textures and in situ chemical and isotopic analyses of...
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©2016 Society of Economic Geologists, Inc.Economic Geology, v. 111, pp. 331–353
Textures and In Situ Chemical and Isotopic Analyses of Pyrite, Huijiabao Trend, Youjiang Basin, China: Implications for Paragenesis and Source of Sulfur
Lin Hou,1,3 Huijuan Peng,2,3† Jun Ding,1 Jinrang Zhang,1 Sibao Zhu,1 Songyang Wu1, Yue Wu4 and Hegen Ouyang5
1 Chengdu Center, China Geological Survey, Chengdu 610081, China2 Key Laboratory of Tectonic Controls on Mineralization and Hydrocarbon Accumulation, College of Earth Science,
Chengdu University of Technology, Chengdu 610059, China3 Economic Geology Research Centre (EGRU), College of Science, Technology and Engineering, James Cook University,
Townsville, QLD 4811, Australia4 School of Earth Environment and Water Resource, Yangtze University, Wuhan 430100, PR China
5 MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, PR China
AbstractMany Carlin-like Au deposits occur within the late Paleozoic and Triassic Youjiang basin of southwest China. The Huijiabao trend in Guizhou Province contains over 300 metric tons (t; 10.6 Moz) of Au at an average grade of 7 to 18 g/t in a narrow corridor that is about 20 km long and 5 km wide. Petrographic and SEM studies of pyrite in barren host rocks and high-grade orebodies led to the recognition of four stages of pyrite. Py1 consists of fine-grained framboidal crystals in black mudstone. Py2 is comprised of coarser grained euhedral-subhedral clusters that are spatially related to organic matter. Py3 is coarse grained, euhedral, and occurs as overgrowths on Py1 and Py2. Py3’s porous texture, inclusion of randomly oriented detrital minerals, and association with quartz recrystallization suggest it was deformed during Late Triassic orogenesis with Py1 and Py2. Py4 generally occurs as rims on Py1 to Py3 and is intergrown with arsenopyrite.
Sensitive high-resolution ion microprobe (SHRIMP) δ34S analyses of each pyrite type and arsenopyrite show that Py1 is related to Py2 and that Py3 is related to Py4 and arsenopyrite. The S isotope compositions of Py1 (–7.5 to +5.9‰) and Py2 (−5.3 to +7.9‰) are bimodal, which suggests that H2S was generated by biogenic sulfate reduction in open marine and sulfate limited systems during sedimentation and/or diagenesis. The compositions of Py3 (−2.6 to +1.5‰), Py4 (−1.2 to +1.5‰), and arsenopyrite (−0.8 to +0.9‰) are homo-geneous and have an intermediate range of values near 0‰ that suggest that H2S was derived either from average pyrite (0.2‰) in sedimentary rocks or from a concealed magmatic source. Laser ablation-inductively coupled plasma-mass spectrometer (LA–ICP–MS) trace element analyses (As, Ni, Co, Cu, Ag, Se, V) support different origins and show that Py3 and Py4 are ore related. The lower w(Co)/w(Ni) and w(S)/w(Se) ratios of Py1 and Py2 are consistent with formation during sedimentation or diagenesis, whereas the higher ratios of Py3, Py4, and arsenopyrite are consistent with a hydrothermal origin. The lower concentrations of Au in Py1 (0.23–2.5 ppm) and Py2 (0.06–12 ppm) show that little Au was added during sedimentation or diagenesis. The higher concentrations of Au in hydrothermal Py3 (1.1–110 ppm) and Py4 (0.34–810 ppm) indicate that most of the Au was introduced during subsequent hydrothermal fluid flow. The low Au contents of arsenopyrite (0.09–0.52 ppm) suggests they formed from Au-depleted fluids. The Au/As ratios of Py1 and Py2 are typical of diagenetic pyrite whereas Py3 and Py4 have ratios that approach those of ore-stage pyrite in Nevada Carlin-type deposits. The fracturing of Py3 and its cementation by Py4 suggests that ore fluid movement was associ-ated with deformation.
Published isochron ages on arsenopyrite (Re-Os ~200 Ma) and late calcite-realgar veinlets (Sm-Nd ~135 Ma) in the Huijiabao trend are older than mafic dikes (84 Ma) exposed ~20 km to the east. If the 200 and 135 Ma ages are valid, H2S and Au may be derived from a sedimentary source because igneous intrusions of this age have not been found. If these ages are not valid and the gold deposits are actually Late Cretaceous in age, then H2S and Au may be derived from a magmatic source. Additional geochronology and isotopic tracer studies are needed to resolve this uncertainty.
IntroductionCarlin-type gold deposits in Nevada, United States, are numerous, large, and known for the consistent occurrence of submicron Au in disseminated trace element-rich pyrite (Hofstra and Cline, 2000; Cline et al., 2005; Reich et al., 2005). Many similar Au deposits have been identified in vari-ous parts of the world. However, the grades or tons of ore are seldom comparable to those in Nevada and the deposits outside of China are generally isolated (Berger et al., 2014).
The clusters of Carlin-like deposits currently being mined in the Youjiang basin of southwest China share many character-istics with the Nevada deposits (Ashley et al., 1991; Hu et al., 2002; Hofstra et al., 2005; Peters et al., 2007; Su et al., 2009a; Cline et al., 2013). The Youjiang basin is a Paleozoic-Mesozoic sedimentary basin located in the southwest Yangtze block and has experienced a Cenozoic orogeny and Au mineralization. Disseminated gold deposits within the basin are both strati-graphically and structurally controlled and contain micron to submicron gold in ~0.1- to 2-mm grains of pyrite and arse-nopyrite. Although the gold deposits have been mined and
0361-0128/16/4380/331-23 331Submitted: May 12, 2014
Accepted: November 1, 2015
† Corresponding author: e-mail, [email protected]
332 HOU ET AL.
studied for decades (Zhang et al., 2003, 2005; Xia, 2005; Liu et al., 2006a; Su et al., 2008; Xia et al., 2009; Xiao, 2012; Wang, 2014), due to conflicting data, uncertainties remain regard-ing their origin, and this has hampered the development of a single genetic model.
Timing of Au mineralization
Table 1 lists geochronological data for Carlin-like gold depos-its in the Youjiang basin. A number of analytical methods have been used that have yielded a wide range of ages between 36 and 267 Ma. Of these, the ~200 Ma Re-Os isochron ages on arsenopyrite (Chen et al., 2015) and the ~135 Ma Sm-Nd isochron ages on calcite (Su et al. 2009b) are the most con-sistent. The arsenopyrite Re-Os ages overlap the end of the Indosinian (257–205 Ma) and the beginning of the Yanshanian (205–65 Ma) orogenies located, respectively, on the western and southeastern margins of the Youjiang basin. The calcite Sm-Nd ages fit with the Yanshanian orogeny. The Youjiang basin was affected by Late Triassic extension, contraction of uncertain age, and a second episode of extension prior to mineralization (Zeng et al., 1995; Yang et al., 2012a). Neither isochron age is contemporaneous with magmatism in the You-jiang basin (Fig. 1), which consists of (1) ~260 Ma extrusive Emeishan alkaline basalts/tuffaceous rocks and small doler-ite dikes on the northwest side of the basin; (2) 77 to 93 Ma intrusive alkaline granites that occur in the southern part of the basin, and (3) 91 to 97 Ma felsic and 84 Ma ultramafic dikes and pipes on the northeastern part of the basin (Chen, 2007; Chen et al., 2011, 2015). Due to a lack of agreement regarding deposit ages, the genetic relationship of magmatism to Au mineralization remains the subject of debate (Liu et al., 2006b; Su et al., 2009b; Zhang et al., 2010; Wang et al., 2013).
Without more robust data on the mineralization age, which is difficult to obtain, it is almost impossible to relate Au min-eralization to specific episodes of deformation or magmatism in southwest China.
Source of the Au
Previous workers have proposed that Au was derived from an initial synsedimentary or syndiagenetic source, followed by extraction from the Permian volcaniclastic rocks (Li, 1994; Xie, 2000; Qin and Liu, 2006). Others have called upon epigenetic input from hidden felsic intrusions (Liu et al., 2006b; Su et al., 2009b; Zhang et al., 2010; Wang et al., 2013). A mixture of sed-imentary and igneous sources has also been proposed (Zhang et al., 2003; Li et al., 2005; Xia, 2005; Wang et al., 2010). One of the most important constraints on the source of Au is pro-vided by the S isotope composition of Au-bearing sulfides. This is because HS– is likely the principal ligand responsible for Au transport in Carlin-type systems (Hofstra et al., 1991; Stenger et al., 1998; Hofstra and Cline, 2000; Cline et al., 2005). There-fore, as Au cannot have been sourced at depths greater than the S, the S isotope data can provide insights into Au sources. However, many studies have analyzed different generations of pyrite together, and since the ore-stage Au-bearing pyrite is seldom the only pyrite present in mineralized rock, the con-tribution from earlier and later formed pyrite may explain the broad S isotope range (Fig. 2). A lack of information on the proportions of ore stage and pre- or postore-stage pyrite lim-its their usefulness as indicators of the isotopic composition of sulfide introduced during mineralization.
