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Volume 31 Issue 2
Apr.  2020
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Xuejuan Sun, Pei Ni, Yulong Yang, Zhe Chi, Shan Jing. Constraints on the Genesis of the Qixiashan Pb-Zn Deposit, Nanjing: Evidence from Sulfide Trace Element Geochemistry. Journal of Earth Science, 2020, 31(2): 287-297. doi: 10.1007/s12583-019-1270-5
Citation: Xuejuan Sun, Pei Ni, Yulong Yang, Zhe Chi, Shan Jing. Constraints on the Genesis of the Qixiashan Pb-Zn Deposit, Nanjing: Evidence from Sulfide Trace Element Geochemistry. Journal of Earth Science, 2020, 31(2): 287-297. doi: 10.1007/s12583-019-1270-5

Constraints on the Genesis of the Qixiashan Pb-Zn Deposit, Nanjing: Evidence from Sulfide Trace Element Geochemistry

doi: 10.1007/s12583-019-1270-5
Funds:

the National Natural Science Foundation of China 1212011220678

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  • The large-scale Qixiashan Pb-Zn Deposit in the eastern Middle-Lower Yangtze metallogenic belt is hosted in carbonate rocks. Based on a detailed mineral paragenesis study, in-situ LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) trace element geochemistry data for pyrite and sphalerite from different stages in the Qixiashan Deposit are reported, the Pb-Zn mineralization processes are reconstructed, and a genetic model is constructed. Four paragenetic stages of Pb-Zn ore deposition are identified:the biogenic pyrite mineralization stage (Stage 1), the early stage of hydrothermal Pb-Zn mineralization (Stage 2), the late stage of hydrothermal Pb-Zn mineralization (Stage 3), and the carbonate stage (Stage 4). Stages 2 and 3 are the main ore stages. The trace element characteristics of the sulfide in stages 2 and 3, such as the higher Co/Ni and lower trace element contents of the pyrite and the Fe, Mn, and Ge contents of the sphalerite, indicate that they were generated by magmatic-hydrothermal processes. Furthermore, the lower Cu, Ag, Sb, and Pb contents of the pyrite and sphalerite of Stage 3 compared to Stage 2 suggest an increase in magmatic-hydrothermal activity from Stage 2 to Stage 3. The hydrothermal fluids leached trace elements (e.g., Cu, Ag, Sb, and Pb) from the previously deposited primary pyrite and sphalerite, which were precipitated in the later hydrothermal stage Cu, Au, Ag, Sb, and Pb bearing minerals and secondary pyrite and sphalerite with lower trace element contents (e.g., Cu, Au, Ag, Sb, and Pb). Compared with the pyrite from stages 2 and 3, the Stage 1 pyrite has relatively higher trace elements contents (Sb, Cu, Zn, Au, Ag, Pb, As, and Ni). However, their lower Co/Ni ratio suggests a syngenetic sedimentary origin. Based on the petrographic features and trace element data, a multi-stage mineralization model is proposed. The Stage 1 biogenic pyrite formed stratiform pyrite layers, which provided reducing conditions and a base for the subsequent Pb-Zn mineralization. During Stage 2, subsequent hydrothermal fluid interacted with the stratiform pyrite layers, which resulted in sulfide precipitation and the formation of stratiform Pb-Zn orebodies. In Stage 3, the hydrothermal fluid replaced the limestone along the fractures, which triggered the formation of Pb-Zn vein orebodies.
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Constraints on the Genesis of the Qixiashan Pb-Zn Deposit, Nanjing: Evidence from Sulfide Trace Element Geochemistry

doi: 10.1007/s12583-019-1270-5
Funds:

the National Natural Science Foundation of China 1212011220678

Abstract: The large-scale Qixiashan Pb-Zn Deposit in the eastern Middle-Lower Yangtze metallogenic belt is hosted in carbonate rocks. Based on a detailed mineral paragenesis study, in-situ LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) trace element geochemistry data for pyrite and sphalerite from different stages in the Qixiashan Deposit are reported, the Pb-Zn mineralization processes are reconstructed, and a genetic model is constructed. Four paragenetic stages of Pb-Zn ore deposition are identified:the biogenic pyrite mineralization stage (Stage 1), the early stage of hydrothermal Pb-Zn mineralization (Stage 2), the late stage of hydrothermal Pb-Zn mineralization (Stage 3), and the carbonate stage (Stage 4). Stages 2 and 3 are the main ore stages. The trace element characteristics of the sulfide in stages 2 and 3, such as the higher Co/Ni and lower trace element contents of the pyrite and the Fe, Mn, and Ge contents of the sphalerite, indicate that they were generated by magmatic-hydrothermal processes. Furthermore, the lower Cu, Ag, Sb, and Pb contents of the pyrite and sphalerite of Stage 3 compared to Stage 2 suggest an increase in magmatic-hydrothermal activity from Stage 2 to Stage 3. The hydrothermal fluids leached trace elements (e.g., Cu, Ag, Sb, and Pb) from the previously deposited primary pyrite and sphalerite, which were precipitated in the later hydrothermal stage Cu, Au, Ag, Sb, and Pb bearing minerals and secondary pyrite and sphalerite with lower trace element contents (e.g., Cu, Au, Ag, Sb, and Pb). Compared with the pyrite from stages 2 and 3, the Stage 1 pyrite has relatively higher trace elements contents (Sb, Cu, Zn, Au, Ag, Pb, As, and Ni). However, their lower Co/Ni ratio suggests a syngenetic sedimentary origin. Based on the petrographic features and trace element data, a multi-stage mineralization model is proposed. The Stage 1 biogenic pyrite formed stratiform pyrite layers, which provided reducing conditions and a base for the subsequent Pb-Zn mineralization. During Stage 2, subsequent hydrothermal fluid interacted with the stratiform pyrite layers, which resulted in sulfide precipitation and the formation of stratiform Pb-Zn orebodies. In Stage 3, the hydrothermal fluid replaced the limestone along the fractures, which triggered the formation of Pb-Zn vein orebodies.

