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Article

Origin and Nature of Parental Magma and Sulfide Segregation of the Baixintan Magmatic Ni–Cu Sulfide Deposit, Southern Central Asian Orogenic Belt (CAOB), NW China: Insights from Mineral Chemistry of Chromite and Silicate Minerals

by
Banxiao Ruan
1,*,
Mingyang Liao
1,
Bingke Sun
2 and
Chao Chen
1,*
1
Institute of Geological Survey, China University of Geosciences, Wuhan 430074, China
2
Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China
*
Authors to whom correspondence should be addressed.
Minerals 2020, 10(12), 1050; https://doi.org/10.3390/min10121050
Submission received: 27 October 2020 / Revised: 16 November 2020 / Accepted: 20 November 2020 / Published: 25 November 2020
(This article belongs to the Special Issue The Formation of Sulfide Ores in PGE-Cu-Ni Deposits)
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Abstract

:
The mineral chemistry of chromite and silicate minerals in the Baixintan magmatic Ni-Cu sulfide deposit in the Northern Tianshan, southern Central Asian Orogenic Belt (CAOB) are reported here. Two types of chromite were identified in mafic-ultramafic rocks. Type I chromite occurs as inclusions encased in olivine and has a primary and magmatic origin and homogeneous composition with Cr# values (49–66). It is characterized by high Ti contents (0.33–1.52 wt%) and small quantities of ZnO (0–0.21 wt%), MnO (0.28–0.45 wt%), and NiO (0.06–0.24 wt%) contents. In contrast, type II chromite with interstitial phase and larger compositional variations has significantly higher TiO2 (up to 6.2 wt%) and FeOt contents (up to 69.3 wt%) and slightly lower Al2O3 (minimum 3.0 wt%) and MgO contents (minimum 0.53 wt%). It is considered to crystallize from a more evolved and fractionated melt and suffers from post-magmatic alteration, such as serpentinization and chloritization. The olivine has forsterite values (Fo) varying from 76.8 to 85.6. The parental magma is characterized by high temperature (1389 °C), high pressure (3.8 Gpa), and high Mg content (11.4 wt%) with oxidized (FMQ + 1.6) and hydrous nature based on compositions of primary chromite and olivine–chromite pairs. The intrusion originated from high-degree partial melting of depleted mantle that had been modified by crustal components and metasomatized by subduction fluid in a post-orogenic extensional setting. Two stages of sulfide segregation have been recognized. Early segregation led to the depletion of platinum group elements (PGE), and disseminated sulfide mineralization was the product of later segregation. The assimilation of crustal Si and S components played more important roles on sulfide segregation rather than fractional crystallization.

1. Introduction

Chromite serves as one of the earliest crystallization phases in mafic-ultramafic magma, in which trivalent irons (Cr3+ and Al3+) diffuse at a much lower rate compared with divalent ions (Mg2+ and Fe2+) [1,2]. It has thus been an important petrogenetic indicator in mantle-origin rocks, such as implications for parental magma conditions, mantle source, and tectonic setting [3,4,5,6,7,8,9]. On the other hand, post-magmatic hydrothermal alteration, metamorphism, and subsolidus re-equilibration have significant effects on chromite composition [10,11,12,13,14]. Modified chromite has fingerprinted the alteration processes, while magmatic chromite with primary composition has traced the nature of parental magma. The olivine–chromite pair can also give constraints on equilibrium condition, such as temperature, pressure, and oxygen fugacity [10,15,16]. These conditions are key factors controlling sulfide immiscibility in magmatic sulfide deposits, and chromite composition has been used to understand Ni–Cu–platinum group element (PGE) mineralization in the Jinchuan [17], the Noril’sk–Talnakh [18], and the Sudbury deposits [19] as well as deposits in the southern Central Asian Orogenic Belt (CAOB) [20,21,22].
Abundant early Permian (275–285 Ma) mafic-ultramafic intrusions associated with Ni–Cu sulfide deposits occur in the Northern Tianshan, southern CAOB [23,24,25]. These deposits are mainly located in the eastern segment; however, there are some breakthroughs of Ni prospecting in the western segment in the past several years [26,27,28,29]. One example is the Baixintan deposit, a medium-sized deposit with 6.49 × 104 tons of Ni and 8.43 × 104 tons of Cu [30]. Preliminary petrogenetic studies have been conducted in the newly discovered deposit, such as geology, geochronology, geochemistry, and Sr–Nd–Hf–O–S isotopic studies [29,31,32,33]. The sulfide mineralization has also been discussed, such as sulfur saturation, quantitative modeling of sulfide segregation, and regional Ni–Cu sulfide prospectivity [26,29,32]. These studies are beneficial for understanding petrogenesis and mineralization; however, mineralogical studies, especially detailed chromite study, have not yet been conducted though it is widespread as accessory mineral in ultramafic-mafic rocks.
In this paper, systematical mineralogical studies of chromite and silicate minerals are conducted. First, two types of chromite have been identified according to mineral texture and occurrence. Then, parental magma composition and physicochemical conditions, including temperature, pressure, and oxygen fugacity, are estimated based on a new dataset of mineral chemistry. Their implications for tectonic setting and mantle source of mafic-ultramafic rocks are consequently presented. Finally, timescale and trigger factors of sulfide segregation are discussed based on olivine crystallization model and magma evolution.

