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Gondwana Research 26 (2014) 557–575

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LA-ICP-MS trace element analysis of pyrite from the Chang'an golddeposit, Sanjiang region, China: Implication forore-forming process☆,☆☆

Jing Zhang a,b,⁎, Jun Deng a,1, Hua-yong Chen b,c,2, Li-qiang Yang a,3, David Cooke b,4,Leonid Danyushevsky b,5, Qing-jie Gong a,6

a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, Chinab CODES ARC Center of Excellence in Ore Deposits, University of Tasmania, 7001, Australiac Key Laboratory for Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

☆ This is an open-access article distributed under the tAttribution-NonCommercial-No Derivative Works License,use, distribution, and reproduction in any medium, provideare credited.☆☆ This article belongs to the Special Issue on OrogenSanjiang Tethyan Domain.

⁎ Corresponding author at: State Key Laboratory of GeResources, China University of Geosciences, Beijing 1004017.

E-mail addresses: [email protected] (J. Zhang),[email protected] (H. Chen), [email protected] (L(D. Cooke), [email protected] (L. Danyushevsky), qjiegon

1 Tel.: +86 10 8232 2301.2 Tel.: +86 20 85292708.3 Tel.: +86 10 8232 2175.4 Tel.: +61 3 62267605.5 Tel.: +61 3 62262469.6 Tel.: +86 10 8232 0895.

1342-937X/$ – see front matter © 2013 The Authors. Pubhttp://dx.doi.org/10.1016/j.gr.2013.11.003

a b s t r a c t
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Article history:Received 10 March 2013Received in revised form 1 November 2013Accepted 21 November 2013Available online 28 November 2013

Keywords:LA-ICP-MSTrace elementsPyriteChang'an gold depositSanjiang Tethyan metallogenic domain

The Chang'an gold deposit, Yunnan province is one of five large gold deposits in the Ailaoshan gold belt that is themost important ore belt of the Sanjiang Tethyanmetallogenic domain. Geochemical study of pyrite in the depositwas conducted using laser ablation inductively coupled plasmamass spectroscopy. Three types of hydrothermalpyrite were identified in the ores and wall rocks, i.e., the coarse euhedral crystals in syenite (Py1), the coarsegrains disseminated in altered sandstones or sandstone ores (Py2), and the fine-grained euhedral pyrite insandstone ores (Py3). Their trace elements exhibit different concentrations, associations and rim–core zoning,implying different geneses and crystallization processes. The cores of Py1 were formed by magmatic fluids andhave the lowest concentrations of Au, As, Cu and Zn. The cores of Py2 were formed by metamorphic fluids andhave relatively high Au, Ag, Ni, Pb and Cu concentrations. The Py3 grains and the growth rims of Py1 and Py2show consistently high contents of Au, As, Pb and Co, suggesting that all of them were rapidly deposited from amixingfluid system thatwasprobably composedoffluids ofmetamorphic andmagmatic origins. This interpretationis supported by the observation that the high-grade ores generally contain lots of Py2 and Py3. Hence we considerthat the deposit was formed during India–Asia collision, slightly postdate the 33–35 Ma tectono-magmatism.

© 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

In the world, China is a unique area accommodating three globalmetallogenic belts, i.e. the Tethyan, Paleo-Asian and Circum-Pacificbelts (Chen, 2002; Chen et al., 2009; Pirajno, 2013). The Tethyan beltcovers the southern half of mainland China, with the Qinling–Dabie–

erms of the Creative Commonswhich permits non-commerciald the original author and source

esis and metallogenesis in the

ological Processes and Mineral083, China. Tel.: +86 10 8232

[email protected] (J. Deng),. Yang), [email protected]@cugb.edu.cn (Q. Gong).

lished by Elsevier B.V. on behalf of In

Sulu orogenic belt as its northernmost unit (Zhang et al., 2009; N. Liet al., 2013), hosting numerous giant mineral systems, such as theJinding Pb–Zn deposit in Yunnan (Hou et al., 2007; Deng et al.,2013b), Yangshan Au deposit in Gansu (Yang et al., 2006, 2009), andthe Mo cluster in Henan (Chen et al., 2000; Li et al., 2007; N. Li et al.,2011) and gold province in eastern Shandong (Chen et al., 2005; Guoet al., 2013). The Sanjiang Tethyan metallogenic domain in southwest-ern China is a significant part of the Tethyan metallogenic belt (Houet al., 2007; Deng et al., 2013b; Lehmann et al., 2013; B.D. Wang et al.,2013; C.M.Wang et al., 2013; Tang et al., 2013), which tectonically con-nects the Yangtze Craton to the east and the Tibet plateau to the west.

The Ailaoshan gold belt (Fig. 1) in Yunnan province is the most im-portant in the Sanjiang Tethyan metallogenic domain. It strikes120 km long, with width of 0.5–5 km, along the Jinshajiang suture.The belt is developed with complicated thrusts and nappes formedfrom the India–Asia continental collision since the Paleocene (Houet al., 2007; Mo et al., 2007; Deng et al., 2010a, 2010b, 2013a; Fanet al., 2010). To date, five large gold deposits, i.e., Laowangzhai (22 t at5.29 g/t Au in metal), Donggualin (45 t at 5.10 g/t), Jinchang (32 t at2.69 g/t), Chang'an (31 t at 5.84 g/t) and Daping (60 t at 14.30 g/t)areas (Yang et al., 2010), and eight medium and numerous small golddeposits have been discovered in the belt (Fig. 1). It also hosts a numberof other ore deposits, including the Hercynian Baimazhai magmatic

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Fig. 1. Tectonic framework of the Ailaoshan orogenic belt, showing locations of main gold deposits.Modified from Li et al. (1998), Hou et al. (2007), and Yang et al. (2010).

558 J. Zhang et al. / Gondwana Research 26 (2014) 557–575

Cu–Ni deposit, the Cenozoic Tongchang porphyry Cu deposit, and theAnding hydrothermal Ni deposit (Ying et al., 2006; Sun et al., 2009).

The Chang'an gold deposit was discovered at the southern segment ofthe Ailaoshan gold belt in 2001. Previous studies paid much attention tothe metallogenic setting and ore geology (Ying et al., 2006; He et al.,2008; Chen et al., 2010; Yang et al., 2010, 2011; Zhang et al., 2010; S.H.Li et al., 2011, 2013), but the ore genesis is still open. In the Chang'angold deposit, native gold and electrumoccur as inclusionswithin the crev-ices of quartz, pyrite and arsenopyrite, or along boundaries of these min-eral grains (Ying et al., 2006; Yang et al., 2010). However, whether pyritehosts ‘invisible’ gold (lattice or nanoparticulate gold) (Cook andChryssoulis, 1990) is also open, but very important to insight Au-enrichment, because pyrite is the dominant gold-bearing mineral. More-over, trace element contents and associations in hydrothermal pyrite canprovide useful information for understanding the ore-forming processesand physicochemical conditions (Li et al., 2004; Zheng et al., 2013).

The laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) has been proven to be a powerful tool for definingseparate pyrite growth zones and hydrothermal mineralization events(Cook et al., 2009; Maslennikov et al., 2009; Large et al., 2009; Sunget al., 2009; Thomas et al., 2011; Ye et al., 2011; Zhao et al., 2011;Winderbaum et al., 2012; Zheng et al., 2013). However, no equivalent re-search has been conducted for deposits in China, especially gold deposits.In this contribution, we report the results obtained from the LA-ICP-MSstudy of the pyrite in the Chang'an gold deposit, and thereby discuss themineralization processes, and present a new understanding of the oregenesis.

2. Geological setting

In the Ailaoshan gold belt, three thrust belts occur from east to west,i.e., the Red River, Ailaoshan and Jiujia–Mojiang faults, respectively.These NW-trending faults subdivide the Ailaoshan gold belt into severalunits with different geological characteristics (Fig. 1) (Li et al., 1998;Burnard et al., 1999; Hou et al., 2007; Qi et al., 2012; Q.F. Wang et al.,2013).West of the Jiujia–Mojiang fault is a foreland unit primarily com-posed of Paleozoic sediments and Upper Triassic molasses. Between theJiujia–Mojiang andAilaoshan faults is a front thrust belt accommodatinga series of nappes (e.g., Jinshan, Jinping and Lvchunnappes) and thema-jority of the gold deposits in the Ailaoshan gold belt. Goldmineralizationusually occurred along the slipping planes. The Ailaoshan fault zoneconstitutes the central thrust belt, controlling multistage granitic intru-sions and lead, copper and gold mineralization that usually occurredalong the contacts betweenmetamorphic rocks and intrusive dikes. Be-tween the Ailaoshan and Red River faults is a high-grade metamorphicunit primarily composed of the Proterozoic Ailaoshan Group that isdominated by two mica schist and biotite amphibole plagiogneiss.

