Isotopic characteristics of gold-silver polymetallic deposits

Due to the in-depth study of mineralization of gold-silver polymetallic deposits, more and more test data have been accumulated, which can be processed and discussed by statistical methods to avoid cognitive deviation caused by a small amount of data. Of course, the current testing technology, especially isotope testing technology, can only determine the source depth of ore-forming materials up to the mantle. The main method is to compare the isotopes of meteorites similar to the mantle with those of crustal materials, and then infer their sources. The deeper part can only be reasonably inferred by studying the geochemical properties of elements, modern geodynamics and modern deep geophysical techniques.

(A) sulfur isotope characteristics analysis

According to the measurement and collection data of sulfur isotopes in 17 mining area 189, the individual samples of x+3s are excluded for statistical analysis (Table 2-3).

Table 2-3 Sulfur Isotope Characteristics of Some Gold, Copper and Molybdenum Deposits in Eastern Hebei Province

① Exclude 2 +3s samples; ② Exclude 1 +3s samples.

1) Except for Huzhangzi, the average value of sulfur isotopes in other mining areas is between-6.4 and+6.3, and the total average value is+1.93; The average value of each mining area is between-1.78 ~+3.3, which has a small positive value and tends to zero, indicating that the overall source of sulfur should have the characteristics of deep mantle-derived sulfur.

2) The evolution trend of sulfur isotope values of symbiotic sulfides in most mining areas is δ 34 sfes2 > δ 34 sfecus2 > δ 34 szns > δ 34 spbs, indicating that the sulfur isotope reaction has basically reached equilibrium.

3) A few mining areas are negative, which is generally considered to be related to the secondary oxidation of sulfide, so that the residual 34S in sulfide is relatively reduced, while the residual 32S is relatively increased. Of course, it is more likely that the sample size is too small.

(2) Analysis of lead isotope characteristics

According to the statistics of 67 pieces of lead isotope data in 13 mining area and 4 granite bodies in different structural positions (Table 2-4), it is found that:

Table 2-4 Lead Isotopic Characteristics of Some Gold Deposits and Granites in Eastern Hebei Province

1) No matter what tectonic position (axial shear zone, main detachment zone or caprock), it is found that the average lead isotope of each mining area is very close and the amplitude is very small. Isotope variation range is 206 Pb/204 Pb14.986 ~16.304, and the range is 1.3 18. 207pb/204pb14.95438+0 ~15.408, interval 0.447; 208 Pb/204 Pb 34.834 ~ 36.787, interval 1.953.

2) The isotopic composition of lead in granitic rocks related to gold and silver mineralization is consistent with that of ore lead, which indicates that they are from the same source, that is, from Yanshanian granitic magmatism. The specific value is 206pb/204pb15.882 ~17.465, and the interval is1.583; 207pb/204pb15.147 ~15.510, with an interval of 0.363; 208 Pb/204 Pb 35.722 ~ 37.454, interval 1.732.

3) Projecting the average lead isotope values of each mining area on the lead isotope composition map (Figure 2-4), it can be clearly seen that the mineral sources of gold and silver deposits in eastern Hebei are mainly distributed between the lower crust and the mantle source area, which further proves the genetic relationship between gold deposits and mantle plume in this area.

Fig. 2-4 Lead Isotope (Average) Evolution Diagram of Jidong Gold Mine

(3) Analysis of isotopic characteristics of hydrogen, oxygen and carbon.

According to the statistics of more than 60 hydrogen, oxygen and carbon isotope samples from different structural parts 18 deposit (Table 2-5), the average δ 18H2O of the representative deposits in Yuerya and Jinchangyu is 6.066 ~ 7.029, which is different from that of the standard magmatic water δ18H2O+5 ~/kloc-0. The average δDSMOW of 12 mining area is -56 ~-88.67, which is also consistent with δ dsmow-40 ~-80 of standard magmatic water. The average value of δ 13C is -4. 18 ~-5.25, which is consistent with δ 13C-5 ~-8 of primary carbon. The average values of oxygen isotopes in eight mining areas are projected on the δD-δ 18O coordinate diagram (Figure 2-5). It can be seen that the oxygen isotopes of all the gold deposits are near the primary magmatic water, but far away from the atmospheric precipitation and metamorphic water, indicating that the hydrogen, oxygen and carbon isotopes all support that the ore-forming solution of the gold deposits in this area mainly comes from magmatic water, and there is indeed the addition of atmospheric precipitation.

Table 2-5 Hydrogen, oxygen and carbon isotopic characteristics of some gold, copper and molybdenum in eastern Hebei Province

sequential

Note: The number of samples is in brackets.

