Geosciences, Beijing, , China b Department of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell

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1 This article was downloaded by: [China University of Geosciences], [Mr Zhaochong Zhang] On: 15 October 2012, At: 20:59 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK International Geology Review Publication details, including instructions for authors and subscription information: Geochronology and geochemistry of the Nantianwan mafic ultramafic complex, Emeishan large igneous province: metallogenesis of magmatic Ni Cu sulphide deposits and geodynamic setting Meng Wang a, Zhaochong Zhang a, John Encarnacion b, Tong Hou a & Wenjuan Luo a a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, , China b Department of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell Boulevard, St. Louis, MO, 63108, USA Version of record first published: 08 Mar To cite this article: Meng Wang, Zhaochong Zhang, John Encarnacion, Tong Hou & Wenjuan Luo (2012): Geochronology and geochemistry of the Nantianwan mafic ultramafic complex, Emeishan large igneous province: metallogenesis of magmatic Ni Cu sulphide deposits and geodynamic setting, International Geology Review, 54:15, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 International Geology Review Vol. 54, No. 15, November 2012, Geochronology and geochemistry of the Nantianwan mafic ultramafic complex, Emeishan large igneous province: metallogenesis of magmatic Ni Cu sulphide deposits and geodynamic setting Meng Wang a, Zhaochong Zhang a *, John Encarnacion b, Tong Hou a and Wenjuan Luo a a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing , China; b Department of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell Boulevard, St. Louis, MO 63108, USA (Accepted 17 February 2012) The Nantianwan mafic ultramafic complex is situated in the northwest part of the Panxi district, southwest China. It consists predominantly of gabbros, gabbronorites, and lherzolites. LA ICP MS U Pb zircon dating of the gabbronorites yields an age of ± 0.6 million years, consistent with the ages of other mafic ultramafic intrusions in the Emeishan large igneous province (ELIP). Gabbronorites and lherzolites host Cu Ni sulphide ores. Cumulus texture is pronounced in these rocks, containing magnesium-rich olivine (up to 81.4% forsterite). SiO 2 contents of the lherzolites range from to wt.%, whereas those of the gabbronorites vary between and wt.%. Analysed samples have low rare earth element (REE) contents ( ppm for lherzolites and ppm for gabbronorites). Both lherzolites and gabbronorites have similar chondrite-normalized REE patterns, suggesting that they are comagmatic. All samples are slightly enriched in large ion lithophile elements (LILEs, e.g. Rb, Ba, and Sr) relative to high field strength elements (HFSEs, e.g. Nb, Ta, and Ti), very similar to those of ocean island basalts (OIBs). The presence of cumulus textures and geochemical signatures indicates that fractional crystallization played an important role in the petrogenesis of these rocks. Initial ( 87 Sr/ 86 Sr) t (t = 260 Ma) ratios and ε Nd (t) values of the mafic ultramafic suite vary from to , and 0.4 to 1.7, respectively. Compared to the Cu Ni-bearing Baimazhai and Limahe intrusions in the ELIP, which were considerably contaminated by variable crustal materials, the Nantianwan complex exhibits much lower ( 87 Sr/ 86 Sr) t.theirε Nd (t) versus (Th/Nb) PM ratios also indicate that the ore-bearing magmas did not undergo significant crustal contamination. In combination with (Tb/Yb) PM versus (Yb/Sm) PM modelling, we infer that the magmas originated from an incompatible elements-enriched spinel-facies lherzolite that itself formed by interaction between the Emeishan plume and the lithospheric mantle. Most plots of NiO versus Fo contents of olivine suggest that sulphides are separated from the parental magma by liquid immiscibility, which is also supported by bulk-rock Cu/Zr ratios of the lherzolites ( ) and gabbronorites ( ). We suggest that the gabbronorites and lherzolites experienced undersaturation to oversaturation of sulphur; the latter may be due to fractional crystallization in a high-level magma chamber, accounting for the sulphide segregation. Keywords: Nantianwan complex; geochemistry; metallogenesis; Cu Ni sulphide; Panxi region Introduction The Emeishan igneous complex is a unique large igneous province (LIP) in China, widely recognized by the international geoscientific community. It is one of the three LIPs on Earth that formed near the end of the Permian in widely separated locations, the others being the Siberian Traps (Sharma 1997; Dobretsov 2005) and the Panjal Traps of northwestern India (e.g. Bhat et al. 1981). The Emeishan large igneous province (ELIP) is thought to be genetically related to a major 260 Ma plume event (e.g. Thompson et al. 2001; Xu et al. 2004; Ali et al. 2010). Although the Permian ELIP is not as large as other LIPs (e.g. Zhang et al. 2006), it is one of the richest in mineral resources and the only province in the world that hosts both magmatic Fe Ti V oxide ores and Cu Ni (PGE) (platinum group element) sulphide deposits (e.g. Hou et al. 2011). Particularly, it is well known for the presence of the world s largest Fe Ti V oxide ore cluster in the Panxi region (e.g. Zhang et al. 2009), where the scale of Cu Ni (PGE) mineralization is relatively small. In contrast, the Siberian LIP is characterized by the world-class Noril sk Cu Ni (PGE) sulphide deposits. The difference in mineralization between these two Permian LIPs raises an important question: what caused the different features of the ore deposits: geologic settings, source composition, depth of melting, lithospheric/crustal contamination, or magma chamber processes? Recently, small-scale Cu Ni (PGE) sulphide mineralization has been recognized in the Nantianwan mafic ultramafic complex in the Pingchuan area, Yanyuan *Corresponding author. zczhang@cugb.edu.cn ISSN print/issn online 2012 Taylor & Francis

