2019 | 72 / Special Issue | 127–143 | 12 Figs. | 3 Tabs. | www.geologia-croatica.hr Journal of the Croatian Geological Survey and the Croatian Geological Society Article history: Manuscript received April 23, 2019 Revised manuscript accepted July 17, 2019 Available online December 20, 2019 Keywords: Quartz vein type gold deposit, Fluid inclusion, δDH2O and δ18OH2O, Fluid immiscibility, Fancha, Henan province 1. INTRODUCTION The Xiaoqinling gold deposit area is an important rock gold pro- ducing areas in China (CHEN et al., 1998; FAN et al., 2000a; MAO et al., 2002; WANG et al., 2010). Although the area has been the subject of a great deal of investigative research, the way in which the gold deposits formed here is still debated. In Xiao- qinling, the primary type of deposit is quartz vein gold of which the Fancha gold is a typical example (LI et al., 1996; REN, 2012; ZHOU et al., 2014). Research on the Fancha gold deposits can therefore, not only potentially identify the metallogenic forma- tion mechanism of such deposits, but also provide critical theo- retical support for the mineral exploration of the area. Previous studies have investigated the ore-forming fluids of the gold deposits in the area (CHEN et al., 2007; FAN et al., 2003; LI et al., 2012b; MAO et al., 2002; ). The phases of the inclusions in the current gold deposits are three-phase CO2-rich, three-phase CO2-bearing, two phase aqueous, liquid, vapours, and three- phase daughter mineral-bearing inclusions (ZHAO et al., 2017; ZHOU et al., 2015). Currently the main theories on the origins of the ore-forming fluids include: (1) metamorphic fluids (WANG and ZHOU, 1996), (2) magmatic fluids (NIE et al., 2001), (3) deep mantle fluids (LU et al., 2003), and (4) mixed fluids (CHEN et al., 2007). Regarding the ore fluid evolution, there are generally two theories: (1) magmatic water gradually evolves into meteoric wa- Ore-forming fluids of the Fancha gold deposit, Lingbao, Henan Province Zhang Yu1*, Cao Yi2 and Li Hongmeng2 1 China University of Geosciences (Beijing), School of Ocean Sciences, 100083 Beijing, P.R.China; (zhangyu_cugb@foxmail.com) 2 China University of Geosciences (Beijing), School of Earth Sciences and Resources, 100083 Beijing, P.R.China doi: 10.4154/gc.2019.22 Abstract The Fancha gold deposit is a representative of the auriferous quartz vein type gold deposits in the Xiaoqingling gold district where the ore-forming fluid is one of the key controlling factors in the deposits formation, and also the main focus of ore deposit research. Here, the geological characteristics and fluid inclusions of the Fancha deposit are presented in detail, with a descrip- tion of the potential origin and evolution of the fluid and the genesis of the deposit. Field investi- gation and petrographic observation show that ore-bodies in the Fancha deposit are preferen- tially hosted in the amphibolites of the Taihua Group and are controlled by brittle fracture. Hydrothermal alteration in the Fancha deposit includes silicification, potash feldspathization, sericitization, pyritization and carbonatization. The mineral assemblage, cross-cutting and re- placement relationships indicate that the ore-forming process can be divided into quartz-K-feld- spar (I), quartz-coarse pyrite (II), quartz-fine pyrite (III), quartz-polymetallic sulfide (IV) and car- bonate stages (V). Gold is mainly precipitated in stages III and IV. Three types of fluid inclusions have been identified in the Fancha deposit, based on their characteristics at room temperature: three-phase CO2-rich, three-phase CO2-bearing and two-phase aqueous inclusions. The fluid inclusions demonstrate the modes of homogenization temperatures of 363.3°C, 325.5°C, 273°C, 258.62°C and salinities of 14.53, 9.59, 11.61, 8.03 wt% NaCl.equiv., for stages II, III, IV, and V respectively. The ore-forming fluid in the Fancha deposit belonged to the CO2-H2O-NaCl-(±CH4) fluid system with mid-low temperature and mid-low salinity. The δ18OH2O of the metallogenic flu- ids was between 1.36 and 6.28, and the δDH2O was between -87 and -53.1. Hydrogen and oxy- gen isotope data indicate that the ore-forming fluids were dominated by metamorphic fluids in the early stage, and that the metamorphic fluid gradually mixed with circulating meteoric water during the later stages. Petrographic observation and microthermometry suggest that fluid mix- ing and immiscibility obviously occurred during the main metallogenic stage. Fluid mixing and fluid immiscibility probably led to gold deposition in the Fancha deposit. ter (FAN et al., 2000a; FAN et al., 2000b), and (2) metamorphic water gradually evolves into meteoric water (QI et al., 2002). There are also various views on the ore sources from: (1) meso- zoic granites (NIE et al., 2001; ZHAO et al., 2017), (2) the Taihua group (FENG et al., 2014; LI et al., 1996), (3) mainly the centro- sphere, and a small proportion from the wall rock (LU et al., 2003), and (4) a multi-material source (CHEN et al., 2007). The placement of basic dykes in Xiaoqinling reflects the tec- tonic setting of the crustal extension. The lithology is mainly di- abase and lamprophyre, which are spatially related to the gold- bearing quartz vein (CHEN et al., 2008; LUAN et al., 1985; ZHANG et al., 2009a). Presently, in terms of the relationship be- tween these two, there is debate: LI et al. (1996) considered that the basic dykes provide a material source for gold mineralization, while CHEN et al. (1998), LI et al. (2012a), LI et al. (2012b) chal- lenged this theory. In addition, some scholars believe that the volatiles in the lamprophye mainly act as mineralizers (NI, 1994). Fluid inclusions can preserve various geological and geo- chemical information during mineralization. Therefore, conduct- ing a detailed physico-chemical and thermodynamic study of fluid inclusions can yield important data on the mineralization process, which may also inform and guide prospecting and ex- ploration (LU, 2004; MELFOS et al., 2016; ROEDDER, 1984; TASEV et al., 2018). Therefore, our main objective focused on studying the Fancha gold deposits, and in particular, thoroughly G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue128 investigating the geology, fluid inclusions and their stable isotope geochemistry. Furthermore, we explored the metallogenic mecha- nism by tracking the source of the ore-forming fluids and analyzing the fluid evolution. 2. REGIONAL GEOLOGY The Xiaoqinling area is located at the northern margin of the eastern Qinling orogenic belt (MAO et al., 2010; WANG et al., 2008). The east side of this area begins in the Zhuyang basin in Henan and ends 50km away in the west at Shanxi Huashan. The southern boundary is the Xiaohe fault, and to the north, the Tai- yao fault. The area is 7 to 15km wide, and overall it appears as an irregular strip (Fig. 1). The emergent stratum of this area is the Taihua Group (LUAN et al., 1990), which in accordance with other studies is divided into an upper and a lower basement. The upper basement is further sub-divided, from bottom to top, into the Guanyintang formation and the Huanchiyu formation (XU et al., 2009). The Guanyintang formation is dominated by biotite- plagiogneiss and plagioclase amphibole gneiss, while the Huanchiyu formation is composed of mainly marble and diopside marble. The lower basement comprises three units; from bottom to top they are the basic supracrustal rock, the Yangzhaiyu for- mation, and the Sifangou formation (FENG et al., 2014). The su- pracrustal rock is mainly composed of amphibolites and plagio- clase amphibole gneiss, most of which is in the form of inclusions in metamorphic granite. The Yangzhaiyu formation is dominated by biotite-plagiogneiss and plagioclase amphibole gneiss while the Sifangou formation is mainly biotite plagiogneiss and biotite monzogneiss. In terms of construction and development, the re- gion’s major fold structures are the Wuli Village anticline, Qishuping syncline and the Laoyacha anticline (LUAN and CHEN, 1990; MAO et al., 2005). The fault structures consist mainly of the approximately E-W trending axial folds, and the similarly trending E-W and nearly S-N trending faults. More than 500 ductile-brittle faults have been discovered in the area (HU et al., 2005; MAO et al., 2005), mostly trending E-W (ZHANG et al., 2009b). From the regional perspective, the E-W trending faults are the main ore-controlling structures. The south directed faults of the middle ore belt and the north directed faults of the northern ore belt are the main ore-controlling structures of large and medium-sized gold deposits in the region, striking several kilometres. Both the ductile shear zone and the brittle shear zone were found to contain gold-bearing quartz veins. The magmatic rocks in the area include Palaeoproterozoic granites, early Meso- zoic granites and late Mesozoic granites (GAO et al., 2012; WANG et al., 2010). Among them, the palaeoproterozoic granites are mainly the Xiaohe and Guijiayu rock masses. From east to west, the Mesozoic granites contain the Niangniangshan, Wenyu and Huashan rock masses. In addition, most gold deposits in the region developed into Palaeoproterozoic and late Mesozoic mafic dykes, with the gold-bearing quartz veins existed as a symbiotic or mutual cross-cutting complex. The lithology is mainly diabase and lamprophyre, and the lamprophyre consisted of mainly camptonite and kersantite (NI, 1994; WANG et al., 2008; WANG et al., 2010). 3. GEOLOGY The administrative division of the Fancha gold mining area be- longs to Zhuyang Town, Lingbao City, Henan Province. The ex- cavation strata in the mining area consist mainly of supracrustal rock (Ar3B), the Yangzhaiyu (Ar3yn), Sifangou (Ar3sf ), and Guanyintang formations (Ar3g) in the Taihua group. Magmatic Figure 1. Map of the research area. a) Tectonic subdivisions of China (modified from LIU et al., 2015) CCO: Central China Orogen; b) tectonic subdivisions of the Qinling Orogen. (modified from ZHOU et al., 2015); c) geological map of the Xiaoqinling gold belt (modified from ZHOU et al., 2015). G eologia C roatica Yu et al.: Ore-forming fluids of the fancha gold deposit, Lingbao, Henan Province 129 rocks exposed in the mining area include diabase porphyrite, monzonitic granite and granitic pegmatite (Fig. 2) (GAO et al., 2012; NI, 1994; WANG et al., 2008; WANG et al., 2010). Diabase porphyrite is gray-green, with a blastoporphyritic texture, blocky structure and strong sericite alteration. It is widely distributed in the eastern mining area, mainly intruded in an ap- proximately N-S orientation, and partially intruded as E-W trend- ing veins. Monzonitic granite is medium-fine grained, has a blocky structure, and is composed mainly of plagioclase, K-feldspar, quartz, biotite, a small amount of amphibole, and magnetite. It is distributed in the centre of the mining area. Granitic pegmatite is grayish white, with a blocky struc- ture. It is composed of microcline, plagioclase, quartz, and biotite and located in the southeast of the mining area. The mining area mainly developed near E-W trending duc- tile-brittle faults. The faults are divided into compression-tor- sional and tensile faults, according to their mechanical properties. The compression-torsional faults are the ore-controlling struc- ture, and the tensile faults have a destructive effect on the ore body. The mining area is located in the eastern tip of the Xiao- qinling anticline, and the anticline trends E-W with a vertical axial plane (CHEN, 2013; REN, 2012). 3.1. Characteristics of the ore body The Fancha gold deposit consists of four gold veins S902, S431, S310 and S301. This study is focused on the S902 vein (Fig. 2). The wall rocks of the S902 vein are mainly composed of striped migmatite, Palaeoproterozoic granites and plagioclase amphibole gneiss of the Taihua group. The ore body is controlled by NNE brittle faults and its maximum depth is c. 710m.The ore body strikes 280° in a SW direction, with an inclination angle of c. 40 °, an average vein thickness of 0.50m, and a plate-like or evenly veined profile (REN, 2012). 3.2. Ore Characteristics The ore minerals are pyrite, chalcopyrite, sphalerite, galena, and a small amount of natural gold. The ores are mainly quartz, K- feldspar, sericite and carbonate. Paragenetic sequences of differ- ent ore stages with corresponding mineral assemblage are shown in Fig. 3i. 3.2.1. The main ore minerals Pyrite is grayish yellow or light yellow and distributed through- out the entire mineralization process. Grain size varies greatly, mostly as fine particles but a small amount exhibits a visible cu- bic crystalline form. It often occurs as agglomerate with fine veins, or sparsely disseminated in quartz veins or mineralized altered rocks. In the early stage of mineralization, it is mostly a coarse pyrite euhedral-semi-shaped structure, with a dissemi- nated or blocky structure in milky quartz (Fig. 3a,b). Cracks of- ten occur at this stage, and the fissures can contain a small amount of gold (LU et al., 2013). During the main mineralization stage, the pyrite is mostly of medium-fine subhedral-allotriomorphic granular form, co-existing with the quartz and various metal sulfides (Fig. 3c,d). The pyrite appears as disseminated veinlets and lumps distributed on the side of the quartz vein or in the mi- cro-cracks of the rock, usually with a higher gold content. At the late stage of mineralization, pyrite is generally fine subhedral- allotriomorphic granular in form, distributed in carbonate veins or altered rocks and is associated with carbonate minerals such as calcite (Fig. 3e). The gold content is low. Gold content is general ly Figure 2. Geological sketch map of the Fancha gold deposit and geological cross section along the No.2 exploration line (modified from REN, 2012). G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue130 positively correlated with the pyrite content in quartz veins or mineralized altered rocks. Chalcopyrite is bright yellow, with a metallic lustre, and is fractured shell-like, and irregularly granular in shape (Figs. 3d,f). Usually it is found in quartz fine-grain pyrite and quartz polym- etallic sulfide stages. It forms as an agglomerate, disseminated or fine veins and is produced in quartz vein fissures or altered rocks, commonly associated with other sulfides. Galena is mid gray with a metallic lustre, and occurs mainly in quartz veins. It is very unevenly distributed, with most at the Figure 3. The main ore types and paragenetic sequences of different ore stages with corresponding mineral assemblages of the Fancha gold deposit. a) sparsely disseminated coarse pyrite in quartz; b) cataclastic coarse pyrite and fine-grained pyrite in quartz; c) fine-grained pyrite and chalcopyrite in smoky gray quartz; d) chalcopyrite encrusts fine-grained pyrite in quartz; e) veinlet pyrite + chalcopyrite + sphalerite + galena in quartz; f ) quartz + pyrite + chalcopyrite + galena in quartz-polymetallic sulfide stage; g) potassium feldspar in early stage cut by quartz coarse-grained veins; h) quartz and calcite at the end of mineralization; i) para- genetic sequences of different ore stages with corresponding mineral assemblages. Q-quartz; Py-pyrite; Ccp-chalcopyrite; Sp-sphalerite; Cal-calcite; Gn-galena; Kf-K-feldspar. G eologia C roatica Yu et al.: Ore-forming fluids of the fancha gold deposit, Lingbao, Henan Province 131 polymetallic sulfide stage. It occurs in veinlets or as mass distri- bution with a euhedral-hypidiomorphic granular structure (Fig. 3e, f). A small amount is fine-grained. It is combined with pyrite, chalcopyrite, sphalerite and quartz as minerals, and is closely re- lated to gold enrichment. Sphalerite is bright silver gray, with a metallic lustre, me- dium-fine grained structure, and is mostly associated with the quartz polymetallic sulphide stage. It is mainly observed in as- sociation with galena and net veins, as agglomerations and is dis- tributed in quartz veins (Fig. 3e). 3.2.2. Main gangue minerals Quartz is the most important gangue mineral in the ore (usually > 90%), and generally occurs in different-sized veins discontinu- ously distributed in the cracks. In the coarse-grained pyrite stage of mineralization, quartz is a milky white dense block assembly, often forming thick veins (Fig. 3a). During the fine pyrite phase, it is the colour of cigarette ash with a fine granular structure, of- ten co-existing with pyrite and chalcopyrite (Fig. 3c). It is mainly distributed in fine vein quartz at micro-cracks in the veins and is slightly more transparent during this phase. At the polymetallic sulphide stage, quartz is generally colourless, transparent, trans- lucent, and euhedral or subhedral, co-existing with polymetallic sulphides (Fig.3e). It is distributed in the early stage quartz veins. At the carbonate stage, the quartz is white and granular and as- sociated with calcite (dolomite) and pyrite. The fine veins or small clumps are scattered at the edges of, or in the altered rocks of the veins (Fig. 3g). K-feldspar is pink and belongs to a group of potash altered minerals. It is generally distributed in the form of aggregates, which are produced in large veins or dip-like forms, and often co-exists with quartz. The type of K-feldspar is microcline, and polysynthetic twins and crosshatched twins are visible under the microscope (Fig. 3g). Sericite forms microscopic scaly aggregates with directional alignment, complete cleavage and a silky lustre. It is closely re- lated to gold mineralization. Carbonate is mainly calcite and it is colourless, transparent or translucent, with complete cleavage in both directions and a low degree of crystallization. It forms as irregular veins (Fig. 3h) among other surrounding rocks with carbonate minerals and very low levels of mineralization. Through detailed ocular and microscopic observation, it can be concluded that the Fancha gold ore includes the following structures: euhedral or subhedral-granular, allotriomorphic gran- ular including, cataclastic and filling structure with a metaso- matic texture. The Fancha gold is mainly a vein-like, dissemi- nated, and agglomerate structure. 3.3. Wall rock alteration The common surrounding rock alterations in the mining area in- clude silicification, beresitization, potash feldspathization, chlo- ritization, epidotization and carbonation. Rock alteration in the area is not strong, and all kinds of alterations are distributed at the boundary of the contact zone between the gold-bearing quartz vein and wall rocks, showing characteristics of linear alteration. Silicide is the most widely developed alteration in the Fancha gold deposit, and it is disseminated in wall rocks within the fault zone as lumps and mesh-vein by metasomatism and infilling. The hy- drothermal fluid is relatively well developed in the early and mid- dle stages, and rare in the later stage. The lithology of beresitiza- tion is accompanied by mineralization, and densely disseminated as lumps and fine veins. It is particularly well developed in the early and middle stages of hydrothermal alteration and is closely associated with mineralization. Potash feldspathization often oc- curs in the early stage of mineralization, is spread in the distal rocks in disseminated or veined forms, and is occasionally ob- served in the adjacent wall rock. Chloritization and epidotization is evidenced by a pale green colour and is distributed in the wall rock in clumped and disseminated forms, and is not closely re- lated to mineralization. Carbonation is formed in the late stage of mineralization with mainly fine veins, usually of quartz car- bonate. The main minerals are calcite, dolomite and ferro-dolo- mite. 3.4. Metallogenic stage Based on comprehensive field and laboratory study, the Fancha gold deposit mineralization process can be divided into five stages recognised by different mineral assemblages (Fig. 4). Quartz-K-feldspar stage (I): early potassium-rich fluid mi- grated along the fractured zones and reacted (water-rock) with the roof and floor of the wall rocks, forming a strong planar mas- sive potassic alteration. This stage is the preparatory stage of mineralization. Quartz coarse pyrite stage (Ⅱ): ore-bearing hydrothermal fluids filled fractures of the wall rock appearing as large milky quartz veins and the ore is a coarse-grained pyrite type. The py- rite is bright yellow, and generally occurs as medium-coarse cubes or pentagonal dodecahedron with a good degree of euhe- dral form. Quartz fine-grain pyrite stage (Ⅲ): this stage is the main mineralization stage. The ore-forming hydrothermal fluids pene- trate into the large quartz vein fractures of the early stage as fine veins, characterized by fine-grained pyrite and smoky quartz. It is accompanied by a small amount of allotriomorphic chalcopy- rite deposition. Quartz-polymetallic sulfide stage (Ⅳ): the ore-bearing hy- drothermal fluids continue to fill the quartz vein fractures and are enriched in the local area by metasomatism. Sulphide ores disseminated by veinlets are typically present in this stage. The main metallic mineral assemblage is pyrite, chalcopyrite, galena, sphalerite and gold. The gangue minerals are smoky quartz, with a granular texture. A small amount of sericite is developed in the fractures. Quartz carbonate stage (V): the last stage of mineraliza- tion with no gold mineralization. The plagioclase amphibole gneiss is mainly filled with carbonate veins (calcite, dolomite), and in open space, the carbonate mineral precipitated as crystal clusters with the quartz. 4. Fluid characteristics 4.1. Samples and analytical methods All the samples were collected from the S902 vein in the Fancha gold deposit. Based on the different mineralization stages, we col- lected 5 samples at each stage for further study (Table 1). The samples were processed by the Hebei Institute of Geology and Mineral Resources Survey laboratory into 10 inclusion sections. Micro thermometric measurements were performed using a LinkamMDSG600 heating-freezing stage attached to a Leitz Or- tholux transmitted light microscope connected to a television camera and screen. The stage was calibrated using synthetic fluid inclusions. The estimated accuracy was ±0.1 ℃ at temperatures below 30 ℃ and ±1 ℃ at temperatures above 30 ℃. The warm- G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue132 ing rate was maintained at 0.2 to 5 ℃ per minute, and the heating rate was reduced to 0.2 ℃ per minute when close to phase-change conditions. Freezing experiments were performed first on all sec- tions to avoid inclusion decrepitation. The measured phase tran- sitions included the melting temperature of ice (Tm-ice), the melt- ing temperature of solid CO2 (Tm-CO2), the temperature of CO2-clathrate dissociation (Tm-cl), the homogenization