The Pobei Cu-Ni and Fe ore deposits in NW China are comagmatic evolution products: evidence from ore microscopy, zircon U-Pb chronology and geochemistry

Y.G. LIU; W.Y LI; X.B. LÜ; Y.H. HUO; B. ZHANG

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Abstract: The Pobei mafic-ultramafic complex in northwestern China comprises magmatic Cu-Ni sulfide ore deposits coexisting with Fe-Ti oxide deposits. The Poshi, Poyi, and Podong ultramafic intrusions host the Cu-Ni ore. The ultramafic intrusions experienced four stages during its formation. The intrusion sequence was as follows: dunite, hornblende-peridotite, wehrlite and pyroxenite. The wall rock of the ultramafic intrusions is the gabbro intrusion in the southwestern of the Pobei complex. The Xiaochangshan magmatic deposit outcrops in the magnetite- mineralized gabbro in the northeastern part of the Pobei complex. The main emplacement events related to the mineralization in the Pobei complex, are the magnetite-mineralized gabbro related to the Xiaochangshan Fe deposit, the gabbro intrusion associated to the Poyi, Poshi and Podong Cu-Ni deposits, and the ultramafic intrusions that host Cu-Ni deposits (Poyi and Poshi). The U-Pb age of the magnetite-mineralized gabbro is 276±1.7Ma, which is similar to that of the Pobei mafic intrusions. The εHf(t) value of zircon in the magnetite-mineralized gabbro is almost the same as that of the gabbro around the Poyi and Poshi Cu-Ni deposits, indicating that the rocks related to Cu-Ni and magnetite deposits probably originated from the same parental magma. There is a trend of crystallization differentiation evolution in the Harker diagram from the dunite in the Cu-Ni deposit to the magnetite-mineralized gabbro. The monosulfide solid solution fractional crystallization was weak in Pobei; thus, the Pd/Ir values were only influenced by the crystallization of silicate minerals. The more complete the magma evolution is, the greater is the Pd/Ir ratio. The Pd/Ir values of dunite, the lithofacies containing sulfide (including hornblende peridotite, wehrlite, and pyroxenite) in the Poyi Cu-Ni deposit, magnetite-mineralized gabbro, and massive magnetite, are 8.55, 12.18, 12.26, and 18.14, respectively. Thus, the massive magnetite was probably the latest product in the evolution of the Pobei mafic-ultramafic intrusions. We infer that the Cu-Ni sulfide and Fe-Ti oxide ores in the Pobei area were products of a cogenetic magma at different evolutionary stages; at the late stage, the magma became iron enriched through crystallization differentiation. The magma differentiation occurred in a deep staging magma chamber emplaced in the upper magma chamber. Earlier crystallized olivine with some interstitial sulfides gathered at the bottom of the staging magma chamber because of its greater density. That is to say, the ultramafic magma hosting the Cu-Ni sulfide formed at the bottom of the staging magma chamber, while the magnetite-mineralized gabbro was in the upper part. However, the magnetite-mineralized gabbro injected into the upper magma chamber first and the ultramafic lithofacies containing the olivine and the interstitial Cu-Ni sulfides were subsequently emplaced in the upper magma chamber as crystal mush.

Keywords:Magmatic Cu-Ni sulfide depositMagmatic Cu-Ni sulfide deposit,Xiaochangshan Fe depositXiaochangshan Fe deposit,Crystallization differentiationCrystallization differentiation,Magnetite-mineralized gabbroMagnetite-mineralized gabbro,BeishanBeishan.

Carátula del artículo

The Pobei Cu-Ni and Fe ore deposits in NW China are comagmatic evolution products: evidence from ore microscopy, zircon U-Pb chronology and geochemistry

Y.G. LIU ltdzgj_2006@126.com

University of Geosciences, China

W.Y LI

Ministry of Land and Resources, China

X.B. LÜ luxb@cug.edu.cn

China University of Geosciences, China

Y.H. HUO huoyh14@lzu.edu.cn

Lanzhou University, China

B. ZHANG zhangbo5015@foxmail.com

Peking University, China

Geologica Acta: an international earth science journal, vol. 15, no. 1, pp. 37-50, 2017
Universitat de Barcelona

This work is licensed under Creative Commons Attribution-ShareAlike 4.0 International.

