Geology, geochemistry, and geochronology of the Dunde iron–zinc ore deposit in western Tianshan, China

Ore Geology Reviews xxx (2013) xxx–xxx

OREGEO-01092; No of Pages 21

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Geology, geochemistry, and geochronology of the Dunde iron–zinc ore deposit inwestern Tianshan, China

Shigang Duan a,b,⁎, Zuoheng Zhang a, Zongsheng Jiang a, Jun Zhao b, Yongping Zhang c,Fengming Li d, Jingquan Tian c

a MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, Chinab College of Earth Science and Resources, Chang'an University, Xi'an 710054, Chinac No. 3 Geological Party, Xinjiang Bureau of Geology and Mineral Resources, Korla 841000, Chinad Xinjiang Bureau of Geology and Mineral Resources, Urumqi 830000, China

⁎ Corresponding author at: MLR Key Laboratory of MetalInstitute of Mineral Resources, Chinese Academy of GeolChina. Tel.: +86 10 6899 9050.

E-mail address: [email protected] (S. Duan).

0169-1368/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.oregeorev.2013.08.019

Please cite this article as: Duan, S., et al., GeChina, Ore Geol. Rev. (2013), http://dx.doi.o

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Article history:Received 23 April 2013Received in revised form 21 August 2013Accepted 26 August 2013Available online xxxx

Keywords:Skarn iron depositFluid inclusionsSulfur isotopesU–Pb datingDundeTianshanChina

The Dunde iron–zinc deposit (185 Mt at 35% Fe), located in the Tianshan orogenic belt, Xinjiang, northwestChina, is hosted in late Carboniferous volcanic–volcaniclastic rocks characterized by prograde skarn formationand retrograde alteration, but the ore lacks any clear spatial link with intrusive rocks. Four stages of skarn forma-tion and ore development can be recognized: (1) a prograde skarn stage that formed grossularitic garnet(Gr39–80; Ad15–58) and diposidic pyroxene (Di63–97); (2) a retrograde skarn stage dominated by the formationof magnetite with minor epidote, ferropargasite, apatite, hematite, and trace amounts of titanite and spinel;(3) a sulfide stage dominated by the formation of arsenide (loellingite and arsenopyrite) and sulfide minerals(sphalerite, pyrrhotite, pyrite, chalcopyrite, and minor galena), calcite, and traces of quartz; and (4) a chlorite–calcite stage mainly characterized by the formation of chlorite, calcite, and traces of sericite. Exsolution ofdroplet-like and patchy chalcopyrite is developed within sphalerite of the sulfide stage, indicating an exsolutiontemperature of 350–400 °C. As such, the crystallization temperature of early stage magnetite may have beenN400 °C. Abundant fluid inclusions occur in calcite, which include daughter-mineral-bearing H2O, H2O, andpure H2O inclusions. The H2O inclusions have a wide range of homogenization temperatures from 147 °C to367 °C with salinities of 2.4–23.4 wt.% NaCl equivalent. The daughter-mineral-bearing H2O inclusions have ho-mogenization temperatures from 172 °C to 347 °C with salinities of 31.9–33.0 wt.% NaCl equivalent. Using thechlorite geothermometer, the temperature of chlorite formation is constrained to be between 152 °C and222 °C (average = 194 °C). Sulfur isotope compositions of pyrrhotite, sphalerite, pyrite, and loellingite havea narrow range of δ34S values from 3.8‰ to 8.1‰ (average δ34S = 6.8‰), suggesting that the sulfur wasmagmatic-derived. Zircon LA–ICP-MS U–Pb dating of wall rock dacite yields a weighted mean 206Pb/238U ageof 316.0 ± 1.7 Ma. Combined with previous dating results of magnetite-mineralized diorite stocks, dioritedikes, and garnet skarn, it can be inferred that the Dunde iron–zinc deposit formed in the late Carboniferousafter 316 Ma and is genetically related to deep dioritic intrusions. During the late Carboniferous, the tectonicsetting of this region changed from subduction–collision to extension, accompanied by mantle-derived magmaunderplating in deep. After the formation of the iron–zinc ore deposit, the Dunde district was intruded by anearly Permian K-feldspar granite that yields a zircon LA–ICP-MS U–Pb age of 295.75 ± 0.71 Ma.

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1. Introduction

The Central Asian Orogenic Belt (CAOB) is the world's largest Phan-erozoic accretionary orogen and crustal growth region (Fig. 1a) (Gaoet al., 2009a,b; Jahn et al., 2000; Mao et al., 2004, 2005, 2008; Sengör,1993; Windley et al., 2007; Xiao et al., 2008), and is also an ideal regionfor studying mineralization related to accretionary processes (Xiao

logeny andMineral Assessment,ogical Sciences, Beijing 100037,

ghts reserved.

ology, geochemistry, and georg/10.1016/j.oregeorev.2013.0

et al., 2009; X. Zhang et al., 2012b). Marine volcanic-hosted iron ore de-posits are one of the most important mineralization types of the CAOB(Chen et al., 2008; Yang et al., 2012, 2013; X. Zhang et al., 2012b),such as the Kachar and Davydovo in Turgay (Belevtsev, 1982); Anzas,Mengku and Abagong in Altai-Sayan (Belevtsev, 1982; Chai et al.,2013; Xu et al., 2010; Yang et al., 2012); Chagangnuoer, Zhibo andYamansu in Chinese Tianshan (Hong et al., 2012a, 2012b, 2012c; Houet al., 2013; Jiang et al., 2012a, 2012b; Zhang et al., 2012b, 2012c). Lotsof marine volcanic-hosted iron deposits and occurrences were discov-ered in the 1970s in the Chinese Tianshan along the southwestern mar-gin of the CAOB (Chen et al., 2008; D.H. Wang et al., 2006, 2007). Sincethen, iron ore exploration in the Chinese Tianshan has experienced a

chronology of the Dunde iron–zinc ore deposit in western Tianshan,8.019

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Fig. 1. (a) Simplified Central Asia Orogenic collage and adjacent structures (modified after Yakubchuk, 2004). (b) Geological map of the Chinese western Tianshan and adjacent region(modified after Gao et al., 2009a). (c) Geological map of the eastern segment of the Awulale Metallogenetic Belt showing the locality of iron deposits (modified after Zhang et al., 2012b).

2 S. Duan et al. / Ore Geology Reviews xxx (2013) xxx–xxx

resurgence with the beginning of a new phase of iron prospecting in2004. This recent exploration has resulted in the discovery of the Songhuiron deposit in 2005, the Zhibo iron deposit in 2006, the Dunde iron–

Please cite this article as: Duan, S., et al., Geology, geochemistry, and geoChina, Ore Geol. Rev. (2013), http://dx.doi.org/10.1016/j.oregeorev.2013.0

zinc deposit in 2007, and the Wuling iron deposit in 2008, all inthe Awulale iron metallogenic belt (AIMB) in western TianshanMountains (Fig. 1b). Many prominent magnetic anomalies were


image of Fig.�1


3S. Duan et al. / Ore Geology Reviews xxx (2013) xxx–xxx

additionally discovered after a 1:50,000 aeromagnetic survey aroundthe AIMB from 2007 to 2009, indicating that this region is a signifi-cant iron ore prospect. Currently, four large iron deposits(N100 Mt), have been identified in the AIMB (Chagangnuoer, Beizhan,Zhibo, and Dunde; Fig. 1c), along with three medium-sized iron de-posits (Shikebutai, Songhu, and Nixintage) (Feng et al., 2010).

The marine volcanic-hosted iron deposits in the CAOB are differentfrom the terrestrial volcanic-hosted iron deposits in the middle-lowerYangtze River Valley in eastern China (Mao et al., 2006; NingwuResearch Group, 1978; J.J. Yu et al., 2011), the Marcona deposit inPeru (Chen et al., 2010a, 2010b), the El Laco deposit in Chile (Nyströmand Henriquez, 1994; Sillitoe and Burrows, 2002) and the marinevolcanic-hosted classic Kiruna type iron deposits in Sweden (Frietsch,1978; Frietsch and Perdahl, 1995) as follows: 1) extensive skarn alter-ation occurred around the iron orebodies; 2) lack any clear spatial linkwith intrusive rocks; 3) apatite content of the ores is in trace amountexcept the Abagong iron deposit in Altai. These reasons lead to debateson the genesis of the marine volcanic-hosted iron deposits in the CAOB,and various genetic models have been proposed, including volcanogenicsedimentary metamorphosed (Belevtsev, 1982), seafloor exhalation(Jiang, 1983), intrusion-related iron skarn (Hong et al., 2012a; Xu et al.,2010), metasomatism by metamorphic fluids from a major shear zone(Wan et al., 2012), metasomatism by fluids that originated from a deepmagma chamber (Hou et al., 2013), ore magma originated from mag-matic liquid immiscibility (Chai et al., 2013).

The newly discovered marine volcanic-hosted iron deposits in theAIMB show more prominent and diagnostic characteristics, and fewprevious studies have investigated their geology (Feng et al., 2010;Wang et al., 2011; Zhang et al., 2012a), mineralogy (Hong et al.,2012a,b; Jiang et al., 2012a; Shan et al., 2009; Wang et al., 2012),ore geochronology (Hong et al., 2012c), and igneous geochemistryand geochronology (Jiang et al., 2012b; Wang and Jiang, 2011;Zhang et al., 2012b, 2012d). Herein, we describe for the first timethe geology, mineralogy, fluid inclusions, sulfur isotope composition,and geochemistry and geochronology of igneous rocks in the Dundeiron–zinc deposit. We use these observations and data to constrainthe genesis of the Dunde iron–zinc deposit and the geodynamic settingof iron ore deposits in the AIMB.

2. Regional geology

The east–west-trendingAwulaleMountains liewithin the Yilimassif(Fig. 1b), which comprises Archean and Paleoproterozoic crystallinebasement (Kröner et al., 2007) that may be a fragment of the Rodiniasupercontinent (Zuo et al., 2008). During the Devonian, the Yili massifwas the overriding plate during subduction of the South TianshanOcean from the south, and subduction of the North Tianshan Oceanfrom the north (Gao et al., 2009a,b; Long et al., 2011). The SouthTianshan Ocean closed between the end of the Devonian and the endof the early Carboniferous, leading to the collision of the Tarim Plateand the Yili Block. This collision finished prior to the Permian (Chenet al., 1999; Gao et al., 2009a,b) and amalgamated the Tarim Plate andYili Block (Bazhenov et al., 2003;Wang et al., 2007a). Due to the closureof theNorth TianshanOcean at the end of the late Carboniferous, the Yilimassif experienced post-collision extension from the early Permian(Allen et al., 1992; Gao et al., 2009a,b; Q. Wang et al., 2006; Wang et al.,2007b; Windley et al., 1990).

The oldest rocks cropping out in the Awulale Mountains areMesoproterozoic metamorphic crystalline basement predominantlycomprising marble, biotite–plagioclase gneiss, and amphibole + two-feldspar gneiss (Feng et al., 2010). The cover rocks include Siluriancarbonate–volcanic rocks, Middle Devonian marine volcaniclastic andsedimentary rocks, Upper Devonian littoral volcanic and terrigenousclastic–carbonate rocks, lower Carboniferous marine volcanic–sedimentary clastic–carbonate rocks, upper Carboniferousmarine vol-canic rocks with limestone interbeds, lower Permian terrigenous clastic


rocks, Lower to Middle Triassic terrigenous clastic rocks, and Jurassicterrigenous clastic rocks with coal seams (Feng et al., 2010; Li et al.,2009a,b; Luan et al., 2008).

Thewall rocks of the iron deposits in the AIMB are part of the LowerCarboniferous Dahalajunshan Formation. However, high-precision zirconU–Pb dating results indicate that the volcanic rocks of the DahalajunshanFormation are, in fact, Middle Devonian to late Carboniferous in age(An and Zhu, 2008; Jiang et al., 2012a; Zhai et al., 2006; Zhu et al.,2005, 2006a,b).

Intrusive rocks in the Awulale Mountains are spatially concentratedaround its central region (Chagangnuoer iron deposit) and, to a lesserextent, along its eastern and western sides. These rocks young fromeast to west (Devonian to Carboniferous–Permian). The intrusiverocks in the east are mainly Devonian quartz diorite; Carboniferousgranite, granodiorite, diorite, and quartz diorite; and Permian granite.The central area contains large outcrops of Permian granite, diorite,and quartz diorite, and Carboniferous diorite and granodiorite (Jianget al., 2012a; Li et al., 2007; Yang et al., 2007; Zhang et al., 2012b). Thewestern area contains Permian granitic and dioritic porphyry, diorite,diabase dikes, and albite porphyry (Zhao et al., 2008). A small numberof late Carboniferous diorite intrusions have an unusually high magne-tite content or iron orebodies developed at the contacts between dioriteintrusions and wall rocks. These include the disseminated magnetite-mineralized diorite of the Wuling prospect located ca. 45 km west ofthe Dunde iron deposit, themagnetite orebodies at the contact betweendiorite and limestone of the Gulungou iron deposit located ca. 30 kmeast of the Dunde iron deposit (Tian et al., 2009), and diorite dikes ofthe Beizhan iron deposit located ca. 16 km east of the Dunde irondeposit.

