(1) Metallogenic Geological Background
Dongzigou silver-copper deposit is located at the turning end of the northwest edge of mantle branch structure in eastern Hebei, close to the secondary detachment slip zone of Changzhou Formation above the main detachment zone (Figure 2-29). It is preliminarily confirmed that Dongzigou Ag-Cu deposit is a large-scale deposit with silver as the main component and copper and gold as the companion.
The strata exposed in the mining area are Archaean Qianxi Group Paomachang Formation and Mesoproterozoic Great Wall System. The Great Wall System is unconformity or fault contact with the Paomachang Formation of Qianxi Group, which is distributed in the east of the mining area and is a set of migmatized amphibolite gneiss and amphibolite. Changzhougou Formation of Great Wall System is composed of conglomerate, glutenite, sandstone and siltstone clastic rocks, Chuanlinggou Formation is composed of sandstone shale, Tuanshanzi Formation is composed of argillaceous rocks and carbonate rocks, and Dahongyu Formation is composed of sandstone, volcanic rocks and carbonate rocks. Dahongyu volcanic rocks are composed of olivine basalt, basalt, trachyte and tuff. The direct host rock of the deposit is the glutenite of Changzhougou Formation.
Figure 2-29 Geological Schematic Diagram of Dongzigou Silver-Copper Mine Area (according to North China Nonferrous Comprehensive Investigation Brigade, 1998)
The ore body is basically controlled by the secondary detachment zone of Changzhou Gou Formation, which is relatively stable in layered output, and the ore body tends to thicken and enrich at the turning point or inclined end of the detachment zone. The ore bodies in the secondary fracture zone are lenticular and lentil-shaped, with obvious expansion and contraction and obvious breakthrough characteristics.
The ore body is layered, with stable thickness and large extension depth, which is basically controlled by horizons. Strike direction is mainly 300, strike direction is mainly northeast, and dip angle is10 ~ 50. 1 and No.2 ore bodies occur in the first layer of pebbled feldspar timely coarse sandstone of Changzhou Gou Formation (chc1-1); No.3 ore body occurs in the grey fine-grained timely sandstone in the third layer of the first section (Chc 1-3); No.4 and No.5 ore bodies occur in the fifth layer (Chc 1-5) purplish red feldspar timely sandstone.
(2) Geological survey of the deposit
1. Ore body shape, occurrence and scale
All ore bodies occur in the interlayer shear slip zone of sandstone in Changzhou Gou Formation, and most of them occur in the first member of Changzhou Gou Formation in layered, layered or lenticular form, which is basically consistent with the occurrence of strata, and some of them occur in veins above and below the detachment fault. At present, five ore bodies have been proved in Dongzigou ore section. The ore body is 130 ~ 1000 m long, 0.4 ~ 1 m thick, extending over 500 m, with the highest average grade of Ag560× 10-6, Au2.35× 10-6 and Cu 0.70% The single highest grade of this project is Ag2750× 10-6, Au3× 10-6 and Cu9.5%.
2. Characteristics of ore fabric
Dongzigou silver-copper deposit has a complex mineral composition, with more than 30 kinds of minerals. Among them, copper minerals include tetrahedrite (3. 1%), bornite (0.46%), chalcopyrite (0.25%), chalcocite, chalcopyrite, chalcopyrite, malachite and chalcocite. A few silver-bearing minerals include tetrahedrite, silver-bearing sky blue and biotite. Gold and silver minerals include Hesse stone, Hesse stone, silver-gold mine, gold-silver mine and spiral silver sulfide mine. Other metal minerals are galena (0.89%), sphalerite (0.8 1%), pyrite (0.5%) and limonite (0.8%), and a small amount of lead alum, magnesite, galena and tellurite. Gangue minerals are mainly timely (82.8%), followed by feldspar (5.5%), carbonate (2.5%), kaolin (1.2%) and a small amount of barite and fluorite.
Ore structure mainly includes grain structure, interstitial structure, dissolution structure, metasomatic residual structure, inclusion structure and marginal structure. The ore structure is relatively simple, mainly including massive, banded, dotted, disseminated, breccia, vein, grid and honeycomb structure.
3. Wall rock alteration
The types of wall rock alteration are complex, including silicification, sericitization, carbonation and kaolin. There is barite mineralization locally and fluorite mineralization occasionally. Among them, silicification is closely related to mineralization, which is an important prospecting indicator of this type of deposit. There are two main manifestations of silicification in this deposit: first, dense network-like timely veinlets are formed in the surrounding rock of the ore body, which are generally 0. 1 ~ 0.5 cm thick, and the most developed place is more than 20 veinlets/meter; The other is timely recrystallization.
(3) Sources of ore-forming materials
1. sulfur isotope
According to the sulfur isotope test results of 2 1 pyrite, chalcopyrite and tetrahedrite (Table 2-22), the variation range of δ34S in this deposit is -0.70 ~ 4.90, with an average value of 1.28 and an estimated value of δ 34σ s of -0.3 (). