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Volume 31 Issue 2
Apr.  2020
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Lei Shu, Kun Shen, Renchao Yang, Yingxin Song, Yuqin Sun, Wei Shan, Yuxin Xiong. SEM-CL Study of Quartz Containing Fluid Inclusions in Wangjiazhuang Porphyry Copper (-Molybdenum) Deposit, Western Shandong, China. Journal of Earth Science, 2020, 31(2): 330-341. doi: 10.1007/s12583-019-1025-3
Citation: Lei Shu, Kun Shen, Renchao Yang, Yingxin Song, Yuqin Sun, Wei Shan, Yuxin Xiong. SEM-CL Study of Quartz Containing Fluid Inclusions in Wangjiazhuang Porphyry Copper (-Molybdenum) Deposit, Western Shandong, China. Journal of Earth Science, 2020, 31(2): 330-341. doi: 10.1007/s12583-019-1025-3

SEM-CL Study of Quartz Containing Fluid Inclusions in Wangjiazhuang Porphyry Copper (-Molybdenum) Deposit, Western Shandong, China

doi: 10.1007/s12583-019-1025-3
Funds:

the National Natural Science Foundation of China 41372086

the Key R & D Program of China 2016YFC0600606

the National Natural Science Foundation of China 41503038

the National Natural Science Foundation of China 41140025

the Key R & D Program of Shandong Province 2017CXGC1603

the National Natural Science Foundation of China 41672084

the Key R & D Program of Shandong Province 2017CXGC1602

the Key R & D Program of Shandong Province 2017CXGC1601

More Information
  • The Wangjiazhuang porphyry copper (-molybdenum) deposit is located at Zouping volcanic basin in Shandong Province, East China and hosted in the Wangjiazhuang intrusive complex emplaced along a late volcanic conduit. There are two types of ores in this deposit:early disseminated and stockwork ores in the ore-bearing intrusion, and late massive sulfide-quartz veins above brecciated quartz monzonite. The ore minerals are mainly pyrite, chalcopyrite, and subordinately magnetite, tennantite, molybdenite with minor bornite, enargite, galena and sphalerite, etc., and gangue minerals including K-feldspar, biotite, quartz, muscovite-sericite, chlorite and calcite. Combined with fluid inclusion study, the scanning electron microscope-cathodoluminescence (SEM-CL) study of quartz in the deposit and wall rocks shows significant differences between the two types of quartz in the ores. In addition, four types of primary-pseudosecondary fluid inclusions in the quartz have been recognized. They are one-or two-phase aqueous inclusions with vapor/liquid ratios less than 30% to 40% (type I); gas-rich inclusions with vapor/liquid ratios more than 50% (type Ⅱ), some of which contain some small opaque minerals, probably chalcopyrite; multiphase fluid inclusions with daughter minerals of halite±anhydrite±opaque (chalcopyrite)±sylvite±hematite±unknown crystal (type Ⅲ); and mica-bearing fluid inclusions (type IV). Quartz containing abundant muscovite-bearing and halite-bearing fluid inclusions in the mineralized quartz monzonite with potassic-silicic alteration, have better oscillatory growth zoning with CL-colors from bright in the core to darker in the rim, indicating variations of element concentrations in the fluid media from which quartz grew during the later period of magmatic-hydrothermal process. In contrast, the quartz in the sulfide-quartz veins contains mainly fluid inclusions of low-to-medium salinities and does not show oscillatory zoning, indicating that there was less fluctuation in composition and element concentrations of the hydrothermal fluids. However, the quartz containing halite-bearing fluid inclusions and being associated with copper-molybdenum mineralization in the sulfide-quartz veins shows zoning in its rims, indicating variations in composition and element concentrations of the hydrothermal fluids.
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SEM-CL Study of Quartz Containing Fluid Inclusions in Wangjiazhuang Porphyry Copper (-Molybdenum) Deposit, Western Shandong, China

doi: 10.1007/s12583-019-1025-3
Funds:

the National Natural Science Foundation of China 41372086

the Key R & D Program of China 2016YFC0600606

the National Natural Science Foundation of China 41503038

the National Natural Science Foundation of China 41140025

the Key R & D Program of Shandong Province 2017CXGC1603

the National Natural Science Foundation of China 41672084

the Key R & D Program of Shandong Province 2017CXGC1602

the Key R & D Program of Shandong Province 2017CXGC1601

Abstract: The Wangjiazhuang porphyry copper (-molybdenum) deposit is located at Zouping volcanic basin in Shandong Province, East China and hosted in the Wangjiazhuang intrusive complex emplaced along a late volcanic conduit. There are two types of ores in this deposit:early disseminated and stockwork ores in the ore-bearing intrusion, and late massive sulfide-quartz veins above brecciated quartz monzonite. The ore minerals are mainly pyrite, chalcopyrite, and subordinately magnetite, tennantite, molybdenite with minor bornite, enargite, galena and sphalerite, etc., and gangue minerals including K-feldspar, biotite, quartz, muscovite-sericite, chlorite and calcite. Combined with fluid inclusion study, the scanning electron microscope-cathodoluminescence (SEM-CL) study of quartz in the deposit and wall rocks shows significant differences between the two types of quartz in the ores. In addition, four types of primary-pseudosecondary fluid inclusions in the quartz have been recognized. They are one-or two-phase aqueous inclusions with vapor/liquid ratios less than 30% to 40% (type I); gas-rich inclusions with vapor/liquid ratios more than 50% (type Ⅱ), some of which contain some small opaque minerals, probably chalcopyrite; multiphase fluid inclusions with daughter minerals of halite±anhydrite±opaque (chalcopyrite)±sylvite±hematite±unknown crystal (type Ⅲ); and mica-bearing fluid inclusions (type IV). Quartz containing abundant muscovite-bearing and halite-bearing fluid inclusions in the mineralized quartz monzonite with potassic-silicic alteration, have better oscillatory growth zoning with CL-colors from bright in the core to darker in the rim, indicating variations of element concentrations in the fluid media from which quartz grew during the later period of magmatic-hydrothermal process. In contrast, the quartz in the sulfide-quartz veins contains mainly fluid inclusions of low-to-medium salinities and does not show oscillatory zoning, indicating that there was less fluctuation in composition and element concentrations of the hydrothermal fluids. However, the quartz containing halite-bearing fluid inclusions and being associated with copper-molybdenum mineralization in the sulfide-quartz veins shows zoning in its rims, indicating variations in composition and element concentrations of the hydrothermal fluids.

Lei Shu, Kun Shen, Renchao Yang, Yingxin Song, Yuqin Sun, Wei Shan, Yuxin Xiong. SEM-CL Study of Quartz Containing Fluid Inclusions in Wangjiazhuang Porphyry Copper (-Molybdenum) Deposit, Western Shandong, China. Journal of Earth Science, 2020, 31(2): 330-341. doi: 10.1007/s12583-019-1025-3
Citation: Lei Shu, Kun Shen, Renchao Yang, Yingxin Song, Yuqin Sun, Wei Shan, Yuxin Xiong. SEM-CL Study of Quartz Containing Fluid Inclusions in Wangjiazhuang Porphyry Copper (-Molybdenum) Deposit, Western Shandong, China. Journal of Earth Science, 2020, 31(2): 330-341. doi: 10.1007/s12583-019-1025-3
  • Scanning electron microscope-cathodoluminescence (SEM- CL) analysis is an effective technical method for studying minerals, especially quartz in rocks and ore deposits. Through detailed study of the microtextures and luminescence intensity of the quartz in the CL images, researchers can reveal the variations of physical and chemical conditions of melt and/or fluid from which quartz crystals grew, and of subsequent deformation and metamorphism to which the quartz crystals were subjected (Chen et al., 2015, 2006; van den Kerkhof and Hein, 2001).

    Many porphyry copper deposits are characterized by types (generations) of quartz containing fluid inclusions that have been trapped from hydrothermal fluids in a wide range of composition, pressure, and temperature. The formation conditions of the ore deposits can be estimated from different methods, one of which is by studying fluid inclusions mainly in quartz of the deposits. Fluid inclusion studies typically show that quartz in different mineralization stages contains different types of fluid inclusions, suggesting that the quartz and the fluid inclusions generally were not formed from one fluid-flow event, instead, were formed during multiple superimposed geologic events that involve fracturing, dissolution, and precipitation of quartz ( Lecumberri-Sanchez et al., 2013; Landtwing et al., 2010; Pan et al., 2009; Audétat et al., 2008; Klemm et al., 2008, 2007; Rusk et al., 2008; Halter et al., 2005; Meng et al., 2005; Cline and Bodnar, 1994; Yang and Bodnar, 1994; Eastoe, 1978; Nash, 1976; Roedder, 1971). However, transmitted light petrographic examination of quartz provides little textural information useful for distinguishing different types (generations) of quartz.

    On the other hand, SEM-CL studies reveal a wide range of textures (e.g., luminescence intensity) of quartz that are useful for deciphering variations in composition and concentration of the hydrothermal fluids from which quartz grew. The variations in luminescence intensity are due to structural defects in the quartz lattice, which, in part, result from trace-element substitution into quartz (Li et al., 2011; Wiebe et al., 2007; Rusk et al., 2006; Bernet and Bassett, 2005; Bignall et al., 2004; Redmond et al., 2004; Ruffini et al., 2002; Rusk and Reed, 2002; Götze et al., 2001; Penniston-Dorland, 2001; Pagel et al., 2000).

    We use SEM-CL texture and intensity of quartz which contains fluid inclusions in alteration/mineralization wallrock and sulfide veins from the Wangjiazhuang porphyry copper (-molybdenum) deposit in Zouping City, East China, to distinguish this two types of quartz precipitated at different stages and to relate the quartz and the fluid inclusions contained to specific fluid-flow events.