Received: 15 July 2016

Accepted: 15 November 2016

Published: 15 February 2017

INTRODUCTION

The Pobei mafic-ultramafic complex is unique because of the coexistence of magmatic Cu-Ni sulfide ore and Fe-Ti oxide deposits. The symbiosis of magmatic sulfide deposits and magmatic titanium iron oxide ore (including magnetite, ilmenite, and titanomagnetite) is visible in some parts of the world (Wang et al., 2010). For example, not only chromium and Platinum Group Elements (PGE) but also magnetite oreoutcrops are found in the Busheveld complex (Cawthorn and Molyneux, 1986). The Xinjie layered intrusion in the Panxi area of China contains both vanadic-titanomagnetite and copper-nickel-platinum. The upper part has two layers of vanadium-titanium magnetite, while the lower part has four layers of Cu-Ni-Pt sulfide ore (Li et al., 2006a; Zhu et al., 2010; Zhong et al., 2011). The Xiangshan intrusion in the eastern Tianshan area of China hosts both Ti-Fe and Cu-Ni sulfide mineralization (Wang et al., 2006, 2009; Xiao et al., 2010).

There are two hypotheses explaining the coexistence of the magmatic sulfide ore deposit and the Fe-Ti oxide deposit. One is that the formation of Fe-Ti oxide ore occurred later than that of the sulfide; the Cu-Ni sulfide ore and the Fe-Ti oxide ore were products of a cogenetic magma at different evolutionary stages; and the liquids were iron enriched through fractionation at the late stage in the layered intrusions (Scoon and Mitchell, 1994; Wang et al., 2006, 2009; Xia, 2009; Xiao et al., 2010; Jiang, 2014). The other hypothesis argues that the Fe-Ti oxide ore was formed earlier than the sulfides, while periodic magma recharge and the continuous mixing of resident and replenishing ferropicritic magma led to the crystallization of minor chromite and Cr-bearing magnetite and, thereafter, to relatively abundant Fe-Ti oxides, with the resultant S saturation giving rise to PGE-enriched sulfide droplets (Zhu et al., 2010; Zhong et al., 2011).

Based on field observations, geochronology, major and trace element geochemistry, and PGE geochemistry, we discuss the petrogenetic relationships between CuNi sulfide and magnetite deposits and the mineralization process in the Pobei complex. A deep understanding of the relationship between the magmatic Cu-Ni sulfide ore and Fe-Ti oxide deposits will help the exploration of mafic- ultramafic intrusions, especially the metallogenic types.

GEOLOGIC SETTING

Early Permian mafic-ultramafic intrusions discovered at the northeastern margin of the Tarim Basin in Xianjiang in northwestern China (Fig. 1), host economic magmatic sulfide deposits. This made Xinjiang the second most important district of nickel resources in China (Mao et al., 2008; Qin et al., 2011; Han et al., 2013; Yang et al., 2014).

The Tarim Basin is the largest sedimentary basin in China (Fig. 1B). According to recent geological mapping, drill core logging, and seismic data, the Permian ultramafic-mafic rocks of the the Large Igneous Province (LIP) in the Tarim Basin, form a NE-SW-trending zone across the basin, with an outcrop area of approximately 250,000km2 (Fig. 1B; Jia, 1997). The Permian igneous units are dominated by basalts, with some peridotites, gabbros, basaltic andesites, and dolerites.

Basalt is mainly distributed in the eastern and southern part of the Bachu area, and the Early Permian Tajilitag V-Ti-Fe deposit is located in the southeast (Fig. 1B)(Li et al., 2001; Zhang et al., 2008a). Early Permian basalt has also been found in the Liuyuan and Pobei areas in Beishan, along the northeastern margin of Tarim (Jiang et al., 2007; Xiao, 2004).

The Cu-Ni deposits along the northeastern margin of the Tarim Basin are mainly distributed in three areas (Fig. 1C): Beishan (Pobei), central Tianshan, and eastern Tianshan (Han et al., 2004; Zhou et al., 2004; Jiang et al., 2006; Chai et al., 2008; Zhang et al., 2008b; Han et al., 2010; Qin et al., 2011; Zhang et al., 2011; Tang et al., 2012; Zhao et al., 2015; Liu et al., 2015, 2016).

FIGURE 1
Tectonic units and distribution of magmatic deposits and Early Permian basalts in northern Xinjiang, northwestern China (modified after BGMRXUAR, 1993; Xiao et al., 2004), and age resources (Qin et al., 2002; Mao et al., 2003; Wu et al., 2005; Li et al., 2006b; Xiao et al., 2010; Jiang et al., 2012).

The Cu-Ni deposits related to the Early Permian maficultramafic complexes in eastern Tianshan include the Huangshandong, Tulargen, Huangshanxi, Hulu, and Xianshan deposits (Fig. 1C). The surrounding rocks are mainly Middle Ordovician and Carboniferous strata (Dong, 2005).

Central Tianshan was formed from a Precambrian block containing both the Tianyu and Baishiquan Cu-Ni deposits (Fig. 1C). The surrounding rocks of these deposits comprise Paleoproterozoic and Mesoproterozoic strata, which are mainly composed of mica quartz schist, gneiss, and migmatite, with ages in the range of 997Ma–1829Ma (Dong, 2005).

The Beishan (Pobei) area is generally considered to be the rift (Xiao, 2004; Su et al., 2011) in which the Poyi and Poshi Cu-Ni deposits formed related to mafic-ultramafic intrusions (Fig. 1C). The surrounding rocks of these intrusions include Paleoproterozoic quartz-biotite schist (2203±74Ma), Mesoproterozoic gneissic biotite-plagioclase granite (1311Ma), mica quartz schist, and marble (Xiao, 2004).

The intergrowth of the fault sag and fault uplift in the Beishan region separates the pre-Permian sedimentary sequences (BGMRXUAR, 1993; Qin et al., 2011); the discovery of Permian bimodal igneous rocks in the Pobei area (Su et al., 2011; Yang, 2011) supports the fact that the Beishan region, in which the Pobei Cu-Ni-Fe deposit is located, is a Permian rift (Xiao, 2004). The Beishan Rift Zone is composed of Precambrian crystalline basement, mainly Paleoproterozoic rocks, overlaid by sedimentary rocks (Xiao, 2004). The rift zone is separated from the Kruktag Block by the Xingxingxia fault in the west and from the Dunhuang Block by the Liuyuan fault in the east (Fig. 1).

The exposed rocks in the Beishan area are mainly part of the Paleoproterozoic Beishan group, the Mesoproterozoic Gudongjing group, the Lower Paleozoic Cambrian Shuangyingshan Formation, the Silurian Tushenbulake Formation, the Carboniferous Hongliuyuan Formation, the Shibanshan Formation, the Shengliquan Formation, the Permian Hongliuhe Formation, the Luotuogou Formation, the Paleogene Taoshuyuan Formation, and the Neogene Kuquan Formation (Xiao, 2004).

The Pobei mafic-ultramafic complex is approximately 35km in length and 8km in width. The mafic-ultramafic intrusions exhibit close relationships with regional faults (Fig. 2). The Baidiwa-Yunihe fault has a long-term effect on the distribution of the Pobei mafic-ultramafic intrusions and iron-copper-nickel ore deposits.

GEOLOGY OF THE POBEI Cu-Ni AND XIAOCHANGSHAN FE DEPOSITS

The Pobei complex is mainly composed of three mafic intrusions: gabbro, magnetite-mineralized gabbro, and leucocratic gabbro, from southwest to northeast (Fig. 2). The wall rock of the Poshi, Poyi, and Podong ultramafic rocks which host the Cu-Ni ore, comprises gabbro intrusions; the Xiaochangshan Fe deposit outcrops in the southern part of the magnetite-mineralized gabbro intrusion (Fig. 2). In the field, the gabbro cross-cutting the magnetite-mineralized gabbro (Fig. 3) indicates that the gabbro intrusion around the Poyi and Poshi Cu-Ni deposits was emplaced later than the magnetite-mineralized gabbro.

The Poyi multiple ultramafic intrusion experienced four stages during its formation. The intrusion sequence was as follows: dunite, hornblende-peridotite, wehrlite, and pyroxenite (Liu, 2015). The Poshi ultramafic intrusion has these four lithofacies together with troctolite. The Cu- Ni ore bodies (Ni% higher than 0.4%) only exists in the hornblende-peridotite in both the Poyi and Poshi deposits. The contact between the peridotite unit and the gabbro intrusions show a clear intrusive relationship. Moreover, gabbro xenoliths of several meters can be observed in the peridotite (Yang, 2011).

Therefore, there were three main emplacement events related to the mineralization in the Pobei complex, from early to late: the magnetite-mineralized gabbro related to the Xiaochangshan Fe deposit, the gabbro intrusion associated to the Poyi and Poshi Cu-Ni deposits, and the ultramafic intrusions that host Cu-Ni deposits (Poyi and Poshi).

FIGURE 2
Geologic map of the Pobei maficultramafic complex

The magnetite-mineralized gabbro is black (Fig. 4). Clinopyroxene (Cpx) accounts for 35%–40% and is euhedral to subhedral, with a particle size of approximately 0.5x1mm; plagioclase (Pl) accounts for 50%–55% and is euhedral to subhedral, with a particle size of approximately 0.5x1mm; and accounting for 2%–4% magnetite (Mt) also exists in intergranular clinopyroxene and plagioclase (Fig. 4); therefore, Mt crystallized later than Cpx and Pl. The body of the Xiaochangshan magnetite ore is mainly located around the contact of the magnetite-mineralized gabbro and marble (Fig. 5). The ore mineral is magnetite, and ilmenite is rare.

FIGURE 3
Gabbro around the Poyi and Poshi Cu-Ni deposits cross- cutting the magnetite-mineralized gabbro.

ANALYTICAL METHODS

Major and rare elements of 11 magnetite-mineralized gabbro smaples were analyzed at ALS Chemex (Guangzhou) Co. Ltd using ME-XRF26 and ME-MS81 methods, respectively. The analysis method for the loss of ignition was ME-GRA05.

The platinum group elements (PGEs) concentrations in 6 magnetite-mineralized gabbro samples and 2 Massive magnetite samples were determined at the Institute of Geochemistry of the State Key Laboratory of Ore Deposit Geochemistry in Guiyang, China, by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) coupled with a modified Carius tube isotope dilution method (Qi et al., 2007).

U-Pb dating and trace-element analysis of zircon from the magnetite-mineralized gabbro were conducted synchronously by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) at the State Key Laboratory of Geological Processes and Mineral Resources of the China University of Geosciences in Wuhan. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are described in Hu et al. (2008, 2012b) and Liu et al. (2008, 2010). The preferred U-Th-Pb isotopic ratios used for the standard Zircon 91500 are from Wiedenbeck et al. (1995). The uncertainty of the preferred values for the external standard 91500 was propagated to the ultimate results. Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_v.3 (Ludwig, 2003). The preferred values of element concentrations for the USGS reference glasses were taken from the GeoReM database. (http://georem.mpchmainz.gwdg.de/).

