Ore Geology Reviews 57 (2014) 589–601

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Platinum-group elemental and Re–Os isotopic geochemistry of theWajilitag and Puchang Fe–Ti–V oxide deposits, northwestern TarimLarge Igneous Province

Dongyang Zhang a, Zhaochong Zhang a,⁎, He Huang a, John Encarnación b, Nengwu Zhou c, Xiaoxue Ding b

a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, PR Chinab Department of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell Avenue, St. Louis, MO 63108, USAc No. 11 Geological Party, Xinjiang Bureau of Geology and Mineral Resources, Changji, Xinjiang 831100, PR China

⁎ Corresponding author at: School of Earth SciencesUniversity of Geosciences, No. 29 Xueyuan Road, Beijing8232 2195; fax: +86 10 82323419.

E-mail address: zczhang@cugb.edu.cn (Z. Zhang).

0169-1368/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.oregeorev.2013.08.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 April 2013Received in revised form 31 July 2013Accepted 8 August 2013Available online 19 August 2013

Keywords:Platinum-group elementsOs isotopeMafic–ultramafic complexFe–Ti oxide depositTarim Large Igneous Province

TheWajilitag and Puchang complexes are two important mafic–ultramafic intrusions hosting Fe–Ti–V oxide oredeposits in the newly discovered Tarim Large Igneous Province (TLIP), NW China. The dominant rocks of theWajilitag complex are clinopyroxenite and gabbro, while the Puchang complex is mainly gabbroic with onlyminor clinopyroxenite and anorthosite components. Fe–Ti oxide ores in the Wajilitag complex are mostly dis-seminated and principally restricted to the ultramafic rocks, whereas the Puchang complex hosts massive to dis-seminated Fe–Ti–V oxide ores mainly within its gabbroic section. The abundances of platinum-group elements(PGE) in the Wajilitag and Puchang silicate rocks and ores are low, with total PGE contents ranging from 0.95to 2.69 ppb and from to 0.15 to 0.44 ppb, respectively. The low total PGE concentrations and extremely highCu/Pd ratios (up to 5 × 106) in both complexes clearly demonstrate that the sulfide mineral segregation mayhaveplayed an important role in PGEdistribution.Weakdepletion of Ru relative to Ir and Rh in theWajilitag sam-ples may have resulted from segregation of Ru-dominant phases during magma evolution. The Wajilitag andPuchang samples exhibit more fractionated primitivemantle-normalized PGE patterns than that of nearly coevalTarim flood basalts. The differences in previously published Sr–Nd isotopic compositions of these intrusive rocksand basalts imply that theymay be derived from distinct mantle sources, although both of them could be relatedto the same magmatic event. The Wajilitag titanomagnetites have lower Re (0.19–0.75 ppb) and higher Os(0.04–0.19 ppb) concentrations than the Puchang titanomagnetite samples that yield relatively high Re(0.63–1.80 ppb) and exceptionally low Os (b0.01 ppb) contents. The positive γOs values (43–387) in theWajilitag complex coupledwith high Re and lowOs contents in the Puchang complex are consistentwith variabledegrees of crustal contamination during magma ascent and emplacement. Different degrees of crustal contami-nation are proposed to have played a key role in causing variable sulfide saturation and segregation. Fractionalcrystallization involving abundant magnetite also may have induced sulfide saturation at the later stages ofmagma evolution. The identification of sulfide mineral segregation during the late stage of magma evolution inthe shallow magma chamber suggests that there is a potential to find economic Cu–Ni sulfide mineralizationin these complexes and other similar types of mafic–ultramafic intrusions in the TLIP.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Large volumes of early Permian volcanic and intrusive rocks have re-cently been identified in the Tarim craton and its surrounding areas innorthwestern China (Ju and Hou, 2013; Pirajno et al., 2011; Tian et al.,2010; Xia et al., 2012; Yu et al., 2011). These volcanic and intrusiverocks make up a new large igneous province that was emplaced duringthe Permian, similar to the ~251 Ma Siberian Traps in Russia (Kamo

and Mineral Resources, China100083, PR China. Tel.: +86 10

ghts reserved.

et al., 2003; Prokoph et al., 2013; Reichow et al., 2009), the ~260 MaEmeishan Large Igneous Province (ELIP) in southwestern China andthe ~289 Ma Panjal Traps of Kashmir (Lo et al., 2002; Shellnutt et al.,2011a; Zhang et al., 2006; Zhou et al., 2002) (Fig. 1). Pirajno et al.(2009) suggested that these three basaltic provinces may be related toa single mantle superplume, although they were formed up to ~40 Maapart and are separated by up to ~3000 km from each other. TheSiberian Traps contain the Noril'sk intrusion, which hosts prodigiousCu–Ni–Platinum group element (PGE) sulfide deposits, which are be-lieved to be related to the Siberian mantle plume event (Lightfoot andKeays, 2005; Starostin and Sorokhtin, 2011). The ELIP is the only prov-ince in the world that hosts both world-class magmatic Fe–Ti–V oxidedeposits and some important Cu–Ni–(PGE) sulfide deposits, which are

Fig. 1. Distribution and inferred extent of the Permian basalts in Asia, showing the locations of Siberian Traps, Tarim and Emeishan provinces.Modified from Li et al., 2011).

590 D. Zhang et al. / Ore Geology Reviews 57 (2014) 589–601

also associated with plume-related mafic–ultramafic intrusions (Donget al., 2013; Ganino et al., 2013b; Howarth and Prevec, 2013; Shellnuttet al., 2011b; Zhang et al., 2009; Zhong et al., 2011; Zhou et al., 2013).The Permian, therefore, is one of the most important periods in Earth'shistory during which many important magmatic Ni–Cu–(PGE) sulfideand Fe–Ti–V oxide deposits formed. However, compared to the Siberianand Emeishan LIPs, little is known about the metallogenesis in the re-cently recognized Tarim Large Igneous Province (TLIP). The discoveryof the Wajilitag and Puchang Fe–Ti–V oxide deposits represents amajor breakthrough in the exploration for magmatic deposits in theTLIP. Although some research efforts have beenmade on these deposits(Li et al., 2012a; C.L. Zhang et al., 2008, 2010), their relation topenecontemporaneous Tarim basaltic magmatism and the potentialfor economic Ni–Cu–PGE mineralization in these deposits remainelusive.

