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

ARTICLE

Paragenesis and geochemistry of ore minerals in the epizonal gold 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-long analyses reveal that different generations of sulfides have char- Yangshan gold belt, Gansu Province are estimated to contain acteristic of major and trace element patterns, which can be more than 300 t of gold at an average grade of 4.76 g/t and used as a proxy for the distinct hydrothermal events. thus define one of China's largest gold resources. Detailed Syngenetic/diagenetic pyrite has high concentrations of As, paragenetic studies have recognized five stages of sulfide Au,Bi,Co,Cu,Mn,Ni,Pb,Sb,andZn.ThePy0 also retains mineral precipitation in the deposits of the belt. Syngenetic/ a sedimentary Co/Ni ratio, which is distinct from hydrothermal diagenetic pyrite (Py0) has a framboidal or colloform texture ore-related pyrite. Early hydrothermal Py1 has high contents of and is disseminated in the metasedimentary host rocks. Early Ag,As,Au,Bi,Cu,Fe,Sb,andV,anditreflectselevatedlevels hydrothermal pyrite (Py1) in quartz veins is disseminated in of these elements in the earliest mineralizing metamorphic metasedimentary rocks and dikes and also occurs as semi- fluids. The main ore stage Py2 has a very high content of As massive pyrite aggregates or bedding-parallel pyrite bands in (median value of 2.96 wt%) and Au (median value of phyllite. The main ore stage pyrite (Py2) commonly over- 47.5 ppm) and slightly elevated Cu, but relatively low values grows Py1 and is typically associated with main ore stage for other trace elements. Arsenic in the main ore stage Py2 arsenopyrite (Apy2). Late ore stage pyrite (Py3), arsenopyrite occurs in solid solution. Late ore stage Py3, formed coevally (Apy3), and stibnite occur in quartz ± calcite veins or are with stibnite, contains relatively high As (median value of disseminated in country rocks. Post-ore stage pyrite (Py4) 1.44 wt%), Au, Fe, Mn, Mo, Sb, and Zn and low Bi, Co, Ni, occurs in quartz ± calcite veins that cut all earlier formed and Pb. The main ore stage Apy2,comparedtolateorestage mineralization. Electron probe microanalyses and laser arsenopyrite, is relatively enriched in As, whereas the later ablation-inductively coupled plasma mass spectrometry Apy3 has high concentrations of S, Fe, and Sb, which is consistent with element patterns in associated main and late ore stage pyrite generations. Compared with pyrite from other Editorial handling: G. Beaudoin stages, the post-ore stage Py4 has relatively low concentrations Electronic supplementary material The online version of this article of Fe and S, whereas As remains elevated (2.05∼3.20 wt%), (doi:10.1007/s00126-013-0498-8) contains supplementary material, − which is available to authorized users. which could be interpreted by the substitution of As for S in the pyrite structure. These results suggest that syngenetic/ : * : < : : N. Li J. Deng ( ) L. Q. Yang R. J. Goldfarb C. Zhang diagenetic pyrite is the main metal source for the Yangshan State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, No. 29 Xueyuan Road, gold deposits where such pyrite was metamorphosed at depth Beijing 100083, People’s Republic of China below presently exposed levels. The ore-forming elements were e-mail: [email protected] concentrated into the hydrothermal fluids during metamorphic devolatilization, and subsequently, during extensive fluid–rock R. J. Goldfarb (*) : E. Marsh : A. Koenig : H. Lowers U.S. Geological Survey, Box 25046, Mail Stop 973, Denver Federal interaction at shallower levels, these elements were precipitated Center, Denver, CO 80225, USA via widespread sulfidation during the main ore stage. e-mail: [email protected]

< S. B. Lei . . . Headquarters of Gold Exploration Branch of Chinese Armed Police Keywords Orogenic gold Geochemistry Pyrite Force, Beijing 100055, People’s Republic of China Yangshan . West Qinling . China 428 Miner Deposita (2014) 49:427–449

Introduction sericite–pyrite, pyrite–arsenopyrite–quartz, arsenopyrite–py- rite–quartz, gold–stibnite–quartz–calcite, and calcite–quartz. China's orogenic belts are prospective areas for gold explora- However, due to the fact that most ore minerals are fine tion, and this has led to China being the world's largest gold grained and disseminated in host rocks, no supporting petro- producer for the past 5 years. The Yangshan gold belt, a major logical evidence of mineral paragenesis has yet been presented gold resource, was discovered between 1993 and 1997 by the in literature. This makes it difficult to convincingly define the People's Armed Police Gold Mining Troops. The 30-km-long composition of the hydrothermal fluids, gold-associated min- Yangshan gold belt in the Qinling-Dabie orogenic belt extends erals, and geochemical relationships that must be based on from Tangpugou in the west to Guzhen in the east (Fig. 1). A meticulous mineral paragenesis work and definition of asso- total of 96 gold-bearing lodes in six main deposits have been ciated ore-forming events. For example, previous researchers recognized in the gold belt and the identified recoverable conducted preliminary investigations of sulfide trace element resource is reported to be 308 tonnes (t) Au. It ranks as one geochemistry (Wu et al. 2008; Mao et al. 2009), but the of the largest gold resources in China, with an average grade relatively high detection limit of trace elements by electron of 4.76 g/t, and the resource is increasing with continued probe microanalyzer (EPMA), as well as the lack of petrolog- exploration (Yan et al. 2010). ical evidence for the paragenetic sequence, limits the possible From west to east, the Yangshan gold belt is divided into geological interpretations from their results. the Nishan, Getiaowan, Anba, Gaoloushan, Guanyinba, and This paper presents detailed petrological evidence to define Zhangjiashan gold deposits (Fig. 1). Among the six gold the mineral paragenesis of the Anba and related gold deposits. deposits, the Anba deposit contains ∼90 % of the total gold It discriminates the different sulfide generations by their char- resource (281 t with an average grade of 4.77 g/t), making it acteristic major and trace element patterns, which can be used the largest gold deposit defined in China. The Guanyinba gold as a proxy for defining the specific hydrothermal events in the deposit has 18 t Au at an average grade of 4.96 g/t, and the gold belt (e.g., Thomas et al. 2011). The results of this paper Getiaowan gold deposit contains 8.6 t Au at an average grade may be useful for further exploration through the identifica- of 4.14 g/t (Yan et al. 2010). Exploration of the Anba gold tion of sulfide minerals in the region that are related to the deposit is almost complete, with both underground and open gold-forming event and for helping to classify the paragenet- pit mining activities scheduled to begin in 2014. Unlike Anba, ically complex deposits of the Yangshan gold belt. 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 has Regional geologic setting focused on ore-controlling structures (Yuan et al. 2004;Li et al. 2008b), magmatic rocks (Qi et al. 2006a; Liu et al. 2008; It was first proposed that the Qinling orogenic belt was built Lei et al. 2010), hydrothermal fluids (Liu et al. 2003;Lietal. through closure of the Shangdan oceanic basin between the 2007a;Lietal.2008a), gold-bearing mineral assemblages North China and Yangtze Cratons (Mattauer et al. 1985; (Wu et al. 2008; Mao et al. 2009;Yangetal.2009), geochro- Zhang et al. 2001). However, more recent recognition of a nology (Qi et al. 2006b;Yangetal.2006; Lei et al. 2010), and South Qinling Block between the cratons, originally a part of deposit classification (Chen et al. 2004;Lietal.2007a;Qi the northern margin of the Yangtze Craton that had been rifted et al. 2008; Liu et al. 2010). Insufficient petrological evidence during the middle Paleozoic, has led to the now widely ac- of mineral paragenesis, however, hinders significance of many cepted “three-plate with two-suture zone” tectonic model previous studies, and there remains extensive debate about the (Zhang et al. 1995; Meng and Zhang 1999; Zhang et al. genesis and classification of the deposits in the belt. 2001). The geologic framework of the Qinling orogen is thus The Anba gold deposit has more extensive workings and viewed as a product of lengthy convergence and the closure of more exposed mineralization compared to the five other gold two basins between the North China Craton, South Qinling deposits, but the mineral paragenesis of the Anba gold deposit Block, and Yangtze Craton, which are separated by the Shang- was only briefly described in previous studies. Guo et al. Dan and Mian-Lue suturing fault systems, respectively (2002) and Yuan (2007) proposed four hydrothermal stages: (Fig. 1b; Meng and Zhang 2000). early unmineralized quartz, pyrite–quartz, pyrite–arsenopy- During Middle Devonian to Early Carboniferous (Zhang rite–quartz, and late quartz–calcite. The widespread stibnite et al. 2004), extension along the northern Yangtze craton was not discussed in these studies. Liu et al. (2003)andLi margin led to rifting of the South Qinling Block. The block et al. (2007b) pointed out the existence of stibnite in a late ore separated the opening Mian-Lue and preexisting Shang-Dan stage and suggested quartz–pyrite, quartz–arsenopyrite–py- paleo-oceans into two basins (Zhang et al. 1996). Rifting was rite, quartz–stibnite, and quartz–calcite stages. Yang et al. associated with the formation of Devonian and Carboniferous (2006), Li et al. (2007a), and Mao et al. (2009)arguedthat continental shelf-basinal sedimentary strata along the Mian- the hydrothermal stages should be divided into quartz– Lue ocean basin margins (Zhang et al. 2004). At the same Miner Deposita (2014) 49:427–449 429

Fig. 1 Simplified regional geology and deposit geology of the Yangshan b Schematic map showing the tectonics and distribution of gold deposits gold belt (modified from Chen et al. 2004;Zhao2009). a Tectonics in West Qinling. c Geology map of the Yangshan gold belt showing the position of West Qinling in the Qinling-Dabie orogenic belt. time, to the north, the middle Paleozoic northern Qinling arc The Mian-Lue ocean basin reached its maximum width terrane was developed along the suture between the South from Early Carboniferous to Early Permian (Zhang et al. Qinling Block and North China Craton as the Shang-Dan 2004). During the Middle Permian to Middle Triassic, the ocean basin closed. Mian-Lue ocean basin began to close, accompanied by 430 Miner Deposita (2014) 49:427–449 subduction of oceanic crust below the South Qinling Block. to 11-km-thick carbonaceous phyllite, limestone, and sand- This led to the Late Permian island arc magmatism along the stone. These sedimentary rocks were deposited during the previously passive continental margin on the southern edge of rifting of the South Qinling Block and were metamor- the South Qinling Block (Zhang et al. 2004). In Middle–Late phosed up to the lower greenschist facies (Li et al. 2003; Triassic, final closure of the Mian-Lue basin along the Mian- Dai et al. 2012). Lue fault system led to the continent–continent collisional Intrusions in the gold belt include minor granite, ap- orogenesis that characterized the West Qinling orogen and is lite, and porphyry dikes. The granitic rocks were sheared associated with the widespread gold mineralization. This in- into lenses along the Anchanghe-Guanyinba fault and cluded intense deformation, metamorphism, and magmatism secondary ENE- or NW-trending faults. Silicification, at the end of the Middle Triassic and development of a marine sericitization, carbonatization, sulfidation, chloritization, facies foreland basin system (Zhang et al. 1996). epidotization, and argillization are common in the hydro- As a result of orogenic uplift during Late Triassic to Middle thermally altered granitic rocks. Most pre-ore granitic Jurassic, particularly characterized by extensive thrusting and rocks are ca. 210 Ma and were subsequently altered folding on the southern side of the Mian-Lue fault, a foreland and mineralized to variable degrees (Qi et al. 2005; basin formed in the front of the foreland fold-and-thrust zone. Yang et al. 2006;Lei2011). Coevally, extensional collapse in the uplifted area to the north The gold belt is structurally located on the Wenxian arcuate of the Mian-Lue fault zone led to the formation of a series of structure, which is part of the 20- to 30-km-wide Mian-Lue Early and Middle Jurassic fault-bounded basins (Zhang et al. suture zone (Fig. 1). The Wenxian arcuate structure is com- 2004). The extension was accompanied by widespread em- prised of three E–W-trending faults; the Songbai-Liping, placement of middle Mesozoic felsic plutons and dikes (Du Majiamo-Weijiaba, and Baima-Linjiang faults, with some N– and Wu 1998). S-trending faults overprinting the arcuate structure. The arcuate The Middle Jurassic to Quaternary reflects a period of post- structure was initially formed during Late Permian to Late orogenic intracontinental tectonism. Intracontinental thrusting Triassic (e.g., Indosinian) tectonism and strongly deformed is marked by Middle Jurassic and Early Cretaceous large- during Jurassic to Early Cretaceous (e.g., Yanshanian) tecto- scale, thin-skinned nappe structures. The Bashan, Kangma, nism as determined from the ages of both hanging wall and foot and southern Dabie arcuate thrust systems define this defor- wall rocks and ages of E–W-trending syntectonic, brecciated mation along the Mian-Lue suture. A continental facies fore- felsic intrusions (Du and Wu 1998; Yan et al. 2010). land basin system also developed during the period. A second The 30-km-long E–W-trending Anchanghe-Guanyinba event is defined by the Cenozoic lateral extrusion of the Tibet fault follows a fold axis, offsets the more regional Songbai- plateau. Final rapid uplift of the central orogenic system was Liping fault, and has a width varying from tens of meters to associated with strike-slip to transtensional motion along several kilometers. The fault mainly dips to the north and the preexisting major structures, such as the Mian-Lue tectonic dip angle changes from 50° to 70°, commonly parallel to zone (Zhang et al. 2004). foliation in the phyllitic country rocks. Devonian phyllite, limestone, and sandstone occur in both the hanging wall and foot wall of the fault. The Anchanghe-Guanyinba fault in- Geology of the gold deposits cludes three distinct splays, namely F1, F2, and F3, dipping to the south, north, and south, respectively, and controlling the The Yangshan gold belt is situated in the Shaanxi–Gansu– no. 401, 305, 402, 403, and 311 ore bodies within the Anba Sichuan “Golden Triangle” region of China, which is the broad and Getiaowan deposits (Fig. 2). junction between the North China Craton in the north, Yangtze Due to the multiple periods of deformation and N–Scon- Craton in the south, and Songpan-Ganzi fold belt in the west vergence, the strata in the gold belt were complexly folded. (Fig. 1; Zhang et al. 1996;DuandWu1998; Pei et al. 2002). The near E–W-trending Getiaowan-Caopingliang anticline (Fig. 2), the largest fold in the gold belt, extends for about Geology of the Yangshan gold belt 10 km, with a width of 1 km. At the Anba gold deposit, both north and south limbs of the anticline are exposed, whereas at The Bikou Group containing the oldest rocks (846∼776 Ma: the Getiaowan gold deposit, the south limb is cut off by the Yan et al. 2003) in this region, at the northern margin of the younger Anchanghe-Guanyinba fault. The Devonian Yangtze Craton, has a maximum thickness of >16 km. It metasedimentary rocks were folded in this anticline and the consists of Mesoproterozoic to Neoproterozoic volcanic–sed- no. 311 ore body is located near the core of the anticline imentary rocks that were metamorphosed to the greenschist (Fig. 2). The Wujiashan syncline is the largest syncline in and, locally, to the amphibolite facies (Chen et al. 1987;Pei the gold belt and the Devonian limestone makes up the core of 1989). Rocks of the Devonian Sanhekou Group, which host the syncline. The syncline is limited in length due to later the deposits of the Yangshan gold belt, consist of a suite of 5- faulting associated with the Anchanghe-Guanyinba fault Miner Deposita (2014) 49:427–449 431