2. Geological Background

2.1. Regional Geology

The CAOB, set between the Siberian and the Tarim-North China cratons, is one of the largest Phanerozoic accretionary belts on the globe and hosts numerous polymetallic deposits [34]. It is formed by multiple subduction-accretion of island arcs, oceanic plateaus, seamounts, ophiolites, and microcontinents that occurred from the late Proterozoic to the Mesozoic [35]. A series of Permian mafic-ultramafic rocks and related Ni–Cu deposits are located in the southern margin of the CAOB from NW China to NE China (Figure 1a, [36]). The Northern Tianshan and Beishan orogens are situated northeast to the Tarim craton and represent the southernmost segments of the CAOB in eastern Xinjiang, NW China (Figure 1b). The Northern Tianshan is separated from Tuha Basin to the north and the Middle Tianshan by the Aqikkuduk fault to the south and comprises the Dananhu-Harlike island arc, the Kangguer intra-arc basin, and the Yamansu arc [29,37,38,39].
The Dananhu-Harlike arc is composed of Ordovician–Carboniferous arc volcanic rocks, siliceous slate, and limestone [29]. Precambrian inherited zircons have been found in volcanic rocks from the Kalatage area [37,39]. The Late Carboniferous pillow basaltic rocks may form in a back-arc tectonic setting [40,41], whereas the Permian bimodal volcanism in the Bogda zone is considered to have formed in a post-collisional extensional environment caused by slab break-off [42,43]. Various types of arc-related ore deposits occur in this area, including the Silurian Huangtan volcanogenic massive sulfide (VMS) Cu–Au deposit, the Silurian Hongshan epithermal deposit, the Middle Devonian Xierqu skarn Fe deposit, the Middle Devonian Yudai porphyry Cu deposit, and the Early Carboniferous Tuwu porphyry Cu deposit [39].
In addition to widespread intermediate-mafic volcanic eruptions, more than 30 mafic-ultramafic intrusions have been identified along the east-trending Kangguer–Huangshan deep fault [26,28,44]. Available age data show that they were emplaced in early Permian with a major peak at approximately 280 Ma due to the interaction between the metasomatized lithospheric mantle and the ascending asthenospheric mantle, as a result of slab break-off or an upwelling mantle plume [23,24,43,45,46,47,48]. A few intrusions are known to host magmatic Ni–Cu sulfide deposits, such as evidenced by the Huangshan, Huangshandong, Tulaergen, and the newly found Yueyawan deposits. Most of them are located to the eastern segment of Northern Tianshan, and the Ni mineralization belt was previously considered not to extend to the western segment. However, the recent Ni prospecting breakthroughs in the Baixintan and Lubei mafic-ultramafic intrusions have extended the metallogenic belt to the west for 180 km (Figure 1c) [26,28,29].

2.2. Deposit Geology

The Baixintan complex occurs along the secondary Dacaotan fault and has an exposed area of 5 km2. It extends to depths of more than 400 m with a flat funnel shape. The immediate country rocks are middle Ordovician dacite, basaltic andesite, volcanic tuff, and sandstone clastic rocks, and later monzogranite and granodiorite are situated in the south. The complex generally consists of olivine gabbro, olivine websterite, and lherzolite. The olivine gabbro occurs in the north, and mineralized lherzolite appears in the southwestern region. The olivine websterite is found between lherzolite and olivine gabbro. The lherzolite and olivine websterite are enveloped by olivine gabbro in the cross section (Figure 2b).
The lherzolite is dark black and composed of olivine (50–60%); pyroxene (25–30%); amphibole (10–15%); and a little bit of plagioclase, chromite, and phlogopite (Figure 3a,d). The olivine websterite is grey black and contains pyroxene (50–70%) olivine (20%), plagioclase (5–10%), and accessory amphibole and chromite (Figure 3b). Significant alteration has been identified in these ultramafic rocks, including serpentinization, iddingsitization, chloritization, and talcization. The olivine in ultramafic rocks are 0.2–3 mm, idiomorphic, and surrounded by clinopyroxene and serpentine. The gabbro and olivine gabbro exhibit a mosaic texture and predominantly consist of plagioclase (approximately 50%), pyroxene (approximately 40%), and occasional olivine (5–10%) (Figure 3c).
The ore bodies occur as lenticles and funnels at depths of 0–200 m (Figure 2b). The sulfide mineralization appears mainly at the bottom of lherzolite with Ni content ranging from 0.27 wt% to 5.45 wt% (average 0.41 wt%) and Cu content ranging from 0.24 wt% to 1.54 wt% (average 0.60 wt%), respectively. They are dominantly disseminated sulfide, and vein-type sulfide is also recently discovered at these depths. Sulfide minerals, including chalcopyrite, pyrrhotite, pentlandite, pyrite, minor bornite, and violarite, occur as interstitial phases between olivine grains (Figure 3e,f).

3. Mineral Occurrence

Chromite, usually the first phase to crystallize from mafic-ultramafic magmas, occurs as an accessory mineral in the Baixintan complex and is more common in ultramafic rocks than in mafic rocks. It has sizes ranging from 20 to 200 μm. Two types of chromite have been identified based on their morphology and occurrence. The first type occurs as euhedral-subhedral inclusions in olivine and pyroxene and has straight borders and a bright white color (Figure 3g). The second type is more common and refers to anhedral or irregular interstitial grain surrounded by alteration minerals, such as serpentine and chlorite (Figure 3h). They usually have rough boundaries produced by later dissolution and alteration. Some of them have bright rims in BSE images.
The olivine dominantly occurs as cumulate grains in lherzolite or is occasionally enclosed by pyroxene and amphibole. The cumulate grains have larger size (0.3–3 mm) and are more fragmented. The enclosed ones are round and fresh with small size (0.2–0.5 mm). Pentlandite and pyrrhotite are found filled among olivine grains in lherzolite. The orthopyroxene mainly occurs as short columnar and granular in olivine websterite and lherzolite, while clinopyroxene is common in various rock types. Alteration minerals are generally found surrounding pyroxene, such as uralite, chlorite, and talc. Plagioclase is characterized by short columnar, perfect cleavage, and polysynthetic twin. Amphibole is common in various rock types and is characterized by uralitization and talcization. Hydrous minerals, including amphibole and phlogopite are widely found in the intrusion.

4. Materials and Methods

Thirty samples of lherzolite, websterite, olivine gabbro, and gabbro were collected from drill cores in profile 7 (Figure 2) and outcrops for petrographic observations and mineral composition analysis. Chromite and silicate minerals, including olivine, pyroxene, plagioclase, and amphibole, in different rock types have been analyzed. The thin sections were carefully observed by transmitted and reflected optical microscopy as well as by environmental scanning electron microscope (ESEM) under backscattered electron (BSE) mode prior to quantitative electron probe microanalysis (EPMA). The latter were performed using a Cameca SX 50 electron microprobe instrument at Wuhan Sample Solution Analytical Technology Co., Ltd., China and the Materials Research and Testing Center, Wuhan University of Technology.
The standards used for EPMA of chromite were spinel for Mg and Al, diopside for Si, ilmenite for Ti, chromite for Cr and Fe, manganese oxide for Mn, niccolite (NiAs) for Ni, sphalerite for Zn, albite for Na, and phlogopite for K. Peak and background counting times were set at 30 and 15 s, respectively. The beam diameter was 3–5 μm, and spot mode was used for tiny particles or minerals with rough surfaces. The experimental conditions were 20 kV of accelerating voltage and 10 nA of beam current. Detection limit was 0.1 wt.%. Iron was determined as total iron (FeOt), and Fe2+ and Fe3+ were calculated by assuming an R2+R3+2O4 formula and by balancing RO: R2O3 = 1 for chromite [49].
The same experimental conditions were applied to silicate minerals. Most olivine and pyroxene were altered, and thus, only fresh grains with crystal integrity were selected for analyses. The standards used were olivine for Mg and Si, garnet for Al, diopside for Ca, omphacite for Na, ilmenite for Ti, chromite for Cr, fayalite for Fe, niccolite (NiAs) for Ni, sphalerite for Zn, phlogopite for K, and manganese oxide for Mn.

5. Results

5.1. Chromite

Type 1 chromite, enclosed within olivine, exhibits compositional variations with 3.9–9.0 wt% MgO, 22.3–35.4 wt % Cr2O3, and 10.9–21.4 wt% Al2O3 (Figure 4 and Table S1). The stoichiometric FeO and Fe2O3 contents are 20.7–27.5 wt% and 12.3–28.8 wt%, respectively. The chromite is characterized by high Ti contents (0.33–1.52 wt%) and small quantities of ZnO (0–0.21 wt%), MnO (0.28–0.45 wt%), and NiO (0.06–0.24 wt%) contents (Table S1). The MgO contents display positive relationships with Cr2O3 and Al2O3 contents and a negative relationship with FeOt contents (Figure 4d–f). Generally, chromite in lherzolite, representing the earliest chromite, has slightly higher MgO and Cr2O3 contents and lower FeOt and TiO2 contents than that in other rocks. Chromite in olivine gabbro is more scattered than that in lherzolite and websterite, which may be because it consists of different stages of chromite. The calculated Cr# values ((100Cr/(Cr + Al), molar) thus range from 49 to 66. The Mg# values ((100 Mg/(Mg + Fe2+), molar)) range from 20 to 44 (Figure 5a,b). In contrast, type II chromite (Chr II) has more dispersed compositions and is characterized by significantly high TiO2 content (up to 6.2 wt%) and FeOt content (up to 69.3 wt%) and slightly low Al2O3 content (minimum 3.0 wt%) and MgO content (minimum 0.53 wt%) (Figure 4). Chr II in olivine gabbro has larger compositional variations than that in lherzolite and olivine websterite, which is similar to type I chromite. Particularly, a few type II chromites in olivine gabbro have the lowest Cr2O3 content (<20 wt%). The Cr# and Mg# ranges are 46–83 and 3–46, respectively (Table S1). As a whole, all the chromites constitute positive relationships between MgO and Al2O3 and between MgO and Cr2O3 (Figure 4d,f), and negative relationships between FeOt and Cr2O3 and between FeOt and MgO (Figure 4c,e). Chr I represents the primary chromite and thus can give constraints to the nature of parental magma.
The chromite compositional comparison between the Baixintan and other mafic-ultramafic intrusions is illustrated in Figure 5. Only Chr I was used for comparison since it represents the primary chromite while Chr II crystallized from a more evolved and fractionated melt (see the discussion in Section 6.1). The Baixintan chromite as well as the chromite in the Northern Tianshan and Beishan display similar Cr# values to the middle ocean ridge basalt (MORB), which is slightly lower than that in island arc-related intrusions, boninites, hot spot, and large igneous provinces (LIPs), while their Mg values are slightly higher than MORB (Figure 5a,b). The Baixintan chromite is characterized by higher Fe3+ than others and some plot in the fields of Alaskan-type intrusions and island-arc tholeiite (Figure 5c,d).

5.2. Silicate Minerals

The olivine in lherzolite has the highest MgO content (41.5–46.0 wt%) compared to that in olivine websterite (38.0–44.4 wt%) and in olivine gabbro (39.2–41.7 wt%). The calculated Fo values of the whole olivine (Fo = 100 * MgO/(MgO + FeO), molar ratio) ranges from 76.8 to 85.6. Olivine in lherzolite has higher Fo values (Fo = 80.9–85.6) than that in olivine websterite (Fo = 76.8–83.4) and olivine gabbro (Fo = 78.7–82.4). The Ni contents in olivine from the lherzolite have a wider range (1149–2837 ppm) than that from olivine websterite (864–1807 ppm) and olivine gabbro (629–1336 ppm). The olivine in all these samples are characterized by low MnO and CaO contents, which vary from 0.17 wt% to 0.36 wt% and from 0.02 wt% to 0.22 wt%, respectively (Table S2, Supplementary Material).
Pyroxene is characterized by tholeiite to calc-alkaline series. Orthopyroxene is classified as enstatite with Mg# = 85.0. Clinopyroxene is more common than orthopyroxene and is classified as augite and diopside. Clinopyroxene in lherzolite displays high Mg# = 86.5 and Wo = 43.7. Its compositional variation seems to be more related to a rift cumulated trend and is different from that of typical Alaskan-type complexes (Figure S1 and Table S3).
The anorthite proportions of plagioclase (An numbers) are from 67.4 to 80.8 for lherzolite, from 72.5 to 82.5 for olivine gabbro, and from 69.1 to 86.1 for gabbro. Except for a few grains of labradorite, all plagioclases are classified as bytownite (Figure S2 and Table S4).
The amphiboles in lherzolite and olivine gabbro are classified as pargasite and endenite, respectively, while that in websterite has larger compositional variations. Amphibole in lherzolite and websterite has higher Mg# (Mg#/(Mg# + Fe2+)) than that in olivine gabbro. They almost all have a mantle origin and similar compositions with other amphibole in ultramafic rocks (Figure S3 and Table S5).

