Paragenesis and geochemistry of ore minerals in the ... · ARTICLE Paragenesis and geochemistry of...

ARTICLE

Paragenesis and geochemistry of ore minerals in the epizonalgold deposits of the Yangshan gold belt, West Qinling, China

Nan Li & Jun Deng & Li-Qiang Yang &

Richard J. Goldfarb & Chuang Zhang & Erin Marsh &

Shi-Bin Lei & Alan Koenig & Heather Lowers

Received: 24 November 2012 /Accepted: 14 November 2013 /Published online: 13 December 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Six epizonal gold deposits in the 30-km-longYangshan gold belt, Gansu Province are estimated to containmore than 300 t of gold at an average grade of 4.76 g/t andthus define one of China's largest gold resources. Detailedparagenetic studies have recognized five stages of sulfidemineral precipitation in the deposits of the belt. Syngenetic/diagenetic pyrite (Py0) has a framboidal or colloform textureand is disseminated in the metasedimentary host rocks. Earlyhydrothermal pyrite (Py1) in quartz veins is disseminated inmetasedimentary rocks and dikes and also occurs as semi-massive pyrite aggregates or bedding-parallel pyrite bands inphyllite. The main ore stage pyrite (Py2) commonly over-grows Py1 and is typically associated with main ore stagearsenopyrite (Apy2). Late ore stage pyrite (Py3), arsenopyrite(Apy3), and stibnite occur in quartz ± calcite veins or aredisseminated in country rocks. Post-ore stage pyrite (Py4)occurs in quartz ± calcite veins that cut all earlier formedmineralization. Electron probe microanalyses and laserablation-inductively coupled plasma mass spectrometry

analyses reveal that different generations of sulfides have char-acteristic of major and trace element patterns, which can beused as a proxy for the distinct hydrothermal events.Syngenetic/diagenetic pyrite has high concentrations of As,Au, Bi, Co, Cu, Mn, Ni, Pb, Sb, and Zn. The Py0 also retainsa sedimentary Co/Ni ratio, which is distinct from hydrothermalore-related pyrite. Early hydrothermal Py1 has high contents ofAg, As, Au, Bi, Cu, Fe, Sb, and V, and it reflects elevated levelsof these elements in the earliest mineralizing metamorphicfluids. The main ore stage Py2 has a very high content of As(median value of 2.96 wt%) and Au (median value of47.5 ppm) and slightly elevated Cu, but relatively low valuesfor other trace elements. Arsenic in the main ore stage Py2occurs in solid solution. Late ore stage Py3, formed coevallywith stibnite, contains relatively high As (median value of1.44 wt%), Au, Fe, Mn, Mo, Sb, and Zn and low Bi, Co, Ni,and Pb. The main ore stage Apy2, compared to late ore stagearsenopyrite, is relatively enriched in As, whereas the laterApy3 has high concentrations of S, Fe, and Sb, which isconsistent with element patterns in associated main and lateore stage pyrite generations. Compared with pyrite from otherstages, the post-ore stage Py4 has relatively low concentrationsof Fe and S, whereas As remains elevated (2.05∼3.20 wt%),which could be interpreted by the substitution of As− for S inthe pyrite structure. These results suggest that syngenetic/diagenetic pyrite is the main metal source for the Yangshangold deposits where such pyrite was metamorphosed at depthbelow presently exposed levels. The ore-forming elements wereconcentrated into the hydrothermal fluids during metamorphicdevolatilization, and subsequently, during extensive fluid–rockinteraction at shallower levels, these elements were precipitatedvia widespread sulfidation during the main ore stage.

Keywords Orogenic gold . Geochemistry . Pyrite .

Yangshan .West Qinling . China

Editorial handling: G. Beaudoin

Electronic supplementary material The online version of this article(doi:10.1007/s00126-013-0498-8) contains supplementary material,which is available to authorized users.

N. Li : J. Deng (*) : L.<Q. Yang :R. J. Goldfarb : C. ZhangState Key Laboratory of Geological Processes and MineralResources, China University of Geosciences, No. 29 Xueyuan Road,Beijing 100083, People’s Republic of Chinae-mail: [email protected]

R. J. Goldfarb (*) : E. Marsh :A. Koenig :H. LowersU.S. Geological Survey, Box 25046, Mail Stop 973, Denver FederalCenter, Denver, CO 80225, USAe-mail: [email protected]

S.<B. LeiHeadquarters of Gold Exploration Branch of Chinese Armed PoliceForce, Beijing 100055, People’s Republic of China

Miner Deposita (2014) 49:427–449DOI 10.1007/s00126-013-0498-8

http://dx.doi.org/10.1007/s00126-013-0498-8

Introduction

China's orogenic belts are prospective areas for gold explora-tion, and this has led to China being the world's largest goldproducer for the past 5 years. The Yangshan gold belt, a majorgold resource, was discovered between 1993 and 1997 by thePeople's Armed Police Gold Mining Troops. The 30-km-longYangshan gold belt in the Qinling-Dabie orogenic belt extendsfrom Tangpugou in the west to Guzhen in the east (Fig. 1). Atotal of 96 gold-bearing lodes in six main deposits have beenrecognized in the gold belt and the identified recoverableresource is reported to be 308 tonnes (t) Au. It ranks as oneof the largest gold resources in China, with an average gradeof 4.76 g/t, and the resource is increasing with continuedexploration (Yan et al. 2010).

From west to east, the Yangshan gold belt is divided intothe Nishan, Getiaowan, Anba, Gaoloushan, Guanyinba, andZhangjiashan gold deposits (Fig. 1). Among the six golddeposits, the Anba deposit contains ∼90 % of the total goldresource (281 t with an average grade of 4.77 g/t), making itthe largest gold deposit defined in China. The Guanyinba golddeposit has 18 t Au at an average grade of 4.96 g/t, and theGetiaowan gold deposit contains 8.6 t Au at an average gradeof 4.14 g/t (Yan et al. 2010). Exploration of the Anba golddeposit is almost complete, with both underground and openpit mining activities scheduled to begin in 2014. Unlike Anba,the five other gold deposits have been poorly explored,through surface outcrops, shallow adits, and a few drill holes.

The recent Chinese literature on the Yangshan gold belt hasfocused on ore-controlling structures (Yuan et al. 2004; Liet al. 2008b), magmatic rocks (Qi et al. 2006a; Liu et al. 2008;Lei et al. 2010), hydrothermal fluids (Liu et al. 2003; Li et al.2007a; Li et al. 2008a), gold-bearing mineral assemblages(Wu et al. 2008; Mao et al. 2009; Yang et al. 2009), geochro-nology (Qi et al. 2006b; Yang et al. 2006; Lei et al. 2010), anddeposit classification (Chen et al. 2004; Li et al. 2007a; Qiet al. 2008; Liu et al. 2010). Insufficient petrological evidenceofmineral paragenesis, however, hinders significance ofmanyprevious studies, and there remains extensive debate about thegenesis and classification of the deposits in the belt.

The Anba gold deposit has more extensive workings andmore exposed mineralization compared to the five other golddeposits, but the mineral paragenesis of the Anba gold depositwas only briefly described in previous studies. Guo et al.(2002) and Yuan (2007) proposed four hydrothermal stages:early unmineralized quartz, pyrite–quartz, pyrite–arsenopy-rite–quartz, and late quartz–calcite. The widespread stibnitewas not discussed in these studies. Liu et al. (2003) and Liet al. (2007b) pointed out the existence of stibnite in a late orestage and suggested quartz–pyrite, quartz–arsenopyrite–py-rite, quartz–stibnite, and quartz–calcite stages. Yang et al.(2006), Li et al. (2007a), and Mao et al. (2009) argued thatthe hydrothermal stages should be divided into quartz–

sericite–pyrite, pyrite–arsenopyrite–quartz, arsenopyrite–py-rite–quartz, gold–stibnite–quartz–calcite, and calcite–quartz.However, due to the fact that most ore minerals are finegrained and disseminated in host rocks, no supporting petro-logical evidence ofmineral paragenesis has yet been presentedin literature. This makes it difficult to convincingly define thecomposition of the hydrothermal fluids, gold-associated min-erals, and geochemical relationships that must be based onmeticulous mineral paragenesis work and definition of asso-ciated ore-forming events. For example, previous researchersconducted preliminary investigations of sulfide trace elementgeochemistry (Wu et al. 2008; Mao et al. 2009), but therelatively high detection limit of trace elements by electronprobe microanalyzer (EPMA), as well as the lack of petrolog-ical evidence for the paragenetic sequence, limits the possiblegeological interpretations from their results.

This paper presents detailed petrological evidence to definethe mineral paragenesis of the Anba and related gold deposits.It discriminates the different sulfide generations by their char-acteristic major and trace element patterns, which can be usedas a proxy for defining the specific hydrothermal events in thegold belt (e.g., Thomas et al. 2011). The results of this papermay be useful for further exploration through the identifica-tion of sulfide minerals in the region that are related to thegold-forming event and for helping to classify the paragenet-ically complex deposits of the Yangshan gold belt.