  • The Wangjiazhuang porphyry copper (-molybdenum) deposit is located at the Zouping volcanic basin, which is situated in the southeastern part of North China Craton and the northern margin of the Luxi uplift, adjacent to Jiyang depression in the north (Fig. 1). Regional faults here include the nearly EW- trending Qihe-Guangrao fault in the north, the NS-trending Jinshan-Yaojiayu fault in the east and the NNW-trending Wenzu fault in the west, which play an important role in controlling the formation and development of the volcanic basin (Lan et al., 2018; Shen et al., 2018; Yang et al., 2016; Wang et al., 2015; Liu et al., 2013; Han et al., 2008; Zhang et al., 2008; Li and Yuan, 1991; Yuan and Li, 1988, 1987). The Early Cretaceous volcanic sedimentary strata covers an area of ~400 km2 with the thickness about 5 km in the volcanic basin (Li et al., 2008; Yuan and Li, 1987).

    Figure 1.  Simplified regional geological map of Zouping area, Shandong Province (after Liu et al., 2013)

    The volcanic rocks in the basin can be divided into three major eruption cycles from south to north, forming three central volcanic structural systems (Yuan and Li, 1987). The lithology of the first cycle includes basalt-andesite, andesite brecciated lava, tuff and agglomerate, which are distributed outside the ringdyke. During the middle period of volcanic activities, due to the ejection of massive materials, the crater collapsed, resulting in caldera, as well as annular/radial faults. The lithology of the second cycle includes ignimbrite breccia, tuff, trachyandesite and basalt- trachyandesite, which are distributed in the inner ringdyke. The volcanic rocks of the third cycle include trachytic brecciated lava, volcanic agglomerate, breccia and tuff, which are distributed in the north of volcanic basin.

    The intrusive rocks in the basin are spatially controlled by volcanic structures, forming a basic-intermediate-alkaline intrusive complex, which can be divided into three phases from early to late: (1) the early phase includes norite-gabbros, diabase porphyrite and diorite porphyrite which are distributed in the southern margin of the basin; (2) the middle phase includes diorite-quartz monzonite, monzonite (ringdyke), syenodiorite which are distributed in Wangjiazhuang, Beilou, Xidong and Zhanggao areas; and (3) the late phase includes monzonite porphyry, syenite porphyry and calc-alkaline subvolcanic rocks (Han et al., 2008; Yuan and Li, 1988).

    The Wangjiazhuang intrusion, a concealed complex, has been emplaced along a volcanic passage into the overlying Cretaceous volcanic sedimentary strata and is composed mainly of diorite, monzonite and quartz monzonite. Among them, the quartz monzonite is the host rock of the Wangjiazhuang Cu (-Mo) deposit that has a medium-grained texture and is mainly composed of plagioclase (45%–55%), K-feldspar (25%–30%), quartz (15%–18%) and biotite (5%–10%) with accessory apatite, zircon, magnetite, monazite and rutile (Li and Yuan, 1991; Yuan and Li, 1987).

    Hydrothermal alteration is widely developed, and three wallrock alteration zones, potassic (K) alteration zone, potassic- silicic (K-Si) alteration zone and intensive potassic-silicic (K-Si) alteration zone occur from the margin of the intrusion towards the inner core (Lan et al., 2018; Tang, 1990; Yuan and Li, 1988). In addition, phyllic and chlorite alterations surround these alteration zones discontinuously (Figs. 2, 3).

    Figure 2.  Simplified geological map of the Wangjiazhuang mineralization field (modified after Yuan and Li, 1988)

    Figure 3.  Sketch diagram showing the cross-section of No. 15 exploration line (modified after Tang, 1990)

    The Wangjiazhuang Cu (-Mo) orebodies exclusively occur in the altered quartz monzonite (Figs. 2, 3). Forty-four large and small orebodies have been found at the depths from -100 to 700 m a.s.l. They can be divided into deep orebodies and shallow ones. (1) The deep orebodies are branch-vein-like ones with steep dipping angle (55°–65°) and are small in scale, large in quantity and low in copper grade, having veinlet-disseminated ores (Fig. 4a); they are mainly distributed in the potassic-silicic alteration zone and intensive potassic-silicic alteration zone. (2) The shallow orebodies are nearly horizontal pegmatitic massive sulfide-quartz veins and lenticules (Fig. 4b) characterized with large scale and high grade; they are distributed above the brecciated quartz monzonite which contains deep orebodies. There is a trend from the lower molybdenum-rich orebodies through quartz core to the upper gold-rich copper orebodies (Tang, 1990).

    Figure 4.  Photographs showing (a) the altered quartz monzonite porphyry with quartz veinlet ore and (b) sulfide quartz ores

    The ore minerals are mainly pyrite, chalcopyrite, and subordinately magnetite, tennantite, molybdenite with minor bornite, enargite, galena and sphalerite, etc., and gangue minerals include K-feldspar, biotite, quartz, muscovite-sericite, chlorite and calcite. The ore textures include vein-dissemination, stockwork, breccia and massive textures, etc., and the mineral paragenesis is given in Table 1.

    Table 1.  Mineral paragenetic sequence

  • Fluid inclusion study has been conducted to reconstruct the evolution of magmatic-hydrothermal fluids that formed the Wangjiazhuang porphyry Cu (-Mo) deposit.

  • Doubly polished thin sections were prepared for petrographic investigation of fluid inclusions, including the phases present in the inclusions, the shape, the size, the mode of occurrence and the distribution, following the techniques outlined by Roedder (1984) and Lu et al. (2004).

    Two types of quartz containing fluid inclusions are distinguished and studied: (1) type Ⅰ quartz in the mineralized quartz monzonite with potassic-silicic alteration (Figs. 5a–5d), and (2) type Ⅱ quartz in the sulfide-quartz veins (Figs. 5e, 5f).

    Figure 5.  Photomicrographs showing quartz and fluid inclusions. (a) Isolated Bt-bearing and aqueous fluid inclusions in type Ⅰ quartz (sample W33C); (b) Ms-bearing and, to a lesser extent, halite-bearing inclusions occur in growth zones of type Ⅰ quartz crystals (W33G); (c) many type IV and some type Ⅲ inclusions occur in the core and intracrystalline fractures of quartz crystals (W33-2A); (d) type IV inclusions occur in the core and 4 rhythmic growth zones of quartz crystals whereas in the clear quartz between the zones some type Ⅲ and type Ⅰ inclusions occur randomly (W33-3A); (e) type Ⅰ and type Ⅲ fluid inclusions occur in the intracrystalline fractures of type Ⅱ quartz crystals in the Cu (-Mo)-quartz vein where types III and IV inclusions are absent (W22B); (f) isolated types I, II and III fluid inclusions occur in type Ⅱ quartz associated with sulfosalt mineralization where type IV inclusion is absent (W7H). Bt. Biotite; FI. fluid inclusions; Ms. muscovite; Qz. quartz

    Four types of primary-pseudosecondary fluid inclusions were recognized in the two kinds of quartz based on the observed phases at room temperature. They are one- or two-phase aqueous inclusions with vapor/liquid ratios less than 30% to 40% (type I); gas-rich inclusions with vapor/liquid ratios more than 50% (type Ⅱ), some of which contain a small opaque mineral, probably chalcopyrite; multiphase fluid inclusions with daughter minerals halite±anhydrite±opaque (chalcopyrite)± sylvite±hematite±unknown crystal (type Ⅲ); and mica-bearing fluid inclusions (type IV) (Fig. 5). It is noted that hematite occurs mainly in matrix quartz of quartz monzonite and in the early quartz veinlet cutting it. The types and abundance of fluid inclusions in type Ⅰ and type Ⅱ quartz of different samples are listed in Table 2.

    Sample Type of quartz containing fluid inclusions Lithology Type Ⅰ Type Ⅱ Type Ⅲ Type Ⅳ
    W33 Quartz monzonite with K-Si alteration +++ ++ +++ +++
    W22 Cu (-Mo) Qz vein ++++ +++ ++
    W7 Sulfosalt-Qz vein +++ ++
    -. Absent; ++. common; +++. many; ++++. abundant; Cu (-Mo); copper (-molybdenum); K-Si. potassic-silicic; Qz. quartz.

    Table 2.  Types and abundance of fluid inclusions in quartz

  • The fluid inclusion microthermometric experiments were conducted on the Linkam THMS600 heating/cooling stage mounted on the microscope at Shandong Institute of Geological Sciences (SIGS), Jinan. Liquid nitrogen was used as a coolant and the stage was routinely calibrated at 0 and 374.1 ℃ based on the ice-melting temperature and homogenization temperature of a pure water synthetic fluid inclusion having the critical density, and at -56.6 ℃ based on the CO2 melting temperature of a pure CO2 fluid inclusion, respectively. The uncertainty is estimated as ±0.2 ℃ below room temperature and 5 ℃ in the temperature range higher than 200 ℃, respectively (Bodnar, 1994). In addition, the melting temperatures of muscovite in type IV fluid inclusions were measured in the hydrothermal diamond anvil cell (HDAC) of the fifth generation Bassett diamond anvil cell (1.0 mm in diameter and 0.25 mm in thickness) at the Laboratory of High Temperature and High Pressure, Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS), Beijing (Li et al., 2017; Li and Li, 2014).

    The ice melting temperatures (Tm ice) and homogenization temperatures to liquid water (Th V-L) for type Ⅰ inclusions in the early quartz veinlet are in the range of -13– -1.5 and 290–417 ℃, respectively, indicating the moderate-to-low salinity and higher temperature for the early hydrothermal fluid. The Tm ice and Th V-L for type Ⅰ inclusions in the Cu (-Mo) quartz vein are in a wide range from -25 to -2 ℃ and from 380 to 120 ℃. The Tm ice and Th V-L for type Ⅱ inclusions in the late quartz related to sulfosalts deposition are in the range of -4.1– -0.8 ℃ and 110–215 ℃, respectively.