6. Discussion

6.1. Primary and Magmatic Chromite

Chromite is sensitive to hydrothermal alteration and prograde metamorphism, which may modify its composition [50,53,54]. Type I chromite mainly occurs as inclusions in olivine and, rarely, in silicate interspaces. They were crystallized earlier and captured by olivine crystals, which had avoided reaction with interstitial evolving liquid. It is noteworthy that magmatic type I chromites are outside any specific field of MORB, IAT, and Alaskan-type intrusions. This may be caused by a possible Fe–Mg subsolidus exchange equilibrium, in which process Fe2+ could incorporate into chromite crystals as an isomorphism of Mg, resulting in lower Mg# values than MORB (Figure 5b). The incorporation of Fe3+ and loss of Al lead to some samples plotting in the Alaskan-type intrusions and IAT (Figure 5c,d). Despite compositional changes, the extent of modification could be evaluated by Mg# ratios and MnO content. Significant subsolidus exchange between chromite and olivine commonly gives rise to low Mg# ratios (<15) and high MnO content (up to 2 wt%) because Fe2+ and Mn could replace Mg during subsolidus exchange [55,56,57]. Type I chromite has Mg# ranging from 20 to 44 and low MnO content (0.28–0.45 wt%) and plots the “filter polygon” for magmatic chromite (Figure 6) [55,58]. The subsolidus exchange may have an effect on type I chromite composition but at a much slower rate. Type I chromite can thus represent magmatic chromite, while type II chromite cannot be treated as primary chromite due to its interstitial phase and large compositional variations. It is considered to crystallize from a more evolved and fractionated melt [20,58]. Some samples have low MgO, Al2O3, and Cr2O3 contents and high FeOt and TiO2 contents, indicating far greater modification than type I chromite (Figure 4). Particularly, Fe-rich chromite rims in zoned chromite formed during serpentinization and chloritization [14,20,54,59]. The loss of Al, Mg, and Cr may be caused by chloritization, which involved the reaction of chromite, olivine, and magnetite to produce Fe-rich chromite rim and chlorite [20].

6.2. Nature of Parental Magma

6.2.1. Temperature and Pressure

The chromite and olivine compositions can give constraints on the nature of parental magma, such as crystallization temperature and pressure, oxygen fugacity, and compositions [7,9,22,60]. As mentioned above, type I chromite with primary and magmatic origin and olivine with the highest Fo value (Fo = 85.6), are appropriate to calculate magma composition and physicochemical condition. The crystallization temperature of chromite is calculated to be 1389 °C using the thermometer proposed by Fabriès [60]: T (°C) = (4250 * Cr# + 1343)/(lnKd + 1.825Cr# + 0.571) − 273, where lnKd = 0.34 + 1.06(Cr#)2. The pressure is estimated as 3.8 Gpa by the empirical formula: P(Gpa) = 0.187 + 2.79(XSplCr + XSplFe3+), where XSpl is the mole fraction in chromite [61].
The crystallization temperature and pressure of olivine, pyroxene, and amphibole are also respectively estimated. The initial crystallization temperature of olivine with the highest Mg# value is estimated to be 1309 °C using the empirical formula: T (℃) = 1066 + 12.067Mg# + 312.3(Mg#)2 [62]. The clinopyroxene has a temperature of 1206 °C and a pressure of 0.62 Gpa using the thermobarometer proposed by Thompson [63]. Amphibole in the Baixintan intrusion characterizes low pressure (≤0.8 Gpa) and relatively high temperature (≥950 °C, Figure 7), with the reference of experimentally synthesized amphibole in various temperature–pressure conditions [64].

6.2.2. Oxygen Fugacity

The magma oxygen fugacity corresponding to the olivine–chromite pairs is calculated as FMQ + 1.1 to FMQ + 2.1, with an average of FMQ + 1.6, using the method proposed by Ballhaus et al. [15] (Table S1). The oxygen fugacity exhibits a generally elevated trend with fractionation of magmas. Specifically, the olivine has relatively higher oxygen fugacity (FMQ + 2.1) than lherzolite (FMQ + 1.5) and olivine websterite (FMQ + 1.7). The result is similar to the other mafic-ultramafic intrusions in Northern Tianshan, such as the Tulaergen and Huangshandong intrusions, and higher than those in the Beishan area, for which the parental magma commonly have higher MgO content (Figure 8).

6.2.3. Compositions

The mafic-ultramafic rocks are classified as subalkaline and tholeiite series [29,33]. The MgO content of parental magma can be quantitatively estimated as follows: wMgO = 0.561Kd * Fo/(1 − Fo) wFeO, where Kd = (Fe/Mg)Ol/(Fe/Mg)melt = 0.3 [56] and wFeO is the FeO content of the cumulative lherzolite (unpublished data). The olivine with the highest Fo number (Fo = 85.6) should be used in the calculations [66], and the result is thus calculated to be 11.4 wt% MgO and 11.4 wt% FeO. The MgO content is similar to that of other intrusions in the Northern Tianshan, such as the Tulaergen (12.5 wt% MgO [67]), the Huangshandong (11.7 wt% MgO [44]), and the Hulu (11.0 wt% MgO [68]). For comparison, the parental magma in the Northern Tianshan is lower than that in the Beishan (14.5–15.1 wt% MgO [36]). The linear relationship between the Al2O3 and TiO2 contents of chromite and the melt from which it crystallized provides an effective tool to estimate the Al2O3 and TiO2 content of parental magma [8,9]: Al2O3-melt = 3.996*(Al2O3-Chr)0.4132 and TiO2-melt = 1.5907 * (TiO2)0.6322. The conjectured melt is characterized by 10.7–14.2 wt% Al2O3 (average 12.1 wt%) and 0.8–2.1 wt% TiO2 (average 1.4 wt%) (Figure 9). The high TiO2 content is also recognized in the Beishan intrusions, southern CAOB [20,51]. In addition, the hydrous nature of parental magma is indicated by the presence of unaltered magmatic amphibole and phlogopite (Figure 3d and Figure 7b).
In summary, the parental magma of the Baixintan intrusions is characterized by high temperature (1389 °C), high pressure (3.8 Gpa), and high Mg content (11.4 MgO wt%) with oxidized (FMQ + 1.6) and hydrous nature.