Regional geologic setting

It was first proposed that the Qinling orogenic belt was builtthrough closure of the Shangdan oceanic basin between theNorth China and Yangtze Cratons (Mattauer et al. 1985;Zhang et al. 2001). However, more recent recognition of aSouth Qinling Block between the cratons, originally a part ofthe northern margin of the Yangtze Craton that had been riftedduring the middle Paleozoic, has led to the now widely ac-cepted “three-plate with two-suture zone” tectonic model(Zhang et al. 1995; Meng and Zhang 1999; Zhang et al.2001). The geologic framework of the Qinling orogen is thusviewed as a product of lengthy convergence and the closure oftwo basins between the North China Craton, South QinlingBlock, and Yangtze Craton, which are separated by the Shang-Dan and Mian-Lue suturing fault systems, respectively(Fig. 1b; Meng and Zhang 2000).

During Middle Devonian to Early Carboniferous (Zhanget al. 2004), extension along the northern Yangtze cratonmargin led to rifting of the South Qinling Block. The blockseparated the opening Mian-Lue and preexisting Shang-Danpaleo-oceans into two basins (Zhang et al. 1996). Rifting wasassociated with the formation of Devonian and Carboniferouscontinental shelf-basinal sedimentary strata along the Mian-Lue ocean basin margins (Zhang et al. 2004). At the same

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time, to the north, the middle Paleozoic northern Qinling arcterrane was developed along the suture between the SouthQinling Block and North China Craton as the Shang-Danocean basin closed.

The Mian-Lue ocean basin reached its maximum widthfrom Early Carboniferous to Early Permian (Zhang et al.2004). During the Middle Permian to Middle Triassic, theMian-Lue ocean basin began to close, accompanied by

Fig. 1 Simplified regional geology and deposit geology of the Yangshangold belt (modified from Chen et al. 2004; Zhao 2009). a Tectonicsshowing the position of West Qinling in the Qinling-Dabie orogenic belt.

b Schematic map showing the tectonics and distribution of gold depositsin West Qinling. c Geology map of the Yangshan gold belt

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subduction of oceanic crust below the South Qinling Block.This led to the Late Permian island arc magmatism along thepreviously passive continental margin on the southern edge ofthe South Qinling Block (Zhang et al. 2004). In Middle–LateTriassic, final closure of the Mian-Lue basin along the Mian-Lue fault system led to the continent–continent collisionalorogenesis that characterized the West Qinling orogen and isassociated with the widespread gold mineralization. This in-cluded intense deformation, metamorphism, and magmatismat the end of the Middle Triassic and development of a marinefacies foreland basin system (Zhang et al. 1996).

As a result of orogenic uplift during Late Triassic toMiddleJurassic, particularly characterized by extensive thrusting andfolding on the southern side of the Mian-Lue fault, a forelandbasin formed in the front of the foreland fold-and-thrust zone.Coevally, extensional collapse in the uplifted area to the northof the Mian-Lue fault zone led to the formation of a series ofEarly and Middle Jurassic fault-bounded basins (Zhang et al.2004). The extension was accompanied by widespread em-placement of middle Mesozoic felsic plutons and dikes (Duand Wu 1998).

TheMiddle Jurassic to Quaternary reflects a period of post-orogenic intracontinental tectonism. Intracontinental thrustingis marked by Middle Jurassic and Early Cretaceous large-scale, thin-skinned nappe structures. The Bashan, Kangma,and southern Dabie arcuate thrust systems define this defor-mation along the Mian-Lue suture. A continental facies fore-land basin system also developed during the period. A secondevent is defined by the Cenozoic lateral extrusion of the Tibetplateau. Final rapid uplift of the central orogenic system wasassociated with strike-slip to transtensional motion alongpreexisting major structures, such as the Mian-Lue tectoniczone (Zhang et al. 2004).

Geology of the gold deposits

The Yangshan gold belt is situated in the Shaanxi–Gansu–Sichuan “Golden Triangle” region of China, which is the broadjunction between the North China Craton in the north, YangtzeCraton in the south, and Songpan-Ganzi fold belt in the west(Fig. 1; Zhang et al. 1996; Du and Wu 1998; Pei et al. 2002).

Geology of the Yangshan gold belt

The Bikou Group containing the oldest rocks (846∼776 Ma:Yan et al. 2003) in this region, at the northern margin of theYangtze Craton, has a maximum thickness of >16 km. Itconsists of Mesoproterozoic to Neoproterozoic volcanic–sed-imentary rocks that were metamorphosed to the greenschistand, locally, to the amphibolite facies (Chen et al. 1987; Pei1989). Rocks of the Devonian Sanhekou Group, which hostthe deposits of the Yangshan gold belt, consist of a suite of 5-

to 11-km-thick carbonaceous phyllite, limestone, and sand-stone. These sedimentary rocks were deposited during therifting of the South Qinling Block and were metamor-phosed up to the lower greenschist facies (Li et al. 2003;Dai et al. 2012).

Intrusions in the gold belt include minor granite, ap-lite, and porphyry dikes. The granitic rocks were shearedinto lenses along the Anchanghe-Guanyinba fault andsecondary ENE- or NW-trending faults. Silicification,sericitization, carbonatization, sulfidation, chloritization,epidotization, and argillization are common in the hydro-thermally altered granitic rocks. Most pre-ore graniticrocks are ca. 210 Ma and were subsequently alteredand mineralized to variable degrees (Qi et al. 2005;Yang et al. 2006; Lei 2011).

The gold belt is structurally located on the Wenxian arcuatestructure, which is part of the 20- to 30-km-wide Mian-Luesuture zone (Fig. 1). The Wenxian arcuate structure is com-prised of three E–W-trending faults; the Songbai-Liping,Majiamo-Weijiaba, and Baima-Linjiang faults, with some N–S-trending faults overprinting the arcuate structure. The arcuatestructure was initially formed during Late Permian to LateTriassic (e.g., Indosinian) tectonism and strongly deformedduring Jurassic to Early Cretaceous (e.g., Yanshanian) tecto-nism as determined from the ages of both hanging wall and footwall rocks and ages of E–W-trending syntectonic, brecciatedfelsic intrusions (Du and Wu 1998; Yan et al. 2010).

The 30-km-long E–W-trending Anchanghe-Guanyinbafault follows a fold axis, offsets the more regional Songbai-Liping fault, and has a width varying from tens of meters toseveral kilometers. The fault mainly dips to the north and thedip angle changes from 50° to 70°, commonly parallel tofoliation in the phyllitic country rocks. Devonian phyllite,limestone, and sandstone occur in both the hanging wall andfoot wall of the fault. The Anchanghe-Guanyinba fault in-cludes three distinct splays, namely F1, F2, and F3, dipping tothe south, north, and south, respectively, and controlling theno. 401, 305, 402, 403, and 311 ore bodies within the Anbaand Getiaowan deposits (Fig. 2).

Due to the multiple periods of deformation and N–S con-vergence, the strata in the gold belt were complexly folded.The near E–W-trending Getiaowan-Caopingliang anticline(Fig. 2), the largest fold in the gold belt, extends for about10 km, with a width of 1 km. At the Anba gold deposit, bothnorth and south limbs of the anticline are exposed, whereas atthe Getiaowan gold deposit, the south limb is cut off by theyounger Anchanghe-Guanyinba fault. The Devonianmetasedimentary rocks were folded in this anticline and theno. 311 ore body is located near the core of the anticline(Fig. 2). The Wujiashan syncline is the largest syncline inthe gold belt and the Devonian limestone makes up the core ofthe syncline. The syncline is limited in length due to laterfaulting associated with the Anchanghe-Guanyinba fault

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system (Fig. 2). The Anchanghe-Guanyinba fault system andthe Getiaowan-Caopingliang anticline, as well as a series ofsecondary ENE-trending faults, control the ore bodies of theYangshan gold belt (Figs. 1 and 2).

Description of the gold ore bodies

Primary gold ores are mainly disseminated in phyllite, graniticdikes, limestone, and sandstone, but also occur in small veinscutting these units. For the most part, the phyllite, with itsenclosed lenses of granite, is multiply deformed between thethick, relatively competent units of massive limestone. Thealtered granites indicate mineralization must be no older thanLate Triassic. Disseminated ores in phyllite and granitic dikesare dominant in all six gold deposits in the belt, althoughlimestone is also an important host rock at the Guanyinbadeposit. Most disseminated ore bodies have an average gradeof about 5 g/t Au. However, the more cataclastically deformedphyllite and granite have grades locally as high as 18 g/t Au(Yan et al. 2010). The mineralized quartz veins, with anaverage grade of 9.1 g/t Au, are localized along the contactbetween phyllite and granitic dikes. Oxidized gold ores arecharacterized by high limonite contents and are present within30m of the surface; they have an average grade of 2 g/t Au (Qiet al. 2003). Based on the observations of outcrops, aditexposures, and diamond drill hole cores of the different golddeposits in the Yangshan gold belt, as well as on microscopicobservations, the key textural and structural characteristics ofthe disseminated and vein ore styles are described below.

Disseminated pyrite and/or arsenopyrite occur in all rocktypes and most paragenetic stages (pre-ore syngenetic/diagenetic, as well as early, main, and late ore stages;Table 1) at the six gold deposits. Two to 15 vol% fine-grained pyrite and arsenopyrite are unevenly disseminated inthe altered host rocks, which vary from essentially barren ofgold to grades of as much as 10.6 g/t Au. Economic goldgrades appear to be associated with two specific generationsof disseminated sulfides, the pyrite- and arsenopyrite-richmain and late ore stages, as defined in more detail below.

