Mineralizing fluids and their O, H and C isotopic compositions

Study of inclusions in the Chahansara gold deposit, the main minerals are calcite and quartz, of which quartz is the sulfide-bearing quartz veinlets in the ore, and calcite is the calcite veinlets in the late non-mineralized perforated ore. In the present work, the petrographic observation of inclusions, homogeneous temperature measurement, laser Raman composition experiment, estimation of fluid salinity, density, pressure, and finally analysis of the evolution of physicochemical conditions of mineralizing fluids and mineralization were completed for the quartz veins and calcite veins.

The Chahansara gold mine is a typical tectonically fractured altered rock-type gold deposit, with relatively undeveloped quartz veins, sulfide-quartz veins, and carbonate veins. The samples for this study were taken from the gold ores in the pit, quarry and drill cores of the II mine section. Five samples of mineralized silicified siltstone and diorite were collected from the Ⅱ ore section, in which quartz fine veins of about 0.8-1.0 cm filled between diorite and silicified siltstone, and the quartz veins contain pyrite and polymetallic sulphides; and five samples of webbed calcite veins of perforated altered-rock-type ore were collected, in which calcite crystals are coarse and basically no sulphide mineralization is seen, which may be the formation of the late stage of mineralization. Inclusion slices polished on both sides were ground for petrographic studies and microtemperature work. The microscopic observation of the morphology and petrographic characteristics of the fluid inclusions at room temperature was done in the Laboratory of Resource Exploration, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing).

The quartz veins in the ore contain a large number of fluid inclusions, which are oval, round, elongated, polygonal and irregular in shape. According to the composition of fluid inclusions at room temperature and the characteristics of the phases present at room temperature (Figure 3-68), the inclusions in the quartz of the Chakhanzara gold ore are classified into three types: ① gas-liquid two-phase inclusions (Ⅰ), composed of two phases of brine solution and aqueous air bubbles; ② CO2-rich three-phase inclusions (Ⅱ), composed of brine solution, liquid-phase CO2, and gas-phase CO2, which may contain other volatilized fractions; ③ liquid-phase-rich inclusions ( Ⅲ), an aqueous solution inclusions, only a single liquid phase exists.

Type I inclusions are widely distributed, the main type of inclusions in the deposit, isolated output or distribution along the growth zone, the size of between 2 to 14 μm, gas-liquid ratio of between 0.05 to 0.8, room temperature is often gas-liquid two-phase, liquid-phase colorless and transparent, the gas-phase is transparent, but with a slight pinkish color. Type II inclusions are less common, and are three-phase at room temperature, with the outermost layer being transparent liquid-phase brine containing liquid CO2, and the innermost layer being CO2 bubbles, with both CO2 phases being dark in color. Most of the type III inclusions are secondary inclusions, a single liquid phase, no bubbles developed therein, elliptical in shape, size between 2 and 5 μm, smaller in volume, mostly distributed along the main mineral cleavage in a linear fashion, and colorless and transparent at room temperature.

Calcite inclusions in the development, most of the gas-liquid two-phase inclusions, often more than one *** with the output, CO2-rich gas-liquid inclusions and pure CO2 inclusions are very rare, often isolated and scattered, and most often secondary inclusions along the healing line of the distribution of the band. The shape of inclusions is more regular, and can be seen round and oval inclusions. The fluid inclusions are large, mostly less than 5 μm × 5 μm, and the largest can reach 10 μm × 14 μm. Primary inclusions can be classified into three categories: Ⅰ for gas-liquid two-phase aqueous solution inclusions, consisting of brine solution and gas bubbles, the gas-phase filling degree is less than 30% and the boundaries between the two phases are clear, inclusions of bubbles are light pink while the liquid phase is transparent and colorless, and the gas-rich inclusions are light pink due to the larger gas-phase filling degree (Figs. 3-69A, B, C, D, and D, Figure 3). 69A,B,C,D); Class II inclusions are CO2-rich three-phase inclusions, consisting of brine solution, liquid-phase CO2 and gas-phase CO2, and may contain other volatile compounds, with sizes ranging from 2 μm×4 μm to 8 μm×8 μm, and gas-phase filling degrees ranging from 30% to 70%, and the liquid-phase CO2 and gas-phase CO2 are pale pink in color (Fig. 3-69E); Class III inclusions are CO2-rich two-phase inclusions, consisting of liquid-phase CO2 and liquid-phase CO2, with liquid-phase bubbles being pale pink in color, and liquid-phase CO2 being transparent and colorless. Class III inclusions are CO2 two-phase inclusions, composed of liquid-phase CO2 and gas-phase CO2, and contains a small amount of other volatile components, pure CO2 gas-liquid two-phase inclusions in the microscopic temperature is usually lower than 31.1 ℃, indicating that its composition is dominated by CO2, the size of the 2 μm × 3 μm to 6 μm × 8 μm, the gas-phase filling degree of 20% to 100%, irregular shape gas-liquid CO2 was dark green (Figure 3-69F).