6.3. Origin of the Complex

6.3.1. Tectonic Setting

The tectonic setting of early Permian mantle-origin rocks in the Northern Tianshan has been a disputed topic. Some research considered that they are the Alaskan-type complexes emplaced in an island-arc environment because of widespread hydrous minerals with no alteration origin and island-arc signature of trace elements [69,70]. However, the presence of orthopyroxene in these complexes in the Northern Tianshan, which is extremely scarce in the Ural-Alaskan-type intrusions [71,72,73], argues against the Alaskan-type origin. Furthermore, a post-orogenic extensional setting has gained growing recognition in the past several years due to increasing studies of geochronology and metallogeny. The Paleo-Asian ocean subducted southward until later Carboniferous (310 Ma) [74,75,76], which predates the early Permian mafic-ultramafic complexes (275–285 Ma) in the Northern Tianshan. The arc-related deposits in this area, such as porphyry-type and VMS deposits in this area, mainly formed from Silurian to Carboniferous rather than in Permian [37,39]. Our mineralogical studies in the Baixintan intrusions also give new constraints on the origin and support the post-orogenic extensional setting.
The Cr# of Baixintan chromite (49–66) as well as chromite in other early Permian intrusions in the northern Tianshan and Beishan are significantly lower than that in island arc-related rocks (Cr# 75–90, Figure 5a). Chromite from island-arc tholeiite (IAT) is characterized by low Al content and significant positive relationship between Al and Ti content [8], whereas chromite in early Permian intrusions in Northern Tianshan and Beishan displays a negative relationship and relatively high Al content. Their compositions are mainly plotted in the mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) area rather than in the IAT area (Figure 5d). Clinopyroxene in this area is mainly augite and exhibits a rift cumulate trend, while clinopyroxene in the Alaskan-type complexes is mainly diopside and exhibit an arc cumulate trend (Figure S1). In addition, a non-arc setting is also indicated by plagioclase composition. Plagioclase from early Permian mafic-ultramafic rocks in Northern Tianshan and Beishan has higher CaO and Al2O3 contents and lower SiO2 and alkaline content than that from typical Alaskan-type intrusions (Figure S4).

6.3.2. Mantle Source

A depleted mantle source modified by a subducted slab has been indicated by Hf-Sr-Nd-O isotope and trace element geochemistry. Our oxygen fugacity calculation provides new evidence to support the conclusion. The parental magma in the Northern Tianshan is relatively oxidized because its fO2 value is clearly higher than those of MORBs (FMQ − 1 to FMQ + 0.5 [77]) and the mantle fO2 of intrusions in the southern CAOB (FMQ − 1 to FMQ + 0.5 [22]), implying incorporation of minor slab-derived oxidized components into to mantle source. The extent of oxidized component involvement in the Northern Tianshan is slightly higher than that in the Beishan (Figure 8). A few chromite samples exhibit subduction signatures, such as plotting in the field of Alaskan-type intrusions and IAT (Figure 5c,d). Chen et al. [28] used the silicate-liquid model pMELTS to simulate magmatic evolution of the adjacent Lubei intrusion, and the result showed that the mantle was hydrous (approximately 0.29 wt % H2O). The hydrous nature may be ascribed to the mantle source having been metasomatized by subduction fluid. The partial melting degree can be estimated by the composition of primary chromite enclosed within olivine according the formula: F = 10 * ln (Cr#) + 24 [5]. The result ranges from 17.0% to 18.9% with average 18.3% and shows a high degree of partial melting.
To sum up, the mafic-ultramafic rocks originated from high-degree partial melting of depleted mantle that had been modified by crustal components and metasomatized by subduction fluid in a post-orogenic extensional setting.

6.4. Implications for Sulfide Segregation

6.4.1. Early Segregation

High Mg tholeiite magma caused by high-degree mantle partial melting is enriched in the chalcophile element, especially Ni, Cu, and PGE [78,79]. The Baixintan intrusion has a high partial melting degree (18.3%), which is sufficient to eliminate sulfide retention in mantle source [79]. Besides, the oxidized nature (FMQ + 1.6, Figure 8) could significantly increase the sulfur solubility [22,80]. The parental magma should be enriched in Ni, Cu, and PGE, which are positive factors to form magmatic Ni–Cu–PGE sulfide deposits. However, the mineralization mainly occurs as a disseminated sulfide with a relatively low Ni–Cu tenor, and the PGE is depleted in the sulfide ore [26,29]. Similar PGE depletion had also been confirmed in other Ni–Cu deposits in the Northern Tianshan with a high degree of partial melting and oxidized parental magma (Table S6), and it was considered to be caused by early sulfide segregation at depths.
Early segregation could remove a majority of PGE because of their higher partition coefficients between sulfide and silicate (103 and 105) than that for Ni (300–1000) and Cu (102–103) [81]. A slightly negative relationship between Cu/Pd and Pd is illustrated by the Baixintan deposits as well as the adjacent Lubei and Huangshandong deposits (Figure 10a), and this may be the effect of early segregation, since the Cu/Pd ratio could increase rapidly when Pd is strongly partitioned into a sulfide liquid during early segregation [82]. The evolution of sulfides formed by early segregation and by PGE-depleted melt is also depicted by the plot of Pd vs. Cu (Figure 10b). Lightfoot et al. [83] illustrated that an assumed initial PGE-undepleted melt of approximately 80 ppm Cu and approximately 10 ppb Pd produced PGE- and Cu-rich sulfide ores (red line). Conversely, sulfides that precipitated from a PGE-depleted melt (approximately 45 ppm Cu and approximately 0.278 ppb Pd) after early segregation would be Cu-rich but PGE-poor (blue trend line). Almost all the sulfide ores plot along the blue trend line, suggesting that they formed from a PGE-depleted melt due to early segregation (Figure 10b). Moreover, the 100% sulfide calculation of PGE in the Baixintan and Lubei deposits suggests that the primary magma underwent 0.007% early sulfide removal at depth and lost much chalcophile content. Then, the sulfide was segregated from PGE-depleted parental magma in a shallow chamber under R factors (R = silicate/sulfide, mass ratio) of 100 to 800 [29]. Though trigger factors needs further study, the early segregation has been widely recognized in the Northern Tianshan [26,29,44,84].