tempera- ture of CO2 (Th-CO2), and the total homogenization temperature (Th). Salinities were calculated using the equations of BODNAR (1993) for aqueous fluids and COLLINS (1979) for aqueous-car- bonic fluids. Six samples were collected for each metallogenic stage. As we failed to collect suitable carbonate samples at the carbonate stage and it is not the main research focus, we did not investigate Figure 4. Characteristics of the different types of mineralization of the Fancha gold deposit. a1) - a3) Photographs of stage I, potassium feldspar vein cut by quartz coarse-grained pyrite vein in hand specimen; b1) - b3) Photographs of stage II, massive or sparsely disseminated coarse pyrite in milky white quartz, and the com- pletely filled fractures of the wall rock as veins; c1) - c3) Photographs of stage III, smoky gray quartz fine-grained pyrite veins of stage III are filled in milky white quartz fractures of stage II; d1) - d3) Photographs of stage IV, polymetallic sulfide veins fill in the fractures of milky white quartz; e1) - e3) Photographs of stage V, polymetal- lic sulfide veins cut by carbonate veins. Q-quartz; Kf-K-feldspar; Py-pyrite; Ccp- chalcopyrite; Gn- galena; Cal-calcite. G eologia C roatica Yu et al.: Ore-forming fluids of the fancha gold deposit, Lingbao, Henan Province 133 it further. The oxygen and hydrogen isotopes were analyzed on a Finnigan MAT253 mass spectrometer in the Analytical Labo- ratory, Beijing Research Institute of Uranium Geology, using the methods outlined by DING (1980). Oxygen gas was generated from the samples by a quantitative reaction with BrF5 in exter- nally heated nickel vessels. Hydrogen in the extracted water from the fluid inclusions within the quartz separates was replaced by zinc at c. 600 ℃ and released for mass spectrometry. The isotope data are reported in ppm relative to the Vienna SMOW standard for oxygen and hydrogen. Total uncertainties were estimated to be better than ±0.2‰ for δ18O and ±1‰ for δD. The oxygen iso- tope ratios of water in equilibrium with the minerals are calcu- lated using the fractionation formula reported by CLAYTON et al. (1972). 4.2. Types of fluid inclusions 4.2.1. Genetic types Based on their genesis, fluid inclusions of the Fancha gold deposit are classified as primary, secondary and pseudo secondary inclu- sions (LU, 2004). Isolated primary inclusions of suborbicular or oval shape with a diameter of 5~40μm often have a discrete dis- tribution (Fig. 5a) and are occasionally observed along the growth zoning of quartz (Fig. 5b). Linear secondary inclusions are often distributed in micro-cracks cutting through the boundary of quartz crystals, with a elongate but irregular in shape and irreg- ular shape (Fig. 5c, d). However, since they cannot represent ore fluid, they are not discussed further. Pseudo secondary inclu- sions, with compositions consistent with the primary inclusions, are elongate and formed as linear groups distributed within quartz micro-cracks during crystal growth and represent the ore- forming fluids. 4.2.2. Phase types Petrographic observations indicated that the fluid inclusions of the Fancha gold deposit have 3 main phases: three-phase CO2- bearing inclusions (VCO2 + LCO2 + LH2O, LH2O > 50%), three- phase CO2-rich inclusions (VCO2+LCO2+LH2O, LH2O<50%) and two-phase aqueous inclusions (LH2O+VH2O). Occasionally there are also vapour inclusions (VCO2 or VH2O) and liquid inclusions (LCO2 or LH2O). Three-phase CO2-bearing inclusions consisted of brine solu- tion, VCO2 and LCO2. These inclusions are widely distributed throughout the entire process of mineralization, representing the main inclusion type of the Fancha gold deposit. The VCO2+ LCO2 phase accounts for about 15 - 50% of the inclusion volume, and the ratio of VCO2 and LCO2 is approximately 5 - 35%. Such inclu- sions are often suborbicular, irregularly-shaped, or negative crys- talline with a diameter of 10 to 40μm (Fig. 6a1-a3). Three-phase CO2-rich inclusions consisted of brine solution, VCO2 and LCO2. These inclusions are mainly found in the polyme- tallic sulfide stage, and rarely in other mineralization stages. The volume of VCO2 + LCO2 accounts for c. 50 – 80% of the total volu me. The ratios of VCO2 and LCO2 are approximately 5 – 15%. Inclusions often are suborbicular or oval, and columnar crystal- line or negative crystalline. Compared with the size of the three- phase CO2-bearing inclusions, they have a slightly smaller diame- ter of usually 5 – 25μm (Fig. 6b1-b3). Two-phase aqueous inclusions are comprised of LH2O and VH2O. The vapour-liquid ratio is less than 15%, and they are of- ten elongated and suborbicular, with a diameter mostly less than 20μm (Fig. 6c1-c2). Vapour inclusions develop in all stages, showing vapour- phase at room temperature with no liquid and no significant change with increasing and decreasing temperature. They are dark black, mostly suborbicular, short cylindrical and regular shapes, with a typical diameter of 5–15μm (Fig. 6e2). Liquid inclusions at room temperature, are only a single liqu id phase (LH2O) with no vapour phase and do not change with changes in temperature. There are only a few of this type of in- Figure 5. Genetic types of fluid inclusions in the Fancha deposit. a) isolated primary inclusions in single quartz; b) a group of primary inclusions distributed along the growth zoning of quartz; c) linear secondary inclusions traverse crystals; d) linear secondary inclusions along cracks of crystal. G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue134 clusion, they are colourless, transparent and filled with aqueous solution. The general size is 5 – 10μm, mainly formed in the late stages of mineralization (Fig. 6e1, e3). In addition, microscopic observation showed that daughter mineral-bearing inclusions are found in the polymetallic sulphide stage (Fig. 6d). However, due to their rarity, they were ignored. There are a few pure LCO2 inclusions, mainly found in the metal- logenic stage. VCO2 occurs during cooling to below 20 °C. The possible mechanism is that the original homogeneous fluid is sub- jected to structural decompression or mixed with meteoric water to cause fluid boiling, so a large amount of vapour escapes and the mineral captures the inclusion. There are many “bottle-neck” shapes in the sample, probably because these inclusions have been subjected to local stress effects associated with the primary mineral after crystallization. It is noteworthy that different types (or one type with various ratios of phases) of inclusions are pre- sent in the field during the main mineralization stage, suggesting the possibility of fluid immiscibility with the ore-forming fluid (LU, 2004; ROEDDER, 1984). 