The behavior of PGE and Re–Os isotopic systematics in mantle-derived igneous rocks can provide very robust constraints on the na-ture of the mantle source, the extent of crustal contamination, andthe sulfide saturation history of the mantle-derived magmas (Barnesand Picard, 1993; Barnes et al., 1985; Peach et al., 1994; Said et al.,2011), and has proven to be of particular significance in the study ofthegenesis ofmagmatic Cu–Ni–PGEdeposits and Fe–Ti–Voxide deposits(Lambert et al., 2000; Li et al., 2011; Morgan et al., 2000; Ripley et al.,2008; Walker et al., 1994). Although there are a few studies of PGE onthe Tarim flood basalts (Li et al., 2012b; Yuan et al., 2012), the PGE andRe–Os isotopic systematics of the mafic–ultramafic intrusive rocks inthe TLIP have not been reported. In this paper, we present the first

whole-rock PGE data, Re–Os isotopic compositions of titanomagnetiteseparates for the Wajilitag and Puchang Fe–Ti–V oxide deposits. Weevaluate PGE fractionation processes, the relationship between these in-trusive rocks and Tarim flood basalts, and the potential for economic Ni–Cu–PGE mineralization.

2. Geological background

2.1. Regional geology

The Tarim block, situated in the northwest China, is surrounded bythe Tianshan, western Kunlun and Altyn Tagh orogenic belts (Fig. 2a).The basement of the Tarim block is composed of late Neoarchean–earlyPaleoproterozoic high-grade metamorphic gneisses and amphibolites,and late Paleoproterozoic–early Neoproterozoic volcano-sedimentaryrocks and late Neoproterozoic low-grade metamorphic volcaniclasticrocks (Long et al., 2010; Zhang et al., 2012a). Abundant Neoproterozoicintrusions, consisting dominantly of mid-Neoproterozoic (830–620 Ma)felsic rocks andminormafic–ultramafic rocks, also occur around themar-gins of the block (Ge et al., 2012; Zhang et al., 2012b; Zhu et al., 2011).The crystalline basement is overlain by thick volcano-sedimentary se-quences of Phanerozoic strata (Guo et al., 2005).

The TLIP is composed of voluminous volcanic successions and associ-ated intrusions. It covers an area ofmore than 3 × 105 km2with a thick-ness ranging from several hundred meters to 3 km (Tian et al., 2010;Zhang et al., 2010; Zhou et al., 2009). The volcanic rocks analyzed sofar consist predominantly of alkaline basalts, with minor trachybasalts,

Fig. 2. (a) Simplified tectonic map of the TC and surrounding areas showing the distribution of Permian basalts and the locations of the Fe–Ti oxide ore-bearing complexes in Tarim. Sim-plified geological maps and cross sections of the Wajilitag complex (b, c) and the Puchang complex (d, e), with sample locations.Panel a: modified after Tian et al. (2010); Panel b: modified from XJGMR (2009, 2010).

591D. Zhang et al. / Ore Geology Reviews 57 (2014) 589–601

592 D. Zhang et al. / Ore Geology Reviews 57 (2014) 589–601

593D. Zhang et al. / Ore Geology Reviews 57 (2014) 589–601

andesites, dacites and rhyolites. Recent geochronological studies andstratigraphic evaluation suggest that most of the TLIP magmatism wasshort-lived (292–287 Ma; Chen et al., 2010; Tian et al., 2010; Yu et al.,2011). It is widely believed that the TLIP resulted from an early Permianmantle plume (Z.L. Li et al., 2012; Tian et al., 2010; Yu et al., 2011; Zhanget al., 2013a). In addition to early Permian Tarim flood basalts, there arenumerousmafic–ultramafic and felsic intrusions that crop out along themargins of the Tarim block. The Wajilitag and Puchang complexes, ex-posed in the northwest part of the TLIP have been recently recognizedas hosting large Fe–Ti–V oxide deposits. Zircon U–Pb dating results ofthese complexes reveal that they were emplaced at ca. 283 Ma and ca.275 Ma, respectively (Zhang et al., 2010, in preparation). They areinterpreted as the products of a Tarim mantle plume (Zhang et al., inpreparation), although crystallization of these intrusive rocks occurredsomewhat later than the main stage of Tarim volcanism.

2.2. Geology and petrology of oxide-bearing intrusive complexes

The Wajilitag complex has a length of ~5.0 km, a width of~1.5–3.0 km, with an exposed area of ~12 km2. The complex intrud-ed the flat-lying meta-sediments of the upper Devonian Keziletagand Yimugangtawu Formations. The complex consists primarily ofclinopyroxenite and gabbro (Fig. 2b). The contact of the two unitsis transitional. The structure of the complex is not consistent withthat of some archetypal layered intrusions, such as the Bushveldand Panzhihua intrusions (Cawthorn and Spies, 2003; Ganino et al.,2013a; Maier and Barnes, 1999; Pêcher et al., 2013; Zhang et al.,2009; Zhou et al., 2005). Most of the clinopyroxenites have fine- tomedium-grained textures with local coarse-grained facies, and arecomposed dominantly of 70–80% clinopyroxene and 10–20% iron–titanium oxides, with minor plagioclase, hornblende and apatite.Some clinopyroxenites also contain up to 20% olivine. Gabbro con-tains variable proportions of fine- tomedium-grained clinopyroxene(30–60%), plagioclase (20–60%), and iron–titanium oxides (5–15%).Some gabbros also contain trace amounts of olivine and apatite.