6.4.2. Later Segregation

The later segregation is represented by disseminated sulfide occurring as the interstitial phase between olivine grains (Figure 3e,f), which indicates that sulfide segregation is posterior to olivine crystallization. The Fo–Ni variation during olivine crystallization has been modelled in the Baixintan deposit (Figure 11 and Table S7). As a whole, the Ni content decreased as olivine crystallization. A few olivine samples in lherzolite have high Ni contents (approximately 2500 ppm) and plot along with the S-unsaturated curve, indicating that no sulfide segregation had occurred in the initial crystallization, while most olivine samples plot below the S-unsaturated line, especially for that in mineralized lherzolite and olivine websterite. The calculated result shows that the Ni depletion begun after about 3% olivine crystallization; then, the Ni content significantly dropped to approximately 1900 ppm, and the missing Ni was partitioned into segregated sulfide, which was evidently indicative of later segregation.

6.4.3. Segregation Factors

Limited fractional crystallization could not induce S-oversaturation and sulfide segregation. Factors controlling sulfur content at sulfide saturation (SCSS) mainly consist of temperature, pressure, and magma composition without considering crustal assimilation [85]:
ln(SCSS) = −1.76 − 0.474(104/T) − 0.021P + 5.559XFeO + 2.565XTiO2 + 2.709XCaO − 3.192XSiO2 + 3.049XH2O
The pressure and temperature are almost unchanged during initial limited crystallization and have opposite effects on SCSS. During fractional crystallization from chromite (1389 °C and 3.8 Gpa) to amphibole (950 °C and 0.8Gpa), the T–P conditions decreased together, and their contributions on SCSS counteracted each other by the following partial differential equations:
∂(ln(SCSS))/∂(P) = −0.021
∂(ln(SCSS))/∂(T) = 4740 ∗ T−2
About 3% olivine crystallization would consume FeO and SiO2 content (Table S7); similarly, their influences on SCSS also offset each other. Therefore, limited and simple fractional crystallization would not trigger sulfur oversaturation.
The contamination of crustal Si and S components are more important factors for sulfide segregation rather than fractional crystallization. The assimilation had been indicated by trace elements and Hf–O–Sr–Nd isotopic compositions [26,29,30,33]. In addition, the lower Ordovician volcanics are potential source of external S since a series of Cu-polymetallic deposits developed in volcanics, such as the Huangtupo and Huangtan VMS deposits, the Hongshan epithermal deposit, and the Heicaotan volcanic hydrothermal deposit [39]. Though the sulfide in the Baixintan deposit is near zero (δ34S = −0.7 to + 1.1) [22], the sulfide in volcanic strata also displayed a mantle signature and the possibility of incorporation of external S could not be excluded. Furthermore, assimilation of the Si component from southern granitoid is also a positive factor for S oversaturation. The fact is different from Ni deposits in the Beishan, for which immediate country rocks consist marble, and the contamination of CaO would hinder S oversaturation [25]. In addition, the relatively higher oxygen fugacity in Northern Tianshan can dissolve more sulfide than the Beishan (Figure 8). They may be key reasons that the Ni mineralization in Beishan is poorer than that in Northern Tianshan.

7. Conclusions

The mineralogical studies of chromite and silicate minerals in the Baixintan magmatic Ni deposit in Northern Tianshan (southern CAOB) have given us the following conclusions:
(1)
Two types of chromite were identified in mafic-ultramafic rocks. Type I chromite occurs as inclusions enclosed in olivine and has a primary and magmatic origin. Type II chromite with an interstitial phase crystallized from a more evolved melt and suffered from post-magmatic alteration.
(2)
The parental magma is characterized by high temperature (1389 °C), high pressure (3.8 Gpa), and high Mg content (11.4 MgO wt%) with oxidized (FMQ + 1.6) and hydrous nature.
(3)
The mafic-ultramafic rocks originated from high-degree partial melting of a depleted mantle that had been modified by crustal components and metasomatized by subduction fluid in a post-orogenic extensional setting.
(4)
Two stages of sulfide segregation have been recognized. The assimilation of crustal Si and S components played more important roles on sulfide segregation rather than fractional crystallization.

Supplementary Materials

The following are available online at https://www.mdpi.com/2075-163X/10/12/1050/s1. Figure S1: The composition of pyroxene in the Baixintan deposit: (a) the classification of pyroxene, (b) the plot of Ti vs. (Ca + Na) of clinopyroxene, (c) the plot of Ti vs. total Al of clinopyroxene, and (d) the plot of Alz vs. TiO2 of clinopyroxene. The shadowed area is from Su [24]. Di—Diopside, He—Hedenbergite, Au—Augite, Pi—Pigeonite, ClEn—Clinoenstatite, and ClFs—Clinoferrosilite, Figure S2: Plagioclase classification in the Baixintan deposit, Figure S3: Composition and classification of amphibole in the Baixintan deposit: (a) diagram of Si–Mg/(Mg + Fe2+) and (b) diagram of Si-(Na + K). (c) The amphibole has a mantle origin. (d) The amphibole is consistent with that in ultramafic rocks, Figure S4: Plagioclase composition compared with that of arc-related intrusions, modified from Ruan [86], (a) Plot of CaO–SiO2; (b) Plot of (Na2O+K2O)–Al2O3. Table S1: Chromite compositions, Table S2: Olivine compositions, Table S3: Pyroxene compositions, Table S4: Plagioclase compositions, Table S5: Amphibole compositions, Table S6: PGE compositions in the Baixintan, Lubei, and Huangshandong Ni–Cu deposits, Table S7: Olivine crystallization simulation.