The syngenetic/diagenetic pyrite is widely disseminated inthe host rocks and, in the area of the ore deposits, forms thecores to many of the early and main ore stage pyrite grains.Early ore stage, semi-massive pyrite aggregates (Fig. 3a), withcrushed subhedral crystals, are particularly abundant in thephyllite and typically define subeconomic gold-bearing zoneswith <0.5 g/t Au. The main ore stage bedding-parallel pyriteand/or arsenopyrite (Fig. 3b), which are common along thephyllitic foliations and form 1 to 7 vol% of the rock, aretypically in zones that vary widely in grade from 0.02 to8.9 g/t Au. These sulfide minerals, where disseminated incountry rocks, grew over cores of syngenetic/diagenetic andearly ore stage pyrite. Some of the late ore stage pyrite,arsenopyrite, and stibnite are disseminated in the host rocks,with the former two sulfides being the major gold-bearingminerals.

Gold is less common in sulfide-bearing quartz ± calciteveins in the early and main ore stages. Early ore stage quartzveins having widths of 1 to 2 cm and containing 15 to 30 vol%pyrite, with individual grains that vary in diameter from 0.001

Fig. 2 Simplified geology of theGetiaowan-Anba gold deposits inYangshan gold belt (after Yanet al. 2010), showing the locationof Figs. 5 and 6. F1 , F2 , and F3are the three splays of theAnchanghe-Guanyinba fault

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to 5 mm, are mainly hosted in phyllite in the northern part of theAnba gold deposit. Main ore stage, folded thin veinlets, 1- to 3-mm wide, are present in phyllite also mainly in Anba golddeposit. They contain euhedral pyrite and arsenopyrite crystals.The pyrite is fine grained and 15 to 25 μm in diameter, whereasarsenopyrite in these veinlets is 0.05 to 2 mm in diameter.

Late ore stage quartz ± calcite veins contain pyrite, arsenopy-rite, and stibnite. The veins are from 2- to 50-cm wide and aremostly present in the Anba gold deposit (Figs. 3c, d and 4).These veins commonly occur in fracture zones in phyllite andwere locally brecciated during final events of the late ore stage(Figs. 3c, 4a, and 5). Some thin veins, 2–5 mm wide, arecommon in granitic dikes (Fig. 4b), with only a few veinsobserved in metamorphosed sandstone. The mineralized latequartz veins, commonly striking NE, cut the earlier formeddisseminated and vein ore bodies (Fig. 5). Gold grade in eightof these veins ranges from 1.6 to 25 g/t. Pyrite, arsenopyrite, andstibnite account for 5 to 10 vol% of the veins. Pyrite has

anhedral–euhedral crystals ranging from 0.005 to 0.05 mm indiameter. Arsenopyrite has euhedral crystals ranging from 0.005to 0.1 mm. Stibnite occurs as anhedral crystals in the void spacesamong quartz/calcite crystals, and the pyrite-arsenopyrite-stib-nite-quartz ± calcite vein cuts the earlier formed pyrite andarsenopyrite mineralization (Figs. 3c, d and 7).

Post-ore stage quartz ± calcite veins contain sparse fine-grained pyrite and cut the earlier formed disseminated andvein ores of all three epigenetic stages at all six gold deposits.Pyrite in these veins has subhedral crystals with averagediameters of 0.01 mm.

Analytical techniques

Mineral paragenesis was complemented by petrological workunder a transmitted and reflected light microscope and ascanning electron microscope (SEM). The SEM images were

Table 1 Sulfides and mineralization styles in different ore stages

Mineralization styles Syngenetic/diageneticstage (Py0)

Early orestage (Py1)

Main ore stage(Py2 + Apy2)

Late ore stage(Py3 + Apy3 + Stn)

Post-orestage (Py4)

Vein or veinlet type Pyrite–quartz vein •

Folded Au-bearing sulfides–quartz vein •

Pyrite–arsenopyrite–stibnite–quartz–calcite vein

•

Pyrite–quartz–calcite vein •

Disseminated type Semi-massive pyrite aggregates •

Bedding-parallel bands ofdisseminated sulfides

• •

Disseminated sulfides • • • •

Center dot means the mineralization style existed in the stage; blank means the mineralization was not found in the stage

Fig. 3 The main styles of sulfidemineralization at Yangshan goldbelt. a Semi-massive pyriteaggregate in gray-green phyllite.b Bedding-parallel bands ofdisseminated pyrite in gray-greenphyllite. c The white dash lineindicates the contact plane ofgranite and phyllite, and the leftpart is granite while the right partis phyllite, both of which are richin pyrite and arsenopyrite. The25-cm wide stibnite-quartz veinoverprints the pyrite–arsenopyritemineralization in phyllite. dPyrite–arsenopyrite–stibnite–quartz vein in phyllite that isadjacent to granite dike

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also used to identify small grains of rutile, galena, sphalerite,and sulfosalt mineral inclusions.

Based on careful examination of the mineral paragenesis,suites of 19 and 11 samples of the granitic dikes, phyllite, andquartz ± calcite veins were selected for EPMA and laserablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses, respectively, at the U.S. GeologicalSurvey, Denver (USA). Phyllite is gray in color, with granularlepidoblastic texture and phyllitic structure. Fine-grainedquartz, sericite, and clay minerals are the dominant minerals.Granitic dikes are gray green in color, with granulitic orporphyritic texture. The primary minerals in the dikes areplagioclase, quartz, and biotite, which were commonly alteredto sericite, chlorite, epidote, and clays. According to thedifferent textures and mineralogy, various granitic dikes canbe classified as granite, plagioclase granite, granite porphyry,and plagioclase granite porphyry.

The samples were studied with an FEI Quanta 450 fieldemission scanning electron microscope operated at 20 kVand2 to 10 nA current. Quantitative chemical analysis was ac-quired with a JEOL JXA 8900 electron probe microanalyzer.Operating conditions were 20 kV, 50 nA, and a focused beamof 0.5 μm in diameter. Grains were analyzed by EPMA, anddetection limits for studied elements are shown in OnlineResource 1. Accuracy and precision were better than 3 %based on replicate analyses of sulfide standards.

Pyrite grains analyzed by LA-ICP-MS were typically larg-er than 100 μm. The LA-ICP-MS analyses were conducted ona Photon Machines Analyte G2 193 nm laser ablation systemattached to a PerkinElmer ELAN DRC-e ICP-MS. Laserablation-ICP-MS methods for pyrite are based on Largeet al. (2007) and Zhao et al. (2011). A 15- and 30-microndiameter laser spot was used for the analyses. A laser fluenceof 2 J/cm2 and a frequency of 5 Hz were used for the spots.Helium was used as a carrier gas. Calibration was conductedusing the USGSMASS-1 sulfide reference material run five toten times at the beginning of each session, following theprocedures of Longerich et al. (1996) and using Fe as theinternal standard element (e.g., Large et al. 2007).Concentration calculations were carried out using off-line dataprocessing following the equations of Longerich et al. (1996).The MASS-1 reference material was run periodically to mon-itor for drift. During these analytical sessions, drift was lessthan 5 % for all elements. A stoichiometric value of 46 % Fewas used for the LA-ICP-MS concentration calculations.Detection limits were calculated as three times the standarddeviation of the blank (Longerich et al. 1996). Data wereexamined for the presence of mineral inclusions or zoningseen in the time-resolved spectra as deviations from a stablesignal (e.g., Large et al. 2007; Zhao et al. 2011).

Because some data are below the detection limits of theanalytical techniques, corrections are necessary for the

Fig. 4 Occurrence of stibnite. aStibnite–quartz vein and quartzbreccias in fracture zone. 1Stibnite–quartz vein and quartzbreccia, 2 phyllite. b Stibnite–quartz vein in granite. Blue inkmarks indicate the directions ofthin sections

Fig. 5 Measured section of theadit YM24-19-2C of Anba golddeposit (see Fig. 2 for location)

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incorporation into the statistical method. Data qualified with a“less than” value were replaced with 0.7 times the detectionlimit. The highly censored elements, which include Cd, Ge,Hg, In, Se, and Sn, are not reported.

Textures of sulfides and paragenetic sequence

The sulfides observed in the gold deposits can be interpretedto have formed in five very distinct stages, which includesyngenetic/diagenetic (stage 0) to late veins (stage 4).Sulfide minerals described below are identified by their par-ticular hydrothermal stage. Five generations of pyrite (Py0 toPy4), two generations of arsenopyrite (Apy2 and Apy3), onestage of stibnite deposition, and minor amounts of othersulfides and sulfosalt minerals from the different ore styleshave been defined by detailed paragenetic studies of the oresand host rocks. All the minerals of the summarized parage-netic sequence (Fig. 8) are hosted in phyllite, granitic dikes,and sandstone. Although limestone in the Guanyinba depositalso exhibits a relatively strong degree of pyritization, thepresence of arsenopyrite, stibnite, and gold is rare. The differ-ent sulfides and ore styles in different stages are summarizedin Table 1, and the individual pyrite types are discussed belowin the order of interpreted paragenetic sequence.

Py0

The earliest pyrite, Py0, is disseminated in phyllite, limestone,and sandstone, and occurs as framboidal or colloform textureof microcrystals in spheres from 1 to 5 μm across (Fig. 9a) oras irregular or rounded aggregates that are overgrown by orestages Py1 and Py2 (Fig. 9d, e).