Oxide ore bodies with theWajilitag complex occur as lenses or podsprincipally hosted in the ultramafic rocks (Fig. 2c). Disseminated oresare dominant, although minor massive ores can be recognized insome industrial boreholes (Fig. 3a). Most contacts between the dissem-inated ores and the adjacent silicate rocks are transitional, whereasmost, if not all, massive ores always have sharp magmatic boundarieswith the barren silicate rocks and disseminated ores (Fig. 3a). Accordingto a report released in 2010 by Xinjiang Bayi Iron and Steel Ltd., a sub-sidiary of Baosteel Group, the deposit contains 146 million tons (Mt)of ore reserves with an average grade of ~17% total FeO, ~7% TiO2, and~0.2% V2O5. Compared with giant Fe–Ti oxide deposits in the ELIP(Zhong et al., 2005), the deposit contains relatively smaller amountsof Fe–Ti oxide ores. Disseminated ore is generally fine to mediumgrained and consists of 25–40% titanomagnetite, 40–50% clinopyroxene,5–25% plagioclase and 10–15% ilmenite, with small amounts of base-metal sulfides (e.g., chalcopyrite and pyrrhotite), olivine, biotite andhornblende (Fig. 3b). These silicate minerals, such as clinopyroxene, ol-ivine, and plagioclase are surrounded by late-crystallizing Fe–Ti oxides.Massive ore is predominantly fine-grained and is composed of ~50%titanomagnetite and ~35% pyrrhotite with small amounts of ilmenite

Fig. 3. Field photos and photomicrographs of rocks (including ores) in the Wajilitag and Puand clinopyroxenite in the Wajilitag deposit; (b) the Wajilitag clinopyroxenite containinlight; (c) clinopyroxene and plagioclase in the Wajilitag gabbro under cross-polarized llight; (e) abundant pyrrhotite grains in the Wajilitag massive ore under reflected light;the marble and the clinopyroxenite in the Puchang deposit; (g) sharp boundary of disseand disseminated ore in the Puchang deposit; (i) cumulus clinopyroxene with interstitial plagicontaining clinopyroxene and plagioclase under cross-polarized light; (k) a Puchang gabbro is c(l) cumulus plagioclase with interstitial olivine and clinopyroxene in the Puchang anorthosite uinated ore under reflected light; (n, o) Pentlandite grains in the Puchang massive ore under rChalcopyrite = Ccp; Pentlandite = Pn; Olivine = Ol; Clinopyroxene = Cpx; Plagioclase = Pl

and clinopyroxene (Fig. 3c). The sparse silicate minerals are completelysurrounded by oxides.

The Puchang intrusive complex has an outcrop area of ~25 km2. Thecomplex intruded the lower Carboniferous Kangkelin Formation, whichconsistsmainly of calcareous slate andmarble (Fig. 2d),with the contactzone frequently containing partially assimilated enclaves of marble(Fig. 3d). The complex is differentiated into mainly gabbro, minorclinopyroxenite, and anorthosite. Internal contacts between these sili-cate rock types are gradational. No visible igneous layering or foliationis present in any of the silicate rock units. Clinopyroxenite always ex-hibits a granular texture and contains approximately 80% clinopyroxeneand 15% plagioclase, which are locally replaced by albite. However, themajority of rocks are unaltered. The gabbro is fine- to coarse-grainedand composed of plagioclase (20–60%), clinopyroxene (10–45%), iron–titanium oxides (5–15%), and minor hornblende and biotite. Somegabbros contain up to 25% olivine in addition to theminerals above. An-orthosite is medium- to coarse-grained composed of ~90% plagioclaseand ~5% clinopyroxene, with minor olivine, hornblende, biotite, andiron–titanium oxides.

The Fe–Ti oxide ore from Puchang is hosted only in the gabbroicrocks. Orebodies mainly display the morphology of irregular lenses,layers or pods, of which the layered bodies are the most common. TheFe–Ti oxide ores generally have sharp contactswith orwithin thebarrengabbro (Fig. 3e). The massive and disseminated ores also have sharpcontacts with each other (Fig. 3f). Such features are similar to those de-scribed in the Panzhihua and Hongge intrusions (Bai et al., 2012b; Houet al., 2012b). The ore reserves have been estimated to be 120 Mt withan average grade of ~20% total FeO, ~11% TiO2, and ~0.8% V2O5 by theNo.2 Geological Party of Xinjiang Bureau of Geology and Mineral Re-sources (local geological report). Disseminated ore is fine- to medium-grained and consists of 20–45% titanomagnetite, 25–40% plagioclase,10–25% clinopyroxene and 5–15% ilmenite, with small amounts of oliv-ine and hornblende. Massive ore is fine-grained and is typicallycomposed of greater than 80% titanomagnetite, and 5–10% ilmenitewith small amounts of the same silicate minerals that occur in the dis-seminated ore as described above. Both types of oxide ores commonlycontain fine-grained pentlandite and chalcopyrite ranging from 1% indisseminated ores to 3% in massive ores (Fig. 3g–i).

3. Sampling and analytical methods

The samples analyzed in this study included oxide–barrenclinopyroxenite, gabbro, disseminated ore,massive ore and titanomagne-tite separates from ore samples. They were collected from surface expo-sures and drilled boreholes. The sampling locations are shown inFig. 2b–d. Fourteen whole-rock samples, eight and six from theWajilitagandPuchang complexes, respectively,were processed by jaw crusher androller mill, and then powdered in agatemortars in order to minimize po-tential transition metal contamination. Nine titanomagnetite separateswere concentrated using a combination of magnetic and heavy liquidtechniques and further purified byhandpicking at themineral separationlaboratory of the Bureau of Geology andMineral Resources of Hebei Prov-ince at Langfang.

Whole rock PGE concentrations were determined by ICP-MS(Perkin–Elmer Sciex Elan 6000) at the Guangzhou Institute of Geo-chemistry after pre-concentration using an improved Fe–Ni sulfide fire

chang deposits. (a) Half-core samples showing sharp contacts between massive oresg clinopyroxene with a few percent of olivine and plagioclase under plain-polarizedight; (d) the chalcopyrite grains in the Wajilitag disseminated ore under reflected(f) marble xenoliths occurring in the clinopyroxenite close to the contact betweenminated ore and gabbro in the Puchang deposit; (h) sharp boundary of massive oreoclase in the Puchang clinopyroxenite under plain-polarized light; (j) the Puchang gabbroomposed dominantly of olivine, clinopyroxene and plagioclase under cross-polarized light;nder plain-polarized light; (m) pentlandite and chalcopyrite grains in the Puchang dissem-eflected light. Abbreviations: Titanomagnetite = Tmn; Ilmenite = Ilm; Pyrrhotite = Po;.