Author Contributions

Conceptualization, B.R. and C.C.; data curation, M.L.; formal analysis, B.R.; investigation, B.R., M.L., and B.S.; methodology, B.R. and C.C.; project administration, B.R.; writing—original draft, B.R., M.L., and C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This study is financially supported by the China Geologic Survey Program (grants DD20179607), by the National Natural Science Foundation of China (grant 41802101), and by the Basic University Research Fund from Ministry of Education of China (grants G1323541879 and G1323541839).

Acknowledgments

Wuhan Sample Solution Analytical Technology Co., Ltd, China is appreciated for EPMA. We would like to thank Li Xiliang for fieldwork support. We are grateful for suggestions and comments from anonymous reviewers, who greatly improved the presentation of the research results.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Location map of the study area and Ni-bearing mafic-ultramafic intrusions in the south margin of the Central Asian Orogenic Belt (CAOB) (modified after Jahn et al. [34]); (b) schematic division of massifs and the location of the Northern Tianshan, NW China (modified after Ruan et al. [36]); and (c) a regional geological map showing the distribution of early Permian mafic-ultramafic complexes in Northern Tianshan (modified after Deng et al. [29]).
Figure 1. (a) Location map of the study area and Ni-bearing mafic-ultramafic intrusions in the south margin of the Central Asian Orogenic Belt (CAOB) (modified after Jahn et al. [34]); (b) schematic division of massifs and the location of the Northern Tianshan, NW China (modified after Ruan et al. [36]); and (c) a regional geological map showing the distribution of early Permian mafic-ultramafic complexes in Northern Tianshan (modified after Deng et al. [29]).
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Figure 2. (a) Simplified geological map and (b) section of line 7 of the Baixintan deposit, showing the distribution of mafic and ultramafic rocks and sulfide mineralization.
Figure 2. (a) Simplified geological map and (b) section of line 7 of the Baixintan deposit, showing the distribution of mafic and ultramafic rocks and sulfide mineralization.
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Figure 3. Photomicrographs of mafic-ultramafic rocks showing the occurrence of silicate minerals, sulfide, and chromite: (a) lherzolite, rounded olivine encased in clinopyroxene; (b) olivine websterite; (c) gabbro, with slight alteration of silicate minerals; (d) amphibole lherzolite, widespread amphibole enveloping olivine grains and exhibiting two groups of complete cleavage; (e,f) interstitial sulfide of pentlandite, pyrrhotite, and chalcopyrite occurring between silicate minerals; and (g) two types of chromite. Type I chromite occurs as inclusions enclosed in olivine (Chr I). Type II chromite occurs as interstitial phase in serpentine (Chr II). (h) A few type II chromites display zonation with bright rims. Ol: olivine, Cpx: clinopyroxene, Opx: orthopyroxene, Amp: amphibole, Pl: plagioclase, Po: pyrrhotite, Pn: pentlandite, Ccp: chalcopyrite, Chr: chromite, Srp: serpentine, and Mag: magnetite.
Figure 3. Photomicrographs of mafic-ultramafic rocks showing the occurrence of silicate minerals, sulfide, and chromite: (a) lherzolite, rounded olivine encased in clinopyroxene; (b) olivine websterite; (c) gabbro, with slight alteration of silicate minerals; (d) amphibole lherzolite, widespread amphibole enveloping olivine grains and exhibiting two groups of complete cleavage; (e,f) interstitial sulfide of pentlandite, pyrrhotite, and chalcopyrite occurring between silicate minerals; and (g) two types of chromite. Type I chromite occurs as inclusions enclosed in olivine (Chr I). Type II chromite occurs as interstitial phase in serpentine (Chr II). (h) A few type II chromites display zonation with bright rims. Ol: olivine, Cpx: clinopyroxene, Opx: orthopyroxene, Amp: amphibole, Pl: plagioclase, Po: pyrrhotite, Pn: pentlandite, Ccp: chalcopyrite, Chr: chromite, Srp: serpentine, and Mag: magnetite.
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Figure 4. Major oxide variations for two types of chromite and different rocks in the Baixintan mafic-ultramafic intrusion (wt%): the shadow area is roughly defined by primary type I chromite (Chr I, circles). (a) Plot of Al2O3-Cr2O3; (b) Plot of TiO2-Cr2O3; (c) Plot of FeOt-Cr2O3; (d) Plot of MgO-Cr2O3; (e) Plot of FeOt-MgO; (f) Plot of Al2O3-MgO.
Figure 4. Major oxide variations for two types of chromite and different rocks in the Baixintan mafic-ultramafic intrusion (wt%): the shadow area is roughly defined by primary type I chromite (Chr I, circles). (a) Plot of Al2O3-Cr2O3; (b) Plot of TiO2-Cr2O3; (c) Plot of FeOt-Cr2O3; (d) Plot of MgO-Cr2O3; (e) Plot of FeOt-MgO; (f) Plot of Al2O3-MgO.
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Figure 5. Chromite composition in the Baixintan (only Chr I) and other mafic-ultramafic intrusions in the Northern Tianshan and Beishan: (a) the Cr# range in island arc-related intrusions, hot spots or LIP, and Ni–Cu deposits; (b) a diagram of Cr#–Mg#, fields for boninites, and the middle ocean ridge basalt (MORB) from Barnes and Roeder [50], and Alaskan-type intrusions from Zhou et al. [19]; (c) a Cr–Al–(Fe3+ + 2Ti) ternary diagram, with the references of fields the same as in (b); and (d) a plot of Al2O3–TiO2 constraining tectonic setting. The chromite compositional fields for MORB, ocean island basalt (OIB), island-arc tholeiite (IAT), supra-subduction zone (SSZ), and large igneous provinces (LIP) are from Kamenetsky et al. [8]. Data of the other intrusions in Northern Tianshan and Beishan are from references [20,51,52].