New in situ analytical methods, such as laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to measure chemical compositions and sensitive high-resolu-tion ion microprobe (SHRIMP) to measure S isotope com-positions, make it feasible to distinguish each generation of pyrite in the ore and identify data related to mineralization.
The Huijiabao trend, usually referred by Chinese geologists as the Shuiyindong deposit, is known for its high-grade Au mineralization (7–18 g/t), and contains six ore blocks with over 300 t (10.6 Moz) of Au. The ore blocks occur along the axis of the Hijiabao anticline within in a narrow corridor <20 km long and 5 km wide. The proven Au reserves in these deposits
Table 1. Mineralization Age Data of the Gold Deposits in the Youjiang Basin
Deposit Dating methods Mineralization age (Ma) Reference
Nibao Fluid inclusion Rb-Sr age 142 ± 2 Liu et al. (2006a)Jinya Ore bulk Pb model age 130 ± 82 Hu et al. (2002) Sericite Rb-Sr age 206 ± 12 Wang et al. (2013) Sulfide Rb-Sr age 267 ± 28 Arsenopyrite 206 ± 22 Chen et al. (2015)Getang Fluorite Sm-Nd age 35.83 ± 0.37 Liu (2003) Quartz electron spin resonance (ESR) age 46 Zhu et al. (2000)Baidi Quartz fission track age 85.5 ± 6.8~90.8 ± 6.4 Zhang et al. (2003)Lannigou Quartz fission track age 82.3 ± 7.5~83.4 ± 8.3 Arsenian pyrite Re-Os age 193 ± 13 Chen (2007) Sericite Ar-Ar age 194.6 ± 2 Chen (2007) Arsenopyrite 204 ± 19 Chen et al. (2015) Fluid inclusion Rb-Sr age 259 ± 27 Hu et al. (2002) 105.6 Su et al. (2008) Quartz electron spin resonance (ESR) age 55.4 Zhu et al. (2000)Danzhai Enriched ore bulk Rb-Sr age 114 Chen (2007)Shijia Basite K-Ar age 137.8 Hu et al. (1995)Shuiyindong Calcite Sm-Nd age 134 ± 3~136 ± 3 Su et al. (2009a) Arsenopyrite 235 ± 33 Chen et al. (2015)Yata Ore bulk Pb model age 138 Chen (2007) Quartz electron spin resonance (ESR) age 63.4 Zhu et al. (2000)
CHEMICAL AND ISOTOPIC ANALYSES OF PYRITE, HUIJIABAO TREND, YOUJIANG BASIN, CHINA 333
are still increasing. The characteristics of the Huijiabao trend deposits are typical of many other Au deposits hosted in Permian fluvial and marine volcaniclastic and carbonate rocks of the Longtan Formation (Nie et al., 2008).
In this study, we present data on the textures, paragen-esis, in situ trace elements, and in situ S isotope geochem-istry of the various stages of pyrite, and related As sulfides, within three representative ore blocks in the Huijiabao trend: Shuiyindong, Nayang, and Taipingdong. On the basis of these results, we conclude that the S and therefore Au in the Hui-jiabao trend is either derived from magma or from average pyrite in the sedimentary rocks. In either case, it provides an important constraint on the source of S and Au as understand-ing of the significance of magmatism and its spatial and tem-poral relationship to hydrothermal activity advance.
Geologic BackgroundThe Youjiang basin is bound to the northwest by the Shizong-Mile fault, to the northeast by the Ziyun-Yadu fault, and to the southeast by the Pingxiang-Nanning fault, which sepa-rates the basin from Cathaysia (Fig. 1). The southern edge of the Youjiang basin is dominated by metamorphic rocks of the North Vietnam terrane, which is separated from the Simao block by the Red River shear zone (Zeng et al., 1995; Qin et al., 1996; Du et al., 2009; Yang et al., 2012b).
Yangtze Craton
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Fig. 1. Simplified geologic map showing the distribution of sediment-hosted disseminated Au deposits in the Youjiang basin: 1 = Huijiabao trend, 2 = Getang trend, 3 = Lannigou trend, 4 = Nibao, 5 = Nage 6 = Lekang, 7 = Langquan, 8 = Baidi, 9 = Longhuo, 10 = Gaolong, 11 = Badu, 12 = Jinya, 13 = Linbu (modified from Su et al., 2009a; Chen et al., 2011, 2015).
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Fig. 2. δ34S isotope data for sulfides and sulfates of Au deposits in the Youji-ang basin (data from Li, 1994; Hu et al., 2002; Xia, 2005; Zhang et al., 2003, 2005; Zhang et al., 2010; Wang et al., 2013).
334 HOU ET AL.
Devonian-Triassic sedimentation is recognized throughout the area and includes platform, open continental shelf, shelf margin, slope, and deep-sea basin faces. The Early-Middle Devonian strata in the Youjiang basin consist of sandstone, siltstone, and shale, along with rift-related basalt and diabase that unconformably overlie Cambrian-Ordovician calcare-ous rocks and shale (Zeng et al., 1995; Du et al., 2009). In turn, they are conformably overlain by a late Paleozoic-Early Triassic succession consisting of deep-sea basinal calcareous/siliceous/volcanic rocks and pelite, interpreted as a passive continental margin sequence (Qin et al., 1996). Also from this period are locally distributed shallow-marine platform carbonates (Fig. 1). Middle Triassic strata are comprised of shallow-marine platform carbonates in the northwest and thick deep-sea siliciclastic turbidites with interbedded sand-stone and shale in the southeast (Zeng et al., 1995; Yang et al., 2012a). Gold deposits generally occur in Upper Permian-Middle Triassic strata, which consist of bioclastic limestone and siltstone interlayered with calcareous pelitic beds and volcaniclastic layers (Su et al., 2009a; Cline et al., 2013).
Description of the DepositsGold deposits in the Youjiang basin are characterized by dis-seminated, Au-bearing, and trace element-rich pyrite that occurs in replacement bodies in the Late Permian-Middle Tri-assic carbonaceous limestone and bioclastic limestone (Fig. 3, Zhu et al., 1998; Wang et al., 2010). The various forms of indi-vidual orebodies, including tabular, strata-bound, T-shaped, and irregular, reflect local zones of porosity and permeability that result from the presence of reactive carbonate, lithologic contacts, disconformity planes, anticlinal hinges, high- and
low-angle faults, and, especially, the intersections of these features (Fig. 4, Xie, 2000; Zhu et al., 2000; Liu, 2003; Qin and Liu, 2006; Peng et al., 2012; Wang et al., 2013; Wang et al., 2014; Wu et al., 2014). Host rocks are typically decarbon-atized, argillized, and variably silicified, in addition to being sulfidized and enriched in Au (Su et al., 2009b).
The E-trending Huijiabao symmetrical anticline (Figs. 3A, 4, 5A), which is ~20 km long and 5 km wide, contains six ore blocks with significant amounts of Au (Xia, 2005). Within the Huijiabao trend, the most significant deposit is Shuiyindong (60 t or 2.1 Moz Au; Liu et al., 2009). As the geology of the Huijiabao trend has been described in detail by other workers (Liu et al., 2006a; Su et al., 2008, 2009a, b; Cline et al., 2013; Wang et al., 2013), it is only described briefly below.
The deposit stratigraphy includes bioclastic limestone of the Middle Permian Maokou Formation (P2m), interbedded sandstone, siltstone, limestone, and mudstone of the Upper Permian Longtan Formation (P3l), the Changxing-Dalong Formation (P3c-d) exposed in the core of the Huijiabao anti-cline, argillite intercalated with marlstone of the Yelang For-mation (T1y), and thick bioclastic limestone and dolomite of the Lower Triassic Yongningzhen Formation (T1yn), exposed in the limbs of the anticline (Figs. 3B, 5B, C). Observations from both outcrop and drill core indicate compression-related deformation, with the intensity of deformation varying with the physical characteristics of the stratigraphy. On the dis-conformity plane between P2m and P3l, weak deformation is evident as bedding-parallel slip (Fig. 4), whereas in the car-bonaceous siltstone, mudstone, and silty limestone of P3l, the wall rock exhibits plastic deformation (Fig. 5D). The marl-stone of T1y is affected by brittle fracturing and brecciation
Limestone
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Fig. 3. Structural map and stratigraphic profile of the study area. A. Map of the Huijiabao anticline, showing the major struc-tural features of the study area (e.g., faults) and the locations of the Taipingdong, Shuiyindong, Nayang, and other ore blocks. B. Stratigraphic column of upper Permian-Lower Triassic strata of the Huijiabao anticline (modified from Su et al., 2009a). The five dominant ore-bearing strata are denoted by I–V.
