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1 Available online at Geochimica et Cosmochimica Acta 75 (2011) Rhenium osmium isotope and platinum-group elements in the Xinjie layered intrusion, SW China: Implications for source mantle composition, mantle evolution, PGE fractionation and mineralization Hong Zhong a,, Liang Qi a, Rui-Zhong Hu a, Mei-Fu Zhou b, Ti-Zhong Gou a, Wei-Guang Zhu a, Bing-Guang Liu c, Zhu-Yin Chu c a State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, 46 Guanshui Road, Guiyang , China b Department of Earth Sciences, University of Hong Kong, Hong Kong, China c Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing , China Received 5 February 2010; accepted in revised form 4 January 2011; available online 13 January 2011 Abstract The Xinjie mafic ultramafic layered intrusion in the Emeishan large igneous province (ELIP) hosts Cu Ni platinum group element (PGE) sulfide ore layers within the lower part and Fe Ti V oxide-bearing horizons within the middle part. The major magmatic Cu Ni PGE sulfide ores and spatially associated cumulate rocks are examined for their PGE contents and Re Os isotopic systematics. The samples yielded a Re Os isochron with an age of 262 ± 27 Ma and an initial 187 Os/ 188 Os of ± (c Os (t)= 0.5 ± 0.1). The age is in good agreement with the previously reported U Pb zircon age, indicating that the Re Os system remained closed for most samples since the intrusion emplacement. They have near-chondritic c Os (t) values ranging from 0.7 to 0.2, similar to those of the Lijiang picrites and Song Da komatiites. Exceptionally, two samples from the roof zone and one from upper sequence exhibit radiogenic c Os (t) values (+0.6 to +8.6), showing minor contamination by the overlying Emeishan basalts. The PGE-rich ores contain relatively high PGE and small amounts of sulfides (generally less than 2%) and the abundance of Cu and PGE correlate well with S, implying that the distribution of these elements is controlled by the segregation and accumulation of a sulfide liquid. Some ore samples are poor in S (mostly <800 ppm), which may due to late-stage S loss caused by the dissolution of FeS from pre-existing sulfides through their interaction with sulfide-unsaturated flowing magma. The combined study shows that the Xinjie intrusion may be derived from ferropicritic magmas. The sharp reversals in Mg#, Cr/FeO T and Cr/TiO 2 ratios immediately below Units 2 4, together with high Cu/Zr ratios decreasing from each PGE ore layer within these cyclic units, are consistent with multiple magma replenishment episodes. The sulfides in the cumulate rocks show little evidence of PGE depletion with height and thus appear to have segregated from successive inputs of fertile magma. This suggests that the Xinjie intrusion crystallized from in an open magma system, e.g., a magma conduit. The compositions of the disseminated sulfides in most samples can be modeled by applying an R factor (silicate sulfide mass ratio) of between 1000 and 8000, indicating the segregation of only small amounts of sulfide liquid in the parental ferropicritic magmas. Thus, continuous mixing between primitive ferropicritic magma and differentiated resident magma could lead to crystallization of chromite, Cr-bearing magnetite and subsequently abundant Fe Ti oxides, thereby the segregation of PGE-rich Cu-sulfide. When considered in the light of previous studies on plume-derived komatiites and picrites worldwide, the close-to-chondritic Os isotopic composition for most Xinjie samples, Lijiang picrites and Song Da komatiites suggest that the ferropicritic Corresponding author. Tel.: ; fax: address: zhonghong@vip.gyig.ac.cn (H. Zhong) /$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi: /j.gca

2 1622 H. Zhong et al. / Geochimica et Cosmochimica Acta 75 (2011) magma in the ELIP were generated from a plume. This comprised recycled Neoproterozic oceanic lithosphere, including depleted peridotite mantle embedded with geochemically enriched domains. The ascending magmas thereafter interacted with minor (possibly <10%) subducted/altered oceanic crust. This comparison suggests that the komatiitic melts in the ELIP originated from a greater-than normal degree of melting of incompatible trace element depleted, refractory mantle components in the plume source. Ó 2011 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Large igneous provinces (LIPs) are the physical manifestation of short-timescale melting events, often associated with continental break-up (Ernst et al., 2005). The tectonic setting of intraplate magmas, typically a plume intersecting a rift, is ideal for the development of Ni Cu platinumgroup element (PGE)-bearing sulfides (Barnes et al., 1997). Layered intrusions are keys to addressing the geochemical signatures of LIPs and the influence of these reservoirs in the formation of mineral deposits in magmatic systems, because they record periods of magmatic activity intimately associated with LIPs. The 260 Ma Emeishan large igneous province (ELIP) is one of the best exposed LIPs, formed as a result of mantle plume activity, and hosts a number of layered intrusions (Chung and Jahn, 1995; Xu et al., 2001; Zhong et al., 2002; Zhou et al., 2002). Previous studies indicate that two major types of magmatic ore deposits occur in the central part of the ELIP. Several large mafic ultramafic layered intrusions host giant Fe Ti V oxide deposits, including the Panzhihua, Hongge, Baima and Taihe intrusions (Zhong et al., 2002, 2005; Zhou et al., 2005, 2008; Pang et al., 2008). In contrast, some relatively small mafic ultramafic intrusions host Ni Cu (PGE) sulfide mineralization, including the Limahe, Baimazhai, Jinbaoshan and Yangliuping intrusions (Song et al., 2003; Wang et al., 2005, 2006; Tao et al., 2007, 2008; Zhou et al., 