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Volume 32 Issue 2
Apr.  2021
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Qinglin Xia, Tongfei Li, Li Kang, Shuai Leng, Xiaochen Wang. Study on the PTX Parameters and Fractal Characteristics of Ore-Forming Fluids in the East Ore Section of the Pulang Copper Deposit, Southwest China. Journal of Earth Science, 2021, 32(2): 390-407. doi: 10.1007/s12583-021-1448-5
Citation: Qinglin Xia, Tongfei Li, Li Kang, Shuai Leng, Xiaochen Wang. Study on the PTX Parameters and Fractal Characteristics of Ore-Forming Fluids in the East Ore Section of the Pulang Copper Deposit, Southwest China. Journal of Earth Science, 2021, 32(2): 390-407. doi: 10.1007/s12583-021-1448-5

Study on the PTX Parameters and Fractal Characteristics of Ore-Forming Fluids in the East Ore Section of the Pulang Copper Deposit, Southwest China

doi: 10.1007/s12583-021-1448-5
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  • In this paper, the east ore section of the Pulang porphyry copper deposit is selected as the research object. The micro-thermometer and laser Raman spectroscopic technique are utilized to study the parameters of ore-forming fluids such as pressure, temperature, and compositions. In the meantime, the fractal models, including the perimeter-area (P-A) model and number-size (N-S) model, are introduced to quantify the shape of fluid inclusions, and distinguish the stages of ore-forming fluids, respectively. The results show that the types of fluid inclusions are diversified, namely two-phase liquid-rich type, two-phase vapor-rich type, three-phase CO2-rich type, three-phase halite-bearing type and pure liquid type. The fluids of main mineralization stage are characterized by medium-high temperature (170.2-421.4℃), medium-high salinity (9.3 wt.%-33.3 wt.%), and low density (0.73-1.06 g/cm3). With the migration and evolution, the temperature, salinity, and pressure of ore-forming fluids gradually decrease, while the density of fluids increases. The liquid-phase compositions mainly include H2O, and the vapor-phase compositions consist of H2O, CH4, N2, and CO2, indicating the characteristics of reducing fluids and the mixing of atmospheric precipitation. In general, the characteristics of ore-forming fluids in the east ore section are similar to those of the first mining area, suggesting that the ore-forming fluids in the east ore section may not migrate from the first mining area. And the east ore section may be a relatively independent metallogenic system. Moreover, the fractal analysis results demonstrate that the shape of fluid inclusions formed in the same hydrothermal activity features self-similarity. The DAP values of fluid inclusions in B veins, ED veins, and D veins are 1.04, 1.06 and 1.10, respectively, showing a gradually increasing trend from the main stage to the late stage of mineralization. Meanwhile, the shape of fluid inclusions ranging from B veins to D veins becomes increasingly irregular. It also reveals that the homogenization temperature satisfies fractal distribution with four scale-invariant intervals, suggesting that all B veins, ED veins, and D veins have experienced at least four hydrothermal activities. Compared with histogram, the N-S fractal model is able to describe the distribution characteristics of the ore-forming fluids' homogenization temperature more precisely. Therefore, it presents a potential tool for the stage division of ore-forming fluids. This research provides information about the characteristics of ore-forming fluids in the east ore section of the Pulang porphyry copper deposit, which is beneficial for further exploration in this region, and the extension of the application of fractal models in the study of fluid inclusions. However, further testing of fractal models on the fluid inclusion study is warranted to fully determine the universality.
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Study on the PTX Parameters and Fractal Characteristics of Ore-Forming Fluids in the East Ore Section of the Pulang Copper Deposit, Southwest China

doi: 10.1007/s12583-021-1448-5

Abstract: In this paper, the east ore section of the Pulang porphyry copper deposit is selected as the research object. The micro-thermometer and laser Raman spectroscopic technique are utilized to study the parameters of ore-forming fluids such as pressure, temperature, and compositions. In the meantime, the fractal models, including the perimeter-area (P-A) model and number-size (N-S) model, are introduced to quantify the shape of fluid inclusions, and distinguish the stages of ore-forming fluids, respectively. The results show that the types of fluid inclusions are diversified, namely two-phase liquid-rich type, two-phase vapor-rich type, three-phase CO2-rich type, three-phase halite-bearing type and pure liquid type. The fluids of main mineralization stage are characterized by medium-high temperature (170.2-421.4℃), medium-high salinity (9.3 wt.%-33.3 wt.%), and low density (0.73-1.06 g/cm3). With the migration and evolution, the temperature, salinity, and pressure of ore-forming fluids gradually decrease, while the density of fluids increases. The liquid-phase compositions mainly include H2O, and the vapor-phase compositions consist of H2O, CH4, N2, and CO2, indicating the characteristics of reducing fluids and the mixing of atmospheric precipitation. In general, the characteristics of ore-forming fluids in the east ore section are similar to those of the first mining area, suggesting that the ore-forming fluids in the east ore section may not migrate from the first mining area. And the east ore section may be a relatively independent metallogenic system. Moreover, the fractal analysis results demonstrate that the shape of fluid inclusions formed in the same hydrothermal activity features self-similarity. The DAP values of fluid inclusions in B veins, ED veins, and D veins are 1.04, 1.06 and 1.10, respectively, showing a gradually increasing trend from the main stage to the late stage of mineralization. Meanwhile, the shape of fluid inclusions ranging from B veins to D veins becomes increasingly irregular. It also reveals that the homogenization temperature satisfies fractal distribution with four scale-invariant intervals, suggesting that all B veins, ED veins, and D veins have experienced at least four hydrothermal activities. Compared with histogram, the N-S fractal model is able to describe the distribution characteristics of the ore-forming fluids' homogenization temperature more precisely. Therefore, it presents a potential tool for the stage division of ore-forming fluids. This research provides information about the characteristics of ore-forming fluids in the east ore section of the Pulang porphyry copper deposit, which is beneficial for further exploration in this region, and the extension of the application of fractal models in the study of fluid inclusions. However, further testing of fractal models on the fluid inclusion study is warranted to fully determine the universality.

Qinglin Xia, Tongfei Li, Li Kang, Shuai Leng, Xiaochen Wang. Study on the PTX Parameters and Fractal Characteristics of Ore-Forming Fluids in the East Ore Section of the Pulang Copper Deposit, Southwest China. Journal of Earth Science, 2021, 32(2): 390-407. doi: 10.1007/s12583-021-1448-5
Citation: Qinglin Xia, Tongfei Li, Li Kang, Shuai Leng, Xiaochen Wang. Study on the PTX Parameters and Fractal Characteristics of Ore-Forming Fluids in the East Ore Section of the Pulang Copper Deposit, Southwest China. Journal of Earth Science, 2021, 32(2): 390-407. doi: 10.1007/s12583-021-1448-5
  • Fluid inclusions refer to small droplets of diagenetic and metallogenic fluids encapsulated and preserved in the mineral defects during growth of the crystal (Lu et al., 2004). They recorded a wide range of paleo-geological environment information during the formation of minerals, making them the key to the study of related geological processes (Klyukin et al., 2019; Frezzotti et al., 2012; Parry, 1986; Roedder, 1984). Based on the fluid inclusions, many physicochemical parameters (such as temperature, pressure, salinity, density, pH, Eh, and compositions) of diagenesis and mineralization can be obtained, which can be used to solve the problems of ore deposit genesis, metallogenic regularity, and prospecting criteria (Qiu et al., 2021; Pan et al., 2020; Chen et al., 2018; Chi et al., 2018; Ni et al., 2018, 2001; Wilkinson, 2001; Roedder, 1984).

    Petrography forms the basis of fluid inclusion study (Chi et al., 2003). In this process, the host minerals, and the characteristics of fluid inclusions (e.g., shape, size, color, attitude, distribution characteristics, and filling factor) are described (Lu et al., 2004). Traditional geologists believed that a small amount of detailed petrographic data might be more valuable than the data obtained from thousands of tests and analyses. For example, according to the shape, attitude, and distribution characteristics, the primary, pseudo-secondary, and secondary inclusions can be distinguished (Roedder, 1984). The color of inclusions is usually related to their compositions. When the fluid inclusion bubble is dominated by organic gas, it often shows the color of light brown (Lu et al., 2004). The size of fluid inclusions, usually described by the average diameters range, is a useful parameter (Bodnar, 1983). Some studies also demonstrate that the ease with which fluid inclusions decrepitate, stretch, or leak is related to the inclusion size (Xu et al., 2008; Roedder and Skinner, 1968).

    Thermodynamic research focuses on the inversion of fluid properties by calculating the corresponding thermodynamic parameters (e.g., temperature, pressure, salinity, density) through micro-thermometry, empirical formulas, and the related phase diagrams. Meanwhile, composition analysis of fluid inclusions can provide detailed element and isotope composition of inclusions. This information is essential for the inference of the origin and evolution of ore-forming fluids, the migration and precipitation mechanism of ore-forming materials, and the environment of deposit formation. As the most practical tool for the non-destructive analysis of single fluid inclusion, laser Raman spectroscopy can be used for qualitative and semi-quantitative analysis of fluid inclusion compositions quickly and conveniently (Frezzotti et al., 2012; Lu et al., 2004). With the aid of laser Raman spectroscopy, it is possible to intuitively distinguish the main components of different stages, as well as the primary and secondary inclusions. In the meantime, laser Raman spectroscopy can effectively avoid the contamination and loss of samples (Zhang and Chen, 1993).

    The frequency distribution histogram of homogenization temperature plays a vital role in interpreting fluid inclusion data, especially in the stage division of fluids (Lu et al., 2004). However, the interpretation of histogram may be influenced by choice of homogenization temperature interval. When the interval is too small, the overall characteristics of homogenization temperature are difficult to highlight; when the interval is too large, the graph's regularity can not be obtained intuitively. In the meantime, when there are enough samples, the bimodal or multimodal distribution represents multi-stage fluids, and unimodal distribution can not rule out the possibility of multi-stage fluids (Zhao et al., 1994). Therefore, it may be challenging to divide the stages of fluid virtually only according to the histogram of homogenization temperature. And the final explanation depends largely on expert judgment. Fluid inclusion assemblage (FIA) is a widely applied method to validate microthermometric data and assist histograms in dividing the stages of fluid (Goldstein, 2001; Touret, 2001). However, this method still relies on expert knowledge and observation scope (Chi and Lu, 2008). For this reason, it is necessary to explore a relatively simple method to assist in the division of fluid stages.

    As mentioned above, size presents the principal parameter to describe the morphology of fluid inclusions. There are unique temperature and pressure parameters in the same stage of hydrothermal activity, and the shape of inclusions may be able to record this critical information (Klyukin et al., 2019). Hence, the analysis of the shape of fluid inclusions may provide useful information for distinguishing the stages of hydrothermal activity.

    The fractal model introduced by Mandelbrot (1983) has been widely applied in non-linear processes of geosciences for decades, such as rainfall (Veneziano and Furcolo, 2002), cloud formation (Schertzer and Lovejoy, 1987), landslide (Zuo and Carranza, 2017) and mineralization (Li et al., 2020, 2018; Carranza and Sadeghi, 2010; Carranza, 2009a; Zuo et al., 2009a; Cheng, 2008; Cheng et al., 2000, 1994; Carlson, 1991). Fractal models have been developed rapidly in mineral exploration, especially in exploration geochemistry. The N-S model, initially proposed by Mandelbrot (1983), characterizes the power-law relationship between the measurement and the cumulative number of objects. This model lays the foundation for other fractal models. Bölviken et al. (1992) first recognized that geochemical landscape plausibly consists of fractals (background and abnormal patterns). Later, according to Cheng et al. (1994), the concentration-area (C-A) model was proposed to describe the power-law relationship between element concentration and the area enclosed by a concentration contour from a fractal/multifractal perspective. This model is regarded as the first significant advancement in fractal/multifractal modeling of geochemical data, and a fundamental technique for geochemical anomaly recognition (Zuo et al., 2012; Carranza, 2009b). The perimeter-area (P-A) model was soon deducted to analyze the geochemical anomalies (Cheng, 1995). Subsequently, the spectrum-area (S-A) model, as an extension of the C-A model in the frequency domain, was proposed to decompose the background and anomalies in intricate geochemical patterns (Cheng et al., 2000). Afzal et al. (2011) developed the concentration-volume (C-V) model, extending the C-A model to the geochemical anomaly separation in three-dimension. Based on the same idea of the N-S model, the concentration-number (C-N) model was also developed to separate different level geochemical anomalies according to their intensity (Hassanpour and Afzal, 2013). The advantage of this model lies in the classification of the geochemical data distribution without any geostatistical estimation or simulation (Farahmandfar et al., 2020; Kouhestani et al., 2020; Saadati et al., 2020). Compared to the conventional statistical method, the fractal model can mine more in-depth information. More importantly, Cheng (2008) defined singular processes as physic or chemical processes that may result in an abnormal amount of energy release or mass accumulation confined to narrow intervals in time or space. The products of these processes can be modeled as fractals or multifractals. The hydrothermal activity in mineralization can be regarded as a singular process in which a large amount of energy is released in a short time. As the medium for recording the singular process, the fluid inclusions should feature self-similarity. Therefore, it is possible to characterize fluid inclusions by fractal models.

    In this paper, the parameters of ore-forming fluids in the east ore section of the Pulang copper deposit are inversed, employing a micro-thermometer and laser Raman spectroscopic analysis. Compared with the first mining area, the potential correlation between ore-forming fluids is revealed, thereby providing guiding to the peripheral prospecting. In addition, the applicability of fractal models in identifying the stages of fluid is discussed. It should be noted that this research has no intension to deny the significance of traditional methods, such as histogram and fluid inclusions assemblage (FIA), instead, it aims to explore the methods of distinguishing the stages of fluid in the study of fluid inclusions from a new perspective.