4.3. Fluid inclusion assemblage The concept of the fluid inclusion assemblage (FIA) was proposed by TOURET (2001) and GOLDSTEIN (2001). FIA represents a set of the representative period inclusions captured in a certain stage. All inclusions of a FIA have similar features and they should have a substantially similar homogenization temperature (the difference does not exceed 15 ℃). However, due to frequent tectonic activity in the Xiaoqinling area, the mineralization and geological complexity means that the same FIA inclusions have a slight temperature variation. The reason for this is that after be- ing captured, the inclusions suffered varying degrees of destruc- tion and transformation (stretching or partially open), with stretch- ing typically resulting in a high temperature (CHI et al., 2008). Therefore, if the temperature difference of the inclusions in one FIA is within a 20–25℃ range, it is still considered that the inclu- sion thermometry data are valid. Robust data was not obtained for potash feldspathization (stage I) so it was excluded from fur- ther study. The distribution of inclusions and the inclusion char- acteristics of each mineralization stage are shown in Fig. 7. Figure 6. Phase types of fluid inclusions in the Fancha deposit. a1) - a2) three-phase CO2-bearing inclusions; b1) - b3) three-phase CO2-rich inclusions; c1) - c2) two- phase aqueous inclusions; d) three-phase daughter mineral-bearing inclusion; e1) aqueous inclusion; e2) vapour inclusions (CO2); e3) liquid inclusion (CO2). G eologia C roatica Yu et al.: Ore-forming fluids of the fancha gold deposit, Lingbao, Henan Province 135 Quartz coarse-pyrite stage (Ⅱ): the main inclusions are three-phase CO2-bearing inclusions, accounting for c. 70 percent of the total number of inclusions. Inclusions are distributed as large strips and large planar forms, as well as visible strips and linear discrete shapes. Their abundance can reach 80%, they have a large volume, and range from 15 to 40 μm in diameter. They are usually negatively crystalline, suborbicular, nearly elliptical and elliptical. The filling degree generally varies from 50% to 80% (Fig. 7a1-a2). A few three-phase CO2-rich inclusions were also discovered. Quartz fine-grain pyrite stage (Ⅲ): mainly composed of three-phase CO2-bearing inclusions (60%), some three-phase CO2-rich inclusions (30%) and a few two-phase aqueous inclu- sions (10%) (Fig. 7b1-b3). Primary inclusions are distributed in discrete isolates, partially banded and occurring in groups. Inclu- sion size is smaller than at stage (Ⅱ), 10–25μm in diameter. Shape morphology is suborbicular, elliptical or irregular. The fill degree is 40% to 60%. Polymetallic sulphide stage (Ⅳ): three-phase mainly CO2- rich inclusions (95%), two-phase liquid-rich inclusions (5%), very small amounts of pure liquid phase (LCO2) inclusions, and occa- sionally some three-phase daughter mineral-bearing inclusions (Fig. 7c1-c3). The inclusions are disordered, distributed in groups, and mostly a primary isolated shape. The abundance of the inclu- sions decreased slightly in comparison with the earlier stages, and the primary inclusions were negative crystalline, and subor- bicular, or elliptical. Inclusion diameter is 5–20μm, with gene- rally less than 50% infill. Carbonate stage (Ⅴ): mainly two-phase liquid-rich inclu- sions (40%), three-phase CO2-bearing inclusions (35%) and a few three-phase CO2-rich inclusions. The inclusions are sparse, dis- ordered and discretely distributed. Occasionally, small-area clus- ters are distributed in a strip. The number of inclusions is small, and nearly circular, elliptical, negative crystal, pointed, or a reg- ular shape. The inclusion volume is small, with a diameter gene- rally from 5–10μm. The fill degree is 20%–80% (Fig. 7d1-d3). Figure 7. Characteristics of FIA in quartz in different stages of mineralization. a1) - a2) three- phase CO2-bearing and three-phase CO2-rich inclusions in stage II; b1) - b3) three-phase CO2-bearing inclusions, three-phase CO2-rich and two-phase aqueous inclusions in stage III; c1)-c3) three-phase CO2-rich inclusions and two-phase aqueous inclusions in stage IV; d1) - d3) two- phase aqueous, three-phase CO2-rich and three phase CO2-bearing inclusions in stage V. G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue136 4.4. Homogenization temperature, salinity and density The microscopic temperature measurement data of fluid inclu- sions in each metallogenic stage in the Fancha gold deposit can be described as follows (Table 2). Quartz coarse-pyrite stage (Ⅱ): the homogenization tem- perature of three-phase CO2-rich inclusions is 255.4–315.1℃, with a mean of 277.5 ℃, equalizing to a homogeneous vapour phase. The homogenization temperature of three-phase CO2- bearing inclusions is 261.4–363.3 ℃, with a mean of 293.1℃, equalizing to a homogeneous liquid phase. The melting tempera- ture of the solid-phase CO2 is -60.0–-56.7℃, which is below the melting temperature of pure CO2 in the solid phase (-56.6 ℃). This indicates that in addition to CO2 inclusions, the vapour may also contain other ingredients (e.g., CH4). The melting tempera- ture of CO2 clathrate in the three-phase CO2-rich inclusion is 3.0–6.5℃. The salinity was determined from a temperature – salinity table for CO2 clathrate melting (COLLINS, 1979). The salinity is 6.54 – 11.89 wt% NaCl equiv., with an average of 9.24 wt% NaCl equiv. The CO2 clathrates of three- phase CO2 bear- ing inclusion has a melting temperature of 0.9 to 7.0 ° C. The sa- linity varies from 5.68 to 14.53 wt% NaCl equiv., with an average of 9.63 wt% NaCl equiv. The homogenization temperature of the CO2 part is 25.0 – 31.0℃. According to the known relationship between the homogenization temperature of each part and the CO2 phase density, the relative density of the CO2 phase was cal- culated as c. 0.70–0.80g/cm3. Quartz fine-pyrite phase (Ⅲ): the homogenization tem- perature of three-phase CO2-rich inclusions is 235.2–325.5℃, with a mean of 275.7℃, equalizing to a homogeneous vapour phase. The homogenization temperature of three-phase CO2- bearing inclusions is 258.5–310.3 ℃, with an average of 288.1℃, equalizing to a homogeneous liquid phase. The melting tempera- ture of the solid-phase CO2 is -60.0–56.7℃, which is below the melting temperature of pure CO2 in the solid phase (-56.6 ℃). For CO2 clathrate of three-phase CO2-rich inclusion, the melting tem- perature is 4.6 – 8.4℃. Salinity is 3.15 – 9.59 wt% NaCl equiv., with an average of 6.44 wt% NaCl equiv. The CO2 clathrates of three-phase CO2 bearing inclusion has a melting temperature of 5.7 to 8.7 ℃. The salinity is from 2.58 to 7.87 wt% NaCl equiv., (average 6.47 wt% NaCl equiv.). The homogenization tempera- ture of the CO2 part is 13.6 – 30.8℃. According to the known re- lationship between the homogenization temperature of each part and the CO2 phase density, the relative density of the CO2 phase was calculated as c. 0.60–0.82g/cm3. Some measured data for the two-phase aqueous inclusions was also obtained. The complete homogenization temperature is 182.0 – 240.9℃, (mean of 206.7 ℃). The freezing point ranges from -5.6 to -1.2 ℃. The corresponding salinity is 2.07–8.86 wt% NaCl equiv., (mean of 4.59 wt% NaCl equiv), from the table in BODNAR, (1993). Polymetallic sulfide stage (IV): the homogenization tem- perature of the three-phase CO2-rich inclusions varies from 203 – 273 ℃, (mean 245.2 ℃), equalizing to a homogeneous vapour phase. For clathrates with a different phase ratio, they have a sim- ilar homogenization temperature. The solid CO2 melting tem- perature varies from -59.1 to -56.5 C; CO2 clathrate melting tem- perature is 3.2–7.2℃. And the corresponding salinity is 5.33 to 11.61 wt% NaCl equiv., (average 8.79 wt% NaCl equiv.). The par- Table 1. Characteristics of fluid inclusion samples in different stages. Sample Stage Host mineral Description Location FX013 I Quartz Quartz coarse-grained pyrite fine vein cutting K-feldspar Adit1100 FX037 II Quartz Quartz coarse-grained pyrite veins in wall rocks 3W10, Adit1050 FX016 III Quartz Smoky quartz vein with fine-grained pyrite filling in the fractures of milky quartz 6E18, Adit1020 FX028 IV Quartz Fine-grained densely disseminated polymetallic sulphide ore W9, Adit1050 FX038 V Quartz A small amount of calcite on the quartz in the late stage 6E16, Adit1020 Table 2. Summary of microthermometric data on fluid inclusions in the Fancha gold deposit. Stage Sample Host mineral Type Content/% Tm,icm/°C Tm,cla/°C Th/°C Average Th/°C