2008). It has been suggested that the oxide-bearing mafic ultramafic intrusions originated from a Fe Ti-rich, more fertile mantle source whereas the sulfide-bearing intrusions were derived from a Fe Ti-poor, more refractory mantle source with signature of crustal contamination (Zhou et al., 2008). The Xinjie layered intrusion in the Panzhihua-Xichang (Pan-Xi) area of the ELIP is a 7.5-km long, km wide, and 1200-m thick mafic ultramafic sill. The intrusion is an unusual example of the mafic ultramafic bodies in the ELIP because it hosts Ni Cu PGE sulfide ore layers within the lower part and major Fe Ti V oxide-bearing layers within the middle part (Fig. 1; Zhong et al., 2004; Wang et al., 2008; Zhang et al., 2009), which is somewhat similar to mineralization in the Bushveld complex. The relatively small size (10 km 2 ) of the Xinjie layered intrusion compared with the Bushveld, Stillwater or Muskox layered intrusions (Irvine, 1977; Eales and Cawthorn, 1996; Day et al., 2008) has helped to preserve primary stratigraphic variations in magmatic textural and geochemical features, which may be obscured in larger intrusions. The elemental and Sr Nd isotopic study of a 90-m thick rock package in the lowermost part suggested that magma mixing might lead to crystallization of Fe Ti oxides which considerably reduced the sulfur capacity of the melt and thereafter caused the segregation of sulfides in the Xinjie intrusion (Zhong et al., 2004). A recent study on the Fe Ti Cr oxides in this intrusion indicated that these minerals crystallized from Fe- and Ti-rich basaltic magmas (Wang et al., 2008). However, it is still unclear why the combined mineralization of Fe Ti V oxide and Ni Cu PGE sulfide occurs only in the Xinjie intrusion. As stated above, it has been supposed that the Fe Ti-rich mafic magmas generally have given rise only to the Fe Ti oxide deposits in the ELIP. Thus, a further investigation in the parental magma compositions and magma evolution processes of the Xinjie intrusion has been conducted to provide an answer to this problem. The behavior of highly siderophile elements (HSE; Os, Ir, Ru, Rh, Pt, Pd, Re and Au) and Re Os isotopic systematics in plume-derived mafic ultramafic intrusions can provide important insights to the origin and mechanics of plumes (Walker et al., 1997; Brügmann et al., 2000; Day et al., 2008; O Driscoll et al., 2009). Morgan et al. (2000) used the Re Os isochron method to date magmatic oxide and sulfide deposits hosted in Suwalki anorthosite and norite, thus directly relating the isotopic results to the age and origin of the Suwalki anorthosite massif. Re Os modeling was also applied to explore the impact of variable crustal contamination and variable R factor for the oxide-sulfide deposits in Suwalki anorthosite complex (Hannah and Stein, 2002). In this study, we focus on the major PGE-enriched stratigraphic levels in the Xinjie layered intrusion. Detailed and precise PGE and Re Os data of different cumulate types at these discrete horizons are used to determine the age of the Xinjie Fe Ti V Ni Cu PGE deposit, to address the characteristic origin of parental magmas, and to provide a better understanding of the magma chamber processes and the formation of Ni Cu PGE-rich layers. An ultimate objective is to characterize the Os isotopic compositions of the mantle source of the Emeishan LIP, thus place further constraints on the mantle evolution curve. 2. GEOLOGICAL BACKGROUND The ELIP is located near the western margin of the Yangtze block, SW China (Fig. 1). The basement of the Yangtze block comprises the Paleo-Mesoproterozoic Huili Group or its equivalents, the Yanbian or Kunyang Groups, which consist of low-grade metasedimentary rocks interbedded with felsic and mafic metavolcanic rocks. Abundant Neoproterozoic igneous rocks, consisting dominantly of mid-neoproterozoic ( Ma) felsic intrusive and volcanic rocks, and minor mafic/ultramafic rocks, including

3 Re Os isotope and PGE in Xinjie layered intrusion, SW China 1623 Fig. 1. Simplified geological map of the Xinjie layered intrusion (modified after Zhong et al., 2004). Inset a shows the distribution of the layered intrusions in the Pan-Xi area (modified after Zhong et al., 2002). Insert b illustrates distribution of major terranes in China and the Pan-Xi area (modified after Chung and Jahn, 1995). Abbreviations: NCB = North China block; YZB = Yangtze block; SG = Songpan- Ganze accretionary complex; QT = Qiangtang; LS = Lhasa; HI = Himalayan; TAR = Tarim; MON = Mongolia; QD = Qaidam; WB = West Burma; STM = Shan-Thai-Malay; IC = Indochina. basaltic lava, sills, dikes and small intrusions, occur in the western margin of the Yangtze block (Li et al., 2003, 2006; Zhou et al., 2006). The basement is overlain by a thick sequence (>9 km) of Sinian ( Ma) to Permian strata composed of clastic, carbonate, and meta-volcanic rocks (SBGMR, 1991). The ELIP comprises the Emeishan continental flood basalts and spatially associated intrusions. The Emeishan basalts are exposed over a rhombic area of km 2, with the volcanic succession ranging from several hundred meters to 5 km in thickness. The volcanic rocks consist predominantly of tholeiites and andesitic bas-