CHEMICAL AND ISOTOPIC ANALYSES OF PYRITE, HUIJIABAO TREND, YOUJIANG BASIN, CHINA 335
along reverse thrusts (Fig. 5E, F). Hydrothermal minerals are abundant in the deformation zones and stratigraphy with high permeability, usually occurring in dilatant veinlets or as stock-works of quartz, calcite, fluorite, and locally barite. However, the intensity of the hydrothermal alteration and gold miner-alization is not entirely cospatial with one another such that some altered rocks have low concentrations of trace elements and gold (Fig. 5G, H).
The most intense Au precipitation and wall-rock alteration (e.g., decarbonatization, silicification, and argillization) are also concentrated in the deformation zones and permeable strata. Carbonate rocks have been dissolved in most of the ore zones and have been locally replaced with quartz, which changed their color into light gray to white pink (Fig, 6A-C). In some places, intense decarbonatization has produced collapse brec-cias, which significantly enhanced porosity, permeability, and fluid-rock reactions, leading to the formation of high-grade ore (Fig. 6B, C). Silicification accompanied some, but not all, Au deposition and is best developed at the P3l3-P2m contact,
and to a lesser extent by fine-grained drusy quartz lining vugs (Fig. 6G-I). Milky quartz veins are also present near the hinge of the anticline that generally crosscut early-formed minerals (Fig. 6M-P). Argillization, which mainly affected aluminosili-cate minerals in volcaniclastic layers, produced assemblages of kaolinite ± dickite ± illite.
Orebodies occur within alteration zones in the Upper Perm-ian-Lower Triassic host rocks (Figs. 3B, 4, 5B). The orebod-ies located along the disconformity between P2m and P3l are named the I orebody, and they show features of interforma-tional brecciation and hydrothermal flow (Figs. 3B, 4, 10A). The three main orebodies (II, III, IV) contain approximately two-thirds of the reserves in the trend and are character-ized by strata-bound disseminated pyrite in decarbonatized carbonates and silicified siltstone of P3l (Figs. 3B, 4, 5G, H, 6J-L). The discordant fault- and breccia-controlled orebody V occurs in the siltstone of T1y (Figs. 3B, 4B, C, 5E, F). Sulfides present in each orebody include pyrite, arsenopyrite, real-gar, and orpiment, along with minor stibnite, marcasite, and
A
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Unit one: calcareous siltstone and sandstone
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Fig. 4. Schematic cross sections along the Huijiabao exploration lines (Fig. 3). A. Cross section through exploration line 7 of the Shuiyindong ore block (Liu et al., 2006a). B. Cross section through exploration line 260 of the Taipingdong ore block (Liu et al., 2012). C. Cross section through exploration line 319 of the Nayang ore block (Liu et al., 2006b). Note that layered ore blocks are buried deeper in the eastern part of the Huijiabao anticline than in the western part.
336 HOU ET AL.
Fig. 5. Field photographs highlighting the geology of the Huijiabao trend. A. Outcrop of the Huijiabao anticline. B. Con-formable contact between the argillite of P3l3, limestone of P3c + d, and marlstone of T1y. C. Lithology of the ore-hosting P3l3 formation of the Huijiabao trend. D. Strong plastic deformation of the clastic rocks of P3l3. E. F. Brecciated zones within marlstone of T1y, showing slip striations in the wall rock and veinlets of quartz (Qtz) and calcite denoted in black. G. Exten-sively silicified and sulfidized calcareous mudstone between the IIIb and IIIc orebodies, showing abundant calcite (Cal) veins, disseminated pyrite (py), and mudstone with low Au grades (denoted in red). H. IIIc ore block of P3l2, showing extensive sulfidation and silicification of limestone, quartz (Qtz), and a calcite (Cal) veinlet, which are interbedded with the siltstone.
CHEMICAL AND ISOTOPIC ANALYSES OF PYRITE, HUIJIABAO TREND, YOUJIANG BASIN, CHINA 337
Fig. 6. Photographs of drill cores and hand specimens, showing the alteration, mineralization, and ore textures of the Huijia-bao trend. A. The gray, black carbonates (limestone, marlstone) were decarbonatized into white, pink, sandstone-like rocks with high porosity. B. Strongly altered carbonates showing layers, Py2 mostly distributed in beddings, while Py3 crosscutting the beddings. C. Decarbonatized carbonate, mineralized by disseminated Py3. D. E. F. Py2 formed as layers in the carbon-ate, showing close relationship with organic matters. G. H. Py3-bearing quartz veins crosscutting early-formed Py2. I. A clear photo showing Py3-bearing quartz vein, Py3 is coarser than Py2 in (E) to (H). J. K. Photos showing very fine grained Py4 dis-seminating altered wall rocks. L. Py4-bearing rocks crosscutting by lateral barren quartz calcite veins. M. Py2-bearing rocks crosscutting by lateral barren quartz calcite veins. N. O. Brecciation of Py3, Py4, and wall rocks triggered by the lateral barren quartz calcite veins. P. Realgar-bearing veins crosscutting Py3 veins, then crosscut by lateral quartz calcite veins.
338 HOU ET AL.
chalcopyrite. Among these, the most significant Au-bearing mineral is arsenian pyrite.
Textures of Pyrite and Other SulfidesAlthough the Huijiabao trend contains a number of distinct structural and textural styles of mineralization, pyrite is the dominant repository of gold. However, the boundaries of the orebodies are defined by Au grade, rather than by the abun-dance of pyrite or by any clear lithologic change. The pyrite content varies from <1 to >10 wt %, both within and outside of the orebodies, and some areas that contain abundant pyrite (Fig. 5G, H) contain little or no Au. The poor correlation between pyrite and gold suggests that gold is only present in certain types of pyrite. Thus, it is critical to distinguish the dif-ferent types of pyrite and other Au-bearing sulfides.
Based on hand specimen and thin section observations of samples from three drill holes along exploration line 7 (Shuiy-indong ore block; Zk716, Zk720, Zk724), one drill hole from exploration line 319 (Nayang ore block; Zk31917), and one drill hole from exploration line 260 (Taipingdong ore block; ZK26007; Fig. 4), four main types of pyrite have been recog-nized based on grain size, texture, and matrix (Table 2). The key textural and structural characteristics of individual pyrite types and associated sulfides are discussed below in parage-netic order.
Pyrite 1
Pyrite 1 (Py1) occurs parallel to bedding in carbonaceous black mudstone (Fig. 7A) and consists of tiny fine-grained framboi-dal pyrite grains (Fig. 7B-E). Minor bands of Py1 (Fig. 7A, C) are present in most of the drill holes sampled in this study, but it is best developed in the strata of P3l2, which is an important host rock. This type of pyrite is commonly spatially related to organic matter (Fig. 7B-E, G, H). Individual Py1 clusters occur as euhedral microcrystals of ~0.1 to 5 μm in size, which form framboids 10 to 30 μm across (Fig. 7F). Py1 grains are overgrown by both Py2 (Fig. 7G) and Py3 (Fig. 7H) and are
considered to be the oldest pyrite type. Framboidal pyrite is generally thought to be synsedimentary or early diagenetic in origin (Liu et al., 2006b).
Pyrite 2
Pyrite 2 (Py2) occurs as clusters of relatively coarse euhedral-subhedral grains that commonly surround and overgrow Py1 (Fig. 7G). Py2 euhedral vary from 20 to 80 μm across and locally form aggregate layers or nodules several centimeters across (Fig. 6D-F, 8A).
Py2 can be further divided into Py2a and Py2b: the former is commonly developed in or surrounding biologic detritus (Fig. 8B), while the latter usually occurs as interlayers in carbonaceous siltstone (Fig. 8D). Both are related to S-rich organic matter in carbonaceous siltstone or mudstone (Fig. 8B-E). Many Py2 grains have overgrowth rims of later pyrite euhedra (Fig. 8C, E), interpreted as Py3. Some are further overgrown by latest Py4, with Py2 present in the core of Py3 euhedra (Fig. 8F).