Py1

In general, the early ore stage Py1 forms euhedral–subhedral cubes or pyritohedral crystals that are dissemi-nated in host rocks (Figs. 9e–i and 10a). The Py1 alsooccurs as aggregates in quartz veins in the northern partof the Anba deposit or is present as semi-massive pyriteaggregates in phyllite (Fig. 3a). The Py1 crystals vary from 5to 10 μm in diameter, with a few being more than 100 μm.There are different mineral inclusions in disseminated Py1,which include zircon, quartz, sphalerite, galena, boulangerite,jamesonite, and famatinite (Fig. 10). The quartz fibers, whichare coexisting with Py1 and parallel to the foliation in phyllite,along with the microfractures in Py1 suggest that a ductile–brittle deformation regime was ongoing during the early orestage.

Py2

The Py2 is the main ore stage pyrite, with euhedral–subhedralcrystals varying from 10 to 200 μm in diameter, with a few ofmore than 500 μm. The Py2 forms pyritohedron and octahedroncubes and/or as a combination of these forms. The Py2 occurs asfine-grained bedding-parallel sulfide, as grains hosted in thefolded quartz veinlets in phyllite (Figs. 3b, 6c, and 7), or asdisseminations in granitic dikes. The Py2 is parageneticallyassociated with Apy2 and overgrows Py0 and Py1 (Figs. 7 and9c–i). It is the most important gold-bearingmineral. Themineralinclusions in disseminated Py2 include zircon, sphalerite, galena,boulangerite, famatinite, rutile, and apatite (Fig. 10).

Apy2

The Apy2 grains occur as euhedral prismatic crystals withlozenge-shaped cross sections that vary from 0.002 to 2 mm.They are widely disseminated in all the host rocks. The Apy2is paragenetically associated with Py2 and quartz (Figs. 6b, c,7, and 9c–e, h, i).

Py3

Late ore stage Py3 varies from 7 to 570 μm in diameter andforms euhedral crystals. The Py3 is disseminated in host rocksor is paragenetically associated with Apy3 and stibnite inquartz ± calcite veins in fracture zones, which indicates abrittle deformation regime during the late ore stage (Figs. 4and 7 and Table 1). The Py3–Apy3–stibnite–quartz ± calciteveins cut the earlier formed Py2–Apy2–quartz veins (Fig. 5).

Apy3

The Apy3 has euhedral prismatic crystals with lozenge-shapedcross sections varying from 0.03 to 0.1 mm. The Apy3 coexistswith Py3 in stibnite-rich quartz ± calcite veins in phyllite or ispresent as disseminated grains in granitic dikes (Figs. 6c and 7).

Stibnite

Anhedral stibnite crystals occur in late ore stage quartz ±calcite veins, which cut the earlier formed pyrite and arseno-pyrite mineralization. Stibnite commonly coexists with Py3and Apy3 and mainly occurs in quartz ± calcite veins infracture zones, and less commonly is disseminated in phylliteor in the dikes (Figs. 3c, d, 4 and 7).

Py4

The Py4 forms subhedral–euhedral crystals that are 5 to 10 μmin diameter. The pyrite commonly occurs in the barren quartz ±calcite veins that cut all earlier formed mineralization (Fig. 6c).

434 Miner Deposita (2014) 49:427–449

Other sulfides and sulfosalt minerals

Chalcopyrite, galena, sphalerite, boulangerite, jamesonite,famatinite, tennantite, and bournonite (Fig. 10) are commonminor metallic minerals in the deposits of the Yangshan goldbelt. They may occur as mineral inclusions in pyrite andarsenopyrite, in ore stage quartz veins, or as disseminationsin country rocks. The sulfosalt minerals form anhedral crystals

ranging from 5 to 10 μm in diameter, with some crystals up totypically 100 μm in diameter.

Gold

Native gold is uncommon in the Yangshan gold belt deposits.Gold is detected by EPMA and LA-ICP-MS analyses in pyriteand arsenopyrite. However, native gold, with irregular shape,occurs locally in the vuggy quartz veins of the late pyrite-arsenopyrite-stibnite-quartz ± calcite stage.

Paragenetic sequence

Based on the detailed field studies of crosscutting relationships,combined with petrological studies, the paragenetic sequenceof the Yangshan gold belt mineralization is summarized inFig. 8. Petrographic observations distinguish different vein orveinlet mineralization types from a syngenetic/diagenetic stageto a post-ore stage (Table 1). They also provide detailed evi-dence for each paragenetic association (Figs. 3, 4, 5, 6, 7, 8, 9,and 10), among which pyrite and arsenopyrite are the majorgold-bearing minerals. Pyrite is the most common sulfidemineral in the gold belt and it precipitated throughout thedifferent ore stages. Our detailed paragenesis now enables athorough documentation of sulfide evolution and description ofthe chemical changes across different stages, from early dia-genesis through to peak and post-ore events.

Fig. 6 Cross-cutting relationbetween quartz veins in theYangshan gold belt. a Profile oflithology, contacts of differentlithologies, and mineralization inAnba gold deposit (see Fig. 2 forlocation). b Apy2 in phyllite. cScanned section of mineralizedphyllite with various stagesulfide–quartz ± calcite veins,showing that the Py2–Apy2–quartz–calcite (Cal) vein type 1(Qz1) is occurring simultaneouslywith the mineralization inphyllite, and the Py3–Apy3–quartz vein type 2 (Qz2) isoccurring after Qz1 and before thePy4–quartz vein type 3 (Qz3)

Fig. 7 Scanned section of mineralized phyllite with different-stage min-eralized veins, showing the curved Py2–Apy2–quartz veinlet type 1 (Qz1)occurring simultaneously with the mineralization in phyllite and therelatively wide and straight Py3–Apy3–Stn–quartz vein (Qz2) cuttingQz1

Miner Deposita (2014) 49:427–449 435

Mineral chemistry of sulfides

Comparison between EPMA and LA-ICP-MS analyses

Elements such as Ag, Au, Bi, Cu, Ni, Pb, Sb, V, and Znare commonly at concentrations lower than the detectionlimits of EPMA, but could be detected by LA-ICP-MS(Tables 2, 3, 4, and 5). Pyrite of different stages, examinedby both EPMA and LA-ICP-MS analyses, are compared inOnline Resource 2. Comparing the As concentrations in pyriteusing both methods, it can be concluded that the results fromthe two methods are relatively consistent and LA-ICP-MSprovides more robust low concentration data (OnlineResource 3).

EPMA major element data of sulfide minerals

The EPMA analysis of minerals in different paragenetic stageshas been undertaken for 19 samples from the Yangshan goldbelt (Tables 2, 3, and 4). The EPMA data show that the Fecontent in pyrite (Fig. 11) is fairly consistent across all fivegenerations. The Fe content in Py2, the main ore stage pyrite,has an average value of 45.4±0.7 wt% and is slightly higherthan that in Py0 (44.2±0.3 wt%) or Py4 (44.6∼44.7 wt%) andslightly lower than Fe in Py1 (46.5±0.5 wt%) or Py3 (46.5±0.4 wt%). The S content in pyrite has a wider variation rangethan the Fe content, from 45.0 to 57.1 wt%. The average Scontents in syngenetic/diagenetic Py0 (53.4±2.5 wt%) andearly ore stage Py1 (52.5±1.0 wt%) are slightly higher thanthose in later ore stages (50.1 ±1.6, 51.5±2.0, and

50.6∼50.7 wt% for Py2, Py3, and Py4, respectively). Arsenicis commonly present in all pyrite stages, and the As contentsin the pyrite vary from <251 ppm to 10.7 wt%, and the mainore stage Py2 has the highest average As content (3.84±1.83 wt%; Fig. 11 and Table 2). There is a negative correlationbetween As and S in pyrite (Fig. 12), indicating substitution ofAs1− for S in the pyrite structure (Fleet et al. 1993; Reich et al.2005; Deditius et al. 2008).

Comparing the chemistry of main ore stage Apy2 and lateore stage Apy3, Apy2 has a lower average content of Fe (35.4±0.4 wt%) and S (22.5±0.8 wt%) and a higher averagecontent of As (40.7±1.3 wt%), whereas Apy3 has a higheraverage content of Fe (35.9±0.9 wt%) and S (23.4±1.0 wt%)and a lower content of As (39.8±2.4 wt%). This is the samerelationship observed for Fe, S, and As contents in Py2 versusPy3 (Fig. 13 and Table 2). In addition to the major elements,Apy3 has a higher average content of Sb compared with Apy2(0.067±0.051 and 0.023±0.022 wt%, respectively; Fig. 13and Table 3). Minor amounts of Au (0.027±0.020 and 0.027±0.020 wt%, respectively), Cu, Ni, and Co are detected in bothApy2 and Apy3 (Table 3).

Minor amounts of As (0.253∼0.341 wt%), Cr(0.028∼0.094 wt%), and Bi (0.044∼0.087 wt%) are pres-ent in stibnite, probably indicating minor substitution ofAs for Sb (Nakai et al. 1986; Neiva et al. 2008), and Cr-bearing silicate or oxide mineral nanometer-sized inclusionsthat may come from the phyllite. Minor Bi in stibnite may bepresent as solid solution within the stibnite structure (Luethet al. 1990; Kyono and Kimata 2004). Gold is not detected instibnite (Table 4).

Fig. 8 Paragenetic sequence ofthe Yangshan gold beltmineralization and alterationinterpreted from texture andsulfide geochemistry. The boldlines indicate high abundance, thethin lines represent the minoramounts, and the discontinuouslines indicate uncertainty in thedetermination of the parageneticsequence due to the lack of cleartextural relationship

436 Miner Deposita (2014) 49:427–449

LA-ICP-MS trace element chemistry of pyrite

The trace element compositions of Py0 to Py3 were determinedby LA-ICP-MS on a suite of 11 representative samples (OnlineResource 4). Table 5 shows summary statistics for analyses ofPy0 to Py3. The difference in composition for many elementsbetween the different pyrite generations is shown by variation inmedian values. The concentrations of some elements, includingGa and Te, are very low in all the pyrite types. Other elements,such as Ag, As, Au, Bi, Co, Cu, Mn, Mo, Ni, Pb, Sb, Tl, V, andZn, can be interpreted to define characteristic signatures thatfingerprint each of the four pyrite types (see Fig. 14).

The syngenetic/diagenetic Py0 has relatively high values ofAs, Au, Bi, Co, Cu, Mn, Ni, Pb, Sb, Tl, V, and Zn. The earlyore stage Py1 has higher contents of Ag, As, Au, Bi, Cu, Sb,and V and lower contents of Co, Mn, Ni, Pb, Tl, and Zn,

compared with Py0. Main ore stage Py2 has higher concentra-tions of As, Au, and Cu, whereas it has lower concentrationsin other trace elements. Late ore stage Py3 has higher values ofAs, Au, Cu, Fe, Mn, Mo, Sb, and Zn and lower values of Bi,Co, Ni, Pb, and Ag compared with Py1 and shows moredetectable trace elements than Py2 (Fig. 14 and Table 5).The post-ore stage pyrite (Py4) was generally too fine grainedfor LA-ICP-MS analyses.

Figure 15a shows that five of the six Py0 samples plotbelow the Au/Ag=1 line, whereas only one Py0 sample isabove the Au/Ag=1 line. Early ore stage Py1 and main orestage Py2 show the broadest fields, with Au/Ag ratios varyingfrom 0.02 to >200. The Au/Ag ratio of Py1 can be below orabove 1, whereas that of Py2 is mostly >1, indicating Au ispreferentially concentrated in Py2 compared with Ag(Fig. 15b). The Au/Ag ratio of late hydrothermal stage Py3

Fig. 9 Ore paragenesis of Yangshan gold belt. a SEM image ofsyngenetic/diagenetic framboid Py0. b Main ore stage Py2. c Main orestage Py2 and Apy2. d Syngenetic/diagenetic framboid Py0 is overgrownby main ore stage Py2 and Apy2. e Syngenetic/diagenetic framboid Py0and early stage Py1 are overgrown by main ore stage Py2 and Apy2. f

Early ore stage Py1 is overgrown bymain ore stage Py2. g Early ore stagePy1 is overgrown by main ore stage Py2. h Early ore stage Py1 isovergrown by main ore stage Py2 and Apy2. i Early ore stage Py1 isovergrown by main ore stage Py2 and Apy2

Miner Deposita (2014) 49:427–449 437

is above 1, which is similar to that of Py2, but with lower Agcompared to Py2.

There is a positive correlation between Co andNi and the Co/Ni ratio mainly falls between 0.5 and 10 for all pyrite genera-tions (Fig. 15c). All Py0 plots above the Co/Ni=1 line, indicat-ing that Py0 has more Ni than Co, which is distinct from otherhydrothermal pyrite that mainly has more Co than Ni (Fig. 15c;Bralia et al. 1979; Cook 1996; Zhao et al. 2011). Bismuth couldbe present in solid solution in the pyrite or in metallic or sulfideinclusions (Large et al. 2007, 2009; Thomas et al. 2011), as thereis positive correlation between Bi and Pb (Fig. 15d).

Discussion

Textural evolution of sulfides: geological and explorationsignificance

Multiple generations of pyrite are disseminated in the countryrocks along the length of the Yangshan gold belt. The earliestPy0 is commonly incorporated into the cores of Py1, Py2, andApy2 (Fig. 9d, e), resulting from the reaction of hydrothermalfluids with the wall rocks and precipitating the gold-richsulfide rims surrounding the preexisting Py0. Early ore stage

Fig. 10 Secondary sulfides inYangshan gold belt. a Quartz(Qz) and boulangerite (Boul)inclusions in Py1. b Quartz andfamatinite (Fm) inclusions in Py1.c Quartz and sphalerite (Sp)inclusions in Py1. d Zircon (Zrn)and rutile (Rt) inclusions in Py2. eSphalerite and tennantite (Tnt) inquartz vein. f Bournonite (Bnn) ingranite porphyry

438 Miner Deposita (2014) 49:427–449

Py1 is unevenly disseminated and often preserved as cores oflater main ore stage Py2 (Fig. 9e–i), although some is alsohosted in quartz veins in the northern part of the Anba deposit.The disseminated Py1 occurs as semi-massive pyrite aggre-gates or forms bedding-parallel bands of disseminated sulfidesin phyllite. The notable increase in As and Au contents in Py1compared with Py0 indicates the hydrothermal fluids thatreacted with wall rocks had high contents of Au and As.This concentration of Au and As in the fluids likely resulted

from metal and sulfur release from other syngenetic/diagenetic pyrite (Py0) grains that were in the country rocksbeing devolatilized in areas below the exposed greenschistfacies rocks and thus at higher metamorphic grades, which is acommon process inherent to most metamorphic belts andleading to the formation of orogenic gold provinces (e.g.,Groves et al. 1998; Pitcairn et al. 2006).

Main ore stage Py2 and Apy2 are generally disseminated,with some forming bedding-parallel bands or folded gold-

Table 3 EPMA analyses of arsenopyrite from different stages (in weight percent)

Stage Item As Au Co Cu Fe Ni S Sb Number of data

Apy2 Maximum 44.5 0.118 0.108 0.044 36.1 0.089 23.7 0.113 29Minimum 38.4 0.020 0.005 0.028 34.5 0.021 20.7 0.013

Average 40.7 0.027 0.019 0.030 35.4 0.025 22.5 0.023

Median 40.8 0.020 0.005 0.028 35.5 0.021 22.8 0.013

SD 1.3 0.020 0.028 0.004 0.4 0.014 0.8 0.022

Apy3 Maximum 45.5 0.086 0.042 0.028 37.1 0.128 24.8 0.154 11Minimum 37.4 0.020 0.005 0.028 33.9 0.021 22.1 0.013

Average 39.8 0.027 0.011 0.028 35.9 0.037 23.4 0.067

Median 39.7 0.020 0.005 0.028 35.9 0.021 23.3 0.063

SD 2.4 0.020 0.014 0 0.9 0.036 1.0 0.051

Detection limit 0.0132 0.0276 0.0070 0.0406 0.0474 0.0302 0.0216 0.0192

Table 2 EPMA analyses of pyrite in Yangshan gold belt (in weight percent)

Stage Item As Au Bi Co Cr Cu Fe Ni S Sb Zn Number of data

Py0 Maximum 3.63 0.0344 0.122 0.371 0.0313 1.06 44.6 0.433 57.1 0.616 0.136 4Minimum 0.0176 0.0344 0.0767 0.165 0.0313 0.0533 43.9 0.0393 51.6 0.0239 0.0158

Average 1.17 0.0344 0.0880 0.282 0.0313 0.305 44.2 0.158 53.4 0.172 0.0567

Median 0.510 0.0344 0.0767 0.297 0.0313 0.0533 44.2 0.0793 52.4 0.0239 0.0374

SD 1.66 0 0.0227 0.087 0 0.502 0.3 0.185 2.5 0.296 0.0567


Average 0.523 0.0351 0.0767 0.0262 0.0316 0.0533 46.5 0.0410 52.5 0.0243 0.0300

Median 0.0608 0.0344 0.0767 0.0106 0.0313 0.0533 46.5 0.0393 52.5 0.0239 0.0158

SD 0.808 0.0054 0 0.0458 0.0026 0 0.5 0.0107 1.0 0.0037 0.1322


Average 3.84 0.0352 0.0769 0.0153 0.0317 0.0619 45.4 0.0398 50.1 0.0246 0.0159

Median 3.49 0.0344 0.0767 0.0106 0.0313 0.0533 45.5 0.0393 50.1 0.0239 0.0158

SD 1.83 0.0045 0.0031 0.0148 0.0070 0.0292 0.7 0.0047 1.6 0.0044 0.0013


Average 1.55 0.0353 0.0767 0.0113 0.0313 0.0533 46.5 0.0393 51.5 0.0239 0.0158

Median 1.81 0.0344 0.0767 0.0106 0.0313 0.0533 46.4 0.0393 50.8 0.0239 0.0158

SD 1.29 0.0055 0 0.0028 0 0 0.4 0 2.0 0 0


Detection limit 0.0251 0.0491 0.110 0.0152 0.0447 0.0762 0.0454 0.0561 0.0228 0.0342 0.0225

Miner Deposita (2014) 49:427–449 439

bearing sulfide–quartz veinlets in phyllite. Minor chalcopyritecoexists with Py2 and Apy2. The Py0 and Py1 may haveprovided sites for precipitation of much of the disseminatedPy2 and Apy2 during fluid–rock interaction, which are local-ized in areas identified by widespread sericite–clay alteration orbleaching of country rocks. Significantly, in contrast to thedisseminated grains, Py2 and Apy2 in the quartz veins haveno Py0 or Py1 cores. This may be caused by direct precipitationof silica, sulfides, and gold from the hydrothermal fluids due topressure decreases during hydrofracturing and vein formation,which is the most common precipitation mechanism for metalsin orogenic gold deposits hosted within vein quartz (e.g.,Goldfarb et al. 1988, 2005; Weatherley and Henley 2013).