Figure 2-5 δD-δ 18O (average) of some gold, copper and molybdenum ores in eastern Hebei (according to Sheppard, 1977)

(4) Characteristics of rare gases

1. Helium isotope

It is a new method to distinguish ore-forming materials by using the isotopic characteristics of rare gases helium and argon in recent years. Rare gases such as helium and argon, as inert gases, basically do not participate in chemical reactions inside the earth, so they are used to study the internal structure of the earth and geodynamic processes, and to explore basic problems in earth science, such as the relationship between the composition of the earth's atmosphere and the formation and deepening of continents, the structural characteristics of mantle convection, and the sources of original materials inside the earth. The results show that there are three main occurrence States of he and Ar in minerals: ① they are enclosed in fluid inclusions; ② The epigenetic radioactivity 4He and 40Ar③ produced by the decay of U, Th and K in the mineral lattice are adsorbed on the mineral surface. The existing research shows that the fluid inclusions containing helium in pyrite will not be obviously lost after being captured.

Stuart et al. (1994) proved that radioactive 4He and 40Ar in the mineral lattice were not released when rare gases were extracted from crushed samples, and pyrite had a very low helium diffusion coefficient, so pyrite was considered as an ideal mineral for preserving helium. The research of Trull et al. (199 1) confirmed that fluid inclusions have a good ability to preserve argon, and argon can also be quantitatively preserved in a long geological history. Qiu (1996) and Turner et al. (1992) studied the yield of in-situ radioactivity 40Ar in fluid inclusions. The results showed that although the superposition of in-situ radioactivity 40Ar in fluid inclusions in potassium or potassium-bearing minerals could not be completely ruled out, in-situ radioactivity 40Ar was found in fluid inclusions for non-potassium-bearing minerals (all samples in this book belong to this category).

He and Ar isotopes were first used in submarine hydrothermal systems, modern geothermal systems and geological fluids such as natural gas and oil wells. Many deposit geologists have done a lot of work on its application in ore-forming fluids (Hu et al., 1997, 1998,1999; Mao Jingwen et al., 2000,2001; Xue et al., 2003; Mao et al., 2002; Zhang Lianchang et al., 2002; Wang et al., 2003; Wang Baode et al., 2003, 2008). The results show that in many important nonferrous metal ore concentration areas and metallogenic belts in China, such as Jiaodong gold deposit, gold (copper) metallogenic belt in the western margin of Yangtze platform, Ailaoshan gold (copper) metallogenic belt and gold concentration area in northwest hebei, He and Ar isotopic studies show that a large number of mantle-derived components were added in the fluid mineralization process.

Previous studies have shown that the isotopic composition of inert gases in fluid inclusions can be used to distinguish the following three ore-forming fluids from different sources. ① Atmospheric saturated water (ASW) mainly includes atmospheric precipitation and seawater, and its standard isotopic compositions of 3He/4He and 40Ar/36Ar are1Ra (Ra: Air3He/4He =1.40×/kloc-0-6) and 295.5 respectively. ② The standard values of 3He/4He and 40Ar/36Ar of deep mantle fluid should be 6 ~ 9Ra and > 40000, respectively. ③ Crustal fluids, including formation water or basin hot brine, are characterized by 3He/4He and 40Ar/36Ar compositions of 0.0 1 ~ 0.05Ra and > 295.5, respectively (Bumard et al., 1999).

Honda et al. (1993) pointed out that the existence of high 3He/4He in the island arc accords with the layered mantle model, that is, the mantle rises directly from the lower high 3He/4He reservoir through the higher low 3He/4He layer; In other words, they originated from the basement of the upper layer, in which case, they must migrate from the lower mantle to the magma source area.

We selected 15 samples of sulfide, Xiaoyingzi granite and Jinchangyu surrounding rock in the representative mining area of Jidong 10 for helium isotope test (Table 2-6). It can be seen from Table 2-6 that the 3He/4He content of pyrite in the ore ranges from 2.50×10-6 to 9.39×10-6, with an average value of 5.43× 10-6. It is hundreds to thousands of times higher than ordinary igneous rocks (0.003×10-6 ~ 0.26×10-6), but it is a more typical mantle-derived material (1.1×10-5 ~) The relatively low 3He/4He content in this area may be due to the fact that most of the samples are in mantle plume. Compared with gold deposits, the formation process of these geological bodies is relatively simple and less disturbed by the outside world, and more (or better preserved) original helium and argon gases are captured in rocks (or water samples). It is found that, except for the super-large deposits such as Baiyunebo, Jinchuan and Shizhuyuan, the proportion of direct intrusion into mineralization in the form of mantle-derived thermal fluid is generally small in most deposits, and a large number of minerals should be carried by some carrier of multi-stage evolution of mantle plume and gradually mineralized in the continuous evolution. During this period, shell-derived substances (including radioactive helium and argon) will inevitably be added, resulting in a decrease in the 3He/4He ratio. During the long migration of He-Ar gas accompanying ore-forming materials, some crust-source fluids are inevitably added, which makes the He-Ar isotope values of the samples measured often lie between the crust and mantle. Therefore, compared with normal rocks, the higher values of 3He/4He and R/Ra reflect that mantle thermal fluid participated in the mineralization process to some extent.