Pyrite 3
Pyrite 3(Py3) is coarser grained than both Py1 and Py2, occurs as subhedral-euhedral grains, and commonly is confined to calcareous siltstone and carbonate (Fig. 9A). Py3 grains vary from 100 μm to 2 mm across and contain abundant inclusions of Py1, Py2, and organic matter (Fig. 9B, C, F-H). In back-scatter electron (BSE) images, Py3 grains are usually brighter than Py1 and Py2 (Figs. 7H, 8F, 9F-H), indicating they con-tain higher concentrations of other elements, such as Au and As. Many Py3 grains occur in quartz veinlets, indicating a hydrothermal origin (Fig. 9B). Nitric acid etching reveals that Py3 overgrowths replace randomly oriented detrital min-erals in the sedimentary rock (Fig. 9B). A porous texture is locally observed in Py3 that is flanked by quartz-rich pres-sure shadows (Fig. 9C, D). Py3 is best developed in strongly deformed zones in the core of the anticline and in breccias along reverse faults (Fig. 8C, D). These characteristics allow
Table 2. Summary of Common Textures and Interpreted Timing for Main Sulfide Types at Huijiabao
Interpretation ofSulfide type Texture Matrix timing and origin Evidence for timing
Py1 Fine-grained stratiform; S-rich organic matters Synsedimentary, Overgrown by all other pyrite types and microeuhedral; framboidal bioconcentration sulfides; typical framboidal texture for biological enrichment
Py2 Bedding-parallel; fine-grained Biologic clasts, interlayer of Syndiagenetic, Intimately related to Py1; bedding parallel; euhedral to subhedral clusters S-riched organic matters organic enrichment overgrow Py1 and is overgrown by Py3
Py3 Coarse-grained subhedral to Calcareous siltstone, Early deformation, Unoriented sediment matrix inclusions; euhedral; unaligned inclusions; carbonate, quartz veinlet hydrothermal quartz veinlets as matrix; quartz are quartz pressure shadow recrystallized as pressure shadows; overgrow Py1 and Py2, overgrown by Py4, or overprinted by arsenopyrite and realgar
Py4 Intermediate anhedral to Calcareous siltstone, Postdeformation, Overgrow all other pyrite types; show no subhedral; oscillatory rim carbonate, quartz veinlet hydrothermal orientation or pressure shadow
Arsenopyrite Acicular subhedral to euhedral Calcareous siltstone, Postdeformation Overgrow Py3; overgrow or overgrown by carbonate, quartz veinlet hydrothermal Py4
Realgar Subhedral to euhedral Quartz veinlet Postdeformation Overgrow Py3, hosted in the latest quartz hydrothermal veinlet
CHEMICAL AND ISOTOPIC ANALYSES OF PYRITE, HUIJIABAO TREND, YOUJIANG BASIN, CHINA 339
5um25um
20um
5um 50um
Py1MudstoneMarlstone
OM
D
A B
Py1
Py1
OM
Clay
Py2
Py1
Py1OM
OM
OM
Clay
Clay
Py3
Py3
E F
OMClay
Py1
50um
OM
Clay Py1OM
20um
C
HG
Fig. 7. Textural features of pyrite 1 (Py1) and its relationship to later pyrite 2 (Py2) and pyrite 3 (Py3). A. Hand specimen of a Py1-bearing mudstone interbedded with marlstone (OM = organic matter). B. Reflected light image of Py1 clusters in siltstone. C. D. E. Backscattered electron (BSE) image of Py1 framboids and clusters within mudstone, which are close to S-rich organic matter. F. Secondary electron image of a Py1 framboid. G. BSE image of Py1 framboids included in nodules of Py2, with both showing a close association with OM. H. BSE image of Py1 framboids, together with OM, hosted within a larger coarse-grained Py3.
340 HOU ET AL.
that Py3 formed during hydrothermal activity and subsequent deformation. Py3 grains are locally overgrown by Py4 with oscillatory zoning (Figs. 8F, 9G, 10B, C), or are overgrown by arsenopyrite and realgar (Figs. 9H, 10D, E).
Pyrite 4
Pyrite 4 (Py4) occurs as anhedral to subhedral grains 50 to 200 μm across, or as 5- to 30-μm-thick subhedral-euhedral rims with oscillatory zoning (Figs. 8F, 9G, 10B, C) and mantles earlier generations of pyrite. It is randomly dissemi-nated throughout the matrix of altered rocks and is present in quartz veins (Fig. 10A). The fact that Py4 overgrows Py3 and Py2, does not contain massive-porous texture, has no adjacent quartz-rich pressure shadows, and occurs mainly
within veinlets of quartz suggests formation after an episode of deformation.
Arsenopyrite
Arsenopyrite generally occurs as disseminated acicular crys-tals (Figs. 9H, 10D, E) 50 to 300 μm in length that either overgrow Py4 or are overgrown by Py4. Thus, it is synchro-nous with Py4.
Realgar
Minor amounts of realgar occur mainly as subhedral grains within later veinlets of hydrothermal quartz (Figs. 6P, 10A, F) and as overgrowths on Py2 and Py3. Although realgar was not observed in contact with Py4 or arsenopyrite, based on
A
1cm
Py2bSiltstone
200um
50um
D
e
E
Py2b
OM
Siltstone
OM
Py2b SiltstonePy3
100um
BC
Py2a
Siltstone
BC
Py2a
Py3
OMOM
OM
c
B
C
40um
50um
F
Py1?
Py2Py2
Rt
Py4
Py3
Py3
Fig. 8. Textural features of pyrite 2 (Py2) and its relationship with later pyrite. A. Hand specimen photograph showing the two subtypes of Py2 in siltstone. B. Reflected-light image of Py2a surrounding biologic detritus and intergrown with organic matter (OM). C. Backscattered electron (BSE) image of Py2a nodules overgrown by pyrite 3 (Py3) rims. D. BSE image of strings of Py2 within siltstone. E. BSE image of Py2b nodules overgrown by Py3. F. BSE image of Py2, probably overgrown by pyrite 1 (Py1) that in turn is overgrown by Py3, pyrite 4 (Py4), and Py3. BC = biological cluster.
CHEMICAL AND ISOTOPIC ANALYSES OF PYRITE, HUIJIABAO TREND, YOUJIANG BASIN, CHINA 341
50um
1cm
100um
80um
200um
100um
BA
DC
F
G H
10cm
Qtz
Py3
Recrystallized Qtz
Siltstone
Siltstone
Py3
Qtz (pressure shadow)
Fracture
Py3
Py2
Py2
Py3
Py4Py2
Py3
Aspy
DM
100um
Py1
Py2Py3
E
Fig. 9. Textural features of pyrite 3 (Py3) and its relationship with later pyrite. A. Photograph of the brecciated zone of a reverse fault in siltstone. B. Reflected-light (left; RL) and transmitted-light (right; TL) images of euhedral Py3 cubes in quartz veins, along with detrital minerals (DM) hosted within the main minerals. C. RL (left) and TL (right) images of fractured Py3 in recrystallized siltstone, showing quartz pressure shadows adjacent to a Py3 cube. D. Photograph of recrystallized wall rock, showing oriented quartz fibers. E. Backscattered electron (BSE) image of Py1 framboids overgrown by subhedral Py2, then surrounded by Py3. F. BSE image of euhedral Py3 overgrown on a pyrite 2 (Py2) nodule. G. BSE image of Py3 overgrown on Py2, in turn overgrown by a pyrite 4 (Py4) rim. H. BSE image of subhedral Py3 overgrown on Py2, in turn overgrown or overprinted by arsenopyrite (Apy).
342 HOU ET AL.
its occurrence in late-stage quartz-free veins, it is likely that realgar formed after Py4 and arsenopyrite.
Internally zoned pyrite
The internal textures of pyrite commonly consist of a core of broken and corroded crystals of Py1 (Figs. 7G, H, 9E), and sometimes of Py2 (Figs. 8C, E, F, 9E-H, 10C-E) that are sur-rounded by euhedral Py3 (Figs. 7H, 8C, E, F, 9E-H, 10B-E), and rimmed (Fig. 7C, E) by oscillatory zoned Py4 (Figs. 8F, 9G, 10B, C). Although not all sulfide types are necessarily present within any given pyrite aggregate, the same parage-netic sequence is observed from core to rim: Py1 → Py2 → Py3 → Py4.
Pyrite nodules
Two types of pyrite nodules are present in the sedimentary rocks of the Huijiabao trend. The first type consists of spheri-cal/framboidal pyrite nodules 10 to 30 μm across composed of Py1 (Fig. 7B-E). The second type consists of layers or nod-ules up to 2 mm across and 5 mm long comprised of coarse-grained, rounded to oval aggregates of Py2 (Figs. 6D-F, 8A, B, D). These two types of pyrite occur close to clasts of organic matter.
Sampling and Analytical TechniquesSamples for textural, chemical, and S isotope analysis were collected from field outcrops and from five drill cores taken
20um
100um
150um
1cm
50um
Py2
Py3
Py4
Py2
Py3Py4
Apy
Py2
Py3
B
C
E F
LimestonePy
Qtz
Re
Re
Py
Qtz
A
D
20um
Apy
Py3
Py2
Fig. 10. Textural features of pyrite 4 (Py4), arsenopyrite (Apy), and realgar (Re), and their relationships with other miner-als. A. Hand specimen photograph of brecciated limestone with pyrite, cemented by hydrothermal quartz (Qtz) and realgar. B. C. Reflected light (RL; B) and backscattered electron (BSE; C) images of subhedral pyrite 3 (Py3) with cores of pyrite 2 (Py2) and rims of Py4; the round black spots in (B) are laser burns from sample analysis. D. E. BSE image of Py3 with a core of Py2, overprinted by arsenopyrite (Apy). F. RL (left) and TL (right; transmitted light) images of realgar in a quartz vein overprinting pyrite.