Xuejuan Sun, Pei Ni, Yulong Yang, Zhe Chi, Shan Jing. Constraints on the Genesis of the Qixiashan Pb-Zn Deposit, Nanjing: Evidence from Sulfide Trace Element Geochemistry. Journal of Earth Science, 2020, 31(2): 287-297. doi: 10.1007/s12583-019-1270-5
Citation: Xuejuan Sun, Pei Ni, Yulong Yang, Zhe Chi, Shan Jing. Constraints on the Genesis of the Qixiashan Pb-Zn Deposit, Nanjing: Evidence from Sulfide Trace Element Geochemistry. Journal of Earth Science, 2020, 31(2): 287-297. doi: 10.1007/s12583-019-1270-5
  • Carbonate-hosted Pb-Zn is the main source of Pb-Zn in the world (Leach et al., 2005). Mississippi Valley type (MVT) and sedimentary exhalative (Sedex) deposits are strata-bound, but, in general, they have no direct relationship to igneous activities (Leach et al., 2010, 2005). Carbonate-hosted Pb-Zn deposits are the main type of Pb-Zn deposit in China (Tu et al., 1996). This type of deposit is widely distributed in the South China Block (Gu et al., 2007): the Nanning region hosts the Shiding Pb-Zn Deposit in Guangxi and the Fankou Pb-Zn Deposit in Guangdong; and the Middle-Lower Yangtze region hosts the Magushan Cu-Pb Deposit in Anhui, the Chengmenshan Cu-Mo-Au Deposit in Jiangxi, and the Qixiashan Pb-Zn Deposit in Jiangsu. However, Mesozoic granitiods are wide spread in this area and generally intrude the bulk deposits, which caused this type of deposit to undergo subsequent superimposed hydrothermal processes (Gu et al., 2007). Therefore, multiple mineralization processes are the main characteristics of this type of deposit in southern China.

    The Qixiashan Pb-Zn Deposit is a typical example of a carbonate-hosted deposit in the South China Block. It is located in the Middle-Lower Yangtze region, which is an important Fe-Cu- Pb-Zn ploymetallogenic belt (Zhou et al., 2017, 2016, 2012, 2008; Tang et al., 1998; Zhai et al., 1992; Chang et al., 1991). This deposit is the largest Pb-Zn polymetallic deposit in the Nanjing- Zhenjiang region, with combined Pb-Zn resources of 260 million tons at grades of 4.57% Pb and 7.49% Zn, and exploitable Ag, Ge, Cd, and Ga. Many studies have been carried out since the discovery of this deposit in 1948, including studies on the regional tectonic setting (Zhang, 2015; Ye, 1983), geophysical characteristics (Liu, 1991; Jiang and Liu, 1990), ore deposit geological features (Gui et al., 2015; Xu and Zeng, 2006; Zhong, 1998), ore texture (Xiao et al., 1996; Cai, 1983; Liu et al., 1979), ore-forming fluid characteristics (Gui, 2012; Ye and Zeng, 2000; Xie and Yin, 1997), mineralization zone (Wei and Gong, 2013; Zhong, 1998), and mineralization material source (Zhang, 2015; Guo et al., 1985). Several genetic models have been proposed for the formation of Qixiashan Deposit, including syngenetic sedimentation with hydrothermal overprinting (Liu and Chen, 1985; Liu et al., 1979), a sedimentary exhalative origin (Gui, 2012), and a magmatic- hydrothermal origin (Zhang, 2015). However, there is still significant debate regarding the genesis of the Qixiashan ore deposit. These previous studies were based on conventional techniques using fluid inclusions, stable isotopes (C, H, O, and S), and sulfide trace elements. Unfortunately, they yielded ambiguous results and provided poor constraints on the mineralization processes and genesis of the deposit.

    In this paper, based on detailed adit observations, drill core observations, the petrographic investigation of the classification of the mineralization stages, and in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element data of sulfides, we identify the ore-forming processes, which can be used to guide prospecting and exploration.

  • The Nanjing-Zhenjiang region, which is located in the eastern part of the Lower Yangtze Block, is one of the most important Fe-Cu metallogenic belts in the Middle-Lower Yangtze River region. From the Silurian to the Middle Triassic, this area was a long and narrow basin, and the crustal movement was dominated by stable subsidence covered by a sequence of marine carbonates, continental siltstone, and continental shales. The strata were strongly deformed during the Indosinian Orogeny, forming three anticlines and two synclines with E-W trending axial traces. During the Yanshanian, multi-stage magmatic intrusions were widespread along the generally E-W trending folds (Fig. 1a).