    The Tm ice and Th L-V (to vapor phase) for type Ⅱ inclusions in the early quartz veinlet and in the Cu (-Mo) quartz vein are in the range of -4.3–0.3 and 270–417 ℃, and around 0 and 293–386 ℃, respectively.

    The Th V-L and halite melting temperatures (Tm halite) for type Ⅲ inclusions in the matrix quartz and the early quartz veinlet are in a wide range of 120–215 and 200– > 400 ℃, and of 150–260 and 346–500 ℃, respectively. Many type Ⅲ inclusions which have been finally homogenized by halite disappearance, may have been re-equilibrated. However, some type Ⅲ inclusions have been finally homogenized with similar temperatures of vapor and halite disappearance. Occasionally, sylvite daughter crystal in type Ⅲ inclusions was observed to melt around 100 ℃ before halite disappearance at 340 ℃. The Th V-L and Tm halite for type Ⅲ inclusions in the Cu (-Mo) quartz vein are in the range of 240–360 and 264–380 ℃. Some of them have been homogenized by halite disappearance whereas others by vapor disappearance or both. Because fluid inclusion petrography has shown that in some quartz crystals adjacent to chalcopyrite and molybdenite type Ⅱ and type Ⅲ inclusions coexist and they have varying but similar homogenization temperature range, the fluid boiling probably occurred during Cu-Mo mineralization.

    Type IV inclusions are only found in quartz monzonite with K-Si alteration and the Th V-L of aqueous phase in the type IV inclusions are mainly concentrated in the range of 300–360 ℃ whereas the melting temperatures of muscovite are in the range of 773–790 ℃. The microthermometry data of fluid inclusions are listed in Table 3.

    Sample FI type Tm ice/Tm hh* (℃) Tm halite* (℃) Th V-L* (℃) Salinity (wt.% NaCl eq.) Remarks
    W33 Type I -16.5– -2.0 245–400 19.84–3.39
    -5.2– -2.0 153–173 8.14–3.39
    Type Ⅲ 170–445 180–360 30.48–52.65
    Type Ⅳ -25.1– -2.0 300–360 > 26–3.39 *Tm Ms > 700 ℃
    W22 Type Ⅰ -5.6– -3.8 210–260 6.16–8.68
    -3.0– -2.2 106–126 3.71–4.96
    Type Ⅱ -4.8– -2.0 298–390 7.59–3.39
    Type Ⅲ 300–380 298– > 400 38.16–45.33 Tm haliteTh V-L
    W7 Type Ⅰ -4.1– -0.8 115–212 1.40–6.59
    (-4.0– -3.0)
    Type Ⅱ Not detected
    *Tm ice/Tm hh. Last melting temperature of ice/hydrohalite; Tm halite. final melting temperature of halite; Tm Ms. muscovite melting temperature; Th V-L. homogenization temperature of vapor to liquid phase.

    Table 3.  Microthermometry of fluid inclusions in the Wangjiazhuang Deposit

  • Doubly polished thin sections containing quartz and fluid inclusions were examined using scanning electron microscope- cathodoluminescence (SEM-CL), as well as secondary electrons (SE) and transmitted light microscopy.

  • SEM-CL analysis of quartz was conducted in SIGS and School of Physics, Peking University, respectively. In SIGS, ZEISS SUPRA55 field emission scanning electron microscope (SEM) and MonoCL cathode fluorescence spectrometer were used; and in the Peking University, FEI Company's Quanta series low vacuum environment scanning electron microscope (ESEM) and MonoCL cathode fluorescence spectrometer were used. Most of the analyzed sections were carbon-coated and carried out in SIGS in order to get a clearer picture whereas some precious samples without coating were analyzed by ESEM to minimize its damage. Instrument parameters were constant as follows: acceleration voltage=15 kV, temperature= 25 ℃, and each CL image was acquired for 40 s. Magnification varied slightly between 100 and 150×.

    Because the apparent luminescence observed in SEM-CL images depends on numerous operating conditions, absolute CL intensity cannot be easily quantified. Luminescence intensity (CL-color) is simply referred to as CL-dark, CL-grey, and CL-bright (Rusk and Reed, 2002).

  • Two types of quartz containing fluid inclusions are distinguished by fluid inclusion petrography and their characteristics are as follows: (1) type Ⅰ quartz crystals, especially some large quartz grains show core-rim textures with growth zoning and usually contain fluid inclusions of types Ⅰ, Ⅲ and Ⅳ (Figs. 5a–5d ,6a, 6c, 6e, 6f, 7a, 7c); (2) type Ⅱ quartz has massive texture and is composed of euhedral to anhedral grains (aggregates) in varying sizes which often show clear fracturing but seldom show growth zoning. The type Ⅱ quartz contains fluid inclusions of types I, II and III except type Ⅳ (Figs. 5e, 5f, 8a, 8c).