CHEMICAL AND ISOTOPIC ANALYSES OF PYRITE, HUIJIABAO TREND, YOUJIANG BASIN, CHINA 343
from different ore blocks (Fig. 4). All drill holes pass through the central high-grade mineralization and marginal mineral-ization. Although complete drill holes were not available for sampling, a suite of samples was collected from the miner-alized and barren strata to provide a complete spectrum of pyrite types. Mineralogical and textural studies were con-ducted on 46 polished thin sections and polished blocks, on which 145 spots were analyzed by in situ LA-ICP-MS for trace element geochemistry, and 100 spots were analyzed by SHRIMP for S isotope compositions. Reflected and transmit-ted light microscopy, BSE imaging, and secondary electron imaging were used to characterize the sulfide mineral assem-blages in the deposit.
In situ S isotope analysis
In situ S isotope analyses were performed by SHRIMP at the Australian National University, Canberra, Australia, utilizing a beam depth of ~1 μm, and spot sizes of ~25 μm for ordinary coarse-grained Py2, Py3, and arsenopyrite, and 6 μm for fine-grained Py1 framboids and Py4 rims. Analytical techniques are as described by Eldridge et al. (1987, 1989). All data are normalized to a δ34S value of 1.2‰ (the Ruttan standard; Crowe and Vaughan, 1996). The internal error (±0.02) is based solely on the individual spot analysis over six scans. The external error includes the internal error and the error in the standards (±0.02).
LA-ICP-MS
Trace element concentrations in pyrite and arsenopyrite were determined by LA-ICP-MS at James Cook University, Townsville, Australia, using a GeoLas 193 nm ArF excimer laser coupled to a Varian 820 quadrupole IC mass spectrom-eter with He as the carrier gas in the ablation chamber. All analyses were performed in spot mode using laser beams of 16 and 32 μm on the same spots that were analyzed for S iso-tope by SHRIMP, using a repetition rate of 10 Hz and a laser energy of ~6 J/cm2. Chemical compositions for other spots on the same type of pyrites were also measured for comparison. Analytical techniques are as described by Large et al. (2007).
All analyses were quantified against the USGS Mass 1 syn-thetic pyrite and NIST 610 standards using Fe as the internal standard. To test the accuracy of internal standards, electron probe microanalysis (EPMA) was performed on different types of pyrites and sulfides at the Advanced Analytical Cen-tre, James Cook University.
Results
Pyrite S isotope compositions
The S isotope compositions measured by SHRIMP are pre-sented in Table 3 and Figure 11. The data show a clear dif-ference between the S isotope compositions of Py1 and Py2 and those of Py3 and Py4. The 47 analyses of Py1 and Py2 are bimodal and similar to each other, ranging from –7.5 to 5.9‰ and –5.3 to 7.9‰, respectively, with the median at –2.5 and +1.3%; all within the field of S isotope compositions from biogenic and abiogenic sulfate reduction (Fig. 12). The 30 analyses of Py3 vary from −2.6 to +1.5‰, with a median of 0.0‰; 73.3% of the data fall between −1 and +1‰. The S isotope compositions of Py4 vary from −1.2 to +1.5‰, with
73.3% of the data falling between −1 and +1‰. The S isotope compositions of arsenopyrite (eight analyses) vary from −0.8 to +0.9‰ (Fig. 11). Published analyses of realgar and stibnite vary from –4.9 to +5.2‰ (Xia, 2005; Zhang et al., 2010; Wang et al., 2013). The calculated S isotope composition of H2S in ore fluids at 210°C (Su et al., 2009a) using the Pyrite-H2S fractionation of Ohmoto (1972) is –1.7 ± 1‰.
Sulfide trace element geochemistry
LA-ICP-MS analyses of the four types of pyrite and arsenopy-rite are listed in Table 4 and displayed in Figures 13, 14, and 15. The data include 12 analyses of Py1, 33 of Py2, 36 of Py3, 26 of Py4, and 11 of arsenopyrite. Due to their fine-grained
0 2 4 6 8 10-2-4-6-8-10
Py1(n=15)Py2(n=32)Py3(n=30)Py4(n=15)Apy(n=8)
δ S (‰)34
Fig. 11. The δ34S values for all pyrite types (Py1-Py4) and for arsenopyrite (Apy; data summarized in Table 3). Note the narrow range of values for Py3, Py4, and arsenopyrite, but the bimodal distribution for Py1 and Py2.
Pa
K
JT
PeCDSOЄ
pЄ Compositions frombiogenic sulfatereduction only
Compositions frombiogenic and abiogenicsulfate reduction
Py1-2 Py1-2
Seawater -20‰
Seawater sulfatecomposition
-15-20 -10 -5 0 5 10 15 20 25 30 35δ S (‰)34
Age
(m
illio
n ye
ars)
0
100
200
300
400
500
600
700
800
Fig. 12. Comparison of δ34S values for Py1 and Py2 with the marine sulfate and biogenic sulfide curves. Note that all data of the Py1 and Py2 matched with the compositions from biogenic and abiogenic sulfate reduction.
344 HOU ET AL.
Table 3. SHRIMP In Situ S Isotope Compositions of Different Types of Pyrites and Arsenopyrite from the Huijiabao Trend
Internal Internal External Measured Internal δ4S error ± error ± error ±Ore block Section Pyrite Spot 34S/32S ratio error ± 1 sem (‰, CDT) 1 sem 95%T ‰ 95%T ‰
Shuiyindong 14ZK724-04 Py2 1 0.04367011 0.00000069 –1.8 0.02 0.04 0.29 2 0.04359241 0.00000075 –3.5 0.02 0.04 0.29 3 0.04356290 0.00000063 –4.2 0.01 0.04 0.29 14ZK724-05 Py1 1 0.04366639 0.00000726 –6.9 0.17 0.46 0.72 2 0.04351168 0.00000672 –5.4 0.15 0.42 0.71 3 0.04347927 0.00000909 –6.1 0.20 0.50 0.81 4 0.04345518 0.00000595 –6.7 0.14 0.39 0.69 Py2 5 0.04378019 0.00000092 3.0 0.02 0.05 0.27 6 0.043835794 0.000000626 2.0 0.01 0.04 0.29 7 0.043820978 0.000000688 1.9 0.02 0.04 0.29 8 0.044058996 0.000013895 1.9 0.32 0.81 0.90 Py3 9 0.04398409 0.00000610 0.3 0.14 0.39 0.69 10 0.04395378 0.00000548 –0.4 0.12 0.35 0.68 14ZK720-5-1 Py2 1 0.04394224 0.00000074 4.5 0.02 0.04 0.29 2 0.04389282 0.00000083 3.3 0.02 0.05 0.29 3 0.04366968 0.00000062 –1.8 0.01 0.04 0.29 4 0.04358030 0.00000102 –1.5 0.02 0.06 0.27 Py3 5 0.04374725 0.00000072 0.0 0.02 0.04 0.29 6 0.04368956 0.00000072 –1.3 0.02 0.04 0.29 7 0.04372296 0.00000068 –0.6 0.02 0.04 0.29 8 0.04371002 0.00000074 –0.9 0.02 0.04 0.29 Py4 9 0.04369360 0.00000072 –1.2 0.02 0.04 0.29 Py4 10 0.04369668 0.00000077 –1.2 0.02 0.05 0.29 11 0.04373666 0.00000084 –0.2 0.02 0.05 0.29 12 0.04375045 0.00000075 0.1 0.02 0.04 0.29 Apy 13 0.04366608 0.00000108 0.5 0.02 0.06 0.27 14 0.04363332 0.00000151 –0.3 0.03 0.09 0.27 14ZK720-5-2 Py2 1 0.04380905 0.00000108 3.7 0.02 0.06 0.27 2 0.04383302 0.00000107 4.3 0.02 0.06 0.27 3 0.04384944 0.00000112 4.7 0.03 0.07 0.27 Py3 4 0.04367033 0.00000099 0.6 0.02 0.06 0.27 5 0.04361731 0.00000102 –0.7 0.02 0.06 0.27 6 0.04360745 0.00000102 –0.7 0.02 0.06 0.27 7 0.04365724 0.00000109 0.3 0.02 0.06 0.27 Py4 8 0.04362103 0.00000109 –0.6 0.02 0.06 0.27 9 0.04365048 0.00000103 0.2 0.02 0.06 0.27 10 0.04373778 0.00000074 –0.2 0.02 0.04 0.29 11 0.04372795 0.00000072 –0.4 0.02 0.04 0.29 14ZK720-5-3 Py2 1 0.04375847 0.00000108 2.6 0.02 0.06 0.27 2 0.04376878 0.00000093 2.7 0.02 0.05 0.27 3 0.04380154 0.00000092 3.5 0.02 0.05 0.27 4 0.04385140 0.00000096 4.9 0.02 0.06 0.27 Py3 5 0.04360836 0.00000087 –0.8 0.02 0.05 0.27 6 0.04360828 0.00000112 –0.8 0.03 0.07 0.27 Apy 7 0.04361103 0.00000159 –0.8 0.04 0.09 0.27 8 0.04371772 0.00000058 –0.9 0.01 0.03 0.29 14ZK716-1-5-2 Py2 1 0.04365584 0.00000072 –2.1 0.02 0.04 0.29 2 0.04365944 0.00000071 –2.0 0.02 0.04 0.29 3 0.04362483 0.00000080 –2.8 0.02 0.05 0.29 4 0.04364101 0.00000071 –2.4 0.02 0.04 0.29 Py3 5 0.04366157 0.00000069 –1.9 0.02 0.04 0.29 6 0.04365927 0.00000062 –2.0 0.01 0.04 0.29 7 0.04365921 0.00000063 –2.0 0.01 0.04 0.29 8 0.04363140 0.00000073 –2.7 0.02 0.04 0.29 Py4 9 0.04369679 0.00000072 –1.2 0.02 0.04 0.29 10 0.04375535 0.00000088 0.2 0.02 0.05 0.29 11 0.04373234 0.00000080 –0.3 0.02 0.05 0.29 12 0.04379119 0.00000090 1.0 0.02 0.05 0.29
Taipingdong TPDZK26007Ia-2 Py2 1 0.043827325 0.000000713 1.8 0.02 0.04 0.29 2 0.043921295 0.000000909 3.9 0.02 0.05 0.29 3 0.043839802 0.000000582 2.1 0.01 0.03 0.29 4 0.043866065 0.000000838 2.7 0.02 0.05 0.29 Py3 5 0.043790819 0.000000844 0.9 0.02 0.05 0.29 6 0.043796743 0.000000701 1.1 0.02 0.04 0.29 7 0.04373071 0.00000077 –0.4 0.02 0.05 0.29 8 0.043811545 0.000000544 1.5 0.01 0.03 0.29
CHEMICAL AND ISOTOPIC ANALYSES OF PYRITE, HUIJIABAO TREND, YOUJIANG BASIN, CHINA 345
Taipingdong TPDZK26007Ia-3 Py2 1 0.043839802 0.000000582 2.1 0.01 0.03 0.29 2 0.043858912 0.000000692 2.6 0.02 0.04 0.29 3 0.043879437 0.000000704 3.0 0.02 0.04 0.29 4 0.043516138 0.000000734 –5.3 0.02 0.04 0.29 Py3 5 0.043762088 0.000000832 0.3 0.02 0.05 0.29 6 0.043723927 0.000000866 –0.5 0.02 0.05 0.29 7 0.043764987 0.000000864 0.4 0.02 0.05 0.29 8 0.043793601 0.000000884 1.1 0.02 0.05 0.29 Py4 9 0.044037653 0.00000672 1.5 0.15 0.42 0.71 10 0.044003388 0.000010904 0.7 0.25 0.69 0.81 11 0.04374493 0.00000073 –0.1 0.02 0.04 0.29 Apy 12 0.043763538 0.000000848 0.4 0.02 0.05 0.29 13 0.04373443 0.00000080 –0.3 0.02 0.05 0.29 14 0.04389849 0.00000589 0.9 0.13 0.37 0.55 15 0.04372918 0.00000083 –0.4 0.02 0.05 0.29