Figure 5. Chromite composition in the Baixintan (only Chr I) and other mafic-ultramafic intrusions in the Northern Tianshan and Beishan: (a) the Cr# range in island arc-related intrusions, hot spots or LIP, and Ni–Cu deposits; (b) a diagram of Cr#–Mg#, fields for boninites, and the middle ocean ridge basalt (MORB) from Barnes and Roeder [50], and Alaskan-type intrusions from Zhou et al. [19]; (c) a Cr–Al–(Fe3+ + 2Ti) ternary diagram, with the references of fields the same as in (b); and (d) a plot of Al2O3–TiO2 constraining tectonic setting. The chromite compositional fields for MORB, ocean island basalt (OIB), island-arc tholeiite (IAT), supra-subduction zone (SSZ), and large igneous provinces (LIP) are from Kamenetsky et al. [8]. Data of the other intrusions in Northern Tianshan and Beishan are from references [20,51,52].
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Figure 6. Mg# vs. MnO diagram for type I chromite: the “filter polygon” is from Evans and Evans [55].
Figure 6. Mg# vs. MnO diagram for type I chromite: the “filter polygon” is from Evans and Evans [55].
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Figure 7. Diagrams of AlVI-AlIV (a) and AlIV-Ti (b) of amphibole in the Baixintan deposit: grey cross symbols are the experimental result of T–P conditions for various compositions of synthesized amphibole [64]. Note: The grey cross symbols are the experimental result of T–P conditions for various compositions of synthesized amphibole.
Figure 7. Diagrams of AlVI-AlIV (a) and AlIV-Ti (b) of amphibole in the Baixintan deposit: grey cross symbols are the experimental result of T–P conditions for various compositions of synthesized amphibole [64]. Note: The grey cross symbols are the experimental result of T–P conditions for various compositions of synthesized amphibole.
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Figure 8. The estimated oxygen fugacity verse Fo value for the Baixintan and other magmatic sulfide Ni–Cu deposits in the Northern Tianshan and Beishan. Data sources: MORB [65], the mantle fO2 of intrusions in the southern CAOB [22], Huangshannan and Tulaergen [22], Poyi [21], and Hongnieshan (unpublished data). MORB: middle ocean ridge basalt.
Figure 8. The estimated oxygen fugacity verse Fo value for the Baixintan and other magmatic sulfide Ni–Cu deposits in the Northern Tianshan and Beishan. Data sources: MORB [65], the mantle fO2 of intrusions in the southern CAOB [22], Huangshannan and Tulaergen [22], Poyi [21], and Hongnieshan (unpublished data). MORB: middle ocean ridge basalt.
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Figure 9. Estimated composition of the parental melt in equilibrium with chromite from the Baixintan complex: (a) plot of Al2O3-melt–Al2O3-Chr and (b) plot of TiO2-melt–TiO2-Chr. The ordinate is the calculated compositions of parental magma. The abscissa is the compositions of chromite. The regression lines and data of MORB, OIB, LIP, and Arc are derived from Kamenetsky et al. [8] and Rollinson [9]. Only primary type I chromite is used in calculations. MORB: middle ocean ridge basalt, OIB: ocean island basalt, IAT: island-arc tholeiite, and LIP: large igneous provinces.
Figure 9. Estimated composition of the parental melt in equilibrium with chromite from the Baixintan complex: (a) plot of Al2O3-melt–Al2O3-Chr and (b) plot of TiO2-melt–TiO2-Chr. The ordinate is the calculated compositions of parental magma. The abscissa is the compositions of chromite. The regression lines and data of MORB, OIB, LIP, and Arc are derived from Kamenetsky et al. [8] and Rollinson [9]. Only primary type I chromite is used in calculations. MORB: middle ocean ridge basalt, OIB: ocean island basalt, IAT: island-arc tholeiite, and LIP: large igneous provinces.
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Figure 10. Platinum group element (PGE) contents and ratio sulfide ores in the Baixintan and other magmatic sulfide Ni–Cu deposits in the Northern Tianshan, indicating early segregation and PGE-depleted parental magma (after Barnes et al. [82]): (a) plot of Cu/Pd vs. Pd (after Lightfoot et al. [83]). The numbers are sulfide percentages removed from the initial magma system. The yellow star represents the composition of the primitive mantle. (b) Pd vs. Cu. Data sources: Baixintan [26,29], Lubei [29], and Huangshandong [44].
Figure 10. Platinum group element (PGE) contents and ratio sulfide ores in the Baixintan and other magmatic sulfide Ni–Cu deposits in the Northern Tianshan, indicating early segregation and PGE-depleted parental magma (after Barnes et al. [82]): (a) plot of Cu/Pd vs. Pd (after Lightfoot et al. [83]). The numbers are sulfide percentages removed from the initial magma system. The yellow star represents the composition of the primitive mantle. (b) Pd vs. Cu. Data sources: Baixintan [26,29], Lubei [29], and Huangshandong [44].
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Figure 11. Model of olivine crystallization showing the variations of Ni content and Fo value in the Baixintan deposit: the detailed modelling principle and process can be seen in Table S7.
Figure 11. Model of olivine crystallization showing the variations of Ni content and Fo value in the Baixintan deposit: the detailed modelling principle and process can be seen in Table S7.
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Ruan, B.; Liao, M.; Sun, B.; Chen, C. Origin and Nature of Parental Magma and Sulfide Segregation of the Baixintan Magmatic Ni–Cu Sulfide Deposit, Southern Central Asian Orogenic Belt (CAOB), NW China: Insights from Mineral Chemistry of Chromite and Silicate Minerals. Minerals 2020, 10, 1050. https://doi.org/10.3390/min10121050

AMA Style

Ruan B, Liao M, Sun B, Chen C. Origin and Nature of Parental Magma and Sulfide Segregation of the Baixintan Magmatic Ni–Cu Sulfide Deposit, Southern Central Asian Orogenic Belt (CAOB), NW China: Insights from Mineral Chemistry of Chromite and Silicate Minerals. Minerals. 2020; 10(12):1050. https://doi.org/10.3390/min10121050

Chicago/Turabian Style

Ruan, Banxiao, Mingyang Liao, Bingke Sun, and Chao Chen. 2020. "Origin and Nature of Parental Magma and Sulfide Segregation of the Baixintan Magmatic Ni–Cu Sulfide Deposit, Southern Central Asian Orogenic Belt (CAOB), NW China: Insights from Mineral Chemistry of Chromite and Silicate Minerals" Minerals 10, no. 12: 1050. https://doi.org/10.3390/min10121050

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