Figure 3-68 Photographs of fluid inclusions in quartz from the Chahansara gold mine

Figure 3-69 Photographs of fluid inclusions in calcite from the Chahansara gold mine

Microtemperature measurements of fluid inclusions at room temperature were done in the Laboratory of Resource Exploration, State Key Laboratory of Geological Processes and Mineral Resources of the China University of Geosciences (Beijing). The laboratory temperature measurement instrument used is Linkam600 hot and cold stage, whose temperature control range is -196℃～600℃, precision is 0.1℃, and temperature rise and fall rate is 0.1～130℃/min. The focus of the temperature measurement is completely homogeneous thermometry (Th) and freezing point (TmIce) for Class I gas-liquid two-phase inclusions in quartz, and completely homogeneous thermometry (Th) and freezing point (TmIce) for Class II CO2-rich three-phase inclusions. CO2 three-phase inclusions were tested for complete homogeneous thermometry (ThTOT), cage disappearance temperature (TmClath), and CO2 gas-liquid phase homogeneous temperature (ThCO2), and secondly, the above method was applied to class I and class II of calcite, and the test of gas-liquid phase homogeneous temperature (Th) was performed for all three classes.

The fully homogeneous temperature range of fluid inclusions in quartz is 142℃~399℃, with the main peaks at 180℃~220℃ and 260℃~340℃ (Figure 3-70), and the fully homogeneous temperature selected for calculating δ18OH2O in quartz is 270℃. The completely homogeneous temperature of type I inclusions is 142°C to 391°C, and the freezing point is -4.9°C to -1.0°C. Type II inclusions had a completely homogeneous temperature of 288°C to 399°C, a CO2 solid phase out melting temperature of -60.0°C to -59.7°C, a CO2-5.75H2O cage disappearance temperature of 8.8°C to 9.4°C, and a CO2 phase homogeneous temperature of 15°C to 21°C (Figure 3-71). Wang Juri et al. (1995) on the Tiangeer gold belt in different sections of pyrite burst method of temperature measurement results show that the mineralization temperature range of 170 ℃ ~ 410 ℃, Chen Yanjing et al. (1998) on the Wangfeng gold deposits fluid inclusions homogeneous temperature of the study shows that the mineralization temperature range of 240 ℃ ~ 360 ℃, and Chahanshara gold ore mineralization homogeneous temperature peak is similar.

Figure 3-70 Completely homogeneous temperature of fluid inclusions in quartz of Chahansara gold deposit

Figure 3-71 Histogram of microscopic temperature measurements of freezing point, partial homogeneity, cages, and solid-phase initial dissolution of fluid inclusions in Chahansara gold deposit

The temperature range of the completely homogeneous temperature of fluid inclusions in calcite is from 118℃ to 375℃, with the main peaks at 150℃ to 180℃, 240°C～300°C (Figure 3-72). The completely homogeneous temperature of type I inclusions ranges from 142°C to 391°C, and the freezing point is -4.2°C to -0.6°C. The freezing point is -4.2°C to -0.6°C. Type II inclusions completely homogeneous temperature of 297 ° C to 341 ° C, CO2 solid phase out of the melting temperature of -59.2 ° C to -59.0 ° C, CO2-5.75H2O cage disappearance temperature of 8.5 ° C to 9.5 ° C, CO2 phase homogeneity of 17.9 ° C to 27.0 ° C temperature. The CO2 complete homogenization temperature of type III inclusions was 15.5°C to 27.0°C, and the CO2 solid phase dissolution temperature was -60.0°C to -59.5°C.

Figure 3-72 Completely homogeneous temperature of fluid inclusions in calcite from the Chahansara gold deposit

In the present study, the laser Raman composition of fluid inclusions was tested for quartz veins in the Chahansara gold ore and calcite veins in the late interspersed ore, and the results are shown in Fig. 3-73.