3 International Geology Review 1747 county of Sichuan province, in the western part of the ELIP. Current exploration drilling is in progress by the 604 Geological Team, Geological and Mineral Resources Bureau, Sichuan, China. Sparsely disseminated Cu Ni sulphide ores with a thickness of 350 m were recognized in one borehole. Unlike other Cu Ni (PGE) sulphide deposits in the ELIP, such as the Limahe, Zhubu, and Baimazhai, which are hosted by small sills, the volume of the Nantianwan mafic ultramafic intrusion is relatively large (39 km 2 ). The prospect for large-scale Cu Ni (PGE) sulphide mineralization resembling those large Cu Ni (PGE) sulphide deposits such as Noril sk and Sudbury (Naldrett 1997, 1999) has aroused extensive interest. However, except for a limited geological survey conducted at the scale of 1:50,000 and 1:200,000 in the Pingchuan area, no other detailed information has been documented to date. In this article, we present the first geochronology, mineral chemical, bulk-rock major + trace element, and Sr Nd isotopic compositions, aimed at constraining the nature of the sources of the intrusions and the petrogenesis, which in turn provide some key constraints on the metallogenesis of Cu Ni sulphide mineralization. These new data not only shed new light on the petrogenesis of the ELIP that we can apply to other provinces, but may also allow refinement of previously proposed exploration models for the same type of deposits in the region and around the world. Regional geological setting The ELIP is exposed over a large part of southwest China, including Yunnan, Guizhou, Sichuan, and Guangxi provinces, and northern Vietnam, from the eastern margin of the Tibetan Plateau to the western margin of the Yangtze Craton (Xu et al. 2004). It forms a massive Permian Triassic succession of volcanic rocks along the western margin of the Yangtze Craton (Chung and Jahn 1995; Xu et al. 2001, 2004; He et al. 2003). The Emeishan volcanic succession comprising predominantly basaltic flows and pyroclastics, with minor amounts of picrite and trachyte/rhyolite, is a several hundred metres to 5 km-thick bimodal suite (SBGMR 1991; Chung and Jahn 1995), exposed in an area of km 2, and is associated with numerous ultramafic mafic to felsic alkaline intrusions (Figure 1). The basement of the Yangtze Craton locally comprises the Archaean-Palaeoproterozoic Kangding Complex, composed of granulite amphibolite facies metamorphic rocks, and the Mesoproterozoic Huili Group or its equivalents, the Yanbian Group, which consists of metasedimentary rocks interbedded with felsic and mafic metavolcanic rocks. The basement is overlain by a thick sequence (>9 km) of Sinian ( Ma) to Permian strata composed of clastic, carbonate, and metavolcanic rocks (SBGMR 1991). The early Sinian consists of clastic rocks and felsic volcanic rocks, while the late Sinian Dengying Formations consist of clastic rocks in the lower part and phosphorous-bearing carbonate rocks in the upper part. The Early Cambrian strata are characterized by clastic and carbonate rocks, whereas the Middle Late Cambrian strata are characterized by limestones (Zhulinping Formation) and dolomitic limestones (Gaojiaping Formation). The Early Silurian strata mainly consist of argillaceous rocks and sandstones (Zhongcao Formation). The Ordovician strata consist of carbonate (Hongshiya Formation) and argillaceous rocks, whereas the Carboniferous strata consist of silicalites (Daopingzi Formation) and limestones (Maping Formation). The Permian Emeishan basaltic succession unconformably overlies the limestones of the Early Permian Maokou Formation (YBGMR 1990). In the Panxi region, extensive erosion and thinning of the Middle Permian limestone in the central part of the ELIP indicate kilometre-scale regional uplift linked to the rising plume (He et al. 2003; Xu et al. 2004). Numerous mafic ultramafic intrusions are exposed along several N S-trending faults. These intrusions host major worldclass Fe Ti oxide deposits (Zhou et al. 2005, 2008). Geology of the Nantianwan complex and associated Cu Ni ores The Nantianwan complex, situated at the northwestern margin of the Panxi region, approximately 35 km to the southwest of Xichang City, is bounded by the Jinhe-Jinghe fault. The WNW-trending complex has a large surface exposure of 39 km 2 that is 9 km in length and 3 5 km in width (Figure 2). There is a fault to the south of the complex running in a NW SE direction. The complex intruded argillaceous rocks and sandstones of the Early Silurian Zhongcao Formation. In the study area, Cambrian to Triassic strata are exposed, such as the Cambrian Zhulinping Formation, Gaojiaping Formation, Ordovician Hongshiya Formation, Carboniferous Maping Formation, Daopingzi Formation, and Triassic strata. The Nantianwan mafic ultramafic complex consists of gabbros ( 60 vol.