  • The Pulang porphyry copper deposit is located on the western side of Ganzi-Litang junction zone and the southern segment of Yidun island arc (Li, 2007; Zeng et al., 2003). The predominant strata exposed in the Pulang mining area is the upper Triassic Tumugou Formation (T3t), which consists of a series of volcanic-sedimentary rocks (Zeng et al., 2004). The NNW-trending Heishuitang fault and the NEE-trending Quanganda fault constitute the basic structural framework and control the outcrop of the Pulang complex (Yang, 2017; Li and Liu, 2015). On the other hand, the Pulang complex formed in the Indosinian belongs to calc-alkaline I-type with the characteristics of adakites (Chen, 2016; Li and Liu, 2015; Cao et al., 2014; Li et al., 2011; Leng et al., 2007; Fan and Li, 2006). It consists of quartz diorite porphyry, quartz monzonite porphyry, and granodiorite porphyry, among them, the quartz monzonite porphyry is most closely related to the copper mineralization (Li, 2007; Fan and Li, 2006) (Fig. 1a). Similar to the typical porphyry copper deposit, the alteration is also well developed. From the inner part outwards, there occurs the potassic silicate alteration (biotite+K-feldspar) zone, silicification (quartz) zone, phyllic (sericite and quartz) zone, and propylitic (chlorite and epidote) zone (Wang, 2017; Li, 2007; Zeng et al., 2004). Moreover, the copper mineralization mainly occurs in the potassic silicate alteration (biotite+K-feldspar) zone, silicification (quartz) zone, and three lenticular copper ore bodies KT1, KT2, and KT3 whcih have been delimited in the first mining area of the Pulang deposit (Zeng et al., 2004).

    Figure 1.  Geological map of the Pulang mining area. (a) Simplified geological map of the Pulang area; (b) simplified geological map of east ore section of the Pulang deposit; (c) profile map of exploration line E4 (modified from Yang, 2017).

    In recent years, the copper ore bodies, such as KT4, KT5, KT6, and KT9, have been found in the east ore section of the Pulang copper deposit (Zhou, 2018; Zhou et al., 2018; Wu, 2011) (Fig. 1b). The ore bodies, developed in intrusions (e.g., quartz monzonite porphyry, and quartz diorite porphyry), are discontinuous and steep, stretching to the south in vein and stratoid shape (Chen and Yu, 2016; Wu, 2011) (Fig. 1c). The total reserves of copper metal reaches about 56 856 tons, and the average grade of Cu is 0.56% (Wang et al., 2016b). The metal minerals mainly include chalcopyrite, pyrrhotite and a small amount of sphalerite, galena, molybdenite, and porphyrite. Meanwhile, the gangue minerals include quartz, K-feldspar, plagioclase, biotite, hornblende, sericite, chlorite, epidote, and calcite. The ore structures are veinlets and disseminated. And the ore textures are mainly crystalline and metasomatic, followed by co-junction and opacifying textures. Compared with the first mining area, the east ore section's alteration lacks a potassic silicate alteration (biotite+K-feldspar) zone (Zhou et al., 2018; Wang, 2017).

  • The evolution of ore-forming fluids in this deposit has been studied extensively. Some researchers argued that there are three stages of ore-forming fluids in the Pulang deposit (namely, high-temperature and high-salinity NaCl-H2O fluids in the early stage, medium-high-temperature, and high-salinity NaCl-CO2-H2O fluids in the middle stage, low-temperature, and low-salinity NaCl-H2O fluids in the late stage), besides, Mo and Cu mineralization of the Yanshanian period might be co-existed (Liu, 2018b; Li et al., 2013). During the evolution of ore-forming fluids, the temperature and salinity increase gradually (Lü, 2014; Li et al., 2013). The vapor-phase compositions of ore-forming fluids mainly include H2O, CO2, CH4, CO, and N2. For the liquid phase of ore-forming fluids, the anions are mostly Cl-, NO- 3 and SO2- 4, while the cations are K+, Na+, Ca2+, and Mg2+ (Lü, 2014; Liu J T et al., 2013; Guo et al., 2009). Different from the typical porphyry copper deposits, they are characterized by high oxygen fugacity and minerals indicating high oxygen fugacity (such as original magnetite, hematite, and anhydrite) (Sillitoe, 2010, 2002; Ballard et al., 2002; Mungall, 2002; Blevin and Chappell, 1995; Burnham and Ohmoto, 1980), in addition, the ore-forming fluids contain more reducing compositions. These reduced compositions (such as CO, CH4) in the ore-forming fluids may originate from carbonaceous phyllite surrounding the Pulang complex mafic magma (Liu X L et al., 2013). Furthermore, the reducing compositions in the ore-forming fluids reduce the solubility of Cu and reacts with SO2 to form S2-, which is beneficial to molybdenite formation (Liu X L et al., 2013). Consequently, the grade of Cu in the deposit is lower, while Mo and other accessory elements are relatively higher. It is also considered that the network and disseminated mineralization are caused by the phase separation of CO2-bearing fluids with low salinity and magmatic fluid with high salinity (Wang S X et al., 2007). As per the evidence from isotope, the ore-forming materials come from the upper mantle and lower crust, which are mainly composed of magmatic water and mixed atmospheric precipitation (Liu, 2018b; Liu X L et al., 2013; Liu et al., 2011). Most studies on the ore-forming fluids focused on the first mining area of the Pulang copper deposit. With the progress of the prospecting work in the peripheral, researchers inferred that the east ore section and the first mining area belong to the same metallogenic system based on the geological and alteration characteristics. The east ore section may be located on the porphyry metallogenic system's propylitic alteration zone, and the ore-forming fluids may migrate from the first mining area along the NEE-trending fault (Zhou, 2018; Zhou et al., 2018; Chen and Yu, 2016). However, the research on the east ore section of the Pulang copper deposit is still less.

  • The samples used in this paper were obtained from the drilling wells (ZKE004, ZKE101, ZKE401, and ZK0416) of the east ore section of the Pulang deposit. Doubly polished sections with the thickness of about 0.3 mm were prepared for fluid inclusion analysis. A total of 27 typical samples were analyzed in this paper. Previous studies have shown that the vein system in the porphyry copper deposit not only records the fluid evolution at different stages, but also reveals the relationship between hydrothermal alteration and metal sulfide precipitation (Wang et al., 2016b; Wang et al., 2012; Pan et al., 2009; Li and Sasaki, 2007; Gustafson and Hunt, 1975). Based on the shape, alteration, mineral assemblage, and cutting relations of the veins in the samples, the veins were observed to be divided into four major types and thirteen sub-types (Table 1). More detailed cutting relations of different veins can be seen in Fig. 2.