Table1

Conc

entrations

ofNi,Cu

andplatinum

grou

pelem

ents

intheGPt-7

referenc

estan

dard

andwho

lerocksfrom

theW

ajilitagan

dPu

chan

gcomplex

es.

Sampleno

.ZK

4503

-7ZK

4503

-9ZK

4503

-31

ZK45

03-39

ZK45

03-45

DW

-24-4

DW

-26-1

ZK45

03-20

D8-1

D8-3

ZK4-2-9-1

ZK4-2-22

-2NPC

-11

ZK4-2-21

-1GPt-7

(Thisstud

y,n=

4)GPt-7

(Certified

)

Location

Wajilitagco

mplex

Puch

angco

mplex

Rock/ore

type

Oliv

ine

clinop

yrox

enite

Disseminated

ore

Clinop

yrox

enite

Oliv

inega

bbro

Disseminated

ore

Os(p

pb)

0.24

60.12

80.03

90.02

20.10

40.22

00.05

30.20

80.00

80.00

80.00

60.00

60.00

20.00

50.51

70.64

Ir(p

pb)

0.04

20.03

50.03

90.02

30.11

00.21

20.06

20.20

00.00

20.00

30.00

70.00

50.00

40.00

51.52

61.20

Ru(p

pb)

0.04

90.04

40.04

30.02

80.07

10.13

20.04

70.19

60.01

50.02

70.04

10.02

00.01

40.02

10.35

20.66

Rh(p

pb)

0.01

80.01

50.02

50.01

60.07

30.12

50.04

70.11

30.00

40.01

30.01

00.00

60.00

80.01

21.10

11.10

Pt(p

pb)

0.95

20.72

80.65

00.49

20.78

80.95

71.36

00.99

80.05

50.09

10.12

60.08

70.15

70.23

312

.468

14.70

Pd(p

pb)

1.11

00.89

90.52

40.37

00.46

10.66

51.12

00.82

80.06

60.15

90.07

10.09

10.13

40.16

012

.823

15.20

ΣPG

E(p

pb)

2.42

1.85

1.32

0.95

1.61

2.31

2.69

2.54

0.15

0.30

0.26

0.22

0.32

0.44

Ni(pp

m)a

329

323

343

255

324

247

249

218

19.3

13.5

237

192

102

253

Cu(p

pm)a

457

240

576

563

495

622

592

208

2023

196

85.3

124

804

MgO

(wt.%

)a13

.12

13.61

11.32

11.20

10.85

12.03

11.01

11.01

7.49

7.23

18.14

16.21

13.02

8.01

Cr(p

pm)a

698

636

94.9

75.6

201

727

108

409

12.6

2.35

173

218

101

45.6

Pd/Ir

26.4

25.7

13.4

16.1

4.2

3.1

18.1

4.1

33.0

53.0

10.1

18.2

33.5

32.0

Cu/Pd(×

105)

4.12

2.67

10.99

15.22

10.74

9.35

5.29

2.51

3.03

1.45

27.61

9.37

9.25

50.25

aTh

eda

taof

Ni,Cu

,MgO

andCr

arefrom

Zhan

get

al.(in

prep

aration)

.Certified

values

ofthereferenc

estan

dard

arefrom

Sunet

al.(20

09).

594 D. Zhang et al. / Ore Geology Reviews 57 (2014) 589–601

assay method (Sun et al., 2009). Sample (~20 g) spiked with 190Os wasmixed with 40 g Na2B4O7, 2.5 g Fe, 1.0 g Ni and 1.0 g S, and then trans-ferred to a fired-clay crucible. The mixture was then covered using 5 gof Na2B4O7 powder and fused at 1050 °C for 45 min. After cooling, thesulfide bead was obtained and transferred to a glass beaker containing15 ml H2O. The bead disintegrated into powder and then 30 ml of con-centrated HCl were added. On a hot plate, the sulfide powder wasdissolved by HCl and insoluble residuewas left. By filtration, the residuewas collected and then transferred into a distillation apparatus with3 ml HNO3. Os was distilled at 110 °C for 30 min as OsO4, which wasabsorbed with 5 ml H2O for ICP-MS determination of Os content. Afterdistillation, the remaining solution was concentrated and then 2 mlHCl were added. The solution was evaporated to a small volume, andmade up to 10 mlwith H2O for analyses of Ru, Rh, Pd, Ir and Pt contentsby ICP-MS. For this method, the limits of detection for Os, Ir, Ru, Rh, Ptand Pd are 0.7, 1, 2, 1.5, 6 and 25 ppt, respectively. As shown inTable 1, the results of the certified reference standard (GPT-7) agreewell with the values reported by Sun et al. (2009). The analytical preci-sion and accuracy for PGE are estimated to be generally better than 10%.

Re–Os isotopic compositions of the titanomagnetite separatesfrom the Wajilitag and Puchang intrusions were determined at theState Key Laboratory of Lithospheric Evolution, Institute of Geologyand Geophysics, Chinese Academy of Sciences. The Carius tube diges-tion technique is reported in detail by Chu et al. (2009). Approxi-mately 2 g of homogenized whole-rock powders and appropriateamounts of a 187Re–190Os mixed spikes were sealed in an externallycooled (−50 °C), single-use, Pyrex® borosilicate Carius tube, with3 ml of purified concentrated HCl and 6 ml of purified concentratedHNO3. The Carius tubes were kept at ~240 °C in an oven for48–72 h. Osmium was extracted from the aqua regia solution intoCCl4 (Cohen and Waters, 1996) and then back-extracted into HBr,followed by purification via microdistillation (Birck et al., 2007). Rewas separated from the matrix and purified by anion exchange chro-matography with about 0.6 ml resin (Biorad AG 1 × 8, 100–200mesh). The samples were loaded onto the columns in 0.8 mol/LHNO3, the matrix elements were eluted with 0.8 mol/L HNO3 and1 mol/L HCl, and then the Re was collected with 8 mol/L HNO3. Osisotopic compositions were measured using a GV IsoProbe-T MassSpectrometer with negative ion mode. Purified Os was loaded ontoplatinum filaments and Ba(OH)2 was used as an ion emitter. All sam-ples were run with a secondary electron multiplier in peak-jumpingmode. The Os isotopic compositions and Os concentrations wereobtained in one mass spectrometric run. The measured Os isotopicratios were corrected for mass fractionation using 192Os/188Os =3.08271 after interference corrections, oxygen corrections andspike subtractions. The isotope dilution analyses of Re wereconducted on a NeptuneMC-ICP-MS using a secondary electronmul-tiplier in peak-jumping mode. Mass fractionations for Re werecorrected using a Re standard that was run alternately with the sam-ples. The reference value for the standard was 185Re/187Re = 0.5975(Rosman and Taylor, 1998). Total analytical blanks were 3–5 pg forRe and 1 pg for Os with a 187Os/188Os ratio near 0.16. The in-run pre-cisions for Os isotopic measurements were better than 0.4% based onanalysis of an in-house standard (Johnson–Matthey standard ofUMD) over a period of several months. Analytical results for stan-dard reference material BHVO-2 are shown in Table 2.

4. Analytical results

4.1. Whole-rock PGE concentrations

The concentrations of PGE and other chalcophile elements in thestudied samples are listed in Table 1. Samples from the Wajilitag com-plex have much higher PGE concentrations than that of the Puchangcomplex, even though the total PGE contents in all the studied samplesare quite low (Fig. 4). The abundances of PGE in the ore-bearing silicate

Table 2Rhenium–osmium isotopic compositions for the BHVO-2 reference standard and titanomagnetite separates from the Wajilitag and Puchang deposits.

Sample no. Location Ore type Re (ppb) Os (ppb) Re/Os 187Re/188Os 187Os/188Osa ±2σ 187Os/188Osib γOs(t)c

DW-24-1 Wajilitag deposit Disseminated ore 0.191 0.192 0.99 4.841 0.227 0.24 0.204 +62.7DW-25-4 0.754 0.053 14.3 73.75 0.667 0.11 0.319 +154DW-25-1 0.264 0.079 3.36 16.46 0.257 0.26 0.180 +43.1DW-28-2 0.706 0.037 19.1 103.7 1.101 0.08 0.612 +387Dp-1-2 Puchang deposit Massive ore 0.631 0.004 141.4 1044.2 4.915 2.4 0.121 −4.1Dp-1-4 1.113 0.004 263.4 2261.9 7.481 1.8 -2.91 −2411Dp-2-2 1.801 0.009 199.8 1328.4 3.260 16.4 -2.84 −2359ZK4-2-1 Disseminated ore 1.211 0.009 139.2 961.9 3.733 0.38 -0.684 -644D56-17 1.114 0.002 554.6 6121.9 18.05 0.9 -10.07 -8105BHVO-2 (This study, n = 2) 0.528 0.120 21.35 0.14617 0.00007BHVO-2 (Certified) 0.543 0.101

Certified values of the standard referencematerial are fromMeisel andMoser (2004). Initial isotopic values (187Os/188Osi) andγOs(t) in this tablewere calculated at 283 Ma and275 Ma forthe Wajilitag and Puchang samples, respectively.

a Present day 187Os/188Os ratios.b Age-corrected 187Os/188Os ratios using a 187Re decay constant of 1.666 × 10−11 yr−1 (Smoliar et al., 1996).c Age-correctedγOs values;γOs(t) is thepercentagedeviation between the age-corrected 187Os/188Os ratio and the age-corrected 187Os/188Os ratio of a chondriticmantlewhere thepres-

ent day chondritic mantle has 187Os/188Os = 0.12757 and 187Re/188Os = 0.3972 (Walker et al., 1989).

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rocks and ore-barren silicate rocks of both complexes are in the samerange. The Wajilitag samples are characterized by moderately fraction-ated patterns between IPGE (Os, Ir and Ru) and PPGE (Rh, Pt and Pd). Allof theWajilitag samples show a slightly negative Ru anomaly relative toIr and Rh (Fig. 5a). In contrast, those from the Puchang complex displayhighly fractionated PGE patterns with enrichments of IPGE relative toPPGE (Fig. 5b), although the possibility that the analytical errors at ex-tremely low IPGE concentrations cannot completely be ruled out. Rela-tive to primitive mantle concentrations (McDonough and Sun, 1995),normalized Cu values within both sets of samples are higher than thenormalized PGE values (Fig. 5), although absolute Cu concentrations(208–622 ppm and 20–804 ppm, respectively) are low. The Wajilitagsamples have relatively higher chalcophile elements and PGE contents,whereas the Puchang samples show similar total PGE contents in com-parison with the Tarim basalts (Fig. 4). It is important to note that theWajilitag and Puchang intrusive rocks exhibit more fractionated primi-tive mantle-normalized PGE patterns and higher Pd/Ir ratios (3.1–53)than in the Tarim flood basalts (Figs. 4 and 5). Compared to the severalwell-knownmagmatic Fe–Ti–V deposits associatedwith Permian conti-nental basaltic magmatism in the ELIP, the abundances of PGE in theWajilitag and Puchang intrusive samples are similar to or slightlylower than the Hongge and Panzhihua oxide-bearing intrusions (Fig. 5).

Fig. 4.Diagramof Pd/Ir versus PGE. Data sources for the Tarimflood basalts are fromD.Y. Liet al. (2012) and Yuan et al. (2012).

4.2. Titanomagnetite Re–Os isotope

Re–Os isotopic compositions of titanomagnetite separates are givenin Table 2. All titanomagnetite samples from the Wajilitag deposit con-tain low Re (0.19–0.75 ppb) and Os (0.04–0.19 ppb) concentrationsand present-day 187Os/188Os ratios (0.227–1.101) with initial 187Os/188Os ratios (187Os/188Osi) ranging from 0.1799 to 0.6121. The γOs(t)values of Wajilitag samples vary between 43 and 387 (mostly b155),

Fig. 5. Primitive mantle-normalized chalcophile element (Ni, Cu and PGE) patterns of theWajilitag and Puchang samples. The primitive mantle normalization values are fromMcDonough and Sun (1995). Data sources: Tarim flood basalts (Li et al., 2012b; Yuanet al., 2012), Hongge oxide-bearing intrusion (Bai et al., 2012a), and Panzhihua oxide-bearing intrusion (Pang et al., 2013).