The Chang'an gold deposit is located in the Jinping nappe (Fig. 1),which is wedge-shaped between the Lvchun nappe and the Ailaoshanbasement nappe, with the Tengtiaohe and Ailaoshan fault being theboundaries. In the Jinping area (Fig. 2), the outcropped strata includethe Lower Ordovician clastic rocks (e.g. quartzo-feldspar sandstone,quartz sandstone, siltstone, argillaceous siltstone and limestones), theUpper Silurian–Lower Permian carbonate (e.g. dolomitic limestone,dolomite and limestone), and locally, the Permian Emeishan basalt

Fig. 2.Geology of the Chang'an gold deposit. (a) Geological sketchmap (modified after Zhang et al., 2010); (b) geological map of orebody V5 at 1606 m ASL; (c) geological profile of No. 0exploration line (modified after He et al., 2008).

559J. Zhang et al. / Gondwana Research 26 (2014) 557–575

(Chen et al., 2010). The primary structures in the area are NW- andNNW-trending faults, followed by the NE-trending faults. These faultsplayed a role of paths and spaces for magma intrusion and fluid flow,for instance, the Chang'an gold deposit is located in the NW-trendingbrittle fracture (Fig. 1; Li et al., 1998; He et al., 2008).

The Cenozoic syenite, syenite porphyry, lamprophyre, gabbro anddiabase are frequently observed in the Jinping unit, including at theChang'an deposit (Zhang et al., 2010). The lamprophyre, gabbro anddiabase usually appear as dikes, while the syenite and syenite porphyryas stock or dikes (Fig. 2a, Fig. 3a and b). The LA-ICP-MS zircon U−Pb dat-ing yielded age of 35.1 ± 0.3 Ma for syenite porphyry (Huang et al.,2009), 34.49 ± 0.14 Ma (No. CA11P32) and 34.47 ± 0.07 Ma (No.CA11P115) for syenite (our unpublished data), and 34.12 ± 0.06 Ma(No. YCP03) and 34.19 ± 0.15 Ma (No. YCP04) for fine grained syenite(our unpublished data).

3. Ore geology

At the Chang'an deposit, main strata are the Lower OrdovicianXiangyang Group lithologically composed of cataclastic siltstone,fine-grained quartz sandstone and quartz conglomerate, and the

Middle–Upper Silurian Kanglang Group dolomites (Fig. 2a). The rocksof the Xiangyang Group have relatively high porosity and permeability,which is favorable of hydrothermal mineralization.

The NW-trending F5 and F6 faults pass through the mining area(Fig. 2a). The F5 fault is steeply dipped with an angle of 80°, associatingwith pyritization and gold mineralization. The F6 fault is a left-lateralstrike–slip thrust that spatially coincides with the unconformitybetween the Silurian and Ordovician strata. The major orebody (V5) ofthe Chang'an deposit is hosted in fault F6 and its combination with theunconformity between Silurian and Ordovician strata, but mainly hostedin the sandstones of the Xiangyang Group (Figs. 2a, b and 3a). Thisorebody strikes about 1800 m long and dips 40°–75° with angles of28°–60° (Fig. 2c). Generally, the boundaries between orebodies and wallrocks are gradational and can only be defined by cut-off grade (1 g/t).

The shallow ores above 1645 m ASL (above sea level) were stronglyoxidized and taken as oxidized ores (Fig. 3a) by the mining geologists.The primary ores are presented silicified and sericitized fine-grainedquartz sandstones and siltstones (Fig. 3b, c, d), and as altered tectonicbreccias (Fig. 3e). Metallic minerals occur as dissemination and thinveinlets (Fig. 3f), and are dominated by pyrite, followed by minor arse-nopyrite, chalcopyrite, sphalerite and galena (Fig. 4). The gangue

Fig. 3. Photographs of orebody and rocks at Chang'an deposit. (a) The deformation and crosscutting relationships of intrusive dikes enveloped by the oxidized ores; (b) mineralized sand-stonewith disseminatedpyrite; (c) sandstones intrudeby syenite dike, and then structurally deformed andmineralized; (d) syenite dike cut by gabbrodike, and then cut by quartz–sulfidevein; (e) orebody comprised by pyrite-bearing sandstone breccia and siliceous cement; (f) sulfide-bearing and ore-barren quartz veins in mineralized sandstone; (g) deformed syenitedike within the orebody. The red stars indicate sample locations.


minerals include quartz, sericite, calcite and feldspar as altered relicts.Silicification and carbonation are common in the Chang'an deposit(Fig. 4).

Goldminerals in the ore are predominantly native gold, with a smallamount of electrum. The native gold occurs primarily as fine grains(micron scale) in the crevices, fissures or interfaces of quartz, pyriteand arsenopyrite grains in primary ores, or associates with limonitegrains with pyrite pseudomorph in the oxidized ores. A small amountof gold grains also occurs in some clay minerals (Chen et al., 2010).

Based on a) the nature of host-rock and host-fractures, b) specific ofmetasomatic replacement, c) mineral morphology, and d) intersectingrelationships of the veins, the ore-forming process can be subdividedinto sedimentary enrichment, structural–magmatic–hydrothermalmineralization and epigenically secondary enrichment epochs (Fig. 5).

4. Pyrite types and textures

Based on themethods developed in previous studies on pyrites fromsediment- and volcanic-hosted gold deposits (Mumin et al., 1994; Large

et al., 2007, 2009), the morphology and internal structure of pyriteshave been used here as a guide to relate the times between pyritegrowth and gold events.

In the Chang'an gold deposit, the pyrites are widely developedand are distinguished from diagenetic, diagenetic–metamorphic andhydrothermal types.

Diagenetic pyrite occurs as euhedralmicrocrystalswith size of 1–5 μmand forms irregularly shaped aggregates with size of 50–150 μm(Fig. 6a and b). This type is preferentially hosted in the sandstone ormineralized sandstone, with texture similar to that of syngenetic orsyndiagenetic pyrite that is common in SEDEX Zn–Pb–Ag depositsand the Au deposits hosted in sedimentary or metamorphic rocks(Large et al., 2007, 2009; Yang et al., 2009; Thomas et al., 2011;Zheng et al., 2013). The diagenetic–metamorphic pyrite mainly occursin strongly deformed sedimentary rocks and has a core composed oforiginal diagenetic pyrite surrounded by euhedral pyrite recrystallizedduring metamorphism (Fig. 6b).

Hydrothermal pyrite is widely distributed in the ore, sandstone,quartz–sulfide vein and syenite and can be subdivided into three

Fig. 4. Textures and mineral assemblages of ores and wall rocks. (a) The quartz veins crosscutting sandstone (CA11P111); (b) silicification, sericitization, calcification and pyritization insandstone ore (CA11P403); (c) pyrite in syenite porphyry (CAZK007B3); (d) pyrite with porous core and euhedral rim (CA11P503); (e) coexisting chalcopyrite, sphalerite and galenain sandstone (CA11P105); (f) coexisting chalcopyrite, tetrahedrite, galena and pyrite in the sulfide–quartz vein (CA11P117). Qtz: quartz, Ser: sericite, Py: pyrite, Apy: arsenopyrite,Ccp: chalcopyrite, Sp: sphalerite, Gn: galena, Tt: tetrahedrite, Cal: calcite, Fsp: feldspar.


subtypes according to its morphology and host rock as below: (1) Py1,distributed in syenite or syenite porphyry, occurs as euhedral crystalsof 100–500 μm. Most pyrites of this subtype usually have a core withirregular dissolution holes and a complete and straight rim (Fig. 6c),and sometimes visible sulfide inclusions occur in its rim; (2) Py2,found in sulfide–quartz veins, mineralized sandstones and breccias-type ores, comprising coarse-grained pyrite with size of 50–200 μm,and commonly showing structural overprint as indicated by cataclastictexture and marginal corrosion texture (Fig. 6d and e); and (3) Py3,distributed in strongly-silicified sandstones (ores), comprising muchfine-grained euhedral pyrite with size of 20–50 μm (Fig. 6f).

5. Samples and analytical technique

In this study, 16 sampleswere collected from the ores andwall rocksat different levels (Figs. 2 and 3) and polished into thin sections forpetrographic study. From them, ten samples were selected for pyritetrace element analysis using LA-ICP-MS technique.

Analytical instrumentation employed in this study consists of a NewWave UP-213 nm Laser Ablation System coupled with an Agilent 7700sQuadrupole ICP-MS, housed in the CODES LA-ICP-MS facility at the

Fig. 5. Paragenetic sequence for major minerals of the Chang'an gold deposit.

University of Tasmania. The methodology is described in detail byLarge et al. (2009) and briefly introduced as below.