    Stages Vein types Mineral assemblage Alteration zones Distribution characteristics References
    Early stage (A veins) Q+Kfs (A1 vein) Q+Kfs KSi Distributed in quartz monzonite porphyry and are cut by later veins Gustafson and Hunt (1975)
    Q (A2 vein) Q
    Bt (A3 vein) Bt KSi/SiSe
    Main stage (B veins) Po±Py (B1 vein) Po KSi/SiSe Distributed in quartz monzonite porphyry and quartz diorite porphyry. B veins cut the earlier A veins and are cut by later ED veins and D veins. Cutting relations also exist in B veins of different sub-stages Gustafson and Hunt (1975)
    Po+Py
    Ccp±Po±Py (B2 vein) Ccp
    Ccp+Po
    Ccp+Py
    Ccp+Po+Py
    Q+Ccp±Po±Py (B3 vein) Q+Ccp
    Q+Ccp+Po
    Q+Ccp+Py
    Q+Ccp+Po+Py
    Middle-late stage (ED veins) Ep±Chl (ED1 vein) Ep ChEp Distributed in the edge of granodiorite porphyry. ED veins cut the earlier A veins and B veins. And they are cut by the later D veins. Cutting relations also exist in ED veins of different sub-stages. Wang (2017)
    Ep+Chl
    Chl±Po±Ccp (ED2 vein) Chl
    Chl+Po SiSe/ChEp
    Chl+Ccp
    Chl+Po+Ccp
    Q±Ep±Chl (ED3 vein) Q ChEp
    Q+Ep
    Q+Chl
    Q+Chl+Po±Ccp (ED4 vein) Q+Chl+Po SiSe/ChEp
    Q+Chl+Ccp
    Q+Chl+Po+Ccp
    Po (ED5 vein) Po ChEp/HS
    Late stage (D veins) Q+Cal±Py (D1 vein) Q+Cal ChEp/HS Distribute in the edge of granodiorite porphyry. D veins cut the earlier A veins, B veins and ED veins. Cutting relations also exist in D veins of different sub-stages. Gustafson and Hunt (1975)
    Q+Cal+Py
    Q+Cal+Py+Po SiSe/ChEp
    Q+Cal+Chl
    Q+Cal+Chl+Po
    Q+Cal+Po
    Q+Cal+Po+Ccp
    Cal±Po±Py (D2 vein) Cal ChEp/HS
    Cal+Po
    Cal+Py
    *Q. Quartz; Kfs. K-feldspar; Bt. biotite; Po. pyrrhotite; Py. pyrite; Ccp. chalcopyrite; Ep. epidote; Chl. chlorite; Cal. calcite; KSi. K-silicate zone; SiSe. sericite-quartz alteration zone; ChEp. propylitization zone; HS. hornfels zone.

    Table 1.  The main types and characteristics of veins in the east ore section of the Pulang deposit

    Figure 2.  Characteristics and the cutting relations of different veins in the east ore section of the Pulang deposit. (a) The quartz-K-feldspar vein (A1 vein) of the early stage; (b) the chalcopyrite-pyrrhotite vein (B2 vein) of the main stage cuts the biotite pyrrhotite vein (A3 vein) of the early stage; (c) the quartz-pyrite-chalcopyrite vein (B3 vein) of the main stage; (d) the quartz-chlorite vein (ED3 vein) of middle-late stage cuts the chalcopyrite-pyrrhotite vein (B2 vein) of the main stage; (e) the quartz-chlorite-pyrrhotite vein (ED4 vein) of middle-late stage cuting the chalcopyrite-pyrrhotite vein (B2 vein) of the main stage; (f) the quartz-chlorite-pyrrhotite vein (ED4 vein) of the middle-late stage; (g) the quartz-calcite vein (D1 vein) of the late-stage cuting the chalcopyrite-pyrrhotite vein (B2 vein) of the main stage; (h) the quartz-calcite vein (D1 vein) of the late-stage cuts the quartz-chlorite- pyrrhotite vein (ED4 vein) of the middle-late stage; (i) the quartz-calcite vein (D1 vein) of the late-stage.

    Due to the fact that the fluid inclusions in quartz of A veins and fluid inclusions in calcite of D veins are too small to be used in microthermometry analyses, the fluid inclusions in quartz of B veins, ED veins, and D veins were analyzed in this paper.

  • Micro-thermometry analysis on the fluid inclusions was carried out using an Axioskop Linkam-THMS 600 heating-freezing stage (from -196 to 600 ℃) at the Experimental Teaching Center for Mineral Exploration and Prospecting, China University of Geosciences in Wuhan. The magnification ranges from 100 to 1 000 times. When the test temperature goes up from -196 to 0 ℃, the precision is approximately ±0.1 ℃. Furthermore, when the test temperature ranges from 0 to 600 ℃, the accuracy becomes approximately ±1 ℃. In general, the heating/freezing rate varies from 5–10 ℃/min, and falls in the range from 0.5–1 ℃/min when the phase transformation of fluid inclusions is about to occur.

    Laser Raman spectroscopic analysis of the fluid inclusions was carried out in the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan using Renishaw RM-1000 Raman micro-spectrometer. And an argon ion laser with the wavelength of 514.5 nm, and the source power of 20 mW×100 % was used in detection. The resolution is 1–2 μm with an accumulation time of 30 s. Besides, the laser Raman spectroscopic analysis was performed at the room temperature of 23 ℃.

  • The geometry fractal concept is introduced initially to characterize the scale-invariant properties (Turcotte, 1992; Mandelbrot, 1983). Number-size (N-S) model is a fundamental model in fractal theory, which is widely applied to quantify the power-law relationship between number and parameters of mineralization or geological features, such as grade, tonnage, vein thickness, faults (Zuo et al., 2009a, b; Cheng, 2008; Li et al., 1996; Blenkinsop, 1995; Sanderson et al., 1994; Carlson, 1991). The N-S model can be described by the following power-law relationship (Mandelbrot, 1983),

    where N(r) refers to the number of objects (in this case, it refers to the fluid inclusions) greater than r, "∝" means "proportional to", r represents the measurement of an item (in this case, it refers to the homogenization temperature), and D denotes the fractal dimension. The value of D ranges from 0 to 1.

    As another fractal model to describe the power-law relationship between the perimeters and areas of objects (Cheng, 1995; Cheng et al., 1994), perimeter-area (P-A) model has been used to distinguish the minerals formed in different processes (Zuo et al., 2009b; Wang Z J et al., 2007b; Wang and Cheng, 2006), and the granitoid with different ages (Zhu et al., 2019). The P-A model can be described by the following formula (Cheng, 1995; Cheng et al., 1994),

    where P and A refer the perimeters and areas of objects (in this case, it refers to the fluid inclusions), respectively. "∝" means "proportional to". DAP ranges from 1 to 2. And the greater the value of DAP, the flatter the shapes. When DAP=1, it means the regular-shaped objects, such as circles. When DAP=2, it denotes the exceptionally irregularly shaped objects.

  • Based on the observation with the microscope, the east ore section's primary fluid inclusions can be divided into Type Ⅰ, Type Ⅱ, Type Ⅲ, Type Ⅳ, and Type Ⅴ. Among them, Type Ⅰ (or LV) inclusions are two-phase, liquid-rich. Type Ⅱ (or VL) inclusions are two-phase, vapor-rich. Type Ⅲ (or LV-CO2) inclusions are three-phase and CO2-rich. Type Ⅳ (or LVH) inclusions are three-phase and halite-bearing. Type Ⅴ (or L) inclusions are pure liquid (as shown in Fig. 3). Specifically, there are certain differences in terms of the distribution, shape, size, and type of fluid inclusions in different stages of quartz veins

    Figure 3.  Characteristics of different types of fluid inclusions in the east ore section of the Pulang deposit. (a) Fluid inclusion group in quartz; (b) LV inclusion; (c) VL inclusions; (d) LV-CO2 inclusions; (e) LVH inclusion; (f) L inclusion (LH2O. salt solution; LCO2. CO2-liquid; VCO2. CO2-vapor; SNaCl. halite crystal).

    The types of fluid inclusions in B veins mainly include LV, LVH, and L, and LV inclusions account for the vast, and exhibit various morphologies, such as regular circle and oval. The sizes of LV inclusions range from 6 to 12 μm, and their proportion of vapor phase ranges from 5% to 20%. The morphologies of LVH inclusions are ellipse and negative crystal. And the sizes of LVH inclusions range from 8 to 14 μm. Besides, L inclusions are rare with the sizes range from 8 to 16 μm, and their shapes are irregular. These inclusions are homogenized to liquid at room temperature.