Fig. 2 Simplified geology of the Getiaowan-Anba gold deposits in Yangshan gold belt (after Yan et al. 2010), showing the location of Figs. 5 and 6. F1, F2,andF3 arethethreesplaysofthe Anchanghe-Guanyinba fault

system (Fig. 2). The Anchanghe-Guanyinba fault system and Disseminated pyrite and/or arsenopyrite occur in all rock the Getiaowan-Caopingliang anticline, as well as a series of types and most paragenetic stages (pre-ore syngenetic/ secondary ENE-trending faults, control the ore bodies of the diagenetic, as well as early, main, and late ore stages; Yangshan gold belt (Figs. 1 and 2). Table 1) at the six gold deposits. Two to 15 vol% fine- grained pyrite and arsenopyrite are unevenly disseminated in the altered host rocks, which vary from essentially barren of Description of the gold ore bodies gold to grades of as much as 10.6 g/t Au. Economic gold grades appear to be associated with two specific generations Primary gold ores are mainly disseminated in phyllite, granitic of disseminated sulfides, the pyrite- and arsenopyrite-rich dikes, limestone, and sandstone, but also occur in small veins main and late ore stages, as defined in more detail below. cutting these units. For the most part, the phyllite, with its The syngenetic/diagenetic pyrite is widely disseminated in enclosed lenses of granite, is multiply deformed between the the host rocks and, in the area of the ore deposits, forms the thick, relatively competent units of massive limestone. The cores to many of the early and main ore stage pyrite grains. altered granites indicate mineralization must be no older than Early ore stage, semi-massive pyrite aggregates (Fig. 3a), with Late Triassic. Disseminated ores in phyllite and granitic dikes crushed subhedral crystals, are particularly abundant in the are dominant in all six gold deposits in the belt, although phyllite and typically define subeconomic gold-bearing zones limestone is also an important host rock at the Guanyinba with <0.5 g/t Au. The main ore stage bedding-parallel pyrite deposit. Most disseminated ore bodies have an average grade and/or arsenopyrite (Fig. 3b), which are common along the of about 5 g/t Au. However, the more cataclastically deformed phyllitic foliations and form 1 to 7 vol% of the rock, are phyllite and granite have grades locally as high as 18 g/t Au typically in zones that vary widely in grade from 0.02 to (Yan et al. 2010). The mineralized quartz veins, with an 8.9 g/t Au. These sulfide minerals, where disseminated in average grade of 9.1 g/t Au, are localized along the contact country rocks, grew over cores of syngenetic/diagenetic and between phyllite and granitic dikes. Oxidized gold ores are early ore stage pyrite. Some of the late ore stage pyrite, characterized by high limonite contents and are present within arsenopyrite, and stibnite are disseminated in the host rocks, 30 m of the surface; they have an average grade of 2 g/t Au (Qi with the former two sulfides being the major gold-bearing et al. 2003). Based on the observations of outcrops, adit minerals. exposures, and diamond drill hole cores of the different gold Gold is less common in sulfide-bearing quartz ± calcite deposits in the Yangshan gold belt, as well as on microscopic veins in the early and main ore stages. Early ore stage quartz observations, the key textural and structural characteristics of veins having widths of 1 to 2 cm and containing 15 to 30 vol% the disseminated and vein ore styles are described below. pyrite, with individual grains that vary in diameter from 0.001 432 Miner Deposita (2014) 49:427–449

Table 1 Sulfides and mineralization styles in different ore stages

Mineralization styles Syngenetic/diagenetic Early ore Main ore stage Late ore stage Post-ore stage (Py0) stage (Py1) (Py2 +Apy2) (Py3 +Apy3 +Stn) stage (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 of •• disseminated sulfides Disseminated sulfides ••••

Center dot means the mineralization style existed in the stage; blank means the mineralization was not found in the stage to 5 mm, are mainly hosted in phyllite in the northern part of the anhedral–euhedral crystals ranging from 0.005 to 0.05 mm in Anba gold deposit. Main ore stage, folded thin veinlets, 1- to 3- diameter. Arsenopyrite has euhedral crystals ranging from 0.005 mm wide, are present in phyllite also mainly in Anba gold to 0.1 mm. Stibnite occurs as anhedral crystals in the void spaces deposit. They contain euhedral pyrite and arsenopyrite crystals. among quartz/calcite crystals, and the pyrite-arsenopyrite-stib- Thepyriteisfinegrainedand15to25μm in diameter, whereas nite-quartz ± calcite vein cuts the earlier formed pyrite and arsenopyrite in these veinlets is 0.05 to 2 mm in diameter. arsenopyrite mineralization (Figs. 3c, d and 7). Late ore stage quartz ± calcite veins contain pyrite, arsenopy- Post-ore stage quartz ± calcite veins contain sparse fine- rite, and stibnite. The veins are from 2- to 50-cm wide and are grained pyrite and cut the earlier formed disseminated and mostly present in the Anba gold deposit (Figs. 3c, d and 4). vein ores of all three epigenetic stages at all six gold deposits. These veins commonly occur in fracture zones in phyllite and Pyrite in these veins has subhedral crystals with average were locally brecciated during final events of the late ore stage diameters of 0.01 mm. (Figs. 3c, 4a,and5). Some thin veins, 2–5 mm wide, are common in granitic dikes (Fig. 4b), with only a few veins observed in metamorphosed sandstone. The mineralized late Analytical techniques quartz veins, commonly striking NE, cut the earlier formed disseminated and vein ore bodies (Fig. 5). Gold grade in eight Mineral paragenesis was complemented by petrological work of these veins ranges from 1.6 to 25 g/t. Pyrite, arsenopyrite, and under a transmitted and reflected light microscope and a stibnite account for 5 to 10 vol% of the veins. Pyrite has scanning electron microscope (SEM). The SEM images were

Fig. 3 The main styles of sulfide mineralization at Yangshan gold belt. a Semi-massive pyrite aggregate in gray-green phyllite. b Bedding-parallel bands of disseminated pyrite in gray-green phyllite. c The white dash line indicates the contact plane of granite and phyllite, and the left part is granite while the right part is phyllite, both of which are rich in pyrite and arsenopyrite. The 25-cm wide stibnite-quartz vein overprints the pyrite–arsenopyrite mineralization in phyllite. d Pyrite–arsenopyrite–stibnite– quartz vein in phyllite that is adjacent to granite dike Miner Deposita (2014) 49:427–449 433

Fig. 4 Occurrence of stibnite. a Stibnite–quartz vein and quartz breccias in fracture zone. 1 Stibnite–quartz vein and quartz breccia, 2 phyllite. b Stibnite– quartz vein in granite. Blue ink marks indicate the directions of thin sections

also used to identify small grains of rutile, galena, sphalerite, Pyrite grains analyzed by LA-ICP-MS were typically larg- and sulfosalt mineral inclusions. er than 100 μm. The LA-ICP-MS analyses were conducted on Based on careful examination of the mineral paragenesis, a Photon Machines Analyte G2 193 nm laser ablation system suites of 19 and 11 samples of the granitic dikes, phyllite, and attached to a PerkinElmer ELAN DRC-e ICP-MS. Laser quartz ± calcite veins were selected for EPMA and laser ablation-ICP-MS methods for pyrite are based on Large ablation-inductively coupled plasma mass spectrometry (LA- et al. (2007) and Zhao et al. (2011). A 15- and 30-micron ICP-MS) analyses, respectively, at the U.S. Geological diameter laser spot was used for the analyses. A laser fluence Survey, Denver (USA). Phyllite is gray in color, with granular of 2 J/cm2 and a frequency of 5 Hz were used for the spots. lepidoblastic texture and phyllitic structure. Fine-grained Helium was used as a carrier gas. Calibration was conducted quartz, sericite, and clay minerals are the dominant minerals. using the USGS MASS-1 sulfide reference material run five to Granitic dikes are gray green in color, with granulitic or ten times at the beginning of each session, following the porphyritic texture. The primary minerals in the dikes are procedures of Longerich et al. (1996) and using Fe as the plagioclase, quartz, and biotite, which were commonly altered internal standard element (e.g., Large et al. 2007). to sericite, chlorite, epidote, and clays. According to the Concentration calculations were carried out using off-line data different textures and mineralogy, various granitic dikes can processing following the equations of Longerich et al. (1996). be classified as granite, plagioclase granite, granite porphyry, The MASS-1 reference material was run periodically to mon- and plagioclase granite porphyry. itor for drift. During these analytical sessions, drift was less The samples were studied with an FEI Quanta 450 field than 5 % for all elements. A stoichiometric value of 46 % Fe emission scanning electron microscope operated at 20 kVand was used for the LA-ICP-MS concentration calculations. 2 to 10 nA current. Quantitative chemical analysis was ac- Detection limits were calculated as three times the standard quired with a JEOL JXA 8900 electron probe microanalyzer. deviation of the blank (Longerich et al. 1996). Data were Operating conditions were 20 kV, 50 nA, and a focused beam examined for the presence of mineral inclusions or zoning of 0.5 μm in diameter. Grains were analyzed by EPMA, and seen in the time-resolved spectra as deviations from a stable detection limits for studied elements are shown in Online signal (e.g., Large et al. 2007;Zhaoetal.2011). Resource 1. Accuracy and precision were better than 3 % Because some data are below the detection limits of the based on replicate analyses of sulfide standards. analytical techniques, corrections are necessary for the

