Journal of Earth Science  2019, Vol. 30 Issue (2): 258-271   PDF    
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Origin of the Fanjingshan Mafic-Ultramafic Rocks, Western Jiangnan Orogen, South China: Implications for PGE Fractionation and Mineralization
Huang Sifang , Wang Wei     
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
ABSTRACT: The Fanjingshan mafic-ultramafic rocks in the west Jiangnan Orogen of South China are considered to be a potential target for mineral exploration. However, the petrogenesis and magma evolution of these rocks are not yet clearly constrained, let along their economic significance. The compositions of platinum group elements (PGE) in the Fanjingshan mafic-ultramafic rocks can provide particular insight into the generation and evolution of the mantle-derived magma and thus the potential of Cu-Ni-PGE sulphide mineralization. The Fanjingshan mafic-ultramafic rocks have relatively high Pd-subgroup PGE (PPGE) relative to Ir-subgroup PGE (IPGE) in the primitive mantle-normalized diagrams. Meanwhile, the Fanjingshan mafic-ultramafic rocks have low Pd/Ir (11-28) ratios, implying relatively low degree of partial melting in the mantle. Low Cu/Pd ratios (545-5 216) and high Cu/Zr ratios (0.4-5.8 with the majority greater than 1) of Fanjingshan ultramafic rocks indicate that the S-undersaturated parental magma with relatively high PGE was formed. Although the Fanjingshan mafic rocks have remarkably higher Cu/Pd ratios (8 913-107 016) likely resulting from sulphide segregation, the degree of sulphide removal is insignificant. Fractionation of olivine rather than chromite and platinum group minerals or alloys governed the fractionation of PGE and produced depletion of IPGE (Os, Ir and Ru) relative to PPGE (Rh, Pt and Pd), as supported by the positive correlation between Pd/Ir and V, Y and REE. Collectively, original S-undersaturated magma and insignificant crustal contamination during magma ascent and emplacement result in the separation of immiscible sulphide impossible and thus impede the formation of economic CuNi-PGE sulphide mineralization within the Fanjingshan mafic-ultramafic rocks.
KEY WORDS: magma differentiation    platinum group elements    mafic-ultramafic rocks    Jiangnan Orogen    South China    
0 INTRODUCTION

The Jiangnan Orogen (also known as the Jiangnan fold belt), formed during the Neoproterozoic collision/amalgamation between the Yangtze and Cathaysia blocks, records the major episode of Precambrian crustal evolution episodes of South China, and is temporally and spatially correlated with assembly and breakup of the supercontinent Rodinia (Wang W et al., 2013; Zhang and Zheng, 2013; Zheng et al., 2013; Zhao and Cawood, 2012; Li X H et al., 2009; Li Z X et al., 2008; Zhou et al., 2008). Mafic-ultramafic intrusions ranging between 830–750 Ma, are extensively emplaced within the Jiangnan Orogen along with abundant granitoids and mafic to felsic volcanic rocks (Xia et al., 2018; Yao et al., 2015; Zhang et al., 2015, 2012, 2008; Wang W et al., 2014; Li X H et al., 2008, 2003; Wang X C et al., 2007; Wang C Y et al., 2006; Wang X L et al., 2006; Li Z X et al., 1999). Different models have been proposed to explain the petrogenesis and tectonic setting of the orogen as well as associated granitoids and mafic and felsic volcanic rocks (Xin et al., 2017; Yao et al., 2015; Zhang Y Z et al., 2015; Zhang S B et al., 2012; Zheng et al., 2008; Wang C Y et al., 2006; Wu et al., 2006; Li et al., 2003). However, it is rarely known about the origin of the mafic-ultramafic intrusions. In addition, potential Cu-Ni sulphide mineralization of the mafic-ultramafic intrusions in the western Jiangnan Orogen (Mao, 2002) necessitates detailed evaluation of the Cu-Ni-(PGE) sulphide saturation histories of the mafic-ultramafic intrusions.

Unlike lithophile elements, the platinum group elements (PGE: Os, Ir, Ru, Rh, Pt and Pd) are preferentially concentrated in sulphide phases and are thus useful tools in providing insight into partial melting process and early differentiation history of basaltic magma, and thus the petrogenesis of mafic-ultramafic rocks, Neoproterozoic geodynamic processes and mantle evolution (Fu et al., 2016; Gao et al., 2012b; Puchtel et al., 2004; Maier, 2003; Shirey and Walker, 1998; Zhou, 1994; Barnes et al., 1985; Morgan, 1985). It is widely accepted that the distribution of PGE in mantle-derived magma is largely controlled by mantle source characteristics, partial melting, magma differentiation, crustal contamination, and especially the sulphide segregation due to extremely high partition coefficients of PGE between sulphide phase and silicate melt (Song et al., 2009; Arndt et al., 2005; Naldrett, 2004; Maier, 2003; Zhou, 1994; Barnes et al., 1985). Therefore, systematic study of PGE geochemistry can help in deciphering the mantle source, melting conditions and magmatic differentiation of the mantle-derived magma, as well as formation of associated Cu-Ni-PGE sulphide deposits (Gao et al., 2012a; Yang et al., 2012; Song et al., 2009; Naldrett, 2004; Lightfoot and Hawkesworth, 1997; Barnes et al., 1985).

One of the best preserved mafic-ultramafic components within the Jiangnan Orogen is outcropped in the Fanjingshan area (Wang et al., 2014; Yao et al., 2014; Zhao et al., 2011; Zhou et al., 2009; Zhang et al., 2008). Previous studies suggest that these mafic-ultramafic rocks were generated by partial melting of the mantle wedge, modified by slab-released fluids and emplaced at ca. 824 Ma (Wang et al., 2014), whereas the generation and evolution of the mantle-derived magma and thus the potential of Cu-Ni-PGE sulphide mineralization are poorly constrained. In this study, we investigate PGE compositions of the Fanjingshan mafic-ultramafic rocks in order to understand the nature and melting regime of mantle source and PGE fractionation during the magma generation and evolution. While the potential of Cu-Ni-PGE sulphide mineralization within the Fanjingshan mafic-ultramafic rocks is also evaluated accordingly.

1 GEOLOGICAL BACKGROUND

The Jiangnan Orogen, located along the southeastern margin of the Yangtze Block is formed through amalgamation between the Yangtze and Cathaysia blocks in South China during Neoproterozoic (Cawood et al., 2017; Zhou et al., 2014; Zheng et al., 2013; Zhang et al., 2012; Zhao et al., 2011; Li et al., 2008). The tectonic events resulted in a series of geological events spanning between 970 and 750 Ma (Jiang et al., 2018; Ding et al., 2017; Zhao and Asimow, 2014; Li L M et al., 2013; Wang W et al., 2013, 2012a, b, 2010; Wu et al., 2006; Li X H et al., 2003). Neoproterozoic sedimentary strata (~830 Ma) including the Sibao, Fanjingshan and Lengjiaxi groups are regarded as the Precambrian sedimentary basement in the Jiangnan Orogen, in turn unconformably overlain by a series of post-orogenic extensional sedimentary sequences (mainly 810–740 Ma) named as the Danzhou, Xiajiang and Banxi groups (Wang W et al., 2018, 2014, 2013, 2012a, b 2010; Yang et al., 2015; Yin et al., 2013; Zhao et al., 2011; Wang and Li, 2003; Li and Naldrett, 1999). Predominantly felsic and minor mafic magmatic rocks (mainly 830–750 Ma), emplaced/intruded into these sedimentary rocks represent extensive magmatism associated with the formation and evolution of the Jiangnan Orogen (Li L M et al., 2016; Wang W et al., 2014; Zhao and Zhou, 2013; Li X H et al., 2008a, 2003; Zheng et al., 2008; Wang X C et al., 2007; Wang C Y et al., 2006; Wang X L et al., 2006, 2004; Wu et al., 2006; Li and Naldrett, 1999).

The studied area is geographically located in northeastern Guizhou Province and geologically in the western segment of the Jiangnan Orogen (Fig. 1a). A variety of volcanic-sedimentary rocks with abundant mafic-ultramafic and minor felsic intrusions, which exposed in this region are collectively described as the Fanjingshan Group covering an area of ~270 km2 (GRGST, 1974), which is unconformably overlain by the Xiajiang Group (Fig. 1b). Based on difference in lithological assemblages, relative proportions of volcanic and intrusive rocks, the Fanjingshan Group is subdivided into the upper and lower units. The lower Fanjingshan Group consists of sandstone, siltstone, tuff, sericite phyllite, slate and mafic lavas, constituting a volcanic-sedimentary system, whereas the upper part consists of coarse-grained turbidite sediments (BGMRGP, 1987; GRGST, 1974).

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Figure 1. Sketch geological map of the studied area (modified from Wang et al., 2014). (a) The framework of South China containing the Yangtze Block in the northwest and the Cathaysia Block in the southeast. Primary Precambrian geological units including sedimentary sequences, mafic to felsic igneous plutons and major faults are highlighted in the Jiangnan Orogen; (b) geological map of the Fanjingshan area showing the distribution of the Fanjingshan mafic-ultramafic intrusions; (c) a general cross section involving major units of the Fanjingshan area

The detrital zircon U-Pb ages constrained initiation of the Fanjingshan Group sedimentation at ca. 830–800 Ma (Zhao et al., 2011; Wang et al., 2010; Zhou et al., 2009), which is consistient with the 824–821 Ma layered mafic-ultramafic rocks (U-Pb zircon ages) (Su et al., 2014; Wang et al., 2014; Xue et al., 2012; Zhou et al., 2009). The zircon U-Pb age of ca. 827 Ma for the granitic plutons, which intruded into the Fanjingshan Group, indicates rapid sedimentation of the Early Neoproterozoic sedimentary (Zhao et al., 2011). In the Fanjingshan area, there are some S-type muscovite granites that were emplaced synchronously with the mafic-ultramafic rocks, and are interpreted to be related to subduction-arc setting (Liu and Zhao, 2018; Xin et al., 2017; Wang et al., 2011; Xie and Zhang, 2009).