%), gabbronorites ( 25 vol.%), and lherzolites ( 15 vol.%). The gabbros are intruded by the gabbronorites in the central part of the complex, and several small lherzolites locally intrude the gabbronorites, with a total outcrop area of 0.5 km 2. Thus, the intrusive sequence for the Nantianwan mafic ultramafic complex is inferred to be as follows: gabbro gabbronorite lherzolite. Ni Cu sulphide ores are only hosted by the lherzolites and gabbronorites (Figures 3A and 3B). Since the Nantianwan complex associated with Cu Ni mineralization is still under exploration, the ore reserves and grade

4 1748 M. Wang et al. Figure 1. Geological sketch map of Cu Ni (PGE)-bearing mafic ultramafic intrusions in western Sichuan Province (modified from Liu et al. 2008a). are not yet established. However, based on petrographic observation, the mineralized rocks contain 2.5 modal% pentlandite, 2 modal% chalcopyrite, and 1 modal% pyrrhotite. These minerals are sparsely to densely disseminated throughout the gabbronorites. The lherzolites are mainly composed of variable sizes (0.3 up to 2.5 mm in length) of cumulus olivine crystals ( 45 vol.%; Figure 3C). Some of the clinopyroxenes (Cpx, 25 vol.%) and orthopyroxenes (Opx, 15 vol.%) are partly replaced by tremolite and chlorite. Minor anhedral plagioclases are observed as interstitial phases. Chromite usually occurs as an accessory mineral. Most cumulus olivines are euhedral and partly altered into serpentine along fractures. Characteristics that are only recognized in olivine as mantle xenocrysts or in xenoliths, such as strained, kink-banded crystals, have not been observed. The gabbronorite predominantly consists of plagioclase (Pl; vol.%), Cpx (35 vol.%), Opx (10 vol.%), and minor olivine and biotite (Bi). Poikilitic texture (Figure 3D) and diabasic texture (Figure 3E) are ubiquitous in these rocks. Petrographic observations suggest that the initial crystallization sequence of the minerals is Ol sulphide Cpx Opx Pl Bi (Figure 3F). The gabbro is dark grey to grey black. Gabbros have a diabasic or granular texture and consist mainly of clinopyroxene ( 50%) and plagioclase (25 30%) with minor hornblende and biotite, as well as accessory Fe Ti oxide.

5 International Geology Review E E N N Triassic sediments Permian Emeishan basalts Limestones of Maping Formation Silicolites of Daopingzi Formation Mudstones of Zhongcao Formation Carbonates of Hongshiya Formation Dolomites of Gaojiaping Formation Sediments of Zhulinping Formation Dolomites of Dengying Formation Gabbros Gabbronorites Lherzolites Picritic porphyries Figure 2. Geological map of Nantianwan complex (modified from the 1:50,000 geological map of Pingchuan area, 1990). Clinopyroxenes are usually euhedral to subhedral and partly replaced by actinolite and tremolite. Plagioclase is subhedral to anhedral and usually partly altered by sericite and saussurite. In other Cu Ni (PGE) sulphide deposits, the ores always occur in the lower parts of the mafic ultramafic intrusions or contact zone between intrusions and country rocks such as in the Panxi area (e.g. Luo 1981; Liang et al. 1998; Zhang et al. 1998; Zhong et al. 2002), where the Baimazhai and Limahe intrusions intruded the Ordovician and the Proterozoic Huili Group (Wang et al. 2006; Zhang et al. 2009), respectively. The Nantianwan complex, in contrast, intruded the Early Silurian sequence, and the ores appear to be quite sparsely disseminated through most of the intrusion. Sampling and analytical method Ten samples were systematically collected from one drilled borehole. The samples were initially checked for weathering and traces of alteration, which were removed before the rocks were reduced to chips. One sample of gabbronorites from the Nantianwan intrusion was chosen petrographically for zircon U Pb isotopic dating, nine for major element and trace element analyses, and six for whole-rock Fault Sr Nd isotope analyses. Details of the preparation and analytical techniques follow. Zircon U Pb dating Zircons were separated from the sample DY (gabbronorite) using heavy liquid and magnetic techniques and then purified by handpicking under a binocular microscope. Zircon grains were picked and mounted on adhesive tape then enclosed in epoxy resin and polished to about half of their diameter. In order to observe textures of the polished zircons, cathodoluminescence (CL) imaging was carried out using a Hitachi S3000-N scanning electron microscope (SEM; Hitachi, Tokyo, Japan) with a Mono CL3 cathodoluminescence system for high-resolution imaging and spectroscopy at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China. Zircon U Pb isotopic analyses were performed on a Finnigan Neptune multi-collector ICP MS with a Newwave UP213 laser-ablation system at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. Helium was used as the carrier gas to enhance the transport efficiency of the ablated material. The analyses were conducted with a beam diameter of 25 µm with a 10 Hz repetition rate and a laser power of 2.5 J/cm 2

6 1750 M. Wang et al. Figure 3. Handspecimen photographs and microphotographs (cross-polarized light) of samples from the Nantianwan complex. (A) Drilling samples, scarce nickel copper sulphide; (B) disseminated structure in gabbronorite; (C) cumulate texture and mineral paragenesis in lherzolite; (D) feldspar-bearing poikilitic texture in gabbronorite; (E) diabasic texture in gabbronorite; (F) opaque minerals filled in the fissures in olivine, and surrounded by orthopyroxene, in gabbronorite. Abbreviations: Opx, orthopyroxene; Cpx, clinopyroxene; Ol, olivine; Pl, plagioclase; Py, pyrite; Po, pyrrhotine; Chl, chalcopyrite. (Hou et al. 2009). The masses 206 Pb, 207 Pb, 204 (Pb + Hg), and 202 Hg were measured by multi-ion-counters, while the masses 208 Pb, 232 Th, 235 U, and 238 U were collected using a Faraday cup. Zircon GJ1 was used as the standard and zircon Plesovice was used to optimize the machine. U, Th, and Pb concentrations were calibrated using 29 Si as the internal standard and zircon M127 (U: 923 ppm; Th: 439 ppm; Th/U: 0.475; Nasdala et al. 2008) as the external standard. The in-house software program, ICPMSDataCal, produced by Liu et al. (2008b), was used for off-line selection and integration of background and analysis signals, time-drift correction and quantitative calibration for the trace element analyses and U-Pb dating. Correction for common Pb was omitted because of the high 206 Pb/ 204 Pb ratios (>1000). Data with abnormally high 204 Pb counts were deleted. The zircon Plesovice was analysed as an unknown and yielded a weighted mean 206 Pb/ 238 U age of 337 ± 2 million years (2SD, n = 12), which is in good agreement with the recommended 206 Pb/ 238 U age of ± 0.37 million years (2SD) (Sláma et al. 2008). The age calculation and plotting of concordia diagrams were performed using Isoplot/Ex 3.0 (Ludwig 2003; Figure 4). The results are presented in Table 1. Electric microprobe analyses Electron microprobe analyses were determined for some olivines in the wehrlites using a JEOL JXA-8230 Superprobe at the EMPA Laboratory of Analysis Centre of Mineral and Rocks of the Institute of Mineral Resources,

7 International Geology Review 1751 Figure Pb/ 235 U 206 Pb/ 238 U concordia diagram of zircons from the Nantianwan complex. Chinese Academy of Geological Sciences. Operating conditions were set at 15 kv at 10 na beam current. Natural minerals and synthetic pure oxides from SPI Supplies Inc. (West Chester, PA, USA) were used as standards. For pyroxene, the calibration standards used were hornblende (for Si, Ti, Al, Fe, Ca, Mg, Na, and K), fayalite (for Mn), and Cr 2 O 3 (for Cr). For plagioclase, the standards used were hornblende (for Si, Ti, Al, Fe, Ca, and Mg), albite (for Na), orthoclase (for K), and fayalite (for Mn). Precision is better than 1% for element oxides. Major and trace element analyses After screening under the microscope, relatively fresh samples were selected and sawn into slabs and the central parts were used for whole-rock analyses. Specimens were crushed in a steel mortar and ground in a steel mill to powders of 200 mesh. Major elements were acquired through analysis of a fused glass disc using a scanning wavelength dispersive X-ray fluorescence spectrometer at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing, China. The analytical uncertainties are less than 1%, estimated from the repeated analyses of two standards (andesite GSR- 2 and basalt GSR-3). Loss on ignition was determined gravimetrically after heating the samples at 980 C for 30 min. Trace elements were determined by solution ICP MS performed at the ICP MS Laboratory at the National Research Centre for Geoanalysis, Beijing, China. After complete dissolution, powders ( 40 mg) were dissolved in distilled HF+HClO 4 in 15 ml Savillex Teflon screwcap beakers. Precision for most elements was typically better than 5% relative standard deviation, and the measured values for Zr, Hf, Nb, and Ta were within 10% of the certified values (Dulski, 1994). The detailed sample preparations, instrument operating conditions, and calibration procedures follow those established by Qi and Grégoire (2000). Two standards (granite GSR-1, basalt GSR-3) were used to monitor the analytical quality. Rb Sr and Sm Nd isotope analyses Rb Sr and Sm Nd isotopic compositions were obtained using a Finnigan MAT-262 multi-collector mass spectrometer at the Institute of Geology, Chinese Academy of Geological Sciences. The samples ( 100 mg) were weighed and spiked before dissolution with mixed isotopic tracers. They were dissolved overnight using HF and HNO 3, evaporated to dryness, and then followed by oven dissolution in fresh HF and HNO 3 for 7 days at 160 C. Separation of Rb and Sr were carried out with a cation-exchange column (packed with AG50Wx8). Sm and Nd were further purified using a second cation-exchange column (packed with AG50Wx12). Total procedural blanks were 200 pg for Sr and 50 pg for Nd. The mass fractionation corrections for Sr and Nd isotopic ratios were based on 88 Sr/ 86 Sr of and 146 Nd/ 144 Nd of , respectively. During our analyses, measured 87 Sr/ 86 Sr ratios for standard NBS987 were ± , and measured 143 Nd/ 144 Nd ratios for the JMC standard were ± (in 2σ uncertainty for 18 analyses).

8 1752 M. Wang et al. Table 1. LA ICP MS U Pb isotope compositions of zircon in gabbronorites from the Nantianwan complex. Contents ( 10 6 ) U Th Pb isotopic ratio Age (million years) Sample Th U Th/U 207 Pb/ 206 Pb 207 Pb/ 206 Pb 207 Pb/ 235 U 207 Pb/ 235 U 206 Pb/ 238 U 206 Pb/ 238 U 207 Pb/ 206 Pb 1σ 207 Pb/ 235 U 1σ 206 Pb/ 238 U 1σ Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy Dy

9 International Geology Review 1753 Results LA ICP MS U Pb dating Zircons in our sample DY (gabbronorite) are generally transparent, euhedral, and prismatic (Figure 5). On the CL images, most zircon grains display long prismatic shapes (more than 100 µm in length) and oscillatory zoning. Twenty analyses were obtained in total with variable U contents of ppm, Th contents of ppm, and high Th/U ratios ( ), showing characteristics of typical igneous zircons (Hanchar and Rundnick 1995; Hoskin and Black 2000; Corfu et al. 2003; Grant et al. 2009). Except for one discordant analysis (DY-14), 19 analyses show a consistent age of ± 0.6 million years (mean square weighted deviation = 0.13). This can be interpreted as the crystallization age for the gabbronorite in the Nantianwan complex. Mineral chemistry Analyses of olivine, clinopyroxene, and plagioclase in lherzolite samples are listed in Table 2. The Mg-number [100 Mg/(Mg + Fe), molar] in the olivine varies from 79.1 to Olivine from lherzolite has much higher CaO content (>0.1 wt.%) than that of typical mantle xenoliths (Thompson and Gibson 2000). The orthopyroxene analyses show small compositional ranges of Wo En Fs belonging to bronzite. The plagioclases show a compositional range of An , Ab , Or , and are, therefore, bytownites to andesines. Bulk-rock major and trace element data According to the major and trace element analyses listed in Table 3, in general, the SiO 2 content of lherzolites ranges from to 44.18%, whereas the SiO 2 content of the gabbronorites is relatively higher, from to 52.76%, as expected. Except for two analyses (HC-1 and HC-4), most of our samples have relatively low TiO 2 contents ( wt.%). For lherzolites, gabbronorites, and gabbros, MgO contents decrease from to 5.45% (Figure 6). Additionally, the SiO 2, Al 2 O 3, TiO 2, and K 2 O+NaO 2 contents increase whereas total FeO contents remain constant with decreasing MgO contents, consistent with the fractionation of olivine and pyroxene (Figure 6). In general, all samples have similar primitive mantlenormalized trace element and chondrite-normalized rare earth element (REE) patterns with chondrite-normalized REE patterns similar to ocean island basalts (OIB) and Lijiang picrites (Figure 7). The total REE contents of the three types of rocks are relatively low ( ppm) and are relatively enriched in light REE relative to heavy LREE. The samples lack significant Eu anomalies suggesting the lack of significant plagioclase fractionation, except for three lherzolite samples (δeu = ; δeu = 2Eu n /(Sm n +Gd n )) with slightly negative Eu anomalies. Most of the samples are slightly enriched in large ion lithophile elements (LILEs, e.g. Rb, Ba, and Sr) relative to high field strength elements (HFSEs) with sizeable Nb Ta and Ti troughs. Sr and Nd isotope compositions Samples from the Nantianwan complex have age-corrected ( 87 Sr/ 86 Sr) t (t = 260 Ma) values varying from to and age-corrected ε Nd (260 Ma) values ranging from 0.4 to 1.7 (Table 4). Obviously, the ( 87 Sr/ 86 Sr) t of the Nantianwan complex is higher than those of typical Figure 5. Cathodoluminescence (CL) images of zircons of the Nantianwan complex (circles are laser points, numbers are points order).

10 1754 M. Wang et al. Table 2. Electron microprobe composition of representative minerals in Nantianwan lherzolites (wt.%). Sample no. Mineral SiO 2 Na 2 O MgO Al 2 O 3 CaO FeO MnO TiO 2 NiO Total Fo DPZ-2-2 Ol Ol DPZ-2-4 Ol Ol DPZ-2-4 Ol Ol DPZ-2-5 Ol Ol DPZ-2-7 Ol Ol DPZ-2-12 Ol 2 Ol DPZ-3-1 Ol Ol DPZ-3-2 Ol Ol DPZ-3-3 Ol 2 Ol DPZ-3-3 Ol 3 Ol DPZ-1-6 Ol DPZ-1-9 Ol DPZ-1-18 Ol Wo En Fs DPZ-2-10 Opx DPZ-2-11 Opx DPZ-2-5 Opx DPZ-3-5 Opx DPZ-1-19 Opx DPZ-1-20 Opx DPZ-1-11 Cpx An Ab Or DPZ-3-5 Pl 1 Pl DPZ-3-5 Pl 4 Pl DPZ-3-5 Pl 5 Pl DPZ-3-5 Pl 6 Pl DPZ-3-5 Pl 7 Pl DPZ-1-1 Pl DPZ-1-2 Pl DPZ-1-3 Pl DPZ-1-24 Pl DPZ-1-25 Pl Note: Fo = 100 Mg/(Mg + Fe 2+ ). E-MORB and OIB (Stille et al. 1983; Saunders et al. 1988). Compared with those of the Noril sk intrusion in the 251 Ma Siberian LIP, the Sr and Nd isotopic ratios of the Nantianwan complex are relatively restricted. These values are also slightly higher than those of EMI, but much lower than those of EMII (Stille et al. 1983; Saunders et al. 1988). The ε Nd (260 Ma) values and ( 87 Sr/ 86 Sr) t ratios of the Nantianwan complex overlap the values for the Emeishan flood basalts (Figure 8). The complex has higher ( 87 Sr/ 86 Sr) t values than those of Siberian trap basalts. However, compared with the Cu Ni-bearing Baimazhai and Limahe intrusion in the ELIP (Xu et al. 2001; Hanski et al. 2004; Xiao et al. 2004; Wang et al. 2006), the Nantianwan complex exhibits much lower ( 87 Sr/ 86 Sr) t and higher ε Nd (t). In contrast, the Sr and Nd isotopic data of the Fe Ti V oxide ore-bearing intrusions in the ELIP (represented by Taihe, Baima, and Panzhihua intrusion) overlap with or are close to the high ε Nd (260 Ma) end of ore-bearing intrusions in the ELIP. Discussion The nature of the mantle source Olivine with higher forsterite (Fo) contents (up to 0.81) in the Nantianwan complex may have crystallized from a Mg-rich melt. The most Mg-rich olivine core (Fo 81 in sample DPZ-2) corresponds to olivine that crystallized from relatively primary mantle melts. Assuming a K D (Fe Mg) ol-liq of 0.30 ± 0.03 (Roeder and Emsile 1970), the parental magma that would have been in equilibrium with olivine with Fo 81 can be estimated by the formula W MgO = K D Fo/(1 Fo) W FeO (Zhang and Wang 2003). The FeO values correspond to the FeO content of the parental magma on the assumption that 15% of the total iron oxide content is ferric. According to such estimations and the MgO content (27.82 wt.%), we conclude that the primary magma is probably basaltic and contains 6.68 wt.% MgO, which is lower than the average of the ELIP flood basalts (8 wt.%; Zhang and Wang 2002).

11 International Geology Review 1755 Table 3. Major (wt.%) and trace (ppm) elements analyses of the Nantianwan complex. Sample DPZ-1 DPZ-2 DPZ-3 DPZ-4 DPZ-7 DPZ-9 NTG-1 NTG-3 NTG-4 Lithology Lherzolite Gabbronorite SiO Al 2 O TFe 2 O CaO MgO K 2 O Na 2 O MnO TiO P 2 O LOI < Total Mg # La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Rb Ba Th U Nb Ta Sr Cs Pb Zr Hf Cr Co Ni Sc V Cu Ga REE Cu/Zr (La/Nb) PM (Th/Ta) PM δeu Note: TFe 2 O 3 is total iron as FeO; Mg # = Mg/(Mg+Fe); δeu = 2Eu n /(Sm n +Gd n ). In a diagram of (Tb/Yb) PM versus (Yb/Sm) PM, melts of spinel peridotite are markedly different from melts of garnet peridotite (Figure 9). Our data plot closer to a melt range for spinel peridotite than to one for garnet peridotite. But the parental magma of the Nantianwan complex was likely generated by partial melting of incompatible element-enriched spinel-facies lithospheric mantle and with moderate partial melting of 10%. This is similar

12 1756 M. Wang et al. SiO 2 (wt.%) Gabbros Gabbronorites Iherzolites TiO 2 (wt.%) Figure 6. CaO (wt.%) TFeO (wt.%) MgO (wt%) K 2 O + Na 2 O (wt.%) Al 2 O 3 (wt.%) MgO (wt%) Variation of MgO versus major elements for mafic ultramafic rocks from the Nantianwan complex. to the Baimazhai intrusion and Emeishan low-ti basalts, appearing to reflect shallower depth and lower temperature and pressure in the spinel stability field (Xu and Chung 2001; Zhang and Wang 2002). Crustal contamination Due to the highly incompatible behaviour of Nb, U, Ce, and Pb during mantle partial melting and crystallization processes, the Nb/U and Ce/Pb ratios are sensitive indexes of crustal contamination (Hofmann 1988). The presence of orthopyroxene in our samples further suggests the involvement of crustal contamination. The Ce/Pb values (1 9.8) and Nb/U values (8 19) of our samples are much lower than those of the typical mantle (25 ± 5, 30 35, respectively) (Hofmann 1988; Rudnick and Fountain 1995). Hence, it seems that the rocks from the Nantianwan complex experienced variable crustal contamination. This conclusion is also supported by slightly negative Nb, Ta, and Ti anomalies coupled with elevated ( 87 Sr/ 86 Sr) t. However, these signatures can also be explained as the result of addition of subduction-related melts/fluids into the mantle source (Wilson 1989). If that is the case, strong negative Nb, Ta, Zr, and Hf anomalies would be expected (Stolz et al. 1996). Since all our samples show slightly negative Nb, Ta, Zr, and Hf anomalies, we tend to agree that the primary magma was likely to have been contaminated by crustal materials. However, the limited range of ε Nd (t) and initial 87 Sr/ 86 Sr indicates that there was little variation in the degree of crustal contamination (Figure 8). This is in contrast to other Ni Cu (PGE) deposits, such as Limahe and Baimazhai (Wang et al. 2006; Tao et al. 2008), and Noril sk in Siberia (Lightfoot et al. 1994), where the magmas were considerably contaminated by variable crustal materials (Figure 8). Such an inference is supported by plots of ε Nd (t) versus (Th/Nb) PM (Figure 10). In this respect, the Nantianwan complex is distinct from other intrusions hosting Ni Cu (PGE) ores in the ELIP. Constraints on the metallogenesis It is generally accepted that Cu Ni (PGE) sulphide deposits are generated by immiscible sulphide melt in response to formation of S-saturated magma from S-undersaturated magma. However, there is debate around the mechanism for S-saturation. Four mechanisms have been proposed: (1) rapid decrease of temperature along the margins of intrusions (Maier et al. 1998); (2) contamination by crustal materials (Lightfoot and Hawkesworth 1997; Ripley et al. 2003; Zhang and Wang 2003); (3) mixing of different magmas (Lambertt et al. 1998; Xiong 1994); and (4) rapid differentiation of magma (Haughton et al. 1974).

13 International Geology Review (A) Lijiang picrite Average OIB Rock/chondrite 10 Rock/primitive mantle La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 1 (B) Iherzolites Gabbronorites Gabbros Lijiang picrite Average OIB 0.1 Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Figure 7. (A) Chondrite-normalized REE patterns of the Nantianwan complex, normalized values after Taylor and Mclennan (1985). The diagram for those of Lijiang picrite (Zhang et al. 2006) is shown for comparison. Primitive mantle-normalizing values and average OIB pattern are from Sun and McDonough (1989). (B) Primitive mantle-normalized trace elements spider diagram of Nantianwan complex. Normalized values after Sun and McDonough (1989); data of gabbros in (A) from Zeng, L.G., personal communication. Table 4. Sr and Nd isotopic geochemistry for the Nantianwan complex. Sample no. Rb Sr 87 Rb/ 86 Sr ( 87 Sr/ 86 Sr) 0 ( 87 Sr/ 86 Sr) t Sm Nd 147 Sm/ 144 Nd ( 143 Nd/ 144 Nd) 0 ( 143 Nd/ 144 Nd) t ε Nd (t) DPZ DPZ DPZ NTG NTG NTG Note: Chondrite uniform reservoir (CHUR) values [( 143 Sm/ 144 Nd) 0 CHUR = , ( 143 Nd/ 144 Nd) 0 CHUR = ] are used for the calculation. ( 87 Sr/ 86 Sr) t,( 143 Nd/ 144 Nd) t,andε Nd (t) were calculated at 260 Ma. Many researchers have suggested that crustal contamination is the most important factor triggering S-saturation of basalt magmas (e.g. Brügmann et al. 1993; Arndt et al. 2003; Naldrett 1973, 2004; Lightfoot and Keays 2005), because it can also lead to the addition of sulphur and silicon from the wallrocks to the magma systems, decreasing iron contents, increasing oxygen fugacity, as well as speeding crystallization, all factors which could make magmas S-oversaturated. However, the degree of crustal contamination in the Nantianwan complex is not as high as expected. The presence of cumulus texture, combined with the negative correlation of MgO with SiO 2, Na 2 O, TiO 2, and A1 2 O 3 and the roughly positive correlation with FeO (Figure 6), indicates the fractionation of

14 1758 M. Wang et al. ε Nd (t) Fe Ti V oxide ore-bearing intusions Siberian Traps 5 Basalts in ELIP Cu Ni (PGE) ore-bearing intusions OIB EMI UCC LC Sr/ 86 Sr t 20 EMII Nantianwan Figure 8. ε Nd (t)versus( 87 Sr/ 86 Sr) t for the Nantianwan complex. Data sources: Cu Ni (PGE) sulphide-bearing intrusions (Wang 2008; Zhang et al. 2009); the Fe Ti V oxide ore-bearing intrusions (Zhang et al. 2009; Pang et al. 2010); Emeishan basalts data (Xu et al. 2001; Hanski et al. 2004; Xiao et al. 2004; Wang et al ); Siberian Traps (Sharma et al. 1992; Lightfoot et al. 1993; Wooden et al. 1993); dotted lines with numbers showing the mixing ratio of the mantle and crust material, as the original magma and the Yangtze upper crust ( 87 Sr/ 86 Sr = 0.72, ε Nd (t) = 10) and Yangtze lower crust ( 87 Sr/ 86 Sr = 0.71, ε Nd (t) = 30) as contaminants (Jahn et al. 1999). Figure 9. Variation of (Tb/Yb) PM versus (Yb Sm) PM (after Shaw 1970; Janney et al. 2000; Salters and Stracke 2004; Ito and Mahoney 2005; Workman and Hart 2005; Zhang et al. 2006; data for Xinjie intrusion after Zhao et al. 2006; data for Limahe intrusion after Zhang et al. 2009; data for Baimazhai after Wang 2008; data for Emeishan low-ti basalts after Xu et al. 2001b). olivine, clinopyroxene, and orthopyroxene. The complex was emplaced into the Silurian Zhongcao Formation along a NW SE-striking fault, suggesting a relatively deeper emplacement depth ( km in the light of regional stratigraphic correlation). Thus, shallow emplacement and rapidly decreasing temperature of the magma are unlikely factors leading to the formation of an immiscible sulphide ε Nd (t) Uncontaminated Crustal contamination Nantianwan Baimazhai Limahe Xinjie Panzhihua (Th/Nb) PM Figure 10. Plots of ε Nd (t) versus (Th/Nb) PM (data for Xinjie intrusion after Zhao et al. 2006; data for Limahe, Panzhihua intrusions after Zhang et al. 2009; data for Baimazhai after Wang 2008; normalized values after Sun and McDonough 1989). melt. Since no evidence for magma mixing (e.g. disequilibrium mineral assemblage in the rocks) has been observed, the possibility of a simple magma-mixing model can be ruled out. Finally, gypsiferous strata have not been recognized in the Panxi district unlike the Noril sk region (Lightfoot and Hawkesworth 1997). Therefore, contamination by S-enriched country rock can be excluded. The only viable mechanism left is the rapid differentiation of magma to facilitate the Cu Ni mineralization, which is further explored in the discussion below. As stated above, the Nantianwan complex experienced fractional crystallization of olivine and clinopyroxene and slight crustal contamination. However, it is not clear at what stage in the crystallization sequence the ore-bearing magmas became saturated in sulphide. The chemical composition of olivine occurring in the Cu Ni sulphide ore-bearing intrusions can be used to infer the presence of an immiscible S-rich liquid in equilibrium with the silicate melt (Keays 1995; Naldrett 1997, 1999). Ni and Mg are compatible in early crystallizing olivine and thus decrease in abundance in later phases as crystallization proceeds (Li et al. 2003). The D Ni value between solid phase and magma, particularly between olivine and magma, is summarized in a recent review by Bédard (2005); Li et al. (2003) suggested an olivine/melt D Ni value of 7 for S-bearing basaltic magmas based on the analyses of MORB samples. In komatiitic magma, the olivine/melt D Ni is between 3 and 5 (Arndt 1977). Based on the available experimental data, a D Ni value of 1 and 0 for clinopyroxene and plagioclase, respectively, is reasonable for basaltic systems. When olivine is in equilibrium with a sulphide liquid after the segregation of liquid sulphide during silicate crystallization, the olivines will contain lower Ni than those that crystallize with no segregation of liquid sulphide at given MgO. This relation can be used to evaluate sulphide liquid composition using olivine from sulphide-bearing rock samples (Simkin and Smith 1970). Our analyses show that the Fo contents in the