  • Transmitted-light photomicrographs and scanning electron microscope-cathodoluminescence (SEM-CL) images of the same area of quartz containing fluid inclusions are shown in Figs. 6, 7, and 8.

    (1) Type I quartz of single quartz grain

    Nearly all this kind of quartz shows core-rim texture, that is an embayed CL-bright to CL-light grey core with oscillatory zoning and small patches is surrounded by CL-dark grey to CL-dark quartz rim (overgrowth), indicating that the quartz grains have experienced a complex forming process of dissolution, fracturing, and re-precipitation (Fig. 6). For example, in transmitted-light photomicrographs (Figs. 6a, 6c) the fluid inclusions can be seen in the core and along growth zones while in the corresponding CL images (Figs. 6b, 6d) the embayed CL-bright or CL-grey quartz core has been broken, eroded and surrounded by CL-dark quartz overgrowth; the fluid-inclusion voids are visible as the fluid in the inclusions has leaked. In Figs. 6f and 6h the irregular quartz core with CL-bright to CL-grey color and oscillatory zoning has been re-filled and surrounded by CL-dark quartz overgrowth, too. The core-rim texture is caused by precipitation of CL-bright and lighter quartz, followed by dissolution, fracturing and then precipitation of CL-grey to CL-dark quartz overgrowth. It is noted that oscillatory zoning is visible mainly in the SEM-CL images.

    (2) Type I quartz composed of quartz aggregate and large zoned quartz grains

    The CL images of some type Ⅰ quartz reveal that the quartz aggregate shows mosaic texture and that some large quartz grains contain a CL-bright core having fragments with oscillatory zoning in different orientations, which is surrounded by CL-dark quartz overgrowth. This indicates that the quartz grains have experienced more complex forming processes of precipitation, dissolution, fracturing, and re-precipitation. However, these features can not be seen in the transmitted-light photomicrographs (Fig. 7).

    (3) Type II quartz in the sulfide-quartz veins

    The sulfide-quartz veins contain more than one generation of quartz in the quartz veins with Cu (-Mo) mineralization (W22). The early quartz contains microfractures with abundant type Ⅰ and some type Ⅲ fluid inclusions and exhibits uniform CL-dark color whereas the late euhedral quartz formed during Cu (-Mo) mineralization contains pseudosecondary type Ⅲ fluid inclusions and shows uneven CL-grey to CL-dark grey color (Figs. 8a, 8b). In the sulfide-quartz vein containing Ccp-Bo-Ten assemblage (W7), the early milky quartz contains small patches with bright CL-color which are surrounded by quartz with grey CL-color whereas the late transparent quartz has uniform dark CL-color. Both the early and late type Ⅱ quartz in the Wangjiazhuang Deposit seldom show oscillatory zoning. This is in contrast to the moderate-low quartz in many deposits which show clear oscillatory zoning.

    Figure 6.  Comparison of scanning electron microscope-cathodoluminescence (SEM-CL) images with transmitted-light photomicrographs for type Ⅰ quartz of single quartz grain. (a) Photomicrograph showing the core-rim texture of a quartz grain in quartz monzonite with K-Si alteration; the quartz core contains abundant Ms-bearing and halite-bearing FIs interrupted by subsequent growth of the quartz core (W33-2A). (b) SEM-CL image of the same quartz grain as in Fig. 6a showing that the CL-light grey quartz core surrounded by CL-dark quartz overgrowth; the voids of fluid inclusions (leaked) and oscillatory growth zoning are visible in the core of the quartz grain. (c) Photomicrograph showing oscillatory growth zones of a quartz crystal along which Ms-bearing fluid inclusions occur (W33-2D). (d) SEM-CL image of the same quartz grain as in Fig. 6c exhibits more clear oscillatory zoning in an embayed CL-bright to CL-grey core surrounded by CL-dark quartz overgrowth. (e) Photomicrograph of an irregular quartz grain (W33-1F). (f) SEM-CL image of the same quartz grain as in Fig. 6e showing an embayed CL-bright quartz core with oscillatory zoning surrounded by CL-gray to CL-dark quartz overgrowth. (g) Photomicrograph showing a quartz grain in quartz monzonite with K-Si alteration (W33-2G). (h) SEM-CL image of the same quartz grain as in Fig. 6g, the CL-color changes gradually from light grey in the core to dark grey towards the edge of the quartz grain. The oscillatory zoning is only visible in the SEM-CL image

    Figure 7.  Comparison of SEM-CL images with photomicrographs for type Ⅰ quartz aggregate and large zoned quartz grains. (a) Photomicrograph of quartz monzonite with K-Si alteration which contains mainly quartz aggregate and K-feldspar (W33-1E). (b) SEM-CL image shows mosaic texture of the quartz aggregate, indicating dissolution of CL-bright quartz cores/patches and recrystallization to CL-grey and CL-dark quartz overgrowth. The image also reveals that the irregular quartz aggregate is composed of small quartz grains with oscillatory zoning in different orientations. (c) Photomicrograph showing a large zoned quartz grain in quartz monzonite with intensive K-Si alteration (W33-G). (d) SEM-CL image shows the core of the large quartz grain contains CL-bright and CL-grey patches with oscillatory zoning in different orientations and is filled and surrounded by CL-dark quartz overgrowth. Ap. Apatite; 1. core of the quartz grain; 2 and 3. growth zones of the quartz grain

