; Zheng et al., 2015). The Yamansu iron deposit is developed in the volcano-sedimentary sequences of the Aqishan–Yamansu back-arc basin (Fig. 1). 3. Ore deposit geology The Yamansu iron deposit is volcanic rock-hosted and unmetamorphosed (Pirajno, 2010). The estimated ore reserve is 35 Mt

90° Urumqi

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with an average grade of 49% total Fe. The high-grade iron ores with total Fe grade >50% are up to 77% (Li and Li, 2013). The exposed strata are composed of parts of the Middle Member and Upper Member of the Lower Carboniferous Yamansu Formation (Fig. 2). The Middle Member comprises from bottom upwards: limestone, crystalline limestone, andesitic volcanic breccia and agglomerate and tuff intercalated with rhyolitic tuff, andesitic volcanic tuff and rhyolitic tuff, basaltic andesite, andesitic tuff intercalated with rhyolitic rocks (magnetite-bearing skarn transformed from the andesitic pyroclastic rocks), limestone and tuffaceous limestone. The Upper Member is composed of andesitic tuff and volcanic breccia. The volcanic rocks are characterized by high K, Na and low Ca contents (Dong et al., 2013). Some basaltic porphyrite, augite andesitic porphyrite, augite dioritic porphyrite with diabase and spessartite veins of Hercynian age also occur. LA–ICPMS U–Pb dating of zircon grains from the volcanic rocks of the Yamansu Formation yielded U–Pb zircon ages of 348 ± 2 Ma (dacite), 336 ± 2 Ma (dacite) and 334 ± 3 Ma (rhyolite) respectively (Luo et al., 2012). Hou et al. (2014b) reported U–Pb zircon ages of 324 ± 1 Ma from the basalt. Zircon SHRIMP U–Pb dating of the diabase dike that cuts across the skarn yielded age of 335 ± 4 Ma, indicating that the skarn and iron mineralization are older than 335 Ma (Li et al., 2014c). The deposit is located in the southern limb of an anticlinal structure which is inclined southward. Secondary faults are well developed in this region. The iron orebodies occur in and near the skarns of the Lower Carboniferous Yamansu Formation as EW-trending stratiform. The skarns have a strike length of 2900 m and width of 100–200 m, within which twenty-two orebodies have been recognized. The Fe1, Fe2 and Fe3 orebodies are the largest and major orebodies of the deposit. The orebodies occur as stratiform to lenticular shapes (Figs. 2 and 3) and are mostly conformable with the country rocks, which show EW-trending strata with southerly dip of 35–60°. The Fe1 orebody is located in the middle part of the ore-bearing skarns, which occurs between the garnet skarns at shallow level and the volcanic rocks at depth. The orebody is 886 m long with an average width of 11 m. The Fe2 orebody occurs to the south of the eastern part of the Fe1 and is located within the garnet skarns with stratoid structure. It is 244 m long with an average width of 16 m. The Fe3 orebody is located at the eastern part of the ore-bearing skarns. It is stratoid with the garnet skarns as the hanging wall and

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Fig. 1. Tectonic framework in the Eastern Tianshan orogen and distribution of iron ore deposits (modified after Qin et al., 2003).

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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H.-M. Li et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

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Fig. 2. Geological map of the Yamansu deposit (after Yao et al., 1993).

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4. Geology of the iron-rich fragments 4.1. Occurrence of the iron-rich fragments

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(35–95%), martite and limonite. The accessory oxides are hematite, manganite, allcharite, and mushketovite. The sulfides are dominated by pyrite (4–35%) together with minor galena, sphalerite, pyrrhotite, chalcocite, marcasite and chalcopyrite. The dominant gangue mineral is garnet (10–30%) with subordinate chlorite, epidote, diopside, hornblende, actinolite, albite, calcite and quartz. The hypergene mineral is jarosite. The ores have subhedral–anhedral granular, metasomatic, unmixing textures and massive, banded and disseminated structures.

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Fig. 3. Profile of the No. 43 line of the Yamansu iron deposit (after Yao et al., 1993).

the limestone and rhyolitic tuff as the footwall. The orebody strikes 815 m with an average of 27 m in width. The ores are mainly garnet–magnetite and diopside–epidote magnetite. Skarn deposits in China are defined as contact-metasomatic deposits developed in the contact zones between igneous intrusions and carbonate or volcano-sedimentary rocks. This definition is different from the general definition of skarn deposits worldwide (Meinert et al., 2005). Therefore, the Yamansu iron deposit has been classified as volcanic rock-hosted type (Li et al., 2014c, 2015). The major iron oxides are magnetite

Iron-rich fragments (IRF) occur mainly in the ignimbrite, volcanic breccia and sedimentary volcanic breccia near the orebodies (Fig. 4). The ignimbrite shows tuffaceous or residual texture. The clasts are mainly lithic fragments with minor crystal pyroclasts and vitroclastics. The lithic fragments are composed of andesitic porphyry, dacite porphyry, fine-grained andesite, tuff and ferrobasalt (Fig. 4a). The ferrobasalts (iron-rich fragments, IRF) show vesicular and amygdaloidal textures, which were filled with chlorite, calcite and quartz, with diabasic texture or hyalopilitic texture of the groundmass (Fig. 4b–e). These textures of the IRF are uniquely developed in volcanic rocks such as ferrobasalts, which are not developed in intrusive, sedimentary and hydrothermal filling rocks. Although rocks formed by metasomatism sometimes show similar textures, they represent relicts of the protolith volcanic rocks. The IRF shows clear but sharp boundary, and the surrounding volcanic material has no iron mineralization, suggesting that the IRF do not represent alteration products of iron mineralization during volcanic eruption. The crystals in the pyroclasts are mainly plagioclase with minor quartz and K-feldspar. Most of the vitroclastics had experienced devitrification and developed comb edges with high Fe content.