The Fanjingshan sills constitute a series of mafic to ultramafic rocks and their primary contact with country rocks are obliterated due to strong shearing and folding resulting in dyke like appearance of these sills (BGMRGP, 1987). These sills are distributed along the stratification plane in the Fanjingshan Group (Fig. 1c). Meanwhile, these layered mafic-ultramafic sills were folded together with the hosted strata and have thickness of several to tens of meters with length commonly extending up to ~20 km (Zhang et al., 2008; BGMRGP, 1987). Primary minerals in the mafic-ultramafic rocks, such as olivine, clinopyroxene, plagioclase and magnetite, show typical hypidiomorphic-idiomorphic granular texture, although some of them have been replaced by serpentine, tremolite, chlorite, epidote and sericite during low-grade metamorphism or hydrothermal alteration (BGMRGP, 1987; GRGST, 1974). Abundant pillow basaltic rocks are associated with the mafic-ultramafic sills showing well developed vesicular and amygdaloidal structures with secondary zeolite, chlorite and epidote. Primary minerals like plagioclase, pyroxene (diopside) and magnetite have been variably altered. Some pillow lavas are named as spilites due to their high Na2O contents (BGMRGP, 1987; GRGST, 1974).

2 PETROLOGICAL CHARACTERISTICS

The Fanjingshan mafic-ultramafic sills consist of wehrlite, olivine pyroxenite, pyroxenite, gabbro-diabase and diabase (Zhang et al., 2008). The wehrlite is characterized by poikilitic and blastoporphyritic texture and mainly contains serpentinezed olivine (50%–60%), clinopyroxene (25%–30%) and phlogopite (2%–3%) with accessory magnetite, chromite and sulphide. Coarser grains of clinopyroxene envelop euhedral to subrounded olivine grains to define typical poikilitic texture. The olivine pyroxenite contains up to 75% clinopyroxene and subordinate olivine (5%–25%), minor biotite (~1%) and accessory amounts of magnetite, chromite and sulphide (Figs. 2a, 2b). Pentlandite is the most abundant sulphide mineral in the studied mafic-ultramafic rocks and is sporadically disseminated as blebs within interstitial spaces between silicate minerals. Pyroxenite occurs is spatial association with olivine pyroxenite and predominantly consists of clinopyroxene (up to 95%) along with minor olivine (~5%).

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Figure 2. Representative photomicrographs of the Fanjingshan mafic-ultramafic rocks. (a) and (b) olivine pyroxenite from the Fanjingshan ultramafic rocks, mineral assemblages: clinopyroxene, olivine and minor spinel, pentlandite; (c) and (d) gabbro-diabase from the Fanjingshan mafic rocks; (e) and (f) photomicrograph of diabase from the Fanjingshan mafic. It consists of clinopyroxene, plagioclase and serpentine. Pl. Plagioclase; Cpx. clinopyroxene; Ol. olivine; Srp. serpentine; Spl. spinel; Pent. pentlandite

Most pyroxenite samples show blastogranular texture with euhedral to subhedral grains of pyroxene and occasional plagioclase. Gabbro-diabase mainly outcrop between the ultramafic units and diabase. These rocks display typical poikilitic and ophitic texture, and mainly contain plagioclase (50%–60%) and pyroxene (30%–40%) with minor quartz and titanite (Figs. 2c, 2d). Diabase with typical ophitic texture and amygdaloidal structure containing dominant plagioclase and pyroxene with minor quartz and K-feldspar is widely distributed along the margin of mafic-ultramafic sills (Figs. 2e, 2f).

3 ANALYTICAL METHOD AND RESULTS

The PGEs were determined using an improved digestion technique available in the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Science (SKLODG, IGCAS), Guiyang. Samples and appropriate amounts of the enriched isotope spike solution containing 101Ru, 193Ir, 105Pd and 194Pt were initially digested by HF in a PTFE (poly tetra fluoro ethylene) beaker on a hot plate to remove silicates. Dried residue was digested in a beaker sealed in stainless steel pressure bomb with HF+HNO3 at 190 ℃ for about 48 h. PGEs were preconcentrated through Te-coprecipitation. Main interfering elements such as Cu, Ni and Zr were removed using a mixed ion exchange column containing a Dowex 50W X8 cation exchange resin and a P507 levextrel resin. Eluted solution was measured using an ELAN DRC-e ICP-MS in SKLODG, IGCAS. Total procedural reagent blanks range from 0.008 ng (Ru) to 0.033 ng (Pd). Detailed method description is given in Qi et al. (2011).

PGE concentrations for the Fanjingshan mafic-ultramafic rocks are presented in Table S1. The whole rock major and trace elements and isotopic (Re-Os) compositions reported by Wang et al. (2014) are also summarized in Table S1 and will be cited in the following discussion. The Fanjingshan ultramafic rocks have high Rh (0.498 ppb–2.810 ppb), Pd (7.259 ppb–22.820 ppb, exception of 72.24 ppb) and Pt (6.176 ppb–17.97 ppb) contents, moderate Ru (1.021 ppb–2.789 ppb, exception of 0.283 ppb) and Ir (0.534 ppb–1.130 ppb). Cu/Pd and Pd/Ir ratios of these rocks vary from 545 to 5 211 (exception of 11 446) and 11 to 75, respectively. However, the mafic rocks have variable and moderate-low PGE contents, i.e., Rh, Pd, Pt, Ru and Ir ranging from 0.011 ppb to 0.594 ppb, 0.449 ppb to 22.92 ppb, 0.127 ppb to 44.44 ppb, 0.013 ppb to 0.487 ppb and 0.011 to 0.851, respectively. Cu/Pd (8 916 to 107 029, exception of 3 656 and 5 747) and Pd/Ir (22 to 54, exception of 184) ratios of mafic rocks are obvious higher than ultramafic rocks.