Faults are extensively developed in the AwulaleMountains, reflectingthe complex tectonic history of the region. The most obvious structuralfeature is major WNW–ESE-striking faults and a series of sub-parallelsecondary faults. These faults are cut by weakly developed NE–SW-striking faults, thereby imparting a rhombic structural fabric to theAwulale Mountains (Zhang et al., 2011). Widespread folds have open,tight, and isoclinal forms, and axial traces trend E–W(Feng et al., 2010).

3. Geology of the Dunde iron–zinc deposit

3.1. Mine geology

The Dunde iron–zinc deposit is situated near the ridge of theAwulale Mountains where it is covered by large areas of glacier. Thenorthern part of the mine is covered by a glacier. Volcanic rocks cropout in the southern part of this area, which are classified as being partof the Dahalajunshan Group. These rocks include dacite, andesite,dacitic crystal tuff, basaltic tuff, and andesitic tuff (Fig. 2). Themagnetiteorebodies are hosted in dacite, rhyolite, basaltic tuff, and andesitic tuff.The dacite is gray, porphyritic,massive, and contains phenocrysts of pla-gioclase (20%), K-feldspar (5%), quartz (5%), and chloritized hornblende(3%). The dacite groundmass has a vitreous to hyalopilitic texture. Therhyolite possesses a mineral assemblage and textural features similarto those of the dacite, but contains phenocrysts of K-feldspar (20%)and plagioclase (5%), and lacks hornblende. The basaltic and andesitictuffs are present between the dacite as layers of b1 m thick. Thetuffs contain a small amount (b8%) of lithic debris and feldspar crystals,with the basaltic tuff being gray–black in color and the andesitic tuffbeing dark green.

K-feldspar granite and minor diabase dikes intrude the volcaniclayers in the southwestern part of the mine. The K-feldspar granitehas a medium- to coarse-grained granitic texture comprising 45%K-feldspar, 25% plagioclase, 20% quartz, 5% biotite, and 5% horn-blende. A small number of E–W trending diorite dikes are present1–2 km east of the Dunde iron deposit, and there is a correspondingmedium-sized aeromagnetic anomaly on the reduction-to-pole aero-magnetic anomaly map of the Awulale area. These moderate-intensity



Fig. 2. (a) Geological map of the Dunde iron–zinc deposit. (b) Geological profile of A–A′ in Fig. 2a.


aeromagnetic anomalies have been interpreted as being outcropping orburied igneous intrusions of intermediate lithology (X.Z. Yu et al., 2011).

The ore-bearing fractures have varied orientations with strikes ofE–W,NE–SW, or N–S strikes, and dips to the S, E, NW, or N. The fracturesare generally branching and short (length b 1 km) along strike, but arewide and associated with composite veins. Faults that developed aftermineralization mainly strike NE–SW, dip vertically, and have offsets ofb3 m.

3.2. Orebodies

Seven iron–zinc orebodies have currently been found in the Dundedeposit (Fig. 2a). Three of these crop out in the west and four are


below ground in the east. The four orebodies in the east host most ofthe iron–zinc resources in the deposit and show good prospects for fur-ther deep exploration. These iron–zinc orebodies are hosted in dacite,basaltic tuff, and andesitic tuff, and have plate-like (Fig. 2b), lenticular,or irregular forms with lengths varying between 56 and 931 m, andthicknesses between 2 and 100 m. The variable shape, length, thickness,and occurrence of the orebodies correspond to their host fractures. Thefour orebodies in the east all dip to the north (dip at 52° to 75° toward015° to 355°). The currently constrained iron ore reserve is 185 Mtwith an average grade of 35.06%, and the zinc reserve is 1.492 Mt withan average grade of 1.26%. Wall rock alteration is obviously associatedwith skarn near the orebody, and epidotization, chloritization, andcarbonatization gradually diminish moving away from the orebodies.


image of Fig.�2



K-feldspar alteration and silicification are discontinuously present in theandesite both close to and distant from (N1000 m) the iron orebody.The genetic relationship between theK-feldspar alteration and silicifica-tion, and the orebodies is unclear.

3.3. Skarn and ore mineral assemblages

Skarn is intensively developed around the eastern orebodies butonly weakly developed around the western orebodies. It was not possi-ble to measure the thickness and specific zones of the skarn due to fre-quent tunnel blasting and the presence of heavy dust during this study.However, the following skarn characteristics were evident: (a) skarnnear the orebody is banded (Fig. 3a) or brecciated and cemented bymagnetite (Fig. 3a); (b) garnet is more abundant than pyroxene;(c) the skarn typically has non-planar and curved contacts withwall rocks; (d) skarn distant from the orebody is patchily dispersedin or occurs as veins cutting wall rocks, and here garnet is stillmore abundant than pyroxene. The primary gangue minerals are

Ad= andradite, Cal= calcite, Di= diopsi

Fig. 3. Representative ore photos. (a) Banded skarn. (b) Brecciated magnetite ore. (c) Ban(f) Semi-massive magnetite ore. (g) Massive ore with coarse anhedral and euhedral magnveins cut fine grained magnetite bands.


grossular–andradite, diopside, epidote, chlorite, calcite, apatite, andsmall amounts of ferropargasite, quartz, and mica, all of which areof typical of calcic iron skarn. The primary ore minerals include magne-tite, sphalerite, loellingite, pyrrhotite, pyrite, chalcopyrite, arsenopyrite,hematite, and trace amounts of titanite and galena. In addition, somesecondary oxides are present on the mine surface, which include limo-nite, malachite, hematite, azurite, and hydrohematite.

The ores are variably brecciated (Fig. 3b), banded (Fig. 3c and d),veined (Fig. 3e), disseminated to densely disseminated (Fig. 3f), ormassive (Fig. 3h). The ore textures are subhedral–anhedral, coarse-(Figs. 3g, h, and 4a) to fine-grained (Figs. 3i, 4e, and p), shred-like(Fig. 4b), reaction-rimmed (Fig. 4c, d, f–i, andm), veined or reticulate(Fig. 4k), relict (Fig. 4l), trellis exsolved (Fig. 4j and n), or emulsion topatchily exsolved (Fig. 4o). Field and petrographic observations indi-cate two periods and five stages of mineral formation (Fig. 5).The early skarn period includes: (1) prograde garnet + diopside;(2) retrograde magnetite + hematite + apatite + titanite + spinel +epidote + ferropargasite; (3) a sulfide stage of sphalerite +

de, Ep= epidote, Mt= magnetite.

ded magnetite ore. (d) Banded and veined magnetite ore. (e) Veined magnetite ore.etite. (h) Massive ore with coarse subhedral magnetite. (i) Coarse grained magnetite


image of Fig.�3


Ad= andradite, Ap= apatite, Apy= arsenopyrite, Cal= calcite, Ccp= chalcopyrite, Chl= chlorite, Di= diopside, Fp= ferropargasite,

Hem= hematite, Spl= spinel, Lo=Loellingite, Mt= magnetite, Po= pyrrhotite, Py= pyrite, Sp= sphalerite

Fig. 4. Photomicrographs (a, b, c, d, f and h, taken with crossed polars; e, plane polarized light; others, reflective plane polarized light). (a) Coarse grained zonal garnet with abnormalinterference color. (b) Fined grained euhedral garnetwrapped by chlorite. (c) Calcite replace diopside. (d)Magnetite replace a simple twin diopside. (e) Fine grained euhedral amphibole.(f) Chlorite replace diopside andmagnetite. (g) Loellingite replace apatite. (h) Loellingite replace apatite (figure g under transmitted single polarization). (i)Magnetite replace garnet, bothare wrapped by calcite. (j) Spinel exsolution lattice in magnetite. (k) Loellingite in veins or stockwork of magnetite. (l) Embayed edge of Loellingite and arsenopyrite implying they arereplaced by sphalerite. (m) Paragenetic association of pyrrohtite and chalcopyrite, and chalcopyrite filling pyrite's fissure. (n) Pyrrohtite exsolution lattice in sphalerite. (o) Exsolutiondroplets of chalcopyrite and pyrrohtite in sphalerite. (p) Radial hematite.


loellingite + pyrrhotite + pyrite + chalcopyrite + arsenopyrite +galena + quartz + calcite; and (4) a chlorite–carbonate stage ofchlorite + calcite + sericite. Late supergene minerals include sec-ondary oxide minerals of limonite + hematite + hydrohematite +malachite + azurite.

The prograde garnet and pyroxene occur as bands of varying width(Fig. 3a), in veins, or disseminated in altered rocks. Garnet in the bandedskarn is reddish brown to pale yellow color, fine-grained, euhedral,and equigranular. The prograde clinopyroxene is subhedral andequigranular. Garnet in the disseminated and veined skarn is brownishred, subhedral, and non-equigranular; large grains are zoned (Fig. 4a).Coexisting clinopyroxene is subhedral–anhedral and non-equigranular.

The most prominent feature of the retrograde stage is the formationof a large amount ofmagnetite.Magnetite occurs in the following forms:(a) as a matrix cementing together fragments of altered dacite, tuff, andgarnet–diopside skarn (Fig. 3b); (b) alternating bands with white calcite(Fig. 3c and d) where the magnetite has crystallized towards the centerof the vein walls of a tensional fracture while coarse-grained calcite,


pyrrhotite, pyrite, and loellingite have filled the vein center (Fig. 3e);and (c) subhedral magnetite megacrysts that have formed in locally de-veloped large voids, along with epidote and late-stage calcite (Fig. 3f–h).The Dunde iron–zinc deposit is unusual in that the coarse-grainedeuhedral magnetite is mostly a combination of pentagonal dodecahe-dra/octahedra with only minor octahedra as compared with other irondeposits in the AIMB. Magnetite has locally replaced early stage pyrox-ene (Fig. 4c and d) and garnet (Fig. 4i) or has spinel trellis exsolutionalong the three cleavage directions of the host mineral (Fig. 4j). Themagnetite can be divided into two sub-stages: (a) early stage magnetitethat is fine-grained and disseminated in or occurs as black bands withindiopside–garnet banded skarn; and (b) late stage magnetite that iscoarse-grained and occurs as veins cutting early banded ore (Fig. 3i) orthat forms massive and brecciated ores.

The minerals deposited during the early sulfide stage are loellingite,arsenopyrite, and pyrite. Loellingite has replaced apatite along itscleavage (Fig. 4g and h) or is distributed in fractures in the magnetite(Fig. 4k). Sphalerite, pyrrhotite, chalcopyrite, and pyrite are anhedral


image of Fig.�4


Fig. 5. Paragenesis of the ore and gangue minerals.


and were deposited after loellingite and arsenopyrite. Sphaleritethat has replaced loellingite and arsenopyrite has produced jaggededges to these minerals (Fig. 4l), or is present as exsolution lamellae(Fig. 4o), patchy, and trellis textures (Fig. 4n) within pyrrhotite andchalcopyrite.

Chlorite, calcite, and traces of sericite crystallized during the chlo-rite–calcite stage. Early garnet, diopside, and magnetite were replacedand enclosed by calcite and chlorite (Fig. 4b, c, and f). Chlorite is charac-terized by its radial texture, colorless to light green color (with low Fecontent), gray–white or blue interference colors, and oblique extinction.

Supergene minerals were formed after iron mineralization by sec-ondary oxidation of the primary ore and occur on the surface or incracks near the surface of the concealed orebody. The supergene min-erals are limonite, hematite, hydrohematite, malachite, and azurite.

4. Analytical methods

The basaltic and andesitic tuff samples are wallrock of orebody 7 inadit 3788 with very weak chlorite alteration, dacite samples from thesurface of orebody 6, K-feldspar granite samples from outcrop in eastof the deposit. All the ore samples are from orebody 4, 5, 7 in adit3788 and orebody 7 in adit 3912.

4.1. Geochemical composition of igneous rocks

Themajor, trace and rare-earth element analyseswere carried out intheNational Research Center for Geoanalysis (NRCG), Chinese Academyof Geological Sciences, Beijing, China. Major elements such as SiO2,Al2O3, TFe2O3, Na2O, K2O, CaO, MgO, TiO2, MnO and P2O5 were testedusing a 3080E X-ray fluorescence spectrometer with relative standarddeviation (RSD) b2–8%; FeO by the titration method (RSD b 10%);


CO2 by mercury sulfate solution Heat-outgassing (RSD b 8%); H2O+

is the weight difference of a double ball glass tube before and afterburning-condensation; Loss On Ignition (LOI) is the weight differ-ence between burning and very high temperature (1000 °C) heating(RSD b 5%).

Trace and rare-earth elements were analyzed by a X-series induc-tively coupled plasma mass spectrometry (ICP-MS; RSD b 2–10%).