Fig. 5 Measured section of the adit YM24-19-2C of Anba gold deposit (see Fig. 2 for location) 434 Miner Deposita (2014) 49:427–449

incorporation into the statistical method. Data qualified with a Py2 “less than” value were replaced with 0.7 times the detection limit. The highly censored elements, which include Cd, Ge, The Py2 is the main ore stage pyrite, with euhedral–subhedral Hg, In, Se, and Sn, are not reported. crystals varying from 10 to 200 μm in diameter, with a few of

more than 500 μm. The Py2 forms pyritohedron and octahedron cubes and/or as a combination of these forms. The Py2 occurs as fine-grained bedding-parallel sulfide, as grains hosted in the Textures of sulfides and paragenetic sequence folded quartz veinlets in phyllite (Figs. 3b, 6c,and7), or as

disseminations in granitic dikes. The Py2 is paragenetically The sulfides observed in the gold deposits can be interpreted associated with Apy2 and overgrows Py0 and Py1 (Figs. 7 and to have formed in five very distinct stages, which include 9c–i). It is the most important gold-bearing mineral. The mineral syngenetic/diagenetic (stage 0) to late veins (stage 4). inclusions in disseminated Py2 include zircon, sphalerite, galena, Sulfide minerals described below are identified by their par- boulangerite, famatinite, rutile, and apatite (Fig. 10). ticular hydrothermal stage. Five generations of pyrite (Py0 to Py4), two generations of arsenopyrite (Apy2 and Apy3), one Apy2 stage of stibnite deposition, and minor amounts of other sulfides and sulfosalt minerals from the different ore styles The Apy2 grains occur as euhedral prismatic crystals with have been defined by detailed paragenetic studies of the ores lozenge-shaped cross sections that vary from 0.002 to 2 mm. and host rocks. All the minerals of the summarized parage- They are widely disseminated in all the host rocks. The Apy2 netic sequence (Fig. 8) are hosted in phyllite, granitic dikes, is paragenetically associated with Py2 and quartz (Figs. 6b, c, and sandstone. Although limestone in the Guanyinba deposit 7,and9c–e, h,i). also exhibits a relatively strong degree of pyritization, the presence of arsenopyrite, stibnite, and gold is rare. The differ- Py3 ent sulfides and ore styles in different stages are summarized in Table 1, and the individual pyrite types are discussed below Late ore stage Py3 varies from 7 to 570 μmindiameterand in the order of interpreted paragenetic sequence. forms euhedral crystals. The Py3 is disseminated in host rocks or is paragenetically associated with Apy3 and stibnite in quartz ± calcite veins in fracture zones, which indicates a

Py0 brittle deformation regime during the late ore stage (Figs. 4 and 7 and Table 1). The Py3–Apy3–stibnite–quartz ± calcite The earliest pyrite, Py0, is disseminated in phyllite, limestone, veins cut the earlier formed Py2–Apy2–quartz veins (Fig. 5). and sandstone, and occurs as framboidal or colloform texture of microcrystals in spheres from 1 to 5 μmacross(Fig.9a)or Apy3 as irregular or rounded aggregates that are overgrown by ore stages Py1 and Py2 (Fig. 9d, e). The Apy3 has euhedral prismatic crystals with lozenge-shaped cross sections varying from 0.03 to 0.1 mm. The Apy3 coexists with Py3 in stibnite-rich quartz ± calcite veins in phyllite or is Py1 present as disseminated grains in granitic dikes (Figs. 6c and 7).

In general, the early ore stage Py1 forms euhedral– Stibnite subhedral cubes or pyritohedral crystals that are dissemi- nated in host rocks (Figs. 9e–i and 10a). The Py1 also Anhedral stibnite crystals occur in late ore stage quartz ± occurs as aggregates in quartz veins in the northern part calcite veins, which cut the earlier formed pyrite and arseno- of the Anba deposit or is present as semi-massive pyrite pyrite mineralization. Stibnite commonly coexists with Py3 aggregates in phyllite (Fig. 3a). The Py1 crystals vary from 5 and Apy3 and mainly occurs in quartz ± calcite veins in to 10 μm in diameter, with a few being more than 100 μm. fracture zones, and less commonly is disseminated in phyllite

There are different mineral inclusions in disseminated Py1, or in the dikes (Figs. 3c, d, 4 and 7). which include zircon, quartz, sphalerite, galena, boulangerite, jamesonite, and famatinite (Fig. 10). The quartz fibers, which Py4 are coexisting with Py1 and parallel to the foliation in phyllite, along with the microfractures in Py1 suggest that a ductile– The Py4 forms subhedral–euhedral crystals that are 5 to 10 μm brittle deformation regime was ongoing during the early ore in diameter. The pyrite commonly occurs in the barren quartz ± stage. calcite veins that cut all earlier formed mineralization (Fig. 6c). Miner Deposita (2014) 49:427–449 435

Fig. 6 Cross-cutting relation between quartz veins in the Yangshan gold belt. a Profile of lithology, contacts of different lithologies, and mineralization in Anba gold deposit (see Fig. 2 for location). b Apy2 in phyllite. c Scanned section of mineralized phyllite with various stage sulfide–quartz ± calcite veins, showing that the Py2–Apy2– quartz–calcite (Cal)veintype1 (Qz1) is occurring simultaneously with the mineralization in phyllite, and the Py3–Apy3– quartz vein type 2 (Qz2)is occurring after Qz1 and before the Py4–quartz vein type 3 (Qz3)

Other sulfides and sulfosalt minerals ranging from 5 to 10 μm in diameter, with some crystals up to typically 100 μmindiameter. Chalcopyrite, galena, sphalerite, boulangerite, jamesonite, famatinite, tennantite, and bournonite (Fig. 10) are common Gold minor metallic minerals in the deposits of the Yangshan gold belt. They may occur as mineral inclusions in pyrite and Native gold is uncommon in the Yangshan gold belt deposits. arsenopyrite, in ore stage quartz veins, or as disseminations Gold is detected by EPMA and LA-ICP-MS analyses in pyrite in country rocks. The sulfosalt minerals form anhedral crystals and 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 sequence of the Yangshan gold belt mineralization is summarized in Fig. 8. Petrographic observations distinguish different vein or veinlet mineralization types from a syngenetic/diagenetic stage to 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 major gold-bearing minerals. Pyrite is the most common sulfide mineral in the gold belt and it precipitated throughout the different ore stages. Our detailed paragenesis now enables a Fig. 7 Scanned section of mineralized phyllite with different-stage min- thorough documentation of sulfide evolution and description of eralized veins, showing the curved Py2–Apy2–quartz veinlet type 1 (Qz1) occurring simultaneously with the mineralization in phyllite and the the chemical changes across different stages, from early dia- relatively wide and straight Py3–Apy3–Stn–quartz vein (Qz2) cutting Qz1 genesis through to peak and post-ore events. 436 Miner Deposita (2014) 49:427–449

Fig. 8 Paragenetic sequence of the Yangshan gold belt mineralization and alteration interpreted from texture and sulfide geochemistry. The bold lines indicate high abundance, the thin lines represent the minor amounts, and the discontinuous lines indicate uncertainty in the determination of the paragenetic sequence due to the lack of clear textural relationship