4 1624 H. Zhong et al. / Geochimica et Cosmochimica Acta 75 (2011) alts, with minor flows and tuffs of trachyte and rhyolite in the uppermost sequence (Chung and Jahn, 1995; Xu et al., 2001; Xiao et al., 2004; Zhang et al., 2006). The basalts are divided into high-ti and low-ti groups that are considered to have been derived from different mantle sources (Xu et al., 2001; Xiao et al., 2004). It is noteworthy that minor picrites associated with the high-ti basalts have been identified in the Pan-Xi and Lijiang areas (Chung and Jahn, 1995; Zhang et al., 2006), whereas some komatiitic rocks interbedded with low-ti olivine basalts were documented in the Song Da region of northern Vietnam (Hanski et al., 2004). Magnetostratigraphic data and field observations suggest that the bulk of the Emeishan volcanic sequence formed within 1 2 million years (Huang and Opdyke, 1998; Ali et al., 2002). Recent SHRIMP and TIMS U Pb dating of zircons from silicic ignimbrite, mafic/ultramafic intrusions and diabasic dikes indicate that the ELIP was voluminously erupted at 260 Ma, consistent with the end-guadalupian (end Middle Permian) stratigraphic age (Zhou et al., 2002, 2005; Guo et al., 2004; Zhong and Zhu, 2006; He et al., 2007). The Pan-Xi area is located in the inner zone of the ELIP (Xu et al., 2004), which is considered the impact site of the rising plume head (He et al., 2003). The area comprises N S trending, fault-controlled, massive basalts, numerous spatially associated mafic ultramafic intrusions, granites, and syenites. The ore-bearing mafic ultramafic intrusions described here are exposed along a 300 km-long and km-wide belt, constituting the most important metallogenic district for Fe Ti V and Ni Cu (PGE) metals in China. Giant Fe Ti V oxide deposits occur in several relatively large layered intrusions (13 60 km 2 ), including the Panzhihua, Hongge, Baima and Taihe intrusions (Fig. 1; Yao and Du, 1993; Zhong et al., 2002, 2003, 2004, 2005; Zhou et al., 2005, 2008). Ni Cu (PGE) sulfide deposits are hosted in the Limahe, Jinbaoshan and Zhubu intrusions (Wang et al., 2005; Tao et al., 2007, 2008; Zhou et al., 2008). In contrast, both Fe Ti V oxide and Ni Cu PGE mineralization were discovered in the Xinjie layered intrusion (Zhong et al., 2004; Wang et al., 2008). 3. PETROGRAPHY OF THE XINJIE INTRUSION The 260 Ma Xinjie intrusion (Zhou et al., 2002), is a sill-like, 7.5 km long, km wide, and 1200 m thick ultramafic mafic layered body, which intruded the late Permian Emeishan flood basalts (Fig. 1). Field investigations reveal that the syenitic intrusions always cut the Xinjie intrusion and adjacent Emeishan basalts. The Xinjie ultramafic mafic intrusion exhibits well-developed igneous layering and has been divided into three cycles containing six lithological zones (A F, Fig. 2). Overall, the intrusion is most ultramafic in its lower part and each cycle includes numerous layers starting with the most ultramafic cyclic units at the base followed by progressively evolved cyclic units. Cycle I is about 400 m thick and comprises, from the bottom to top, peridotite, plagioclase peridotite, olivine clinopyroxenite, plagioclase clinopyroxenite, gabbro and quartz-bearing gabbro. Cycle II is 190 m thick and mainly consists of plagioclase-bearing peridotite, olivine clinopyroxenite, gabbro and quartz-bearing gabbro, while Cycle III is greater than 600 m thick, and is dominated by plagioclase clinopyroxenite, gabbro and quartz diorite. A 20 m- thick fine-grained gabbroic and olivine-gabbroic Marginal Unit is at the base of the intrusion and in contact with the country rocks (Mao and Sun, 1981), containing up to several percent hornfelsed and partially digested inclusions of the underlying Emeishan basalts. The petrographic features of the different rock types within each individual cycle have been described in detail by Zhou (1982) and Zhong et al. (2004). In the Xinjie intrusion, the Fe Ti oxide ore layers occur mainly at the top of Cycles I and II, composed of Ti-bearing chromite, Ti-bearing chrome-magnetite, magnetite and ilmenite (Fig. 2), whereas the stratiform-type PGE mineralization is located in the Marginal Unit and the lower part of the intrusion where it is associated with disseminated copper and nickel sulfides with interbedded thin Ti-bearing chrome-magnetite and Ti-bearing chromite layers (Luo, 1981; Zhu et al., 2010). In this study, our samples come from borehole ZK411 that intersected Cycle I of the Xinjie intrusion, which comprises a rock package about 380 m thick and includes the main PGE mineralization occurrences. The location of borehole ZK411 is shown in Fig. 1 and the positions of the samples are given in Table 1. Four major PGE-enriched sulfide ore layers (PGE Layer 1 to Layer 4; Fig. 2) were discovered in borehole ZK411, although an additional layer of PGE mineralization also occurs within the uppermost unit of this drill hole. The dominant PGE-bearing layered sequence has been divided into four units (Fig. 2). It should be pointed out that as the samples were collected primarily to study the PGE mineralization occurrences, the proportion of mineralized samples is not representative of the core as a whole, as more samples were taken in mineralized sections. In the following discussion, we will focus on the main Cu Ni PGE sulfide mineralization occurrences, which are hosted by plagioclase peridotite and plagioclase clinopyroxenite in the lower part of Cycle I. These rocks consist of cumulus olivine in modal amounts of up to 50%, titanaugite (15 60%), Ti-bearing chromite and/or chrome-magnetite (5 15%), and intercumulus titanaugite (5 40%) and plagioclase (10 30%). The stratiform PGE mineralization in the Xinjie intrusion occurs in the form of disseminated PGE-rich sulfides. The sulfide content within the PGE mineralization zone ranges from 0.1% to 1%, and locally, up to 2%. The dominant base-metal sulfides (BMS) comprise chalcopyrite (50 60%), pyrrhotite (20 25%), and pentlandite (15 20%). Sperrylite and Pd Pt Bi Te minerals (merenskyite, moncheite, and michenerite) are present in the PGE-enriched layers. The contents of the Fe Ti oxides correlated with PGE mineralization vary from 5% to 15%, and in places, up to 20%. These platinum-group minerals (PGMs) are commonly associated with the BMS, or magnetite coexisting with BMS in the PGE mineralization zone (Zhu et al., 2010). 4. ANALYTICAL METHODS Platinum, Pd, Ir, and Ru were determined by isotope dilution (ID)-ICP-MS using an improved Carius tube tech-