    Figure 1.  Geological sketch maps showing (a) the intrusions in the Ningzhen region (modified from Mao et al., 1990) and (b) the Qixiashan Pb-Zn Deposit (modified after Wei et al., 2016)

    From the Silurian to the Quaternary, the regional stratigraphy was mostly exposed in this region. The Carboniferous and Silurian rocks are the most widely distributed. The Silurian–Triassic strata mainly consist of marine carbonates, minor marine- terrigenous sandstone and shale, and continental clastic rocks as well as a parallel unconformity or a conformable contact between the stratigraphic units. The Jurassic and Cretaceous strata are mainly continental clastic rocks with minor volcanic rocks.

    The intrusive rocks in this area are well-developed in the middle section, minor in the western part, and rare in the eastern part (Fig. 1a). The compositions of the intrusive bodies vary as follows from west to east: an intermediate-mafic intrusion (Bancang pluton), an intermediate-silicic intrusion (Anjishan pluton), and a silicic pluton (Jianbi pluton). These intrusive rocks occur as E-W striking stocks, covering an area of ~700 km2. This region hosts a number of economic ore deposits, including the Qixiashan Pb-Zn Deposit, the Funiushan Cu Deposit, the Tongshan Cu-Mo Deposit, and the Zhujiafeng and Weigang Fe deposits. Their metallogenesis is considered to be roughly related to the Yanshanian magmatism (Wei et al., 2016).

  • The Qixiashan Pb-Zn Deposit is located at the intersection of the northwestern part of the Longtan-Cangtou anticlinorium and the western part of the Tangshan-Dongyang fault (Fig. 1b). From east to west, the Qixiashan District includes six ore sections with a total area of 15 km2: Sanmaogong, Pingshantou, Huzhaoshan, Beixiangshan, Ganjiaxiang, and Xiku sections.

    The exposed strata in the Qixiashan area includes the Middle Silurian shale of the Fentou Formation, the Upper Devonian quartz sandstone of the Wutong Formation, the limestone of the Upper Carboniferous Chuanshan Formation and the Lower Permian Qixia Formation, the Permian sandstone of the Longtan Formation, the Lower Triassic marlite of the Qinglong Formation, and the Lower–Middle Jurassic sandstone of the Xiangshan Formation. The strata in the mining area can be divided vertically into the upper and lower structural layers separated by a high angle unconformable contact (Fig. 2). The upper structural layer is anticline cover composed of Jurassic continental clastic rocks and pyroclastic rocks. The lower structural layer is Qixiashan-Ganjiaxiang anticlinorium. The anticlinorium curves slightly in the axial direction with a missing north-west wing and a reversed south-east wing. Its axial plane dips steeply (85°) to northwest. The strata of the lower layer are composed of Silurian–Triassic marine, continental, and marine-continental carbonates and clastic rocks. The Middle Carboniferous carbonates of the Huanglong Formation are the main host rocks of the Qixiashan Pb-Zn orebodies.

    Figure 2.  Geological cross-sections along line 46 in the Qixiashan Pb-Zn Deposit (modified after Wei et al., 2016)

    The mainly east-northeast striking and northwest striking faults in this area are well-developed. The east-northeast striking faults occur between the Wutong-Gaolishan formations and the Huanglong-Qixia formations with an along-strike length of over 5 000 m.

    Except for a few dioritic porphyrite dikes exposed in the drilling hole in the Ganjiaxiang area, evidence of igneous activity is conspicuously absent. The Yanshanian granitoid (Anjishan pluton) outcrops about 6 km southeast of the ore district, and the diorite (the Bancang pluton) outcrops about 9 km southwest of the ore district. The geophysical exploration data (Wang and Zhou, 1993; Liu, 1991; Yang, 1989) indicate that a concealed body exists beneath the Qixiashan area.

    The main orebody, represented by the #1 ore body in the Huzhaoshan ore section, accounts for 90% of the combined Pb-Zn resources of this section. This orebody is stratiform to lenticular in shape, has an along-strike (45°–58°) length of 1 100 m, and dips steeply (70°–90°) to the northwest. Drilling demonstrates that the orebody extends vertically for 1 079 m (Fig. 2). It is hosted in the Huanglong Formation carbonates and is controlled by an east-northeast striking fault. They have swell, shrink, wedge out, and reappearance features in the direction of the strike and dip. They generally run parallel to the bedding of the host rocks. The Pb-Zn orebody is distributed along the central axis of the main orebody, while the Fe-Mn orebodies are distributed on both sides of the Pb-Zn orebody. The thickness of the orebodies varies from 3.5 to 90.5 m. The ore deposit dips 60°–80° northwest from -500 m deep to -625 m above sea level (a.s.l.), dips approximately 90° northwest at depths of -625 to -750 m, and dips 70°–85° southeast below 750 m.

  • The main ore minerals of the Qixiashan Deposit include pyrite, rhodochrosite, galena, sphalerite, chalcopyrite, tetrahedrite, magnetite, electrum, and multiple Ag-bearing sulfides. The main gangue minerals are calcite, quartz, sericite, and chlorite. The ore textures are mainly massive, disseminated, brecciated, banded, finely laminated, and veinlets. The ore textures mainly consist of granular, mosaic, and metasomatic texture. Based on detailed adit and drill core observations and the microscopic mineral contact relationships, the Qixiashan Deposit has undergone the biogenic pyrite mineralization stage (Stage 1), the early stage of hydrothermal Pb-Zn mineralization (Stage 2), the late stage of hydrothermal Pb-Zn mineralization (Stage 3), and the carbonate stage (Stage 4).