Depending on the pyrite size, analyses were performed by laser-ablating spot diameters of 10–35 μm and at repetition rate of 2–5 Hz.The laser beam energy was maintained between 1.6 and 2.5 J cm−2.Analysis time for each sample was 90 s, which includes 30 s of back-ground measurement with laser off and 60 s of analysis with laser on.Acquisition time for all masses was set to 0.02 s, with a total sweeptime of ~0.6 s. Data reduction was performed according to standardmethods (Longerich et al., 1996), with Fe as the internal standard.Calibration was performed using the in-house standard (STDGL2b-2),comprising powdered sulfides doped with certified element solutionsand fused to a lithium borate glass disk (Danyushevsky et al., 2011).The standard was analyzed twice every 1.5 h with a 100-μm beamsize at 10 Hz to correct for instrument drift. Accuracy is expected to bebetter than 20% for most elements (Danyushevsky et al., 2011). A setof 28 elements was chosen for spot analysis (Al, Ti, V, Cr, Mn, Fe, Co,Ni, Cu, Zn, As, Se, Zr, Mo, Ag, Sn, Sb, Te, Ba, La, W, Pt, Au, Tl, Pb, Bi, Th,and U) in this study.

For imaging trace element distribution within pyrite, a set of touch-ing parallel lines arranged in a grid was ablated. A beam size of 10 μmwas used for sample CA11P403 and 15 μm for sample CAZK007B2.Rastering speed for each line was equal to the beam size per second(i.e., 10 μm/s and 15 μm/s, respectively). A set of 30 elements was cho-sen for image analysis (Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,Zn, As, Se, Mo, Ag, Sb, Te, Hf, Ta, Pt, Au, Hg, Tl, Pb, Bi, and U). Acquisitiontime was set to 0.002 s for most elements, except Se (0.004 s), and Ag,Te and Au (0.04 s), resulting in a total sweep time of ~0.2 s. Unpro-cessed effective image resolution along each line is ~1.5–2 times thebeam sizes. The complete maps were generated over a period of 1 to2 h to keep instrument drift in sensitivity to the minimum.

6. Results

A total of 57 LA-ICP-MS spot analyses were completed on pyrites se-lected fromdifferent samples at Chang'an deposit (Table 1), including 55of hydrothermal pyrites, one of diagenetic pyrite and one of diagenetic–metamorphic pyrite. The diagenetic and diagenetic–metamorphicpyrite grains are so small that the other kinds of minerals might be in-corporated during laser ablation. Therefore, only the data obtained

Fig. 6. Textures of pyrite types at the Chang'an gold deposit. (a) Fine grained diagenetic pyrite (DP) (CA11P111); (b) clusters of diagenetic pyritemicrocrystals partly overgrown by coarsereuhedral metamorphic/hydrothermal pyrite (MHP) (CA11P113); (c) hydrothermal pyrite (Py1) in syenite (CA11P32); (d) broken coarse pyrite (Py2) in sandstone ore (CA11P125);(e) coarse grained pyrite (Py2) with rupture in sandstone ore (CA11P121), (f) fine grained euhedral pyrite (Py3) in sandstone ore (CA11P403).


from hydrothermal pyrite analyses are discussed in this section. These55 analyses of hydrothermal pyrites include 19 spots on Py1, 21 spotson Py2 and 15 spots on Py3.

In addition to spot analyses, trace element distribution within twopyrite samples was imaged by LA-ICP-MS. Sample CAZK007B2 (Fig. 7)is from drill hole ZK007 at a depth of 336 m (Fig. 2c); and sampleCA11P403 (Fig. 8) is from the ores at the 1606 m ASL (Fig. 2b).

6.1. LA-ICP-MS profile characteristics of pyrite

In this study, no attempt wasmade to remove the effects of micron-sized inclusions on the pyrite. However, inspection of the LA-ICP-MSoutput traces from each laser spot analysis enables an estimation ofwhether a particular trace element occurs within a homogeneous invis-ible or nano-sized inclusion or as larger isolatedmicron-sized inclusionsin the pyrite (Maslennikov et al., 2009). Representative time-resolveddepth profiles for pyrites are illustrated in Fig. 9.

A total of 52hydrothermal pyrite spots containmeasurable quantitiesof gold. A feature of the dataset for Au is that Au concentrations are rela-tively constant in each sample, but with certain variation of an order ofmagnitude between pyrites from some single sample (Table 1). Audistribution patterns for most samples are relatively smooth (Fig. 9band e) with no spikes, indicating that gold occurs primarily as solidsolution or as nanoparticles in pyrite lattice.

Time-resolved depth profiles for arsenic are rarely flat, and arsenic isabundant in most samples, suggesting that the pyrites at Chang'an areAs-bearing. The parallel trends of smooth or spiky pattern between Auand As (Figs. 9a, b, e, f, 10) support the close correlation of theirconcentrations.

Measured values for Pb vary over several orders of magnitude. Themajority of the higher concentrations (numerous spikes in Figs. 9a, b,d, 10b) can be readily attributed to inclusions of galena. However, forthe sample CA11P403, consistent concentrations and flat time-resolveddepth profile (Fig. 9c and e) indicate incorporation of Pb in solid solutionup to several hundreds of ppm.

All pyrites containmeasurable quantities of Ag (Table 1). Figs. 9(a, b,d) and 10(a, b) show that the trend for Ag is parallel to those ofPb and Sb, which indicates that Ag may occur as a S–Sb compound

(e.g. argyrythrose) in galena (Cook et al., 1998). The Bi spike is similarto that of Ag in several samples (Figs. 9a and 11a), indicating thatmatildite probably occurs as solid solution or inclusion in galena.

Co and Ni are the common trace elements in pyrites (Table 1) andare usually included in pyrite lattice. The flat and similar time-resolved depth profiles (Fig. 9) for Co, Ni and Fe prove the existence ofCo and Ni in pyrites. Cu is detectable in most pyrites, but Zn is generallylower than the detection limit (Table 1), especially in Py1. Spikes of Cuor Zn occur in time-resolved depth profiles (Fig. 9c, d and f), indicatingthe probability of sphalerite or chalcopyrite inclusions.

Most time-resolved depth profiles for Al are wavy, with severalspikes (Figs. 11a, 9b and c), suggesting the occurrence of Al in pyriteas silicate inclusions. Similarly, Ti and V are often detected in pyrites(Fig. 11b and c) and possibly occur as oxide or silicate inclusions.

The concentrations of Zr, Ba, La, W, Tl, Th and U in about half of thepyrite samples are lower than detection limits (Table 1), but can reachtens or hundreds ppm in some samples. The element characteristicsand the line patterns shown in Fig. 11 suggest that these elements areascribable to microscopic inclusions.

In general, the concentrations of Cr, Mn, Se,Mo, Sn, Te and Pt inmostpyrite samples (even to 100%) are below detection limits (Table 1),although those of individual elements can be as high as hundreds ofppm, e.g. the maximum Sn content of Py2 in sample CA11P121 isN700 ppm (Fig. 11d). The dataset and line patterns (Fig. 11) indicatethat: (1) Cr is ascribable to oxide inclusion; (2) Sn and Mn may occurin pyrite lattice or as oxide inclusion; and (3) Se and Te enter the pyritelattice via isomorphism.

6.2. Data trends among different pyrite types

Element contents and their variation of the pyrites of different typesin ores and wall rocks are partly shown in Fig. 12.

From Py1 through Py2 to Py3, the Au contents gradually increase(Table 1, Fig. 12a). Au contents in Py1 range from b0.03 to 31.4 ppm(n = 19), similar to those in Py2 that range from 0.05 to 99.9 ppm(n = 21). The Py3 type pyrites have the narrowest and highest Auconcentrations of b1.3 to 91.3 ppm (n = 15).


Arsenic is themost abundant trace element in pyrite in the Chang'andeposit, with As concentrations ranging from 3.3 to 68,931 ppm, span-ning four orders of magnitude (Table 1). The As–Au plot (Fig. 12a)shows that the Py1 and Py2 pyrites have the lowest (3.3–24,651 ppm)and the highest (42.0–68,931 ppm) As distributions, respectively;while the Py3 has a relatively restricted range (Fig. 12a). A positive cor-relation exists between Au and As in both Py1 and Py3 (Fig. 12a), whichis similar to that known for pyrite in the Carlin, orogenic and epithermalgold deposits (Li et al., 2004; Reich et al., 2005; Large et al., 2009; Sunget al., 2009).

The Ag contents for Py1, Py2 and Py3 were b0.03–59.5 ppm,2.3–337 ppm and 1.3–40.2 ppm, respectively (Table 1). The Py2 andPy3 samples from ores have higher Au and Ag concentrations comparedto Py1 samples from syenite (Fig. 12b). The Pb vs. Ag and Sb vs. Ag plots(Fig. 12c and d) showprominent positive correlations amongAg, Pb andSb, which further confirms the silver occurrence.