Late ore stage Py3, Apy3, and stibnite occur in the quartz ±calcite veins or are disseminated in the host rocks, also in areasthat have experienced sericite–clay alteration or bleaching.During this hydrothermal stage, which was structurally con-trolled by the NE-striking faults, the ore fluids had somehowevolved to deposit more Sb. In addition, Au may have beenleached from the earlier formed Py3 and Apy3, or even themainstage sulfide structures, to form free gold. The common occur-rence of stibnite and the possible release of Au from oldersulfide grains may reflect the decline in the fluid temperaturein the late ore stage episode, which is typical for many epizonalorogenic gold deposits where Sb solubility decreases signifi-cantly at the lower temperatures (Boyle 1979; Groves et al.1998). This can occur during the ascent of later pulses ofhydrothermal fluid into uplifting metamorphosed strata, whichare progressively cooling as they move along a clockwise P–Tpath (e.g., Stuwe 1998). Post-ore stage Py4 occurs in latestquartz ± calcite veins, cutting all earlier formed mineralization,with calcite fluid inclusion homogenization temperatures of160–210 °C (Li et al. 2007a) indicating a further decrease intemperature during final hydrothermal activity.

The main and late ore stages contain most of the goldresources in the Yangshan gold belt. Therefore, defining thedistribution and characteristics of Py2, Py3, Apy2, Apy3, andstibnite is a useful guide for gold exploration in the region.Most significantly, our mineral parageneses (Fig. 8), the abun-dant arsenopyrite and or stibnite, the presence of significantchalcopyrite in pyrite grains, visible sericite–clay alteration orbleaching of the phyllite or enclosed igneous rocks, and (or)some of the above stated geochemistry unique to Py2 and Py3,are distinctive characteristics helpful to gold exploration in theYangshan gold belt.

Trace metals in pyrite

Trace metals in pyrite may occur in several forms: (1) in solidsolution in pyrite structure, (2) in nanometer-sized inclusionsof mineral or metallic grains, (3) within visible micron-sizedinclusions of other sulfides, or (4) within visible micron-sizedinclusions of silicate or carbonate minerals (Large et al. 2007,2009, 2011; Thomas et al. 2011; Agangi et al. 2013).However, the pronounced effects from the latter two formson the output of the pyrite analyses were easily identified andavoided when selecting analytical spots. Furthermore, if in-clusions were encountered, then they were removed duringthe data processing.

1. As and Au

There are two different forms of arsenian pyrite: (1) As1–-pyrite [Fe(S,As)2] in which the arsenic substitutes for sulfur asAs1– (e.g., Fleet and Mumin 1997; Simon et al. 1999;Blanchard et al. 2007) and (2) As3+-pyrite [(Fe,As)S2] inwhich arsenic, mainly As3+, substitutes for Fe (Deditiuset al. 2008, 2009). The negative correlation between As andS concentrations in arsenian pyrite (Fig. 12) suggests that thearsenian pyrite is [Fe(S,As)2] in which As1− substitutes for Sin the pyrite structure (Fleet et al. 1993; Reich et al. 2005;Deditius et al. 2008).

In the Yangshan gold belt, Au is mostly present as “invis-ible gold” within pyrite and arsenopyrite, although somenative gold also occurs in the late ore stage. All the pyrite(Py0–Py3) analyses plot below the solubility limit line, show-ing that most Au is present in solid solution (Au1+) in arsenianpyrite (Fig. 15a; Reich et al. 2005).

2. Co/Ni ratio

Many studies of trace elements have related the Co/Ni ratioin pyrite to ore deposit type (Hawley and Nichol 1961; Loftus-Hills and Solomon 1967; Bralia et al. 1979; Mookherjee andPhilip 1979). Volcanogenic pyrite without accompanyinglead- and zinc-bearing minerals shows Co/Ni values greaterthan 1 (Loftus-Hills and Solomon 1967; Price 1972; Braliaet al. 1979). Pyrite of sedimentary origin is characterized by avalue of less than 1 (Loftus-Hills and Solomon 1967), 0.63being typical (Price 1972). Conversely, pyrite with highlyvariable Co/Ni ratios, typically greater than 1, is consideredto be of hydrothermal origin (Bralia et al. 1979; Cook 1996;Zhao et al. 2011). In the Yangshan gold belt, the Py0, althoughit has been metamorphosed to the greenschist facies, retainsthe characteristic sedimentary Co/Ni ratio (<1, averaging0.572), whereas Py1 to Py3 have variable Co/Ni ratios(0.0454∼19.8, averaging 2.5, Table 5), which is typical ofhydrothermal pyrite (Fig. 15d). In a study on a number oforogenic gold deposits (e.g., Sukhoi Log, Bendigo, Spanish

Table 4 Two EPMA analyses of stibnite (in weight percent)

Sample no. As Bi Cr S Sb

SM4-2 no. 9 0.341 0.087 0.094 29.3 70.7

YS-AB-4haodong-01-No1 0.253 0.044 0.028 29.0 69.8

Detection limit 0.0161 0.0611 0.0334 0.0202 0.0216

440 Miner Deposita (2014) 49:427–449

Tab

le5

LA-ICP-M

Sanalyses

ofpyritefrom

differentstages(inpartspermillion)

Stage

Item

Ag

As

Au

Bi

Co

Cu

Ga

Mn

Mo

Ni

Pb

Sb

TeTl

VZn

Co/Ni

Au/Ag

Num

berof

data

Py 0

Maxim

um4.16

12,943

32.8

72.2

980

98.6

3.58

227

15.2

5,661

2,075

112

19.8

5.97

40.2

1,175

0.903

7.9

6Minim

um1.18

169

0.0900

0.300

123

28.0

1.95

31.7

1.68

159

107

8.41

15.5

0.230

7.12

29.5

0.173

0.1

Average

1.74

2,910

5.69

13.0

376

62.4

3.31

117

6.22

1,332

683

50.8

19.1

4.18

22.0

220

0.572

1.5

Median

1.27

765

0.345

0.755

255

56.9

3.58

81.0

3.71

528

328

34.7

19.8

4.79

21.2

29.5

0.679

0.3

SD1.19

4,980

13.3

29.0

315

29.2

0.67

855.62

2,135

791

38.7

1.7

2.20

10.5

468

0.304

3.1

Py 1

Maxim

um20.1

6,0563

511

4,052

2,262

951

6.38

97.3

111

2,582

3,089

871

112

23.2

534

2,732

19.8

234

49Minim

um0.250

109

0.0500

0.370

0.370

2.42

0.750

6.02

1.04

2.79

0.460

0.410

5.47

0.0800

4.52

5.90

0.0454

0.0

Average

3.64

9,250

19.9

133

265

117

3.37

19.3

6.22

211

413

104

22.8

1.89

29.0

130

3.15

9.6

Median

1.92

4,488

1.69

17.0

196

54.8

3.58

13.7

3.11

82.1

126

45.8

19.8

0.230

18.2

29.5

1.79

1.5

SD4.06

13,339

73.5

587

366

188

1.46

16.6

16.2

414

667

177

24.0

4.68

74.2

436

3.84

34.5

Py 2

Maxim

um8.99

48,107

424

89.1

947

853

4.57

37.9

118

606

825

321

28.8

0.790

39.2

1,262

17.0

883

106

Minim

um0.220

2,103

0.0700

0.0800

0.520

5.43

0.850

5.68

0.640

2.39

0.190

0.510

5.66

0.0700

4.30

6.59

0.0562

0.0

Average

1.67

28,357

62.9

6.97

84.1

268

3.38

10.8

4.22

53.0

102

75.2

18.0

0.215

16.3

41.8

2.29

73.1

Median

1.27

29,569

47.5

3.90

44.6

175

3.58

9.92

3.11

26.7

56.9

46.0

19.8

0.230

16.6

29.5

1.43

33.1

SD1.40

9,870

71.2

10.6

145

231

1.34

4.2

11.3

89.8

123

80.9

5.2

0.094

6.7

120

2.80

111

Py 3

Maxim

um1.27

20,924

77.0

7.87

62.1

227

3.58

203

141

48.0

234

677

19.8

0.230

21.2

465

2.24

63.5

7Minim

um0.390

5,955

16.0

0.210

9.61

21.4

3.58

9.02

3.11

4.38

0.620

2.71

19.8

0.230

21.2

29.5

1.09

40.0

Average

0.876

13,364

42.6

3.01

34.7

98.1

3.58

82.0

52.0

23.9

50.0

193

19.8

0.230

21.2

170

1.66

49.6

Median

1.08

14,367

43.9

2.19

44.7

83.4

3.58

58.0

19.7

21.2

25.4

54.5

19.8

0.230

21.2

52.9

1.40

48.3

SD0.438

5,198

22.2

2.97

20.3

69.8

079.5

60.6

17.0

82.2

266

00

0179

0.54

9.8

Miner Deposita (2014) 49:427–449 441

Mountain) and Carlin-type gold deposits, Large et al. (2009)noted that hydrothermal pyrite has a characteristically higherCo/Ni ratio than syngenetic/diagenetic pyrite. Thus, the Co/Niratio in pyrite is an important indicator to discriminate be-tween pre-ore and gold-related pyrite in complexly deformedparts of the Yangshan gold belt.