5 Re Os isotope and PGE in Xinjie layered intrusion, SW China 1625 Fig. 2. Variations of Mg#, Cr/FeO T, Cr/TiO 2, Cu/Zr, Cu/Pd, Pt, Pd, Pt + Pd, Cu, Ni, S, and Pt/S with depth in the main PGE mineralization horizon within the Xinjie intrusion. Stratigraphy of the Xinjie intrusion is modified after Zhong et al. (2004). nique (Qi et al., 2007). The mono-isotope element Rh was measured by external calibration using a 194 Pt spike as the internal standard (Qi et al., 2004). Ten grams of rock powder and appropriate amount of enriched isotope spike solution containing 101 Ru, 105 Pd, 193 Ir, 194 Pt were digested with 35 ml aqua regia in a 75 ml Carius tube, which was placed in a sealed, custom-made, high pressure autoclave filled with water. The internal pressure of the Carius tube is balanced by the external pressure produced by the water when heated. Thus, this method not only avoids a possible explosion of the Carius tube but also allows for relatively high-temperature (300 C) digestion, a greater volume of aqua regia (35 ml) and a larger sample mass (10 g). After digestion at 300 C for 10 h, the solution was transferred

6 Table 1 Major, trace and highly siderophile element distribution in the cumulates and sulfide ores from the Xinjie intrusion. Sample HZK Cyclic unit Upper Unit Depth (m) MgO (%) FeO T (%) TiO 2 (%) Cr (ppm) Zr (ppm) Y (ppm) Cu (ppm) HZK HZK HZK a HZK HZK Unit HZK HZK HZK HZK a HZK HZK a HZK HZK HZK a HZK HZK Unit HZK HZK HZK HZK HZK HZK HZK HZK HZK a HZK a HZK a HZK a HZK a HZK HZK Unit HZK HZK HZK HZK a HZK HZK a HZK Unit HZK HZK Line missing Ni (ppm) S (ppm) Ir (ppb) Ru (ppb) Rh (ppb) Pt (ppb) Pd (ppb) 1626 H. Zhong et al. / Geochimica et Cosmochimica Acta 75 (2011)

7 HZK a HZK a HZK a HZK HZK HZK PGE-rich sulfide ores. a Re Os isotope and PGE in Xinjie layered intrusion, SW China 1627 to 50 ml centrifuge tube and then used for pre-concentrating PGE by Te-coprecipitation, as described in Qi et al. (2004). The total procedural blanks were lower than ng/g for Ru, Rh and Ir; ng/g for Pd; and ng/g for Pt. The reference standards, WPR-1, WGB-1 and TDB-1, were simultaneously used for analytical quality control. The results for WPR-1 are in good agreement with the certified values. The results of Ru, Rh, and Ir for WGB-1 and TDB-1 are lower than the recommended values, but agree well with values reported by Meisel and Moser (2004). The analytical results of the samples are given in Table 1. Re Os isotopic compositions of the Xinjie intrusion (Table 2) were determined at the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The Carius tube digestion technique is similar to those described by Shirey and Walker (1995), which is reported in detail by Chu et al. (2009). Approximately 2 g of homogenized whole-rock powders and appropriate amounts of a 187 Re 190 Os mixed spike were sealed in an externally cooled ( 50 C), single-use, Pyrex Ò borosilicate Carius tube, with 3 ml of purified concentrated HCl and 6 ml of purified concentrated HNO 3. The Carius tubes were kept at 240 C in an oven for h. Osmium was extracted from the aqua regia solution into CCl 4 (Cohen and Waters, 1996) and then back-extracted into HBr, followed by purification via microdistillation (Birck et al., 1997). Re was separated from the matrix and purified by anion exchange chromatography with about 0.6 ml resin (AG 1 8, mesh). The samples were loaded onto the columns in 0.8 mol/l HNO 3, the matrix elements were eluted with 0.8 mol/l HNO 3 and 1 mol/l HCl, and then the Re was collected with 8 mol/l HNO 3. Os isotopic compositions were measured using a GV Isoprobe-T Mass Spectrometer with negative ion mode. Purified Os was loaded onto platinum filaments and Ba(OH) 2 was used as an ion emitter. All samples were run with nine Faraday cups in static mode. The Os isotopic compositions and Os concentrations were obtained in one mass spectrometric run. The measured Os isotopic ratios were corrected for mass fractionation using 192 Os/ 188 Os = after interference corrections, oxygen corrections and spike subtractions. The isotope dilution analyses of Re were conducted on a Neptune MC-ICP-MS using a secondary electron multiplier in peak-jumping mode. Mass fractionations for Re were corrected using a Re standard that was run alternately with the samples. Total analytical blanks were 2 pg for Re and 3 5 pg for Os with a 187 Os/ 188 Os ratio near The reference values for the standard was 185 Re/ 187 Re = The in-run precisions for Os isotopic measurements were better than ±0.2% (2rm) for all the samples. During the period of measurements of our samples, the 187 Os/ 188 Os ratio of Johnson Matthey standard of UMD was ± 4 (2r, n = 5). To calculate the age, Re Os data were regressed using the ISOPLOT program (Ludwig, 2003) and assuming an error correlation coefficient of 0.9. Error input was determined by multiple analyses of the in-house Os and Re standards to be 0.2% on Os isotopic composition and 1% on Re/Os ratio. The errors of two samples (HZK411-