    The primary type of fluid inclusions in ED veins is LV, and LVH inclusions are rare. The shapes of LV inclusions are regular and display morphologies such as circle and oval. The sizes of LV inclusions range from 6 to 16 μm, and the proportion of vapor phase ranges from 10% to 20%.

    All LV, LV-CO2, and VL inclusions occur in D veins, and LV inclusions are the dominant ones in D veins with various sizes (6–20 μm), shapes (regular and irregular), and various proportions of vapor (10%–20%). The sizes of LV-CO2 inclusions range from 6 to 20 μm, and the proportion of vapor ranges from 20% to 40%. VL inclusions are characterized by various sizes (6–12 μm) and, proportions of vapor phase (50%–70%), which are rare. The shape of this kind of inclusions is oval and negative crystal.

  • The freezing point and homogenization temperature (Th) of the primary fluid inclusions in the veins at three different stages (i.e., B veins, ED veins, and D veins) were measured (Fig. 4). A total of 716 sets of micro-thermometric data were obtained. In general, the freezing point shows a positive-skewed distribution (Figs. 4a, 5a, 5c, 5e). And it seems that the homogenization temperature of all veins satisfies a normal distribution (Fig. 4b). After that, the Shapiro-Wilk (S-W) test was carried out at all homogenization temperatures. And the p-value at all the homogenization temperatures is smaller than 0.05, indicating that all the homogenization temperatures fail to obey a normal distribution. More specifically, both the homogenization temperature of B veins and that of ED veins are characterized by bimodal distribution (Figs. 5b, 5d).

    Figure 4.  Histograms of freezing point and homogenization temperature of fluid inclusions in the east ore section of the Pulang deposit. (a) Histogram of freezing point of fluid inclusions; (b) histogram of homogenization temperature of fluid inclusions.

    Figure 5.  Histograms of freezing point and homogenization temperature of fluid inclusions in different veins. (a) Histogram of freezing point of fluid inclusions in B veins; (b) histogram of homogenization temperature of fluid inclusions in B veins; (c) histogram of freezing point of fluid inclusions in ED veins; (d) histogram of homogenization temperature of fluid inclusions in ED veins; (e) histogram of freezing point of fluid inclusions in D veins; (f) histogram of homogenization temperature of fluid inclusions in D veins.

    In contrast, the distribution of D veins' homogenization temperature indicates that there are at least four stages of hydrothermal activity (Fig. 5f). The division of vein stages based on the sample observation shows that there are at least four stages of hydrothermal activity in the east ore section. However, according to the histograms of homogenization temperatures, it may not be able to effectively divide the stages of ore-forming fluids. A more detailed discussion will be shown in Section 5.3.

    In the B veins (the main mineralization stage), the LV inclusions are homogenized to the liquid phase in various ways eventually. While in LVH inclusions, the bubbles disappear later than the halite, and homogenize to the liquid phase, indicating that the primary fluids are unsaturated solutions. The freezing point of LV inclusions ranges from -21.2 to -6.1 ℃ with an average value of -15.8 ℃, and the peak values of freezing point temperature are concentrated within the range from -18.1– -13.8 ℃. The homogenization temperature of LV inclusions ranges from 170.2 to 421.4 ℃ with an average value of 269.3 ℃, and the peak values of LV inclusions are concentrated within the range from 233.5–296.7 ℃. The temperature at which the mineral disappears for LVH inclusions ranges from 173.7 to 226.5 ℃, and the utterly homogenized temperature of LVH inclusions ranges from 284.1 to 323.4 ℃.

    In the ED veins (the middle-late stage of mineralization), the homonization ways of fluid inclusions are relatively simple. The freezing point of LV inclusions ranges from -20.8 to -5.6 ℃, with an average value of -14.5 ℃, and the peak values are concentrated within the range from -17.3– -11.8 ℃. The homogenization temperature of LV inclusions ranges from 122.1 to 324.7 ℃, with an average value of 219.9 ℃, and the peak values are concentrated within the range from 189.5–249.1 ℃.

    In the D veins (the late stage of mineralization), the homonization ways of fluid inclusions are also diversified, that is, the LV inclusions and VL inclusions are homogenized to the liquid phase, while in the LV-CO2 inclusions, the CO2-vapor disappears at first, and the CO2-liquid homogenizes to the aqueous liquid eventually. The freezing point of two-phase inclusions (including LV and VL inclusions) ranges from -20.4 to -5.2 ℃ with an average value of -14.1 ℃, and the peak values are concentrated within the range from -17.0– -11.7 ℃. The homogenization temperature of two-phase inclusions ranges from 115.4 to 357.4 ℃ with an average of 220.5 ℃. For the LV-CO2 inclusions in D veins, the clathrates melting temperature ranges from -9.7 to -0.3 ℃, and the complete homogenization temperature ranges from 230.4 to 367.3 ℃. Moreover, the inclusion groups of LV, VL, and LV-CO2 within the same microscope field of view show the same homogenization temperature range, indicating that fluid boiling occurred in the late stage of mineralization (Lu et al., 2004). More detailed results of the micro-thermometer are shown in Table 2.

    Mineralization stages Sample Types T(m, ice) (℃) T(m, halite) (℃) T(m, clathrate) (℃) Th (℃) Salinity (wt.%) Density (g/cm3)
    Main stage (B veins) PLD-19 LV -21.2– -6.1 207.4–293.2 9.3–23.2 0.88–1.03
    PLD-23 LV -20.3– -11.2 170.2–280.2 15.2–22.6 0.91–1.05
    PLD-25 LV -20.7– -11.6 236.3–421.4 15.6–22.8 0.73–0.95
    PLD-44 LV -20.6– -8.6 245.3–337.4 12.4–22.8 0.86–1.00
    ZKE004-07 LV -21.0– -11.2 194.3–325.1 15.2–23.0 0.88–1.05
    ZKE101-13 LV -19.9– -9.4 191.6–352.3 13.3–22.3 0.86–1.01
    ZKE101-15 LV -21.2– -10.3 172.8–309.7 14.3–23.2 0.91–1.05
    ZKE101-28 LV -20.5– -12.8 219.7–416.4 16.7–22.7 0.77–0.98
    ZKE101-50 LV -20.1– -13.4 186.2–369.8 17.3–22.4 0.82–1.04
    LVH -24.3– -21.3 173.7–226.5 284.1–323.4 30.6–33.3 1.01–1.06
    Middle-late stage (ED veins) ZKE004-09 LV -17.1– -6.2 204.1–293.3 9.5–20.3 0.84–0.99
    ZKE004-10 LV -20.5– -9.8 164.2–290.1 13.7–22.7 0.87–1.05
    ZKE004-14 LV -20.1– -9.7 122.4–219.8 13.6–22.4 0.97–1.09
    ZKE101-16 LV -20.8– -13.8 122.1–227.2 17.6–22.9 1.00–1.10
    ZKE101-25 LV -20.5– -9.3 186.8–324.7 13.2–22.7 0.88–1.03
    ZKE101-30 LV -19.3– -11.8 147.3–294.7 15.8–21.9 0.89–1.07
    ZKE101-36 LV -20.2– -9.9 152.1–248.6 13.8–22.5 0.98–1.06
    ZKE101-39 LV -18.5– -5.6 187.5–255.4 8.7–21.3 0.92–1.00
    Late stage (D veins) PLD-45 LV -20.2– -11.2 115.4–194.7 15.2–22.5 0.99–1.10
    PLD-48 LV, VL -20.2– -7.5 155.6–295.6 11.1–22.5 0.94–1.05
    ZKE004-06 LV -20.2– -8.7 198.9–348.3 12.5–22.5 0.87–1.03
    LV-CO2 -8.8– -0.9 274.9–325.3 16.4–20.4 1.00–1.08
    ZKE004-15 LV -20.0– -10.2 128.6–318.7 14.1–22.4 0.84–1.08
    ZKE101-37 LV, VL -20.4– -7.6 164.3–310.1 11.2–22.6 0.89–1.06
    LV-CO2 -9.7– -0.3 242.6–312.6 15.8–20.5 0.97–1.09
    ZKE101-40 LV, VL -17.8– -5.7 182.5–318.4 8.8–20.8 0.78–1.02
    LV-CO2 -9.7– -9.7 295.3– 295.3 20.5–20.5 1.05–1.05
    ZKE101-41 LV -20.1– -5.2 167.3–332.6 8.1–22.4 0.86–1.03
    LV-CO2 -6.9– -0.4 230.4–333.4 15.9–20.1 1.02–1.06
    ZKE101-42 LV, VL -17.5– -7.9 142.3–357.4 11.6–20.6 0.82–1.03
    ZKE101-43 LV, VL -20.4– -15.7 270.3–349.8 19.2–22.6 0.89–0.95
    LV-CO2 -8.1– -1.1 243.6–367.3 16.8–20.3 1.01–1.09
    ZKE101-44 LV -17.3– -5.2 190.2–251.2 8.1–20.4 0.88–1.02
    *T(m, ice) is the freezzing point; T(m, halite) is the halite melting temperature; T(m, clathrate) is the clathrate melting temperature; Th is the homogenization temperature.