All of the Fanjingshan ultramafic rocks have constantly high concentration of PGE with slight depletion of IPGE relative to PPGE in the primitive mantle-normalized diagrams (Fig. 3a). The mafic rocks have relatively lower abundances but similar patterns of PGE compared to ultramafic rocks. Slight Ir and Pt depletion is observed in the Fanjingshan ultramafic rocks only. In contrast, slight Ru depletion is limited to mafic rocks and absent in ultramafic rocks. Cu and Ni are depleted relative to PPGE and IPGE, respectively, in the primitive mantle-normalized diagrams (Fig. 3b). The Fanjingshan ultramafic rocks have significantly enriched Pd, whereas mafic rocks do not have any significant Pd anomaly. In the MgO versus PGE diagram (Figs. 4a4f), the mafic rocks show strong positive correlation between PGE and MgO, however, the ultramafic rocks display positive trends for Os and Rh but negative trends for Ir, Ru, Pt and Pd with MgO. Whatever mafic or ultramafic rocks, the Pd/Ir and Pd/Rh ratios increase with MgO decreasing (Figs. 4g, 4h).

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Figure 3. Primitive mantle normalized PGE patterns of the Fanjingshan mafic-ultramafic rocks
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Figure 4. PGE (Os, Ir, Ru, Rh, Pt and Pd) vs. MgO variation diagrams for bulk-rock geochemistry of the Fanjingshan mafic-ultramafic rocks. Symbols are the same as in Fig. 3. The major element data are from Wang et al. (2014)
4 DISCUSSION 4.1 The Role of Alteration and Metamorphism

The Neoproterozoic Fanjingshan Group mafic-ultramafic rocks and host rocks experienced low temperature hydrothermal alteration, lower greenschist facies metamorphism and strong tectonic deformation (Wang et al., 2014; GRGST, 1974). Therefore, the whole rock chemical compositions of the Fanjingshan mafic-ultramafic rocks are likely modified. Nevertheless, immobile elements, such as HFSE (high field strength elements) and REE (rare earth element) have been successfully used to constrain the petrogenesis of mafic-ultramafic rocks being insensitive to these secondary processes (Wang W et al., 2014; Zhao and Zhou, 2007; Wang C Y et al., 2006). Studies have also shown that IPGE remain relative immobility during alteration and low-grade metamorphism (Wood, 2002; Barnes et al., 1985), whereas Pt and Pd might be variably mobile during fluid transportation under wide range of pressure and temperature (Peregoedova et al., 2006; Augé et al., 2002; Zhou, 1994). Uniform Pd/Pt ratios and coherent primitive-mantle normalized PGE patterns of ultramafic varieties (Fig. 3), however, indicate little effect of alteration and metamorphism on PGE. Collectively, PGE in the Fanjingshan mafic-ultramafic rocks have remained immobile during the later geological overprints.

4.2 Assimilation and Fractional Crystallization

It has been recognized that the parental magma for the Fanjingshan mafic-ultramafic rocks was sourced from heterogeneous mantle components and experienced limited crustal contamination during the magma ascent and evolution (Wang et al., 2014). Two diabase samples, however, have high γOs values (179 and 243) that are negatively correlated with Mg#, implying significant crustal assimilation during the magma emplacement. Low Os content in evolved magma renders Os isotope much more sensitive to crustal contamination relative to Nd and Pb isotopes (Wang et al., 2014). Therefore, an important parameter for modeling crustal contamination in cumulate rocks is the Os contents of the parental melts to individual cumulate rocks.

A simple modeling involving mantle-derived magma and crustal contaminant is approached following the formula as below

$ \begin{array}{l} {\left({^{187}{\rm{Os}}{/^{188}}{\rm{Os}}} \right)_{{\rm{mix}}}} = [{C^s}_{{\rm{Os}}} \times (1-f) \times {\left({^{187}{\rm{Os}}{/^{188}}{\rm{Os}}} \right)_{\rm{s}}} + {C^{\rm{c}}}_{{\rm{Os}}} \times f \times \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;{\left({^{187}{\rm{Os}}{/^{188}}{\rm{Os}}} \right)_{\rm{c}}}\left] / \right[({C^{\rm{s}}}_{{\rm{Os}}} \times (1-f) + {{\rm{C}}^{\rm{c}}}_{{\rm{Os}}} \times f)] \end{array} $