ω(NaCl)/% Average Salinity Ω(NaCl)/% Homogenization II FX037 Quartz Three-phase CO2-rich 30 3.0~6.5 255.4 ~315.1 277.5 6.54 ~11.89 9.10 vapor Three-phase CO2-bearing 70 0.9~7.0 261.4 ~363.3 293.1 5.68 ~14.53 9.63 liquid III FX016 Quartz Three phase CO2-rich 30 4.6~8.4 235.2 ~325.5 275.7 3.15 ~9.59 6.44 vapor Three-phase CO2-bearing 60 5.7~8.7 258.5 ~310.8 288.1 2.58 ~7.87 6.47 liquid Two-phase aqueous 10 -5.6~-1.2 182 ~240.9 206.7 2.07 ~8.86 4.6 liquid IV FX028 Quartz Three-phase CO2-rich 95 3.2~7.2 203 ~273 245.2 5.33 ~11.61 8.79 vapor Two-phase aqueous 5 -2.0~-0.7 238.7 ~257.6 245.4 1.23 ~3.39 2.45 liquid V FX038 Quartz Three-phase CO2-rich 40 4.6~8.4 190 ~258.6 216.9 3.15 ~9.59 7.85 vapor Three-phase CO2-bearing 35 5.6~9.4 201.3 ~243.5 226.1 1.22 ~8.03 5.79 liquid Two-phase aqueous 25 -4.8~-1.2 182.4 ~225.6 203.5 2.07 ~7.59 5.78 liquid G eologia C roatica Yu et al.: Ore-forming fluids of the fancha gold deposit, Lingbao, Henan Province 137 tial homogenization temperature is 6.1– 23.8 ℃, and thus the es- timated CO2 phase density is c. 0.83–0.97g/cm3. Only 3 two-phase aqueous inclusions, which formed in the same stage as three-phase CO2-rich inclusions, were measured, and their homogenization temperatures are 257.6 ℃, 238.7 ℃, and 240.0 ℃, with a mean of 245.4 ℃, equalizing to a homoge- neous liquid phase. However, at this stage, the homogenization temperature of the three-phase CO2-rich inclusions is concen- trated between 235 and 265 ℃, with a mean of 245.5 ℃, equali- zing to a homogeneous vapour phase. The homogenization tem- peratures of these two are similar. The freezing point of two-phase aqueous inclusions are -2.0, -0.7 and -1.6 ℃, with estimated sa- linities of 3.39, 1.23 and 2.74 wt% NaCl equiv., with a mean of 7.85 wt% NaCl equiv. 4.5. Pressure and depth To estimate pressure using inclusion mineralization we measured CO2 partial homogenization temperature Th with a micro ther- mometer. Then, according to SHEPHERD et al. (1985), the CO2 phase density (ρco2) was be determined from a graph of the cor- relation between homogenization temperature and density. To determine the salinity of the fluid, the CO2 clathrate melting tem- perature Tm, cla, co2 is initially measured, followed by measurement of the complete homogenization temperature Th of the inclusions. Since the salinity largely influences the pressure estimation, and our measured average salinity at various stages of mineralization of the Fancha gold deposit is nearly 6 wt% NaCl equiv., we uti- lized the correlation graph listed in SCHWARTZ (1989), and checked the Xco2-Vco2-ρ-Th in CO2-H2O-NaCl inclusion with Figure 8. Histograms showing homogenization temperature a) and salinity b). G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue138 6% salinity. The resulting Th and ρco2 values were used to obtain the Xco2 and Vco2. Again, utilizing the graph, we determined the minimum trapping pressure of the inclusion. In this paper, we estimated the pressure by directly choosing inclusions with a sa- linity of 6 wt% NaCl equiv. in nearly all stages. From the graph we estimated the pressure in stage (Ⅱ) at 120-180MPa; stage (Ⅲ), 85-140MPa; and stage (Ⅳ), 70-120MPa; a declining trend in ore- forming pressure. Since in our estimation process, we considered that the ore-forming fluid system is approximated as a CO2-H2O- NaCl system, and ignoring the impact of CH4, our estimation may be slightly different from the actual value. However, since the level of CH4 is rather low, its impact on our estimation is probably trivial. As the excavation stratum of the Fancha gold deposit is mainly the Taihua Group, we used a rock density of 2.7 g/cm3 in the lithostatic pressure calculations. The lithostatic pressure depth in the quartz coarse-pyrite stage varies from 4.5 to 6.7 km. In the quartz fine-pyrite stage, it is from 3.5 to 5.2 km, and from 3.5 to 4.0 km in the quartz polymetallic sulphides stage. In gene- ral, the depth decreased as mineralization progressed. 4.6. Hydrogen and oxygen isotopes Hydrogen and oxygen isotope analysis is a reliable method for determining the origin of the mineralising fluid. To determine the ore-forming fluid source of the Fancha gold deposit, samples were collected from all stages (except for the carbonate mineralization stage) for hydrogen and oxygen isotope experiments. The oxygen isotopic composition was obtained using the oxygen isotopic frac- tionation equilibrium of the mineral and water as follows: 1000lnαquartz - water =3.38×106T-2 -2.90 (200~500℃) (CLAYTON et al., 1972), where T is the formation temperature of the mineral, which is the average homogenization temperature of the inclusion. The δ18OH2O of metallogenic fluids is between 1.36 and 6.28, and the δDH2O is between -87 and -53.1 (Table 3). From stage Ⅰ to stage Ⅳ, the δDH2O initially declined and then gradually rose. 5. DISCUSSION 5.1. Diagenesis and mineralization age Using multiple techniques, much research has focused on the chronology of the Xiaoqinling gold field (GUO et al., 2009; HE et al., 2009; WANG et al., 2008; ZHANG et al., 2009a). The 40Ar/39Ar ages of biotite and sericite at the Yangzhaiyu gold de- posit area indicate two stages of gold mineralization (LI et al., 2012b), one from 134.5 to 132.3 Ma and the other from 124.3 to 123.7 Ma. In comparison, the ages of gold-bearing quartz veins at the Dongchuang gold deposit exhibit multi-stage mineraliza- tion: 142.9±2.9Ma, 132.2±2.6Ma and 128.3±6.2Ma (sericite 40Ar/39Ar age) (LI et al., 2002). REN (2012) suggested that the 40Ar/39Ar age of the Fancha gold deposit was 130.5 Ma or 120.2 Ma, based on two sericite Table 3. The O-H isotopic compositon of quartz in the Fancha gold deposit. Sample Host mineral Stage homogenization temperature/°C δDH 2 O (‰) δ18OQz(‰) δ18OH 2 O(‰) FX013 Quartz I 380 -87 10.8 6.28 FX027 Quartz II 320 -66.1 10.7 4.49 FX032 Quartz II 320 -58.5 10.5 4.29 FX015 Quartz III 270 -53.1 10.5 2.44 FX016 Quartz III 270 -54.1 11.4 3.34 FX028 Quartz IV 240 -79 10.8 1.36 Figure 9. a) XCO2 - VCO2 (40°C) - ρ - Th diagram of CO2-H2O-NaCl inclusions with a salinity of 6%; b) XCO2 - VCO2 (40°C) - P diagram of CO2-H2O-NaCl inclusions with a salinity of 6% (after SCHWARTZ, 1989). G eologia C roatica Yu et al.: Ore-forming fluids of the fancha gold deposit, Lingbao, Henan Province 139 samples. These estimates suggest formation in the early Creta- ceous, during the Yanshanian period. It is noteworthy that in Yangzhaiyu and Dongchuang, similar gold mineralization events occurred in the same period. Considering the isotopic composi- tion and micro-thermometer evidence of the fluid inclusion data, it can be concluded that the Fancha gold deposit experienced two stages of hydrothermal activity. 5.2. Metallogenic conditions 5.2.1. Geological conditions According to previous reports, the main mineral assemblages of the Fancha gold deposit are quartz-pyrite-chalcopyrite, quartz- pyrite-chalcopyrite-galena-sphalerite-gold and quartz-carbonate- pyrite, of predominantly fine-grained and metasomatic texture. Wall rock alteration mainly includes silicification, beresitization, potash feldspathization and carbonation, where beresitization is an important criterion for ore prospecting. Other studies also show that the Fancha gold deposit belongs to the Yanshanian (REN, 2012), a period when lithospheric thinning and astheno- sphere upwelling events occurred very frequently in eastern China (MAO et al., 2010; MAO et al., 2005). In the regional tec- tonic setting, extension stretch, Archean metamorphic core uplift and granite emplacement resulted in strengthening regional metamorphism, causing formation of metamorphic fluids from early plate collision and continued activity. Activated deep fluids or the early metamorphic fluid rise along the well-formed frac- ture zone, extracting minerals of the wall rock during migration. Later, they formed into hydrothermal ore and, minerals, resulting in the Taihua Group stratum. The ore-bearing hydrothermal fluid was enriched within the fractures and formed the Fancha gold deposit. 