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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H.-M. Li et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

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Fig. 4. Photomicrographs illustrating the features of the IRF. The IRF (black color) in the submarine volcanic breccia of 515-23 (a), 515-27 (b), YMS-50 (c) and in the ignimbrite of YMS-51 (d). (e and f) The albite-rich crystals and the IRF in the ignimbrite, with the amygdale consisting of chlorite and the groundmass consisting of magnetite and albite (sample 515-34); (g and h) IRF with albite being metasomatised by K-feldspar and magnetite. The groundmass is composed of albite and magnetite (YMS-50); (i and j) IRF in the ignimbrite, with the amygdales consisting of chlorite and calcite and the groundmass consisting of magnetite and albite (YMS-51); (k and l) IRF in the ignimbrite consisting of albite phenocrysts and the chlorite and the groundmass composed of albite and magnetite (YMS-62). (a–e, g, i, k) plane-polarized light; (f, h, j, l) reflectedpolarized light; Ab – albite; Cc – Calcite; Chl – chlorite; Mt – magnetite; Or – K-feldspar.

The matrix of the ignimbrite is composed of volcanic glass and ash, within which the volcanic glass has also experienced devitrification and shows comb-edge textures. The matrix has been strongly altered to carbonate and chlorite. The volcanic breccia shows brecciated structure. The clast is mainly andesite with hyalopilitic texture. Other secondary clasts include dacite porphyry, rhyolite porphyry, crystal tuff and some bioclastic limestone with crinoid stems. The IRF is composed of ferrobasaltic andesite and ferroandesite (Fig. 4f) with phenocrysts and amygdaloidal structures (Fig. 4g–j). The groundmass of the IRF has hyalopilitic texture and is composed of magnetite and albite (Fig. 4h and j). The surrounding volcanic material shows no iron mineralization, suggesting that the IRF is not an alteration product of iron mineralization during volcanic eruption. The matrix of the volcanic breccia is composed of volcanic material with epidote, magnetite, K-feldspar and carbonate minerals. Some fluorite and tourmaline are occasionally found in the IRF. The sedimentary volcanic breccia is crimson in color and shows brecciated structure. The clasts show a large range of sizes and are composed of volcanic and sedimentary rocks. The volcanic rock clast is mainly crimson rhyolitic ignimbrite, within which there are some rigid lithic fragments and slurry pyroclasts which have experienced devitrification and show felsitic texture and pseudoflow structure. The other volcanic rock clasts are andesite and andesitic porphyrite. The sedimentary rock clasts are carbonate and volcanic material-bearing limestone, suggesting that the carbonate and tuff have transitional relationship and are syndepositional. The matrix is carbonate with minor iron-rich volcanic glass and ash. There are porphyritic iron-rich andesite clasts (IRF) with carbonate almonds (Fig. 4k and l).

4.2. Types of iron-rich fragment Here we subdivide the iron-rich fragment (IRF) into five types based on a detailed investigation of 57 IRF samples collected from the wall rocks of ignimbrite, volcanic breccia and sedimentary volcanic breccia. They are: oligoclase-iron oxide type (OIO), oligoclase-albite-iron oxide type (OAIO), albite-iron oxide type (AIO), albite-K-feldspar-iron oxide type (AKIO) and K-feldspar-iron oxide type (KIO). Among these, the OAIO and AKIO are the transitional varieties of the other three categories. 4.2.1. Oligoclase-iron oxide type (OIO) Only three oligoclase-iron oxide (OIO)-type of fragments were recognized in the polished thin sections of our samples. These fragments are composed by oligoclase and iron oxides, and are characterized by porphyritic texture (Fig. 5a). The phenocrysts of the fragments are mainly oligoclase and account for 10% of the fragments. The phenocryst oligoclase (Ab = 81) occurs as euhedral laths of 0.2 mm in diameter. The groundmass of the fragment shows hyalopilitic texture that composed of oligoclase and magnetite (Fig. 5b). The lath-shaped groundmass oligoclase is generally 50 lm long and 5 lm wide. The groundmass magnetite distributed between the groundmass oligoclase is 1 lm in diameter. The oligoclase and magnetite in the groundmass display phenocryst texture, suggesting a magmatic origin. 4.2.2. Oligoclase-albite-iron oxide type (OAIO) This type has been found only in three of the polished thin sections of our samples and is composed of oligoclase, albite and magnetite. The phenocrysts of OAIO are mainly oligoclase (Ab = 88)

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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a

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Fig. 5. BSE images of the OIO-type fragments. Mt – magnetite; Pl – plagioclase. The EDS scanning area is marked by red rectangle and the EDS analyzed area is marked by yellow circle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

with minor albite (Ab = 93). These phenocrysts are present as euhedral laths up to 1 mm in length and are locally altered into magnetite, calcite, epidote and chlorite along cleavage traces or grain margins (Fig. 6a and b). The groundmass of OAIO is composed of albite and magnetite and shows diabasic or hyalopilitic texture. The groundmass albite shows lath-shaped microcrystal framework with magnetite grains in between (Fig. 6c and d). The grain size of the groundmass is less than 1 lm, suggesting formation under hy pabyssal–ultrahypabyssl conditions.

or hyalopilitic texture. The groundmass albite occurs as euhedral lath ranging in length from 10 to 20 lm and with a width of 1 lm. The groundmass magnetite grains are distributed within the framework formed by the albites, displaying an overall diabasic texture or hyalopilitic texture (Fig. 7c and d, g and h). Albite is replaced by calcite (Fig. 7g and h). The amygdales are only composed of quartz and chlorite, thus the magnetite and albite were formed earlier than the amygdales. It is indicated that the albite and magnetite are magmatic, whereas the quartz and chlorite are products of later hydrothermal fluids. Occasionally, the albite and magnetite are found to be replaced by hematite in some fragments.