where COss and COsc represent the Os concentration of source mafic magma and the contaminant, (187Os/188Os)s, (187Os/188Os)c and (187Os/188Os)mix represent Os isotopic compositions of source mafic magma, the contaminant and the mixture of these two end member. "f" refers to mass fraction of the contaminant. According to the estimation of Barnes and Lightfoot (2005), high Mg basaltic magma may have Os concentration ranging from 0.1 ppb to 0.2 ppb. As early removal of sulphide was insignificant, the primitive magma is believed to have high Os content, e.g., 0.15 ppb. Initial 187Os/188Os (830 Ma) of mantle magma is assumed to the same with subcontinental lithosphere mantle (SCLM) of the Yangtze Block (187Os/188Os (830 Ma) = 0.116 9), which is estimated based on the Raobazhai peridotite in the Dabie-Sulu orogenic belt (Zheng et al., 2009). A possible contaminant is the country rock, the Fanjingshan siliciclastic varieties, whose Re-Os isotopic compositions are assumed to be represented by the average upper crust (COs = 0.05 ppb, 187Os/188Os = 1.204 and 187Re/188Os = 50) (Esser and Turekian, 1993). The modeling yields an estimated less than 20% contribution of contaminant for all of the Fanjingshan ultramafic rocks. Nevertheless, diabase requires at least 50%–60% contribution from contaminant to generate their high 187Os/188Osi ratios of 0.347 3 and 0.427 9 even if the Os content of the parental melts is assumed to be as low as 0.015 ppb (Fig. 5a), which hints at an alternative contaminant with higher 187Os/188Osi ratio than average upper crust. The Archean Kongling TTG Complex may probably be the contaminant (Gao and Zhang, 1990). We assume COs = 0.019 ppb and 187Os/188Os = 4.54 for the Kongling TTG based on the Re-Os data of one granitic gneiss from the Archean Kolar schist belt (Walker et al., 1989). Modeling indicates that less than 7% contaminant is required to produce the Os isotopic compositions of the ultramafic rocks (Fig. 5b), whereas 8%–9% contaminant is needed to generate the high 187Os/188Osi ratios of diabase when their parental magma is assumed to have low Os abundance of 0.015 ppb.

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Figure 5. Modeling calculation for crustal contamination for Os isotope (the Os isotope data are from Wang et al., 2014). The f means the degree of contamination. End members used in the modeling are described in the text. The Os isotopic compositions of the Fanjingshan mafic-ultramafic rocks are shown as the shaded areas. Os abundances for parental melts of ultramafic and mafic rocks are assumed in a range of 0.15 ppb to 0.015 ppb. (a) Contaminant is the country rocks of the Fanjingshan sills; (b) contaminant is the Kongling TTG
4.3 Partial Melting of the Mantle Source

In a low degree of melting, Ir, Os and Ru are most compatible during mantle melting and tend to be concentrated in the mantle residue due to their greater bulk partition coefficient and higher melting points than Pt and Pd, while the PPGE are relatively incompatible among which Pd maintains maximum incompatibility and trend to be concentrated in the melting (Lorand and Alard, 2001; Alard et al., 2000; Rehkämper et al., 1999). In contrast, magma formed by high degree of partial melting contains relatively low abundance of Cu and PPGE (owing to dilution) and high abundance of Ni and IPGE (owing to continued melting of olivine and PGE alloys) (Saha et al., 2015). The Fanjingshan mafic-ultramafic rocks are depleted in IPGE relative to PPGE and have the highest Pd contents relative to other PPGE (Fig. 3), therefore, the Fanjignshan rocks were likely derived from low degree of partial melting of the mantle.

On the other hand, fractionation of PGE during partial melting of mantle source has been widely demonstrated in several case studies (Lorand and Alard, 2001; Alard et al., 2000; Barnes et al., 1985) and experimental modeling (Bockrath et al., 2004). Improved understanding on the host phases of the PGE in mantle show that IPGE (i.e., Os, Ir, Ru) are hosted mainly in monosulphide solid solution that commonly incorporated within cumulus silicate minerals, whereas PPGE (i.e., Pd and Pt) tend to reside in Cu-rich sulphide (intermediate solid solution) occurring between silicate minerals (Lorand and Alard, 2001; Luguet et al., 2001; Alard et al., 2000; Handler and Bennett, 1999; Lorand et al., 1999; Rehkämper et al., 1999). Besides, Ir is thought to be partly controlled by Cr-spinel and olivine (Righter et al., 2004; Puchtel and Humayun, 2001), resulting in gradual increase in Ir abundance of the melt during progressive partial melting, whereas Pd content is initially low but increases sharply at ca. 20% partial melting as sulphide is consumed (Barnes and Lightfoot, 2005). Therefore, these variations imply low Pd/Ir ratios at low degree of partial melting but high Pd/Ir ratios at high degree of partial melting. The Fanjingshan rocks have low and nearly constant Pd/Ir ratios (11–28), indicating low degree of partial melting, which is consistent with the conclusion that the Fanjingshan rocks were derived from low degree of partial melting of mantle wedge (enriched spinel lherzolite mantle) (Wang et al., 2014). In addition, Ni is more compatible than Cu during mantle melting and Ni/Cu ratio of the melt would increase progressively with increasing degree of melting. On these grounds, Ni/Cu and Pd/Ir ratios can be used to estimate the degree of partial melting of the mantle (Yang et al., 2012; Naldrett, 2004; Barnes and Maier, 1999). Ni/Cu ratios of the Fanjingshan rocks are rather scattered (0.1–40, average of 16), which is considered to be the result of olivine fractionation as Ni is preferentially partitioned into olivine than Cu. Although ultramafic rocks with variable olivine cumulate have elevated Ni/Cu ratios relative to mafic rocks (Fig. 6), which represent evolved magma that has experienced olivine fractionation, the primary magma of the Fanjingshan rocks is estimated to similar to high Mg basalts (Fig. 6), which is consistent with the previous conclusion.