8 1628 H. Zhong et al. / Geochimica et Cosmochimica Acta 75 (2011) Table 2 Re and Os isotopic data for the Xinjie intrusion. Sample Re (ppb) Os (ppb) 187 Re/ 188 Os 187 Os/ 188 Os 2r ( 187 Os/ 188 Os) i c Os (t) HZK HZK HZK HZK HZK HZK HZK HZK HZK HZK HZK HZK HZK HZK HZK HZK HZK HZK and 239) having quite high Os concentrations were assumed to be 5% on Re/Os ratio. Major elements were measured by wet chemical analyses at the Center of Analysis and Test, Institute of Geochemistry, Chinese Academy of Sciences (CATIGCAS), with the analytical precision better than 5%. Sulfur was analyzed at the CATIGCAS by Leco induction furnace-titration, with the accuracy better than 10%. Trace elements were determined using a VG PQ Excell inductively coupled plasma mass spectrometer (ICP-MS) at the University of Hong Kong. The powdered samples (50 mg) were dissolved in high-pressure Teflon bombs using HF + HNO 3 mixture for 48 h at 190 C (Qi et al., 2000). Rh was used as an internal standard to monitor signal drift during counting. The international standards AMH-1, GBPG-1 and OU-6 were used for analytical quality control. The analytical precision is generally better than 5% for trace elements. The contents of selected major and trace elements are listed in Table 1. The complete major and trace element dataset is presented in Electronic Annex. 5. RESULTS 5.1. Variations in major and trace elements All the studied samples from the Xinjie intrusion are characterized by high but variable MgO ( wt.%), FeO T ( wt.%) and TiO 2 ( wt.%) contents (Table 1). Variations in Mg-number (Mg#), Cr/FeO T and Cr/TiO 2 ratios with stratigraphic height are shown for the main PGE mineralized horizon of the Xinjie intrusion in Fig. 2. The samples from cyclic Units 1, 3 and 4 display consistently increasing trends in Mg# followed by steadily decreasing trends upwards, whereas those from cyclic Unit 2 exhibit a decreasing trend in Mg# upsection. It is interesting that abrupt reversals of Mg# occur at the tops of Units 1, 2 and 3 (Fig. 2a). The samples from Units 1 4 have variable Cr contents of ppm, ppm, ppm, and ppm, respectively. In contrast, the cumulate rocks and PGE ores from Upper Unit exhibit high Cr contents ( ppm; Table 1). Notably, much higher Cr/FeO T and Cr/TiO 2 ratios occur immediately below the PGE Layers 2, 3 and 4, while those in cyclic Unit 1 are relatively constant and low (Fig. 2b and c) Variations in chalcophile elements and PGE As shown in Fig. 2, four major PGE-rich sulfide layers occur near or at the bottom of each cyclic unit. The Cu, Ni, PGE and sulfur contents of the PGE-enriched layers and their host lithologies vary by two to three orders of magnitude (Table 1). Sulfur concentrations are highest in Unit 1 ( ppm), with the exception of one sample (480 ppm S) in the uppermost part of this unit. Apart from one ore sample (HZK ) with a S content of 1400 ppm, most host rocks and PGE-rich sulfide ores within Unit 2 contain significantly less sulfur ( ppm). The PGE ores in Unit 3 have much higher sulfur concentrations ( ppm) than the cumulate rocks ( ppm), with one exceptional rock sample containing 1900 ppm S. The cumulate rocks and PGE-rich sulfide ores within Unit 4 are mostly rich in sulfur ( ppm) except two rock samples with ppm S. In addition, the samples from Upper Unit contain significantly lower sulfur from 100 to 560 ppm, with the exception of one sample (HZK411-42) having 1900 ppm S. Copper concentrations exhibit a similar distribution pattern as sulfur (Fig. 2), with elevated Cu concentrations in the PGE-rich sulfide ores and sulfide-enriched cumulate rocks. Apart from two ore samples (HZK and HZK411-06) having significantly lower Cu contents (332 and 75 ppm), the ores from the PGE Layers 1, 3 and 4 and one from the PGE Layer 2 (HZK ) are characterized by high Cu contents between 1367 and 5476 ppm (Table 1; Fig. 2i). In contrast, the S-poor (<800 ppm) cumulate rocks contain much lower Cu ( ppm;

9 Re Os isotope and PGE in Xinjie layered intrusion, SW China 1629 mostly <800 ppm). The Cu/Zr ratios of the samples within Units 1, 3 and 4 are highly variable (2.4 60, and , respectively), while the Cu/Zr ratios in Unit 2 and Upper Unit are relatively low ( and ). It is noteworthy that the Cu/Zr ratios in Unit 1 increase with height whereas those in Units 3 and 4 generally decrease upward. In contrast, the samples within Unit 2 have slightly decreasing Cu/Zr ratios with height. In this study, the Ni concentrations in the PGE-rich sulfide ores vary from 396 to 1509 ppm, which are similar to those of the cumulate rocks ( ppm). Meanwhile, there is no systematic variation observed throughout the PGE-enriched horizon (Fig. 2j). As shown in Table 1 and Fig. 2, the disseminated sulfide ores from the PGE Layers 1 4 contain between ppb Pt and ppb Pd, in contrast to ppb Ir, ppb Ru and ppb Rh. In addition, one ore sample within Upper Unit has 445 ppb Pt and 507 ppb Pd, and 7.59 ppb Ir, 6.71 ppb Ru and 9.60 ppb Rh. Comparatively, most silicate rocks from the studied Xinjie intrusion contain highly variable PGE, with a similar range of abundances for Ir ( ppb), Ru ( ppb) and Rh ( ppb) but more restricted ranges in Pt from 32.8 to 347 ppb and Pd from 36.0 to 311 ppb. Three exceptional samples (HZK411-42, HZK and HZK ) have much lower PGE contents. Pt/Pd ratios of the Xinjie samples are slightly below or around unity ( ), comparable to Pt/Pd ratios in other magmatic sulfides elsewhere. In summary, the above results indicate that significant PGE depletion in the cumulate rocks upsection and downsection of the main PGE-rich layers is lacking in the Xinjie intrusion (Pt + Pd mostly >100 ppb; Fig. 2h). Moreover, it is notable that abundant PGE-enriched rocks (several hundred ppb Pt + Pd) and two ore samples (HZK and HZK ) are relatively poor in S ( ppm) and Cu ( ppm, mostly <800 ppm), with relatively high Pt/S ratios (Fig. 2l). Primitive mantle-normalized plots of Ni, Cu and PGE in the Xinjie intrusion exhibit low Ni, Os, Ir, Ru compared to Rh, Pt, Pd and Cu (Fig. 3a e). Most of the PGE-enriched sulfide ores and cumulate rocks have broadly similarly- Fig. 3. Primitive-mantle normalized Cu, Ni, and PGE patterns for cyclic Units 1 4 and Upper Unit in the Xinjie intrusion (normalization factors from Barnes and Maier, 1999). The filled-diamond symbol represents the PGE ore sample and the hollow-diamond symbol represents the rock sample. The thick black lines depict the compositions modeled as containing different percentages of sulfides formed at various R factors.