  • During Stage 1, the ores occur as fine laminations within the host limestone (Fig. 3a), and the pyrite (Py1) occurs as irregular aggregates of framboidal (Fig. 4a) and colloform pyrite aggregates (Fig. 4b). The surface of this pyrite has many pores filled with clay and organic matter. Most of them have recrystallized to a certain extent due to the later stage hydrothermal processes, and the coarse-grained pyrite (Py2) occurs around or within the Py1 (Fig. 4b). The folded rhodochrosites were replaced by the late fine-grained pyrite along the fractures within the rhodochrosite (Fig. 3b).

    Figure 3.  Photographs showing the representative samples from the Qixiashan Pb-Zn Deposit. (a) Fine layered pyrites with carbonate (Py1); (b) folded stratiform rhodochrosites replaced by pyrite of stage 2; (c) massive Pb-Zn ore composed of the brown sphalerite (Sph1), pyrite (Py2), and minor galena (Gn1); (d) breccia-type ore with pyrite (Py3) and sphalerite (Sph2) replacing clasts of limestone; (e) yellow-brown euhedral coarse-grained sphalerite (Sph2) and minor galena (Gn2); (f) massive lead-zinc ore replaced by late chalcopyrite vein; (g) disseminated pyrite (Py3) ore replaced by late calcite vein; (h) galena (Gn2) and pyrite (Py3) vein cutting massive Pb-Zn ore. Py. Pyrite; Sph. sphalerite; Gn. galena; Ccp. chalcopyrite; Rds. rhodochrosite; Qz. quartz; Cal. calcite

    Figure 4.  Microphotographs showing ore samples from the Qixiashan Pb-Zn Deposit. (a) Microscopy of pyrite (Py1) framboids in carbonate rock; (b) the primary diagenetic colloform pyrite partially replaced by euhedral-subhedral pyrite (Py2); (c) hydrothermal pyrite (Py2) and sphalerite (Sph1) in massive Pb-Zn ore; (d) pyrite (Py2) replaced by fine-grained sphalerite (Sph1); (e) coarse grained pyrite (Py2) with rupture in ore; (f) bedding parallel sphalerite (Sph1) and pyrite (Py2) with calcite; (g) euhedral pyrite (Py3) with oscillatory zoning and subhedral sphalerite (Sph2) in disseminated ore; (h) the coarse-grained euhedral pyrite intergrowth with chalcopyrite, sphalerite (Sph2), galena (Gn2) and tetrahedrite; (i) coarse grained sphalerite (Sph2) and galena (Gn2) in ore; (j) galena (Gn2) vein cutting the sphalerite (Sph1) and pyrite (Py2) in Stage 2; (k) backscattered electron image of electrum closely intergrown with pyrite (Py3); (l) backscattered electron image of anhedral grains of matildite in galena of Stage 3. Tet. Tetrahedrite; Ele. electrum; Mtl. matildite

  • During Stage 2, most of the ores have massive (Fig. 3c) and banded textures, and the main ore minerals are sphalerite, pyrite, and galena. Most of the pyrite (Py2) in this stage has fine-grained euhedral-subhedral crystals (Figs. 4c and 4d). The 50–200 μm Py2 generally exhibits structural overprinting, which is indicated by its cataclastic texture and marginal corrosion texture (Fig. 4e). Some of this type of pyrite has irregular dissolution holes filled with sphalerite and galena (Fig. 4c). In this stage, the sphalerite (Sph1) contains fine- to medium- grained brown aggregates (Figs. 3c and 3d) and bedding parallel sphalerite (Sph1) aggregates (Fig. 4f), and minor galena (Gn1) are commonly intergrown with the pyrite (Py2) (Fig. 4d) accompanied by silicification and carbonatization.

  • In Stage 3, the ores mainly have veins (Figs. 3f and 3h), disseminated (Fig. 3g), and brecciated textures (Fig. 3d). The ore minerals in this stage mainly consist of pyrite, galena (Fig. 4j), sphalerite, chalcopyrite, tetrahedrite (Fig. 4h), electrum (Fig. 4k), and matildite (Fig. 4l). Galena and chalcopyrite commonly occur as fine or coarse veins crosscutting the massive Pb-Zn ore (Figs. 3f and 3h). In Stage 3, small sulfide grains and calcite cement the wall rock and ore breccias (Fig. 3d). The pyrite (Py3) in this stage occurs as coarse-grained euhedral-subhedral crystals (Figs. 3g and 4g), but some samples contain euhedral pyrite cores surrounded by a fine-grained pyrite rim (Fig. 4g). In this stage, the sphalerite (Sph2) occurs as yellow-brown euhedral coarse-grained crystals (Fig. 3e). In Stage 3, the chalcopyrite (Ccp) is interstitial between the Pb-Zn ores or in veins, which distinguishes it from the early mineralization phases, and it has an evident spatial association with the tetrahedrite (Fig. 4h). Gold-bearing-pyrite is dominant in Stage 3 (Fig. 4k). The backscattered electron images show that the galena contains fine inclusions of matildite coexisting with pyrite and sphalerite (Fig. 4l). The magnetite-tremolite-epidote- chlorite assemblage exposed in the drilling holes is characterized by skarnization, which is also associated with veins or disseminated Pb-Zn mineralization.

  • Quartz and calcite are predominant minerals in Stage 4. Noticeably, there are no ore minerals, e.g., sulfides, in Stage 4. Quartz and calcite typically occur as veins or veinlets that cut the earlier mineral assemblages (Figs. 3a, 3d, and 3g). In thin section, several calcite veinlets replace the Stage 2 Py2 and Sph1 (Figs. 4c, 4d, and 4f). Calcite also occurs as veinlets between the Py3 euhedral crystals (Fig. 4g).

  • We collected over 80 samples from the Huzhaoshan orebody, sampled from depths of -525 to -1 000 m, and prepared double-polished petrographic thin-sections. Detailed petrographic investigation of the sulfides from stages 1, 2, and 3 was carried out. And in situ LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) trace element analysis was performed on representative sulfides (pyrite and sphalerite) from paragenetic stages 1, 2, and 3.

    The in situ trace elements were analyzed using a Photon Machines Excite Excimer laser ablation system coupled to an Agilent 7700x quadrupole inductively coupled plasma mass spectrometer at the Nanjing Focums Technology Co. Ltd., following the procedure of Gao et al. (2015). In order to enhance the sensitivity and lower the instrumental background, two routine pumps were used for the vacuum system. Each analysis was performed using a 40 μm diameter ablation spot at 8 Hz with an energy of ~5 mJ per pulse for 40 s after measuring the gas blank for 15 s. To allow for the quantification of multi-phase mineral assemblages, standardization was achieved via external calibration against standard GSE-1G and an ablation yield correction via normalization to 100% total element abundance (Halicz and Gunther, 2004). To extend the calibration to S, which is not present in standard GSE-1G, surrogate calibration was applied using the S/Fe sensitivity ratios determined by ablation of natural pyrite (NMC-12744). The precision of each analysis is better than 15% for the most element (> 1 ppm).

  • The results of in situ trace element analyses of all of the pyrite samples are listed in Table 1 and shown in Fig. 5a. A striking feature of the pyrite data is the close correlation between the paragenetic stage and the trace element values, e.g., Co/Ni ratio, Cu, Zn, Ag, Au, and Sb. For example, the early fine-grained framboidal or colloform pyrite (Py1) from Stage 1 has extremely low Co/Ni ratios (0.002 5–0.032 7) and high concentrations of Ni (80.99 ppm–152.66 ppm), Cu (72 ppm–212 ppm), Zn (3.28 ppm–32.20 ppm), Ag (130.43 ppm–135.05 ppm), Au (0.09 ppm–0.57 ppm), and Sb (45.23 ppm–442.38 ppm), whereas the coarser euhedral-subhedral pyrite (Py2 and Py3) from stages 2 and 3 have relatively high Co/Ni ratios (0.095 0–1.131 1 and 1.285 1–4.275 8, respectively). Compared with the Stage 2 Py2 (Pb=87.21 ppm–2 660.99 ppm; As=694 ppm–5 571 ppm), the Stage 3 pyrite (Py3) has relatively low Pb (9.46 ppm–93.61 ppm) and As (226 ppm–300 ppm) contents.

    Sample No. Co Ni Cu Zn As Ag Se Sb Te Au Pb Bi Co/Ni Stage of mineralization
    QXS7-01 2.65 80.99 72 3.28 1 696 135.05 0.64 45.23 0.55 0.09 126.57 0.01 0.032 7 Py1
    QXS5-03 0.38 152.66 212 32.2 4 119 137.72 0.72 442.38 0.16 0.49 56 449.74 0.00 0.002 5
    QXS5-04 0.4 117.14 116 21.04 6 931 130.43 0.26 175.33 0.00 0.57 962.08 0.01 0.003 4
    KK4603-02 126.86 112.15 4.3 1 393 694 10.49 0.36 7.87 0.00 0.05 87.21 0.00 1.131 1 Py2
    QXS25-02 0.12 1.26 44 17 5 571 34.13 0.70 59.64 0.40 0.05 1 026.1 0.00 0.095 0
    QXS25-03 0.17 0.87 31 34 4 707 30.48 1.13 57.57 0.09 0.03 1 317.2 0.00 0.193 4
    QXS25-05 0.18 0.23 75 1.2 4 838 44.25 1.49 62.21 0.35 0.04 2 660.99 0.00 0.782 6
    QXS25-11 0.09 0.11 12 0.9 1 291 65.17 0.45 18.28 0.08 0.01 2 718.59 0.00 0.818 2
    KK4004-01 22.27 5.21 4 0.73 283 1.37 0.15 3.3 0.16 0.22 30.58 0.00 4.275 8 Py3
    KK4004-02 4.08 3.17 6 0.69 300 1.28 0.00 3.83 0.00 0.05 93.61 0.03 1.285 1
    KK4004-03 2.39 1.79 1 0.84 226 0.33 0.00 0.75 0.15 0.03 9.46 0.00 1.338 8

    Table 1.  In situ LA-ICP-MS analyses data (ppm) for pyrite from the Qixiashan Pb-Zn Deposit

    Figure 5.  Bulk continental crust normalized trace element diagrams for the different stage pyrites (a) and for the different stage sphalerites (b) (modified after Zhang et al., 2016; normalization values form Rudnick and Gao, 2003)

  • The results of the in situ trace element analyses of all of the sphalerite samples are listed in Table 2 and shown in Fig. 5b. The trace element compositions (Co, Ni, Zn, Se, and Bi) of the sphalerite (Sph1) from Stage 2 are similar to those of the sphalerite (Sph2) from Stage 3. These trace elements values have small variation ranges. Compared to the Sph1 in Stage 2 (e.g., As=3.27 ppm–12.00 ppm, Au=0–0.08 ppm, Ag=42.38 ppm– 238.68 ppm, Sb=6.09 ppm–115.43 ppm, and Pb=17.42 ppm– 103 720.83 ppm), the Stage 3 Sph2 has relatively low As (1.07 ppm–2.45 ppm), Au (0–0.03 ppm), Ag (1.10 ppm–3.46 ppm), Sb (0.03 ppm–0.57 ppm), and Pb (0.26 ppm–0.71 ppm) contents.