Mineral chemistry of sulfides 50.6∼50.7 wt% for Py2,Py3,andPy4,respectively).Arsenic is commonly present in all pyrite stages, and the As contents Comparison between EPMA and LA-ICP-MS analyses in the pyrite vary from <251 ppm to 10.7 wt%, and the main

ore stage Py2 has the highest average As content (3.84± Elements such as Ag, Au, Bi, Cu, Ni, Pb, Sb, V, and Zn 1.83 wt%; Fig. 11 and Table 2). There is a negative correlation are commonly at concentrations lower than the detection between As and S in pyrite (Fig. 12), indicating substitution of limits of EPMA, but could be detected by LA-ICP-MS As1− for S in the pyrite structure (Fleet et al. 1993; Reich et al. (Tables 2, 3, 4,and5). Pyrite of different stages, examined 2005;Deditiusetal.2008). by both EPMA and LA-ICP-MS analyses, are compared in Comparing the chemistry of main ore stage Apy2 and late Online Resource 2. Comparing the As concentrations in pyrite ore stage Apy3,Apy2 has a lower average content of Fe (35.4 using both methods, it can be concluded that the results from ±0.4 wt%) and S (22.5±0.8 wt%) and a higher average the two methods are relatively consistent and LA-ICP-MS content of As (40.7±1.3 wt%), whereas Apy3 has a higher provides more robust low concentration data (Online average content of Fe (35.9±0.9 wt%) and S (23.4±1.0 wt%) Resource 3). and a lower content of As (39.8±2.4 wt%). This is the same

relationship observed for Fe, S, and As contents in Py2 versus EPMA major element data of sulfide minerals Py3 (Fig. 13 and Table 2). In addition to the major elements, Apy3 has a higher average content of Sb compared with Apy2 The EPMA analysis of minerals in different paragenetic stages (0.067±0.051 and 0.023±0.022 wt%, respectively; Fig. 13 has been undertaken for 19 samples from the Yangshan gold and Table 3). Minor amounts of Au (0.027±0.020 and 0.027± belt (Tables 2, 3,and4). The EPMA data show that the Fe 0.020 wt%, respectively), Cu, Ni, and Co are detected in both content in pyrite (Fig. 11)isfairlyconsistentacrossallfive Apy2 and Apy3 (Table 3). ∼ generations. The Fe content in Py2, the main ore stage pyrite, Minor amounts of As (0.253 0.341 wt%), Cr has an average value of 45.4±0.7 wt% and is slightly higher (0.028∼0.094 wt%), and Bi (0.044∼0.087 wt%) are pres- than that in Py0 (44.2±0.3 wt%) or Py4 (44.6∼44.7 wt%) and ent in stibnite, probably indicating minor substitution of slightly lower than Fe in Py1 (46.5±0.5 wt%) or Py3 (46.5± As for Sb (Nakai et al. 1986; Neiva et al. 2008), and Cr- 0.4 wt%). The S content in pyrite has a wider variation range bearing silicate or oxide mineral nanometer-sized inclusions than the Fe content, from 45.0 to 57.1 wt%. The average S that may come from the phyllite. Minor Bi in stibnite may be contents in syngenetic/diagenetic Py0 (53.4±2.5 wt%) and present as solid solution within the stibnite structure (Lueth early ore stage Py1 (52.5±1.0 wt%) are slightly higher than et al. 1990; Kyono and Kimata 2004). Gold is not detected in those in later ore stages (50.1±1.6, 51.5±2.0, and stibnite (Table 4). Miner Deposita (2014) 49:427–449 437

Fig. 9 Ore paragenesis of Yangshan gold belt. a SEM image of Early ore stage Py1 is overgrown by main ore stage Py2. g Early ore stage syngenetic/diagenetic framboid Py0. b Main ore stage Py2. c Main ore Py1 is overgrown by main ore stage Py2. h Early ore stage Py1 is stage Py2 and Apy2. d Syngenetic/diagenetic framboid Py0 is overgrown overgrown by main ore stage Py2 and Apy2. i Early ore stage Py1 is by main ore stage Py2 and Apy2. e Syngenetic/diagenetic framboid Py0 overgrown by main ore stage Py2 and Apy2 and early stage Py1 are overgrown by main ore stage Py2 and Apy2. f

LA-ICP-MS trace element chemistry of pyrite compared with Py0. Main ore stage Py2 has higher concentra- tions of As, Au, and Cu, whereas it has lower concentrations

The trace element compositions of Py0 to Py3 were determined in other trace elements. Late ore stage Py3 has higher values of by LA-ICP-MS on a suite of 11 representative samples (Online As, Au, Cu, Fe, Mn, Mo, Sb, and Zn and lower values of Bi,

Resource 4). Table 5 shows summary statistics for analyses of Co, Ni, Pb, and Ag compared with Py1 and shows more Py0 to Py3. The difference in composition for many elements detectable trace elements than Py2 (Fig. 14 and Table 5). between the different pyrite generations is shown by variation in The post-ore stage pyrite (Py4) was generally too fine grained median values. The concentrations of some elements, including for LA-ICP-MS analyses.

Ga and Te, are very low in all the pyrite types. Other elements, Figure 15a shows that five of the six Py0 samples plot suchasAg,As,Au,Bi,Co,Cu,Mn,Mo,Ni,Pb,Sb,Tl,V,and below the Au/Ag=1 line, whereas only one Py0 sample is Zn, can be interpreted to define characteristic signatures that above the Au/Ag=1 line. Early ore stage Py1 and main ore fingerprint each of the four pyrite types (see Fig. 14). stage Py2 show the broadest fields, with Au/Ag ratios varying The syngenetic/diagenetic Py0 has relatively high values of from 0.02 to >200. The Au/Ag ratio of Py1 can be below or As, Au, Bi, Co, Cu, Mn, Ni, Pb, Sb, Tl, V, and Zn. The early above 1, whereas that of Py2 is mostly >1, indicating Au is ore stage Py1 has higher contents of Ag, As, Au, Bi, Cu, Sb, preferentially concentrated in Py2 compared with Ag and V and lower contents of Co, Mn, Ni, Pb, Tl, and Zn, (Fig. 15b). The Au/Ag ratio of late hydrothermal stage Py3 438 Miner Deposita (2014) 49:427–449

Fig. 10 Secondary sulfides in Yangshan gold belt. a Quartz (Qz) and boulangerite (Boul) inclusions in Py1. b Quartz and famatinite (Fm) inclusions in Py1. c Quartz and sphalerite (Sp) inclusions in Py1. d Zircon (Zrn) and rutile (Rt) inclusions in Py2. e Sphalerite and tennantite (Tnt)in quartz vein. f Bournonite (Bnn)in granite porphyry

is above 1, which is similar to that of Py2, but with lower Ag Discussion compared to Py2. There is a positive correlation between Co and Ni and the Co/ Textural evolution of sulfides: geological and exploration Niratiomainlyfallsbetween0.5and10forallpyritegenera- significance 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 other Multiple generations of pyrite are disseminated in the country hydrothermal pyrite that mainly has more Co than Ni (Fig. 15c; rocks along the length of the Yangshan gold belt. The earliest

Bralia et al. 1979;Cook1996;Zhaoetal.2011). Bismuth could Py0 is commonly incorporated into the cores of Py1,Py2,and be present in solid solution in the pyrite or in metallic or sulfide Apy2 (Fig. 9d, e), resulting from the reaction of hydrothermal inclusions (Large et al. 2007, 2009;Thomasetal.2011), as there fluids with the wall rocks and precipitating the gold-rich is positive correlation between Bi and Pb (Fig. 15d). sulfide rims surrounding the preexisting Py0. Early ore stage Miner Deposita (2014) 49:427–449 439

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 4 Minimum 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

Py1 Maximum 3.07 0.0808 0.0767 0.337 0.0557 0.0533 47.5 0.126 55.1 0.0587 1.27 90 Minimum 0.0176 0.0344 0.0767 0.0106 0.0313 0.0533 44.2 0.0393 50.0 0.0239 0.0158 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

Py2 Maximum 10.7 0.0743 0.128 0.130 0.145 0.237 46.9 0.0862 54.0 0.0676 0.0284 270 Minimum 0.683 0.0344 0.0767 0.0106 0.0313 0.0533 43.0 0.0393 45.0 0.0239 0.0158 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

Py3 Maximum 3.59 0.0658 0.0767 0.0234 0.0313 0.0533 47.1 0.0393 55.1 0.0239 0.0158 22 Minimum 0.0176 0.0344 0.0767 0.0106 0.0313 0.0533 45.9 0.0393 48.1 0.0239 0.0158 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

Py4 Maximum 3.20 0.0344 0.0767 0.0565 0.0313 0.0533 44.7 0.0903 50.7 0.0239 0.0158 2 Minimum 2.05 0.0344 0.0767 0.0218 0.0313 0.0533 44.6 0.0666 50.6 0.0239 0.0158 Detection limit 0.0251 0.0491 0.110 0.0152 0.0447 0.0762 0.0454 0.0561 0.0228 0.0342 0.0225

Py1 is unevenly disseminated and often preserved as cores of from metal and sulfur release from other syngenetic/ later main ore stage Py2 (Fig. 9e–i), although some is also diagenetic pyrite (Py0) grains that were in the country rocks hosted in quartz veins in the northern part of the Anba deposit. being devolatilized in areas below the exposed greenschist

The disseminated Py1 occurs as semi-massive pyrite aggre- facies rocks and thus at higher metamorphic grades, which is a gates or forms bedding-parallel bands of disseminated sulfides common process inherent to most metamorphic belts and in phyllite. The notable increase in As and Au contents in Py1 leading to the formation of orogenic gold provinces (e.g., compared with Py0 indicates the hydrothermal fluids that Groves et al. 1998; Pitcairn et al. 2006). reacted with wall rocks had high contents of Au and As. Main ore stage Py2 and Apy2 are generally disseminated, This concentration of Au and As in the fluids likely resulted 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 29 Minimum 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 11 Minimum 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 440 Miner Deposita (2014) 49:427–449

Table 4 Two EPMA analyses of stibnite (in weight percent) Trace metals in pyrite Sample no. As Bi Cr S Sb Trace metals in pyrite may occur in several forms: (1) in solid SM4-2 no. 9 0.341 0.087 0.094 29.3 70.7 solution in pyrite structure, (2) in nanometer-sized inclusions YS-AB-4haodong-01-No1 0.253 0.044 0.028 29.0 69.8 of mineral or metallic grains, (3) within visible micron-sized Detection limit 0.0161 0.0611 0.0334 0.0202 0.0216 inclusions of other sulfides, or (4) within visible micron-sized inclusions of silicate or carbonate minerals (Large et al. 2007, 2009, 2011;Thomasetal.2011;Agangietal.2013). bearing sulfide–quartz veinlets in phyllite. Minor chalcopyrite However, the pronounced effects from the latter two forms coexists with Py2 and Apy2. The Py0 and Py1 may have on the output of the pyrite analyses were easily identified and provided sites for precipitation of much of the disseminated avoided when selecting analytical spots. Furthermore, if in- Py2 and Apy2 during fluid–rock interaction, which are local- clusions were encountered, then they were removed during ized in areas identified by widespread sericite–clay alteration or the data processing. bleaching of country rocks. Significantly, in contrast to the disseminated grains, Py2 and Apy2 in the quartz veins have 1. As and Au no Py0 or Py1 cores. This may be caused by direct precipitation of silica, sulfides, and gold from the hydrothermal fluids due to There are two different forms of arsenian pyrite: (1) As1–- pressure decreases during hydrofracturing and vein formation, pyrite [Fe(S,As)2] in which the arsenic substitutes for sulfur as which is the most common precipitation mechanism for metals As1– (e.g., Fleet and Mumin 1997;Simonetal.1999; 3+ in orogenic gold deposits hosted within vein quartz (e.g., Blanchard et al. 2007) and (2) As -pyrite [(Fe,As)S2]in Goldfarb et al. 1988, 2005; Weatherley and Henley 2013). which arsenic, mainly As3+, substitutes for Fe (Deditius Late ore stage Py3,Apy3, and stibnite occur in the quartz ± et al. 2008, 2009). The negative correlation between As and calcite veins or are disseminated in the host rocks, also in areas S concentrations in arsenian pyrite (Fig. 12) suggests that the – 1− that have experienced sericite clay alteration or bleaching. arsenian pyrite is [Fe(S,As)2] in which As substitutes for S During this hydrothermal stage, which was structurally con- in the pyrite structure (Fleet et al. 1993; Reich et al. 2005; trolled by the NE-striking faults, the ore fluids had somehow Deditius et al. 2008). evolved to deposit more Sb. In addition, Au may have been In the Yangshan gold belt, Au is mostly present as “invis- leached from the earlier formed Py3 and Apy3, or even the main ible gold” within pyrite and arsenopyrite, although some stage sulfide structures, to form free gold. The common occur- native gold also occurs in the late ore stage. All the pyrite rence of stibnite and the possible release of Au from older (Py0–Py3) analyses plot below the solubility limit line, show- sulfide grains may reflect the decline in the fluid temperature ing that most Au is present in solid solution (Au1+)inarsenian in the late ore stage episode, which is typical for many epizonal pyrite (Fig. 15a; Reich et al. 2005). orogenic gold deposits where Sb solubility decreases signifi- cantly at the lower temperatures (Boyle 1979;Grovesetal. 2. Co/Ni ratio 1998). This can occur during the ascent of later pulses of hydrothermal fluid into uplifting metamorphosed strata, which Many studies of trace elements have related the Co/Ni ratio are progressively cooling as they move along a clockwise P–T in pyrite to ore deposit type (Hawley and Nichol 1961; Loftus- path (e.g., Stuwe 1998). Post-ore stage Py4 occurs in latest Hills and Solomon 1967;Braliaetal.1979;Mookherjeeand quartz ± calcite veins, cutting all earlier formed mineralization, Philip 1979). Volcanogenic pyrite without accompanying with calcite fluid inclusion homogenization temperatures of lead- and zinc-bearing minerals shows Co/Ni values greater 160–210°C(Lietal.2007a) indicating a further decrease in than 1 (Loftus-Hills and Solomon 1967; Price 1972;Bralia temperature during final hydrothermal activity. et al. 1979). Pyrite of sedimentary origin is characterized by a The main and late ore stages contain most of the gold value of less than 1 (Loftus-Hills and Solomon 1967), 0.63 resources in the Yangshan gold belt. Therefore, defining the being typical (Price 1972). Conversely, pyrite with highly distribution and characteristics of Py2,Py3,Apy2,Apy3,and variable Co/Ni ratios, typically greater than 1, is considered stibnite is a useful guide for gold exploration in the region. to be of hydrothermal origin (Bralia et al. 1979; Cook 1996;

Most significantly, our mineral parageneses (Fig. 8), the abun- Zhao et al. 2011). In the Yangshan gold belt, the Py0,although dant arsenopyrite and or stibnite, the presence of significant it has been metamorphosed to the greenschist facies, retains chalcopyrite in pyrite grains, visible sericite–clay alteration or the characteristic sedimentary Co/Ni ratio (<1, averaging bleaching of the phyllite or enclosed igneous rocks, and (or) 0.572), whereas Py1 to Py3 have variable Co/Ni ratios some of the above stated geochemistry unique to Py2 and Py3, (0.0454∼19.8, averaging 2.5, Table 5), which is typical of are distinctive characteristics helpful to gold exploration in the hydrothermal pyrite (Fig. 15d). In a study on a number of Yangshan gold belt. orogenic gold deposits (e.g., Sukhoi Log, Bendigo, Spanish ie eoia(04 49:427 (2014) Deposita Miner – Table 5 LA-ICP-MS analyses of pyrite from different stages (in parts per million) 449

StageItemAgAsAuBiCoCuGaMnMoNiPbSbTeTlVZnCo/NiAu/AgNumberofdata

Py0 Maximum 4.16 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 6 Minimum 1.18 169 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 SD 1.19 4,980 13.3 29.0 315 29.2 0.67 85 5.62 2,135 791 38.7 1.7 2.20 10.5 468 0.304 3.1

Py1 Maximum 20.1 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 49 Minimum 0.250 109 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 SD 4.06 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

Py2 Maximum 8.99 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 Minimum 0.220 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 SD 1.40 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

Py3 Maximum 1.27 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 7 Minimum 0.390 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 SD 0.438 5,198 22.2 2.97 20.3 69.8 0 79.5 60.6 17.0 82.2 266 0 0 0 179 0.54 9.8 441 442 Miner Deposita (2014) 49:427–449

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

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

Mountain) and Carlin-type gold deposits, Large et al. (2009) notedthathydrothermalpyritehasacharacteristicallyhigher Co/Ni ratio than syngenetic/diagenetic pyrite. Thus, the Co/Ni ratio in pyrite is an important indicator to discriminate be- tween pre-ore and gold-related pyrite in complexly deformed parts of the Yangshan gold belt.