4.2. Zircon U–Pb dating

Zircon grains were separated using conventional heavy liquid andmagmatic techniques, then mounted in epoxy and polished. Photomi-crography were taken under cathodoluminescence (CL), transmittedand reflected light to examine the inner structures, fluid inclusionsand cracks for further selecting the appropriate test points. The U–Pbisotope analyses were dong using a Finnigan Neptune inductivelycoupled plasma mass spectrometry that connected to a NewWaveUP-213 laser ablation at the LA–ICP-MS laboratory of Institute ofMineral Resources, Chinese Academy of Geological Sciences. Detailsabout the instrument are described by Hou et al. (2007, 2008). Thelaser beam is with a spot diameter of 25 μm, frequency of 10 Hz andenergy density of about 2.5 J/cm2. Helium gas was used as carrier gas toenhance transport efficiency of the ablated materials. The external stan-dard GJ-1 was used to monitor the age of zircon and the standard M127(U: 923 × 10−6, Th: 439 × 10−6, Th/U: 0.475; Nasdala et al., 2008) wasused to calibrated U and Th concentrations. For a detailed parametersetting and analysis steps please refer to Hou et al. (2009). Data wereprocessed according to the procedure of Liu et al. (2010), and assessedusing Isoplot 3 (Ludwig, 2003). The analytical data are presented as 1σon the concordia plots. Uncertainties in mean age are quoted at the 95%confidence level.


image of Fig.�5



4.3. Electron microprobe analysis

The electron microprobe analysis was carried out at the Institute ofMineral Resources, Chinese Academy of Geological Sciences using aJEOL JXA-8230 probe instrument with 5 V accelerating voltage, 2 nAgalvanic current and 5 μm light diameter. The natural mineral or syn-thetic metal was adopted as the standard and with accuracy betterthan 0.01%.

4.4. Fluid inclusion

Dozens of fluid inclusion sections are prepared aiming at parts withmore transparent minerals of all mineralizing stages. Then it is carriedout petrographic observation under the microscope, identificationtypes and stages of the fluid inclusions, and selection the suitable fluidinclusion for microthermometry. Microthermometry was done at theMineralizaiton and Land Resources Key Laboratory of Ministry of Landand Resources using a LINKAM THMSG600 heating–freezing stage.The measureable temperature range is between −196 °C and+600 °C (precision of freezing data and homogenization temperatureis of ±0.1 °C and ±1 °C, respectively). The sections were soaked in ac-etone andwashedwith alcohol to separate the thin rockflake from slideand resin. The heating rate close to the phase transitions was 1 °C perminute throughout the measurements. Apparent salinities for H2O in-clusion are expressed as weight percent NaCl equivalent (wt.% NaClequiv.) calculated from the final ice melting temperature (Bodnar,1993). For daughter mineral bearing H2O inclusion, salinities are alsoexpressed as wt.% NaCl equiv. calculated using final melting tempera-tures hydrohalite (Sterner et al., 1988).

4.5. Stable isotope

The sulfur isotopic compositions were analyzed at the AnalyticalLaboratory of Beijing Research Institute of Uranium Geology on aFinnigan MAT-251 isotope mass spectrometer. The sulfide sampleswere directly oxidized to SO2 to feed into the instrument using Cu2O(Robinson and Kusabe, 1975). The results are expressed relative to in-ternational standards CDT with precision better than 0.2‰.

5. Analytical results

5.1. Geochemistry of the volcanic and intrusive rocks

The volcanic rocks can be classified using a total alkalis–silicadiagram as trachybasalt, basaltic trachyandesite, dacite, and rhyolite,whereas the K-feldspar granite is rhyolitic in composition (Fig. 6a). AK2O–SiO2 diagram shows that the volcanic rocks are calc-alkalineto high-K calc-alkaline and the K-feldspar granite is high-K calc-alkaline (Fig. 6b). In a plot of A/NK versus A/CNK (A/CNK = molarAl2O3/[CaO + K2O + Na2O]), the tuff samples are metaluminous, thedacite samples vary from metaluminous to slightly peraluminous, andthe K-feldspar granite samples plot at themetaluminous–peraluminousboundary (Fig. 6c). Chondrite-normalized rare earth element (REE)patterns are light REE enriched without δEu anomalies for the tuff,light REE enriched with flat heavy REE patterns and moderate negativeδEu (0.54–0.80) anomalies for the dacite, and light REE enriched withflat heavy REE patterns andmarkednegative δEu (0.09–0.31) anomaliesfor the K-feldspar granite (Fig. 6d; Table 1). Relative to primitivemantle,all the samples are enriched in incompatible trace elements. The tuffsamples have negative Ta and Nb anomalies. The dacite samples areenriched in Cs, Rb, Ba, U, and Th, but have marked negative Ba, Tb, andNb anomalies. The K-feldspar granite is most enriched in Rb, U, and Th,but has the largest negative Ba, Ta, Nb, and Sr anomalies (Fig. 6e). In atriangular Th–Hf–Ta plot, the basaltic and andesitic tuff samples fallin the field of island arc calc-alkaline basalts (Fig. 6f). However, in aplot of factor R1 versus R2, the K-feldspar granite samples fall in the


post-orogenic field (Fig. 6h), which is similar to where they plot(intraplate field) on a Rb versus Y + Nb diagram.

5.2. Zircon U–Pb ages of the volcanic and intrusive rocks

Most zircon grains from the dacite (sample DD-145) are colorless,euhedral, and exhibit clear oscillatory zoning in CL images (Fig. 7a).The zircons are short and columnar, are N100 μm in length, and havea length:width ratio from 1.2:1 to 2:1. Th/U ratios of the analyzed zir-cons vary from 0.58 to 1.02 (Table 2), which is indicative of a magmaticorigin (e.g., Rubatto, 2002). 206Pb/238U apparent ages of 11 of a total19 analyses vary between 314 and 323 Ma, yielding a weightedmean 206Pb/238U age of 317.7 ± 2.3 Ma (MSWD = 2.1) (Fig. 8a).However, 9 of the 11 analyses yield a weighted mean 206Pb/238Uage of 316.0 ± 1.7 Ma (MSWD = 0.89). Analysis 1.14 (306 Ma) wasdiscarded as it results in the MSWD being high (MSWD = 3.8). Wespeculate that the other analyses with 206Pb/238U apparent agesfrom 266 to 288 Ma result from later thermal events related to theyounger K-feldspar granite (see below). Therefore, a weightedmean age of 316.0 ± 1.7 Ma is considered our best estimate of theage of the dacite.

Zircon grains from the K-feldspar granite (sample DD-117) that in-trudes the dacite are colorless and display clear oscillatory zoning inCL images (Fig. 7b). These zircons are columnar, N100 μm in length,and have a length:width ratio of 1.4:1 to 2:1. Th/U ratios of the analyzedzircons vary from 0.61 to 1.03 (Table 2). 206Pb/238U apparent ages of all18 analyses vary between 294 and 301 Ma, yielding a weighted mean206Pb/238U age of 295.75 ± 0.71 Ma (MSWD = 1.17) (Fig. 8b).

5.3. Skarn mineral chemistry

5.3.1. GarnetGarnet compositions are listed in Table 3 and shown in Fig. 9. The

garnet is grossularitic (39.32–79.62 mol%; average = 58.55 mol%)rather than andraditic (14.61–8.37 mol%; average = 35.85 mol%).The garnet pyrope + spessartine + almandine content is low(0.85–8.85 mol%; average = 5.53 mol%). Total Fe expressed as FeOvaries between 5.90 and 17.67 wt.% (average = 12.23 wt.%). MnOcontents vary from 0.27 to 1.81 wt.% (average = 0.73 wt.%). TheMn/Fe ratio ranges from 0.016 to 0.266 (average = 0.071).

5.3.2. ClinopyroxeneClinopyroxene compositions are listed in Table 4 and presented

in Fig. 10. The clinopyroxene is dominated by diopside (62.52–97.08 mol%; average = 85.26 mol%), with lesser hedenbergite(2.36–35.72 mol%; average = 13.20 mol%), and minor amounts ofjohannsenite (0.25–6.13 mol%; average = 1.53 mol%). Total Feexpressed as FeO ranges from 1.06 to 11.19 wt.% (average =3.83 wt.%). Clinopyroxene MnO contents are low (0.08 to 1.78 wt.%;average = 0.48 wt.%) and the Mn/Fe ratio varies between 0.03 and0.57 (average = 0.15).

5.3.3. AmphiboleRepresentative amphibole analyses are given in Table 5 and plotted

in Fig. 11. The amphibole is calcic and falls in the ferropargasite fieldbased on the classification of Leake et al. (1997) (Fig. 11). Amphiboleis rare in the Dunde iron–zinc deposit but has a high chlorine content(0.93 to 3.08 wt.%; average = 1.88 wt.%).

5.3.4. MagnetiteRepresentative magnetite analyses are listed in Table 6 and plotted

in Fig. 12. Al, Mn, Ti, Mg, Zn, Si, Cr, and Na are typically above detectionlimits, whereas K, Ca, Ni, Cu, and V have lower concentrations thatare generally near or below detection limits. In a Ca + Al + Mnversus Ti + V discriminant diagram for iron ore genesis (Dupuis and



0

4

8

12

37 47 57 67 77w (SiO2)%

w(N

a2O

+K

2O)%

picro-basalt

basalt basalticandesite

andesite dacite

tephrite

phono-tephrite

tephri-phonolite

phonolite

tachy-basalt

basaltic trachyandesite

trachy-andesite

trachyte rhyolitefoidite

0

1

2

3

4

5

6

45 55 65 75 85w(SiO2) %

w(K

2O)

%

Tholeiite Series

Calc-alkaline Sereis

High-Kcalc-alkaline

Series

Shoshonite Series

0.4

0.8

1.2

1.6

2

2.4

0.5 0.7 0.9 1.1 1.3

A/CNK

A/N

K

Metaluminous

Pera

lum

inou

s

Peralkaline

1

10

100

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Roc

k/C

hond

rite

1

10

100

1000

Cs Rb Ba U Th TaNb La Ce Sr Nd Zr HfSmTb Y Pb

Roc

k / P

rim

itive

man

tle

a b

c d

e f

g h

Fig. 6. Geochemical characteristics of volcanics and intrusion in the Dunde iron–zinc deposit. (a) Total alkali–silica diagram (after Le Bas et al., 1986) and dividing line for alkaline andsub-alkaline rocks (dashed line; after Irvine and Baragar, 1971). (b) The subalkaline rocks further classification using the K2O–SiO2 diagram (solid line after Peccerillo and Taylor, 1976;dashed line after Middlemost, 1985). (c) Aluminum saturation index. (d) Chondrite-normalized REE patterns (use values of Boynton, 1984). (e) Primitive mantle-normalized trace ele-ment patterns (use values of McDonough et al., 1992). (f) Th–Hf–Ta discrimination diagram of basalt (after Wood, 1980). (g) Factor R1–R2 diagram of igneous rock (after Batchelorand Bowden, 1985). (h) Rb vs. Y + Nb geotectonic discrimination diagram (Pearce et al., 1984). Solid circle-tuff samples, hollow square-dacite samples, cross-rhyolite samples, solidtriangle-K-feldspar granite samples.


Please cite this article as: Duan, S., et al., Geology, geochemistry, and geochronology of the Dunde iron–zinc ore deposit in western Tianshan,China, Ore Geol. Rev. (2013), http://dx.doi.org/10.1016/j.oregeorev.2013.08.019

image of Fig.�6


Table 1Major oxide, trace and rare earth elements for the volcanic and intrusive rocks from the Dunde Fe–Zn deposit.

