Fengning Ag-Au polymetallic deposit is located in Guyuan Mesozoic volcanic basin at the junction of Zhang Xuan mantle branch structure and Fengcheng mantle branch structure, and it is a medium-large hydrothermal Ag-Au polymetallic deposit related to cryptoexplosion (Figure 3-24). Only Proterozoic Hongqiyingzi Group can be seen in the basement metamorphic rocks in this area, and it remains on the top and south edge of Baiyingou granite in the form of irregular roof pituitary, which is generally folded and migmatized. Mesozoic is mainly composed of Donglingtai Group of Upper Jurassic and Luanping Group of Lower Cretaceous. It is a complex continental volcanic-sedimentary rock assemblage, which is accumulated in the NE-trending volcanic fault basin and unconformity with Proterozoic metamorphic rock series. The Shanghuangqi-Wulonggou super-shell fault, which is closely related to Fengning silver deposit, is divided into two branches: the west branch is called Yudaokou fault, which strikes 25; The east branch is called Qipanshan fault, which strikes 50. The two faults control the distribution of Baiyingou-Laohugou Gate and Houwopu-Shanghuangqi magmatic belt respectively. The basement structure in this area is dominated by folds and faults, which extend in the east-west direction as a whole. Yanshanian folds are mostly wide and gentle anticlines extending from NNE to NNE. The NNE-trending Huangqi-Wulonggou deep fault controlled the formation and development of the Upper Huangqi magmatic belt, and a series of shallow-medium plutonic intrusions such as granite, monzogranite, granodiorite, monzoporphyry and quartz diorite developed along this deep fault. Tectonic and magmatic activities provide necessary power, heat source and minerals for mineralization in this area.
Magmatic activity in this area has been extremely intense since the late Paleozoic, and intrusive rocks have been widely exposed and distributed in bands in space. The formation times are mainly Hercynian and Yanshan. Hercynian intrusive rocks are mainly granite, followed by granodiorite and quartz diorite, mostly bedrock and rock-like. Among them, the long axis of Baiyingou granite (γ34) in horseshoe-shaped batholith is NE (covering an area of 285km2), its isotopic K-Ar age is 152.6 ~ 222.9 Ma, and zircon U-Pb coincidence age is (24 1 8) Ma, which belongs to the late Hercynian intrusive rock. Yanshanian magmatic intrusion was the strongest in late Jurassic Zhangjiakou period and early Cretaceous Dabeigou period. The former is mainly granite porphyry, syenite porphyry and adamellite porphyry. The latter are granite, granodiorite and syenite porphyry.
The main fault structures are F 1, F2, F3, F4, etc. The F 1 fault runs through the whole area and is about 8km long. It is a part of Shanghuangqi-Wulonggou deep fault, and has close temporal and spatial distribution relationship with Fengning silver (gold) deposit. The whole fault fracture zone is stable in occurrence, deep in extension, flat in profile and gentle in strike and dip, and consists of fractured breccia (fracture zone), fault gouge and coarse-grained mylonite. Strike 10, dip SE, dip angle 60 ~ 75, with the characteristics of multiple activities.
Figure 3-24 Geological Schematic Diagram of Fengning Ag-Au Deposit
(2) Geological characteristics of the deposit
1. Scale, occurrence and shape of the deposit
Fengning Ag (Au) metallogenic belt mainly occurs in cryptoexplosive breccia. The whole ore belt is NNE, with a length of 478m and a width of10 ~ 34m. From north to south, it can be divided into three ore sections: north, middle and south, but they are interconnected at underground 150 ~ 260 m, which has the spatial zoning characteristics of upper silver and lower gold. There are 26 ore bodies in the mining area, including silver ore body 16 and gold ore body 16. Most single ore bodies are distributed along the NNE structural belt in the form of veins or lenses, with an "S" shape on the plane and a "splay" shape on the section, with a length of 40-450m, a thickness of 0.73-8. 1m, an inclined depth of 180-350m and a maximum of 4/kloc-0. Most silver (gold) ore bodies tend to the southeast with an inclination of 3165. No.2 ore body is a large-scale and high-grade ore body in the area, accounting for about 98% of all ore reserves. The exposed length of the ore body is about 240m, and the horizontal thickness varies greatly, ranging from 1 ~ 18m, with an average thickness of 8.02m The silver content ranges from 50×10-6 to 980×10-6, with an average grade of 5/kloc. The gold content is 0.89×10-6 ~10.63×10-6, and the average value is10-6. The change of ore body thickness is related to the occurrence of ore body, from steep to slow, and the ore body becomes thicker.
2. Alteration and mineralization characteristics
The hydrothermal alteration of the main ore bodies in Fengning silver (gold) deposit is characterized by wide distribution, many types and high intensity. The alteration zone is nearly north-south, with a length of 1000 m and a width of 100 ~ 300 m. The main hydrothermal alteration types are silicification, alkalization, sericitization, kaolin, montmorillonite, chloritization and carbonation. From the center of ore body to the outside, altered rocks can be divided into three zones: silicified zone, sericitized zone and Panqing mineralized zone. Among them, the surrounding rock of the roof of the silver (gold) ore body is the most developed in the silicified zone, and its distribution range and occurrence form are generally similar to those of the silver (gold) ore body.