    Sample No. Co Ni Cu Zn As Ag Se Sb Te Au Pb Bi Fe Mn Ge Stage of
    (ppm) mineralization
    KK4603-16-SP-02 0.16 0 200 559 150 5.12 83.08 5.17 15.4 0.14 0 87.49 0 71 698 32 849 0.44 Sph1
    KK4603-16-SP-03 0.05 0.11 51 557 922 4.15 42.38 2.81 13.28 0.19 0.03 17.42 0.01 73 179 34 805 0.54
    KK4603-16-SP-04 0.56 0.07 588 548 745 5.70 96.06 2.52 10.53 0.35 0.01 94.4 0.01 76 305 31 695 0.28
    KK4603-13-SP-01 1.82 0.08 125 581 980 5.27 238.68 2.8 11.06 0.48 0.02 1 492.85 0.01 51 557 34 229 0.71
    KK4603-13-SP-02 2.97 0.11 109 587 195 3.27 194.11 2.18 7.11 0.30 0 377.14 0.01 49 845 26 899 0.83
    KK4603-13-SP-03 1.79 0.08 41 565 167 5.35 133.77 2.88 51.79 0.40 0.03 43 977.94 0.16 46 710 18 432 0.51
    KK4603-13-SP-04 2.69 0.24 67 552 410 6.22 102.34 1.56 14.26 0.37 0.06 15 645.06 0.01 56 258 50 816 0.39
    KK4603-13-SP-05 4.06 0.27 67 567 799 3.67 84.49 2.62 6.09 0.31 0.01 4 934.75 0.01 54 630 43 689 0.69
    KK4603-13-SP-06 0.67 0.41 134 579 623 5.13 226.13 2.99 36.4 0.47 0.01 23 628.86 0.03 45 308 32 437 0.88
    QXS5-SP-01 6.18 0.64 456 596 544 12 86.71 0.92 101.18 0.54 0.08 23 322.18 0.01 22 437 59 197 1.18
    QXS5-SP-02 8.82 0.08 1 651 634 310 11 120.27 2.38 115.43 0.46 0.03 9 228.34 0.01 21 447 26 790 1.01
    QXS5-SP-03 5.39 0.01 976 646 886 7.50 95.34 2.3 62.67 0.55 0.06 7 488.98 0.01 13 541 32 325 0.55
    QXS5-SP-05 2.28 0.17 861 647 999 3.30 50.41 2.28 9.51 0.39 0.04 578.68 0.02 19 731 34 239 0.73
    QXS5-SP-07 6.79 0.02 1 011 567 661 6.60 92.1 1.46 74.61 0.14 0.03 103 720.8 0.01 18 114 24 714 1.11
    QXS5-SP-08 5.14 0.24 1 320 581 353 3.60 83.1 2.83 68.19 0.34 0.02 74 518.8 0 18 534 39 330 0.88
    QXS13-SP-02 0.04 0 34 568 248 2.45 1.87 1.2 0.19 0.60 0.03 0.62 0 12 760 18 732 0.39 Sph2
    KK4004-1-SP-01 15.36 0.55 8 626 226 1.11 1.10 5.13 0.57 0.03 0 0.31 0.02 9 124 15 383 0.48
    KK4004-1-SP-03 0.02 0.1 502 657 619 1.07 3.46 4.88 0.27 0.40 0 0.71 0.03 2 392 1 557 0.52
    QXS25-SP-03 0 0.28 34 663 581 1.29 3.12 2.43 0.03 0.53 0.01 0.36 0.01 16 698 4 457 0.67
    QXS25-SP-04 0 0.03 43 659 251 1.23 2.87 3.4 0.18 0.23 0.02 0.26 0.01 14 973 4 720 0.80

    Table 2.  In situ LA-ICP-MS analyses data (ppm) for sphalerite from the Qixiashan Pb-Zn Deposit

  • The early-stage pyrite (Py1) occurs as fine-grained laminated layers in the host limestone (Fig. 3a) and has a framboidal or colloform habit in thin-section (Figs. 4a and 4b). The ore textures of the pyrite are similar to those of the other sediment- hosted Pb-Zn deposits, e.g., the Sullivan (Lydon, 2004), Rammelsberg (Large and Walcher, 1999), McArthur River (Large et al., 2002), and Red Dog deposits (Kelley et al., 2004). Notably, bacteria and algae occur widely throughout the host limestone and the Pb-Zn ores in the Qixiashan Deposit, while framboidal pyrite and rhodochrosite occur in the nodular and laminated ores (Liu and Chen, 1985; Liu et al., 1979). Therefore, we infer that the early-stage pyrite in the Qixiashan Deposit has a biogenic origin.