Sample no. DD-86 DD-121 DD-135 DD-136 DD-147 DD-2 DD-30 DD-59 DD-117 DD-118 DD-119

Rock type D D D D D B tuff B tuff A tuff MKG MKG CKG

wt.%SiO2 70.96 67.35 70.99 66.64 71.40 50.30 48.99 54.60 74.77 75.05 76.76Al2O3 14.37 16.20 14.75 15.78 14.61 16.23 17.32 15.83 12.94 12.44 11.94TiO2 0.33 0.38 0.35 0.65 0.35 1.28 1.19 1.17 0.27 0.28 0.14CaO 2.58 3.12 1.87 4.95 1.60 7.12 8.97 6.47 1.06 1.08 1.13Fe2O3 1.02 1.09 1.06 0.70 0.46 3.11 3.97 2.95 1.11 1.02 0.57FeO 1.19 1.62 1.81 1.33 1.96 6.13 5.14 4.69 0.62 0.79 0.31K2O 3.23 1.89 3.87 2.84 3.59 1.03 1.80 1.54 5.41 5.01 4.87Na2O 4.20 4.41 3.63 4.54 4.06 4.30 3.77 4.44 3.28 3.29 2.67MgO 0.56 1.41 0.51 0.92 0.50 5.09 4.66 4.71 0.27 0.29 0.17MnO 0.06 0.04 0.06 0.06 0.07 0.25 0.29 0.22 0.04 0.04 0.03P2O5 0.08 0.11 0.08 0.11 0.08 0.66 0.53 0.54 0.05 0.05 0.01CO2 0.43 0.78 0.47 0.09 0.25 0.60 0.52 0.44 0.17 0.34 0.69H2O+ 0.90 1.68 1.00 0.96 0.94 3.22 2.26 2.54 0.44 0.46 0.66LOI 0.98 2.39 0.99 1.89 1.13 3.43 2.45 2.64 0.60 0.67 1.41Total 100.89 102.47 101.44 101.46 101.00 102.75 101.86 102.78 101.03 100.81 101.36

ppmCs 0.81 3.51 2.65 1.38 1.49 1.59 1.36 1.38 2.42Ba 470 321 540 416 525 185 419 218 360 342 140Ni 3.09 8.22 5.18 4.32 2.62 61.2 60.4 34.0 2.02 2.85 1.79Cu 7.64 32.9 54.9 5.57 8.99 2.50 21.1 5.40 26.7 11.2 4.30Zn 25.8 47.0 26.6 89.3 87.9 257 58.7 41.8 22.3 25.1 11.4Ga 12.6 17.0 12.8 14.3 15.4 18.4 21.3 19.6 14.7 14.1 15.5As 28.1 5.05 31.4 37.9 54.8 93.0 46.7 19.6 1.90 1.95 1.77Rb 103 76.8 128 99.4 121 39.8 62.1 55.4 213 185 244Sr 195 520 240 394 238 476 822 436 96.5 89.8 24.1Zr 147 100 153 214 206 237 230 264 263Hf 4.25 2.91 4.30 5.80 4.72 4.79 5.19 5.01 6.76 7.64 9.43Th 11.2 2.83 9.98 10.2 10.8 2.59 2.38 2.26 22.4 22.5 67.6U 4.74 0.99 3.35 2.72 3.35 0.72 0.69 0.68 4.97 4.86 15.1Pb 5.78 8.17 16.7 35.0 53.9 24.3 5.53 3.79 2.93 2.55 4.50Nb 5.36 2.97 4.92 6.43 5.54 11.4 11.8 11.0 9.99 9.92 15.7Ta 0.50 0.28 0.44 0.51 0.52 0.73 0.67 0.56 0.96 0.91 1.39Tl 0.56 0.25 0.56 0.47 0.64 0.20 0.34 0.28 0.61 0.60 0.74Y 18.1 9.10 17.1 22.8 18.9 26.7 26.8 30.2 60.2 55.6 44.2La 16.4 13.8 14.4 11.1 15.8 35.2 33.5 28.2 36.0 39.5 16.6Ce 32.6 25.4 28.3 27.9 28.4 77.3 72.4 61.2 78.8 88.1 31.2Pr 3.81 2.91 3.49 3.58 3.41 9.62 8.63 7.34 10.3 11.0 4.38Nd 13.9 10.6 13.0 14.8 12.4 37.8 34.0 29.9 39.0 40.8 15.9Sm 2.82 2.04 2.63 3.50 2.56 7.31 6.77 6.10 8.56 8.55 3.79Eu 0.76 0.61 0.69 0.64 0.58 2.00 2.00 1.67 0.90 0.81 0.12Gd 2.91 1.79 2.81 3.72 2.55 6.77 6.34 5.64 9.00 8.70 4.64Tb 0.48 0.27 0.44 0.58 0.44 0.93 0.92 0.91 1.59 1.46 0.96Dy 2.87 1.51 2.78 3.66 2.88 5.12 5.20 5.11 10.1 9.35 7.05Ho 0.61 0.29 0.57 0.77 0.61 0.97 0.98 1.00 2.18 1.96 1.59Er 2.04 0.89 1.92 2.32 2.00 2.76 2.91 2.96 6.91 6.34 5.35Tm 0.28 0.13 0.28 0.36 0.28 0.35 0.38 0.39 0.97 0.90 0.81Yb 2.18 0.77 2.06 2.36 2.18 2.37 2.45 2.62 6.33 6.14 5.73Lu 0.34 0.12 0.31 0.35 0.33 0.34 0.36 0.39 0.92 0.87 0.91ΣREE 82.00 61.13 73.68 75.64 74.42 188.84 176.84 153.43 211.56 224.48 99.03δEu 0.80 0.96 0.77 0.54 0.69 0.86 0.92 0.86 0.31 0.28 0.09LaN/YbN 5.07 12.08 4.71 3.17 4.89 10.01 9.22 7.26 3.83 4.34 1.95

D — dacite, B tuff — basaltic tuff, A tuff — andesitic tuff, MKG— medium grained K-feldspar granite and CKG — coarse grained K-feldspar granite.


Beaudoin, 2011), data for magnetite from the Dunde ore deposit plot inthe skarn field (Fig. 12).

5.3.5. ChloriteChlorite compositions are listed in Table 7 and presented in Fig. 13.

Chemical data for chlorite fall in the sheridanite, pycnochlorite,clinochlore, and diabantite fields using the classification scheme ofHey (1954). The chlorite basal spacing value (d001) can be obtainedfrom the equation of Nieto (1997) modified after Rausell-Colom et al.(1991): d001 = 14.339 − 0.1155AlIV − 0.0201FeII (based on 14 oxy-gen atoms). The temperature of chlorite formation can then be calculat-ed using the formula of Battaglia (1999) that describes the relationshipbetween basal spacing and temperature: t (°C) = (14.379 − d001) /0.001. Calculated temperatures of chlorite formation vary between151.5 °C and 222.2 °C (average = 194.0 °C) (Table 7).


5.4. Fluid inclusion microthermometry

A large number of fluid inclusion sections were selected fromthose ores containing transparent minerals and representing differ-ent mineralizing stages. Petrographic observations at room temper-ature revealed that: (1) no fluid inclusions are present in diopside,apatite, and epidote, whereas some isolated, irregular, H2O fluid in-clusions are present in garnet (Fig. 14a), although microthermometricresults indicate that these fluid inclusions are secondary; (2) densefluid inclusions are present in several narrow quartz veins but aretoo small to investigate further; (3) there are abundant fluid inclu-sions in calcite, although it is difficult to determine whether these in-clusions are related to the sulfide or chlorite–carbonate stages. Fluidinclusions in garnet and calcite were selected for microthermometryinvestigation.



Fig. 7. Cathodoluminescence images of zircon grains in dacite sample (DD-145) and K-feldspar granite sample (DD-117). The circles are analysis spots marked with ages.


Fluid inclusions in calcite include daughter-mineral-bearing H2Oinclusions, H2O inclusions, and pure H2O inclusions. These inclusionsare rectangular, oval, or irregularly shaped, and are either isolated

Table 2LA–ICP-MS U–Pb analysis results for zircon of the volcanic and intrusive rocks from the Dunde

Spot no. Concentration(×10−6) Th/U 206Pb/238U 207Pb/235U 207Pb/

206PbC Th U Ratio 1σ Ratio 1σ Ratios

DD-1451.1 202.6 128.86 177.56 0.73 0.04452 0.00042 0.3311 0.01295 0.05401.2 134.14 80.71 112.06 0.72 0.04212 0.00049 0.30634 0.00794 0.05281.3 155.73 67.52 101.75 0.66 0.04239 0.00051 0.31925 0.01242 0.05461.4 73.61 93.58 157.56 0.59 0.04521 0.0003 0.33702 0.00446 0.05411.5 197.1 99.42 159.05 0.63 0.04566 0.00036 0.34354 0.00696 0.05451.6 156.36 83.75 144.02 0.58 0.0512 0.00022 0.3872 0.00714 0.05481.7 170.04 114.65 139.85 0.82 0.05033 0.0004 0.37865 0.01208 0.05451.8 88.04 65.67 77.02 0.85 0.05293 0.00131 0.42902 0.03403 0.05871.9 89.24 24.93 38.88 0.64 0.05103 0.00089 0.39216 0.02448 0.05561.10 130.73 42.2 61.58 0.69 0.04989 0.00039 0.37827 0.00827 0.05491.11 114.64 34.44 49.31 0.7 0.04567 0.00052 0.33837 0.0124 0.05391.12 0.88 25.14 38.58 0.65 0.05096 0.00065 0.39304 0.01305 0.05591.13 333.25 109.52 109.96 1 0.05053 0.00052 0.36419 0.00647 0.05231.14 297.07 104.52 102.11 1.02 0.04856 0.00041 0.3663 0.00572 0.05461.15 243.55 63.14 100.82 0.63 0.04984 0.00032 0.368 0.00513 0.05351.16 354.32 131.2 158.19 0.83 0.05006 0.00033 0.37767 0.00442 0.05461.17 248.34 72.58 113.52 0.64 0.05064 0.0003 0.37375 0.00448 0.05351.18 156.81 57.18 64.79 0.88 0.05138 0.00142 0.35771 0.01551 0.05051.19 261.47 37.74 49.65 0.76 0.04993 0.00075 0.35281 0.01312 0.0511

DD-1172.1 374.67 163.89 269.67 0.61 0.0468 0.00019 0.33843 0.00315 0.05252.2 250.03 116.57 177.17 0.66 0.04675 0.00021 0.34161 0.00346 0.05302.3 146 98.61 144.31 0.68 0.04676 0.0002 0.34195 0.00406 0.05312.4 336.87 193.88 216.35 0.9 0.04666 0.0002 0.33959 0.00276 0.05282.5 128.3 112.34 149.31 0.75 0.04679 0.00021 0.34079 0.00397 0.05292.6 202.11 140.66 210.3 0.67 0.04702 0.00021 0.33931 0.00321 0.05242.7 154.37 152.26 158.07 0.96 0.04664 0.00023 0.33537 0.00385 0.05222.8 254.77 257.89 280.82 0.92 0.04727 0.0002 0.34094 0.00282 0.05232.9 200.06 250.29 265.4 0.94 0.04708 0.00024 0.33952 0.00293 0.05232.10 236.8 333.21 340.68 0.98 0.04762 0.00026 0.34352 0.00364 0.05222.11 88.26 151.58 196.82 0.77 0.04694 0.00025 0.34376 0.00417 0.05312.12 118.67 196.26 238.39 0.82 0.04712 0.00029 0.34263 0.00361 0.05272.13 78.35 128.01 154.43 0.83 0.04716 0.00027 0.34028 0.00409 0.05232.14 88.31 131.11 166.65 0.79 0.04723 0.00029 0.34189 0.00403 0.05252.15 110.12 200.47 195.57 1.03 0.04685 0.00028 0.33726 0.00403 0.05212.16 79.57 127.31 154.66 0.82 0.04701 0.0003 0.34116 0.00412 0.05262.17 34.32 81.58 107.23 0.76 0.04782 0.00027 0.34719 0.0051 0.05282.18 3.94 103.24 148.54 0.7 0.04701 0.00104 0.34113 0.00777 0.0815


(Fig. 14b, d, and e), form clusters (Fig. 14c and f), or occur along frac-tures. The isolated and clustered inclusions appeared to be primaryfluid inclusions and were selected for analysis. Microthermometry

Fe–Zn deposit.

206Pb 208Pb/232Th 206Pb/238U 207Pb/235U 207Pb/206Pb

1σ Ratio 1σ Age (Ma) 1σ Age (Ma) 1σ Age (Ma) 1σ

9 0.00232 0.00186 0.00048 280.8 2.6 290.4 9.9 376 100.97 0.00135 0.00326 0.00056 266 3.0 271.3 6.2 324.1 623 0.00198 0.00434 0.00068 267.6 3.1 281.3 9.6 398.2 76.8

0.00068 0.0032 0.0004 285.1 1.8 294.9 3.4 376 60.21 0.00099 0.00297 0.00039 287.8 2.2 299.8 5.3 390.8 40.73 0.00099 0.00392 0.00047 321.9 1.4 332.3 5.2 405.6 45.41 0.00164 0.00357 0.00055 316.5 2.5 326 8.9 390.8 66.7

0.0041 0.00523 0.00261 332.5 8.0 362.5 24.2 566.7 153.78 0.00322 0.01137 0.00436 320.8 5.4 335.9 17.9 438.9 129.65 0.00108 0.00883 0.00108 313.8 2.4 325.8 6.1 409.3 44.46 0.00215 0.0099 0.00186 287.9 3.2 295.9 9.4 368.6 90.71 0.00167 0.01 0.00225 320.4 4.0 336.6 9.5 450 66.73 0.00084 0.00476 0.00046 317.8 3.2 315.3 4.8 298.2 41.75 0.00064 0.00401 0.00039 305.6 2.5 316.9 4.2 398.2 30.69 0.00071 0.00587 0.00064 313.5 2.0 318.2 3.8 353.8 29.66 0.00046 0.00367 0.00033 314.9 2.0 325.3 3.3 398.2 13.98 0.00062 0.00527 0.00046 318.5 1.8 322.4 3.3 353.8 30.64 0.00211 0.00509 0.00204 323 8.7 310.5 11.6 220.4 100.96 0.00151 0.01258 0.00245 314.1 4.6 306.8 9.8 255.6 66.7

0.00043 0.00237 0.00033 294.9 1.2 296 2.4 305.6 18.55 0.00047 0.00276 0.00038 294.5 1.3 298.4 2.6 331.5 20.4

0.00058 0.00275 0.00038 294.6 1.2 298.6 3.1 331.5 28.77 0.00039 0.00215 0.00027 294 1.2 296.9 2.1 324.1 16.71 0.00059 0.00251 0.00034 294.8 1.3 297.8 3 324.1 25.9