    Table 2.  Summary of micro-thermometer of fluid inclusions of the east ore section of the Pulang deposit

  • For the LV and VL inclusions, the fluid can be treated as a NaCl-H2O system. In this case, the salinity can be estimated by the empirical formula (Hall et al., 1988; Sterner et al., 1988)

    where w refers to the salinity (wt.%), T(m, ice) denotes the freezing point ranging from -21.2 to 0 ℃.

    For the LVH inclusions, the salinity is obtained according to the following formula (Sterner et al., 1988)

    where ψ=T/100, T refers to the melting temperature of halite ranging from 0.1 to 801 ℃.

    For the LV-CO2 inclusions, the fluid can be regarded as a H2O-NaCl-CO2 system. Hence, the salinity is calculated using the following formula (Roedder, 1984)

    where T(m, cla) refers to the melting temperature of CO2 clathrate.

    According to the results, the salinity of fluids ranges from 8.1 wt.% to 33.3 wt.% with an average value of 18.4 wt.%. While, the salinity peak values are concentrated within the range from 16.6 wt.%–20.5 wt.%, indicating the characteristic of medium-high salinity fluids.

    Specifically, in the B veins, the salinity of LV inclusions ranges from 9.3 wt.% to 23.2 wt.% with an average value of 19.1 wt.%, and the peak values are concentrated within the range from 17.6 wt.%–21.1 wt.%. Compared with the LV inclusions, the salinity of LVH inclusions ranges from 30.6 wt.% to 33.3 wt.% with an average value of 32.4 wt.%. In the ED veins, the salinity of LV inclusions ranges from 8.7 wt.% to 22.9 wt.% with an average value of 18.0 wt.%, and the peak values are concentrated within the range from 15.8 wt.%–20.4 wt.%. In the D veins, the salinity of two-phase inclusions (LV and VL inclusions) ranges from 8.1 wt.% to 22.6 wt.% with an average value of 17.6 wt.%, and the peak values are concentrated within the range from 15.7 wt.%–20.2 wt.%. For LV-CO2 inclusions, the salinity ranges from 15.8 wt.% to 20.5 wt.% with an average value of 18.9 wt.%, and the peak values are concentrated within the range from 17.5 wt.%–20.3 wt.%. Generally, the salinity range of B veins varies widely, which indicates a process of fluids immiscibility. The salinity decreases as the fluids evolve.

    The evolution of ore-forming fluids is discussed by using the bivariate diagram (Wilkinson, 2001). The homogenization temperature and salinity of B veins, ED veins, and D veins are scattered in the diagram. After that, the standard deviation ellipses with 90% confidence are drawn. It shows clearly that the homogenization temperature and salinity decrease gradually, indicating that the fluids may experience a process of the mixing with low-temperature and low-salinity fluid. In the meantime, the process may be accompanied by boiling of low salinity, CO2-bearing fluids (as shown in Fig. 6). This result is consistent with the fact that there are many boiling fluid inclusions in D veins.

    Figure 6.  Homogenization temperature-salinity plot of fluid inclusions in different veins (modified from Wilkinson, 2001). The ellipses in the plot represent the standard deviation ellipses with 90% confidence.

    The density of fluids of NaCl-H2O system and H2O-NaCl-CO2 system can be calculated by the following empirical formula (Liu, 2001; Liu and Duan, 1987)

    where ρ refers to the density (g/cm3), Th represents the homogenization temperature (℃), and A, B, and C are the parameters and functions of salinity. More detailed information about these parameters can be found in the related papers (Liu, 2018a).

    For the LV-CO2 inclusions, the following formulas are used to calculate the density of the fluids (Liu, 2018a; Touret, 2001; Sterner and Bodnar, 1991)

    where ρtotal refers to the total density of the fluids (g/cm3), ΦCO2 represents the filling degree of CO2 phase when LV-CO2 inclusions reach partial homogeneity, ρCO2 and ρaq denotes the density of the CO2 phase (g/cm3) and brine solution phase (g/cm3), respectively. And m and w are the molalities of NaCl in an aqueous solution and salinity of aqueous solution, respectively.

    The calculated results show that the density of fluids ranges from 0.73–1.10 g/cm3 with an average value of 0.97 g/cm3, and the peak values are concentrated within the range from 0.93–1.02 g/cm3. For the B veins, the density ranges from 0.73 to 1.06 g/cm3, with an average value of 0.95 g/cm3, and the peak values are concentrated within the range from 0.91–0.99 g/cm3. For the ED veins, the density ranges from 0.84 to 1.10 g/cm3, with an average value of 0.98 g/cm3, and the peak values are concentrated within the range from 0.94–1.03 g/cm3. For the D veins, the density ranges from 0.78 to 1.10 g/cm3, with an average value of 0.99 g/cm3, and the peak values are concentrated within the range from 0.94–1.03 g/cm3. The density increases with the evolution of the fluids.