Both Co and Ni are abundant trace elements in pyrite. Ni contentsrange from 1 to 12,000 ppm. The plot (Fig. 12e) reveals positive correla-tion between Co and Ni. Co and Ni concentrations are the highest in Py3samples. The Co/Ni ratios in Py2 and Py3 pyrites are less than 1, but inhalf of Py1 samples are more than 1, up to 12 (Table 1). On the Co/Nivs. Au/Ag plot, the Py1 spots are primarily in the first and secondquadrants with high Co/Ni values and widely-varied Au/Ag ratios;almost all Py2 spots are located in the third quadrant with Co/Ni b 1and Au/Ag b 1, but Py3 spots are mainly in the third and fourth quad-rants with Co/Ni b 1 and Au/Ag N 0.1 (Fig. 12f).

The Py1 in syenite has the lowest Cu and Zn contents comparedwithpyrites in ores. The fine-grained euhedral pyrites (Py3) from ores showthe most concentrated Cu–Zn distribution range (Fig. 12g). Thegood correlation between Cu and Zn is likely linked to inclusions ofco-existing sphalerite and chalcopyrite.

Compared to Py1 and Py2, the Py3 has more abundant traceelements as below: Al (329–8485 ppm), Ti (b21.6–2880 ppm), V(0.42–37.8 ppm), Zr (b1.1–297 ppm), Ba (b1.2–164 ppm), La(b0.03–119 ppm), W (b0.56–129 ppm), Tl (b0.56–11.7 ppm), Th(b0.06–31.1 ppm) and U (b0.06–10.5 ppm) (Tables 1 and 2).

6.3. Data trends in single pyrite grain

Spot analyses conducted on both rim and core of pyrite grains showthat the rims are enriched in certain trace elements compared to cores,especially for the coarse Py1 and Py2 grains. The Py3 grain size, ranging20−50 μm, is too small to carry laser ablation on core and rim, respec-tively (Fig. 13). Therefore, the dataset for Py3 was handled as ‘whole’pyrite (Tables 1, 2).

A total of eleven Py1 grains were analyzed on both rim and core,respectively, and the results show that the rims have higher Au, As, Coand Sb concentrations and higher Co/Ni and Au/Ag ratios than theirenveloping cores (Table 2; Figs. 13 and 14). In rims and cores of Py1pyrite grains, the Au concentrations range 0.16–31.4 ppm andb0.03–2.1 ppm, with averages of 7.4 ppm and 0.65 ppm, respectively;while the As concentrations range 3.3–24,651 ppm (av. 10,426 ppm)and 120–15,605 ppm (av. 3810 ppm), respectively. The LA-ICP-MSmapping of Py1 also supports this phenomenon that the rim has higherAu, As, Co, Ni, Sb, Ag and Pb contents than the corresponding core(Fig. 7).

A total of eleven Py2 grains were analyzed on both rim and core.The average Au and As concentrations in rims are 13.8 ppm and24,083 ppm, respectively, but in the cores, are only 2.3 ppm and7750 ppm, respectively (Table 2; Figs. 13, 14a, b). The rims are enrichedin Co, Ti and Pb (Fig. 14b, c, d). The cores and rims also show contrastingelement associations, i.e. the cores are enriched in Ag, Cu, Zn and Ni,whereas the rims enriched in Au, As, Co and Pb (Table 2).

The LA-ICP-MS mapping of Py3 (Fig. 8) does not exhibit any clearcyclic zoning of certain trace elements (e.g. As, Ag, Co, Pb, Bi, Te andAu), indicating that it was formed in a single hydrothermal event.

7. Discussions

7.1. Trace element distribution in pyrite

Trace metals in pyrite may occur in several ways: (1) as invisiblesolid solution within the crystal lattice, (2) within invisible nano-particles of sulfides (Ciobanu et al., 2012), (3) within visiblemicron-sized inclusions of sulfides, or (4) within visible micron-sized inclusions of silicate or oxide minerals (Thomas et al., 2011).According to the analytical results, pyrites of different types showsome similarities and differences in element compositions as ad-dressed below.

Siderophile and chalcophile elements, including Co, Ni, As, Se and Te,are commonly distributed in pyrite. Ni and Co enter the lattice viaisomorphous replacement of Fe, and As, Se and Te enter the lattice viareplacing S. The LA-ICP-MS time-resolved depth profiles for Co, Ni andAs are generally smooth and consistent with Fe (Fig. 9), indicatingthat these elements occur in different pyrite types via isomorphism.The concentrations of Se and Te in the Chang'an deposit are below de-tection limits (Table 1) and probably are caused by the high As contents(1000 ppm to 6.8 wt.% in general).

The metallogenetic elements are dominated by Au, Ag, Cu andPb, with minor Zn in the Chang'an gold deposit. The Au distribu-tion in most samples illustrates a relatively smooth pattern similarto As (Fig. 9b and e), indicating that gold occurs primarily as invis-ible solid solution or as nanoparticles in the pyrite lattice. Cu andPb are primarily distributed in pyrite as invisible or visible chalco-pyrite or galena inclusions (Figs. 4 and 9). The correlation betweenAg, Pb, Sb, and sometimes Bi (Section 6.1) indicates that mostAg occurs as solid solution or inclusion of Sb- or Bi-compounds(e.g. argyrythrose, matildite) in galena (Cook et al., 1998), in addi-tion to native Ag.

The other trace elements, including high-field-strength elementsand lithophile elements, such as Zr, La, Th, U, Al and Ba, usually existin the pyrite as oxide or silicate inclusions.

7.2. Pyrite genesis

As introduced in Section 4, three subtype hydrothermal pyritespresent contrasting sizes, crystal forms and textures (Fig. 6) as well ashost rocks. Furthermore, the LA-ICP-MS analyses proved their differ-ences in trace-element concentrations and occurrences. Factors thatcaused these differences are discussed below.

The Co and Ni contents in pyrite can reflect the origin of pyriteand the geological setting (Bralia et al., 1979; Cook et al., 2009).The Co/Ni ratios of Py1 range 0.11−12.55 (Table 1), with an averageof 2.06, which is the character of magmatic hydrothermal pyrite andwell consistent with its host rock (Cenozoic magmatic rock).Undoubtedly, the Py1 in syenite is closely associated with magmatichydrothermal activity. In contrast, the majority of Co/Ni ratios of Py2and Py3 in ores range 0.01–0.56 (av. 0.29), with only 3 ranging1.02–1.09 (Table 1, Fig. 12e, f), which is attributed to metamorphicand/or epizonogenic (Chen et al., 2009) hydrothermal related tosedimentary facies (Li and Zeng, 2005; Mao et al., 2009). The ore-forming fluids have high CO2 contents (up to 21.2–32.8 mol%)and low salinities (6–18 wt.% in general) (S.H. Li et al., 2011),which is a common character of metamorphic hydrothermal systems(Groves et al., 1998; Goldfarb et al., 2001; Chen et al., 2007b; Pirajno,2009; Chen, 2013; Zhang et al., 2013) and suggests that the Chang'andeposit is possibly metamorphic hydrothermal (i.e. orogenic type) inorigin.

The cores of Py1 pyrites hosted in syenite have the lowest contentsof Au, As, Ag, Cu, Pb, Zn and Ni (Figs. 13 and 14), indicating that the ini-tial syenite and/or magmatic fluids were not enriched in these ore-forming elements. However, the rims of Py1 pyrite grains are enrichedin these ore metals, with the element contents and associations being

Table 1LA-ICPMS analyses of selected pyrite types from the Chang'an gold deposit.

Sample no. Spot position Spot size(μm)

Au(ppm)

Al(ppm)

Ti(ppm)

V(ppm)

Cr(ppm)

Mn(ppm)

Co(ppm)

Ni(ppm)

Py1CA11P32-1 Core 35 2.1 3471 24.2 0.97 b2.2 0.58 31.9 38.0CA11P32-1 Rim 35 0.06 b0.87 3.3 b0.09 b2.5 1.5 0.13 0.92CA11P32-2 Core 35 1.9 4026 25.1 1.5 b2.9 1.9 9.7 91.1CA11P32-3 Core 35 1.3 9688 60.5 5.2 b2.2 3.1 19.9 36.7CA11P32-3 Rim 35 7.9 895 23.9 0.52 b1.8 b0.56 762 60.7CA11P32-4 Core 35 0.10 b0.77 2.6 b0.08 b1.8 b0.53 65.4 35.6CA11P32-4 Rim 35 17.3 2870 8.8 0.61 b2.1 0.76 149.7 31.0CAZK007B2-1 Core 35 b0.03 b0.80 b3.08 b0.08 b2.5 0.82 41.9 319CAZK007B2-1 Rim 35 1.1 118 2.1 b0.11 b2.8 b0.78 272 108CAZK007B2-2 Core 35 b0.03 b0.70 2.4 b0.08 b2.1 b0.69 4.3 4.6CAZK007B2-2 Rim 35 0.45 1.43 2.7 b0.09 b2.2 b0.84 117 134CAZK007B2-3 Core 35 1.2 0.73 2.3 b0.07 b1.9 b0.60 13.5 1.7CAZK007B2-3 Rim 35 31.4 b0.77 3.5 b0.07 b2.2 b0.69 83.5 49.4CAZK007B2-4 Core 35 0.09 b0.80 2.6 b0.08 b2.5 b0.53 29.3 222CAZK007B2-4 Rim 35 1.0 1152 8.1 0.25 b2.2 0.83 74.0 70.5CA11P114-1 Core 35 0.06 61.2 4.5 b0.09 b3.0 b0.75 3.5 4.2CA11P114-2 Core 35 0.22 2285 30.1 1.22 b2.6 b0.87 38.6 43.3CA11P114-2 Rim 35 0.16 718 157 0.34 b3.2 15.1 52.5 81.0CA11P114-3 Core 35 0.05 2971 25.3 0.47 3.4 0.86 604 1039