Nayang 14ZK31917-1 Py1 1 0.04396676 0.00000074 5.0 0.02 0.04 0.29 2 0.04392033 0.00000083 3.9 0.02 0.05 0.29 3 0.0440053 0.00000062 5.9 0.01 0.04 0.29 4 0.04399173 0.00000102 5.6 0.02 0.06 0.27 5 0.04393303 0.00000108 4.2 0.02 0.06 0.27 14ZK31917-2 Py1 1 0.04350949 0.00000112 –5.4 0.03 0.07 0.27 2 0.04345649 0.00000610 –6.6 0.14 0.39 0.69 Py3 3 0.04375354 0.000000757 0.1 0.02 0.04 0.29 4 0.043764015 0.000000652 0.4 0.01 0.04 0.29 5 0.043784944 0.000000723 0.9 0.02 0.04 0.29 6 0.043776011 0.000000622 0.7 0.01 0.04 0.29 14ZK31917-3 Py1 1 0.04352307 0.00000548 –5.1 0.12 0.35 0.68 2 0.04347094 0.00000072 –6.3 0.02 0.04 0.29 3 0.04341663 0.00000072 –7.6 0.02 0.04 0.29 4 0.04349241 0.00000068 –5.8 0.02 0.04 0.29 14ZK31917-4 Py2 1 0.043878923 0.000000671 3.0 0.02 0.04 0.29 2 0.044091146 0.000000796 7.9 0.02 0.05 0.29 Py3 3 0.04373339 0.00000075 –0.3 0.02 0.04 0.29 4 0.043773873 0.000000812 0.6 0.02 0.05 0.29
Table 3. (Cont.)
Internal Internal External Measured Internal δ4S error ± error ± error ±Ore block Section Pyrite Spot 34S/32S ratio error ± 1 sem (‰, CDT) 1 sem 95%T ‰ 95%T ‰
Py1(n=12)Py2(n=33)Py3(n=36)Py4(n=26)Apy(n=11)
0.01 0.1 1 10 100Au (ppm)
1000
Fig. 13. Au contents (ppm) for all pyrite types (Py1-Py4) and for arsenopyrite (data summarized in Table 4). Note the gen-eral increase in Au content of pyrite from Py1 and Py2, to Py3 and Py4. Arsenopyrite contains less Au than the pyrite types.
346 HOU ET AL.Ta
ble
4. S
elec
ted
LA
-IC
P-M
S A
naly
ses
of D
iffer
ent P
yrite
and
Ars
enop
yrite
from
the
Hui
jiaba
o Tr
end
(ppm
, unl
ess
note
d)
Ore
blo
ck
Sam
ple
Pyri
te
Spot
A
u S
(%)
V
Se
Te
Fe
(%)
Mn
W
Sn
Co
Ni
Cu
Zn
Ag
As
Sb
Hg
Tl
Shui
yind
ong
14ZK
724-
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Py2
1 0.
51
52.8
1.
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0.
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45.9
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10.7
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2
9.97
53
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8
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31
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6
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6
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2.34
54
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45
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3
53.8
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2
CHEMICAL AND ISOTOPIC ANALYSES OF PYRITE, HUIJIABAO TREND, YOUJIANG BASIN, CHINA 347
Apy
10
0.
30
19.9
61
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5.
91
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3
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5
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10
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30
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61
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9 27
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5 19
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6 29
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52
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8 30
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5 51
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19
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2,
652.
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4
14ZK
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1-5-
2 Py
2 1
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53
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18
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50
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82.4
88
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0.5
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93.6
15
9.0
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1.3
2
1.02
52
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8.0
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0.8
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60
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10
1.2
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23
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75
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3.0
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54
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11
1.8
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97
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5 1.
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9
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7
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53
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5 0.
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7 87
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56.3
21
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06.9
30
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2.48
53
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5.5
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1.
3 0.
3 1.
0 40
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18.7
14
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6.2
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60
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4.0
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9 73
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7 4.
6 0.
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4.
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0
10
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13
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12
7.
60
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11
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3 1.
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85.0
8.
3 5.
8
Taip
ingd
ong
TPD
ZK26
007I
a-2
Py2
1 0.
18
54.3
1.
9 2.
3 0.
2 45
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2.4
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282.
1 63
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60.1
32
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42.1
26
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2.6
2
1.72
53
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26.6
2.
7 0.
5 45
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3,11
5.6
0.1
0.6
71.6
17
1.8
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51
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8.9
29.9
1.
4 0.
9
3 0.
22
53.1
13
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12
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0.3
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5 4.
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5 23
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39.6
2.
9 2.
3
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53.9
10
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119.
4 14
5.4
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2 10
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3.8
11,1
75.8
15
5.5
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786.
4 4,
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5 10
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T
PDZK
2600
7Ia-
3 Py
2 1
1.26
53
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30.0
3.
5 1.
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6 9.
1 6.
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2 10
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2
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54
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7 1.
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8 9.
3 7.
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513.
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105.
4
3 0.
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8 9.
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1 4.
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3 5
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4 9
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7
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50
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0 10
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1.
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21
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5.
3 8.
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1,09
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7 4.
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8 49
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4.5
2.4
Tabl
e 4.
(C
ont.)