4.2.3. Albite-iron oxide type (AIO) The albite-iron oxide (AIO) type is the main type and is found in 22 fragments in the polished thin sections of our samples. The AIO exhibits porphyritic texture and amygdaloidal structure (Fig. 7a–f). The phenocrysts of AIO are dominated by albite (Ab = 90–100). The amygdale mineral assemblage comprises chlorite and quartz from the core to margin (Fig. 7c and d) and shows hydrothermal filling features along the cavities of vesicles. The groundmass of AIO is composed of albite and magnetite aggregates, showing diabasic

a

4.2.4. Albite-K feldspar-iron oxide type (AKIO) Ten fragments of this type were found in the polished thin sections of our samples. The AKIO shows amygdaloidal structure (Fig. 8a–l). The groundmass is dominated by albite, magnetite and metasomatic K-feldspar (Fig. 8c and d, k and l). Outward from the amygdales are K-feldspar, albite and magnetite (Fig. 8c and d) or chlorite, albite and magnetite (Fig. 8g and h) or chlorite,

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Fig. 6. Photomicrographs of the OAIO-type fragments (L10-2). (a and b) IRF consisting of altered oligoclase phenocrysts and groundmass of albite and magnetite. (a) Planepolarized light, (b) BSE image; (c and d) IRF consisting of oligoclase phenocrysts which were replaced by magnetite and chlorite and groundmass of albite and magnetite. (c) BES image, (d) X-ray scanning image with Mg marked by red and Na marked by green and Ca marked by blue color; Ab – albite; Chl – chlorite; Mt – magnetite; Pl – plagioclase. The EDS scanning area is marked by red rectangle and the EDS analyzed area is marked by yellow circle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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Fig. 7. Photomicrographs of the AIO-type of fragments (515-34). (a and d) the vesicles were filled with albite and magnetite to form amygdales. (a and c) BSE images, (b) X-ray scanning image with Na marked by blue and Mg marked by red and Fe marked by green color, (d) X-ray scanning image with Na marked by blue and Si marked by red and Ca marked by green color; (e–h) IRF consisting of carbonated albite phenocrysts and groundmass of magnetite and albite. (e and g) BES images, (f and h) X-ray scanning images with Na marked by blue and Fe marked by red and Ca marked by green color. Ab – albite; Cc – calcite; Chl – chlorite; Mt – magnetite; Q – quartz. The EDS scanned area is marked by red rectangle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. Photomicrographs of the KAIO-type fragments (YMS-51). (a, e, i) Plane-polarized light. (b and c, f and g, j and k) BSE images. (c), (g) and (k) are detail view of (b), (f) and (j), respectively. (d) X-ray scanning image with K marked by blue, Na marked by red and Fe marked by green color. (h) and (l) X-ray scanning images with K marked by blue, Na marked by red and Mg marked by green color. Mt – magnetite, Ab – albite, Chl – chlorite. The EDS scanned area is marked by red rectangle and the EDS analyzed area is marked by yellow circle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

K-feldspar, albite and magnetite (Fig. 8k and l). Iron oxides such as magnetite are not found in amygdales. This type of IRF shows obvious hydrothermal features. When the vesicles were filled, minerals near the wall crystallized earlier than those in the inner part. Thus, the crystallization sequence of the hydrothermal mineral is albite, then K-feldspar, and then chlorite. Iron oxides such as magnetite

showing no features of hydrothermal origin are more likely to be magmatic. Some albite laths are cut by the later stage calcite veins. 4.2.5. K-feldspar-iron oxide type (KIO) We recognized 19 fragments of this type in the polished thin sections of our samples. They exhibit porphyritic texture and

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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amygdaloidal structure (Fig. 9a–d), suggesting hypabyssal–eruptive nature of the magma. Phenocrysts of the KIO are dominated by the K-feldspar (Fig. 9c and d). Groundmass of the KIO is composed by lath-shaped feldspar and hematite, displaying diabasic texture. The amygdales were filled by chlorite, K-feldspar and drusy hematite from inner outward (Fig. 9c and d), indicating that they are hydrothermal minerals and the hematite formed earlier than K-feldspar and the chlorite is the latest phase. The K-feldspar has irregular boundary and hematite inclusion, suggesting a result of hydrothermal metasomatism. Sphene sometimes occurs in the groundmass associated with K-feldspar and hematite (Fig. 9e and f). Minor fluorspar and calcite veins occur occasionally in the groundmass, suggesting hydrothermal process. Porphyritic albite–hematite fragments occur in the sample L8-8, in which the albite phenocrysts are partly replaced by hematite and the groundmass is dominated by schistose hematite (Fig. 9g and h).

The EDS results of the IRF are listed in Table 2. The IRF shows high contents of Si, Al, Fe, Ca, Na and K, which corresponds with the mineral assemblages of iron oxides, albite, K-feldspar and oligoclase. The content of MgO is relatively low. The average total FeO content is up to 26 wt.%. The high TiO2 content is due to the presences of iron oxides. Based on the plots in Fig. 10, we summarize the salient geochemical features of the IRF as follows: (1) The Al2O3 content of the fragments shows an obvious positive correlation with SiO2 due to the partitioning of Al and Si in the feldspars. The total FeO content shows a negative correlation with SiO2 consistent with the feldspars and Fe-oxide paragenesis. (2) The MgO and CaO contents are low and variable with no correlation with SiO2 content due to the presences of altered minerals such as chlorite and calcite. (3) The Na2O content of the AIO and AKIO shows positive correlation with SiO2 whereas no correlation is displayed in the KIO. The K2O content of the KIO and AKIO has positive correlation with SiO2, whereas the low K2O content in the AIO shows no correlation with SiO2. (4) In AIO, the TiO2 content shows obviously negative correlations with SiO2 content and positive correlation with total FeO content, suggesting the genetic relationships between Fe and Ti as well as the Ti partitioning in magnetite. Nevertheless, the TiO2 has no correlation with total FeO and SiO2 in KIO and AKIO, possibly due to hydrothermal activity and low content of Ti in magnetite. (5) The total FeO content of AKIO has negative correlation with MgO content whereas in other types, a minor negative correlation is observed.