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Figure 6. Plot of Ni/Cu vs. Pd/Ir of the Fanjingshan rocks. Reference fields after Barnes et al. (1985). Symbols are the same as in Fig. 3. Ol. Olivine, CHR. chromite, MSS. monosulphide solid solution

The major and trace elements as well as Sr-Nd-Pb isotopic characteristics of the Fanjingshan mafic-ultramafic rocks suggest that their parental magma was derived from a mantle wedge (enriched spinel lherzolite mantle) previously metasomatized by slab-derived fluids and melts. The parental magma compositions are similar to high Mg basaltic melts generated by multiple pulses magmatism (Wang et al., 2014). The clinopyroxenes of the Fanjingshan ultramafic rocks have diopsidic to augitic compositions (Wang et al., 2014), which are similar to clinopyroxenes of the typical Alaskan-type intrusions (Helmy and Mahallawi, 2003). The subduction-related geodynamic setting prior to the final amalgamation between the Yangtze and Cathaysia blocks is constrained by the volcanic-sedimentary sequence of the Fanjingshan Group as well as geochemical evidences of the Fanjingshan mafic-ultramafic rocks. Moreover, associated pillow basalts show LREE enriched patterns with (La/Yb)N ratios of 2.38–4.83 and flat HREE, and display prominent negative Nb, Ta, and Ti but positive Pb anomalies, the typical geochemical signatures of arc basalts (Xue et al., 2012; Zhou et al., 2009). Such a scenario is broadly analogous to the emplacement of Alskan-type intrusions (Wang et al., 2014). Relatively lower degree (5%–10%) of partial melting of mantle source has been estimated based on trace elemental compositions of bulk rocks and clinopyroxene of the Fanjingshan sills (Wang et al., 2014). Nevertheless, this low degree of partial melting could be underestimated to represent the minimum value because the chemical composition of accumulated clinopyroxene may change during its re-equilibration with the residual liquid, resulting in slightly higher trace element contents relative to the original composition (Wang et al., 2014).

4.4 Behavior of PGE during the Evolution of the Fanjingshan Magma 4.4.1 Early segregation of sulphide and cumulus sulphide

Early segregation of sulphide phase, which may effectively remove PGE from the mantle-derived magma, could be promoted by decreasing temperature and Fe2+ content of the magma and addition of external sulfur by combination of crustal assimilation and fractional crystallization during magma ascent and emplacement (Liu et al., 2017; Mavrogenes and O'Neill, 1999; Lightfoot and Hawkesworth, 1997). As the PGE and a lesser content Cu, partition strongly into sulphide melts (Barnes and Lightfoot, 2005; Peach et al., 1994), extreme depletion of PGE relative to Cu can be produced by very low amount of sulphide segregation (Song et al., 2009; Barnes and Maier, 1999). The Fanjingshan ultramafic rocks have Cu/Pd ratios (545–5 216, except 11 471 of FJS11-69) significantly lower than primitive mantle value (Fig. 7a), indicating that the parental magma was S-undersaturated and fertile in terms of PGE. Dramatic decrease in Pt/Cr, Pt/Y and Pd/Y relative to constant Pd/Pt is expected to result from early removal of sulphide, which is inconsistent with the near-zero slopping trend in the Pt/YPM versus Pd/PtPM diagram (Figs. 7b, 7c). The insignificant correlation between Pd and Ir (Fig. 7d) also suggests that the distribution of PGE cannot be controlled by sulphide segregation, during which Pd would change accordingly with Ir. Cu/Zr ratio is another sensitive indicator of sulphide removal/addition because Cu and Zr have opposite behavior during sulphide segregation but similar incompatibility during fractionation of S-undersaturated mantle magma. The Fanjingshan ultramafic rocks have Cu/Zr ratios ranging from 0.4 to 5.8 with the majority greater than 1, again indicating no sulphide segregation preceding emplacement (Wang C Y et al., 2006). Negative Pt anomaly (Fig. 3) and high Pd/Pt ratios (0.9–27.9) reveal fractionation between Pd and Pt, which was largely due to silicate fractionation of an S-undersaturated magma rather than fractionation of S-saturated melt because the former process would be accompanied by depletion of Pt relative to Pd in the remaining melt due to higher partition coefficient of Pt between silicate magma and olivine phenocrysts relative to Pd (Momme et al., 2002). Alternatively, the low Cu/Pd and high Cu/Zr ratios of the Fanjingshan ultramafic rocks may suggest the presence of cumulus sulphide, as evidenced by the presence of pentlandite associated with olivine. Coupled increase between Pd/YPM and Pt/CrPM is also supportive of sulphide cumulus. The Fanjingshan diabases have Cu/Pd ratios (8 913–107 016, with two exceptions of 5 743 and 3 655) higher than primitive mantle value (Fig. 7a) indicating segregation of sulphide. Meanwhile, the diabases also show PGE depletion when MgO content is low (Fig. 4). All of these features suggest that the primary magmas of diabase have experienced segregation of sulphide in deeper level magma conduits or magma chambers, which could potentially result in the PGE depletion in the mafic rocks. Meanwhile, the mafic and ultramafic rocks were derived from the same material source, and therefore, it is possible that sulphide have remained in ultramafic rocks resulting in depleting PGE in diabase.