Ore
blo
ck
Sam
ple
Pyri
te
Spot
A
u S
(%)
V
Se
Te
Fe
(%)
Mn
W
Sn
Co
Ni
Cu
Zn
Ag
As
Sb
Hg
Tl
348 HOU ET AL.
nature, a laser beam diameter of 16 μm was used to analyze aggregates of Py1, which generally contain >80 vol % Py1. Py1 contains a wide range of trace elements, the most abun-dant of which are V, Te, Sn, and Hg, while elements with the lowest concentrations include Se, Mn, W, Co, Ni, Cu, As, Sb, and Tl. Compared with Py1, Py2 contains less V, Te, Sn, and Hg, but more Mn, W, Co, Ni, Ag, As, Sb, and Tl. Py3 contains much more Se, Sn, Co, Ni, Cu, and As than Py2, but less V, Mn, W, Ag, Sb, Hg, and Tl. A laser beam diam-eter of 16 μm was also used to analyze the trace element composition of Py4 rims. Py4 contains significant W, Co, and Ni, and abundant Se, Cu, As, Sb, Hg, and Tl. Arsenopyrite is enriched in Cu and Zn, but is relatively low in many other trace elements that are abundant in Py4, including W, Sn, Co, Ni, Hg, and Tl.
Figure 13 shows the results of LA-ICP-MS analyses for Au in Py1 to Py4 and arsenopyrite. Although the mini-mum abundance of Au in each pyrite type is low (0.06–1.1 ppm), the maximum concentrations progressively increase from 2.5 ppm in Py1, to 12 ppm in Py 2, to 110 ppm in Py3, and 810 ppm in Py4. Arsenopyrite contains little Au (0.09–0.52 ppm). In binary Au versus As, Ag, Cu, Mn, Ni, and Se diagrams (Fig. 14), Py1 to Py2 and Py3 to Py4 data cluster in two groups with some overlap, whereas arsenopy-rite data plot in a third cluster. In the Au versus As diagram (Fig. 14A), most of the Py1 to Py2 data plot in the diagenetic pyrite field and Py3 to Py4 data plot in the intrusion-related pyrite field of Large et al. (2011). One data point plots in the Carlin field and another plots above the gold saturation line of Reich et al. (2005).
The Co/Ni ratio is sensitive to environmental change and is widely used to identify the origins of sulfides, especially pyrite (Bralia et al., 1979; Campbell and Ethier, 1984; Bajwah et al., 1987). Generally, synsedimentary pyrites have w(Co)/w(Ni) ratios of <1, averaging 0.63. The w(Co)/w(Ni) ratio of hydro-thermal (vein) pyrite is >1, usually 1.17 to 5. Pyrite of volca-nogenic massive sulfide (VMS) origin has w(Co)/w(Ni) ratios of 5 to 50, averaging 8.7. The w(Co)/w(Ni) ratios of Py1 and Py2 from the Huijiabao trend are 0.09 to 0.94, averaging 0.39, which are all within the synsedimentary range. However, the w(Co)/w(Ni) ratios of Py3 and Py4 are 0.27 to 5.93, averaging 1.39, with most falling within the hydrothermal range. The w(Co)/w(Ni) ratios of arsenopyrite are 0.83 to 1.48, averaging 1.12 (Fig. 15).
The Se/S ratio in sulfide minerals is generally believed to be dependent on the fluid Se2–/S2– ratios, redox, pH, and tem-perature (Yamamoto, 1976), and is used to interpret the ori-gin of pyrite (Tischendorf and Ungethum, 1964; Zhao, 1998; Matthews et al., 2008; Zhang et al., 2013). Normally, when the fluid temperature is above 200°C, H2S and H2Se are the dominant sulfur and selenium species in most hydrother-mal fluids, and H2Se/H2S approximates SSe/SS (Huston et al., 1995a, b), under this situation, hydrothermal pyrite has w(Se)/w(S) ratios of >1 × 10–5, while synsedimentary pyrite has w(Se)/w(S) ratios of <0.5 × 10–6. The w(S)/w(Se) ratios of Py1 and Py2 are mainly <0.5 × 10–6, while those of Py3, Py4, and arsenopyrite are mainly >1 × 10–5, indicating that Py1 and Py2 are synsedimentary, while Py3, Py4, and arsenopyrite are hydrothermal in origin.N
ayan
g 14
ZK31
917-
1 Py
1 1
0.35
51
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47.1
2.
9
3.8
44
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4.9
0.
2
0.9
38
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103.
5
64.3
25
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84
7.9
6.
7
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2
2 2.
50
60.1
39
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3
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44
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0
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13
1.6
55
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14
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52
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073.
3
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127
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3
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7
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9
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2 Py
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7
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3
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4
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1
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73
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4
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4
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0
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8
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82
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3
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0
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29
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3
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8
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7
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6
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9 2
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0
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8
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4
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22
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0
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0
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9
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7
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6
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3 1
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6
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50
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6
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15
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98
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9
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0
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43.1
11
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1
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2.
1
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0
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3
393.
0
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8
620.
6
4.5
0.
0
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09.6
39
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0.
2
14ZK
3191
7-3
Py1
1 0.
25
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2.
1
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4.
0
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0.
6
67.4
16
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0.
6
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6
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30
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2
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5
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7
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4
90.9
32
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12
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1,
655.
7
17.7
76
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3
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2.
4
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44
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0.
1
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70
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9
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9
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6.
0
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4
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6
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0
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3
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2
28.5
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2
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3191
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1 46
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0
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1
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0
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0.
1
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79
0.5
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0.
1
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28.2
18
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0.
2
Py
4 3
4.51
52
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0.6
9.
2
2.2
44
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0.
1
0.5
2,2
45.0
1,
054.
8
334.
9
3.4
0.
1
52,5
19.9
26
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0.
3
4 19
.00
51
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2.7
4.
3
0.5
44
.7
8.8
7.
5
0.4
50
0.0
16
1.6
36
4.5
6.
1
0.0
56
,747
.4
17.4
1.
6
0.3
Tabl
e 4.
(C
ont.)
Ore
blo
ck
Sam
ple
Pyri
te
Spot
A
u S
(%)
V
Se
Te
Fe
(%)
Mn
W
Sn
Co
Ni
Cu
Zn
Ag
As
Sb
Hg
Tl
CHEMICAL AND ISOTOPIC ANALYSES OF PYRITE, HUIJIABAO TREND, YOUJIANG BASIN, CHINA 349
10 100 1000 104 105 1060.01
0.1
1
10
100
1000
104
Py1Py2Py3Py4Apy
As(ppm)
Au(
ppm
)
Au saturation line
0.01 0.1 1 10 100 1000
0.1
1
10
100
1000
104
Ni(p
pm)
Au(ppm)
0.1
1
10
100
1000
0.01
0.01 0.1 1 10 100 1000
Au(ppm)
Ag(
ppm
)
0.01 0.1 1 10 100 1000Au(ppm)
10
100
1000
104
Cu(
ppm
)
0.01 0.1 1 10 100 1000Au(ppm)
0.1
1
10
100
Se(
ppm
)
0.01 0.1 1 10 100 1000Au(ppm)
0.1
1
10
100
1000
104
Mn(
ppm
)
A B
C D
E F
Carlin-type pyrite
Intrusionrelated pyrite
Diagenetic pyrite
Py1Py2Py3Py4Apy
Py1Py2Py3Py4Apy
Py1Py2Py3Py4Apy
Py1Py2Py3Py4Apy
Py1Py2Py3Py4Apy
Fig. 14. Interelemental correlations for all pyrite types (Py1-Py4) and arsenopyrite, the purple polygons represent Py3 and Py4, the yellow polygons represent Py1 and Py2, and the black polygons represent arsenopyrite. (A) As vs. Au, (B) Au vs. Ni, (C) Au vs. Ag, (D) Au vs. Cu, (E) Au vs. Se, and (F) Au vs. Mn. Note that in (A), the content of Au increases sharply in Py3 and Py4, but is low in the arsenopyrite; furthermore, all data plot below the Au saturation line, excepting one data point of Py4. Comparing with the fields Large et al. (2011) suggested, all data of Py1 plot within the field of diagenetic pyrite, Py2 plot within a mixed field of diagenetic- and intrusion-related pyrite, while most plots of Py3 and Py4 fall in the intrusion-related pyrite field. In general, Ni, Ag, Cu, Se, and Mn correlate with Au for Py1, Py2, and Py3; Py4 data plot in different areas of the diagram and are slightly enriched Ni and Cu. From Py1 to Py4, Se increases and Mn decreases. Elements in arsenopyrite behave independently of those in pyrite.
350 HOU ET AL.
Discussion
Sulfide and Au paragenesis
Overprinting and overgrowth relationships define a clear para-genetic succession for the various styles and stages of pyrite (Fig. 16; Table 2), as well as for their relationship to quartz veins, arsenopyrite, and realgar. This paragenetic sequence can be divided into three main stages of development: Py1 and Py2 sedimentation and diagenesis, hydrothermal quartz veinlets, and predeformation Py3 and postdeformation Py4, arsenopyrite, and realgar (Fig. 17).
