Jinshan Gold Mine in Dexing, Jiangxi Province
Wherein R2+ represents Fe2+ or Mg2+, R3+ represents Al3+ or Fe3+, ◇represents the number of holes in the octahedral position of chlorite, and x represents Si4+ or Al3+ coordinated by tetrahedron.
The composition of chlorite can reflect the physical and chemical characteristics of chlorite during crystallization, such as temperature (Cathelineau et al.,1985; Cathelineau, 1988), oxygen fugacity, sulfur fugacity (Bryndzia et al., 1987) and chemical composition of the whole rock (Bevins et al.,1988; DeCaritat et al., 1993), therefore, the composition of chlorite is of great significance to the study of hydrothermal alteration mechanism, geological conditions related to temperature (such as geothermal) and physical and chemical parameters of the environment. In this section, the chemical composition of chlorite in Jinshan altered mylonite, phyllite and gold-bearing quartz vein is studied, and the hydrothermal alteration mechanism and physical and chemical conditions of water-rock reaction in Jinshan Gold Mine are discussed.
1. Analysis method and test results of chemical composition of chlorite.
Due to the fine particles and complex structure of chlorite, it is difficult to analyze the composition of chlorite in rocks by conventional electron probe technique (Peacor,1992; Warren et al., 1992), especially the lining, inclusion, mixed layer structure and complex * * * relationship of minerals, will cause errors in the analysis of chlorite composition by standard electron probe technology or conventional methods. Therefore, (Na2O+K2O+CaO) < 0.5% is the standard to judge whether it meets the requirements of chlorite electron probe test results. If (Na2O+K2O+Cao) > 0.5%, it means that the components of chlorite are mixed (Foster,1962; Hillier et al., 199 1).
The chemical composition of chlorite in this book was tested by JE-OLJXA-8800 M electron probe in the State Key Laboratory of Metallogenic Mechanism of Endogenous Metal Deposits of Nanjing University. The test conditions are: accelerating voltage 15kV and current1×10-8a; The standard samples used are mineral standard samples, timely silicon standard samples, hematite iron standard samples, fayalite manganese standard samples and amphibole standard samples from the National Bureau of Standards. According to the results of electron probe analysis of all chlorite particles, the structural formula of chlorite was calculated with 28 oxygen atoms as the standard. Because the electron probe can't detect Fe3+, iron ions are treated as Fe2+ in the calculation of chlorite structural formula. The content of Fe3+ ion in low-grade chlorite is very small, and its existence has little effect on the composition comparison between chlorites.
This section analyzes the chemical composition of chlorite in phyllite, altered mylonite, altered ultramylonite and chloritization gold-bearing quartz vein, and the results are shown in Table 6-3. As can be seen from Table 6-3, the chemical composition of different types of chlorite has certain changes, the most obvious ones are FeO (20.75% ~ 34.70%) and MgO (7.13% ~15.91%). The FeO and MnO contents of chlorite in chloritization phyllite are lower than those of altered mylonite, while the Al2O3, TiO2 and MgO contents are higher than those of altered mylonite. The contents of Na2O and K2O are basically the same.
Second, the composition characteristics of chlorite
Figure 6-4 Classification Diagram of Chlorite (according to Hey, 1952)
Classification of chlorite
The diagram of Si content and Fe/(Fe+Mg) ratio in chlorite (Figure 6-4) (Hey, 1954) is often used for the classification and naming of chlorite. The ratios of Si Ⅳ and Fe/(Fe+Mg) of chlorite of different rock types in Jinshan Gold Mine range from 5.46 to 5.91and from 0.42 to 0.71respectively (Table 6-3). As can be seen from Figure 6-4, chlorite mainly consists of three types: ① peristalsis chlorite; ② Fe-Mg chlorite; ③ Compact chlorite. Metamorphic genesis of Jinshan gold deposit The chlorite in chloritization phyllite is vermiculite; The chlorite in the auriferous quartz vein is dense chlorite; Mylonite contains iron-magnesium chlorite, vermiculite and dense chlorite. Even in the same sample, there are iron-magnesium chlorite and dense chlorite, which shows that the ion exchange in the crystal structure of chlorite minerals in altered mylonite is not completely balanced, and also shows the difference in physical and chemical environment when they are formed. According to the contents of iron and magnesium in the chemical composition of chlorite, the chlorite in chloritization phyllite is magnesium-rich chlorite, with Mg > Fe. While other types of rocks with Mg < Fe are iron-rich chlorite. This shows that the occurrence environment of phyllite in chloritization is different from other rocks, reflecting that gold mineralization may be related to iron-rich chlorite.