Table 2-6 Helium Characteristics of Some Gold Deposits in Eastern Hebei Province

Note: * is the ratio of 3He/4He in pyrite to 3He/4He in air (Ra: 3He/4He in air =1.40×10-6).

It can also be seen from Table 2-6 that the isotopic content of sediments in different structural parts is not much different, reflecting that they come from the same source. The 3He/4He value of gneiss and granite in the periphery of the mining area is only 0.00/kloc-0 /×10-6 ~ 0.55×10-6, which reflects the obvious provenance difference.

Comparing 3He/4He in pyrite with 3He/4He in air, the range is 1.93 ~ 6.76 Ra, with an average of 3.90Ra, which is slightly lower than that of typical mantle materials (6 ~ 9 Ra), but much higher than that of crustal materials (0.0 1 ~ 0.05 Ra).

If the ore-forming fluid is regarded as a simple binary mixed model, the ratio of mantle fluid to crustal fluid can be calculated by the ratio of 3He/4He. Among them, the proportion of mantle source 4He is calculated according to the following formula:

Mantle helium = [(R-RC)/(RM-RC)] × 100%.

Where: RM =1.1×10-5; RC = 2× 10-8； R represents the helium isotopic composition of mantle fluid, crustal fluid and sample respectively.

It is concluded that the proportion of mantle fluid in ore-forming fluid of gold-silver polymetallic deposits in mantle branch structure in eastern Hebei is 22.59% ~ 85.34%, with an average of 52.5 1%. It shows that the fluid from deep sources accounts for a considerable proportion.

Comparing the sulfide isotope data of pyrite in 10 deposit with the helium isotope concentration map (Figure 2-6), compared with the surrounding rock and granite, the falling points are all located near the mantle helium. It reflects that helium should mainly come from the mantle, and there is degassing or radioactive 4He (crust-derived material) added during the ascent.

2. Argon isotope

See Table 2-7 for the argon isotope analysis results of the studied deposit. 40Ar/36Ar = 365 ~ 1304, with an average value of 74 1.58, which is obviously higher than the 40Ar/36Ar value of crustal fluid (40Ar/36Ar=295.5). 40Ar/38Ar = 1606 ~ 6 189； 36Ar/38Ar = 5.2 ~ 5.5； 40ar = 0.48× 1 0-7 ~ 18.53× 10-7cm3 STP/g, excluding1ultrahigh sample (18.53),10. 4He/40Ar = 0. 10 ~ 6 1.40。 Excluding 1 ultra-high samples (6 1.40), the average value of 1 1.74 is slightly higher than that of Schwarczman (65438). Therefore, the low 4He and 4He/40Ar values of pyrite in Jidong gold mine should indicate that there are gas components from deep earth.

Figure 2-6 Helium Isotope Concentration Map of Jidong Gold Mine (Tolsikhin, 1978)

Table 2-7 argon characteristics of some deposits in eastern Hebei Province

Note: The data source is the same as Table 2-6.

The data of various mines in eastern Hebei are projected on the 3He/4He-40Ar/36Ar diagram (Figure 2-7). The helium and argon isotopic composition of ore-forming fluids in this area is mainly located in the mantle fluid area, indicating that ore-forming fluids mainly come from the deep earth.

Figure 2-7 3He/4He(R/Ra)-40Ar/36Ar ratio diagram.

(V) Analysis of the characteristics of inclusions in gold deposits

According to the collected data of 20 inclusions in 5 mining areas (Table 2-8), it shows that H2O accounts for more than 90% of inclusions in each mining area, and CO2/H2O = 0.003 ~ 0.732；; Cl-F-(F-/Cl-= 0.074 ~ 0.230), Na+ > K+(K+/Na+= 0.009 ~ 0.683), Ca2+> Mg2+(Mg2+/Ca2+= 0.038 ~1. It shows that magmatic water should be the main ore-forming solution (forest,1991; Fan et al., 1983), only a small amount of Tianshui was added. PH = 6.40 ~ 6.60, indicating that the ore-bearing solution is weakly acidic.