0.00045 0.00223 0.0003 296.2 1.3 296.6 2.4 301.9 20.45 0.00058 0.0019 0.00027 293.8 1.4 293.7 2.9 298.2 −6.54 0.00038 0.00168 0.00024 297.7 1.2 297.9 2.1 301.9 16.75 0.00041 0.00167 0.00026 296.6 1.4 296.8 2.2 301.9 18.59 0.00046 0.00183 0.00032 299.9 1.6 299.8 2.7 298.2 18.5

0.00056 0.00198 0.00031 295.7 1.5 300 3.1 331.5 28.77 0.00048 0.00215 0.0003 296.8 1.8 299.2 2.7 320.4 20.46 0.00057 0.00239 0.0003 297 1.7 297.4 3.1 301.9 24.14 0.00056 0.00293 0.00033 297.5 1.8 298.6 3 309.3 24.11 0.00049 0.00186 0.00018 295.1 1.8 295.1 3.1 300.1 −5.66 0.00057 0.00276 0.00026 296.2 1.8 298.1 3.1 322.3 19.48 0.00083 0.00309 0.00031 301.1 1.7 302.6 3.8 324.1 39.81 0.0234 0.00175 0.00036 296.1 6.4 298 5.9 1235.2 556.3


image of Fig.�7


Fig. 8. LA–ICP-MS U–Pb ages and concordia diagrams (a refers to sample DD-145 andb refers to sample DD-117).


results are summarized in Table 8 and shown in Fig. 15. The H2O inclu-sions typically had a gas volume of 5% to 30%, wide homogenizationtemperatures from 147 °C to 367 °C, and salinities of 2.4–23.4 wt.%NaCl equivalent. The daughter-mineral-bearingH2O inclusions typicallyhad gas volumes of 5% to 10% and homogenization temperaturesfrom 172 °C to 347 °C. The daughter minerals are cubic in shapeand were identified as being hydrohalite. However, many of thedaughter minerals had a dissolution temperature higher than the ho-mogenization temperatures of the host inclusions, particularly forthe low-homogenization-temperature inclusions. This may implythat theseminerals are not actually true daughter minerals that crystal-lized from the liquid of the host inclusions, but were in fact trappedfrompreviously crystallized salt. Those inclusions forwhich the dissolu-tion temperatures of the daughter mineral were lower than the inclu-sion homogenization temperature have salinities of 31.9–33.0 wt.%NaCl equivalent.

Fluid inclusions in garnet are irregular in shape, have gas volumesfrom 5% to 10%, liquid homogenization temperatures of 163 °C to208 °C, and salinities of 6.3–9.9 wt.% NaCl equivalent. These garnetinclusions are classified as being secondary, given their low homog-enization temperatures.


5.5. Sulfur isotope geochemistry

A total of nine sulfide samples were analyzed and yielded δ34Svalues from 3.8‰ to 8.1‰ (average = 6.8‰; Table 9). Three pyr-rhotite samples gave δ34S values from 3.8‰ to 8.1‰ (average =5.8‰). Three sphalerite samples gave δ34S values from 6.5‰to 7.1‰ (average = 6.7‰), whereas two pyrite samples and oneloellingite sample have δ34S values of 7.6‰–8.0‰ and 7.8‰, re-spectively. δ34S data for the nine samples are normally distributed(Fig. 16).

6. Discussion

6.1. Geochronological significance of the volcanic and intrusive rocks

The volcanic–volcaniclastic rocks in the Dunde iron–zinc deposithave previously been classified as part of the early CarboniferousDahalajunshan Formation (Feng et al., 2010). However, zircon LA–ICP-MS U–Pb dating of the dacite yields a weighted mean 206Pb/238U ageof 316.0 ± 1.7 Ma that is late Carboniferous rather than early Carbonif-erous in age. These dating results confirm our previous understandingof the Dahalajunshan Formation, in that it includes late Carboniferousvolcanic rocks (An and Zhu, 2008; Jiang et al., 2012a; Zhai et al., 2006;Zhu et al., 2005, 2006a,b).

Late Carboniferous volcanic rocks are also present in other iron de-posits in the AIMB. For example, the wall rock of the Chagangnuoer de-posit contains rhyolite dated at 301.8 ± 0.9 Ma (Jiang et al., 2012a).Dacite of late Carboniferous age (300.3 ± 1.1 Ma; Jiang et al., 2012a)has also been reported in the Zhibo iron deposit, in addition to early Car-boniferous rocks intruded by granites with ages of 320.3 ± 2.5 to294.5 ± 1.6 Ma (Zhang et al., 2012b). The wall rock of the Beizhaniron deposit comprises dacite and rhyolite with ages of 301.3 ± 0.8and 303.7 ± 0.9 Ma, respectively (Zhang et al., 2012d). These chrono-logical results demonstrate that the volcanic–volcaniclastic rocks thathost the iron deposits in the AIMB may be the product of multiplephases of early to late Carboniferous magmatism. Thus, we cautionagainst considering all the volcanic wall rocks of the iron deposits inthe AIMB as being the same unit(s), as their lithological similaritiesmay be purely coincidental. As such, the status of the “DahalajunshanFormation” needs to be reassessed.

The age of the Dunde iron–zinc deposit is constrained to be youngerthan 316.0 ± 1.7 Ma, given that the orebodies occur in fractures in thedacite. The K-feldspar granite in the Dunde ore deposit has a zirconLA–ICP-MS weighted mean 206Pb/238U age of 295.75 ± 0.71 Ma, al-though it is not possible to determinewhether the granite is geneticallyassociated with the iron–zinc mineralization. However, the age of theDunde iron–zinc deposit can be inferred from other similar deposits inthe AIMB that may have formed contemporaneously in relation tothe same geological event (Zhang et al., 2012b). A zircon LA–ICP-MSU–Pb dating study of the disseminated magnetite-mineralized dioritein the Wuling iron deposit yielded a weighted mean 206Pb/238U ageof 307.7 ± 0.8 Ma (MSWD = 0.77) (Duan et al., unpubl. data). ASm–Nd isochron of seven garnet samples from the Chagangnuoeriron deposit resulted in an age of 316.8 ± 6.7 Ma (Hong et al.,2012c). Zhang et al. (2012b) reported that a granite dike with anage of 320.3 ± 2.5 Ma cross-cuts the no. 15 orebody of the Zhiboiron deposit. However, on the basis of field observations, we suggestthat there are two types of granite dikes that are spatially related tothe orebodies. One type pre-dates mineralization and has chloriteveins and alteration, and may correspond to the dikes with ages of320.3 ± 2.5 Ma of Zhang et al. (2012b) (a picture in their papershows the granite dike is occurred with epidote–chlorite alteration).The other type is represented by granite porphyry and diorite dikesthat post-date mineralization and have no discernable alteration.The granite porphyry dikes are porphyritic, K-feldspar- and albite-phyric, and have a microcrystalline matrix, which may correspond


image of Fig.�8


Table 3Electron microprobe analyses results of the representative garnet from the Dunde Fe–Zn deposit.

Sample Oxide composition (wt.%) Cations on the basis of 12 oxygen atoms Garnet components (%)

SiO2 TiO2 Al2O3 Cr2O3 FeOa MnO MgO CaO Total Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Ad Gr Prp Sps Alm Prp + Sps + Alm

DD-45-1 39.55 0.03 18.24 bdl 5.90 0.77 0.08 35.19 99.76 3.02 0.00 1.64 0.00 0.34 0.04 0.05 0.01 2.88 17.20 79.62 0.29 1.67 1.21 3.17DD-45-2 38.31 bdl 16.23 0.02 7.96 0.67 0.11 35.06 98.36 2.99 0.00 1.49 0.00 0.51 0.01 0.04 0.01 2.94 25.34 72.28 0.44 1.47 0.41 2.32DD-75-1 39.79 0.06 18.39 0.03 6.60 1.43 0.34 33.04 99.68 3.04 0.00 1.66 0.00 0.31 0.11 0.09 0.04 2.71 15.69 75.93 1.32 3.15 3.83 8.30DD-75-2 38.85 bdl 18.68 bdl 6.23 1.47 0.40 33.03 98.66 3.01 0.00 1.70 0.00 0.29 0.11 0.10 0.05 2.74 14.61 76.90 1.54 3.22 3.74 8.50DD-75-2 38.04 0.01 17.62 bdl 6.80 1.81 0.39 32.19 96.86 3.01 0.00 1.64 0.00 0.35 0.10 0.12 0.05 2.73 17.66 73.48 1.55 4.05 3.25 8.85DD-75-4 38.52 0.02 12.16 0.19 13.39 0.85 0.31 33.30 98.74 3.04 0.00 1.13 0.01 0.83 0.05 0.06 0.04 2.81 42.11 52.38 1.22 1.92 1.77 4.91DD-75-5 38.02 0.11 11.33 0.04 14.08 0.36 0.35 34.32 98.61 3.01 0.01 1.06 0.00 0.93 0.00 0.02 0.04 2.91 46.68 50.85 1.37 0.82 0.15 2.34DD-87-1 38.30 bdl 9.85 bdl 16.32 0.35 0.22 34.66 99.70 3.02 0.00 0.92 0.00 1.07 0.00 0.02 0.03 2.93 53.93 44.28 0.88 0.78 0.14 1.80DD-87-2 39.09 bdl 13.09 bdl 12.78 0.83 0.27 34.70 100.76 3.02 0.00 1.19 0.00 0.80 0.03 0.05 0.03 2.87 40.12 56.11 1.03 1.81 0.93 3.77DD-87-3 38.58 0.03 13.14 bdl 12.59 1.08 0.33 34.55 100.30 3.00 0.00 1.20 0.00 0.80 0.02 0.07 0.04 2.87 39.93 55.83 1.27 2.36 0.62 4.25DD-87-4 39.07 bdl 12.87 bdl 12.99 0.82 0.25 34.73 100.73 3.02 0.00 1.17 0.00 0.82 0.02 0.05 0.03 2.88 41.04 55.42 0.95 1.80 0.80 3.55DD-87-5 39.33 0.12 13.59 0.07 12.27 1.04 0.22 34.28 100.92 3.03 0.01 1.23 0.00 0.74 0.05 0.07 0.03 2.83 37.47 57.55 0.86 2.28 1.62 4.76DD-87-6 39.79 0.02 13.41 bdl 12.26 1.29 0.33 33.52 100.62 3.06 0.00 1.22 0.00 0.74 0.05 0.08 0.04 2.76 37.83 56.36 1.29 2.87 1.66 5.82DD-87-7 38.88 bdl 8.84 0.09 17.67 0.29 0.30 34.56 100.63 3.04 0.00 0.82 0.01 1.15 0.01 0.02 0.04 2.90 58.37 39.32 1.20 0.64 0.19 2.03DD-98-1 39.45 0.03 15.93 bdl 9.51 0.44 0.40 34.39 100.15 3.03 0.00 1.44 0.00 0.54 0.07 0.03 0.05 2.83 27.27 67.85 1.54 0.96 2.35 4.85DD-123-1 38.15 0.16 13.30 0.03 13.04 0.64 0.12 33.33 98.77 3.01 0.01 1.24 0.00 0.75 0.11 0.04 0.01 2.82 37.74 56.58 0.46 1.44 3.68 5.58DD-123-2 39.12 0.10 14.56 0.02 11.96 0.60 0.17 32.71 99.24 3.05 0.01 1.34 0.00 0.63 0.15 0.04 0.02 2.73 31.84 60.86 0.65 1.34 5.25 7.24DD-123-3 38.32 0.12 12.97 0.02 14.57 0.66 0.19 32.60 99.45 3.01 0.01 1.20 0.00 0.79 0.17 0.04 0.02 2.74 39.52 52.44 0.73 1.48 5.76 7.97DD-123-4 38.68 0.16 13.78 0.03 12.60 0.66 0.14 33.33 99.38 3.02 0.01 1.27 0.00 0.71 0.12 0.04 0.02 2.79 35.77 58.23 0.54 1.46 3.92 5.92DD-123-5 38.70 0.10 15.01 0.05 11.64 0.47 0.10 33.43 99.50 3.01 0.01 1.38 0.00 0.61 0.15 0.03 0.01 2.79 30.56 62.84 0.37 1.03 5.06 6.46DD-123-6 38.28 0.07 14.77 0.01 11.81 0.48 0.11 33.15 98.68 3.01 0.00 1.37 0.00 0.62 0.15 0.03 0.01 2.79 31.32 62.03 0.43 1.08 5.10 6.61DD-123-7 39.17 0.04 15.46 0.01 11.68 0.77 0.14 32.59 99.86 3.03 0.00 1.41 0.00 0.56 0.19 0.05 0.02 2.70 28.52 62.72 0.53 1.70 6.51 8.74DD-123-8 37.83 0.12 13.70 bdl 12.95 0.63 0.10 32.80 98.13 3.00 0.01 1.28 0.00 0.71 0.15 0.04 0.01 2.79 35.71 57.53 0.39 1.42 4.93 6.74DD-123-9 38.21 0.12 13.83 0.03 13.30 0.62 0.14 32.58 98.83 3.01 0.01 1.28 0.00 0.70 0.17 0.04 0.02 2.75 35.33 56.81 0.56 1.38 5.83 7.77DD-123-10 38.66 0.11 13.93 bdl 12.64 0.47 0.10 33.78 99.69 3.01 0.01 1.28 0.00 0.71 0.12 0.03 0.01 2.82 35.61 59.06 0.37 1.05 3.92 5.34DD-123-11 37.79 0.14 12.03 0.03 14.21 0.56 0.13 33.15 98.04 3.01 0.01 1.13 0.00 0.85 0.09 0.04 0.02 2.83 42.99 51.98 0.51 1.28 3.15 4.94DD-123-12 38.21 0.04 14.60 0.06 11.91 0.77 0.13 32.89 98.61 3.01 0.00 1.35 0.00 0.64 0.15 0.05 0.01 2.77 31.87 60.77 0.49 1.71 4.99 7.19DD-123-13 38.34 0.08 14.04 0.03 12.26 0.61 0.14 33.14 98.64 3.02 0.00 1.30 0.00 0.68 0.13 0.04 0.02 2.79 34.28 59.49 0.54 1.35 4.25 6.14DD-123-14 38.59 0.05 14.29 bdl 11.98 0.46 0.08 33.51 98.96 3.02 0.00 1.32 0.00 0.66 0.12 0.03 0.01 2.81 33.44 61.14 0.31 1.02 4.09 5.42DD-123-15 38.00 0.05 12.19 0.02 15.41 0.74 0.22 32.35 98.98 3.01 0.00 1.14 0.00 0.85 0.17 0.05 0.03 2.74 42.84 48.99 0.87 1.66 5.60 8.13

a Total iron as FeO; bdl = below detection limit; Ad = andradite, Gr = grossular, Prp = pyrope, Sps = spessartine, Alm = almandine.