    Figure 8.  Comparison of SEM-CL images with photomicrographs for type Ⅱ quartz. (a) Photomicrograph showing two generations of quartz: the early quartz contains microfractures along which abundant type Ⅰ and some type Ⅲ halite-bearing fluid inclusions occur whereas the late euhedral quartz adjacent to molybdenite contains trials of pseudosecondary type Ⅲ fluid inclusions along intracrystalline microfractures (W22-H). (b) SEM-CL image of the same sample shows that the early quartz has uniform dark CL-color whereas the late euhedral quartz shows uneven CL-color of grey to dark grey and growth zoning is visible on the edge of the crystal. (c) Photomicrograph shows that a sulfide veinlet penetrate and separate early milky quartz and late transparent quartz; the latter is related to sulfosalt mineralization and contains many types Ⅰ, Ⅱ and Ⅲ fluid inclusions (W7-H). (d) SEM-CL image of the same sample shows that the milky quartz contains small patches with bright CL-color which are surrounded by quartz with grey CL-color whereas the transparent quartz has uniform dark CL-color. Cpy. Chalcopyrite; Ten. tennantite; Bo. bornite; Mo. molybdenite

    It is noted that in these two sulfide veins the late clear quartz crystals are related to the deposition of sulfides and contain fluid inclusions of types Ⅰ, Ⅱ and Ⅲ.

  • The combination of fluid inclusion study and SEM-CL study of its host quartz can more effectively gain insight into the nature and evolution of the magmatic-hydrothermal fluids responsible for the wall rock alteration and ore-forming process in the Wangjiazhuang Cu (-Mo) deposit.

    Textures of hydrothermal quartz revealed by cathodoluminescence using a scanning electron microscope (SEM-CL) reflect the physical and chemical environment of quartz formation. Variations in intensity of SEM-CL can be used to distinguish among quartz from superimposed mineralization even in single vein (Rusk et al., 2006) and to reveal growth history of the quartz. Previous studies (Wark and Spear, 2005; Watt et al., 1997; Demars et al., 1996) discussing the relationship of SEM-CL intensity with trace-element contents in quartz and the higher intensity of SEM-CL generally indicate higher trace-element concentrations in quartz. However, Rusk et al. (2006) have found that CL intensity is also related to temperature of quartz formation. For example, plutonic quartz from the Butte quartz monzonite that crystallized at temperatures near 750 ℃ has the highest CL intensity whereas quartz that precipitated at ~250 ℃ in main stage veins has least CL intensity. Primary CL textures in quartz also represent stages of growth zoning and are useful to identify primary fluid inclusions in quartz.

    (1) The SEM-CL images of quartz reveal the following textural characters. The luminescence intensity of type Ⅰ quartz ranges from CL-bright, to CL-grey, to CL-dark and nearly all type Ⅰ quartz shows core-rim texture, that is a CL-bright to CL-light grey core with oscillatory zoning and small patches surrounded by CL-dark grey to CL-dark quartz overgrowth.

    Some quartz grains have embayed cores with oscillatory zoning, which are surrounded by rims with CL-dark quartz. This suggests that the growth of quartz cores was once interrupted and experienced dissolution and re-precipitation. In particular, Figs. 7c and 7d reveal that one large quartz grain showing core-rim texture contains CL-bright and CL-grey patches with oscillatory zoning in different orientations in the core which are filled and cemented by CL-dark quartz overgrowth. This indicates a complex growth history of type Ⅰ quartz, that is initial growth of the quartz grain was followed by dissolution, fracturing and fragmentation due to the changes in physical conditions; then the fragments of the quartz grain were cemented by subsequent overgrowth of quartz. Such a complex formation process of quartz can not be seen in common optical microscope.

    Type I quartz usually shows more clear oscillatory zoning in SEM-CL images than in transmitted light pictures, in combination with the presence of type IV fluid inclusions along growth zones of the quartz, suggesting that variations in temperature and/or trace-element concentrations of the fluid from which the quartz grew occurred periodically during quartz growth, reflecting the replenishment of fluid into the rock system.

    In contrast, type Ⅱ quartz seldom shows oscillatory zoning but has more uniform SEM-CL colors mainly from CL-grey to CL-dark, which may indicate that the type Ⅱ quartz formed in a more stable physico-chemical condition or has experienced recrystallization after its formation (Bernet and Basset, 2005).