3. Au/Ag ratio

The hydrothermal pyrite has higher Au/Ag ratios (median

1.5, 33.1, and 48.3 for Py1,Py2,andPy3, respectively) than Py0 (median 0.3; Table 5), which is consistent with other results that showed hydrothermal pyrite had higher Au/Ag ratios than the syngenetic/diagenetic pyrite (e.g., Large et al.

2009; Thomas et al. 2011). Main and late ore stage Py2 and Py3 have significantly higher Au/Ag ratios than the early ore stage Py1, which is consistent with the recognition that Py2 and Py3 are part of the main ore-bearing generations that are Fig. 12 The negative correlation between As and S in arsenian pyrite characterized by pyrite overgrowths. Miner Deposita (2014) 49:427–449 443

Fig. 14 LA-ICP-MS analyses of representative trace elements (As, Au, Cu, Sb, Co, Ni) for the different pyrite paragenesis in Yangshan gold belt

4. Pb, Zn, Cu, Sb, Bi that the main ore stage fluid has slightly higher Cu contents than the early and late ore stage fluids. This would be consis- The Pb and Zn contents of pyrite can be attributed to the tent with the observation that reduced sulfur in low to moder- presence of nanometer-sized inclusions of Pb- and Zn-bearing ate salinity hydrothermal fluids can play a role in transporting minerals, such as galena and sphalerite, respectively, or to copper (e.g., Liu and McPhail 2005). Alternatively, if the solid solution in the pyrite structure (Large et al. 2009; hydrothermal fluids did not transport significant copper, a Cabral et al. 2011;Deditiusetal.2011;Thomasetal.2011). greater amount of sulfidation during fluid–rock interaction Such Pb and Zn are likely metal enrichments that were char- associated with the main ore stage may have caused precipi- acteristic of the original syngenetic/diagenetic pyrite. Galena tation of more copper phases in the ore bodies. and sphalerite occur as discrete sulfide inclusions in the hy- Correlations between Pb and Bi are consistent with Bi-rich drothermal stages (Py1–Py3; Fig. 8), and it is inferred that the galena and/or Pb-Bi sulfosalt nanometer-sized inclusions as metamorphic hydrothermal fluids may thus have carried mi- the major repository of Bi in pyrite (e.g., Large et al. 2009). nor amounts of Pb or Zn. The Py1,Py2,andPy3 show much stronger correlation be- Copper may be present in solid solution or as nanometer- tween Pb and Bi compared to Py0 (Fig. 15d), consistent with sized inclusions in pyrite (e.g., Large et al. 2007, 2009, 2011; the existence of galena and/or sulfosalt mineral inclusions in

Deditius et al. 2011;Thomasetal.2011). One high Cu value hydrothermal stages. The relatively high Sb contents in Py3 in Py2 suggests a chalcopyrite inclusion supported by the (Table 5) and Apy3 (Table 3), as well as the abundance of observation of minor chalcopyrite coexisting with Py2. stibnite in the late ore stage, indicate that the late hydrothermal Because Py2 has the highest Au and Cu concentrations, which fluid had a high Sb concentration and (or) a lower temperature are markedly different from Py0 and Py1, this could indicate that reduced solubility of Sb. 444 Miner Deposita (2014) 49:427–449

Fig. 15 Element variation plots of in situ LA-ICP-MS analysis of different stage pyrite (Py0 to Py3). a The correlation between Au and As contents in pyrite; the line in a is the inferred solubility limit of gold for arsenian pyrite as documented by Reich et al. (2005). b The Au and Ag contents and Au/Ag ratios in pyrite. c The Co and Ni contents and Co/Ni ratios in pyrite. d The positive correlation between Bi and Pb

Chemical evolution of sulfides and potential metal source sandstone was pervasively metamorphosed to the greenschist reservoir facies (Dong 2004). Thus, it is permissive to argue that the original syngenetic/diagenetic pyrite, disseminated in the

A summary of the petrological and geochemical characteris- greenschist facies strata and now locally overgrown by Py1 tics of the different sulfide mineral types is given in Online and Py2, may have had even greater concentrations of gold Resource 5. The geochemical evolution of ore-forming fluids and related trace elements than we have observed here. The and related sulfides are interpreted as follows: Py0 in the greenschist facies country rocks nevertheless still has relatively high concentrations of As, Au (up to 1,384 and 1. Syngenetic/diagenetic stage 8,000 times rock background levels, respectively; Feng et al. 2005), Bi, Co, Cu, Mn, Ni, Pb, Sb, V, and Zn, and many of In metasedimentary rock-hosted orogenic gold deposits, these elements would have been released deeper in the sedi- Au, As, and related elements are mobilized from syngenetic/ mentary pile as the strata were metamorphosed at higher diagenetic pyrite during metamorphism tens of millions of grades. Most importantly, the Py0 at depth therefore provided years after initial sedimentation. Under mainly greenschist an obvious source for the gold now present at economic and amphibolites facies conditions, these elements are most concentrations in deposits in the Yangshan gold belt. Our consistently released from syngenetic/diagenetic pyrite and hypothesis is that Py0 grains at such deeper crustal levels were concentrated in metamorphic ore-forming fluids (Pitcairn further metamorphosed at higher temperatures than character- et al. 2006, 2010; Large et al. 2007, 2009, 2011, 2012). In istic of the Yangshan greenschist facies and eventually were the Yangshan gold belt, the Devonian phyllite–limestone– converted to pyrrhotite during desulfidation, during which Miner Deposita (2014) 49:427–449 445 significant concentrations of many of the enriched trace ele- and Chryssoulis 1990; Huston et al. 1995; Abraitis et al. ments were released into the hydrothermal fluid (e.g., 2004). Goldfarb et al. 2005; Pitcairn et al. 2006, 2010). Many other trace elements, particularly Co, Ni, Pb, and Zn, 4. Late ore stage are concentrated in the syngenetic/diagenetic pyrite and the organic-rich Devonian strata (Feng et al. 2005). However, Compared with Py1,Py2, and Apy2, the late ore stage Py3 sulfide minerals with these metals are not abundant in the ore and Apy3 contain relatively high concentrations of antimony bodies, and thus, they may not have been released from the (Tables 3 and 5). Stibnite in this stage is abundant and thus the pyrite and transported in the ore fluids in a manner similar to late ore fluid had a high content of Sb and (or) reached an area the gold and arsenic. This reflects the limited solubility of these where uplifting rocks were passing through relatively lower other trace elements in hydrothermal fluids that are rich in temperature retrograde conditions. The lower concentrations reduced sulfur and of low salinity (e.g., Wood et al. 1987; of As and Au in Py3 compared with Py2 and the lower As Hemley et al. 1991).AccordingtoQinandZhou(2009), the concentration in Apy3 compared with Apy2, as well as the free mean organic carbon content in the Devonian country rocks gold observed in the late ore stage sulfide–quartz ± calcite (phyllite, carbonaceous phyllite, sandy phyllite, silty phyllite, veins, may indicate the release of Au from pyrite and/or and limestone) is about 0.93 wt% in the Yangshan gold belt. arsenopyrite structures to form free gold, under slightly lower The organic carbon in the metasedimentary rocks may have temperature and/or pressure conditions (e.g., Boyle 1979; played a significant role in the fluid redox reactions and pre- Groves et al. 1998). cipitation of much of the disseminated pyrite with a suite of The Py3 shows a roughly similar geochemical signature to elements typically enriched in organic shales (Feng et al. 2005). Py1, but with notably higher concentrations of Au, Mn, Mo, and Sb. The Mo and Mn are likely leached from the country 2. Early ore stage rocks, as such elements are typically enriched in carbonaceous metasedimentary rocks, but not very mobile in low salinity