5.2.2. Metallogenic fluid properties Micro-thermometer measurements and petrographic observa- tions show that the Fancha gold mineralization fluid system ex- perienced regular changes: ore-forming fluids from the early stage of a H2O-CO2-NaCl fluid system to the CO2-H2O fluid sys- tem of the main metallogenic stage, and finally evolving into the H2O-NaCl fluid system. From stages Ⅱ to V, the average homoge- nization temperature declined from 283.6 ℃, to 273.8 ℃, to 245.5 ℃ and finally 217.7 ℃. In general, the fluid salinity was low and changed little (mean 9.24, 6.45, 8.79 and 5.78 wt% NaCl equiv., respectively). As discussed above and according to some previous reports, the Fancha ore-forming fluids show the follow- ing characteristics: 1. Low homogenization temperatures, between 180 ℃ and 360 ℃. 2. Low salinity range from 5 to 10 wt% NaCl equiv. with lit- tle variation. 3. Medium density range from 0.50 to 0.97g/cm3, with little variation. 4. Fluid mineralization pressure from 70 to 180MPa, and mineralization depth from 3.5 to 6.7km. 5. Wall rock alteration including silicification, chlorite and sericitization, and an assumed weakly acidic its ore-form- ing environment. In summary, the main metallogenic fluid of the Fancha gold deposit is a low-temperature, low-salinity, medium-density NaCl-H2O-CO2 fluid system. The mineralization environment had low pressure, and was weakly acidic. 5.2.3. Source of the metallogenic fluid The source of the metallogenic fluid is inferred from the hydro- gen and oxygen isotope composition of the fluid inclusions. The isotope data of the Fancha gold deposit in various stages of mine- ralization were mapped on the δ18OH2O–δDH2O diagram (Fig. 10). At only one stage (the quartz feldspar stage), does the data point fall below the primary magmatic water, while for the other stages, namely quartz coarse pyrite and quartz fine pyrite stages, all four data points were located near the left boundary of metamorphic water. The data point of stage (Ⅳ) is below the low left bound- ary of metamorphic water indicating that the metallogenic fluid was mainly metamorphic water (Fig 10). There is a trend towards the meteoric water line, indicating the addition of meteoric fluid. Furthermore, according to the discussion above, the Fancha gold metallogenic fluids are characterized by low temperature, low salinity and high CO2 levels, in contrast to typical magmatic hydrothermal fluids (high temperature and high salinity). Thus, we suggest that the metallogenic fluid may be hydrothermal fluid of metamorphic origin. In summary, at the early stages of mineralization, metallo- genic fluids were mainly metamorphic fluids, but during the main mineralization stage, the fluid was a mix of metamorphic hydro- thermal fluids and meteoric fluids. At the final stage, the fluid is mostly meteoric water. 5.3. Mineralization process 5.3.1. The evolution of the metallogenic fluids During the quartz coarse-grained pyrite stage, there were prima- rily three-phase CO2-bearing inclusions and some three-phase CO2-rich inclusions. The inclusions discovered at the quartz fine- grained pyrite stage were mostly three-phase CO2-rich inclu- sions, three-phase CO2-bearing inclusions and two-phase aque- ous inclusions. Petrographic observation shows that three-phase CO2-bearing inclusions with a different vapour-liquid ratio co- existed with the two-phase aqueous inclusions in this stage (Fig. 7b1-b3). During the polymetallic sulphides stage, three-phase CO2-rich inclusions were dominant, and some other visible inclu- sions or the same type of inclusions with different vapour-liquid ratios were also observed in the same field of view. The homoge- nization temperature of inclusions was similar. The fact that in- clusions with different gas-liquid ratios have a similar homogeni- zation temperature suggests that in the primary mineralization Figure 10. δDH2O(‰)-δ18OH2O(‰)diagram of the Fancha gold deposit (after TAYLOR, 1974). G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue140 stage, the inclusions were captured in an uneven state (LU, 2004, 2008, 2014). The inclusions in the carbonate stage are generally small and sparsely distributed, and mainly composed of two- phase aqueous inclusions and a few three-phase CO2-bearing in- clusions. Occasionally three-phase CO2-rich inclusions were also observed. Evidence above reflects both the initial metallogenic fluid of NaCl-H2O-CO2 injected along the fracture, and the sub- sequent mixing of meteoric fluid leading to a temperature drop. Furthermore, the pressure was reduced due to changes in the tec- tonic environment. Consequently, fluid immiscibility occurred, characterized by CO2 escaping, and the whole system evolved into a NaCl-H2O and H2O-CO2 fluid system. As can be seen from the salinity/temperature graph (Fig. 11), the inclusion temperatures overlapped and the salinity showed a declining trend. At various stages, mineral compositions repre- sent both different fluid temperatures and crystallized mineral compositions. The results also show that declining salinity has a positive effect on the crystallization of the minerals. In addition, from the microscopic temperature measurement, we observed combinations of the same salinity but different temperatures, or of different salinities but the same temperature. At the quartz fine-grain pyrite stage, the quartz polymetallic sulfide stage, and the main metallogenic stage, the salinity data are clearly sepa- rated into low and high salinity groups, suggesting immiscibility of the ore-forming fluids. That, together with the hydrogen and oxygen isotope results, suggests that in the early stage, fluids were metamorphic and hydrothermal, of low temperature, and high salinity. In the primary mineralization stage, the main fluids were metamorphic hydrothermal fluid mixed with meteoric water. In the late mineralization stage, the main fluids were meteoric water of low-temperature and low-salinity. 5.3.2. The gold mineralization mechanism Boiling and fluid immiscibility are considered to be the main gold precipitation mechanisms in many deposits. Boiling should have the following indicators: (1) explosive breccia exists in ore bo- dies; (2) gas-liquid ratio and homogeneous temperature of inclu- sions vary greatly in the same stage (HAYBA et al., 1986). There is no obvious sign of boiling in the Fancha gold deposit, and the existence of multi-stage fluids can be seen in field observations. As discussed earlier, the homogenization temperature of the fluid inclusions in the deposit is low and has shown little change. Therefore, this paper explains the enrichment of gold in the de- posit from the point of view of fluid immiscibility. Figure 11. Salinity-homogenization temperature diagram of fluid inclusions in the Fancha gold deposit. Figure 12. Schematic diagram of metallogenic model of the Fancha gold deposit. G eologia C roatica Yu et al.: Ore-forming fluids of the fancha gold deposit, Lingbao, Henan Province 141 The coexistence of two or more fluids in a system is called the immiscibility of fluids (LU, 2004; ROEDDER, 1992). Schol- ars discovered the mineralization of hydrothermal gold deposits is closely