    The trace element geochemistry of the pyrite can serve as an indicator of its genetic type (Berner et al., 2013; Large et al., 2009). Our results show that the syngenetic pyrite has very high trace element contents and a low Co/Ni ratio compared with the hydrothermal pyrite. The Stage 1 pyrite (Py1) is significantly enriched in Ni, Cu, Ag, Au, Sb, As, and Zn compared to the later pyrite (Py2 and Py3) (Fig. 5a). This is consistent with the results of previous studies (Large et al., 2009). Large et al. (2009) investigated the trace element geochemistry of the genetic pyrite from the Bendigo, Spanish Mountain, and Sukhoi Log (Large et al., 2009). They found that pyrite enriched in various trace elements can be ascribed to the rapid crystallization of fine-grained or framboidal pyrite during deposition, which caused various trace elements to occur as nano-particles in the pyrite. Their results revealed that the As, Ni, and Cu contents of the pyrite range from 500 ppm to 14 000 ppm, 20 ppm to 10 000 ppm, and 30 ppm to 12 600 ppm, respectively. These results are similar to the trace elements concentrations (As=1 696 ppm–6 931 ppm, Ni=80.99 ppm–152.66 ppm, and Cu=72 ppm–212 ppm) of the Stage 1 pyrite, indicating that the Stage 1 pyrite was formed by rapid precipitation of fine-grained pyrite during the syngenetic stage. The Co and Ni contents of pyrite are used as an indicator to distinguish the formation environment (Chen et al., 1987; Bralia et al., 1979; Loftus-Hills and Solomon, 1967). Sedimentary pyrite has low Co contents, high Ni contents, Co/Ni ratios of less than 1; igneous pyrite has high Co contents, low Ni contents, and Co/Ni ratios of greater than 5; and volcanic pyrite has intermediate Co and Ni contents and Co/Ni ratios ranging from 1 to 5. In the Qixiashan Deposit, the Stage 1 pyrite (Py1) has very low Co/Ni ratios (Co/Ni < 1), suggesting a sedimentary origin.

    Additionally, as the concentrations of the three stages are high, there is a positive correlation between the Au and As contents in the pyrite of many gold deposits (Large et al., 2009). Compared to the bulk silicate Earth (BSE) (Fig. 4), the late hydrothermal pyrite (stages 2 and 3) in the Qixiashan Deposit contains inclusions of free gold filling the micro-cracks in the pyrite, which provides the source materials for the subsequent metal mineralization (e.g., Au, Ag).

    Recent studies (Sun et al., 2018) on the sulfur isotopes of the early pyrite indicated that it has a very low and wide range of δ34S values (-13.8‰ to -4.0‰). These mostly negative, highly variable δ34S values can be explained by bacterial sulfate reduction (BSR) (Ohmoto and Goldhaber, 1997). Therefore, we infer that the formation of the first-stage fine-grained disseminated pyrite layer was related to a biological process.

  • In the Qixiashan Deposit, there are indications of later hydrothermal fluid overprinting. Compared to the Stage 1 pyrite (Py1), the pyrite formed in stages 2 and 3 (Py2, Py3) has high Co/Ni ratios, which is consistent with the characteristics of hydrothermal pyrite. The trace element contents of the sphalerite (Sph1 and Sph2) formed in stages 2 and 3 also indicate a hydrothermal origin. On a plot of Fe-Ge versus Fe-Mn (Figs. 6a and 6b), the trace element contents of the Sph1 and Sph2 are similar to that of hydrothermal sphalerite, and they are significantly different from the MVT sphalerite. These data suggest the contribution of hydrothermal fluids in the formation of the sulfides in stages 2 and 3.

    Figure 6.  Binary plots of Fe vs. Ge, Fe vs. Mn in sphalerite from Qixiashan and other Pb-Zn deposits (data of skarn MVT and massive sulfide Pb-Zn deposits from Yuan et al., 2018; and the data of hydrothermal Pb-Zn deposits from Cook et al., 2009)

    It should be noted that there are several differences between the two stages of sulfides (pyrite and sphalerite). Compared with the Py1, the Py2 and Py3 have low Ni, Cu, Zn, Ag, Sb, and Pb contents, which decrease from Stage 2 to Stage 3 (Fig. 5a). Additionally, the trace elements (Cu, Ag, Sb, and Pb) contents of the sphalerite are similar to those of the pyrite, decreasing from Stage 2 to Stage 3 (Fig. 5b). These characteristics suggest that recrystallization of the early-stage pyrite and sphalerite produced the later stage pyrite and sphalerite, which was accompanied by a release of certain trace elements from the crystal textures of the pyrite and sphalerite.

    Several studies (Large et al., 2009, 2007; Butler and Rickard, 2000; Huston et al., 1995) have suggested that the slow crystallization of pyrite during hydrothermal processes can drive trace elements (e.g., Zn, Pb, and Cu) out of the pyrite, which could help to incorporate trace elements into other sulfide phases, decreasing the concentrations of these elements in the pyrite.