Table 2-8 Composition Characteristics of Inclusions in Some Gold Deposits in Eastern Hebei Province

From the existing temperature data (Table 2-9), it can be seen that the temperature values measured by homogenization method and blasting method in each mining area are relatively concentrated, in which the temperature variation range of homogenization method is 120 ~ 4 10℃, and the average temperature is 230 ~ 3 18℃, so it can be determined that the metallogenic temperature of this kind of deposit is medium warm liquid. In addition, the temperature data of steeply inclined gold veins (such as Jinchangyu and Yuerya) tend to gradually decrease from the deep to the surface in space, which also reflects the mineral precipitation caused by the temperature drop and the change of physical and chemical conditions during the upward movement of ore-forming fluids. The salinity change of ore-forming solution should belong to medium level.

Table 2-9 Characteristics of Temperature Measurement Data of Some Gold Deposits in Eastern Hebei Province

(6) Discussion on migration of gold and silver polymetallic and enrichment of integrated ore.

The above data show that the gold in this area should come from the deep part of the earth, but we think that the gold is likely to come from the D "layer of the core-mantle boundary. When the internal and external factors such as thermal disturbance, temperature and pressure conditions and astronomy are superimposed, the heat flow in the core will quickly break through the resistance of the boundary between the core and the mantle with the accumulation of energy, forming a mantle plume, which will be ejected upward through cracks with different depths (Wang Baode, 2002).

According to Huo Mingyuan's research (199 1), gold should exist in gaseous state in the ultra-high temperature and high pressure environment of the core. In the intense outer nuclear current and the differential movement between the core and the mantle, a large amount of gold vapor gathers near the core-mantle interface. Once the mantle plume moves upward, gold moves upward in the state of dispersed gas, and gradually concentrates in the mantle branch structure with the multi-stage evolution of the mantle plume. Yanshanian granitic magmatism in the core of mantle branch structure in this area plays an important role, because only this large-scale geological process can bring so much gold. Most data prove that the Yanshanian granitic magma in this area has the characteristics of mantle source or crust-mantle source, and its own gold content is obviously high, so the consistency between diagenesis and mineralization time can be confirmed.

As far as eastern Hebei is concerned, during the Yanshan movement, North China entered the stage of multi-stage intense evolution of mantle plume. The sub-mantle plume of the Hehuai mantle rises to the bottom of the lithosphere, shielding and separating outwards in an umbrella shape. Due to the thermal thinning of the Hehuai mantle plume, the North China fault depression gradually formed. At the same time, the depth of Maoshan-Jinchangyu-Shuangshanzi ductile shear zone in eastern Hebei makes the originally melted mantle soft sheet decompress and release its load to form deep lava slurry, and makes some surrounding rocks melt to form linear magma chamber, especially at the intersection of transverse faults, which becomes a good place for magmatic activity. Magmatic activity leads to the overall uplift of the block, the exposed top uplift of metamorphic rocks, and the large detachment and slippage of the peripheral cover, forming a typical mantle branch structure. With the large-scale emplacement of a series of Mesozoic granitoids distributed along the line of Happy Valley-Jinchangyu-Banbishan, the deep ore sources are connected, and many gold and silver deposits are formed in the granite magmatic active zone and its surrounding rock detachment zone and caprock.

With the continuous evolution of mantle branch structure, especially in the middle and late Yanshanian granitic magma evolution, gold gradually transformed and enriched from gas to liquid (or gas-liquid), and accumulated and mineralized under suitable space and physical and chemical conditions, which had no obvious exclusive relationship with strata (or surrounding rocks). The prospecting practice of ancient metamorphic rocks, magmatic rocks and sedimentary caprocks in eastern Hebei has repeatedly proved this point.

Of course, in the multi-stage evolution of the mantle plume, especially after entering the crust through the mantle, a large number of crust-derived materials will inevitably be mixed. This makes the measured isotope data often near the typical mantle source area, but not only in the mantle source area, but generally far from the crustal source area.

To sum up, the gold in this area should mainly come from the core, the multi-stage evolution of mantle plume is the main driving force for the upward migration of gold, and the large-scale granitic magmatism in Mesozoic is the main carrier for the upward migration of gold. Under the ultra-high temperature and high pressure environment in the deep part of the earth, gold and its sulfide can only exist in gas phase, and continue to rise with the multi-stage evolution of mantle plume, and are enriched in ore-forming thermal fluid during the evolution of magma condensation, and then unloaded and mineralized under suitable space and physical and chemical conditions. The favorable ore-guiding and ore-controlling space is the axial shear zone of mantle branch structure, the main detachment zone of basement and Mesoproterozoic and the secondary detachment zone of caprock.