Fig. 6.Diagramof Re versusOs (a) andγOs versus Re/Os (b). Data sources: Duluth complex(Ripley et al., 2008) and Suwałki complex (Morgan et al., 2000).

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and are similar to or slightly lower than those of themagnetite separatesof the Duluth complex, Minnesota (177–532; Ripley et al., 2008). How-ever, the values are significantly lower than those reported from theSuwałki complex, Poland (759–975;Morgan et al., 2000),where a crust-al source has been proposed (Fig. 6). In contrast, the Puchangtitanomagnetite samples show significantly high Re (0.63–1.80 ppb)and exceptionally low Os (b0.01 ppb) contents with very radiogenicmeasured 187Os/188Os ratios (3.26–18.05) with larger uncertainties(Table 2, Fig. 6). The data of Puchang titanomagnetite separates may in-dicate underestimation of analytical uncertainty or post-magmatic dis-turbance of the Re–Os isotope system (Gleißner et al., 2012) and donot permit calculation of reliable initial Os isotopic compositions at275 Ma, the time of emplacement of the Puchang complex.

5. Discussion

5.1. Controls on PGE distribution and fractionation

It has been documented that the PGE systematics in mafic and ultra-mafic rocks is predominantly controlled bymantle source characteristics,degrees of partial melting, sulfide segregation during ascent and/or em-placement, and fractional crystallization of olivine, chromite, platinum-group minerals (PGM) and PGE-rich alloys (Barnes et al., 1985;

Capobianco et al., 1994; Keays, 1995; C.S. Li et al., 2012; Maier andBarnes, 1999; Rehkämper et al., 1999; Said et al., 2011; Zhang et al.,2005). As noted above, the Tarim basalts have lower abundances oftotal PGE than those observed in the Wajilitag samples, but have valuesvery close to the Puchang samples (Fig. 4).

It has been suggested that a relatively high-degree (mostly N10%) ofpartial melting is required to dissolve all the sulfides in the mantle(Naldrett, 2010a). At low degree partial melting, sulfides may beretained in the mantle source along with almost all the PGE, leavingthe partial melts significantly depleted in PGE, because PGE have veryhigh partition coefficients to sulfide (Keays, 1995; Naldrett, 2010b).Many studies have suggested that the degree of partial melting wasprobably less than ∼5% for the parental magmas of the Tarim basalts,which is consistent with their alkaline nature (Li et al., 2012b; Z.L. Liet al., 2012; Yu et al., 2011), whereas higher degrees (N10%) of mantlepartial melting have been proposed for theWajilitag and Puchang intru-sive rocks (Zhang et al., 2010). Based on these considerations, it could beargued that sulfide retention in the source mantle can adequately ex-plain the extremely low PGE abundances in the Tarim flood basalts.This inference is supported by the relatively constant total PGE contents(mostly from0.1 to 0.4 ppb; Li et al., 2012b) andnearflat primitiveman-tle-normalized patterns of the Tarimbasalts (Fig. 5), because S-saturateddifferentiation would have resulted in highly variable PGE contents ofthe rocks. Furthermore, the Tarim basalts have notably different Sr–Ndisotopic compositions from the Wajilitag and Puchang intrusive rocks.The (87Sr/86Sr)i and εNd(t) values of the complexes are both close tothose of depleted mantle (C.L. Zhang et al., 2008, 2010; Zhang et al., inpreparation), whereas these values for the Tarim basalts are close toenriched mantle (Z.L. Li et al., 2012; Tian et al., 2010; Yu et al., 2011;D.Y. Zhang et al., 2012; Zhou et al., 2009). These geochemical differencesmake it difficult to ascribe theWajilitag and Puchang intrusive rocks andthe nearby Tarim basalts to a common mantle source. Rather, they ap-pear to be derived from distinct mantle sources. Thus, both source man-tle compositions and degrees of partial melting probably played animportant role in producing differences between the Tarim basalts andnearly coevalWajilitag intrusive rocks. The otherwise similar Sr–Ndgeo-chemical features of the Wajilitag and Puchang intrusive rocks suggestthat their contrasting PGE abundances may be caused by anotherprocess.

The Wajilitag and Puchang samples exhibit fractionated mantle-normalized PGE patterns (Fig. 5) and high Pd/Ir ratios (Fig. 4), indi-cating that the parental magma(s) are highly evolved magma(s),which are more fractionated than the nearly coeval basalts (Keays,1995; Said et al., 2011). Previous studies suggested that segregationof sulfidemineral from a crystallizing silicatemagma is considered tobe crucial in the distribution of PGE in mafic and ultramafic rocks(Keays, 1995). Owning to much higher sulfide–magma partition co-efficients for Pd than Cu [DPd ~105, DCu ~103, c.f. Naldrett (2011)], Pdpartitions more strongly into a sulfide than Cu during sulfide segre-gation from magma, and therefore the Cu/Pd ratio is a sensitive indi-cator of sulfide mineral segregation from magma. If a silicate meltexperiences sulfide mineral saturation and segregation, the Cu/Pd ratiosrapidly increase as themagma evolves.Without sulfidemineral segrega-tion, copper behaves in a manner similar to palladium, and the Cu/Pdratio for both fractionated magma and cumulates will be similar to thatof the primitive mantle and the primary magma (Cu/Pd = 103–104;Barnes, 1993; McDonough and Sun, 1995). All samples of the Wajilitagand Puchang complexes have significantly higher Cu/Pd ratios(2.5 × 105–15 × 105 and 1.4 × 105–50 × 105, respectively) than theexpected mantle values (Table 1; Fig. 7), similar to the Hongge andPanzhihua intrusions from the ELIP. Such high Cu/Pd ratios, togetherwith their low PGE abundances, and the evolved nature of the rocks,strongly indicate that the magma(s) experienced early sulfide mineralsegregation during the evolution of the magma(s). It is notable that theevolved Puchang intrusive rocks tend to have markedly lower totalPGE abundances and relatively higher Cu/Pd ratios than the Wajilitag

Fig. 7.Diagram of Pd versus Cu (a) and Cu/Pd versus Pd (b). Data sources: Tarim flood ba-salts, Hongge and Panzhihua oxide-bearing intrusions as in Fig. 5; Xinjie oxide and sulfide-bearing intrusion (Zhong et al., 2011). Symbols as in Fig. 4.