Py2CA11P125-1 Core 35 0.39 206 5.5 0.19 1.9 b0.64 15.3 61.9CA11P125-1 Rim 35 2.0 3773 84.8 4.1 b6.5 b2.10 44.8 319CA11P125-2 Core 35 0.91 0.99 3.3 b0.08 b2.2 b0.85 82.3 4745CA11P125-2 Rim 35 0.45 432 7.8 0.36 b2.6 b0.92 30.1 94.3CA11P125-3 Core 35 0.22 485 5.2 0.25 b2.2 b0.73 25.3 303CA11P210-1 Core 25 0.06 2532 30.8 2.5 b2.4 b0.56 3.3 8.0CA11P210-2 Core 35 0.05 643 36.7 1.04 b7.0 b2.05 40.8 86.9CA11P210-3 Whole 35 0.37 738 504 1.2 b2.9 8.1 4.8 25.8CA11P210-3 Whole 35 0.27 1786 41.9 1.3 b6.2 5.5 9.2 24.1CA11P121-1 Core 35 0.19 b0.68 b2.26 0.12 b3.0 b0.79 110 107CA11P121-1 Rim 35 1.9 140 2.4 0.78 3.9 2.0 45.1 106CA11P121-2 Core 35 15.2 224 41.1 1.4 359 3.5 110 797CA11P121-2 Rim 35 26.6 1167 28.7 5.4 65.3 64.9 11.9 63.3CA11P121-3 Core 35 5.9 b0.74 b1.9 b0.11 b2.7 5.8 77.4 1326CA11P121-3 Rim 35 99.9 b0.72 2.8 b0.10 b2.4 b0.69 131 404CA11P121-4 Core 35 0.80 b0.76 2.8 b0.10 b2.6 b0.76 68.1 121CA11P121-4 Rim 35 0.76 b0.95 2.9 b0.06 b2.9 0.90 6.7 55.8CAZK007B5-1 Core 25 0.94 791 4.2 0.34 b6.8 b2.20 0.45 79.7CAZK007B5-2 Rim 35 2.7 39.6 154 0.56 b2.5 56.5 157 317CAZK007B5-3 Core 35 0.64 6.8 2.2 b0.09 b2.3 b0.91 0.07 88.9CAZK007B5-3 Rim 35 2.6 241 464 1.5 b2.8 0.79 9.7 34.8

Py3CA11P403-1 Whole 20 24.8 2797 1719 13.1 b12.0 8.6 730 7361CA11P403-1 Whole 20 15.5 4506 171 14.1 b46.6 b18.5 1057 6112CA11P403-1 Whole 20 44.3 4740 118 19.1 110 455 15.9 844CA11P403-1 Whole 20 b1.3 1281 324 8.9 b130 b28.3 2079 5698CA11P403-2 Whole 35 62.3 6496 2880 37.8 49.7 20.4 357 8105CA11P403-2 Whole 35 8.9 2076 801 14.5 9.8 10.2 587 8906CA11P403-2 Whole 35 58.1 2716 1248 9.6 11.4 11.1 1408 12,604CA11P213-1 Whole 25 8.2 8484 133 7.4 11.1 5.2 35.9 119CA11P213-1 Whole 25 2.4 487 873 0.42 b6.2 b1.9 2.4 16.5CA11P213-2 Whole 10 7.8 1509 b67.2 b3.30 b124 b27.4 56.7 627CA11P213-2 Whole 10 2.5 6966 b36.2 3.53 b61.7 131.4 3.2 13.5CA11P213-3 Whole 10 3.5 329 b21.6 b1.48 b60.2 b13.9 17.4 158CA11P213-4 Whole 10 2.5 792 b77.4 b3.50 b86.9 b32.6 113 489CA11P504-1 Whole 25 39.8 3454 1938 4.4 4.4 b2.0 157 154CA11P504-1 Whole 25 91.3 2061 386 4.6 5.8 b1.9 479 440

DP-DMPCA11P504-3 Rim 25 b0.07 1187 990 19.2 8.5 12.8 15.8 34.0CA11P504-3 Core 25 6.8 714 1262 34.9 8.4 b1.7 29.6 85.3

Note: DP—diagenetic pyrite, DMP—diagenetic–metamorphic pyrite.


similar to the Py2 and Py3 pyrites (Figs. 13 and 14; see below). This sug-gests that the rims of Py1 were formed from a fluid system contrastingto that forming the cores of Py1, but same to those forming thepyrites ofPy2 and Py3 types.

The cores of Py2 pyrites have high contents of Au, Cu, Pb andAg, and low Co/Ni ratios, which is similar to the features of dia-genetic and metamorphic pyrites (Tables 1 and 2) addressed inprevious studies (e.g., Li et al., 2004; Cook et al., 2009; Large


Cu(ppm)

Zn(ppm)

As(ppm)

Se(ppm)

Zr(ppm)

Mo(ppm)

Ag(ppm)

Sn(ppm)

Sb(ppm)

Te(ppm)

Ba(ppm)

Py13.1 0.86 9853 b7.0 0.59 b0.07 1.3 b0.16 4.2 b0.77 7.1

237 73.1 3.3 b10.3 b0.04 0.75 4.9 b0.16 2.8 b0.38 b0.024.0 1.0 1841 b8.4 b0.04 0.18 46.6 0.21 25.5 0.51 6.23.8 1.4 1584 b9.6 b0.06 b0.13 8.3 0.23 11.5 b0.81 16.93.4 b0.68 19,807 b7.7 0.11 b0.05 2.1 b0.15 6.2 b0.44 2.70.79 b0.74 382 b7.0 b0.04 0.87 37.8 b0.11 4.6 b0.31 b0.099.1 b0.87 18,443 b8.7 b0.06 b0.05 1.4 b0.14 6.2 b0.73 10.10.60 b0.97 307 b10.9 b0.07 b0.07 1.3 b0.17 b0.15 3.7 b0.061.64 b0.93 6954 b9.7 b0.04 b0.09 0.24 b0.13 0.20 1.2 0.65

b0.47 b1.19 2379 b9.4 b0.04 b0.08 0.03 b0.14 b0.13 4.8 b0.060.84 b1.15 5480 b8.5 b0.02 b0.07 b0.03 b0.13 0.33 b0.44 b0.094.9 b1.10 15,605 b9.3 b0.04 b0.03 0.20 b0.12 0.41 b0.33 b0.107.2 b1.33 24,651 b8.3 b0.05 b0.06 b0.06 0.15 0.22 b0.54 b0.061.1 b1.17 120 b9.0 b0.07 b0.07 1.6 b0.16 1.04 9.7 b0.124.8 b1.21 5823 b10.2 b0.07 b0.09 59.5 0.32 57.8 0.62 3.1

b0.61 b0.95 1406 b11.2 4.6 b0.07 0.08 b0.17 0.15 b0.62 0.982.7 b1.38 1077 b10.2 b0.06 b0.09 2.4 b0.17 5.7 b1.0 13.30.77 b1.06 2245 b10.9 0.26 b0.04 0.35 b0.16 0.76 b0.46 6.3

13.6 1.5 432 b12.8 2.4 0.62 9.2 0.17 26.0 4.8 65.3

Py2869 13.3 2100 b13.5 b0.03 b0.03 13.2 2.2 241 b0.16 1.1195 94.7 28,536 b27.2 0.33 b0.31 98.4 b0.29 137 b1.81 24.7809 5330 3629 b9.8 b0.03 b0.06 39.2 0.81 453 b0.41 0.25637 38.3 5944 b9.6 b0.04 b0.06 18.6 1.3 348 b0.31 2.1652 48.5 2705 b10.5 b0.05 b0.06 13.0 2.1 413 b0.75 1.1402 1.9 38,347 b9.0 0.36 0.03 2.3 0.22 26.5 0.53 4.941.1 b3.2 14,786 b24.3 0.45 b0.20 3.5 b0.26 68.9 0.68 1.2