13S.D

uanetal./O

reGeology

Reviewsxxx

(2013)xxx–xxx

Pleasecite

thisarticle

as:Duan,S.,et

al.,Geology,geochem

istry,andgeochronology

ofthe

Dunde

iron–zinc

oredeposit

inwestern

Tianshan,China,O

reGeol.Rev.(2013),http://dx.doi.org/10.1016/j.oregeorev.2013.08.019


Fig. 9. Ternary plots of garnet composition from the Dunde iron–zinc deposit and othermajor skarn types. The data for Sawusi are from Liu et al. (2012), for Chagangnuoerfrom Hong et al. (2012a), for Yamansu from Hou et al. (2013), and for world typical ironskarn from Meinert et al. (2005).

Fig. 10. Ternary plots of clinopyroxene composition from theDunde iron–zinc deposit andother major skarn types. The data for Chagangnuoer are from Hong et al. (2012a), and forworld typical iron skarn are from Meinert et al. (2005).


to the 294.5 ± 1.6 Ma granite dike described by Zhang et al. (2012).The diorite dikes may be similar to the diorite dike with an age of305.0 ± 1.1 Ma described by Jiang et al. (2012a). All these lines ofevidence suggest that the Zhibo iron-ore mineralizing event tookplace between 320 and 305 Ma. Furthermore, a zircon LA–ICP-MSweighted mean 206Pb/238U age of 299.2 ± 1.4 Ma (MSWD = 0.81)

Table 4Electron microprobe analyses results of the representative clinopyroxene from the Dunde Fe–Z

Sample Oxide composition (wt.%) Catio

SiO2 TiO2 Al2O3 FeOa MnO MgO CaO Na2O K2O Total Si

DD-45-1 50.73 0.01 4.58 6.42 0.26 13.61 25.55 0.03 bdl 101.17 1.87DD-75-1 54.59 0.03 0.17 1.06 0.18 17.76 25.87 0.02 0.01 99.70 1.99DD-75-2 52.22 bdl 0.16 1.23 0.19 16.32 24.06 0.01 bdl 94.19 2.01DD-87-1 55.22 bdl 0.97 2.20 0.50 16.41 25.26 bdl 0.01 100.56 2.00DD-87-2 52.62 bdl 2.32 3.30 1.15 15.26 25.52 bdl 0.02 100.19 1.93DD-87-3 53.76 0.06 2.66 2.70 0.97 15.58 24.83 0.01 bdl 100.57 1.95DD-87-4 54.48 bdl 1.21 3.58 0.75 15.79 25.16 bdl 0.01 100.98 1.98DD-87-5 54.47 bdl 0.30 7.61 0.54 13.04 24.72 0.09 bdl 100.77 2.01DD-87-6 54.24 bdl 1.44 2.84 0.95 16.17 24.50 0.03 0.02 100.19 1.98DD-87-7 54.63 bdl 0.30 7.84 0.62 12.83 24.77 0.09 0.01 101.09 2.02DD-98-1 54.59 bdl 0.46 3.01 0.27 16.22 25.49 bdl bdl 100.04 1.99DD-98-2 55.30 bdl 0.11 2.69 0.13 17.27 25.73 0.02 bdl 101.24 1.99DD-98-3 55.44 0.03 0.14 2.70 0.19 16.99 25.85 bdl 0.01 101.34 2.00DD-98-4 53.89 0.02 0.39 2.94 0.08 16.74 25.85 0.02 bdl 99.91 1.98DD-98-5 53.48 bdl 0.10 3.11 1.78 13.69 25.13 0.09 0.02 97.39 2.02DD-100-1 54.27 bdl 0.06 3.03 0.14 16.54 26.04 bdl bdl 100.09 1.99DD-100-2 53.92 0.01 0.61 4.05 0.10 15.80 25.65 bdl 0.01 100.16 1.98DD-100-3 55.26 bdl 0.01 2.27 0.16 16.63 25.71 0.04 bdl 100.08 2.01DD-142-1 54.73 0.03 0.37 2.78 0.20 16.77 25.83 0.01 bdl 100.72 1.99DD-142-2 51.76 0.02 0.18 11.19 0.53 10.77 24.59 0.08 bdl 99.12 1.99

a Total iron as FeO; bdl = below detection limit; Di = diopside, Hd = hedenbergite, Jo =


(Duan et al., unpubl. data) has been obtained for the disseminatedmagnetite-mineralized dioritic porphyry dikes that intrude thewall rock dacite of the Beizhan iron deposit. This age is consistentwithin analytical uncertainties with the age of the dacite (301.3 ±0.8 Ma; Zhang et al., 2012d), which indicates that the dioritic por-phyry dikes may be a coeval, shallower expression of a deeper

n deposit.

ns on the basis of 6 oxygen atoms Clinopyroxenecomponents (%)

AlIV AlVI Ti Fe3+ Fe2+ Mn Mg Ca Na K Di Hd Jo

0.15 0.05 0.00 0.10 0.10 0.01 0.75 1.01 0.00 0.00 74.30 24.89 0.810.01 0.00 0.00 0.03 0.01 0.01 0.96 1.01 0.00 0.00 97.08 2.36 0.560.00 0.01 0.00 0.00 0.04 0.01 0.94 0.99 0.00 0.00 94.64 4.72 0.640.00 0.04 0.00 0.00 0.07 0.02 0.88 0.98 0.00 0.00 88.07 10.39 1.530.07 0.03 0.00 0.05 0.05 0.04 0.84 1.01 0.00 0.00 83.53 12.88 3.580.05 0.07 0.00 0.00 0.08 0.03 0.84 0.97 0.00 0.00 82.46 14.63 2.920.02 0.03 0.00 0.00 0.11 0.02 0.85 0.98 0.00 0.00 84.06 13.66 2.280.00 0.01 0.00 0.00 0.24 0.02 0.72 0.98 0.01 0.00 73.05 25.24 1.710.02 0.04 0.00 0.00 0.09 0.03 0.88 0.96 0.00 0.00 84.98 12.16 2.850.00 0.01 0.00 0.00 0.24 0.02 0.71 0.98 0.01 0.00 72.00 26.03 1.970.01 0.01 0.00 0.00 0.09 0.01 0.88 1.00 0.00 0.00 88.52 10.66 0.820.00 0.00 0.00 0.02 0.07 0.00 0.93 0.99 0.00 0.00 92.07 7.55 0.380.00 0.00 0.00 0.00 0.08 0.01 0.91 1.00 0.00 0.00 91.00 8.44 0.570.02 0.00 0.00 0.05 0.04 0.00 0.91 1.02 0.00 0.00 92.20 7.55 0.250.00 0.00 0.00 0.00 0.10 0.06 0.77 1.02 0.01 0.00 82.84 11.02 6.130.00 0.00 0.00 0.03 0.06 0.00 0.90 1.02 0.00 0.00 91.73 7.82 0.440.02 0.00 0.00 0.02 0.10 0.00 0.86 1.01 0.00 0.00 86.93 12.75 0.320.00 0.00 0.00 0.00 0.07 0.00 0.90 1.00 0.00 0.00 92.38 7.11 0.510.02 0.00 0.00 0.02 0.07 0.01 0.91 1.00 0.00 0.00 90.93 8.45 0.630.01 0.00 0.00 0.03 0.33 0.02 0.62 1.01 0.01 0.00 62.52 35.72 1.76

johannsenite.


image of Fig.�9

image of Fig.�10


Table5

Electron

microprob

ean

alyses

resu

ltsof

therepresen

tative

amph

ibolefrom

theDun

deFe

–Zn

depo

sit.

Sample

Oxide

compo

sition

(wt.%

)Ca

tion

son

theba

sisof

25ox

ygen

atom

s

SiO2

TiO2

Al 2O3

FeOa

MnO

MgO

CaO

Na 2O

K2O

FCl

Total

SiAlIV

AlV

ITi

Fe3+

Fe2+

Mn

Mg

CaNa

KCa

BNa A

KA

DD-123

-140

.77

0.15

11.88

27.43

0.48

4.21

11.61

1.46

1.54

bdl

0.93

100.44

6.34

1.66

0.51

0.02

0.19

3.37

0.06

0.98

1.93

0.44

0.31

1.87

0.44

0.31

DD-123

-240

.60

bdl

11.56

28.41

0.39

3.84

11.54

1.27

1.45

0.20

0.95

100.20

6.35

1.65

0.48

0.00

0.25

3.46

0.05

0.89

1.93

0.38

0.29

1.86

0.38

0.29

DD-123

-336

.88

0.04

13.36

28.67

0.37

2.91

11.39

1.08

2.76

bdl

3.08

100.52

5.90

2.10

0.43

0.00

0.14

3.70

0.05

0.69

1.95

0.33

0.56

1.95

0.30

0.56

DD-123

-438

.09

0.23

13.47

28.15

0.45

3.07

11.63

1.17

2.44

0.24

2.54

101.47

5.98

2.02

0.47

0.03

0.23

3.47

0.06

0.72

1.96

0.36

0.49

1.96

0.31

0.49

DD-123

-541

.15

bdl

11.34

28.31

0.43

3.87

11.59

1.30

1.63

0.14

1.11

100.86

6.39

1.61

0.47

0.00

0.25

3.42

0.06

0.90

1.93

0.39

0.32

1.90

0.39

0.32

DD-123

-637

.41

0.10

14.33

27.30

0.41

3.06

11.40

1.26

2.43

bdl

2.65

100.34

5.92

2.08

0.60

0.01

0.19

3.43

0.05

0.72

1.93

0.39

0.49

1.93

0.32

0.49

aTo

talironas

FeO;b

dl=

below

detectionlim

it.

CaB>1.50; (Na+K)A>0.5; VIAl>Fe3+

0

0.2

0.4

0.6

0.8

1

4.555.566.577.5

Si

Mg/

(Mg+

Fe2+

)

PargasiteMagnesio-sadanagaite

Sadanagaite

Ferropargasite

Ferroedenite

Edenite

Fig. 11. Classification diagram for amphibole from the Dunde iron–zinc deposit (Leakeet al., 1997).



mineralized intrusion. The geochronological evidence summarizedabove indicates that the iron ore mineralization age in the AIMBwas dominantly late Carboniferous and, thus, the age of the Dundeiron–zinc deposit is inferred to have been in the middle to later stagesof the late Carboniferous.