Compared with Py0,Py1 has greater Ag, As, Au, Bi, Cu, fluids that form orogenic gold deposits. The high Au reflects Fe, Sb, and V and relatively lower concentrations of Co, Mn, either preexisting concentrations from the main ore stage and Ni, Pb, Tl, and Zn. This indicates the initial hydrothermal (or) a still gold-rich late hydrothermal fluid migrating to fluids, which were likely derived from country rocks during shallower crustal levels. their prograde metamorphism at depth, would have contained at least some significant amount of the trace elements. Comparison between Yangshan and similar gold deposits and implications for deposit genesis 3. Main ore stage A comparison of textural and pyrite geochemistry between the

Main ore stage Py2 has low concentrations of most of the Yangshan gold belt and other orogenic gold, as well as Carlin- trace elements, but has high concentrations of As and Au like deposits (Bendigo, Spanish Mountain, Sukhoi Log, and compared to Py1. The high As and Au concentrations may Carlin Trend), is shown in Online Resource 6 and Table 6.The be related to a widespread hydrothermal pulse of metamorphic Py0 has similar textural and geochemical characteristics to fluid derived from a depth and temperature regime where those pre-gold pyrites in orogenic and Carlin gold deposits. devolatilization involved significant loss of Au, As, and S As discussed in previous studies (Goldfarb et al. 1997, 2005; from the syngenetic/diagenetic pyrite. The main stage hydro- Pitcairn et al. 2006, 2010; Large et al. 2007, 2009, 2011, 2012; thermal fluid, pervasive along the Yangshan gold belt, reacted Thomas et al. 2011), the sedimentary country rocks were an widely with the earlier pyrite generations and the host rocks. important source for As and Au in the orogenic and Carlin

The main ore stage Py2 has the highest As values, averag- gold deposits. Such elements, at least for the orogenic gold ing 3±1 wt% (Fig. 14 and Table 5) and most of the pyrite is deposits, were released from their original enrichment in fine grained, indicating that the As in arsenian pyrite occurs as syngenetic/diagenetic pyrite during metamorphism and later a metastable Fe(As,S)2 solid solution and the As-rich pyrite were precipitated in the epigenetic ores. was rapidly precipitated (Cook and Chryssoulis 1990; Huston The hydrothermal pyrite in the Yangshan gold belt has a et al. 1995; Simon et al. 1999). The dianion substitution fine-grained texture that is similar to that of ore-related pyrite mechanism could explain the strong positive correlation be- within the Carlin Trend. Overall, the trace element signatures tween the Au and As in the pyrite because the AsS3− dianion in the early and main ore stage pyrite are similar to those of the may be charge compensated by Au3+ in the mineral lattice. North Carlin Trend and the Bendigo orogenic gold deposit

Thus, the correlation between As and Au may reflect a (Table 6; Large et al. 2009). The main ore stage Py2 has a high coupled substitution mechanism in which Au3+ substitutes content of As, which is similar to both orogenic and Carlin 2+ 3− 2− for Fe and AsS substitutes for the S2 dianion (Cook Trend gold deposits. It has a similar low Ag content to pyrite 446 Miner Deposita (2014) 49:427–449

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 in Trace elements enriched Trace elements enriched Reference syngenetic/diagenetic pyrite in metamorphic and/or in outermost pyrite rim (>100 ppm) hydrothermal pyrite (>100 ppm) (>100 ppm)

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, As, Sb, Tl, Cu, ±Ag, Pb As, Au, Cu, Sb, Tl, ±Pb, Ag Pb, Tl, Ag, Ti 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 This paper (average 63 ppm)

in orogenic gold deposits, but a similar Mo content to deposits The early through late stages of pyrite were deposited after peak of the Carlin Trend. It has higher Zn and lower Se than both metamorphism of the phyllitic host rocks, and gold was depos- orogenic and Carlin Trend deposits. Concentrations of the ited in the area of the transition from ductile to brittle deforma- 18 other elements (V, Ni, Mn, As, and Au), as well as the Au/ tion regime. The ore fluids are characterized by CO2-and O- Ag ratio, are between those of the two types of gold deposits rich (9.5–15.3‰;Lietal.2008a) compositions, with low to (Online Resource 6;Fig.16). moderate salinities (<2∼5wt%;Lietal.2007a). All the above The Yangshan gold belt was formed during the late stage of characteristics are typical of orogenic gold deposits located the West Qinling orogen. It is located adjacent to the Mian-Lue throughout many of the world's orogenic belts (Groves et al. suture zone, and the ore bodies are strictly controlled by the 1998; Goldfarb et al. 2005). Trace element patterns of ore stage Anchanghe-Guanyinba fault system and the Getiaowan- pyrites in the Yangshan deposits are similar to those of the Caopingliang anticline. The host rocks are moderately metamor- Bendigo orogenic gold deposits (Large et al. 2009). The phosed sediments and displaced blocks of granitic dikes (Dong Yangshan gold belt, with mineralization that is dominated by a 2004; Yan et al. 2010). The chemistry of the early gold-bearing widespread disseminated style, is best defined by geological and

Py1 and later ore-bearing pyrite generations was established geochemical features, discussed above, which resemble most synchronously with prograde metamorphism at depth and ret- well-studied orogenic gold provinces. 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 (Zhang Conclusions et al. 1996). Nevertheless, the syngenetic/diagenetic pyrite is likely the source of gold, other metals, and sulfur released into This study demonstrates the value of EPMA and LA-ICP-MS the hydrothermal fluids at greater depth than present exposures. studies of the major and trace element compositions of

Fig. 16 Comparison of elements variations among the Yangshan gold belt, Bendigo, Spanish Mountain, Sukhoi Log, and Carlin Trend (Online Resource 6) Miner Deposita (2014) 49:427–449 447 sulfides, particularly the complex variations in pyrite geo- References chemistry, in the gold deposits of the Yangshan gold belt. Pyrite is formed/recrystallized throughout the syngenetic/ Abraitis PK, Pattrick RAD, Vaughan DJ (2004) Variations in the compo- diagenetic and hydrothermal stages of the West Qinling sitional, textural and electrical properties of natural pyrite: a review. orogen. We have documented its geochemical and textural Int J Miner Process 74:41–59 evolution from a syngenetic/diagenetic stage through to hy- Agangi A, Hofmann A, Wohlgemuth-Ueberwasser CC (2013) Pyrite zon- ing as a record of mineralization in the Ventersdorp Contact Reef, drothermal varieties formed during main and post-ore stages. Witwatersrand Basin, South Africa. 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Cline of the University (in Chinese with English abstract) of Nevada, Las Vegas, for her helpful suggestions on earlier drafts of this Fleet ME, Mumin AH (1997) Gold-bearing arsenian pyrite and marcasite paper. Thanks are due to David Adams in the USGS for assistance with and arsenopyrite from Carlin Trend gold deposits and laboratory the experiments. In particular, we would like to thank the 12th Gold synthesis. Am Mineral 82:182–193 Detachment of Chinese People's Armed Police for its cooperation. This Fleet ME, Chryssoulis SL, MacLean PJ, Davidson R, Weisener CG research was jointly supported by the National Basic Research Program (1993) Arsenian pyrite from gold deposits: Au and As distribution of China (no. 2009CB421008), the Geological investigation work project investigated by SIMS and EMP, and color staining and surface of China Geological Survey (no. 1212011121090), the Program for New oxidation by XPS and LIMS. Can Mineral 31:1–17 Century Excellent Talents (no. 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