1137 56.5 68,931 b9.8 1.7 b0.08 27.8 0.19 68.6 b0.39 3.7446 70.7 64,112 b20.9 0.51 b0.20 20.5 b0.34 51.6 b0.99 5.7

2.1 b1.0 47.1 b9.9 b0.05 0.09 3.2 b0.11 7.6 b0.58 b0.06225 221 1467 b8.9 b0.04 b0.09 28.7 1.1 45.1 0.46 0.1017.8 40.1 17,817 b8.4 15.5 b0.06 10.7 0.29 18.6 0.69 0.25

219 113 6498 b11.0 0.21 b0.07 51.8 0.31 131 1.3 4.83206 16,870 471 b11.1 b0.04 b0.03 336 74.8 270 4.6 0.07

56.6 6.4 28,914 b8.7 b0.03 b0.06 15.8 0.12 31.4 1.0 b0.05830 1392 1498 b11.6 b0.02 b0.05 51.3 705 40.5 4.3 b0.06

1028 4577 42.0 b13.9 b0.06 b0.07 97.5 774 35.7 3.2 b0.06949 13,609 2544 b23.3 b0.18 b0.11 12.6 1.8 139 b0.89 3.272.0 b6.3 11,087 b9.0 21.6 0.12 9.3 b0.12 62.5 1.2 1.5

1036 71.8 1309 b10.7 b0.02 b0.09 10.2 9.0 159 0.49 0.08333 b2.5 25,302 b8.5 2.0 0.09 8.7 0.11 107 b0.71 1.7

Py3193 b6.3 12,227 b41.6 86.6 1.9 18.2 b0.60 101 b2.6 6.8143 b49.6 7362 b173 10.3 303 10.0 b1.8 69.3 b7.7 9.7136 b31.3 11,944 b354 5.0 b2.3 6.8 b2.9 34.1 8.9 16452.2 b34.2 1618 b428 49.4 b5.5 4.7 b4.5 26.5 b19.0 18.5

274 6.2 14,561 b11.9 296 2.7 26.6 0.53 178 b0.36 4.6123 5.6 5806 b13.2 171 4.0 8.3 b0.13 69.2 b0.27 15.4270 4.4 12,394 b13.4 231 7.7 24.8 0.17 197 0.94 4.1104 b8.4 9270 b26.7 10.1 b0.11 17.6 b0.28 296 b1.4 11.180.0 b2.8 21,204 b26.5 4.3 b0.10 8.4 b0.24 44.6 b0.97 1.3

174 b31.5 9116 b349 4.6 b2.3 40.2 b3.9 215 b20.1 b2.172.4 b25.1 22,744 b232 b1.1 b1.4 8.9 2.5 33.1 b7.7 7.970.4 b18.4 17,177 b210 b1.1 b1.3 12.1 b2.0 104 b7.5 b1.266.3 b41.3 20,343 b336 b1.4 b2.5 11.2 b3.8 66.4 b6.4 7.592.6 6.0 2590 b47.5 6.3 1.4 1.3 b0.45 20.8 2.4 7.6

188 b1.7 8461 b39.2 36.9 5.1 3.5 b0.44 39.7 9.5 8.8

DP-DMP14.2 4.8 12.9 b42.5 37.8 b0.20 0.19 b0.46 2.5 2.9 1.5

125 3.0 467 b44.6 62.9 0.45 0.90 b0.44 10.7 16.2 4.0

(continued on next page)


et al., 2009; Maslennikov et al., 2009). Therefore, we can deducethat the Py2 cores were formed by the metamorphic fluidssourced from metamorphic devolatilization of sedimentarystrata.

Py3 grains and the rims of Py1 and Py2 pyrites share many com-mon features, including the Au, As, Cu, and Co contents and Au/Agand Co/Ni ratios. They are plotted in similar areas in Fig. 14, sug-gesting that they were deposited from the same fluid system.


Sample no. La(ppm)

W(ppm)

Pt(ppm)

Tl(ppm)

Pb(ppm)

Bi(ppm)

Th(ppm)

U(ppm)

Au/Ag Co/Ni

Py1CA11P32-1 0.20 0.06 b0.02 0.08 14.7 0.05 0.06 0.06 1.60 0.84CA11P32-1 b0.002 b0.03 b0.05 b0.02 16.4 0.46 b0.01 b0.002 0.01 0.14CA11P32-2 b0.002 b0.04 b0.03 0.10 3182 56.0 b0.01 b0.01 0.04 0.11CA11P32-3 0.03 0.09 b0.03 0.20 652 6.7 0.004 b0.01 0.16 0.54CA11P32-3 0.01 b0.03 b0.01 b0.02 54.6 0.10 0.03 0.02 3.82 12.55CA11P32-4 b0.002 b0.01 b0.02 0.01 2896 70.9 b0.004 b0.01 0.003 1.83CA11P32-4 0.04 b0.04 b0.01 0.05 41.0 0.31 0.04 0.01 12.1 4.82CAZK007B2-1 b0.01 b0.02 b0.03 b0.01 3.0 5.7 b0.01 b0.01 – 0.13CAZK007B2-1 b0.01 b0.03 b0.01 b0.01 1.6 b0.02 b0.003 b0.01 4.37 2.51CAZK007B2-2 b0.01 b0.02 b0.02 b0.02 b0.04 0.02 b0.01 b0.01 – 0.91CAZK007B2-2 b0.01 b0.03 b0.04 b0.02 1.1 b0.03 b0.01 b0.01 – 0.88CAZK007B2-3 b0.01 b0.01 b0.03 b0.02 8.1 b0.02 b0.01 b0.01 6.07 8.00CAZK007B2-3 b0.01 b0.12 b0.02 b0.02 1.3 b0.02 b0.01 b0.01 – 1.69CAZK007B2-4 b0.01 b0.05 b0.03 0.03 5.9 b0.05 b0.01 0.01 0.05 0.13CAZK007B2-4 0.05 b0.05 b0.02 0.15 23,777 27.74 b0.01 b0.01 0.02 1.05CA11P114-1 b0.01 b0.02 b0.04 b0.03 2.1 4.9 1.7 0.49 0.81 0.81CA11P114-2 0.18 0.02 b0.04 0.15 30.1 22.9 0.02 0.02 0.09 0.89CA11P114-2 0.15 0.54 b0.02 0.03 6.3 13.5 0.06 0.06 0.44 0.65CA11P114-3 0.18 0.06 b0.02 0.28 168 20.2 0.47 12.4 0.01 0.58

Py2CA11P125-1 10.91 0.07 b0.02 3.2 496 1.9 2.4 0.03 0.03 0.25CA11P125-1 b0.01 1.00 b0.09 0.49 7013 12.2 0.05 b0.03 0.02 0.14CA11P125-2 b0.01 b0.03 b0.03 5.1 708 1.7 b0.01 b0.01 0.02 0.17CA11P125-2 b0.01 0.09 b0.03 4.2 668 2.3 b0.004 b0.01 0.02 0.32CA11P125-3 b0.01 0.06 b0.04 5.2 678 4.9 b0.003 b0.01 0.02 0.08CA11P210-1 0.13 0.17 b0.01 0.48 42.1 0.61 0.28 0.02 0.02 0.42CA11P210-2 0.05 0.29 b0.04 1.0 72.1 0.84 0.13 0.02 0.02 0.47CA11P210-3 0.82 1.63 b0.02 1.6 60.2 2.2 0.40 0.14 0.01 0.19CA11P210-3 0.69 0.72 b0.06 2.4 96.2 3.3 0.06 0.02 0.01 0.38CA11P121-1 b0.01 b0.03 b0.03 b0.02 115 1.2 b0.004 b0.01 0.06 1.03CA11P121-1 0.01 b0.04 b0.03 0.05 1221 9.8 b0.01 b0.004 0.07 0.42CA11P121-2 0.53 1.9 b0.04 0.23 471 0.99 0.95 0.59 1.42 0.14CA11P121-2 0.20 0.15 b0.02 0.04 2753 7.4 0.05 b0.01 0.51 0.19CA11P121-3 b0.01 b0.03 b0.05 0.11 3843 6.8 b0.01 b0.01 0.02 0.06CA11P121-3 b0.01 b0.03 b0.03 0.02 2533 3.4 b0.01 b0.004 6.32 0.32CA11P121-4 b0.02 b0.03 b0.03 0.05 222 0.09 b0.01 b0.01 0.02 0.56CA11P121-4 0.02 b0.03 b0.04 0.02 86.9 0.09 b0.003 b0.01 0.01 0.12CAZK007B5-1 b0.02 b0.08 b0.11 1.1 114 3.2 b0.01 b0.02 0.07 0.01CAZK007B5-2 0.18 0.76 b0.03 1.1 55.8 2.3 0.86 0.60 0.29 0.50CAZK007B5-3 b0.02 b0.04 b0.02 1.5 151 4.2 b0.01 b0.003 0.06 0.001CAZK007B5-3 0.02 3.4 b0.03 1.0 383 1.2 0.23 0.14 0.30 0.28