    Due to the influence of pressure, the measured homogenization temperature of fluids often represents the lowest value of the capture temperature, thereby failing to reflect the actual capture temperature (Potter, 1977). In this case, the pressure correction should be considered. Previous research shows that the consolidation pressure of the Pulang complex ranges from 31 to 117 MPa, with an average value of 75 MPa (Li et al., 2013). At the pressure of 75 MPa, the pressure correction is applied for the homogenization temperature of LV inclusions in B veins and ED veins based on the relationship between the temperature of solutions and the pressure (Potter, 1977). The results indicate that the correction values range from 70 to 82 ℃, with an average value of 75 ℃. According to the results of the microthermometer above, the capture temperature of B veins ranges from 245.2 to 496.4 ℃ with an average value of 344.3 ℃. The peak values are concentrated within the range of 308.5–371.7 ℃. In the meantime, the capture temperature of ED veins ranges from 197.1 to 399.7 ℃, with an average value of 294.9 ℃. And the peak values concentrated within the range from 264.5–324.1 ℃. As mentioned above, many boiling inclusion groups in D veins prove that the external pressure is equal to the saturated vapor pressure inside the fluids (Lu et al., 2004). There is no need to correct the temperature; namely, the homogenization temperature is equal to the capture temperature. Therefore, the capture temperature ranges from 115.4 to 357.4 ℃, with an average value of 220.5 ℃. And the peak values are concentrated within the range from 181.9–247.6 ℃. Based on the the capture temperature of veins in different mineralization stages, it shows clearly that the ore-forming fluids in the east ore section of the Pulang deposit experienced a significant cooling process.

    Due to the fact that the LVH inclusions almost only appear in B veins and the number of these types of inclusions is small, this paper mainly uses LV (or VL) inclusions in B veins and ED veins to calculate the trapping pressure. For two-phase inclusions, the following formula can be used to calculate the trapping pressure (Xu, 1987),

    where T0 refers to the initial temperature (℃), w represents the salinity (wt.%), P0 denotes the initial pressure (105 Pa), P is the trapping pressure (105 Pa), and Th refers to the homogenization temperature (℃).

    Due to the existence of many boiling inclusions in the D vein in the late stage of mineralization, the maximum homogenization temperature can be employed to estimate the trapping pressure and the depth (Bouzari and Clark, 2006; Bodnar et al., 1985). In this paper, the phase diagram of the NaCl-H2O system is utilized for the estimation of the capture temperature (Fig. 7).

    Figure 7.  Pressure estimates for the fluid inclusions in the D veins (modified from Bouzari and Clark, 2006; Bodnar et al., 1985).

    According to the results, the trapping pressure of fluid inclusions in B veins ranges from 47.7 to 118.0 MPa with an average value of 75.4 MPa, while the trapping pressure of fluid inclusions in ED veins ranges from 34.3 to 91.1 MPa, with an average value of 61.5 MPa. The trapping pressure of fluid inclusions in D veins is within the range from 7.0 to 19.0 MPa. This result reveals that the ore-forming fluids in the east ore section of the Pulang deposit experienced decompression during the mineralization.

    The formation depth of fluid inclusions in B veins and ED veins are calculated based on the following empirical formula, thereby estimating the formation depth of fluid inclusions (Xu, 1987),

    where H refers to the formation depth (km), and P represents the trapping pressure (105 Pa).

    As per the results, the formation depth of fluid inclusions in B veins ranges from 1.6 to 3.9 km, with an average of 2.5 km. Because the B veins formed in the main mineralization stage, the formation depth of fluid inclusions in B veins can be deemed as the mineralization depth. And the formation depth of fluid inclusions in ED veins ranges from 1.1 to 3.0 km, with an average value of 2.0 km. As to D veins, because the system is open when the fluid is boiling, hydrostatic pressure refers to the primary fluid pressure (Mao, 2012). According to the empirical hydrostatic pressure, such as 10 MPa/km, the formation depth of fluid inclusions in D vein can be estimated. The results show that the formation depth of fluid inclusions in D veins ranges from 0.7 to 1.9 km. In general, the mineralization depth is consistent with the results obtained from the previous studies (Liu et al., 2013b).

  • The compositions of fluid inclusions in the east ore section of the Pulang deposit are analyzed using the laser Raman spectroscopic technique. And the inclusions in B veins (LV and LVH inclusions), ED veins (LV inclusions), and D veins (LV, VL, and LV-CO2 inclusions) are selected as the research objects.

    It can be seen from the results of the analyses that the liquid-phase compositions mainly include H2O, however, the vapor-phase compositions are diversified. The vapor-phase compositions of the fluid inclusions in B veins include H2O, CH4, and N2. While the compositions of the vapor-phase of the fluid inclusions in ED veins are H2O and CH4. The vapor-phase compositions of the fluid inclusions in D veins include H2O, CO2, and CH4 (as shown in Fig. 8). Additionally, compared with the deep samples, the CH4 content in the vapor-phase of the fluid inclusions in shallow samples decreased obviously, suggesting the existence of CH4 loss or chemical reactions that consume CH4. Generally, it is evident that the CH4 in the vapor-phase exists in the process of fluid evolution, indicating a characteristic of reducibility of fluids. And the existence of N2 in the vapor-phase of the fluid inclusions in B veins may reflect the mixing of atmospheric precipitation.

    Figure 8.  Laser Raman spectra of fluid inclusions in quartz veins of different stages in the east ore section of the Pulang deposit. (a), (b) The laser Raman spectra of the vapor phase of fluid inclusions in B veins; (c), (d) the laser Raman spectra of the vapor phase of fluid inclusions in ED veins; (e) and (f) represent the laser Raman spectra of the vapor phase of fluid inclusions in D veins.

  • It is revealed from the above results that the fluid features of the east ore section, including the types of fluid inclusions, the range of homogenization temperature, the content of salinity, the compositions of the vapor phase in the inclusions, and the evolution mechanisms, are similar to those of the main mineralization stage of the first mining area (as shown in Table 3). More importantly, predecessors argued that the east ore section and the first mining area belong to the same metallogenic system based on geological and alteration characteristics. The east ore section may be located on the propylitic alteration zone of the porphyry metallogenic system, and fluids of the east ore section may migrate from the first mining area due to the deep lithology of the east ore section and the lack of complete alteration zoning, especially the potash and silicic zone (Wang et al., 2016a). If this assumption is correct, the homogenization temperature of fluid inclusions of the main mineralization stage in the east ore section should be higher than that of the first mining area due to the fact that the temperature falls with the migration and evolution of fluids. In the meantime, a previous study also shows that the homogenization temperature range of the potash and silicic zone is from 249.7–293.3 ℃, with an average value of 267.2 ℃, and the homogenization temperature range of the propylitic alteration zone is from 173.2–245.3 ℃, with an average value of 203.0 ℃ (Guo et al., 2009). However, the homogenization temperature in the main mineralization stage of the east ore section ranges from 170.2 to 421.4 ℃ with an average value of 269.7 ℃ which is closer to that of potash and silicic zone rather than propylitic zone. Moreover, there is a weak potassic in the east ore section through geological observation (as shown in Fig. 2a). These results suggest that the east ore section's ore-forming fluids are similar to that of the first mining area, which, however, not migrated from the first mining area. Overall, according to the analysis in this paper, it can be inferred that the east ore section belongs to a relatively independent metallogenic system, and the ore-forming fluids in the east ore section may come from deep magma. During the migration and evolution along the NEE-trending Quanganda Fault, the temperature and salinity of fluids decrease while the density of fluids increases. Besides, the ore-forming fluids have experienced fluids immiscibility and atmospheric precipitation. However, more evidence is necessary to support this inference.