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Figure 7. Variation diagrams of Pd/Ir vs. Cu/Pd (a), (Pt/Cr)PM vs. (Pd/Cr)PM (b), (Pt/Y)PM vs. (Pd/Pt)PM (c) and Pd vs. Ir (d). Cu/Pd = 7 000 is the ratio of primitive mantle (after Barnes and Maier, 1999). Symbols are same as in Fig. 3. The trace element data are from Wang et al. (2014)
4.4.2 Partitioning of IPGE into alloy and/or chromite

The depletion of IPGE (Os, Ir and Ru) relative to PPGE (Rh, Pt and Pd) is interpreted to be due to either residual of silicate enclosed sulphide in the mantle source or early fractionation of laurite, chromite and olivine, because all of these minerals have DIPGEhigher than DPPGE (Righter et al., 2004; Puchtel and Humayun, 2001; Alard et al., 2000; Amosse et al., 1990). The interpreted high degree of partial melting in the mantle source is supposed to minimize the fractionation between IPGE and PPGE during magma generation (Lorand and Alard, 2001; Luguet et al., 2001), indicating that early fractionation of crystallized phase is the likely mechanism of depleted IPGE in the evolved magma. Laurite and Os-Ir-Ru alloys enclosed as submicroscopic grains in early fractionating phases, such as chromite and olivine, would effectively remove IPGE relative to PPGE and produce strong fractionation of PGE in the evolved melt (Zhou, 1994; Capobianco and Drake, 1990; Stockman and Hlava, 1984). Compared to the steep primitive mantle-normalized PGE patterns and prominent negative Ru anomalies due to removal of laurite and/or Os-Ir-Ru alloys (Qi and Zhou, 2008; Philipp, 2001; Vogel et al., 1999), the Fanjingshan ultramafic rocks have mild depletion of IPGE relative to PPGE and enriched Ru relative to Ir (Fig. 3) precluding any role of early removal of laurite and/or Os-Ir-Ru alloys in PGE differentiation during magma evolution.

Chromite is believed to be a potential concentrator of IPGE because of their compatibility in this mineral (Righter et al., 2004; Puchtel and Humayun, 2001). Platinum and Pd were highly incompatible with olivine (D = 0.08–0.03), and moderately compatible with chromite (D = 1.6–3.3) (Puchtel and Humayun, 2001). Euhedral Cr-spinel, commonly enclosed in olivine and clinopyroxene, may be a likely concentrator for IPGE whereas a non-linear correlation between Cr and PGE (Pd, Pt, Ir and Ru) implies that PGE were not controlled by chromite/Cr-spinel during crystal fractionation (Fig. 8). Since Ru is more compatible in chromite as compared to Ir (Locmelis et al., 2011; Righter et al., 2004; Puchtel and Humayun, 2001; Zhou et al., 1998), fractionation of chromite would result in Ru depletion relative to Ir in the evolved magma, which contradicts the features of Fanjingshan rocks (Fig. 3), thus indicating that chromite was not a major crystal phase during early stages of magma evolution. However, the Fanjingshan ultramafic rocks have high Cr contents (1 662 ppm–2 674 ppm, exception of 845 ppm and 868 ppm), implying that the parental magma of these rocks was likely to be accumulates with chromite that contains Ru higher than Ir.

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Figure 8. Cr vs. Ir, Ru, Pt and Pd of the Fanjingshan mafic-ultramafic rocks. Ol. Olvine; Cr-sp. Cr-spinel. Symbols are the same as in Fig. 3. The trace element data are from Wang et al. (2014)
4.4.3 PGE accumulation during olivine fractionation

Although several studies reveal incompatible behavior of Ir during crystallization of olivine (Puchtel et al., 2004; Maier, 2003), olivine has been considered as one of the IPGE carrier or to constitute an IPGE sink relative to PPGE (Puchtel et al., 2004; Crocket, 2002; Zhou, 1994). Variation between major/trace elements and MgO content of the Fanjingshan rocks, as well as abundant cumulate olivine grains, indicate that the magma differentiation was largely controlled by olivine fractionation (Wang et al., 2014). In the MgO versus Os and Rh diagrams, the Fanjingshan ultramafic rocks display positive trends, indicating compatible nature of these elements during magma differentiation. On the contrary, Ir, Pt and Pd behave incompatibly during crystallization of olivine, as evidenced by the slightly negative correlation between Ir and Mg, and the obviously negative correlations of MgO with Pt and Pd (Fig. 4).

This observation is consistent with the partition coefficients (DPGEOl/liq) of PGE between olivine and silicate liquid (e.g., 0.77 for Ir, 1.8 for Rh, 0.08 for Pt and 0.03 for Pd) (Righter et al., 2004; Puchtel and Humayun, 2001), implying that olivine could govern the distribution of PGE during magma evolution. Discrimination between olivine- and Cr-spinel control on PGE distribution can be made using the V-Pd/Ir diagram, in which olivine-control PGE defines a positive correlation, whereas Cr-spinel control PGE defines a negative correlation due to compatibility of V in Cr-spinel (Canil, 1999). Y and REE define similar geochemical trends with V. Therefore, the covariance between Pd/Ir and V, Y and REE (Fig. 9) also underline olivine control on the distribution of PGE during Fanjingshan mafic-ultramafic magma evolution.