Table 6-3 Electron Probe Analysis Results and Eigenvalues of Clay Mineral Chlorite in Jinshan Gold Mine
sequential
sequential
Note: H099 is chlorite with vein precipitation, and the analysis result is calculated according to the structural formula of chlorite containing 28 O and OH; FeO stands for total iron; ◇ represents the number of octahedral holes; R2+ represents Fe2++Mg2++Mn2+; H097 is a gold-bearing quartz vein sample.
2. Mg/(Mg+Fe) and Al/(Al+Mg+Fe) of chlorite.
The relationship between the chemical composition of chlorite and parent rock has aroused the research interest of many scholars (Zang et al.,1995; Martinez-Serrano et al., 1998). Laird's diagram of (1988)Mg/(Mg+Fe) and Al/(Al+Mg+Fe) has been widely used to judge the relationship between chlorite and its parent rock. The ratio of chlorite Mg/(Mg+Fe) in Jinshan gold mine ranges from 0.319 to 0.578, while the ratio of Al/(Al+Mg+Fe) ranges from 0.317 to 0.442, which is between Mg/(Mg+Fe) and al/(al+).
In Laird( 1988) diagram, chlorite transformed from argillaceous rocks has a higher Al/(Al+Fe+Mg) than chlorite transformed from mafic rocks, generally greater than 0.35. The Al/(Al+Mg+Fe) ratio of chlorite in chloritization phyllite in Jinshan Gold Mine is 0.4 13 ~ 0.4 17, and that of chlorite in mylonite is 0.3 19 ~ 0.442, which contains gold. This shows that chlorite in phyllite in chloritization mainly comes from argillaceous rocks, while chlorite in altered mylonite may partly come from argillaceous rocks and partly from mafic rocks, while chlorite in gold-bearing quartz veins comes from mafic rocks.
Fig. 6-5 Relationship diagram of chlorite magnesium/(magnesium+iron)-aluminum/(aluminum+magnesium+iron)
3. Chlorite Al ⅳ, Al ⅵ and Fe/(Fe+Mg)
The range of Al Ⅳ in chlorite in Jinshan Gold Mine is 2.042 ~ 2.759, while the range of Al Ⅳ is 2.247 ~ 3.653 (Table 6-3; Figure 6-6), Al Ⅳ > Al Ⅳ, slightly different from chlorite (Foster, 1962) of metamorphic origin. The ratio of Al Ⅳ/Al Ⅳ varies from 0.66 to 0.96, which shows that Al Ⅳ replaces Si4+ and Al Ⅳ replaces Fe or Mg in octahedral position to achieve charge balance, and also shows that the content of Fe3+ in these chlorites is relatively low.
Figure 6-6 Relationship Diagram of Chlorite Al ⅳ-Al ⅵ
From the diagram of Alⅵ-Fe/(Fe+Mg) (Figure 6-7), it can be seen that there are two kinds of relationships between Alⅵ and Fe/(Fe+Mg): one is that with the increase of Alⅵ content, the ratio of Fe/(Fe+Mg) also increases, indicating that Alⅵ must replace Mg2+ with a lot of Fe2+, in other words. The other is that with the increase of Alⅳ content, the ratio of Fe/(Fe+Mg) decreases, which means that with the increase of Fe/(Fe+Mg) ratio, there is no corresponding more Alⅳ instead of Si 4+. It shows that the ore-forming fluid contains more Si4+, which may be related to the silicification of Jinshan Gold Mine.