6.2. Tectonic implications

Although the igneous rocks (granite, basalt, andesite, dacite, andrhyolite) have been variably attributed to a rift (Che et al., 1996; Xiaet al., 2004a, 2008), post-collisional (Han et al., 2004; Wang and Xu,2006), or mantle plume setting (Xia et al., 2004b), an island arc modelfor the Carboniferous tectonic setting of western Tianshan, China, hasbeen favored by most studies (e.g., Gao et al., 1998, 2009a,b; Q. Wanget al., 2006; Windley et al., 1990; Xiao et al., 2008; Zhu et al., 2006a;Zuo et al., 2008). This model is also supported by the presence of:(1) ophiolites in the northern margin of western Tianshan that containcumulate gabbro with zircon SHRIMP U–Pb age of 344 Ma (Xu et al.,2006a) and radiolarian cherts with radiolarian and conodont microfos-sils of late Devonian to early Carboniferous (Xiao et al., 1992) and areintruded by plagiogranite with zircon SHRIMP U–Pb age of 325 Ma(Xu et al., 2006b); (2) adakites and Nb-enriched arc basalts and basalticandesites of early Carboniferous (ca. 320 Ma) and arc calcalkalineandesites–dacites–rhyolites of late Carboniferous (ca. 306–310 Ma) inthe Alataw area, northern margin of western Tianshan (Q. Wang et al.,2007). Subduction–collision should have ceased prior to the Permian(Gao et al., 2009a,b), as increasing evidences from alkaline intrusions,bimodal basalt–rhyolite, paleomagnetic and continental molasse havebeen presented to support a Permian post-collisional extensionalsetting for this region (Allen et al., 1992; Gao et al., 2009a,b; Q.Wang et al., 2006; Wang et al., 2007b). The transformation fromsubduction–collision to extension probably took place in the lateCarboniferous, and this may be accompanied by considerable amountof mantle-derived magma underplating in deep (Long et al., 2011; Sunet al., 2008; D.Y. Zhang et al., 2012a). In this tectonic setting, igneousrocks were generated and emplaced or erupted, and associated irondeposits of late Carboniferous age formed in the AIMB. Volcanicand volcaniclastic rocks from the Dunde iron–zinc deposit have geo-chemical features similar to those of island arc rocks. For example,they have high contents of large ion lithophile elements, low con-tents of high field strength elements, negative Nb–Ta anomalies,and plot in the island arc calc-alkaline basalt field on discriminantdiagrams (Fig. 6f). Data for samples of the Early Permian K-feldspar gran-ite plot in the fields for post-collisional (Fig. 6g) and intracontinental(Fig. 6h) settings.


image of Fig.�11


Table 6Electron microprobe analyses results of magnetite from the Dunde Fe–Zn deposit.

Sample Magnetite type Na2O MgO Al2O3 K2O SiO2 CaO TiO2 NiO CuO Cr2O3 FeOa MnO V2O3 ZnO Total

DD-13-1 Fine-grained/disseminated ore 0.077 0.04 0.285 bdl 0.046 0.018 0.083 0.012 0.093 0.035 91.572 0.311 0.026 bdl 92.598DD-13-2 Fine-grained/disseminated ore bdl 0.239 2.122 bdl 0.019 bdl 0.024 bdl bdl 0.027 91.807 0.325 bdl bdl 94.597DD-13-3 Fine-grained/disseminated ore 0.052 0.024 0.242 bdl 0.053 0.01 0.158 bdl 0.036 0.008 91.979 0.422 bdl 0.071 93.064DD-13-4 Fine-grained/disseminated ore 0.07 0.001 0.409 bdl 0.036 bdl 0.047 bdl bdl 0.04 92.133 0.123 bdl bdl 92.884DD-13-5 Fine-grained/disseminated ore 0.028 0.036 0.623 bdl 0.019 bdl 0.275 bdl bdl 0.026 92.855 0.531 bdl 0.011 94.405DD-45-1 Coarse-grained/massive ore 0.045 0.494 2.603 bdl 0.014 bdl 0.418 bdl 0.036 0.019 88.77 1.39 bdl 0.138 93.927DD-45-2 Coarse-grained/massive ore 0.02 0.371 1.818 bdl 0.038 bdl 0.365 0.052 0.093 0.054 89.361 1.447 bdl 0.214 93.833DD-45-3 Coarse-grained/massive ore 0.051 0.611 3.001 bdl 0.025 bdl 0.1 bdl bdl 0.028 90.79 0.79 bdl bdl 95.396DD-45-4 Coarse-grained/massive ore 0.036 0.409 1.967 bdl 0.058 bdl 0.259 bdl bdl 0.056 89.76 1.383 bdl 0.089 94.017DD-75-1 Coarse-grained/massive ore 0.044 0.606 2.254 bdl 0.033 bdl 0.045 0.042 bdl bdl 90.811 0.941 bdl bdl 94.776DD-81-1 Fine-grained/half massive ore 0.015 0.428 2.558 bdl 0.03 bdl 0.025 bdl bdl 0.006 91.234 1.791 bdl 0.159 96.246DD-81-2 Fine-grained/half massive ore 0.025 0.461 2.487 bdl 0.042 bdl 0.067 bdl bdl bdl 90.497 1.628 bdl 0.108 95.322DD-81-3 Fine-grained/half massive ore 0.013 0.49 2.788 bdl 0.028 bdl 0.017 bdl bdl 0.004 90.581 1.473 bdl 0.125 95.519DD-87-1 Fine-grained/banded ore 0.04 0.798 2.725 bdl 0.008 bdl 0.343 bdl 0.017 0.021 87.74 1.884 0.027 0.277 93.888DD-87-2 Fine-grained/banded ore 0.045 0.56 2.699 bdl 0.016 bdl 0.582 bdl 0.038 bdl 88.43 1.606 bdl 0.16 94.136DD-87-3 Fine-grained/banded ore bdl 0.731 2.941 bdl 0.039 bdl 0.229 bdl bdl 0.064 87.14 1.67 0.036 bdl 92.858DD-87-4 Fine-grained/banded ore 0.045 0.585 2.902 bdl 0.052 bdl 0.301 bdl 0.041 0.034 88.453 1.606 bdl 0.227 94.251DD-98-1 Coarse-grained/banded ore 0.081 0.289 0.877 0.073 0.074 0.075 bdl bdl bdl 0.07 93.034 0.454 bdl bdl 95.027DD-98-2 Coarse-grained/banded ore 0.09 0.552 3.039 bdl 0.033 0.012 0.075 bdl bdl 0.018 88.697 0.999 bdl 0.398 93.913DD-98-3 Coarse-grained/banded ore bdl 0.312 0.811 bdl 0.057 bdl 0.1 0.012 bdl 0.015 92.162 0.838 0.022 bdl 94.329DD-98-4 Coarse-grained/banded ore 0.026 0.476 2.08 bdl 0.052 bdl 0.025 0.042 bdl 0.051 91.307 0.936 0.03 0.11 95.135DD-100-1 Fine-grained/banded ore 0.064 0.283 1.479 bdl 0.038 0.01 0.141 bdl 0.03 0.02 89.57 1.066 0.014 0.12 92.839DD-100-2 Fine-grained/banded ore bdl 0.054 bdl bdl 0.145 bdl bdl 0.042 bdl 0.06 92.666 0.349 bdl bdl 93.316DD-100-3 Fine-grained/banded ore 0.028 0.115 0.653 bdl 0.038 0.002 0.241 bdl 0.011 0.073 90.623 1.257 0.024 bdl 93.065DD-100-4 Fine-grained/banded ore 0.049 0.903 1.073 bdl 0.338 bdl bdl bdl bdl 0.054 90.493 0.667 bdl bdl 93.582DD-100-5 Fine-grained/banded ore 0.029 0.209 1.544 bdl 0.036 bdl 0.108 bdl bdl 0.064 89.309 1.133 0.024 0.088 92.548DD-142-1 Coarse-grained/banded ore 0.123 0.22 1.039 0.025 0.059 0.136 0.117 bdl 0.041 0.023 91.41 0.775 0.08 bdl 94.048DD-142-2 Coarse-grained/banded ore 0.125 0.18 0.847 0.014 0.058 0.108 0.225 bdl bdl 0.021 91.754 0.538 0.057 bdl 93.934DD-142-3 Coarse-grained/banded ore 0.018 0.699 1.52 0.011 0.044 bdl bdl bdl bdl 0.038 90.246 0.553 bdl bdl 93.129DD-142-4 Coarse-grained/banded ore 0.17 0.216 1.118 bdl 0.429 0.051 0.491 bdl bdl 0.051 90.948 0.81 bdl bdl 94.292

a Total iron as FeO; bdl = below detection limit.


6.3. Genesis of the Dunde iron–zinc deposit

6.3.1. Skarn genesisThe calcic iron skarn in the Dunde iron–zinc deposit occurs within

volcanic and tuff rocks, and does not have a clear link with intrusiverocks. Similar examples of iron skarn mineralization are widely distrib-uted in the Tianshan and Altay-Sayan of the CAOB, for example,the Anzas, Mengku, Tuomuerte, Yamansu, and Chagangnuoer skarns(Belevtsev, 1982; Hong et al., 2012a; Hou et al., 2013; Yang et al.,2013). Several competing models have been proposed to explain the

Fig. 12. The Ca + Al + Mn vs. Ti + V discriminant diagram showing analyses of magne-tite for samples from the Dunde iron–zinc deposit (Dupuis and Beaudoin, 2011).


origin(s) of these skarns: (a) volcanogenic sedimentary metamor-phosed (Belevtsev, 1982); (b) seafloor exhalation (Jiang, 1983); (c)intrusion-related (Hong et al., 2012a; Xu et al., 2010); (d) metasoma-tism by metamorphic fluids related to a major shear zone (Wan et al.,2012); and (e) metasomatism by fluids that originated from a magmachamber (Hou et al., 2013). The volcanogenic sedimentary metamor-phosed model and seafloor exhalation model suggest that the ironskarn is stratiform (Belevtsev, 1982; Jiang, 1983), and the latter alsosuggests the iron skarn is syn-volcanic, which is not consistent withthe skarn geology of the Dunde deposit in that the skarn is related tofractures that cut through the bedding of the wall rocks. In addition,skarn-related fractures in the Dunde iron–zinc deposit are short (butwide), have varied orientations, and are incoherent, and can beinterpreted as being volcano-related radial fractures (Feng et al.,2010). There is no evidence to suggest that the iron skarn in theDunde deposit is related to a shear zone, and no regional shear zonehas yet been recognized in the Dunde area. Consequently, the originof the Dunde skarn cannot be explained by metasomatism of meta-morphic fluids related to a major shear zone either.

The composition of skarnminerals reflects the physical and chemicalconditions under which they formed (Meinert et al., 2005), and cantherefore be used to constrain the mode of genesis of the skarn. Typicalvolcanic exhalation and sedimentation stratiform skarn mainly com-prises Mn- and Fe-rich spessartine and almandine garnet, such as theSedex type Sawusi lead–zinc deposit (Liu et al., 2012). In contrast,intrusion-related iron skarn predominantly comprises andradite–grossularite garnet and diopside–hedenbergite pyroxene (Meinertet al., 2005), which are both iron-rich (Purtov et al., 1989). The Dundeskarn contaINS grossularite-rich garnet (Gr39–80; Ad15–58) and predom-inantly diopsidic pyroxene (Di63–97; Hd2–36), which is similar to otherglobal examples of iron skarn (Einaudi and Burt, 1982; Einaudi et al.,1981;Meinert et al., 2005), although the garnet and pyroxene are a littlemore grossularitic and diopsidic than the Chagangnuoer and Yamansuskarns (Hong et al., 2012a;Hou et al., 2013). However, theDunde garnet


image of Fig.�12


Table7

Electron

microprob

ean

alyses

resultsan

dform

ationtempe

rature

oftherepresen

tative

chlorite

from

theDun

deFe–Zn

depo

sit.

Sample

Oxide

compo

sition

(wt.%

)Ca

tion

son

theba

sisof

18ox

ygen

atom

st(°C)

SiO2

TiO2

Al 2O3

FeOa

MnO

MgO

CaO

Na 2O

K2O

FCl

Cr2O

3NiO

Total

SiAlIV

AlV

ITi

Cr(Fe3

+)V

I(Fe2

+)V

IMn

Mg

Ni

CaNa

KF

ClFe/(Fe

+Mg)

DD-13-1

29.81

0.11

15.5

24.4

0.73

15.2

0.39

0.01

0.60

nn

0.04

0.01

86.80

3.15

0.85

1.09

0.01

0.00

0.11

2.05

0.07

2.39

0.00

0.04

0.01

0.16

0.04

179.69

DD-13-2

30.82

0.05

15.3

20.94

0.93

16.99

0.08

0.00

2.09

nn

0.10

0.01

87.31

3.19

0.81

1.07

0.00

0.01

0.03

1.78

0.08

2.62

0.00

0.01

0.00

0.55

0.41

169.73

DD-45-1

27.23

0.03

25.62

4.01

0.68

28.62

0.04

0.01

bdl

nn

0.02

bdl

86.26

2.60

1.40

1.48

0.00

0.00

0.05

0.27

0.06

4.07

0.00

0.00

0.00

0.00

0.47

207.41

DD-75-1

28.14

0.01

22.55

6.51

0.29

28.85

0.04

0.01

0.01

nn

0.07

0.02

86.50

2.72

1.28

1.28

0.00

0.01

0.00

0.52

0.02

4.15

0.00

0.00

0.00

0.00

0.41

198.79

DD-87-1

28.7

0.00

20.05

22.23

0.65

18.27

0.00

0.00

0.06

bdl

0.01

bdl

0.03

90.00

2.88

1.12

1.25

0.00

0.00

0.07

1.80

0.06

2.73

0.00

0.00

0.00

0.02

0.00

0.00

0.36

205.90

DD-87-2

28.65

0.00

21.01

19.74

0.56

19.76

0.06

0.02

0.05

0.17

0.03

bdl

bdl

90.05

2.82

1.18

1.28

0.00

0.00

0.09

1.54

0.05

2.90

0.00

0.01

0.01

0.01

0.11

0.01

0.11

206.86

DD-87-3

28.12

0.03

17.23

26.42

4.07

12.77

0.17

0.04

0.01

0.06

0.01

0.16

bdl

89.09

2.97

1.03

1.13

0.00

0.01

0.08

2.26

0.36

2.01

0.00

0.02

0.02

0.00

0.04

0.00

0.54

203.83

DD-100

-131

.71

0.02

16.16

2.69

0.4

33.26

0.06

0.03

bdl

nn

0.10

0.04

84.47

3.07

0.93

0.92

0.00

0.01

0.00

0.22

0.03

4.8

0.00

0.01

0.01

0.00

0.07

151.48

DD-142

-126

.39

0.00

19.33

25.67

0.29

14.75

0.00

0.15

0.05

nn

bdl

bdl

86.63

2.82

1.18

1.26

0.00

0.00

0.02

2.27

0.03

2.35

0.00

0.00

0.06

0.01

0.49

222.18

aTo

talironas

FeO;n

=no

ttested

;bdl

=be

low

detectionlim

it;t

(°C)

:formationtempe

rature

ofch

lorite

asex

plaine

din

thetext.