5. Geochemistry 5.1. Geochemistry of iron-rich fragments (IRF) The analyses of the iron-rich fragments (IRF) were performed by Energy Dispersive Spectrometer (EDS). In order to obtain the bulk geochemical features, the EDS was set to cover the whole fragment (shown by the ranges of the yellow circles in Figs. 5a, 6b, 8b, f, j and 9b and g. In order to test the accuracy of the EDS, the same homogeneous samples were analyzed both by X-ray Fluorescence Spectrometer (XRF) and EDS. The results are presented in Table 1. The XRF experiments performed at the CNNC Beijing Research Institute of Uranium Geology. The EDS measurements were carried out using a TM3000 Tabletop SEM (Hitachi) equipped with a Quantax 70 EDS attachment (Bruker). Results obtained by the two different methods are similar. The contents of SiO2 and Al2O3 obtained by EDS are higher by 1.54–6.21 wt.% and 1.06–4.36 wt.% than those obtained by XRF, respectively. The error is considered to be related to the loss of ignitions (LOI). Taking the LOI into account, the error would be less. Thus, the result of EDS analysis is taken to reflect the geochemical features of the fragments.

a

5.2. Geochemistry of the iron oxides The EMPA data on iron oxides in sample 515-23 (AIO) is listed in Table 3. For comparison, the magnetite in the YMS-65 garnet-bearing ore was also analyzed. The EMPA analysis was

c

b

d Hem

Hem+Or

Hem+Or

Hem Or Chl

Fig. 9c, d

Or

Or Or

Chl

Hem

Hem

Hem

Or Hem Or

Or 500µm

500µm

HV: 15kv

e

f

Hem

50µm

HV: 15kv Ttn

g

50µm

HV: 15kv h

Ab

Hem+Or Fig. 9h

Ab

Ttn Hem Or Ttn HV: 15kv

Hem

Ttn 25µm

HV: 15kv

25µm

HV: 15kv

250µm

HV: 15kv

25µm

Fig. 9. Photomicrographs of the KIO-type fragments (YMS-50 and L8-8). (a) and (b) represent the same area of plane-polarized light and BSE images. (c) and (d) are enlarged views of (b) where X-ray scanning image with K marked by blue, Mg marked by green and Fe marked by red color, illustrating the K-feldspar phenocrysts, iron oxide groundmass and amygdaloidal texture. (e) and (f) represent the same area where X-ray scanning image with K marked by blue, Fe marked by green, Ti marked by red and Ca marked by red color, illustrating the close association of iron oxide, titanite and K-feldspar. (g) is a BSE image showing that the albite phenocrysts replaced by hematite and the groundmass consisting of lath-shaped hematite aggregates. (h) is an enlarged view of (g) showing the texture of hematite. Ab – albite; Chl – chlorite; Hem – hematite; Or – orthoclase; Ttn – titanite. The EDS scanning area is marked by red rectangle and the EDS analyzed area is marked by yellow circle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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H.-M. Li et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

Table 1 A comparison of the major element compositions for the same homogeneous samples analyzed by XRF and EDS methods. Sample

Rock type

Method

SiO2

Al2O3

TFeO

MgO

CaO

Na2O

K2O

MnO

TiO2

P2O5

LOI

L2-1

Keratophyric tuff Dacitic tuff

L2-7

Dacitic keratophyre

L4-9

Diabase

L5-1

Tuffaceous siltstone

60.7 62 58.8 62.4 69.9 72.6 52.2 53 67.1 69

19.59 20.33 19.32 19.54 16.02 16.19 17.33 17.56 16.66 17.39

1.13 1.64 4.63 3.34 0.61 0.38 7.53 8.17 1.32 1.23

3.7 4.8 3.2 2.7 1.3 0.5 5 6.3 2 1.8

1.7 1.2 1.6 2.1 1.1 0.9 7.5 6.9 1.5 0.7

8.32 8.57 2.8 3.22 8.51 7.66 5.55 6.42 8.56 9.4

1.58 0.67 5.3 5.83 1.01 1.76 0.23 0.17 0.37 0.1

0.1 0 0 0 0 0 0.1 0.2 0 0

0.81 0.78 0.86 0.87 0.59 0 1.02 1.3 0.74 0.5

0.07 0 0.06 0 0.1 0 0.2 0 0.13 0

2.1

L2-2

XRF EDS XRF EDS XRF EDS XRF EDS XRF EDS

3.3 0.8 3.3 1.5

Table 2 Major element compositions of the iron-rich magmatic fragments analyzed by EDS method. Sample

Fragment

SiO2

Al2O3

TFeO

MgO

CaO

Na2O

K2O

MnO

TiO2

515-24-2 515-24-8 515-24-3 L10-2-3 L10-2-4 515-24-1 515-24-4 515-24-5 515-27-4 515-30-1 515-32-1 515-34-1 515-34-2 515-34-3 515-34-4 515-34-6 515-34-7 515-34-8 515-34-9 515-35-1 L10-2-1 L10-2-2 L8-4-1 L8-4-2 L8-8-1 L8-8-2 YMS-51-3 515-27-1 515-27-2 515-27-3 L9-3-5 YMS-51-1 YMS-51-2 YMS-51-4 YMS-51-6 YMS-51-7 YMS-51-8 L9-3-1 L9-3-10 L9-3-11 L9-3-12 L9-3-2 L9-3-3 L9-3-4 L9-3-7 L9-3-8 L9-3-9 YMS-50-1 YMS-50-10 YMS-50-2 YMS-50-3 YMS-50-4 YMS-50-5 YMS-50-6 YMS-50-8 YMS-51-5

Oligoclase-iron oxide type Oligoclase-iron oxide type Oligoclase-albite-iron oxide type Oligoclase-albite-iron oxide type Oligoclase-albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type

46.03 53.50 58.55 37.90 45.52 56.91 48.60 59.67 57.30 59.49 51.88 44.14 49.35 34.35 22.23 30.89 42.80 23.89 38.33 62.23 42.59 41.60 58.46 70.07 33.59 62.90 19.29 42.85 58.94 39.26 33.90 35.08 21.34 35.86 44.91 54.18 42.62 40.10 56.34 33.33 40.72 48.10 41.54 47.23 39.73 55.03 48.43 48.69 45.53 52.60 44.53 36.36 45.72 51.56 51.07 41.40

15.05 17.11 12.10 18.10 19.10 18.61 15.49 16.88 17.73 19.95 11.67 14.01 16.10 11.23 6.31 9.58 12.71 8.56 12.53 20.08 15.60 19.68 7.67 14.76 9.22 13.85 11.83 16.00 15.20 14.09 12.02 13.01 7.54 13.52 15.25 17.40 14.69 14.28 18.31 11.94 15.62 15.00 14.34 18.00 13.21 17.73 17.70 14.23 14.51 15.51 14.01 11.46 14.52 15.41 15.50 13.34