    However, in the sulfide-quartz vein sample of W22H, the luminescence intensity of the late type Ⅱ euhedral quartz associated with Cu (-Mo) mineralization (Fig. 8a) has uneven SEM-CL color of grey to dark grey and shows growth zoning in the rim, reflecting variations in physico-chemical conditions of the hydrothermal fluid from which the quartz grew. The CL intensity of type Ⅱ quartz in sample W7H is CL-dark for late quartz crystals without obvious zoning.

    (2) The SEM-CL images of type Ⅰ quartz containing muscovite-bearing fluid inclusions reveal clearly that these inclusions belong to primary fluid inclusions rather than melt inclusions (Shen et al., 2018; Bodnar and Student, 2006; Roedder, 1984; Sobolev and Kostyuk, 1975). They were heterogeneously trapped from an aqueous fluid rich in K-Al-Si components, volatiles, salts and metals that were separated from a crystalline intermediate-acidic magma. With the opening of fractures and quick drop of pressure and temperature of the rock system, muscovite (and biotite) would crystallize from aqueous fluid rich in K-Al-Si components and deposit in the core or growth surface of crystallizing quartz grains and then were trapped with surrounding fluid media as mica-bearing fluid inclusions.

    (3) The composition of hydrothermal fluids responsible for K-Si alteration of the quartz monzonite can be deduced from types I, III and IV inclusions in this rock, that is initially K-Al-Si-rich and high-salinity aqueous solution containing NaCl-Fe2O3-CaCO3-KCl-CaSO4 and metals (such as Cu, Mo) with minor CO2. These indicate that the early hydrothermal fluid was very saline with a salinity of ~50 wt.% NaCl eq. and contains multiple components, including ore-forming elements. The presence of hematite and sulfate indicates the fluid was more oxidized in the early stage.

    The composition of hydrothermal fluids responsible for sulfide-quartz veins of the main mineralization stage can be obtained from types I, II and III inclusions in the sulfide-quartz veins. However, the abundance of type Ⅲ inclusions and the solids (or daughter minerals) in them decreased considerably, suggesting that the magmatic-hydrothermal fluids were diluted by mixing with meteoric-ground water, and by deposition of ore-forming elements from the hydrothermal fluids in this mineralization stage. In type Ⅱ quartz the coexistence of type Ⅱ and type Ⅲ inclusions and their similar homogenization temperature range suggest that fluid boiling once occurred during Cu (-Mo) mineralization.

    In the sulfosalt mineralization stage after the deposition of sulfosalt minerals the hydrothermal fluid was diluted greatly as evidenced by the predominance of type Ⅰ inclusions with low salinity and low homogenization temperature.

  • (1) Type I quartz grains usually show core-rim texture and better growth zoning than type Ⅱ quartz grains. The changes in CL intensity from CL-bright in the core to CL-grey in the rim of the quartz grains reveal a complex history of quartz formation, including initial precipitation, followed by dissolution, fracturing and re-precipitation in response to changes of temperature and pressure.

    The existence of oscillatory zoning in type Ⅰ quartz may also indicate periodic variations in the composition (i.e., trace elements) and concentration of hydrothermal fluids during quartz growth.

    (2) Type II quartz grains in the sulfide-quartz veins show mainly CL-grey to CL-dark color without oscillatory zoning, indicating that type Ⅱ quartz formed at low temperature and that during the quartz growth in mineralization stage no obvious changes in composition and concentration of the hydrothermal fluids occurred.

    However, the SEM-CL images of the type Ⅱ quartz associated with Cu (-Mo) mineralization show zoning sometimes, indicating variations in composition and concentration of the hydrothermal fluids once occurred.

    (3) There is a close relationship between the SEM-CL textures of quartz and fluid-inclusion data.

    The type IV muscovite-bearing inclusions are densely distributed in the core or growth zones of type Ⅰ quartz crystals in quartz monzonite with potassic-silicic alteration, which indicate that they are primary fluid inclusions trapped during the quartz growth.

    The occurrence of muscovite-bearing fluid inclusions along oscillatory growth zones of type Ⅰ quartz as revealed by SEM-CL images of the quartz suggests that they were trapped several times in the process of quartz growth when the magmatic- hydrothermal fluids periodically flowed into the rock system.

    In contrast, type Ⅱ quartz seldom shows growth zoning and contains mainly types I, II and III fluid inclusions. In particular, the coexistence of type Ⅱ and type Ⅲ inclusions in the type Ⅱ quartz crystals related (adjacent) to chalcopyrite and molybdenite, and their similar homogenization temperature range suggest that fluid boiling once occurred during Cu (-Mo) mineralization.

  • We thank Prof. Chen Li from Peking University for assistance with the scanning electron microscope analysis. This study was financially supported by the Key R & D Program of China (No. 2016YFC0600606), the Key R & D Program of Shandong Province (Nos. 2017CXGC 1601, 2017CXGC 1602, 2017CXGC 1603), the National Natural Science Foundation of China (Nos. 41140025, 41672084, 41372086, 41503038) and the Special Fund for "Taishan Scholars" Project in Shandong Province. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1025-3.

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