4. The number of holes in the octahedral position of chlorite is the same as Na+K+2Ca.
The number of holes (◇) at the location of chlorite octahedron can be seen from the diagram of ◇-Na+K+2Ca (Figure 6-8), but there is no obvious correlation between ◇ and Na+K+2Ca, indicating that the change of chlorite composition during hydrothermal alteration is not caused by illite generated by it.
Figure 6-7 Relationship Diagram of Chlorite Al ⅳ-Fe/(Mg+Fe)
Figure 6-8 Schematic diagram of chlorite Na+K+2ca-◇
Three, chlorite geological thermometer
Chlorite, as a stable mineral in the medium-low temperature and medium-low pressure environment, has attracted people's attention because of its great variability in structure and non-metric composition (Bailey et al.,1962; Walker,1989; Martinea-Serrana et al.,1998; DeCaritatP et al.,1993; Zang et al.,1995; Battaglia, 1999).Hayes( 1970) noticed that there was a qualitative relationship between the polymorphism of chlorite and its formation temperature. Crutisetal。 (1985) points out the possible evolution route of chlorite polymorphs in sedimentary basins with the increase of burial depth. Cathelineau et al. (1985) systematically studied the relationship between chlorite composition and temperature in the geothermal systems of LosAzufres and SaltonSea in Mexico, and found that Al ⅳ was positively correlated with temperature, so a chlorite solid solution geological thermometer was put forward. Battaglia (1999) proposed to discuss the formation temperature of chlorite by XRD diffraction results. In order to explore the relationship between the chemical composition of chlorite in Jinshan Gold Mine and its formation temperature, this book uses the relationship between the chemical composition of chlorite proposed by Lausel-Collom et al. (19 1) and d00 1 revised by Nieto (197) to calculate d00 1.
d 00 1 = 14.339-0. 1 15Alⅳ-0.020 1fe 2+
Then, according to the relation equation between surface grid spacing d00 1 proposed by Battaglia (1999) and temperature, the formation temperature of chlorite is calculated:
t(℃)=( 14.379-d 00 1)/0.00 1
The calculation results are listed in Table 6-3. It can be seen from the table that the formation temperature of chlorite in chloritization phyllite in Jinshan Gold Mine is 265,438+09 ~ 225℃, while the temperature of chlorite formed by hydrothermal alteration is 206 ~ 258℃, which is basically consistent with the uniform temperature measured by fluid inclusions.
4. Physical and chemical conditions of chlorite formation
Just as temperature affects the chemical composition of chlorite, so do the physical and chemical conditions (such as fO2 and fS2) when chlorite is formed. The influence of fO2 and fS2 on the chemical composition of chlorite is mainly manifested in the ratio of Fe/(Fe+Mg). Brindiatel (1987) The influence of fO2 and fS2 on the chemical composition of chlorite was studied experimentally, and a method for calculating fO2 and fS2 from the chemical composition of chlorite was established. Walshe( 1986) proposed to use chlorite six-element model to calculate the physical and chemical conditions of chlorite formation in hydrothermal system. Xiao Zhifeng et al. (1993) studied the formation conditions of chlorite in the wall rock alteration of Baoban gold ore field in Hainan by using its chemical composition. According to the method proposed by Bryndzia et al. (1987), the physical and chemical conditions of chlorite formation in Jinshan Gold Mine are calculated, and the results are listed in Table 6-3. It can be seen from Table 6-3 that in the process of water-rock interaction in Jinshan Gold Mine, the fO2 of the fluid is10-29.56 ~10-31.48, while the fO2 of the fluid forming metamorphic chlorite is10-28.69 ~.