23.56 15.00 12.05 24.83 15.90 10.02 24.22 11.38 14.37 4.88 11.02 30.72 24.43 42.36 57.21 50.26 33.37 59.13 33.04 5.20 27.45 14.59 26.27 5.82 47.73 14.00 48.55 21.91 11.21 22.21 44.32 39.85 63.47 36.71 25.97 13.97 26.16 31.48 8.54 42.85 29.10 11.62 31.80 16.83 34.11 11.26 17.68 14.80 26.71 16.32 28.31 41.44 26.36 18.30 21.49 34.60

0.58 1.20 0.52 2.92 2.14 1.81 0.73 2.27 3.99 0.53 0.73 1.82 0.00 0.75 0.76 0.50 2.92 1.26 0.52 0.45 4.68 1.44 0.00 0.00 3.77 0.05 11.26 1.39 1.89 1.13 1.09 3.04 0.56 1.27 1.57 1.29 1.39 4.65 2.86 4.09 5.25 0.43 0.81 7.80 0.89 2.42 4.50 0.00 0.19 0.12 0.11 0.24 0.45 0.00 0.18 0.19

5.27 2.83 9.44 10.97 11.15 2.70 1.44 1.25 0.98 3.88 16.77 0.68 1.18 1.21 7.98 0.86 0.43 0.66 6.70 0.69 0.88 18.10 0.58 0.64 0.46 0.53 3.01 9.35 4.24 16.85 1.18 0.64 0.30 2.07 0.85 0.74 1.27 0.78 0.62 1.69 1.42 13.46 1.59 0.56 1.31 1.00 0.87 4.10 0.92 0.95 0.44 0.72 0.69 0.67 0.45 0.13

8.19 8.83 6.10 2.55 4.09 9.02 8.42 7.62 4.30 8.77 6.28 7.42 7.28 7.70 4.38 6.39 5.19 4.75 7.90 9.42 6.52 2.37 5.07 7.91 4.26 8.13 0.59 6.43 5.77 3.91 0.61 2.57 2.15 2.74 3.79 4.57 4.03 0.39 0.52 0.50 1.17 0.68 0.57 0.72 1.26 1.33 0.84 0.01 0.40 0.26 0.24 0.33 0.30 0.08 0.39 0.61

0.24 0.53 0.53 1.44 1.68 0.25 0.12 0.28 1.01 1.98 0.63 0.41 0.86 0.14 0.26 0.37 1.36 0.24 0.13 1.31 1.25 1.04 0.15 0.17 0.15 0.32 0.77 1.54 0.79 1.87 6.51 3.95 2.25 4.48 5.65 6.80 8.29 7.62 12.35 5.19 5.71 9.63 8.86 8.10 8.08 10.60 9.37 13.31 10.54 12.45 10.63 7.68 10.72 13.51 9.88 8.43

0 0 0.15 0.18 0 0 0 0 0 0.08 0.24 0 0 0 0 0 0 0 0 0 0.20 0.19 0 0 0 0 1.02 0 0 0 0 0 0 0 0.18 0.17 0 0 0.29 0.24 0.28 0 0 0.53 0 0 0.37 0 0 0 0 0 0 0 0 0

1.08 1.00 0.56 1.10 0.42 0.68 0.99 0.64 0.31 0.44 0.79 0.81 0.80 1.55 0.87 1.15 1.22 1.51 0.85 0.62 0.83 0.98 1.80 0.63 0.82 0.22 3.69 0.54 1.97 0.68 0.37 1.86 2.38 3.34 1.83 0.88 1.55 0.70 0.18 0.18 0.72 1.08 0.49 0.23 1.41 0.63 0.23 4.85 1.19 1.79 1.72 1.77 1.23 0.47 1.03 1.30

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

9

H.-M. Li et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

70

25

60

TFeO(%)

Al 2 O 3(%)

20 15

50 40 30 20

10

10 5

10

20

30

40

50

70

60

0

80

10

20

30

SiO 2 (%) 20

10

16

8

CaO(%)

MgO(%)

12

6 4

50

60

70

80

60

70

80

12 8 4

2 0 10

0 20

30

40

50

70

60

80

10

20

30

SiO 2(%)

40

50

SiO 2(%)

10

15 AIO

8

KIO

12

K 2 O(%)

Na 2 O(%)

40

SiO 2(%)

6 AKIO

4 2

9 6 AKIO

3

AIO

KIO

0 10

20

30

40

50

70

60

0

80

10

20

30

40

50

70

60

80

SiO 2(%)

5

5

4

4

TiO 2(%)

TiO 2(%)

SiO 2(%)

3 2

3 AIO

2

AIO

1

1 0

10

20

30

40

50

70

60

0

80

0

10

20

SiO 2(%) 15

50

60

70

10

KIO

9

MgO(%)

K 2 O(%)

40

12

12

AKIO

6

8 6 4

AIO

3 0

30

TFeO(%)

AKIO

2 0

2

4

6

8

10

0

0

10

Na 2 O(%) Oligoclase-iron oxide type (OIO)

20

30

40

50

60

70

TFeO(%) Oligoclase-albite-iron oxide type (OAIO)

Albite-K-fledspar-iron oxide type (AKIO)

Albite-iron oxide type (AIO)

K-feldspar-iron oxide type (KIO)

Fig. 10. Binary diagram of oxides for the iron-rich magmatic fragments.