Although Su et al. (2008) reported the occurrence of micron to submicron particles of visible Au in the Shuiyin-dong ore block, no native Au was found either under micro-scope or BSE in this study, even for the pyrites from which the maximum concentrations we have detected (110 ppm in Py3, 810 ppm in Py4). In this case, we agree with Liu et al. (2003) who considered that gold occurs in the lattice of the pyrite. However, the LA-ICP-MS trace element data allow us to infer the paragenesis of Au in sulfides based on the Au com-position. As shown in Figure 13, Au contents were strongly enriched during the precipitation of Py3 and Py4. Consider-ing that these pyrites are also paragenetically related to other hydrothermal gangue minerals, including quartz, calcite, and fluorite, we suggest that the evolution/reaction of the fluids was accompanied by the appearance of Au and other metal-logenic elements. In the Au versus As diagram (Fig. 14A), the data of Au-enriched Py1 and Py2 and Au-depleted Py3 and Py4 plotted in two distinct fields representing diagenesis- and intrusion-related origin, respectively. The plots of Py3 and Py4 form an elongate vertical pattern located at the Au-As-rich end of the trend. Arsenopyrite forms an independent cluster in the lower right part of the diagram, where Au is depleted and As is enriched. These results may indicate that Au con-centrations in the Huijiabao trend are related with intrusions and are not correlated with As.
Origin of sulfides
On the basis of petrographic observations, the pyrite is divided into four types according to morphological characteristics and overgrowth relationships. Geochemical data strengthen these distinctions and indicate that Py1-Py2 and Py3-Py4-arsenopy-rite formed in different geochemical environments: syndiage-netic and hydrothermal. Reich et al. (2013) and Deditius et al.
Py1(n=12)Py2(n=33)Py3(n=36)Py4(n=26)Apy(n=11)
101
102
103
104
104103102101100
w(C
o)/1
0-6
w(Ni)/10 -6
Co/Ni=10
Co/Ni=1
Co/Ni=0.1
Fig. 15. Relationship between w (Co) and w (Ni) of different sulfides from the Huijiabao trend.
B C
DE
Py1(framboid) Py1(framboid)
Py2(nodules)
Py1(framboid)Py2(nodules)Py1(framboid)Py2(nodules)
Py3(euhedra-subhedra)Py4(euhedral rim) Py3(euhedra-subhedra)
50um
A
Fig. 16. Stages of pyrite paragenesis. A. Backscatter electron image of multistage pyrite. B. Schematic diagram showing the sequence of formation of the four types of pyrite.
CHEMICAL AND ISOTOPIC ANALYSES OF PYRITE, HUIJIABAO TREND, YOUJIANG BASIN, CHINA 351
(2014) reported that in undersaturated S-bearing fluids, Au is generally transported as Au(HS), and there is a maximum Au/As molar radio of ~0.02 for arsenian pyrite. In the present study, the Au/As molar ratios of all but one pyrite are much less than 0.02, indicating that the Au-bearing solutions were generally undersaturated with gold (Table 4; Fig. 14A). The porous textures in Py3 are typical of pyrite that inverted from a marcasite precursor, which forms at pH less than 5 and T <240°C (Murowchick, 1992). The paucity of Au in arsenopy-rite suggests that ore fluids were depleted in Au by the time it reached saturation at lower fS2 conditions.
Su et al. (2009a) reported that during the main-stage min-eralization in the Huijiabao trend, fluids had temperatures of 210° ± 20°C, CO2 and CH4 were present, and the Fe contents of fluids were below the detection limit, indicating that the oxygen fugacity of the fluids was low enough for H2S to be dominant. In Hofstra et al. (2005) synopsis of H and O isotope data from gold deposits in the Youjiang basin (including Shuy-indong), they concluded that metamorphic ore fluids mixed with external meteoric ground water and, in places, organic water. The paucity of igneous intrusions in the Youjiang basin led them to discount the potential importance of magmatic fluids.
Sources of S in sulfides
Previous estimates of the isotopic composition of S in Au-bearing pyrite and arsenopyrite in the Huijiabao trend yielded a wide range of values (–8.41 to +27.17‰; Xia, 2005). Our S isotope data has a narrower range from –7.5 to +7.9‰ during the syndiagenetic Py1 and Py2 stages to –2.6 to +1.5‰ during the hydrothermal Py3 and Py4 stages.
The S isotope compositions of syndiagenetic Py1 and Py2 have a range of 15.4‰. The large range in δ34S indicates that H2S was generated by bacterial reduction off marine sulfate during sedimentation and diagenesis in open (lower mode) and sulfate-limited (higher mode) systems (Aharon and Fu, 2000; Machel, 2001; Shen et al., 2001), which is consistent with the texture of the pyrite (e.g., framboids and nodules) and the abundance of organic matter.
The δ34S values (−2.6 to +1.5‰) of ore-related Py3 and Py4 in the Huijiabao anticline have a much more restricted range compared to data reported for ore-related pyrite in other Car-lin-type deposits in the Youjiang basin (−30 to +27‰; Fig.
1). Most of those data were from pyrite separates contami-nated by variable amounts of synsedimentary or syndiagenetic pyrite. The absence of a wide range in δ34S values for Py3 and Py4 indicates there was little or no contamination from Py1 or Py2 and that the narrow range of values is representative of ore-stage S. A magmatic source may account for the narrow range of near 0‰ Py3-Py4 and arsenopyrite S isotope values (Ohmoto, 1972). Similar results were obtained by secondary ion mass spectrometry (SIMS) analyses of ore-stage pyrite from the Betze-Post deposit, in the Carlin trend (Kesler et al., 2005), which also has a small range in values (−1 to +7‰), that have been interpreted to reflect a magmatic source of S. However, the H2S in ore fluids may have been derived from an average sedimentary pyrite source, because the mean of Py1-Py2 (0.16‰) is essentially the same as the mean of Py3-Py4 and arsenopyrite (–0.15‰). Organic S compounds, another labile source of reduced S, should also be considered.
The paucity of igneous intrusions in the Youjiang basin is at odds with a magmatic source for S. As are the best estimates for the age of mineralization (~200 and ~135 Ma), which are older than the nearest set of small 84 Ma lamprophyre dikes located ~20 km to the east of Shuiyindong. A concealed intru-sion could be an explanation, but aeromagnetic surveys have failed to detect a short amplitude anomaly near Huijiabao that might reflect one (Bai et al., 2014). If magmatic fluids were involved, they must be derived from a small deeply buried intrusion. If not, then S and Au were probably derived from sedimentary rocks, as Su et al. (2009a) suggested.
Summary and ConclusionsThe results of this study of pyrite paragenesis and S isotope and trace element geochemistry at Huijiabao trend support the model of Muntean et al. (2011), who suggested that Au and S were introduced by magmatic fluids. However, the absence of geologic, geophysical, or independent isotopic tracer evidence in support of a magmatic source make it pos-sible that S and Au were derived from sedimentary rocks.
Our results, based on textural analyses, show that pyrite can be divided into four types that formed in the order Py1 → Py2 → Py3 → Py4. SHRIMP S isotope analyses suggest that the H2S fixed in Py1 and Py2 was produced by bacterial reduction of marine sulfate in open and sulfate-limited syndiagenetic environments. In contrast, the H2S and Au in Py3 and Py4 is hydrothermal in origin and was derived either from a mag-matic source or an average sedimentary source. The fractur-ing in Py3 and it cementation by Py4 show that mineralization was coeval with an episode of deformation. Additional geo-chronologic and isotopic tracer studies are needed to ascer-tain the relation of the Huijiabao trend to specific episodes of deformation and magmatism in the Youjiang basin.
AcknowledgmentsThis research was supported financially by the National Natural Science Foundation of China (Grant 41402074), the Applied Fundamental Research Funding of Sichuan Prov-ince, China (Grant 2015JY0055), and the National Geologi-cal Survey Foundation of China (Grant 1212011094400). The authors thank Zhaoshan Chang and Yi Hu of James Cook University, and Richard Armstrong and Bin Fu of Australia National University for their support during experimental
Sedimentation-diagenesis
Deformation Lateral Hydrothermal
StageSulfides
Pyrites
Py1Py2Py3Py4
Invisible Au
ArsenopyriteRealgar
Quartz veinlets
InitialHydrothermal
Fig. 17. Paragenetic sequence of Au-related minerals that formed dur-ing the Huijiabao mineralization, as interpreted from textures and pyrite geochemistry.
352 HOU ET AL.
work in Australia. Suggestions by David Huston of Geoscience Australia led to significant improvements in this paper. This manuscript benefited greatly from the two reviewers, John Muntean of the NBMG and Albert Hofstra of the USGS, whose constructive suggestions helped to clarify our purpose and ideas. Qi Zhou and Jianzhong Liu of the Guizhou Bureau of Geology and Mineral Exploration are thanked for provid-ing assistance with field sampling.
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