related to the immiscibility of H2O-CO2-NaCl fluid (CRAW, 1992; HU et al., 2005; LU, 2014). Immiscibility causes a phase separation of the single fluid phase, breaking the balance of the original fluid system, resulting in precipitation of valuable components. Microscopic observation of the Fancha gold deposit inclu- sions shows a group of primary inclusions (formed simultane- ously at a single mineral, including two-phase aqueous inclu- sions, three-phase CO2-bearing inclusions and three-phase CO2-rich inclusions) can co-exist in the same field of view, al- though the filling degree is very different. They formed a con- tinuous series, and occasionally there are daughter mineral-bear- ing inclusions. The above phenomenon illustrates the petrographic characteristics of fluid immiscibility. At the primary mineralization stage, there are two-phase CO2-bearing inclusions and three-phase CO2-rich inclusions with different vapour-liquid ratios. While these two types of inclusions have different means of homogenization, their homogenization temperatures are similar. For the polymetallic sulfide stage, the homogenization temperature of three two-phase aqueous inclu- sions is 257.6 ℃, 238.7 ℃ and 240.0 ℃, with a mean of 245.4 ℃, equalizing to a liquid phase. The homogenization temperature of two-phase CO2-rich inclusions ranges from 235 to 265 ℃, with an average of 245.5℃, equalizing to a vapour phase. These two inclusions formed at the same time but with different end-mem- bers, and with similar homogenization temperatures but different means of homogenization. Considering both the definition of im- miscible inclusions and the microscopic features of the observed inclusions, it can be concluded that they are immiscible inclu- sions. Fluid immiscibility is the main metallogenic mechanism of the Fancha gold deposit. Furthermore, H-O isotope mapping (Fig. 10) and the homog- enization temperature-salinity data (Fig. 11), suggest that metal- logenic fluids in the primary mineralization stage were mixed significantly with meteoric water. Therefore, the mixture of fluids may also be one of the metallogenic mechanisms for the Fancha gold deposit. 5.3.3. The metallogenic process Geological and mineralogical data presented here support a pre- liminary idealized fluid-based mineralization model for the Fan- cha gold deposit. The Fancha gold deposit is located in the North China Craton. The collision of the Yangtze and North China Cratons occurred 245–210Ma ago (AMES et al., 1993; LI et al., 2011). At the end of the mid-Triassic, the strong compressional environment led to dif- ferent degrees of metamorphic deformation of the strata. Severe dehydration of a large number of water-bearing minerals resulted in the formation of metamorphic hydrothermal fluid with K, Na and Si, and various volatiles. The metallogenic hydrothermal fluid rose along fractures, accompanied by a potash alteration that is closely related to the gold-bearing quartz veins (stage Ⅰ). With the Qinling Orogenic Belt thrust from north to south, the strata experienced varying degrees of deterioration and de- formation, and metamorphic fluids were formed once again (CHEN et al., 1998; MAO et al., 2005). As the temperature de- clined in the high-pressure ductile shear environment, metamor- phic fluids rich in CO2 and other volatiles formed large, weakly mineralized milky quartz veins in some parts (stage Ⅱ). Fluid inclusions were mainly highly filled three-phase CO2-bearing in- clusions and some three-phase CO2-rich inclusions. Large quartz veins that formed early, developed a series of small fractures due to tectonic stress. Ore-bearing metamorphic hydrothermal fluid and some meteoric water filled the fractures, forming fine-pyrite smoky quartz veins (stage Ⅲ). In this stage, the main inclusions are three-phase CO2-rich inclusions, three- phase CO2-bearing inclusions and two-phase aqueous inclusions. Fluid mixing may have occurred in this stage. In the late mineralization stage, a small amount of metamor- phic fluid and meteoric water mixed and filled the fractures of the silica fine-pyrite veins, forming polymetallic sulphide veins (stage Ⅳ). At this stage, the main inclusions were three-phase CO2-rich inclusions and two-phase aqueous inclusions; different types of inclusions but with similar homogenization temperature. For the same type of inclusions, the gas-liquid ratio continuously changes. At this stage, the tectonic environment altered from compressive to tensile, and the decompression caused the origi- nal fluid to decompress and potentially boil. In addition, the mix- ture of meteoric water caused the temperature of the original high-temperature metallogenic fluid to decline, resulting in im- miscibility of the fluid. The metallogenic fluid is separated from the NaCl-H2O-CO2 fluid system into a H2O-NaCl and CO2-H2O fluid system, with CO2 acting as a pH buffer in the metallogenic fluid. The metallogenic elements are in a weak acid environment. The Au-S complex had the highest solubility (LU, 2008), then CO2 escape caused Au to precipitate from the fluid. The immis- cibility caused the metallogenic material to precipitate and be enriched at favourable locations. As the meteoric water continued mixing, the fluid properties changed further. The temperature and pressure continued to de- cline, associated with further escape of CO2. In the late minerali- zation stage, meteoric water formed some weakly developed car- bonate veins (stage Ⅴ). 6. CONCLUSIONS Both detailed field investigations and laboratory research were undertaken on the Fancha gold deposit. The geological charac- teristics of the deposit were identified and insights into the fluid inclusions at various stages of mineralization in terms of petrog- raphy, thermometry and stable isotope geochemistry were ob- tained. The main conclusions are: 1. The Fancha gold deposit is a typical quartz vein type gold deposit in the Xiaoqinling gold field. The main ore minerals are pyrite, chalcopyrite, native gold, sphalerite and galena, gangue minerals include quartz and calcite. According to the combination of minerals and vein interpenetration, the metallogenic process can be divided into five metallogenic stages: the quartz-K-feldspar (I), quartz-coarse pyrite (KAGAMI et al., 1995), quartz-fine pyrite (III), quartz-polymetallic sulfide (IV) and carbonate stages (V). Gold was mainly precipitated in stages III and IV. 2. The petrographic observations of fluid inclusions show that in the Fancha gold deposit, there are mainly three-phase CO2- bearing, three-phase CO2-rich and two-phase aqueous inclusions. Regarding the composition of inclusions, there are mainly three- phase CO2-bearing and a few three-phase CO2-rich inclusions in stage Ⅱ; mainly three-phase CO2-bearing inclusions, two-phase aqueous inclusions and a small amount of three-phase CO2-rish inclusions in stage Ⅲ; in stage IV, the main types are three-phase CO2-rich and a few two-phase aqueous inclusions; for stage V the main ones are two-phase aqueous, a small number of two- phase CO2-bearing and three-phase CO2-rich inclusions. G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue142 3. The fluid inclusions temperature data show that metallo- genic fluids are generally of low temperature and salinity, fea- tures of CO2-rich metamorphic fluids. In the early mineralization stage, the main fluids were metamorphic water, while in the pri- mary mineralization stage, the main fluids were mixtures of met- amorphic and meteoric water. In the late stage, most fluids were meteoric water. 4. 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