    East ore section First mining area (Yang, 2017) First mining area (Lü, 2014)
    Mineralization stages (main stage) B veins B veins B veins
    Types of fluid inclusions LV, LVH LV, LVH, LV-CO2 LV, LVH, LV-CO2
    The range of homogenization temperature (℃) 170.2–421.4 138.7–395.0 256.4–416.3
    The range of salinity (wt.%) 9.3–33.3 4.6–24.7 16.8–40.6
    Vapor-phase compositions H2O, CH4, N2 H2O, CO2, CH4 H2O, CO2, CH4
    Sources of fluids Magma water, atmospheric precipitation

    Table 3.  Comparison of fluid characteristics between the east ore section and the first mining area of the Pulang deposit

  • The microscopic photos of liquid-vapor two-phase fluid inclusions taken during the petrographic observation were digitized as vector maps. And the shape of fluid inclusions was outlined with AutoCAD. After that, the corresponding perimeter and area of each fluid inclusion were calculated according to the scale. Among them, 112 fluid inclusions in each type of veins (B veins, ED veins, and D veins) were randomly selected. The perimeters and areas of fluid inclusions in B veins, ED veins, and D veins were scattered into the log-log plot, respectively. And the scatter in each plot fitted by a straight line following the P-A model.

    As shown in Fig. 9, all the scatters in each plot fitted by a straight line with the values of R2 are greater than 0.90, indicating that the morphology of fluid inclusions formed in the same mineralization stage holds the property of self-similarity. Additionally, the DAP values of fluid inclusions in B veins, ED veins, and D veins are 1.04, 1.06, 1.10, respectively, which shows a gradually increasing trend from the early stage to the late stage of mineralization, and the shape of fluid inclusions from B veins to D veins tend to be more irregular. It is evident that the temperature gradually decreases from the center of intrusion outwards. In the main mineralization stage (B veins), the ore-forming temperature is relatively high, and the pressure is relatively stable. In this case, the quartz crystallizes slowly, showing a good crystal form that is beneficial to fluid inclusions with regular shapes such as circles and negative crystals. However, in the late stage of mineralization (D veins), the fluid has undergone a unique cooling process. Consequently, the quartz crystallizes fast, which is not conducive to forming of regular-shaped fluid inclusions. Therefore, the shapes of fluid inclusions in D veins vary, which is consistent with the observation with the microscope. This research also suggests that the P-A fractal model based on the shape of fluid inclusions can be used as potential tool to distinguish the formation stages of fluid inclusions.

    Figure 9.  Log-log plot of perimeter and area of two-phase fluid inclusions in different veins. (a) B veins; (b) ED veins; (c) D veins.

  • As has mentioned in the section of Introduction, the histogram may suffer from some subjective and objective limitations in determining the stages of ore-forming fluids. It also shows that the homogenization temperature of all veins illustrates a unimodal distribution. In contrast, the homogenization temperature of individual veins is characterized by bimodal or multimodal distribution. One possible reason for this phenomenon lies in the value of homogenization temperature interval (Lu et al., 2004). It also shows that it may be a challenging task to divide the stages of ore-forming fluids by histogram only.

    For this reason, the number-size (N-S) fractal model is introduced in this paper. According to this model, the homogenization temperature of two-phase fluid inclusions of total samples, B veins, ED veins, and D veins is scattered into the corresponding log-log plot. It is clear that the homogenization temperature of fluid inclusions in the east ore section of the Pulang deposit satisfies fractal distribution and shows four scale-invariant intervals(as shown in Figs. 10a, 10b, 10c, 10d). The results also suggest that all the formation of B veins, ED veins, and D veins may experience at least four hydrothermal activities. Specifically, the B veins may experience four stages of hydrothermal activities, i.e., high-temperature stage with the temperature greater than 370 ℃, middle-high temperature stage within the temperature range from 330 to 370 ℃, a middle-temperature stage with the temperature ranging from 235 to 330 ℃, and low-temperature stage at the temperature lower than 235 ℃ (as shown in Fig. 10b). On the other hand, the ED veins may experience four stages of hydrothermal activities, that is, high-temperature stage with the temperature greater than 290 ℃, middle-high temperature stage within the temperature range from 250 to 290 ℃, a middle-temperature stage with the temperature ranging from 200 to 250 ℃, and low-temperature stage at the temperature lower than 200 ℃ (as shown in Fig. 10c). When comparing with the B veins and ED veins, the D veins show also experience four stages of hydrothermal activities, i.e., relative high-temperature stage with the temperature greater than 315 ℃, middle-high temperature stage within the temperature range from 280 to 315 ℃, a middle-temperature stage with the temperature ranging from 220 to 280 ℃, and relatively low stage at the temperature lower than 220 ℃ (as shown in Fig. 10d). Compared with histogram, the N-S fractal model is available for a more precise description of the distribution characteristics of the ore-forming fluids' homogenization temperature. Therefore, it provides a potential tool for the stage division of ore-forming fluids.

    Figure 10.  Log-log plot of homogenization temperature of two-phase fluid inclusions in different veins. (a) all veins; (b) B veins; (c) ED veins; (d) D veins.

    The temperature intervals corresponding to different hydrothermal activities show a decreasing trend, indicating that the fluids' temperature gradually decreases during the evolution process. Moreover, from the early to the late mineralization stage, each hydrothermal activity's fractal dimension is close. For example, the fractal dimensions in the high-temperature stage of B veins, ED veins, and D veins are 15.88, 17.94, and 17.94, respectively. In general, hydrothermal activities are divided into several stages, ranging from relatively high temperature to low temperature represented by different veins. Although the formation temperature of veins may differ during the mineralization, the veins' fluid inclusions recorded the evolution process from high to low temperature. And this problem will become more complicated in mineralization with multi-stage. Therefore, the mineralization temperature cannot wholly correspond to the formation temperature of veins. The application of fractal models and microthermometer of metal minerals may provide more detailed information for mineralization.

  • The following conclusions can be drawn from this paper:

    (1) The fluids of main mineralization stage are characterized by medium-high temperature (170.2–421.4 ℃), medium-high salinity (9.3 wt.%–33.3 wt.%), and low density (0.73–1.06 g/cm3). With the migration and evolution, the temperature, salinity, and pressure of ore-forming fluids gradually decrease, while the density of fluids increases. The liquid-phase compositions mainly include H2O, and the vapor-phase compositions consist of H2O, CH4, N2, and CO2, indicating the characteristics of reducing fluids and the mixing with atmospheric precipitation.

    (2) The ore-forming fluids characteristics of the east ore section are similar to those of the first mining area, which suggests that the ore-forming fluids of the east ore section may not migrate from the first mining area. And the east ore section may be a relatively independent metallogenic system.

    (3) The shape of fluid inclusions formed by the same hydrothermal activity holds the property of self-similarity. And the P-A model presents a potential tool to distinguish the fluid inclusions formed in different mineralization stages. From the main to the late stage of mineralization, the value of DAP increases gradually, indicating that the shape of fluid inclusions becomes more irregular as the fluids evolve.

    (4) The homogenization temperatures of all veins, i.e., B veins, ED veins, and D veins satisfy fractal distribution with four scale-invariant intervals, indicating that there are at least four hydrothermal activities during the mineralization process, and all the B veins, ED veins, and D veins experienced these hydrothermal activities. Compared with histogram, the N-S fractal model is available for a more precise description of the distribution characteristics of the ore-forming fluids' homogenization temperature. Therefore, it provides a potential tool for the stage division of ore-forming fluids.

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