In situ Hf isotope ratio analysis of zircon was conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas 2005 excimer ArF laser ablation system (Lambda Physik, Göttingen, Germany) at the State Key Laboratory of Geological Processes and Mineral Resources of the China University of Geosciences in Wuhan. The detailed operating conditions are described in Hu et al. (2008, 2012a, b). The offline selection, integration of the signals, and mass bias calibrations were performed using ICP- MSDataCal (Liu et al., 2010).

FIGURE 4
Characteristics of the magnetite-mineralized gabbro in a hand specimen and under the optical microscope.

FIGURE 5
Relationship between the magnetite-mineralized gabbro and the magnetite ore body.

RESULTS

Geochronology

Zircon grains from all samples are transparent, mostly subhedral, and 50–150μm in length. The internal structure of the selected zircon crystals from the magnetite-mineralized gabbro was studied with cathodoluminescence images (Fig. 6). Some of the grains exhibit concentric zoning, and others are structureless. The analytical results of the selected zircon crystals are listed in Table I (see Electronic Appendix at www. geologica-acta.com). The concordia plots of the analyses are shown in Figure 7. The U, Th, and Pb contents of the zircons vary from 196–1348ppm, 112–947ppm and 32– 264ppm, respectively, and the Th/U ratios are ~0.46–0.82. All analyses give concordant U-Pb ages within analytical errors (Fig. 7), yielding a concordia age of 276±1.7Ma (MSWD=0.2) for the magnetite-mineralized gabbro.

Lutetium-Hafnium (Lu-Hf) isotopes

The Lu-Hf isotopes of the selected zircon crystals from the magnetite-mineralized gabbro are listed in Table II. The calculated εHf(t) values of these zircon crystals range from 4.0 to 6.1, and is 5.3 on average. While, the εHf(t) of the zircon crystals of the Pobei gabbro enclosing the Poyi and Poshi Cu-Ni deposit ranges from -0.8 to 5.3, with 3.2 on average (Su et al., 2011); thus, the magnetite-mineralized gabbro has a εHf(t) of zircon close to that of the Pobei gabbro around the Poyi and Poshi Cu-Ni deposits.

Major and Rare Earth Elements (REEs)

The concentrations of major and Rare Earth Elements (REEs) in the magnetite-mineralized gabbros are listed in Table III. The magnetite-mineralized gabbros contain 47.6%–55.8% SiO., 15.2%–21.3% Al.O., 3.2%–10.9% MgO, 0.10%–0.45% MnO, 5.85%–16.26% Fe.O. (total), 5.07%–12.95% CaO, 1.77%–3.97% Na.O, 0.67%–2.79% TiO., 0.20%–1.1% K.O, 0.01%–1.26 P.O., 0.02%–0.07% V.O., and 29.19–194.9ppm REEs.

Platinum Group Elements (PGEs)

The concentrations of PGEs in the magnetite- mineralized gabbro and massive magnetite ores are listed in Table IV. The magnetite-mineralized gabbro and magnetite ores have extremely low PGE concentrations (0.01–0.05ppb Ir, 0.015–0.036ppb Ru, 0.007–0.023ppb Rh, 0.119–0.292ppb Pt and 0.165–0.540ppb Pd) and show left-leaning primitive mantle-normalized PGE patterns (Fig. 8). The ∑PGE content of the magnetite ores ranges between 0.644 and 0.746ppb and is 0.695 on average, whereas that of the magnetite-mineralized gabbro is 0.4–0.69, with a mean value of 0.49. Both the magnetite-mineralized gabbro and magnetite ores have negative Ru anomalies in the primitive mantle-normalized PGE diagrams (Fig. 8) relative to primitive mantle concentrations (Barnes and Maier, 1999). The Pd/Ir ratio of the magnetite ores is 18.14, which is higher than that of the magnetite- mineralized gabbro.

FIGURE 6
Cathodoluminescence images of selected zircon crystals of the magnetite-mineralized gabbro.