10 1630 H. Zhong et al. / Geochimica et Cosmochimica Acta 75 (2011) Fig. 4. Cr vs. Ir, Ru, Pt, and Pd for the Cr-rich, sulfide-bearing and sulfide-poor samples from the Xinjie intrusion. shaped PGE patterns, characterized by significant Ni depletion, obvious Pd enrichment relative to Cu, and a marked fractionation between Os, Ir and Ru on the one and Rh, Pt and Pd on the other. These samples have Cu/Pd ratios ( ) similar to or lower than that of the mantle ( ,000; Barnes et al., 1993). In contrast, several samples from Unit 1 (HZK and HZK ), Unit 3 (HZK , HZK and HZK ), Unit 4 (HZK , HZK , HZK , HZK and HZK ) and Upper Unit (HZK411-42) exhibit obvious Pd depletion relative to Cu (Fig. 3), which have Cu/Pd ratios (11, ,773) greater than that of the mantle. Interestingly, the sharp increases of Cu/Pd ratios occur immediately above the PGE Layers 1, 3 and 4 (Fig. 2e). It is also noteworthy that most ores and cumulate rocks within Units 1 4 show obviously negative Ru anomalies (Fig. 3a d), with the exception of three samples (HZK , HZK129 and HZK ). Throughout the section of the intrusion examined, Ir, Ru, Pt and Pd of the Cr-rich, sulfide-poor and sulfide-bearing samples exhibit poor correlations with Cr (Fig. 4a d). In contrast, the variations in Cu, Pt, Pd, Ir and Ru concentrations correlate well with S, whereas Ni poorly correlates with S (Table 1; Fig. 5) Re Os isotope Re Os isotope data for the Xinjie intrusion are reported in Table 2 and plotted on the Re Os isochron diagram in Fig. 6. The Xinjie cumulate rocks and PGE-rich sulfide ores exhibit high but variable Os contents ( ppb), with sample HZK , at 599 ppb being an exception. In contrast, Re concentrations are low in the range of ppb. Most samples have near-chondritic initial Os isotopic compositions, with c Os (t) values (corrected to 259 Ma) ranging from 0.7 to 0.2 and 187 Re/ 188 Os ratios varying from to (Table 2). Exceptions include the two uppermost samples from the Upper Unit (HZK and HZK411-06) and HZK from Unit 4 (Table 2) which have slightly higher initial 187 Os/ 188 Os ( ), with radiogenic c Os (t) values (+0.6 to +8.6) and higher 187 Re/ 188 Os ratios ( ). All data, except for the three uppermost samples and sample HZK which have slightly elevated 187 Re/ 188 Os ( ), define an isochron age (MSWD = 0.8) of 262 ± 27 Ma (Fig. 6) that is consistent, within the uncertainty, with the SHRIMP U Pb zircon age of the Xinjie intrusion (259 ± 3 Ma; Zhou et al., 2002). The calculated initial 187 Os/ 188 Os of ± (c Os (t)= 0.5 ± 0.1) is approximately chondritic for Permian and attests to minimal crustal contamination. The high Os and low Re contents and chondritic Os component of the Xinjie intrusion are quite similar to those of the Lijiang picrites (Fig. 7; <0.2 ppb Re, ppb Os and c Os (t)= 2.4 to 0.4; Zhang et al., 2008) and Song Da komatiites (Fig. 7; ppb Re, ppb Os and c Os (t)= 0.5 to +0.6; Hanski et al., 2004) in the Emeishan large igneous province. However, the Re Os isotopic characteristics of the Xinjie intrusion are significantly different to those for the Emeishan basalts of the ELIP, which have low Os contents of ppb (mostly <0.10 ppb) and highly variable c Os (t) values ranging from 2.8 to +747 (mostly >+10) (Fig. 7; Hanski et al., 2004; Xu et al., 2007; Zhang et al., 2008). The high c Os (t) of these Emeishan basalts must be attributed to variable degrees of crustal contamination (Hanski et al., 2004; Xu et al., 2007; Zhang et al., 2008). In addition, the Re Os isotopic compositions of the Xinjie intrusion are also different from those of the Pt Pd mineralized Jinbaoshan ultramafic intrusion (c Os (t) = ; Tao et al., 2007) and the Limahe Ni Cu deposit (c Os (t) = ; Tao et al., 2010) in the ELIP, and the Bushveld complex (c Os (t) = ; McCandless

11 Re Os isotope and PGE in Xinjie layered intrusion, SW China 1631 Fig. 5. Covariations of Cu, Ni, and PGE with sulfur content in the Xinjie intrusion. Cu and PGE exhibit positive correlations with S, whereas Ni shows poor correlation with S. et al., 1999), Stillwater complex (c Os (t) = ; Horan et al., 2001) and Noril sk-talnakh intrusion (c Os (t) = ; Walker et al., 1994; Arndt et al., 2003) (Fig. 7), which have been proposed to reflect outer core or recycled oceanic crust contributions (Walker et al., 1994), or crustal assimilation processes (McCandless et al., 1999; Horan et al., 2001; Arndt et al., 2003; Tao et al., 2007, 2010). Fig Re/ 188 Os vs. 187 Os/ 188 Os isochron figure for the Xinjie intrusion. Insert includes all the analyzed samples but four samples having slightly radiogenic Os compositions are excluded from age plotting. Analytical uncertainties are the size of the symbols or smaller. Fig. 7. c Os (t) vs. Os concentration for magmatic sulfides showing data for Cu Ni PGE mineralization from the Xinjie (this study), Jinbaoshan (Tao et al., 2007), Limahe (Tao et al., 2010), NT: Noril sk-talnakh (Walker et al., 1994; Arndt et al., 2003), Bushveld (McCandless et al., 1999) and Stillwater (Horan et al., 2001) intrusions. Data for the Lijiang picrites, Song Da komatiites and Emeishan basalts (HTB: high-ti basalt; LTB: low-ti basalt) are from Zhang et al. (2008), Hanski et al. (2004) and Xu et al. (2007). Samples with c Os (t) > +120 are not plotted in this figure for clarity.