Fig. 8. Diagram of MgO versus Ru (a) and Cr versus Ru (b). Symbols as in Fig. 4.

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samples, likely suggesting that the Puchang rocks experienced largeramounts of sulfide mineral segregation during magma ascent and/oremplacement, and would have lost a majority of PGE to the segregatingimmiscible sulfide melts (Said et al., 2011; Wang et al., 2007).

Another notable feature is that all Wajilitag samples are weakly de-pleted in Ru relative to Ir and Rh (Fig. 5a). A number of studies have pro-posed that sulfide segregation was not the primary cause of such ananomaly, because all PGE have a similar affinity for immiscible sulfidemelts (Bezmen et al., 1994; C.S. Li et al., 2012; Peach et al., 1994;Zhong et al., 2002). Negative Ru anomalies are also widely documentedin the Emeishan flood basalts (Qi and Zhou, 2008), Kerguelen Plateaubasalts (Chazey Iii andNeal, 2005) and theXinjie and Agnew layered in-trusions (Vogel et al., 1999; Zhong et al., 2011),which are commonly at-tributed to removal of laurite and/or Os–Ir–Ru alloys, together withchromite or olivine, from the parental magmas. Some experimentaland theoretical studies have shown that IPGE are more compatiblethan PPGE in olivine and chromite and may fractionate from eachother during fractional crystallization of these minerals (Barnes andPicard, 1993; Brenan et al., 2003, 2005; Capobianco et al., 1994;Puchtel andHumayun, 2001). That is, fractional crystallization of olivineand chromite could be responsible for the negative Ru anomalies in theWajilitag samples. However, no correlations are found between Ru andMgO or Cr (Fig. 8), suggesting that olivine and chromite did not play asignificant role in the distribution of ruthenium. This interpretation isalso supported by many recent experimental and geochemical studies

showing that both olivine and chromite are not enriched in IPGE andwould not be responsible for the fractionation of ruthenium (Godelet al., 2007; Lorand et al., 1999; Puchtel et al., 2004). Instead, it hasbeen well documented that laurite and Ru-rich alloys can be an earlyprimary magmatic phase because of their high thermal stability, andcan be entrapped in other crystallizing minerals phases, such as olivineand spinel (Brenan and Andrews, 2001; Hiemstra, 1979; Merkle, 1992;Righter et al., 2004). Some researchers have also advocated that deple-tions of IPGE relative to Pt and Pd in some mafic–ultramafic magmacould be attributed to the occurrence of potential Ru-alloys in the man-tle or Ru-rich phases (e.g., PGM and/or alloy) that fractionated from themagmas during ascent (Bai et al., 2012a; C.S. Li et al., 2012; Said et al.,2011). On the basis of the PGE systematics of theWajilitag and Puchangintrusive rocks, and the available experimental results outlined above, itis unlikely that the IPGE, especially Ru, were controlled by the fraction-ation of olivine and chromite. Rather, it is probable that discrete IPGE-bearing phase(s) fractionated from the mafic–ultramafic magmasprior to their final emplacement. In summary, variable sulfide segrega-tion and fractionation of Ru-dominant phase(s) may have largelygoverned the contrasting PGE abundances in theWajilitag and Puchangintrusive rocks.

5.2. Petrogenetic implications

It is generally accepted that some mafic–ultramafic intrusions andcoeval continental flood basalts in many LIPs belong to a genetically re-lated intrusive–extrusive association (Hou et al., 2012a; Lightfoot and

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Keays, 2005; Zhong et al., 2011). The magmatic events that producedthe Wajilitag and Puchang complexes and the Tarim basalts took placepenecontemporaneously, and the spatial and temporal correlation hasprompted some researchers to suggest that both complexes and theTarim basalts are related to a single event of basaltic magmatism(Zhou et al., 2009). However, as elaborated above, thedistinct geochem-ical characteristics of the Tarim basalts and the Wajilitag and Puchangintrusive rocks may reflect different compositions of mantle sourcesand degree of partial melting. Therefore, there is no direct genetic rela-tionship between these intrusive rocks and Tarim basalts in terms of ashared magma chamber, although both of them may be related to thesame thermal event, caused by a mantle plume. These differences arealso highlighted by the fact that the Wajilitag and Puchang intrusiverocks have a tholeiitic affinity, whereas the Tarim basalts are alkaline(Yu et al., 2011; Zhang et al., in preparation; Zhou et al., 2009).

Previous studies proposed that the Wajilitag intrusive rocks mightbe derived from an asthenospheric mantle source with negligible crust-al contamination based on incompatible trace element characteristicsand Sm–Nd isotopic system (Z.L. Li et al., 2012; C.L. Zhang et al.,2008). During the evolution of the Earth's mantle and development ofthe crust, Os was preferentially retained in the mantle, whereas Rewas moderately enriched in most crustal rocks. Because Re is more in-compatible during mantle melting than Os, crustal material tends tohave distinctively high Re and lowOs concentrations and high Re/Os ra-tios and, with time, very radiogenic Os isotopic compositions (Meiselet al., 2001; Walker et al., 1989). As a result, small amounts of crustalcontamination will strongly elevate the Os isotopic ratios of magmaswith low Os concentrations (Reisberg et al., 1993; Widom and Shirey,1996;Widom et al., 1999; Z.C. Zhang et al., 2008). Hence, the Re–Os iso-topic system is a highly sensitive tracer of crustal contamination ofman-tle-derived magmas. Samples with high Re and low Os concentrations,very radiogenic present-day 187Os/188Os ratios, and positive γOs valuesmost likely experienced assimilation of crustal materials (Reisberget al., 1993; Walker et al., 1994; Z.C. Zhang et al., 2008). The γOs valuesfor theWajilitag samples are all elevated relative to the uncontaminatedasthenospheric mantle melts whichwould have a γOs value near 0. Thisis consistent with the involvement of modest quantities of crustal con-taminants in the Os source of theWajilitag oxide ores. The contaminantmay alter the Os isotope compositions of a mafic–ultramafic magmawith low Os concentrations, but not easily disturb the lithophile ele-ment isotopic systematics (e.g., Rb–Sr, Sm–Nd and Lu–Hf). Further-more, the relatively higher oxygen isotopic composition of zircons and(87Sr/86Sr)i and lower εNd(t) values of whole rocks within the Puchangsamples in comparison with those in the Wajilitag samples (Zhanget al., in preparation) suggest that relatively more crustal materialshave been involved in the formation of the Puchang Fe–Ti–V oxide de-posit. Because crustal contamination has been widely regarded as animportant trigger for sulfide saturation and formation of immiscible sul-fidemelts duringmagma evolution (Li et al., 2011), the idea of generallyhigher degree of crustal contamination in the Puchang intrusive rockswould be consistentwith the above inference that the amount of sulfidesegregated must have been relatively large in the Puchang magma. Insummary, the evolvedWajilitag and Puchang samples experienced var-iable degrees of crust contamination. Thismost likely reflects the assim-ilation of crustal materials that may have played a critical role ininducing variable sulfide segregation in the mafic–ultramafic magmas.