Py3CA11P403-1 5.1 44.9 b0.24 2.6 392 14.9 2.5 1.5 1.36 0.10CA11P403-1 2.0 17.6 b0.96 2.7 372 15.6 0.43 b0.12 1.54 0.17CA11P403-1 10.2 b0.56 b0.75 1.7 138 b0.78 0.23 0.26 6.49 0.02CA11P403-1 54.8 2.4 b1.2 b0.56 181 15.4 3.7 0.38 – 0.36CA11P403-2 27.8 129 b0.05 7.2 567 5.1 12.6 9.8 2.34 0.04CA11P403-2 90.5 12.8 b0.02 2.7 220 14.6 9.8 10.5 1.08 0.07CA11P403-2 82.7 27.5 0.02 6.8 567 11.4 11.3 5.5 2.34 0.11CA11P213-1 0.03 1.1 b0.08 11.7 456 5.0 0.60 0.54 0.47 0.30CA11P213-1 b0.03 1.4 b0.08 0.55 115 1.3 0.12 0.16 0.28 0.15CA11P213-2 b0.24 b1.1 b1.53 4.4 460 7.8 b0.10 b0.28 0.19 0.09CA11P213-2 b0.15 0.78 b0.70 0.51 89.1 0.75 0.31 b0.06 0.28 0.24CA11P213-3 b0.14 b0.65 b0.67 1.3 170 2.1 b0.06 b0.08 0.29 0.11CA11P213-4 b0.24 b1.1 b1.1 2.3 348 3.0 b0.21 0.26 0.22 0.23CA11P504-1 118 5.4 b0.12 2.1 68.3 8.8 31.1 1.4 31.41 1.02CA11P504-1 6.2 1.2 b0.12 2.9 147 22.2 3.5 0.64 25.76 1.09

DP-DMPCA11P504-3 1.8 2.7 b0.13 0.03 45.9 3.7 2.9 1.7 – 0.47CA11P504-3 1.9 7.1 b0.11 0.77 151 52.6 3.6 2.0 7.57 0.35

Table 1 (continued)


These analyses overlap the domain for the Py2 cores by a large por-tion, and the domain for the Py1 cores by a smaller portion. Thisphenomenon possibly indicates that the fluids forming Py3 andthe rims of Py1 and Py2 were mainly evolved from what formed

the Py2 cores, and mixed with fluids evolved from what formedthe Py1 cores.

According to the texture characteristics and trace-element geo-chemistry discussed above, the hydrothermal pyrites in the Chang'an

Fig. 7. LA-ICP-MS images of trace element distribution (in counts per second) in pyrite (Py1) from syenite CAZK007B2 in the Chang'anmine. This pyrite contains a clean core ofmagmatichydrothermal origin, surrounded by a rim of metamorphic–magmatic hydrothermal origin. The LA-ICP-MS maps show zonation of various elements from core to rim. Compared to thecore, the outer rim is enriched in Au, As, Co, Ni, Ag, Pb and Sb (warm colors, yellow and red). There is very minor Cu, Zn, and Bi enrichment (cool colors, green and blue) in rim, andnone in core.


deposit belong to different geneses, i.e. the core of Py1 formed by mag-matic fluids, the core of Py2 bymetamorphic fluids, and the Py3 and therims of Py1 and Py2 were related to mixture of evolved metamorphicand magmatic fluids.

7.3. Gold source

The gold source of the Chang'an gold deposit has not been wellconstrained yet. Based on element geochemical study of the deposit,He et al. (2008) proposed that both the intrusive and sedimentaryrocks could provide gold for the metallogenic system. Chen et al.(2010) suggested that the Cenozoic magmatic intrusions (e.g. t syenite,lamprophyre) played a key role due to the similarity in sulfur and leadisotopic compositions between the ores and intrusive rocks. S.H. Liet al. (2011) argued that although Cenozoic magmatic activities provid-ed heat for the metallogenic system, but the gold and associatingore-forming components were mainly sourced from sedimentary wallrocks via hydrothermal processes. However, these previous under-standings have been drawn from data obtained from massive pyritegrains, without genetic and rim–core distinguishing. The in-situ traceelement analyses of single pyrite grains using LA-ICP-MS method pro-vided us powerful constraints on the metal source of the Chang'angold deposit.

The ore-forming metals, such as Au, As, Ag, Cu, Pb and Zn, were notrich in initial intrusive rocks and magmatic fluids as indicated by thecores of Py1 in syenite, but were remarkably enriched in metamorphicfluids represented by the cores of Py2 grains that formed by diagenesisto metamorphism of the sedimentary host-rocks. Moreover, the Py3grains with highest Au and Ag contents only occur in sandstone-type

ores, which strongly supports that the ore-forming metals must bemainly sourced from the sedimentary host-rocks via metamorphicdevolatilization. This understanding can also be supported by thecontrasting contents of Au and other ore-forming metals between therims and cores of Py1 pyrite grains.

7.4. Ore-forming process

So far no isotope age has been obtained for the Chang'an gold depos-it, because minerals suitable for dating are not observed at the deposit.However, as mentioned in Section 3, the syenite and syenite porphyrywhich pre-date but may be genetically related to the formation ofthe orebodies at the Chang'an deposit yield zircon U−Pb ages of34–35 Ma, suggesting that the mineralization occurred slightlylater than 34 Ma. Fully in accordance, the hydrothermal sericite frommylonitized and sericitized diorite in the Daping gold deposit yieldeda 40Ar–39Ar plateau age of 33.76 ± 0.65 Ma (Sun et al., 2007), whilethe Tongchang and Chang'anchong Cu–Mo deposits yielded molybde-nite Re–Os ages of 34.38 ± 0.5 Ma and 34.54 ± 0.69 Ma, respectively(Wang et al., 2005). This shows that a significant tectono-magmaticand metallogenic event occurred around 33–35 Ma in the Jinping re-gion, including the formation of the Chang'an deposit, which is coevalwith the regional metallogenic event in the Sanjiang Tethyanmetallogenic domain (Chen, 2002; Chen et al., 2007a; Hou et al., 2007).

According to the previous studies on tectonic setting and evolution(Hou et al., 2007; Xia et al., 2011; Deng et al., 2013b; Zhu et al., 2013),the India–Asia collision since the Paleocene resulted in the large-scalestrike–slip fault system, nappe–thrust system and tectonic–magmaticbelts in the Sanjiang Tethyan domain. Correspondingly, the Chang'an

Fig. 8. LA-ICP-MS images of trace element distribution (in counts per second) in pyrite (Py3) from ore sample CA11P403. These pyrites are fine grained euhedral and distributed as cluster.The images show that thepyrite core contains relatively elevatedCo, Ni, and Bi. As awhole, theAu, As, Ag, Sb, Pb, Zn andCudistribution in counts shows no difference between core and rim.


deposit, as well as other large deposits in the Ailaoshan gold ore belt,formed in this favorable setting. From the point of trace elements inpyrite, we propose its ore-forming process in the following.

(1) The India–Asia continent collision (65–40 Ma) caused thrustfaults and several nappes in the Ailaoshan gold belt (such asthe Jinping nappe), as well as the reactivity of the Ailaoshanand Mojiang faults (Fig. 1) and their subsidiary faults F5 and F6(Fig. 2). During this period, some original diagenetic pyrite inthe Lower Ordovician Xiangyang Group was recrystallized andthemetamorphic pyrite formed (Fig. 6a and b). Driven by region-al structural system, the fluids sourced from metamorphicdevolatilization of the sedimentary strata upwardly migratedand extracted ore-forming metals from the host rocks. In thebrittle zones along faults F6 and F5 and their subsidiary splay,the pyrite of metamorphic hydrothermal origin (the core ofPy2) was precipitated (Fig. 15a). It is expected to be coarserand clear grained, and to seldom bear other mineral inclusions(Figs. 6d, e and 13) owing to the higher temperature (N200 °C)and slower growth, which allow the trace elements to bepartitioned into separate mineral phases rather than incorporat-ed into pyrite in solid solution or as tiny (b5 μm) inclusions(Butler and Rickard, 2000; Li et al., 2004; Large et al., 2009).

(2) During 40–26 Ma, decompression of the India–Asia continentalcollision caused structural extension and strike–slip faulting(Fig. 1) (Hou et al., 2007; Mo et al., 2007; Deng et al., 2013b),resulting in fragmentation of the already formed Py2 core(Figs. 6d, e and 15b). At the same time, magma intruded alongfaults, forming numerous dikes and stocks with various scales.Accompanied with the subsequent magmatic hydrothermal ac-tivity, the primary Py1was formed and occurred as coarse grainswith euhedral shape (Figs. 6c, 15b) because of the high temper-ature and slow cooling process.