3. Au/Ag ratio

The hydrothermal pyrite has higher Au/Ag ratios (median1.5, 33.1, and 48.3 for Py1, Py2, and Py3, respectively) thanPy0 (median 0.3; Table 5), which is consistent with otherresults that showed hydrothermal pyrite had higher Au/Agratios than the syngenetic/diagenetic pyrite (e.g., Large et al.2009; Thomas et al. 2011). Main and late ore stage Py2 andPy3 have significantly higher Au/Ag ratios than the early orestage Py1, which is consistent with the recognition that Py2and Py3 are part of the main ore-bearing generations that arecharacterized by pyrite overgrowths.

Fig. 11 Average values and SD bars of Fe, S, and As contents in thepyrite of different stages, showing generously consistent Fe contents in allpyrite stages and the highest As values in Py2

Fig. 12 The negative correlation between As and S in arsenian pyrite

Fig. 13 Contents and correlations of As, S, Au, and Sb in different stagesof arsenopyrite of the Yangshan gold belt, showing that the Apy3 hashigher contents of S and Sb and lower contents of As than Apy2

442 Miner Deposita (2014) 49:427–449

4. Pb, Zn, Cu, Sb, Bi

The Pb and Zn contents of pyrite can be attributed to thepresence of nanometer-sized inclusions of Pb- and Zn-bearingminerals, such as galena and sphalerite, respectively, or tosolid solution in the pyrite structure (Large et al. 2009;Cabral et al. 2011; Deditius et al. 2011; Thomas et al. 2011).Such Pb and Zn are likely metal enrichments that were char-acteristic of the original syngenetic/diagenetic pyrite. Galenaand sphalerite occur as discrete sulfide inclusions in the hy-drothermal stages (Py1–Py3; Fig. 8), and it is inferred that themetamorphic hydrothermal fluids may thus have carried mi-nor amounts of Pb or Zn.

Copper may be present in solid solution or as nanometer-sized inclusions in pyrite (e.g., Large et al. 2007, 2009, 2011;Deditius et al. 2011; Thomas et al. 2011). One high Cu valuein Py2 suggests a chalcopyrite inclusion supported by theobservation of minor chalcopyrite coexisting with Py2.Because Py2 has the highest Au and Cu concentrations, whichare markedly different from Py0 and Py1, this could indicate

that the main ore stage fluid has slightly higher Cu contentsthan the early and late ore stage fluids. This would be consis-tent with the observation that reduced sulfur in low to moder-ate salinity hydrothermal fluids can play a role in transportingcopper (e.g., Liu and McPhail 2005). Alternatively, if thehydrothermal fluids did not transport significant copper, agreater amount of sulfidation during fluid–rock interactionassociated with the main ore stage may have caused precipi-tation of more copper phases in the ore bodies.

Correlations between Pb and Bi are consistent with Bi-richgalena and/or Pb-Bi sulfosalt nanometer-sized inclusions asthe major repository of Bi in pyrite (e.g., Large et al. 2009).The Py1, Py2, and Py3 show much stronger correlation be-tween Pb and Bi compared to Py0 (Fig. 15d), consistent withthe existence of galena and/or sulfosalt mineral inclusions inhydrothermal stages. The relatively high Sb contents in Py3(Table 5) and Apy3 (Table 3), as well as the abundance ofstibnite in the late ore stage, indicate that the late hydrothermalfluid had a high Sb concentration and (or) a lower temperaturethat reduced solubility of Sb.

Fig. 14 LA-ICP-MS analyses ofrepresentative trace elements (As,Au, Cu, Sb, Co, Ni) for thedifferent pyrite paragenesis inYangshan gold belt

Miner Deposita (2014) 49:427–449 443

Chemical evolution of sulfides and potential metal sourcereservoir

A summary of the petrological and geochemical characteris-tics of the different sulfide mineral types is given in OnlineResource 5. The geochemical evolution of ore-forming fluidsand related sulfides are interpreted as follows:

1. Syngenetic/diagenetic stage

In metasedimentary rock-hosted orogenic gold deposits,Au, As, and related elements are mobilized from syngenetic/diagenetic pyrite during metamorphism tens of millions ofyears after initial sedimentation. Under mainly greenschistand amphibolites facies conditions, these elements are mostconsistently released from syngenetic/diagenetic pyrite andconcentrated in metamorphic ore-forming fluids (Pitcairnet al. 2006, 2010; Large et al. 2007, 2009, 2011, 2012). Inthe Yangshan gold belt, the Devonian phyllite–limestone–

sandstone was pervasively metamorphosed to the greenschistfacies (Dong 2004). Thus, it is permissive to argue that theoriginal syngenetic/diagenetic pyrite, disseminated in thegreenschist facies strata and now locally overgrown by Py1and Py2, may have had even greater concentrations of goldand related trace elements than we have observed here. ThePy0 in the greenschist facies country rocks nevertheless stillhas relatively high concentrations of As, Au (up to 1,384 and8,000 times rock background levels, respectively; Feng et al.2005), Bi, Co, Cu, Mn, Ni, Pb, Sb, V, and Zn, and many ofthese elements would have been released deeper in the sedi-mentary pile as the strata were metamorphosed at highergrades. Most importantly, the Py0 at depth therefore providedan obvious source for the gold now present at economicconcentrations in deposits in the Yangshan gold belt. Ourhypothesis is that Py0 grains at such deeper crustal levels werefurther metamorphosed at higher temperatures than character-istic of the Yangshan greenschist facies and eventually wereconverted to pyrrhotite during desulfidation, during which

Fig. 15 Element variation plotsof in situ LA-ICP-MS analysis ofdifferent stage pyrite (Py0 to Py3).a The correlation between Au andAs contents in pyrite; the line in ais the inferred solubility limit ofgold for arsenian pyrite asdocumented by Reich et al.(2005). b The Au and Agcontents and Au/Ag ratios inpyrite. c The Co and Ni contentsand Co/Ni ratios in pyrite. d Thepositive correlation between Biand Pb

444 Miner Deposita (2014) 49:427–449

significant concentrations of many of the enriched trace ele-ments were released into the hydrothermal fluid (e.g.,Goldfarb et al. 2005; Pitcairn et al. 2006, 2010).

Many other trace elements, particularly Co, Ni, Pb, and Zn,are concentrated in the syngenetic/diagenetic pyrite and theorganic-rich Devonian strata (Feng et al. 2005). However,sulfide minerals with these metals are not abundant in the orebodies, and thus, they may not have been released from thepyrite and transported in the ore fluids in a manner similar tothe gold and arsenic. This reflects the limited solubility of theseother trace elements in hydrothermal fluids that are rich inreduced sulfur and of low salinity (e.g., Wood et al. 1987;Hemley et al. 1991). According to Qin and Zhou (2009), themean organic carbon content in the Devonian country rocks(phyllite, carbonaceous phyllite, sandy phyllite, silty phyllite,and limestone) is about 0.93 wt% in the Yangshan gold belt.The organic carbon in the metasedimentary rocks may haveplayed a significant role in the fluid redox reactions and pre-cipitation of much of the disseminated pyrite with a suite ofelements typically enriched in organic shales (Feng et al. 2005).

2. Early ore stage

Compared with Py0, Py1 has greater Ag, As, Au, Bi, Cu,Fe, Sb, and V and relatively lower concentrations of Co, Mn,Ni, Pb, Tl, and Zn. This indicates the initial hydrothermalfluids, which were likely derived from country rocks duringtheir prograde metamorphism at depth, would have containedat least some significant amount of the trace elements.

3. Main ore stage

Main ore stage Py2 has low concentrations of most of thetrace elements, but has high concentrations of As and Aucompared to Py1. The high As and Au concentrations maybe related to a widespread hydrothermal pulse of metamorphicfluid derived from a depth and temperature regime wheredevolatilization involved significant loss of Au, As, and Sfrom the syngenetic/diagenetic pyrite. The main stage hydro-thermal fluid, pervasive along the Yangshan gold belt, reactedwidely with the earlier pyrite generations and the host rocks.

The main ore stage Py2 has the highest As values, averag-ing 3±1 wt% (Fig. 14 and Table 5) and most of the pyrite isfine grained, indicating that the As in arsenian pyrite occurs asa metastable Fe(As,S)2 solid solution and the As-rich pyritewas rapidly precipitated (Cook and Chryssoulis 1990; Hustonet al. 1995; Simon et al. 1999). The dianion substitutionmechanism could explain the strong positive correlation be-tween the Au and As in the pyrite because the AsS3− dianionmay be charge compensated by Au3+ in the mineral lattice.Thus, the correlation between As and Au may reflect acoupled substitution mechanism in which Au3+ substitutesfor Fe2+ and AsS3− substitutes for the S2

2− dianion (Cook

and Chryssoulis 1990; Huston et al. 1995; Abraitis et al.2004).