DISCUSSION

The U-Pb age of the magnetite-mineralized gabbro is 276±1.7Ma (n=16, MSWD=0.2; Fig. 7). The U-Pb ages of the zircon crystals separated from the gabbro intrusions, which formed the wallrock of the Poshi, Poyi, and Podong ultramafic rocks, where the Cu-Ni ores exist, are 274±4 and 278±2Ma (Jiang et al., 2006; Li et al., 2006c). The fact that these two types of rock show the same age indicates that the rocks hosting the Cu-Ni and magnetite ore deposits probably originated from the same parental magma.

The average εHf(t) value of the zircons from the magnetite-mineralized gabbro and the Pobei gabbro around the Poyi and Poshi Cu-Ni ore deposits is 5.3 and 3.2, respectively, which indicate that they probably have the same origin. We have also calculated the degree of contamination of the surrounding rocks using the two-endmember isotope-mixing model (Fig. 9). The surrounding rocks of the gabbros are the Mesoproterozoic Baihu Group (Fig. 2) which is mainly composed of gneiss. So we choosed the Mesoproterozoic gneiss with 8.85ppm Hf and εHf (t= 276Ma) of -16.2 (He et al., 2015, calculated from εHf (t= 1409Ma)= 7.0) as one of the endmembers. Based on the Sr-Nd isotopes and trace elements, Yang (2011) suggested that the Pobei mafic- ultramafic rocks may have been derived from a depleted mantle source. Therefore, we chose the depleted mantle as the endmember. The Hf content of the depleted mantle (Fig. 9) is 2.05ppm (Sun and McDonough, 1989), and the εHf (t= 276Ma) of the depleted mantle is 19.4, roughly calculated from Salters and Stracke (2004) and Griffin et al. (2000). The other endmember comprises the Beishan Mesoproterozoic gneiss with 8.85ppm Hf and εHf (t= 276Ma) of -16.2 (He et al., 2015, calculated from εHf (t= 1409Ma)= 7.0). The comparison of the εHf(t) values with those of the depleted mantle and a potential contaminant, the Mesoproterozoic gneiss from Beishan (He et al., 2015), reveals that 13% and 16% of contamination are permitted for the magnetite- mineralized gabbro and the gabbro around the Poyi and Poshi Cu-Ni deposits, respectively (Fig. 9). The degree process of crystallization differentiation (Bowen and Schairer, 1956). MgO is negatively related to SiO. and positively related to TFe.O. (Fig. 10), indicating the crystallization differentiation of olivine. CaO and Al.O. show a negative correlation with MgO (Fig. 10), indicating the crystallization differentiation of clinopyroxene.

TiO. and MgO are negatively correlated (Fig. 10). A relationship between K.O, P.O., and MgO was not noted. The Harker diagrams of the Pobei mafic-ultramafic rocks, especially the relationships of SiO., TFe.O., CaO, Al.O., and MgO, reveal the crystallization differentiation evolutionary trend from the ultramafic rocks related to Cu-Ni deposits to the magnetite-mineralized gabbro related to the Xiaochangshan magmatic deposit. Although the Harker diagrams are not robust evidence to support the hypothesis of the same parental magma of the different intrusions, they at least do not contradict the crystallization differentiation of contamination of these two types of gabbros with almost the same outcrop area is, in general, 15%. If the outcrop area of the two types of gabbros resulting from homologous evolution is roughly the same, the degree of contact with the surrounding rock is probably also the same; therefore, they may have a similar degree of crustal contamination.

FIGURE 7
Zircon U-Pb isotope concordia plot for the magnetite-mineralized gabbro.

To gain a better understanding of the relationship between the magnetite-mineralized gabbro and the mafic- ultramafic rocks related to the Cu-Ni ore deposits in the Pobei area, the major elements and REEs of mafic- ultramafic rocks related to Cu-Ni ore deposits in Pobei were compiled (Fig. 10).