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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H.-M. Li et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

performed using a JEOL JXA-8230 Superprobe at the MLR Key Laboratory of Metallogeny and Mineral Assessment, China. Some of the magnetite domains display high contents of SiO2, Al2O3 and Na2O due to the effect feldspars surrounding the fine-grained magnetite. Based on the data plots in Fig. 11, the magnetite in AIO is enriched in TiO2 (1.84 wt.% in average and 4.86% the highest) and V2O3 (1.04 wt.% in average and 3.20% the highest). The contents of TiO2 and V2O3 have evidently negative and positive correlations with total FeO content. The magnetite of the garnet skarn has lower TiO2 (0–0.12 wt.%, 0.03 wt.% in average) and V2O3 (0– 0.08 wt.%, 0.02 wt.% in average) contents. Compared with the skarn deposits of hydrothermal origin, the obviously different TiO2 and V2O3 compositions may suggest a magmatic origin. The TiO2 content of the magnetite in different types of IRF was analyzed by EDS for comparison and the results are listed Table 4. The Ti content of the magnetite varies in different types of the fragments: the magnetite in AIO has TiO2 content of 0.28–3.39 wt.% with an average of 1.13 wt.%. The magnetite in KIO has variable content of TiO2, with most being low (from 0 to 0.08 wt.%) and some being high (from 2.75 to 4.38 wt.%). One point of OIO has TiO2 content of 2.93 wt.%. Two points of OAIO have TiO2 content of 0.85 wt.% and 6.85 wt.%. Three points of AKIO have TiO2 content of 0–5.38 wt.%. In summary, magnetite in the AIO, OAIO and OIO has high TiO2 content, showing magmatic affinity. Duo to intense hydrothermal alteration, the Ti of the magnetite in KIO and AKIO has been leached out to form ilmenite and titanite, resulting in the low Ti in the magnetite. Tiny ilmenite and titanite grains in the magnetite may account for the high content of TiO2 of the tested points in the KIO and AKIO magnetite. 6. Discussion 6.1. Genesis of the iron-rich fragments The iron-rich fragments (IRF) are characterized by porphyritic texture and amygdaloidal textures, with albite constituting the

major porphyritic mineral. The groundmass of IRF consists of magnetite and euhedral microlite of albite or K-feldspar. The feldspar is distributed irregularly or shows minor orientation with intergranular magnetite, showing diabasic or hyalopilitic texture. The vesicles are filled by quartz, K-feldspar, chlorite and calcite. All of the above characteristics indicate that the IRF shows typical volcanic textures. These IRFs experienced late hydrothermal replacement of different degree. The OAIO and AIO were slightly influenced by late hydrothermal fluids. Hydrothermal quartz and chlorite filled in the vesicles (Fig. 7a–d). Hydrothermal calcite replaced feldspar phenocryst (Fig. 7e–h). Although magnetite, oligoclase and albite also replaced feldspar phenocryst (Fig. 6a–d and Fig. 7e–h), these are not found in amygdales, and the feldspar phenocryst in the groundmass shows complete crystal shape (Figs. 5a and b, 6c and d, 7c and d, g and h). Magnetite in the groundmass is high in TiO2 and V2O3 contents which is different with the hydrothermal magnetite (Tables 3 and 4). Furthermore, the mineral assemblage of magnetite, oligoclase and albite is not spatially related to hydrothermal minerals such as quartz and chlorite, indicating that the magnetite, oligoclase and albite crystallized earlier than amygdales and are probably of magmatic origin. The AKIO and KIO were intensely influenced by late hydrothermal fluids. Besides hydrothermal chlorite, the filling minerals in vesicles of the AKIO consist of K-feldspar and albite (Fig. 8c and d, g and h, k and l). The hydrothermal albite in the amygadales occurs as anhedral aggregates, which is different from the euhedral lath-shaped oligoclase and albite of magmatic origin. Magnetite in the AKIO is fine-grained, which is similar to that in the OIO, OAIO and AIO, but show lower contents of TiO2 and V2O3. Outward from the wall of the amygdales to the groundmass, mineral aggregates of K-feldspar, albite and magnetite gradually changes to mineral aggregates of albite and magnetite (Fig. 8k and l), indicating that the hydrothermal metasomatism was gradually weakened. The KIO experienced the most intense hydrothermal metasomatism, with the near-total disappearance of albite. Magnetite was replaced by hematite with low TiO2 and V2O3 contents (Table 4). Iron oxides crystallized as druse along the wall of

Table 3 EPMA analyses of iron oxides in the 515-23 AIO-type fragments and YMS-65 garnet–magnetite ore. Sample

Fe-oxides

Na2O

MgO

Al2O3

K2O

CaO

P2O5

SiO2

ZnO

FeO

MnO

TiO2

Cl

Cr2O3

NiO

V2O3

Total

515-23-10 515-23-11 515-23-12 515-23-14 515-23-16 515-23-17 515-23-18 515-23-20 515-23-21 515-23-22 515-23-23 515-23-24 515-23-25 515-23-26 515-23-27 515-23-28 515-23-30 515-23-31 515-23-32 515-23-33 YMS-65-11 YMS-65-12 YMS-65-13 YMS-65-14 YMS-65-15 YMS-65-16 YMS-65-17 YMS-65-18

Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite Magnetite

0.04 0.65 0.15 0.21 0.14 0.00 0.03 0.06 0.06 0.10 0.04 0.12 0.09 0.09 0.00 0.64 0.08 0.08 0.00 0.73 0.04 0.15 0.11 0.00 0.07 0.03 0.05 0.06

0.59 0.37 0.04 0.08 0.16 1.40 1.40 0.00 0.04 0.00 0.01 0.00 1.56 1.54 0.03 0.06 0.00 0.03 0.01 0.08 0.02 0.05 0.05 0.00 0.00 0.00 0.01 0.00

0.67 1.56 0.39 0.51 0.50 0.45 0.50 0.28 0.25 0.41 0.59 0.41 1.23 1.59 0.44 0.96 0.75 0.70 0.34 1.39 0.14 0.32 0.53 0.03 0.02 0.01 0.02 0.05

0.00 0.01 0.04 0.02 0.01 0.00 0.01 0.04 0.01 0.02 0.02 0.05 0.03 0.07 0.05 0.02 0.07 0.04 0.00 0.09 0.00 0.03 0.04 0.00 0.00 0.00 0.00 0.01

0.11 0.05 0.02 0.00 0.04 0.01 0.00 0.00 0.01 0.11 1.36 0.25 0.33 0.19 0.00 0.17 0.00 0.05 0.06 0.11 0.08 0.24 0.40 0.00 0.06 0.16 0.00 0.00

0.01 0.00 0.00 0.04 0.03 0.00 0.01 0.00 0.05 0.02 0.01 0.00 0.00 0.00 0.00 0.11 0.03 0.00 0.00 0.01 0.00 0.00 0.03 0.00 0.00 0.00 0.01 0.00