Formation mechanism of chlorite in verb (abbreviation of verb)
1. Analysis of formation conditions of chlorite
The formation of chlorite is a water-rock reaction controlled by reaction kinetics. In addition to temperature and pressure, the formation of chlorite is also restricted by water-rock ratio, fluid and petrochemical composition (Harvery et al.,1991; Inoue et al.,1994; Polastro, 1993).Harvery et al. (199 1) think that illite and chlorite are directly precipitated from fluids in high-permeability geothermal areas. Praise Guetal. (1995) It is considered that the formation of chlorite is theoretically related to the content of Mg2+ in the system. It is found that at 250℃, high water-rock ratio (> 50) is beneficial to the formation of chlorite-isochronous rocks, while low water-rock ratio is beneficial to the formation of chlorite-isochronous rocks (mot). Bowles et al., 1985). It was found that the change of Mg/(Fe+Mg) ratio was related to the vulcanization and oxidation of the system (Bryndzia et al., 1987). Zan et al. (1998) think that the whole rock composition controls the composition of metamorphic chlorite. Hillier (1993) pointed out that the existence of Fe-Mg chlorite shows that the system is unbalanced under microscopic conditions. Inoue (1995) thinks that the conditions of low oxidation and low pH value in hydrothermal alteration of vein deposits are favorable for the formation of magnesium-rich chlorite, while the reducing environment is favorable for the formation of iron chlorite, which may be formed. The composition of chlorite in Jinshan Gold Mine is basically the same as that proposed by Martinez-Serrand et al. (1998). The Fe/(Fe+Mg) of chlorite in altered rocks in Jinshan Gold Mine is 0.50 ~ 0.69, which is iron-rich chlorite. The above research shows that the chlorite precipitation environment in the process of rock alteration in Jinshan Gold Mine is a reducing environment. In the reaction of diagenesis and hydrothermal alteration in geothermal active area, the transformation of montmorillonite into chlorite and chlorite can often be seen. The (00 1) diffraction peak of chlorite is 29? ( 1? = 10- 10m) is treated with hexanediol and becomes 3 1? Have the characteristics of. Whether the transformation from montmorillonite to chlorite is a continuous process has always been controversial (Bateson-Vogel,1991; Shau et al.,1992; Roberson et al., 1999a) and Shau et al.( 1992) point out that the transformation from montmorillonite to chlorite is a continuous process under the condition of incomplete crystallization. However, under the condition of high water-rock ratio, it tends to be a discontinuous process, mainly in a single state. Robinson et al. (1999b) think that in the discontinuous transformation process from montmorillonite to chlorite, the stable temperature range of chlorite is 150 ~ 200℃, and the highest upper limit of the stable temperature of chlorite is 230 ~ 250℃. Barrenechea et al. (2000) think that the oxidation environment is favorable for the formation of chlorite. The appearance of chlorite+chlorite is related to the reducing environment. Montmorillonite was not found in Jinshan Gold Mine (Figure 6-9). Therefore, it can be considered that the occurrence of chlorite in Jinshan Gold Mine is discontinuous. Its existence with chlorite indicates that the metallogenic environment is a reducing environment. It is the result of chlorite degradation during the degeneration of ductile shear zone under the action of fluid.
Fig. 6-9 X-ray diffraction patterns of chlorite in AD tablets of altered ultramylonite clay minerals (< 2 microns) and EG tablets treated with hexanediol in Jinshan Gold Mine; Q- timely; Feldspar-Feldspar; Corr—- montmorillonite
2. Formation mechanism of chlorite
The chlorite content in altered mylonite in Jinshan Gold Mine is lower than that in altered ultramylonite, indicating that the contents of Fe2+ and Mg2+ in fluid in mylonite zone are lower than those in ultramylonite zone. The source and migration of Fe2+ and Mg2+ are related to the differentiation of ferromagnesian rocks and structures. The chlorite in the alteration zone is mainly iron chlorite, indicating that Fe2+> Mg2+ in the fluid. From the microscope, it can be seen that chlorite is filled in the cracks in the form of veins, or in the isotonic parts of the buckling and turning ends in the ductile shear zone. Combined with the previous discussion, the formation temperature of chlorite is 206 ~ 258℃, and the water-rock ratio is high during the fluid action. Therefore, this book thinks that the chlorite formation mechanism of hydrothermal alteration in Jinshan Gold Mine, like illite, is formed by dissolution-migration-precipitation.