12 1632 H. Zhong et al. / Geochimica et Cosmochimica Acta 75 (2011) DISCUSSION 6.1. Estimation of a Xinjie parental melt composition The parental magma compositions of the Xinjie intrusion can be obtained using the compositions of the chilled margins. Previous study has shown that one fine-grained olivine-bearing gabbro sample (CSXJ26) from the Marginal Unit within the Xinjie intrusion has high MgO (14.6%), FeO T (15.5%), and TiO 2 (3.6%) contents (Zhong et al., 2004), This sample is also characterized by the highest Y concentration of 17 ppm (Fig. 8a), indicating that it contain distinctly more trapped liquid than other samples in the Marginal Unit. Sample CSXJ26 can be then taken as a mixture of the primary magma and cumulus olivine. The most primitive olivine observed in the Xinjie chilled margins contains 84 mol percent Fo (Mao and Sun, 1981). Fig. 8b illustrates the compositions of the Xinjie parental melt estimated from the method of Chai and Naldrett (1992). The coexisting liquid calculated using the ratio of (FeO/ MgO) olivine /(FeO/MgO) liquid = 0.3 (Roeder and Emslie, 1970) contains 13.9 wt.% MgO and 15.8 wt.% FeO. The calculated MgO content and 17 ppm Y of the initial liquid are applied to model the fractionating liquid changes using MELTS (Ghiorso and Sack, 1995). The model line roughly accounts for the compositional variations in the high-ti Emeishan basalt wall rocks of the Xinjie intrusion (Fig. 8a; Zhong et al., 2004), suggesting that the Xinjie parental magma could share a common mantle source with the nearby high-ti basalts. The Xinjie cumulate rocks are also enriched in highly incompatible lithophile elements (Zhong et al., 2004). Therefore, the mantle source of the Xinjie parental magma should have unusually high FeO and TiO 2 contents and enrichment of highly incompatible elements. The compositions are comparable to those of the coeval Lijiang picrites in the ELIP, which are enriched in MgO ( %), FeO T ( %) and TiO 2 ( %; Zhang et al., 2006). The Xinjie intrusion also exhibits similar primitive mantle-normalized PGE distribution patterns (Fig. 3) to those of the Lijiang picrites (Zhang et al., 2005), characterized by enrichment of Pt and Pd relative to Os, Ir and Ru. Moreover, the near-chondritic initial Os isotope values for most Xinjie samples (Table 2) show no effects of crustal contamination, similar to those of the Lijiang picrites (Zhang et al., 2008; Fig. 7). It has been suggested that the high temperature, high magnesium komatiitic and picritic magmas injected into the upper crust are the only types of magmas that can form the major magmatic Ni Cu PGE sulfide deposits in the world, which are generally S-undersaturated due to high degrees of partial melting or derivation from a S-poor plume source (Keays, 1995; Arndt et al., 2005). The modeling of Naldrett (2010) has also shown that only a magma generated by high degree of melting (P15%) would be rich in PGE and Cu, implying that the Xinjie parental magma was derived through high-degree partial melting of the ELIP mantle source. As demonstrated above, the occurrence of Cu Ni PGE and Fe Ti V mineralization within the Xinjie intrusion thus requires that the parental magmas are not only enriched in Fe and Ti but also in magnesium, incompatible elements and PGE, which have similarities to the characteristics of the ferropicritic magmas (e.g., Brügmann et al., 2000; Hanski et al., 2001) PGE behavior during the evolution of the Xinjie magma Fig. 8. (a) Plot of MgO vs. Zr in the Xinjie chilled margin samples and Emeishan basalt wall rocks (Zhong et al., 2004); (b) modeling of primary olivine and coexisting liquid composition. Point A is the composition of olivine Fo84 for the Xinjie chilled margins (Mao and Sun, 1981). Point B is the compositions of a fine-grained olivine-bearing gabbro from the Xinjie chilled margins (Zhong et al., 2004). Point C is the estimated compositions of trapped liquid in equilibrium with olivine Fo 84 for the Xinjie intrusion. The layered series of the Xinjie intrusion are cumulate rocks and as such the minerals they accumulate dominantly control their major and trace element compositions. In the Cr-rich, sulfide-poor and sulfide-bearing samples, Ir, Ru, Pt and Pd are negatively correlated with Cr (Fig. 4), showing that these elements are unlikely to be controlled by chromite during crystal fractionation. Instead, the PGE concentrations exhibit broadly positive correlations with S (Fig. 5), suggesting that disseminated sulfide could be the main collector phase. This is confirmed by the observation that the Xinjie platinum group minerals (PGMs) are mostly hosted in the base metal sulfides (Zhu et al., 2010). Notably, the occurrence of significantly higher Cr/FeO T and Cr/TiO 2 ratios immediately below the PGE Layers 2, 3 and 4 (Fig. 2b and c) reflects chromite or Cr-spinel accumulation before sulfide deposition. As pointed out by Li and Naldrett (1999) for the Voisey s Bay intrusion, the Cu/Zr ratio is a good measure of chalcophile depletion. Most of the analyzed samples (except