0.21 3.08 3.04 1.16 1.07 0.17 0.10 1.01 0.98 1.09 1.20 1.11 0.12 0.17 0.79 2.43 1.55 2.12 0.76 3.54 0.47 1.10 1.44 0.28 0.39 0.15 0.19 0.45

0.00 0.00 0.02 0.00 0.00 0.23 0.03 0.06 0.00 0.00 0.03 0.08 0.00 0.00 0.00 0.04 0.09 0.00 0.21 0.00 0.00 0.00 0.04 0.00 0.00 0.00 0.00 0.00

82.84 81.08 82.89 86.15 83.68 83.95 83.78 87.36 85.86 87.14 84.99 85.93 81.61 81.71 86.75 83.31 86.65 86.62 88.02 81.38 93.66 91.64 91.79 94.16 93.08 92.23 94.03 93.11

0.18 0.17 0.08 0.02 0.11 0.62 0.58 0.04 0.07 0.06 0.08 0.09 0.27 0.18 0.07 0.01 0.05 0.10 0.01 0.15 0.01 0.03 0.08 0.12 0.00 0.00 0.00 0.07

3.37 2.92 1.84 2.03 2.81 2.64 2.91 0.00 0.03 0.18 0.03 0.15 4.86 4.12 1.88 1.93 0.81 0.53 1.20 2.50 0.04 0.08 0.00 0.00 0.01 0.00 0.12 0.00

0.00 0.00 0.01 0.01 0.03 0.01 0.00 0.02 0.03 0.03 0.02 0.01 0.00 0.02 0.00 0.02 0.02 0.00 0.01 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.00 0.00

0.07 0.06 0.06 0.03 0.04 0.01 0.02 0.15 0.16 0.13 0.15 0.12 0.05 0.09 0.03 0.03 0.05 0.03 0.04 0.01 0.04 0.04 0.04 0.04 0.04 0.07 0.02 0.04

0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.02

0.67 0.50 0.66 0.45 0.27 0.04 0.10 2.73 3.20 2.64 2.17 2.31 0.41 0.38 0.33 0.39 0.91 1.21 0.75 0.69 0.00 0.01 0.00 0.06 0.04 0.00 0.00 0.08

88.77 90.44 89.23 90.69 88.88 89.54 89.46 91.75 90.74 91.92 90.70 90.61 90.56 90.12 90.35 90.12 91.05 91.54 91.44 90.69 94.49 93.68 94.54 94.69 93.71 92.65 94.45 93.87

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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H.-M. Li et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

5

5

Albite-ironoxide type (AIO)

Albite-iron oxide type (AIO)

Garnet-magnetite ore (garnet-Fe)

Garnet-magnetite ore (garnet-Fe)

4

3

V 2 O 3 (%)

TiO 2 (%)

4

AIO 2

1

3

AIO

2

1

Garnet-Fe

Garnet-Fe 0

0 80

85

90

95

100

80

90

85

95

100

TFeO (%)

TFeO (%)

Fig. 11. Plots of total FeO vs. TiO2 and V2O5 for magnetite in the AIO and the garnet-magnetite ore.

Table 4 EDS analyses of the magnetite in different types of IRF. Sample

Fragment

SiO2

Al2O3

TFeO

MgO

CaO

Na2O

K2O

TiO2

515-24-2 515-24-3 L10-2-3 515-24-1 515-34-1 515-34-2 515-34-3 515-34-4 515-34-6 L10-2-1 L8-4-1 L8-8-1 515-27-3 L9-3-5 YMS-51-1 L9-3-1 L9-3-2 L9-3-3 L9-3-7 YMS-50-10 YMS-50-3 YMS-50-6 YMS-51-5

Oligoclase-iron oxide type Oligoclase-albite-iron oxide type Oligoclase-albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type Albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-albite-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type K-feldspar-iron oxide type

16.11 15.09 5.89 12.79 8.96 7.46 11.89 8.94 9.41 8.94 13.41 6.69 6.3 9.32 9.69 12.02 15.24 13.76 9.45 6.64 9.24 10.09 11.55

14.09 5.02 6.91 8.12 7.37 7.29 7.97 5.52 6.8 8.84 4 0 0 11.07 7.74 9.9 9.6 11.03 7.82 4.87 8.69 7.78 8.16

70.48 77.58 73.29 72.76 84.05 75.59 72.23 75.54 73.93 74.92 76.62 85.24 73.11 79.84 78.07 77.72 68.66 71.87 79.35 81.95 78.8 80.85 76.8

1.05 0 1.8 0 0 0.68 0 0.65 0 1.65 0 1.15 0 0 0 0.43 0.92 0.82 0 0 0 0 0

3.01 1.2 1.9 1.11 0 0.38 0.78 1.5 0 0 0 0 0 0 0 0 2.31 0 0 0.62 0 0 0

4.8 3.02 0.54 5.82 4.85 7.52 4.77 2.59 2.7 5.09 0.97 0 0 0 2.35 0 0 0 0 0 0 0 1.4

0 0 1.04 0 0 0.72 0.65 0 0 0 0 0 0 2.51 1.98 0 5.54 4.39 2.77 2.41 3.81 3.21 3.71

2.93 0.85 6.85 1.42 1.32 1.08 2.07 0.47 2.08 1.88 5.65 1 5.38 0 2.65 0 0 0.08 0 3.07 4.38 0 2.75

the vesicles (Fig. 9c and d) or occur as laths in groundmass coexisting with sphene (Fig. 9e and f). The formation of sphene is probably related to the replacement of magnetite to hematite. The K-feldspar grains show irregular boundary, suggesting hydrothermal origin. Albite, chlorite, epidote and carbonate minerals are common in marine volcanic rocks, such as the spilite–keratophyre series. This mineral assemblage has been widely considered to indicate the interaction between volcanic rocks and seawater (Seyfried et al., 1978; Rosenbauer et al., 1988). In general, Na of albite is sourced from seawater, whereas Fe is leached from the basalts and thus sourced from primitive magma (Humphris and Thompson, 1978; Reed, 1983). Iron-rich magma showing similar features is common, such as the ferrobasalt (up to 20% total FeO) as reported from Large Igneous Provinces (e.g. Zhang et al., 2006; Zhou et al., 2008; Charlier et al., 2013; Song et al., 2013) and Proterozoic massif anorthosites (Owens and Dymek, 1992; Ashwal, 1993; Vander Auwera et al., 1998). The iron-rich igneous rocks are typically generated by fractional crystallization (Duchesne, 1999; Lindsley, 2003; Dymek and Owens, 2001; Tollari et al., 2008). Alternatively, magma immiscibility is also considered as a