The composition changes during the evolution of the magma because of the separation of crystals. The compositional change trends with MgO can reflect the process of crystallization differentiation (Bowen and Schairer, 1956). MgO is negatively related to SiO2 and positively related to TFe2O3 (Fig. 10), indicating the crystallization differentiation of olivine. CaO and Al2O3 show a negative correlation with MgO (Fig. 10), indicating the crystallization differentiation of clinopyroxene. TiO2 and MgO are negatively correlated (Fig. 10). A relationship between K2O, P2O5, and MgO was not noted. The Harker diagrams of the Pobei mafic-ultramafic rocks, especially the relationships of SiO2, TFe2O3, CaO, Al2O3, and MgO, reveal the crystallization differentiation evolutionary trend from the ultramafic rocks related to Cu-Ni deposits to the magnetite-mineralized gabbro related to the Xiaochangshan magmatic deposit. Although the Harker diagrams are not robust evidence to support the hypothesis of the same parental magma of the different intrusions, they at least do not contradict the crystallization differentiation evolutionary trend.

FIGURE 8
Primitive mantle-normalized PGE pattern of magnetite- mineralized gabbro, massive magnetite ores, and dunite in the Poyi deposit. Primitive mantle values are from Barnes and Maier (1999). The dunite data were taken from Liu et al. (2015).

Because Mn, P, and REEs are not compatible with olivine and pyroxene during crystallization differentiation (Anderson and Greenland, 1969; Nagasawa et al., 1980; Hirokazu et al., 1984; Gaetani and Grove, 1997; Jones and Layne, 1997; Zack and Brumm, 1999; Adam and Green, 2006), they tend to be enriched in the residual phase. The magnetite-mineralized gabbro has the highest P, Mn, and REE contents (Fig. 10), which indicates that it is probably a later product of the magma evolution relative to the mafic-ultramafic rocks (i.e. dunite, hornblende-peridotite, wehrlite, and pyroxenite) that host the Cu-Ni deposits.

FIGURE 9
Quantitative simulation of the contamination degree of Paleoproterozoic rocks for two types of gabbro.

The PGEs, including Os, Ir, Ru, Rh, Pt, and Pd, have special chemical properties, leading to a different geochemical behavior compared to other trace elements. Therefore, the PGEs have a unique and irreplaceable geochemical tracer importance for the study of the

FIGURE 10
Harker diagrams of the Pobei mafic-ultramafic rocks (Jiang et al., 2010; Ling, 2011; Liu, 2011; Yang, 2011; Guo et al., 2012; Jiang et al., 2012; Liu, 2015; magnetite-mineralized gabbro: data from this paper).

mantle-derived magma origin and evolution and the genesis of magmatic deposits (Qi et al., 2007; Lorand et al., 2008; Song et al., 2009). Ruthenium is a compatible element during crystallization evolution (Capobianco et al., 1994; Hill et al., 2000; Puchtel and Humayun, 2001; Righter et al., 2004) and thus will gradually be lost in the residual magma, magnetite-mineralized gabbro, and massive magnetite ores, which show a loss of Ru relative to dunite in the Poyi Cu-Ni ore deposits (Fig. 9), indicating that they may be later products compared to the Poyi ultramafic rocks (dunite). During the crystallization of the silicate minerals, Ir is compatible, while Pd is incompatible (Capobianco et al., 1994; Hill et al., 2000; Puchtel and Humayun, 2001; Righter et al., 2004). Therefore, along with the crystallization of olivine, chromite, and pyroxene, the Ir content will decrease, whereas the Pd content will increase in the residual phase, causing an increase in the Pd/Ir ratio. Therefore, the more complete the magma evolution is, the greater is the Pd/ Ir. However, the Pd/Ir ratio can also be influenced by the sulfide fractional crystallization. Experimental studies indicate that the first phase to crystallize from a sulfide melt is a monosulfide solid solution (Mss; Naldrett et al., 1967; Kullerud et al., 1969; Misra and Flet, 1973, 1974). The Mss/sulfide liquid partition coefficient of Ir is 3.4–11, and that of Pd is 0.09–0.2 (Fleet et al., 1993; Barnes et al., 1997). If Ir were compatible during the fractionation of Mss in the rocks, Ir would show a negative correlation with Pd in the whole rock (Song et al., 2008). It is clear that Ir has a positive relationship with Pd in the Pobei area (Fig. 11), indicating that the Mss fractional crystallization was weak in Pobei. Therefore, the Pd/Ir values of the Pobei mafic-ultramafic rocks are mainly influenced by the crystallization of silicate minerals. The Pd/Ir values of dunite without sulfide, the lithofacies containing sulfide (including hornblende-peridotite, wehrlite, and pyroxenite) in the Poyi Cu-Ni deposit, magnetite-mineralized gabbro, and massive magnetite ores are 8.19, 12.18, 12.26, and 18.14, respectively. This finding indicates that the massive magnetite was probably the latest product in the evolution of the Pobei mafic- ultramafic intrusions, as the more complete the magma evolution is, the greater is the Pd/Ir.