Figure 3-73 Composition of quartz fluid inclusions in the ore of Chahansara gold mine

Liu Yangjie et al. (1994), Chen Yanjing et al. (1998) studied the fluid inclusion composition of Wangfeng and Sazhidala tectonically fractured altered rock-type gold ores in the neighboring Yilianhabirga tectonic zone, and the gas-phase composition of fluid inclusions mainly consisted of H2O, CO2, CH4, CO, H2 and N2. The liquid phase composition of fluid inclusions is mainly K+, Na+, SO2-4, followed by Cl-, F-, and more SO2-4 in the mineralized fluid indicates the existence of (HS)-. The compositions in the quartz fluid inclusions of Chahansara gold mine are mainly CO2-3, SO2, CO2, CH4, H2S and N2, which are similar to the compositions of the fluid inclusions of the above mentioned deposits, reflecting that the mineralizing fluid is a weakly reducing fluid, and at the same time, N2 is also found in the inclusions, which may indicate that the fluid is not of a single source, and there is a mixing of other sources of fluids.The presence of H2S indicates that the gold in the gold hydrothermal fluid may mainly migrate in the form of sulfur complexes, which is similar to the inclusions composition of most tectonically fractured altered rock type gold deposits in China.

Based on the results of microtemperature measurement of fluid inclusions combined with the relevant formulas summarized by previous authors, the salinity, density and homogeneous pressure of inclusions in the mineralized fluid samples can be calculated to understand the physicochemical environment of mineralization.

Salinity of inclusions

For type I gas-liquid two-phase inclusions, the salinity formula summarized by Liu Bin et al. (1999) is used:

S=0.00+1.78t-0.0442t2+0.000557t3 (0-23.3% NaCl solution)

In the formula: S is the salinity (%), t is the temperature at which the freezing point is lowered (°C)

For type II CO2-rich three-phase inclusions, the value of the cages dissolution temperature Tc was calculated using the formula of Bozzo et al. (1973):

S=15.52022-1.02342t-0.05286 t2(-9.6℃≤t≤10℃)

In the formula:S is the salinity (%), t is the dissolution temperature of CO2 hydrate (℃)

The freezing point of type I inclusions in the quartz veins of the ore is from -4.9℃ to -1.0℃, and the salinity is from 2.24% to 7.73%, type II inclusions CO2-5.75H2O cage disappearance temperature of 8.8 ℃ ~ 9.4 ℃, salinity of 1.22% ~ 2.39%; late non-mineralized calcite veins in the type I inclusions freezing point of -4.2 ℃ ~ -0.6 ℃, salinity of 1.05% ~ 6.74%, type II inclusions CO2-5.75H2O cage disappearance temperature is 8.5℃～9.5℃, salinity is 1.02%～2.96%.

Density of inclusions

For type I gas-liquid two-phase inclusions, the empirical formula summarizing the density of inclusions in aqueous solution according to Liu Bin et al.(1999)

ρ=A+Bt+Ct2 (salinity S between 1% and 30%)

In which:ρ is the density of saline solution (g/cm3), t is the homogeneous temperature (℃). a, b, c are functions of salinity

Prediction of copper-molybdenum-gold mineralization associated with porphyry along the Lelisgauer-Dabat in the Western Tien Shan

For type II CO2-rich three-phase inclusions, the formula for calculating the total density of the fluid in the inclusions was proposed according to Sterner et al. (1991):

ρ=0.999839×(1000+58.4428×m)/{ 1000+0.999839×(12.43×m+3.07×m1.5-0.02×m2)

+5.2777×10-5×tc-1.0113×10-5×tc2+ 9.3537×10-8×tc3}

Where:ρ is the density of the aqueous solution phase at CO2 gas-liquid homogenization (g/cm3), m is the mass molar concentration of NaCl in the aqueous solution, and tc is the temperature of CO2 gas-liquid homogenization (°C).

Type III pure CO2 inclusions fluid density, calculated with reference to Gong Qingjie (2004) Geofluid1.0 software

The density of type I inclusions in the ore quartz veins is 0.669～0.983g/cm3, and the density of type II inclusions is 0.67～0.926g/cm3; the density of type I inclusions in the late ore-free calcite veins is 0.669 ~0.983g/cm3, and the density of type II inclusions is 0.858-0.924g/cm3 and 0.178-0.815g/cm3.