15 International Geology Review 1759 Ni in olivine (ppm) Figure ). No sulphide removal Sulphide removal Olivine from Iherzolites Fo in olivine Fo NiO diagram of olivine (after Simkin and Smith olivine from the lherzolites vary from 79.1 to 81.4 ppm, and the corresponding Ni contents vary from 0.11 to 0.23 ppm (Figure 11). Most olivines plot within and below the field of normal olivine compositions, strongly suggesting that some liquid sulphides have separated from the ore-bearing magma (Naldrett 1989; Naldrett et al. 1992). The Cu/Zr ratios compare the concentrations of two highly incompatible elements, one being a highly chalcophile element (Cu) and the other (Zr) a non-chalcophile, lithophile element (Li and Naldrett 1999). These two elements are similarly incompatible during the early stages of fractional crystallization of sulphide-undersaturated mafic ultramafic magmas. Typically, chalcophile metalundepleted continental flood basalts have Cu/Zr ratios around 1, whereas lavas depleted in chalcophile metals due to sulphide segregation have Cu/Zr ratios less than <1 (Lightfoot and Keays 2005). The bulk-rock Cu/Zr ratios in lherzolites ( ) and gabbronorites ( ) of the Nantianwan complex indicate that the parental magmas of the gabbronorites and lherzolites went from undersaturation to oversaturation of sulphur, which may have been due to fractional crystallization in a high-level magma chamber. This can account for the sulphide segregation, although the role of crustal contamination cannot be ignored for the formation of the Cu Ni ores in the Nantianwan complex. Interestingly, a similar scenario has also been recognized in the Uitkomst intrusion, Mpumalanga, South Africa (Maier et al. 1998). Assessment of ore potential As stated above, the primary basaltic magma underwent fractional crystallization of Opx+Cpx+Ol and segregation of sulphides. Because of the emplacement depth, the fractional crystallization of olivine and clinopyroxene will lead to the formation of sulphide droplets that crystallize. The dispersed sulphide droplets would form the disseminated Cu Ni sulphide ores. Globally, it is generally accepted that the generation of magmatic Cu Ni sulphide ore deposits requires crustal contamination of mantlederived magmas (Lesher and Keays 2002; Lightfoot and Keays 2005; Wilson and Chunnett 2006). More controversial is the role of crustal versus. mantle S in both triggering and maintaining the S-saturated state of the magma. At issue is whether very large ore deposits require a significant crustal contribution of S. As stated above, the Nantianwan intrusion is distinct from other large-scale Cu Ni sulphide deposits in which crustal contamination played an important role in generating an immiscible sulphide melt, such as Sudbury in Canada (Keays and Lightfoot 2004), Noril sk in Siberia (Lightfoot and Keays 2005), and Jinchuan in China (Tang 1990). The large quantity of Cu Ni PGE sulphide ores that occur in the Noril sk intrusion has high initial 87 Sr/ 86 Sr ratios, which are attributed in part to the assimilation of anhydriterich, evaporitic sediments. These sediments are abundant in the sedimentary sequence that underlies the volcanic sequence (Lightfoot and Hawkesworth 1997). The parental magmas that formed the Noril sk intrusion ascended to a higher level chamber within the sedimentary sequence, where it encountered evaporites, assimilated sulphur, and segregated immiscible sulphide melts (Arndt et al. 2003). Previous studies suggested that the capacity of a magma to form an economic Ni Cu (PGE) deposit is controlled mainly by (1) the abundance of ore metals in the magma; (2) the sulphide saturation state of the magma; and (3) the capacity of the magma to interact with its surroundings (see discussion by Lesher and Campbell 1993). In other words, to form an economic Ni Cu (PGE) deposit, the primary mantle-derived magma must contain sufficient ore metals and must be capable of being driven to sulphide saturation. If the magma is too undersaturated in sulphide and/or does not interact with wallrocks, it may not reach sulphide saturation until crystallization is well advanced, resulting in only small amounts of fine-grained disseminated sulphides and very fine grained cloudy or dusty sulphides. Hence, although an immiscible sulphide phase is a normal segregation product in most mafic ultramafic systems, it usually segregates in only very small amounts because of limitations on the abundance of S in the magma and usually at a late stage during the crystallization of the magma. Thus, Arndt et al. (2005) concluded that the magma must be driven to sulphide saturation by contamination, with or without the addition of external S, to not only saturate the magma in sulphide but also to form large enough amounts of a molten immiscible sulphide liquid. As a consequence, considering the insignificant crustal contamination in the Nantianwan mafic ultramafic complex and relatively homogeneous fine-grained disseminated sulphides in the complex, we infer that it does not have much potential for large Cu Ni sulphide deposits.