FIGURE 11
Pd-Ir diagram for different lithofacies.

Based on all the evidence, we think that the magnetite-mineralized gabbro was a late product during crystallization differentiation compared to Pobei ultramafic rocks, but field evidence shows that the former was emplaced earlier. Hence, the question is, what happened? We infer that the magma differentiation occurred in a deep staging magma chamber. The olivine crystals sank to the bottom of the magma chamber, leaving the less mafic residual magma in the upper part of the chamber (Fig. 12I). At the late stage, the magma became iron enriched through crystallization differentiation. The ultramafic rocks hosting the Poyi and Poshi Cu-Ni sulfide deposits belong to the lower part of the magma chamber, while the magnetite-mineralized gabbro is located in the upper part (Fig. 12I). The magnetite-mineralized gabbro in the upper part of the deep chamber was injected first into the upper chamber (Fig. 12II). The earlier crystallized heavy olivine and some interstitial sulfides were at the bottom of the staging magma chamber and thus were emplaced last (Fig. 12IV), forming the intrusive ultramafic bodies.

CONCLUSIONS

The Cu-Ni sulfide and Fe-Ti oxide ores in the Pobei area were products of a cogenetic parent magma in the Early Permian. The parent magma experienced crystallization differentiation in a deep staging magma chamber. The earlier crystallized olivine and some interstitial sulfides were located at the bottom of the staging magma chamber because they were denser. The magnetite-mineralized gabbro was formed in the upper part of the staging magma chamber at the late stage of the crystallization differentiation process. The magnetite- mineralized gabbro then escaped from the deep staging magma chamber and injected first into the upper magma chamber. Subsequently, the olivine and the interstitial sulfides left the deep staging magma chamber and were finally emplaced in the upper magma chamber.

FIGURE 12
Cartoon showing the evolution in the deep staging magma chamber and the emplacement history of the Pobei complex in NW China.

Supplementary material

Appendices

Apéndice

TABLE I
LA-ICP-MS zircon U-Pb dating data from the magnetite-mineralized gabbro

TABLE II
Hf isotopic data of zircons from the magnetite-mineralized gabbro

εHf(t) was calculated using the method of Blichert-Toft and Albarède (1998); the 176Lu decay constant is λ=1.865 x 10-11 y-1 (Söderlund et al., 2004)

TABLE III
Major elements and REEs of magnetite-mineralized gabbros

TABLE IV
PGE contents in the magnetite-mineralized gabbros, massive magnetite ores, and lithofacies related to the Cu-Ni deposit in the Pobei area

Agradecimientos

This study was financially supported by the Pobei Cu-Ni Sulfide Deposits Metallogenic Regularity and Location Prediction of Rich Ores of the Xinjiang Province Project (XGMB2012012); the National Science and Technology Support Plan 305 project in Xinjiang (2011BAB06B04-0); the China Geological Survey Projects (DD20160013; 121201011000150005-19); the State

Scholarship Fund (201608610016) from the China Scholarship Council; and the China Postdoctoral Science Foundation (2015M582762XB).

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Notes