potential mechanism for the generation of the iron-rich igneous rocks (Philpotts, 1967; Kolker, 1982; Charlier et al., 2011; Holness et al., 2011; Jakobsen et al., 2011; Namur et al., 2012; VanTongeren and Mathez, 2012). Osborn (1979) proposed two modes of fractional crystallization based on different oxygen fugacity conditions. (1) In the open system with persistent high fO2 during fractional crystallization, the magnetite crystallizes with olivine and then pyroxene, resulting in the high contents of Si in the residual magmas (Bowen trend). (2) In the lower fO2 system, the firstly crystallized mineral is generally olivine without magnetite, thus the residual magma is Fe enriched with relatively lower Si contents (Fenner trend). Experimental results show that the total Fe content of the iron-rich magma that results from the extreme Fe enrichment is less than 22 wt.% (Veksler et al., 2007; Zhang et al., 2014b). The actinolite (or tremolite)-biotite bearing magnetite orebodies in the world-class Marcona deposit in Peru has been considered as the product of iron oxide-rich melt (Chen et al., 2010b). The Yamansu iron deposit shows different characteristics when compared to the Marcona deposit. For example, no Fe-poor and Si-rich igneous rocks have been observed in our study area, and the total FeO contents of the iron-rich rocks show

Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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significant variation. Thus, the iron-rich rocks could not have been generated by immiscibility. Iron-rich magmatic fragments in Yamansu are rich in iron oxides and albite but lack actinolite, tremolite and biotite with low contents of MgO. The total FeO contents are negatively correlated with MgO, which corresponds to the Fenner trend which is different from the Ca–Mg–K-rich iron oxide-rich melt of the Marcona deposit. The magnetite is enriched in V and Ti, indicating a high-temperature magmatic origin.

(2)

6.2. Geological implications The host rock of the Yamasu deposit is Na-rich submarine volcanics (Fu et al., 1986). The presence of iron-rich magmatic fragments in these rocks indicates the role of Fe- and Na-rich residual magma. The erupted iron-rich residual magma formed the iron-rich intrusions or volcanics, where the total Fe content reached more than 20 wt.% and formed low-grade iron orebodies. The Manyang deposit in Yunnan province is genetically related to the spilite, where the albite in the orebodies commonly occurs as phenocryst in the magnetite-rich groundmass (Song et al., 1981). The vesicles of Manyang iron ores are filled with hydrothermal minerals such as tourmaline, apatite, albite, chlorite and calcite. All the above features are similar to those of the magmatic fragments in Yamansu deposit. The iron-rich magmatic fragments in the Yamansu occur near the orebodies. The high-grade orebodies mainly consist of magnetite and garnet without albite-magnetite ores which were likely destroyed by late hydrothermal activities. The volcanic and sedimentary-volcanic rocks were altered to the garnet skarns with common potassic alteration (Li et al., 2014c). The ores with magnetite poor in Ti indicate hydrothermal origin that formed by the dissolution-precipitation of the early-formed iron-rich igneous rocks or magmatic low-grade orebodies that formed by iron-rich melts. The magnetite in KIO lacks Ti, corresponding to late metasomatism. The iron ores resulted from the late metasomatism and enrichment. The total FeO content of IRF is up to 63.47 wt.% with an average of 26.01 wt.%. The corresponding total Fe content is 49.37 wt.% and 20.23 wt.%, respectively. These fragments belong to the low-grade ores (20% 6 TFe < 50%), suggesting that the ore deposits of magmatic origin are low-grade in this area. The high-grade ores in El Laco of Chile and Kiruna in Sweden which were considered as magmatic origin are now interpreted as a hydrothermal origin (Sillitoe and Burrows, 2002; Smith et al., 2013; Dare et al., 2015). The high-grade orebodies are genetically related to skarn alteration. Hu et al. (2014) has proposed the high-grade orebodies are the result of the dissolution-precipitation of the early-formed magnetite, when the impurities in the ores were filtered. According to Table 3, the magnetite in the garnet skarn has higher total FeO contents than those in the iron-rich magmatic fragment, which could be interpreted by the dissolution-precipitation model. Submarine volcanic rock-hosted iron deposits such as the Yamansu are widely distributed in the Eastern Tianshan area. The Awulela mineralization belt in Western Tianshan area forms another cluster of the SVIO deposits. The finding of the iron-rich magmatic fragment in Yamansu has important implications for the ore genesis, as well as in future exploration of the SVIO deposits in Eastern Tianshan and other areas. 7. Conclusions (1) The iron-rich fragments in the Yamansu iron deposit are observed in the volcanic breccia, sedimentary volcanic breccia and ignimbrite. The fragments are subdivided into

(3) (4)

(5)

oligoclase-iron oxide type, oligoclase-albite-iron oxide type, albite-iron oxide type, albite-K-feldspar-iron oxide type and K-feldspar-iron oxide type. The fragments commonly exhibit porphyritic texture and amygdaloidal structure with phenocrysts of feldspars. The groundmass consists of magnetite and microlite feldspar which occur as disoriented laths. The magnetite is distributed in the framework formed by the feldspars. The magnetite and feldspar constitute diabasic texture or hyalopilitic texture. The vesicles are filled with quartz, chlorite and calcite from margin to interior. The fragments show high contents of Si, Al, Fe, Na and Ti and low content of Mg. The total FeO content is up to 26 wt.%. The magnetite in the fragment has TiO2 content up to 4.86 wt.% with an average of 1.84 wt.% and V2O3 content up to 3.20 wt.% (1.04 wt.% in average). The iron-rich fragments are of magmatic genesis, possibly generated by the emplacement of residual magma.

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Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026

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Please cite this article in press as: Li, H.-M., et al. Iron-rich fragments in the Yamansu iron deposit, Xinjiang, NW China: Constraints on metallogenesis. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.026