4. Late ore stage

Compared with Py1, Py2, and Apy2, the late ore stage Py3and Apy3 contain relatively high concentrations of antimony(Tables 3 and 5). Stibnite in this stage is abundant and thus thelate ore fluid had a high content of Sb and (or) reached an areawhere uplifting rocks were passing through relatively lowertemperature retrograde conditions. The lower concentrationsof As and Au in Py3 compared with Py2 and the lower Asconcentration in Apy3 compared with Apy2, as well as the freegold observed in the late ore stage sulfide–quartz ± calciteveins, may indicate the release of Au from pyrite and/orarsenopyrite structures to form free gold, under slightly lowertemperature and/or pressure conditions (e.g., Boyle 1979;Groves et al. 1998).

The Py3 shows a roughly similar geochemical signature toPy1, but with notably higher concentrations of Au, Mn, Mo,and Sb. The Mo and Mn are likely leached from the countryrocks, as such elements are typically enriched in carbonaceousmetasedimentary rocks, but not very mobile in low salinityfluids that form orogenic gold deposits. The high Au reflectseither preexisting concentrations from the main ore stage and(or) a still gold-rich late hydrothermal fluid migrating toshallower crustal levels.

Comparison between Yangshan and similar gold depositsand implications for deposit genesis

A comparison of textural and pyrite geochemistry between theYangshan gold belt and other orogenic gold, as well as Carlin-like deposits (Bendigo, Spanish Mountain, Sukhoi Log, andCarlin Trend), is shown inOnline Resource 6 and Table 6. ThePy0 has similar textural and geochemical characteristics tothose pre-gold pyrites in orogenic and Carlin gold deposits.As discussed in previous studies (Goldfarb et al. 1997, 2005;Pitcairn et al. 2006, 2010; Large et al. 2007, 2009, 2011, 2012;Thomas et al. 2011), the sedimentary country rocks were animportant source for As and Au in the orogenic and Carlingold deposits. Such elements, at least for the orogenic golddeposits, were released from their original enrichment insyngenetic/diagenetic pyrite during metamorphism and laterwere precipitated in the epigenetic ores.

The hydrothermal pyrite in the Yangshan gold belt has afine-grained texture that is similar to that of ore-related pyritewithin the Carlin Trend. Overall, the trace element signaturesin the early and main ore stage pyrite are similar to those of theNorth Carlin Trend and the Bendigo orogenic gold deposit(Table 6; Large et al. 2009). The main ore stage Py2 has a highcontent of As, which is similar to both orogenic and CarlinTrend gold deposits. It has a similar low Ag content to pyrite

Miner Deposita (2014) 49:427–449 445

in orogenic gold deposits, but a similar Mo content to depositsof the Carlin Trend. It has higher Zn and lower Se than bothorogenic and Carlin Trend deposits. Concentrations of theother elements (V, Ni, Mn, As, and Au), as well as the Au/Ag ratio, are between those of the two types of gold deposits(Online Resource 6; Fig. 16).

The Yangshan gold belt was formed during the late stage ofthe West Qinling orogen. It is located adjacent to the Mian-Luesuture zone, and the ore bodies are strictly controlled by theAnchanghe-Guanyinba fault system and the Getiaowan-Caopingliang anticline. The host rocks are moderatelymetamor-phosed sediments and displaced blocks of granitic dikes (Dong2004; Yan et al. 2010). The chemistry of the early gold-bearingPy1 and later ore-bearing pyrite generations was establishedsynchronously with prograde metamorphism at depth and ret-rograde metamorphism along rocks exposed in the gold belt(e.g., Stuwe 1998); the syngenetic/diagenetic pyrite, however,predates ore formation by roughly 200 million years (Zhanget al. 1996). Nevertheless, the syngenetic/diagenetic pyrite islikely the source of gold, other metals, and sulfur released intothe hydrothermal fluids at greater depth than present exposures.

The early through late stages of pyrite were deposited after peakmetamorphism of the phyllitic host rocks, and gold was depos-ited in the area of the transition from ductile to brittle deforma-tion regime. The ore fluids are characterized by CO2- and

18O-rich (9.5–15.3‰; Li et al. 2008a) compositions, with low tomoderate salinities (<2∼5 wt%; Li et al. 2007a). All the abovecharacteristics are typical of orogenic gold deposits locatedthroughout many of the world's orogenic belts (Groves et al.1998; Goldfarb et al. 2005). Trace element patterns of ore stagepyrites in the Yangshan deposits are similar to those of theBendigo orogenic gold deposits (Large et al. 2009). TheYangshan gold belt, with mineralization that is dominated by awidespread disseminated style, is best defined by geological andgeochemical features, discussed above, which resemble mostwell-studied orogenic gold provinces.

Conclusions

This study demonstrates the value of EPMA and LA-ICP-MSstudies of the major and trace element compositions of

Table 6 Summary of high-value trace elements (>100 ppm) in major pyrite types at Sukhoi Log, Spanish Mountain, Bendigo, Northern Carlin Trend(Large et al. 2009), and Yangshan gold belt (elements in order from maximum to minimum)

Deposits Trace elements enriched insyngenetic/diagenetic pyrite(>100 ppm)

Trace elements enrichedin metamorphic and/orhydrothermal pyrite (>100 ppm)

Trace elements enrichedin outermost pyrite rim(>100 ppm)

Reference

Sukhoi Log As, Ni, Mn, Pb, Co, Ti, Cu, Zn As, Ni, ±Co Ni, ±Co Large et al. (2009)Spanish Mountain As, Ni, Cu, Pb, Se, Ti As, Ni, Se, ±Co Ni, Co

Northern Carlin Trend Cu, As, Ni, Se, V, Mo, Sb, Mn,Pb, Tl, Ag, Ti

As, Sb, Tl, Cu, ±Ag, Pb As, Au, Cu, Sb, Tl, ±Pb, Ag

Bendigo As, Ni, Pb, Co, Ti, Cu, Sb, Bi As, Ni, ±Co, Pb, Sb As, Pb, minor Au

Yangshan As, Ni, Pb, Co, Zn, Mn As, Pb, Co, Bi, Cu, Zn, Sb As, Cu, Pb, minor Au(average 63 ppm)

This paper

Fig. 16 Comparison of elementsvariations among the Yangshangold belt, Bendigo, SpanishMountain, Sukhoi Log, andCarlin Trend (Online Resource 6)

446 Miner Deposita (2014) 49:427–449

sulfides, particularly the complex variations in pyrite geo-chemistry, in the gold deposits of the Yangshan gold belt.Pyrite is formed/recrystallized throughout the syngenetic/diagenetic and hydrothermal stages of the West Qinlingorogen. We have documented its geochemical and texturalevolution from a syngenetic/diagenetic stage through to hy-drothermal varieties formed during main and post-ore stages.The geochemistry of other related sulfides, interpreted withinthe framework of the well-supported paragenetic sequence,supplements the geochemical study of the pyrite events.

Disseminated mineralization is the main ore style in theYangshan gold belt, with lesser quartz vein and veinlet styles.Main and late ore stage Py2 and Apy2 are commonly finegrained and have internal cores of Py0 and Py1, indicatingreactions between hydrothermal fluids and host rocks. Stibniteand native gold commonly occur as anhedral crystals in late,brittle quartz ± calcite veins.

The Py0 has relatively high values of As, Au, Bi, Co, Cu,Mn, Ni, Pb, Sb, and Zn. This syngenetic/diagenetic pyrite isinterpreted to have been the source for the gold resources inthe Yangshan gold belt, when it was metamorphosed at highertemperature conditions existing below presently exposedcrustal levels in the belt. Compared with hydrothermal pyrite,Py0 contains more Pb and Zn in solid solution and retainscharacteristic sedimentary Co/Ni ratios. Early hydrothermalPy1 has relatively high contents of As, Au, Bi, and Sb, and wesuggest that these high concentrations of ore-related elementsin the hydrothermal fluids were released into solution viametamorphism of country rocks at depth. Main ore stagePy2 has highest concentrations of As and Au, showing theintense fluid–rock interaction during the most important gold-forming event. Arsenic in main ore stage Py2 occurs in meta-stable Fe(As,S)2 solid solution and the arsenian pyrite wasrapidly precipitated. A coupled substitution mechanism couldexplain the correlation between As and Au. Late ore stage Py3contains high contents of Sb, which is consistent with thewidespread stibnite. Comparing the main and late ore stagearsenopyrite, the former has higher As levels, whereas thelatter has higher concentrations of S, Fe, and Sb, which isconsistent with main and late ore stage pyrite. The late hydro-thermal event likely occurred at slightly lower temperaturesand/or shallower depths, due to ongoing uplift of the orogenicbelt (Li et al. 2007a; Yan et al. 2010).

Acknowledgments We thank Professor Jean S. Cline of the Universityof Nevada, Las Vegas, for her helpful suggestions on earlier drafts of thispaper. Thanks are due to David Adams in the USGS for assistance withthe experiments. In particular, we would like to thank the 12th GoldDetachment of Chinese People's Armed Police for its cooperation. Thisresearch was jointly supported by the National Basic Research Programof China (no. 2009CB421008), the Geological investigation work projectof China Geological Survey (no. 1212011121090), the Program for NewCentury Excellent Talents (no. NCET-09-0710), and the 111 Project (no.B07011). Comments from the two anonymous reviewers and the editor,Prof. Georges Beaudoin, are greatly appreciated.

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