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Figure 9. Covariations of Pd/Ir ratios with V, Y and REE concentrations of the Fanjingshan mafic-ultramafic rocks. Symbols are the same as in Fig. 3, the trace element data are from Wang et al. (2014)
4.4.4 Potential of Cu-Ni-PGE sulphide mineralization

From the view of geodynamic/tectonic setting, the Fanjingshan sills are thought to be similar to Alaskan-type mafic-ultramafic intrusions (Wang et al., 2014), which is commonly associated with Cu-Ni-PGE sulphide mineralization (Pettigrew and Hattori, 2006). In addition, the Fanjingshan sills show lithological and geochronological signatures similar to the Cu-Ni sulphide bearing Baotan mafic-ultramafic intrusions in the Guibei area, ca. 200 km west to the study area (Wang et al., 2014; Zhou et al., 2009; Mao, 2002), indicating the possibility of Cu-Ni-PGE sulphide mineralization in the Fanjingshan mafic-ultramafic rocks. As the parent magma for Fanjingshan mafic-ultramafic rocks were S-undersaturated, early segregation of immiscible sulphide is unlikely as also evidenced by relatively low Cu/Pd but high Cu/Zr ratios of the Fanjingshan ultramafic rocks. Although the presence of cumulate sulphide may reflect exsolution from monosulphide solid solution during magma cooling, the primitive mantle normalized PGE pattern of the Fanjingshan mafic-ultramafic rocks deviates from typical "M-shaped" pattern (peaking at Ir and Pt with intervening trough at Ru) of Alaskan-type complexes. Further, the Alaskan-type Cu-Ni sulphide complexes formed by exsolving of sulphide and platinum group minerals (PGM) from monosulphide solid solution (Helmy and Mogessie, 2001). Therefore, sulphide and PGM exsolving from monosulphide solid solution was not the dominant PGE controller during magma evolution. On the other hand, later sulfur saturation can be attained by the introduction of sulfur into the primary magma by devolatilization of crustal sulphide or digestion of bulk crust (Gao et al., 2012a; Yang et al., 2012; Ripley et al., 1998; Lesher and Stone, 1996; Keays, 1995).

The Cu-Ni sulphide bearing Baotan mafic-ultramafic intrusions were derived from low degree of partial melting of depleted mantle and undergone contamination of crustal sulphide (Ge et al., 2001). However, the Nd-Os isotope characteristics and generally low SiO2, K2O and Na2O contents as well as the absence of Zr and Hf positive anomalies in the Fanjingshan ultramafic rocks rule out any significant crustal contamination (Wang et al., 2014). Nevertheless, selective contamination by crustal sulphide can facilitate S-saturation in mantle-derived magma without affecting Sr-Nd isotopic and trace elemental signatures of primary magma whereas 187Os/188Os of the magma would be substantially elevated due to highly radiogenic Os isotopic compositions in the crust and high Os concentration in crustal sulphide (Yang et al., 2012). The subchondritic to slightly radiogenic initial Os isotopic signatures of the Fanjingshan ultramafic rocks can be largely attributed to the enriched and heterogeneous mantle source and thus indicate insignificant selective contamination of crustal sulphide (Wang et al., 2014).

It has been suggested that if high temperature and high magnesium komatiitic, or picritic S-unsaturated magma ascends into the upper crust and becomes S-saturated, it would trigger the formation of magmatic Cu-Ni-PGE sulphide mineralization (Arndt et al., 2005; Keays, 1995). The parental magma of the Fanjingshan mafic-ultramafic rocks is interpreted to be the multiple pulses of high Mg basaltic melts generated by relatively low degree of partial melting (Wang et al., 2014 and references therein). Meanwhile, the low Cu/Pd ratios of Fanjingshan ultramafic rocks indicate that the parental magma was S-undersaturated. In addition, the parental magma undergoes S-satruation phase resulting in the PGE extremely depleted. Taking into account all the above considerations we conclude that the Fanjingshan mafic-ultramafic rocks are unlikely to host economic Cu-Ni-PGE sulphide mineralization.

5 CONCLUSION

The Fanjingshan sills are interpreted to be derived from relatively low degree of partial melting of subduction lithospheric mantle source with minor crustal assimilation. The primary melt is considered to be S-undersaturated due to the insignificant addition of crustal materials during early magmatic differentiation, thus arguing against sulphide melt immiscibility. The PGE distribution and fractionation was largely controlled by fractionation of silicate minerals such as olivine rather than PGM/alloy and chromite. Although cumulate sulphide is present, economical mineralization of Cu-Ni-PGE sulphide seems unlikely due to the limited crustal assimilation during evolution of S-undersaturated magma.

ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (No. 41572170), "Thousand Youth Talents Plan" grant to Wei Wang and MOST Special Fund from the State Key Laboratory of Geological Processes and Mineral Resources (No. MSFGPMR11 and 01-1). We would like to thank Liang Qi and Xiaowen Huang for the PGE analyses. Jian-Feng Gao is thanked for discussion on an early version of this manuscript. The language is polished and improved greatly by Prof. Pandit Manoj. Two anonymous reviewers are thanked for their constructive comments. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-1201-x.

Electronic Supplementary Material: Supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-018-1201-x.


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