Fig. 13. Classification diagram for chlorite from the Dunde iron–zinc deposit (Hey, 1954).



and pyroxene are markedly different from those of typical Sedex typestratiform skarns such as the Sawusi lead–zinc deposit (Figs. 9 and10). Changes in the composition of calc-silicateminerals record the con-ditions of skarn formation (Meinert, 1997). The host rocks of the Dundeiron–zinc deposit are predominantly dacite and rhyolite, whichmay ex-plain why the Dunde skarn ismore grossularite- and diopside-rich thanthe Chagangnuoer (andesitic wall rock) and Yamansu skarns (basalticwall rock). Garnet from the Dunde skarn is relatively rich in aluminum,whichmay also be related to themetasomatized dacite wall rock that isaluminum-rich but iron-poor.

Given that the Dunde skarnmineralization is controlled by fracturesand that no related intrusion crops out or has been encountered in drillholes, a magmatic fluid source from a deep intrusion is considered to bemost reasonable hypothesis to explain the origin of the skarn. Fluids de-rived from a deep intrusion can travel considerable distances upwardsthough fractures to metasomatize overlying country rocks to produceskarn (Meinert et al., 2005). Whether this intrusion is located justbelow the Dunde skarn or at greater depth (Hou et al., 2013) is difficultto constrain. However, a regional comparison of other deposits in thisarea may shed some light on the nature of the intrusion. As describedpreviously, a disseminated magnetite-mineralized diorite crops out inthe Wuling District west of the Dunde deposit. Similar magnetite-mineralized diorite dikes also crop out at the Beizhan deposit east ofthe Dunde deposit. The magnetite orebodies of the Gulungou depositoccur at the contact between diorite and limestone, ca. 30 km east ofthe Dunde deposit (Tian et al., 2009). A reduction-to-pole aeromagneticanomaly map of the Awulale area suggests that themoderate aeromag-netic anomaly located 1–2 km east of the Dunde deposit may be an in-trusion of intermediate composition (X.Z. Yu et al., 2011). All of theseiron deposits are believed to be of similar age and related to the samegeological event (Zhang et al., 2012b). Thus, we envisage a dioriticintrusion located just below the Dunde deposit that caused skarn for-mation and mineralization.

6.3.2. Iron mineralizationMagnetite in the Dunde iron–zinc deposit crystallized during the

retrograde alteration stage after skarn formation and replaces or en-closes early formed garnet and pyroxene (Figs. 3f, 4d, and i). It alsocrystallized in open voids (Fig. 3e and g) and may have precipitatedfrom hydrothermal fluids. Macroscopically, the magnetite orebodiescross-cut the layers of volcanic–volcaniclastic rocks. Chemical datafor magnetite from the Dunde deposit fall in the skarn field in theCa + Al + Mn versus Ti + V discriminant diagram for iron ore genesis(Fig. 12). Magnetite may have crystallized at high temperatures, asdemonstrated by spinel exsolution lamellae within the magnetite(Fig. 4j), and may have been overprinted by later stages of arsenide–


image of Fig.�13


Fig. 14. (a) Secondary H2O inclusions in garnet. (b) Isolated daughter mineral-bearing H2O inclusions in calcite. (c) Multiple daughter mineral-bearing H2O inclusions in calcite. (d) H2Oinclusion and pure H2O inclusion in calcite. (e) Rectangular H2O inclusion in calcite. (f) Rich H2O inclusion in calcite.


sulfidemineralization and low-temperature hydrothermal calcite–chlo-rite alteration. Sphalerite contains emulsion, patchy, and trellis texturedpyrrhotite and chalcopyrite exsolution features. In general, the decom-position temperature for the solid solution sphalerite–chalcopyrite pairis 350 °C to 400 °C (Xu, 1986), which confirms the high temperature ofmagnetite crystallization (N400 °C). The abundant fluid inclusions incalcite have homogenization temperatures from 147 °C to 367 °C. Thechlorite geothermometer yields chlorite formation temperatures be-tween 152 °C and 222 °C (average = 194 °C). In summary, the hydro-thermal fluids responsible for the Dunde iron–zinc deposit precipitatedmagnetite at high temperatures (N400 °C) after crystallization of garnetand pyroxene, followed by arsenides and sulfides at medium to hightemperatures (350 °C to 400 °C), and then calcite and chlorite at lowto medium temperatures (150 °C to 350 °C).

Sulfur isotope compositions of pyrrhotite, sphalerite, pyrite, andloellingite define a narrow range from 3.8‰ to 8.1‰ (average = 6.8‰),

Table 8Summary of fluid inclusion microthermometric data in garnet and calcite from the Dundeiron-zinc deposit.

Sample Type Tice(°C) Td(°C) Th(°C) Salinity

DD-18-1 I −14.9 to −2.7 187.2 to 359.2 4.5 to 18.6II 221.8 347.2 33.0III 162.9

DD-18-2 I −6.9 to−1.5 155.8 to 241.2 2.6 to 10.4DD-46 I −5.7 to−4.7 253.3 to 367.4 7.4 to 8.8

II 199.8 to 347.6 191.7 to 249.8 31.9 to 32.6DD-81 I −13.6 to −1.4 147.3 to 242.6 2.4 to 17.4

II 174.5 to 292.3 172.9 to 173.5DD-93 III −6.5 to−3.9 162.7 to 206.9 6.3 to 9.9DD-98 I −16.2 to −11.3 171.1 to 258.0 15.3 to 19.6DD-104 I −21.6 to −7.3 161.1 to 226.2 10.9 to 23.4

II 172.1III 188.9 to 207.7

Note: I = H2O inclusions in calcite; II = daughter mineral-bearing H2O inclusions incalcite; III = secondary H2O inclusions in garnet. Tice = melting temperature of ice;Td = dissolution temperature of halite; Th = total homogenization temperature; salinityin weight % NaCl equiv.


indicating a single sulfur source. Isotopic fractionations characterize sul-fide compounds during hydrothermal processes (Sakai, 1968), and thesulfur isotopic composition of a mineral is a function of the sulfur isotopecomposition of the hydrothermal fluid, fO2, pH, ion strength, and temper-ature (Ohmoto, 1972). Therefore, the δ34S values of sulfide mineralscannot simply be equated as the δ34S values of the hydrothermal fluids.However, when pyrrhotite is present in the paragenetic mineral assem-blage, the hydrothermal fluid must have a pH N 6, low fO2 (at tempera-tures of b500 °C), H2S as the dominant sulfur species, and the δ34Svalues of the sulfide minerals must be close to that of the hydrothermalfluid (Zheng and Chen, 2000). Pyrrhotite was crystallized during thesulfide stage of the Dunde iron–zinc deposit and, hence, the δ34S of thesulfide minerals is likely to be close to that of the hydrothermal fluids(i.e., δ34Sfluid ≈ 6.8‰). This δ34S value for the hydrothermal fluid is clear-ly different from that of sedimentary sulfide (negative δ34S) and marinesulfate (δ34S ≈ 14‰ for the late Carboniferous ocean) (Zheng andChen, 2000). As such, we infer that the sulfur in the hydrothermal fluid

Fig. 15. Plot of homogenization temperature vs. salinity of fluid inclusions.


image of Fig.�14

image of Fig.�15


Table 9S isotope data for sulfide minerals from the Dunde iron–zinc deposit.

Sample Mineral assemblage Mineral δ34SV-CDT‰

DD-22 Di + Po + Mt + Py Po 8.1DD-23 Di + Mt + Po Py 8DD-104 Cal + Po + Ep Po 5.5DD-106 Po + Sp + Cal Sp 6.5DD-107 Po + Ccp + Cal Po 3.8DD-108 Mt + Po + Cal Sp 6.5DD-112 Mt + Py + Cal + Ep Py 7.6DD-116 Po + Sp + Ccp + Mt + Cal Sp 7.1DD-139 Mt + Lo Lo 7.8

Ccp = chalcopyrite, Di = diopside, Ep = epidote, Lo = Loellingite, Mt = magnetite,Po = pyrrhotite, Py = pyrite and Sp = sphalerite.


that produced the Dunde iron–zinc deposit was probably magmatic-derived and from an unexposed diorite intrusion. A diorite intrusionwithmagmatic δ34S = 0‰ ± 1‰ that exsolved late-stage hydrothermalfluids under high-fO2 conditions, where the dominant sulfur species isSO2, could generate a hydrothermal fluidwith δ34S that can be 5‰ higherthan the melt (Ohmoto and Rye, 1972).

In summary, the Dunde iron–zinc deposit may have formed throughthe metasomatization of volcanic–volcaniclastic rocks by hydrothermalfluids derived from a diorite intrusion, and it can be classified as a skarnore deposit. We speculate that the general mineralizing process was asfollows: (a) diorite magma formed by interaction between mantle-derived magmas and crustal rocks in a setting of mantle-drived magmaunderplating (Long et al., 2011; Sun et al., 2008; Zhang et al., 2012a);(b) the magma ascended and intruded late Carboniferous volcanic–volcaniclastic rocks, and some may have been erupted at the surface;(c) the diorite intrusion began to crystallize, consolidate, and releasedhydrothermal fluids; (d) the fluid flowed along fractures and/or pene-trated into the wall rocks, carrying with it Fe, Ca, andMg into the basalticand andesitic tuff and dacite, resulting in the crystallization of grossular–andradite garnet and diopside; (e) with decreasing temperature the hy-drothermal fluid precipitated magnetite instead of garnet and pyroxene,and degraded earlier-formed skarn, causing the banded skarn to formbreccias cemented by magnetite; (f) at temperatures b400 °C the fO2

suddenly decreased, leading to the precipitation of arsenide and sulfidesthat replacedmagnetite; and (g) oremineral crystallization ceasedwhenthe temperatures fell below 350 °C and gangue calcite–chlorite formed.

7. Conclusions

(1) Volcanic–volcaniclastic rocks in the Dunde iron–zinc deposit arelate Carboniferous and not early Carboniferous in age, as demon-strated by zircon LA–ICP-MS U–Pb dating of wall rock dacite,which yields a weighted mean 206Pb/238U age of 316.0 ± 1.7 Ma.

Fig. 16. δ34S histogram of sulfides from the Dunde iron–zinc deposit.


Coupled with other dating results for magnetite-mineralizeddiorite stocks, diorite dikes, and garnet skarn, it is inferred thatthe Dunde deposit formed in the late Carboniferous, after316 Ma. The late Carboniferous tectonic setting of the regionchanged from subduction–collision to extension, and thiswas accompanied by mantle-derived magma underplating indeep.

(2) The Dunde iron–zinc deposit may have formed throughmetasomatization of volcanic–volcaniclastic rocks by hydrother-mal fluids derived from a dioritic(?) intrusion. The hydrothermalfluid altered the volcanic–volcaniclastic rocks to skarn contain-ing garnet (Gr39–80; Ad15–58) and pyroxene (Di63–97; Hd2–36),and precipitated magnetite at high temperatures (N400 °C) andsphalerite at medium to high temperatures (350 °C to 400 °C).These results classify the Dunde iron–zinc deposit as being askarn ore deposit.

(3) A K-feldspar granite with a zircon LA–ICP-MS U–Pb age of295.75 ± 0.71 Ma intruded the Dunde iron–zinc deposit duringthe early Permian in a post-collisional extension setting.

Acknowledgments

We thank Professor Zhaochong Zhang, Huayong Chen and DoctorTong Hou for their constructive reviews that helped to improve thecontent and presentation of this manuscript. This work was jointlysupported by the project 41203035 supported by NSFC, the NationalBasic Research Program (2012CB416803), the National S&T SupportProgram (2011BAB06B02-05) and the Chinese Geological SurveyProgram (1212011085060).

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Documents

Geology, geochemistry, and geochronology of the Dunde iron–zinc ore deposit in western Tianshan, China