(3) Finally, the multiply sourced fluids circulated in host rocks andextracted ore-forming metals from the epidote-metamorphicstrata (Zhang et al., 2010). Theymightmix in favorable structuralpositions, e.g. brittle fractures, and quickly discharge theirtransported metals owing to the drastic changes in physical andchemical conditions, which resulted formation of Py3 and rimsof Py1 and Py2. The rapid formation caused the trace elementsto incorporate into pyrite in solid solution or as very small(b5 μm) inclusions rather than separate sulfide phases. There-fore, Py3 in this stage is fine euhedral grains, usually 20−50 μmelongated (Figs. 6 and 13), and enriched in trace elements suchas Au (Tables 1 and 2); and coincidently, the Py1 and Py2 pyritesoften have Au–As-rich rims (Figs. 7, 13 and 15c).

Fig. 9.Representative time-resolved depth profiles for pyrite analyzed in this study indicating the occurrences of gold and othermajormetal elements. Fe exhibits a relatively flat responsetypical of major elements in homogeneous pyrite. The parallel trends of the counts for Au and As suggest that these elements occur as solid solution in pyrite or as nanoparticles. Lead iselevated and spiky, suggesting abundantmicro-inclusions of galena in the pyrite. Silver shows a very similar pattern, suggesting that the Ag is present in the structure of galena inclusions.See text for additional explanation.


8. Concluding remarks

The Chang'an gold deposit, Yunnan province, is one of five largegold deposits in the Ailaoshan gold belt that is the most importantpart of the Sanjiang Tethyan metallogenic domain. The orebodies ofthe deposit are structurally controlled and hosted in the Early Ordovi-cian Xiangyang Group meta-sandstones.

Hydrothermal pyrite is the dominant Au-bearing mineraland can be distinguished between three types: the coarseeuhedral grains in syenite (Py1), the coarse grains in sandstonesor sandstone ores (Py2), and the fine-grained euhedral grains insandstone ores (Py3). Types Py1 and Py2 show clear rim–corestructures.

Analysis using laser ablation inductively coupled plasmamass spectroscopy shows that gold in pyrite at the Chang'an de-posit occurs mainly in lattice, as invisible solid solution and as

nanoparticles. Co, Ni and As occur in pyrite as isomorphism. Ag,Cu, Pb and Zn are distributed primarily as invisible or visible sulfideinclusions. Al, Cr and Ti primarily occur as oxide or silicateinclusions.

The ore metals are enriched in Py2 cores, but not in Py1 cores,implying that gold mineralization was mainly related to the meta-sedimentary rocks or metamorphic fluids, instead of the initialintrusive rocks and magmatic fluids. This interpretation can be fullysupported by the Au-enrichment in Py3 pyrites that only occur insandstone ores, and by the Au-enriched rims of Py1 and Py2, whichformed coevally with Py3 pyrites.

The formation of the Chang'an deposit slightly postdates the 34−35 Ma syenite intrusion, coeval with the strike-slipping along theAilaoshan fault. It was related to the regional metallogenic eventaround 33−35 Ma in the Sanjiang Tethyan metallogenic domain,which resulted from the India–Asia continental collision.

Fig. 10. LA-ICP-MS counts for a laser line burn across pyrite aggregate in ore sampleCA11P403 (a) and pyrite grain in syenite sample CAZK007B2 (b), showing the varioustypes of response possible for major metallogenetic elements in pyrite and matrix. Thered line on the photographs indicates the position of laser line. See text for additionalexplanation.


Acknowledgments

This research was financially supported by the National BasicResearch Program (No. 2009CB421006), the National Nature ScienceFoundation of China (No. 41030423), the Fundamental ResearchFunds for the Central Universities (Nos. 2652013017 and 2010ZD11)and the Chinese Scholarship Council (support for Jing Zhang duringher stay in Australia). We especially thank Senior Engineers Jian-rongLi for support in the field survey, and Sarah Gilbert and Jay Thompsonfor their assistance with the LA-ICP-MS trace element analyses.Professor Shao-yong Jiang and Cristiana L. Ciobanu are thanked fortheir critical reviews and constructive comments.

Fig. 11. Representative time-resolved depth profiles showing distribution of Mn, Se, Tl, Biand other trace elements in pyrite from Chang'an; see text for additional explanation.

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Fig. 12. Binary plots of (a) Au vs. As, (b) Au vs. Au, (c) Pb vs. Ag, (d) Sb vs. Ag, (e) Ni vs. Co, (f) Co/Ni vs. Au/Ag, and (g) Cu vs. Zn in different pyrite types. The trace element concentrationsare from Table 1, or calculated using 1/3 mdl values for all measurements bmdl (minimum detection limit).


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Table 2Summary of LA-ICP-MS data on elemental concentrations in pyrite (in ppm).

Au As Co Ni Cu Zn Mo Ag Sn Sb Te Tl Pb Bi Th U

Py1Core (11)MEAN 0.65 3180 78.4 167 3.2 0.67 0.17 9.9 0.09 7.2 2.3 0.08 633 17.1 0.21 1.2S.D. 0.83 4944 176 306 3.9 0.46 0.29 16.4 0.07 9.8 3.2 0.09 1207 24.5 0.5 3.7MIN b0.03 120 3.5 1.7 b0.47 b0.68 b0.06 b0.06 b0.11 b0.13 b0.31 b0.01 b0.04 b0.02 b0.004 b0.01MAX 2.1 15,605 605 1039 13.6 1.5 0.87 46.7 0.23 26.0 9.7 0.28 3182 71.0 1.8 12.4

Rim (8)MEAN 7.4 10,426 189 67.0 33.1 9.4 0.11 8.6 0.09 9.3 0.35 0.03 2987 5.3 0.02 0.01S.D. 11.4 9164 245 42.1 82.6 25.7 0.26 20.6 0.10 19.8 0.36 0.05 8400 10.2 0.02 0.02MIN 0.06 3.3 0.13 0.92 b0.47 b0.68 b0.03 b0.03 b0.12 0.20 b0.33 b0.01 1.1 b0.02 b0.003 b0.002MAX 31.4 24,651 762 134 237 73.1 0.75 59.5 0.32 57.8 1.2 0.15 23,777 27.7 0.06 0.06

Py2Core (11)MEAN 2.3 7750 48.6 314 802 3398 0.03 45.1 72.5 167 1.1 1.6 629 2.4 0.35 0.06S.D. 4.6 11,753 43.0 410 888 6109 0.02 97.9 211 159 1.7 2.0 1094 2.1 0.75 0.18MIN 0.05 47.1 0.07 8.0 2.1 b1.0 b0.03 2.3 b0.11 7.6 b0.16 b0.02 42.1 0.09 b0.003 b0.004MAX 15.2 38,347 111 1326 3207 16,871 0.09 337 705 453 4.6 5.2 3843 6.8 2.4 0.59

Rim (10)MEAN 13.8 24,083 45.1 145 435 518 0.05 37.7 77.8 102 0.86 1.1 1487 4.4 0.16 0.09S.D. 31.3 24,844 54.7 144 383 1428 0.04 34.0 245 94.5 0.93 1.4 2190 4.0 0.27 0.19MIN 0.27 42.0 4.8 24.1 56.7 b2.5 b0.06 8.7 b0.12 31.4 b0.31 0.02 55.8 0.09 b0.01 b0.003MAX 99.9 68,931 157 404 1138 4577 0.12 98.4 774 348 3.2 4.2 7013 12.2 0.86 0.60

Py3(15)MEAN 24.8 11,788 473 3444 136 6.9 22.1 13.5 0.69 99.9 5.4 3.3 287 8.6 5.1 2.1S.D. 28.0 6476 618 4240 71.3 4.5 78.0 10.4 0.71 83.7 4.4 3.1 174 6.8 8.4 3.5MIN b1.3 1618 2.4 13.5 52.2 b1.7 b0.10 1.3 b0.13 20.8 b0.27 b0.56 68.3 b0.78 b0.06 b0.06MAX 91.3 22,744 2079 12,604 274 b49.6 303 40.2 2.5 296 9.5 11.7 567 22.2 31.1 10.5

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Fig. 13. Spot LA-ICP-MS analyses of selected hydrothermal pyrites from the Chang'an gold deposit. Spot position and size of the laser analyses are shown in red circles.


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Fig. 14. Core and rim compositions of different pyrite types; (a) Auvs. Ag; (b) Pb vs. As; (c)Co vs. Ni; (d) Ti vs. Cu.


Fig. 15. Schematic textural and chemical evolution of hydrothermal pyrite in ore and syenite at Chang'an.


la-icp-ms trace element analysis of pyrite from the chang ... · la-icp-ms trace element analysis...

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