5.3. The potential for Ni–Cu mineralization

It has been widely accepted that segregation of sulfidemineral froma silicate magma plays an important role in the formation of magmaticNi–Cu–PGE mineralization in mafic–ultramafic suites (Keays, 1995;Zhong et al., 2002). As described above, theWajilitag and Puchang sam-ples may have experienced variable sulfide segregation prior to theirfinal emplacement, which should be favorable to the enrichment ofCu–Ni–(PGE). Moreover, many researchers have proposed that the

timing of sulfidemineral segregation duringmagma evolution is criticalto the formation of economic magmatic Ni–Cu–(PGE) sulfide deposits,and is therefore a useful guide in local or regional Ni, Cu and PGE explo-ration (Arndt et al., 2005; Barnes and Lightfoot, 2005; Barnes et al.,1993; Song et al., 2011; Wang et al., 2011). There are many base-metal sulfides (e.g., pyrrhotite and pentlandite) occurring as dissemi-nated grains intergrown with titanomagnetite (Fig. 3) in the Wajilitagand Puchang deposits, similar to the scenario reported for the sulfide–magnetite mineralization in the Xinjie, Hongge and Skaergaard mafic–ultramafic intrusions (Andersen et al., 1998; Bai et al., 2012a; Brookset al., 1999; Li et al., 2011; Park et al., 2013; Zhong et al., 2011). This ob-servation may imply that fractional crystallization involving abundantmagnetite at the Wajilitag and Puchang complexes could have accom-panied decreased Fe content, oxygen fugacity, and temperature in themagmas and thus increased the sulfur activity of the fractionatedmagmas, resulting in sulfide saturation and segregation at the latestage of magma evolution. The sulfide inclusions are present in therim and mantle of clinopyroxene and plagioclase crystals or associatedwith interstitial minerals (Fig. 3), indicating that sulfide saturationoccurred late relative to the growth of theseminerals. From these obser-vations, theWajilitag and Puchangmagmasmayhave experienced two-stage sulfide liquid segregation. A similar conclusion is reached for thesulfide–magnetite mineralization in some mafic–ultramafic intrusions(Bai et al., 2012b; Tang et al., 2011). The early stage of sulfide segrega-tion event resulted from crustal contamination and occurred at depth,whereas the late stage took place during extensive fractional crystalliza-tion ofmagnetite in the shallowmagma chamber. Therefore, we suggestthat the complexesmay have the potential for Ni–Cu sulfidemineraliza-tion, which was related to the second stage of sulfide liquid segregationin the upper crust, albeit there are only twowell-known unusual exam-ples of the mafic–ultramafic bodies, the Bushveld and Xinjie intrusions,that host Ni–Cu–PGE sulfide ore layers within the lower part of the in-trusion and major Fe–Ti–V oxide-bearing layers within the middlepart (Kruger, 2005; Maier et al., 2000; Zhang et al., 2009; Zhong et al.,2011). Numerous observations and exploration work have revealedthat economically important magmatic Ni–Cu–(PGE) mineralizationtends to occur in magma conduit systems, and the Ni–Cu–PGE sulfideores are usually present in the lower parts of intrusions (Arndt et al.,2005; Naldrett et al., 1992; Said et al., 2011; Song et al., 2011; Zhonget al., 2011). Therefore, a potential exists for finding sulfide-richmineraldeposits in the TLIP, and exploration for economicmagmatic sulfide de-posits should probably concentrate on the magma conduits and at thelow portions of these complexes.

6. Conclusions

(1) The Wajilitag intrusive rocks have much higher total PGE abun-dances than those in the Puchang, implying that the former expe-rienced smaller amounts of sulfide mineral segregation duringmagma ascent and/or emplacement. Both these intrusive rocksexhibit more fractionated mantle-normalized PGE patternsthan the Tarim basalts.

(2) Sulfide mineral segregation was the main control on PGE distri-bution in the Wajilitag and Puchang complexes, whereas Ru-dominant phase(s) crystallization likely resulted in Ru depletion.The two-stage sulfide saturation events in magmas were likelyinduced by crustal contamination and subsequent fractionalcrystallization of abundant magnetite.

(3) The complexes are not directly related to the Tarim basalticvolcanism, although they are all likely related to the same ther-mal/plume event. These intrusive bodies and other similartypes of mafic–ultramafic intrusions formed by high-degreepartial melting in the TLIP should be considered as prospectivetargets for mineral exploration.

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Acknowledgments

We thank Mr. Donglin Ma and Mr. Jun Cheng of the Wajilitag mine,and Mr. Xu of the Puchang mine for their kind help during field work.We also thank Dr. Zhuyin Chu, Dr. Yali Su and Dr. Shengling Sun for tech-nical support. Constructive reviews and suggestions by Dr. GregoryShellnutt of the National Taiwan Normal University and two anonymousreviewers helped to improve the revised version. This work wasfinancially supported by the National Basic Research Program ofChina (973 Program, No. 2012CB416806) and 305 Project of theState Science and Technology Program of China (Nos. 2007BAB25B05and 2011BAB06B02-04).

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