13 Re Os isotope and PGE in Xinjie layered intrusion, SW China 1633 HZK411-06) have Cu/Zr ratios higher than 2.0, suggesting that cumulus sulfides are present almost throughout the examined package. Meanwhile, the increase of Cu/Zr ratio with height in Unit 1 reflect increasing sulfide/trapped silicate liquid ratios, whereas the Cu/Zr ratios decrease upward in Units 3 and 4 imply decreasing sulfide/trapped silicate liquid ratios. In Unit 2, the Cu/Zr ratios slightly decrease from the PGE Layer 2, implying insignificant variations in sulfide/trapped liquid ratio. The Cu/Pd ratio is particularly sensitive to S-saturation because Pd has a much larger partition coefficient than Cu (Barnes et al., 1993). In the present study, most samples have Cu/Pd ratios close to or lower than that in primitive mantle (Fig. 2e), further indicating that they contain varying amounts of cumulus sulfides. In contrast, several samples above the PGE Layers 1, 3 and 4 having Cu/Pd ratios significantly higher than the mantle value, suggesting their crystallization from differentiated magma that had experienced prior sulfide liquid segregation. The sharp increases in Cu/Pd ratio above these PGE Layers (Fig. 2e) support the derivation of at least some of the PGE within the sulfide-enriched layers from the overlying magma. As shown above, the marked PGE depletion in the cumulate rocks upsection and downsection of the main PGE-rich layers is lacking in the Xinjie intrusion (Pt + Pd mostly >100 ppb; Fig. 2h). The rocks overlying the Merensky Reef within the Bushveld Complex are depleted in PGE relative to Ni and Cu, which is interpreted as a result of PGE extraction from the overlying magma (Maier and Barnes, 1999; Barnes and Maier, 2002). The absence of consistent PGE depletion in most of the Xinjie host rocks indicates that they formed in an open magmatic system. It may occur in a dynamic conduit system (Li et al., 2000; Evans- Lamswood et al., 2000) due to metal upgrading of early formed sulfide melt by continued influx of the later, fresh, sulfide-unsaturated and PGE-undepleted magma (Kerr and Leitch, 2005). Another important observation that some PGE-rich ores and PGE-enriched cumulate rocks from various units within the Xinjie intrusion are poor in S (<800 ppm), is best explained by late-magmatic S loss from these samples, perhaps in a way similar to that proposed by Naldrett and Lehmann (1988) for sulfides in the chromitites of the Bushveld Complex: 1/3 FeS + 4/3 Fe 2 O 3 =Fe 3 O 4 + 1/6 S 2. It has been suggested that new pulse of sulfide-unsaturated magma will dissolve pre-existing sulfide to tend towards saturation, but that, because of the much more chalcophile nature of both the PGE and Ni and Cu in comparison with Fe, the bulk of dissolve sulfide will be FeS, with the more chalcophile metals remaining with the sulfide until very little of this remains (Kerr and Leitch, 2005; Li et al., 2009; Naldrett et al., 2009). Some of the sulfur would then leave the system and the remaining sulfide melt would become S poor, in which case PGM could crystallized from the sulfide melt. In the Xinjie intrusion, S loss is supported by the predominance of copper sulfides over iron sulfides, and the high Pt/ S ratios (Fig. 2l). It is notable that most of the analyzed samples exhibit obvious negative Ru anomalies (Fig. 3), accounting for the original characteristics of the parental magmas generating the Xinjie intrusion. Negative Ru anomalies are known in the Emeishan basalts (Qi et al., 2008), SDRS basalts in Greenland (Philipp et al., 2001), Kerguelen Plateau basalts (Chazey and Neal, 2005) and the Agnew layered intrusion in Canada (Vogel et al., 1999), which are generally attributed to removal of laurite and/or Os Ir Ru alloys, together with chromite or olivine, from the parental magmas. Capobianco and Drake (1990) found that Ru was strongly portioned into spinel (D = 22 25). Furthermore, Puchtel and Humayun (2001) showed that Ru, Os and Ir were slightly compatible to moderately incompatible in olivine (D = ), and were compatible with chromite (D = ). Moreover, early higher temperature spinel condensates are likely to be enriched in Ru over Ir (Righter and Downs, 2001). We thus suggest that the early removal of laurite and and/or Os Ir Ru alloys, together with chromite before the magma emplacement could be responsible for the negative Ru anomalies in the Xinjie samples. It may reveal that the chromite D Ru is higher than D Ir/Os for the Xinjie case. Considering the obvious depletion of Ni over PGE (Fig. 3) and the overall low Ni concentrations in these samples (Table 1; Fig. 2j), it could also be possible that olivine removal from the primary magma simultaneously depleted Ni. This is consistent with the fact that the Fo contents of olivine from the lower part within the Xinjie intrusion range between 69.9 and 74.4 (Wang et al., 2008), suggesting their crystallization from an evolved magma. In summary, small amounts of chromite and olivine removal could have occurred during ascent through the crust, prior to emplacement of the Xinjie intrusion. The PGE-enriched cumulate rocks and PGE ores show broadly similar metal tenors and distribution patterns (Table 1; Fig. 3). If the rocks had crystallized in a closed magmatic system, one would normally expect a pattern of progressive chalcophile metal depletion with height. Instead, the observed metal patterns indicate that the rocks formed in an open magmatic system, from magma flowing through a magma conduit Modeling of the role of cumulus sulfides It has been demonstrated above that cumulus sulfides are the major phases controlling the PGE. The PGE contents in the sulfides are critically dependent on the volume of magma with which the sulfide liquid interacted. The compositions of the studied Xinjie samples were modeled using the following mass balance equation of Campbell and Naldrett (1979): C S = C L D(R + 1)/(R + D), where C S = the final concentration of a metal in a sulfide liquid; C L = the initial concentration of the metal in the silicate magma; D = the partition coefficient between the sulfide liquid and the silicate magma; and R = the mass ratio of silicate magma to sulfide liquid. In this study, small amounts of laurite and/or Os Ir Ru alloys, together with chromite and olivine were removed during ascent of the magma, which may produce lower Os, Ir and Ru concentrations in the Xinjie magma than in the ferropicritic magma in the ELIP. Zhang et al. (2006) suggested that the Lijiang basalts were generated from picritic magmas, by crystal