    This process suggests that the release of certain trace elements (Au, Ag, Pb, Cu, and Zn) from the Py1 in the Qixiashan Deposit formed the micro-inclusions of free gold, gold-silver bearing minerals, galena, chalcopyrite, and sphalerite in the newly formed Py3. According to the SEM analyses (Fig. 4), several inclusions of silver-bearing minerals occur in the galena and tetrahedrite in the chalcopyrite, while electrum grains occur within the cracks in the pyrite (Py3). Therefore, the differences between the two stages of sulfides may suggest an increase in the magmatic-hydrothermal activity from Stage 2 to Stage 3. This also suggests that from Stage 2 to Stage 3, the hydrothermal fluids leached trace elements (Cu, Au, As, Ag, Sb, and Pb) from the previously deposited primary pyrite and sphalerite and precipitated in the later hydrothermal stage Fe, Cu, Au, As, Ag, Sb, and Pb-bearing minerals and in the secondary pyrite and sphalerite with low trace element contents (Cu, Ag, As, Sb, Au, and Pb).

    Based on our petrographic observations and in situ LA-ICP-MS sulfide geochemistry, even though the color of the sphalerite formed in stages 2 and 3 differs (brown and yellow- brown, respectively), there is no significant difference in the iron contents of the sphalerite formed in stages 2 and 3. Therefore, it is unlikely that the slight iron difference between the stages 2 and 3 sphalerite reflects the evolution of the fluids. Due to the main substitution mechanism of Zn in sphalerite caused by Fe, it is significant that the two stages of sphalerite in the Qixiashan Deposit are rich in other trace elements such as Mn, Ge, Ag, and Sb. This signature suggests that the fluid involved in the formation of the Qixiashan Deposit may be an intermediate-lower fluid.

    These trace element contents resemble those of hydrothermal pyrite from Bendigo, Spanish Mountain, and Sukhoi Log (Large et al., 2009). Therefore, the formation of chalcopyrite and tetrahedrite may be caused by the release of trace elements from the early-stage pyrite rather than from the sphalerite (Fig. 4h). Additionally, the Fe and Mn (Fig. 6b) are positively correlated. Thus, the simple substitution mechanism between Zn2+ and Fe2+ is doubly verified and is the most important substitution for sphalerite itself since iron is the most ubiquitous minor element in sphalerite.

  • As stated above, the texture and trace elements of the pyrite and sphalerite formed in stages 1, 2, and 3 indicate that the Stage 1 pyrite has a sedimentary origin, while in stages 2 and 3, the early pyrite and sphalerite are overprinted by the later hydrothermal fluid, which indicates an increase in magmatic- hydrothermal activity from Stage 2 to Stage 3. Therefore, a sole mineralization model, e.g., a SEDEX origin (Gui, 2012) and magmatic-hydrothermal origin (Zhang, 2015), cannot explain the ore-forming process of the Qixiashan Deposit.

    We propose a composite, multi-stage model of syn- sedimentary deposition followed by hydrothermal overprinting for the formation of the Qixiashan Deposit. In this model, the mineralization process of this deposit contains two different stages: syn-sedimentary and hydrothermal. During the syn- sedimentary stage, BSR resulted in the precipitation of the early fine-grained framboidal and colloform pyrite with low Co/Ni ratios and high trace element contents (e.g., Sb, Cu, Ag, and Pb). This occurred almost simultaneously with the deposition of rhodochrosite and the host limestone. During the early stage of hydrothermal mineralization, magmatic fluid derived from the concealed intrusion beneath the Qixiashan Deposit became relatively enriched in Pb and Zn as it migrated along the faults. The ore fluids reacted with the syn-sedimentary pyrite layers, causing the precipitation of the Pb-Zn minerals (as fine-grained brown sphalerite with high Fe and Mn contents) and the formation of the stratiform Pb-Zn orebody. In the later stage of hydrothermal mineralization, the further leaching of hydrothermal fluids caused veins of Pb-Zn sulfides, silver-bearing minerals, galena, and chalcopyrite overprinting the earlier hydrothermal mineralization stage. Deep exploration drilling has revealed skarn minerals and Cu-Fe mineralization (e.g., magnetite and chalcopyrite) assemblages, which coexist with Pb-Zn mineralization (Gui et al., 2015). This mineral assemblage suggests that the hydrothermal fluid migrated from the southwestern part of the main orebody, and there is a prospecting potential for Pb-Zn ore in the deeper part of the Pb-Zn orebody (Sun et al., 2019).

  • Based on in situ trace element data from pyrite and sphalerite of stages 1, 2, and 3, we have reached the following conclusions concerning the Qixiashan Deposit.

    (1) The early pyrite (Stage 1) with low Co/Ni ratios and high trace element contents (Sb, Cu, Zn, Ag, Pb, As, and Ni) has a sedimentary origin.

    (2) The trace element compositions of the pyrite (e.g., Co/Ni ratio) and sphalerite (e.g., Fe, Mn, and Ge) formed in stages 2 and 3 suggest a hydrothermal origin. The high Co/Ni ratios of the Stage 3 pyrite suggest an increase in magmatic- hydrothermal activity from Stage 2 to Stage 3.

    (3) The textural and trace element data indicate that the Qixiashan Pb-Zn Deposit is a hybrid syngenetic sedimentary and hydrothermally overprinted deposit, rather than a simple SEDEX deposit or a magmatic-hydrothermal deposit.

  • We would like to thank the reviewers for their critical comments and constructive reviews which significantly improved the manuscript. This study was supported by the National Natural Science Foundation of China (No. 1212011220678). We are very grateful to Jiangsu East China Basic Geological Exploration Co., Ltd. for providing basic geological data and assistance in the field. The authors express thanks to Feng-Jian Gao from Nanjing FocuMS Technology Co. Ltd for his help during sulfides in situ LA-ICP-MS trace element analyses. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1270-5.

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