Inclusion pressure

For type I gas-liquid two-phase inclusions, the homogeneous pressure is calculated using the summarized formula of Liu Bin et al. (1999):

Western Tien Shan Leresgauer-Dabat area associated with the Porphyry-related copper-molybdenum-gold mineral prediction

In the formula:w=(TH2O+0.01)2-2.937×105;Y=(647.27-TH2O)1.25;em=ln10;z=TH2O+0.01;TH2O=exp[lnT/(A+B×T )],

where

A=1+5.93582×10-6×m-5.19386×10-5×m2+1.23156×10-5×m3

B=m×( 1.1542×10-6+1.4125×10-7×m-1.92476×10-8×m2-1.70717×10-9×m3+ 1.0539×10-10×m4); E0=12.50849, e1=-4616.913, e2=3.193455×10-4, e3=1.1965×10-11, e4= -1.013137 × 10-2, e5 = -5.7148 × 10-3; T is the homogeneous temperature (K) and m is the salinity (mass molar concentration), which is related to the mass percent as:m = 1000 × S ÷ 58.4428 ÷ (100-S), and S is salinity (%).

For type II CO2-rich three-phase inclusions, the fluid inclusion homogeneous pressure was approximated according to Gong Qingjie (2004) Geofluid 1.0 software.

The type III pure CO2 inclusions homogeneous temperature is calculated with reference to Gong Qingjie (2004) Geofluid1.0 software. The mean pressure of type I inclusions in the ore quartz veins is 7.09×105～163.55×105Pa, and the mean pressure of type II inclusions is 1870.29×105～2407.83×105Pa; the mean pressure of type I in the late ore-bearing calcite veins is 1.94×105～215.55×105Pa, and the mean pressure of type II inclusions is 1976×105～2031.46×105Pa, and the mean pressure of type III pure CO2 inclusions is 1.94×105～215.55×105Pa. 105～2031.46×105Pa, and the mean pressure of type Ⅲ inclusions is 51.5×105～67.3×105Pa.

The H and O isotope composition of quartz monomineral samples was analyzed in the Isotope Geology and Geology Open Research Laboratory of the Ministry of Land and Resources of the People's Republic of China.

The H isotope analysis was carried out on the fluid inclusions in the quartz, and the fluid inclusions in the quartz were firstly de-aerated at 150℃ for more than 4h, to remove the fluid in the minerals. H isotope analysis of quartz fluid inclusions was carried out by firstly degassing at 150℃ under vacuum for more than 4h to remove adsorbed water and secondary fluid inclusions in the minerals, then extracting H2O from primary fluid inclusions by heating and bursting at 200℃~350℃, and then reacting with Zn for 30min at 400℃ to produce H2, and finally determining the isotope ratios on a MAT-251EM mass spectrometer. δ18D was calculated by using V-SMOW with an accuracy of ±2%. For quartz minerals, mineral oxygen was extracted by reaction between BrF5 and quartz samples in a vacuum at 500°C and converted to CO2 gas by combustion with a scorched graphite rod, and the O isotopic compositions were analyzed on a MAT-253 mass spectrometer; δ18O was calculated using SMOW as a standard with an accuracy of ±0.2‰; the δ18OH2O value of water in the equilibrium fluid with the quartz was determined from the quartz δ18O based on 1000lnα quartz-water = 3.38 × 106/T2-3.4 (Zheng Yongfei and Chen Jiangfeng, 2000). The H and O isotopic compositions of quartz and water in the associated mineralizing fluids are shown in Table 3-29.

Table 3-28 Estimated micrometry and physicochemical parameters of fluid inclusions at the Chahansala gold mine, Xitianshan, Xinjiang

Analysis of C and O isotopic compositions of calcite samples from the ore in the late stages of mineralization was completed at the State Key Laboratory of Geological Processes and Mineral Resources. The analyses were performed using an IRMS (Isoprime) instrument, and δ18C and δ18O were calculated using the V-PDB and V-SMOW standards, respectively, both with an accuracy of ±0.2‰. The analytical results are shown in Table 3-29.

Table 3-29 Carbon, hydrogen and oxygen isotope compositions of Chakhanzara Gold Mine, Xitianshan, Xinjiang

Note:δ18OH2O is calculated based on 1000lnα(quartz-water)=3.38×106/T2-3.4 (T=270℃); quartz samples were analyzed by the Isotope and Geology Open Research Laboratory of the Ministry of Land and Resources of China; quartz samples were analyzed by the Isotope and Geology Open Research Laboratory of the Ministry of Land and Resources of China; δ18O was calculated by V-PDB and V-SMOW standards, respectively. Geology Open Research Laboratory, Ministry of Land and Resources; calcite samples were analyzed by the State Key Laboratory of Geological Processes and Mineral Resources.