Representative silver (gold) ores have typical metasomatism, inclusion, solid solution separation and fragmentation structure, breccia-like, vein-like disseminated, banded and massive structure. The metallic minerals in the ore include pyrite, galena, sphalerite, chalcopyrite, argentite, natural silver, silver-gold ore, arsenopyrite, pyrrhotite and silver-rich copper-silver ore. Gangue minerals include quartz, mica, barite, fluorite, feldspar, carbonate and clay minerals. Metal sulfides often occur in different forms, such as automorphism, semi-automorphism and heteromorphism, among which metasomatism, dissolution, encapsulation and encapsulation are very obvious. Sulfide skeleton can occasionally be seen in the light section of a single sample. Among all the above metal sulfides, pyrite has the characteristics of wide distribution and high content, which can not only appear in all stages of mineralization, but also be an important carrier of silver and gold. In addition, some silver and gold exist in sphalerite, galena and arsenopyrite in the form of isomorphism.
The independent minerals of silver mainly include argentite, silver-gold ore, spiral chalcopyrite and silver tetrahedrite, which are wrapped in galena and sphalerite and precipitated as solid solution, while interstitial silver and fissure silver exist in metal sulfide and gangue minerals. In terms of correlation, there is a high correlation between silver and lead. The forms of silver minerals are complex and diverse, mainly round, wheat-shaped, strip-shaped and leaf-shaped.
3. Time limit of diagenesis and mineralization
The K-Ar isotopic age of Baiyingou granite is 152.6 ~ 222.9 Ma, and the zircon U-Pb isotopic age is (24 1 8) Ma, which belongs to the late Hercynian intrusive rock. Invasion of alkaline fine-grained granite (108Ma) and cryptoexplosion penetrating into breccia (120.66Ma) (Liu Fengshan et al., 1997) are most closely related to the time and space of mineralization. Nie et al. (2007) determined the 40Ar/39Ar isotopic ages of fine-grained potash feldspar granite strains and potash feldspar samples in ore-bearing cryptoexplosive breccia in Fengning silver-gold mining area, Hebei Province. The isotopic isochron age of fine-grained potash feldspar granite is (216.6 3.2) ma, and the ore-bearing cryptoexplosive breccia bodies are (227.9±4.8)Ma and (216.7 6.1) ma. Based on this, it is determined that the formation age of fine-grained potash feldspar granite and ore-bearing cryptoexplosive breccia is 2 16Ma, which is closely related to Indosinian potassium-rich magmatism.
(3) Sources of ore-forming materials
1. Sulfur isotope characteristics
The analysis results of 24 sulfur isotope samples in this mine (Table 3-25) show that all sulfur isotopes in this area are positive and belong to heavy sulfur enrichment type. Δ 34s =+3.2 ~+5.7, with an average value of +4.09, with a small variation range of 2.5, which is the standard deviation. The sulfur isotope characteristics are δ 34S pyrite > sphalerite δ34S sphalerite > δ34S galena, indicating that sulfur (34S) in ore-bearing thermal fluid has reached thermodynamic isotope equilibrium fractionation. The homogenization degree of sulfur isotope is high, and the diagram shows obvious tower effect, which has a positive sulfur isotope composition close to that of meteorites. It shows that sulfide sulfur source is single and has the characteristics of deep source sulfur.
Table 3-25 Sulfur Isotope Composition of Fengning Ag-Au Deposit
2. Lead isotope characteristics
See Table 3-26 for lead isotope determination of pyrite and galena. It can be seen from the table that the lead isotopic composition of Fengning silver-gold deposit has little change, and the sample values are relatively concentrated, which is characterized by normal lead content. Its 206Pb/204Pb is 16.48 ~ 17.83, with an average of16.95; 207Pb/204Pb is15.18 ~15.56, with an average of15.45438+0; 208Pb/204Pb is 36.98 ~ 38.06, with an average of 37.65. Projection of lead isotope data to B.R.Doe lead structure diagram 207pb/204pb-206pb/204pb (Figure 3-25) shows that the projection point is mainly located between the mantle and the lead evolution line of the lower crust, indicating that the lead isotope of Fengning silver-gold deposit has the same characteristics, that is, lead should mainly come from the deep part of the earth.
Table 3-26 Lead Isotope Characteristics of Fengning Ag-Au Deposit
sequential
Note: ① North China Nonferrous Metals Group 5 14, geological report on exploration of Fangyingyin lead-zinc mine in Sichakou Township, Fengning Manchu Autonomous County, Hebei Province, 1993.
Fig. 3-25 Lead Isotope Composition of Fengning Ag-Au Deposit (According to Zartman et al., 198 1)
3. Isotopic characteristics of hydrogen and oxygen
The determination of hydrogen and oxygen isotope samples of inclusions in the altered mineral (chalcedony) shows that (Zhang Baiming, 1996), δ18o =1.31‰ ~ 4.15 ‰, with a range of 2.84. δ d =-98.7 ‰ ~-116.9 ‰, with a range of-18.2‰ and an average of-106.35‰. The sample data is projected on the composition diagram of fluid δ(D)-δ 18O, and its projection point is located at the lower left of magmatic water, indicating that magmatic water and atmospheric precipitation may have played an important role in the mineralization process. However, because the ore body is located in a relatively open expansion space (cryptoexplosive breccia), the water/rock ratio is large, the water-rock reaction is fierce, and the oxygen isotope drift is obvious, so that the δ 18O value in the fluid increases slightly. Compared with meteorites (δ 18O is 3.7 ‰ ~ 6.3 ‰) and mantle (δ 18O is 5.7 1.00 ‰), the ore-forming fluid of this deposit belongs to deep-seated migmatite magma.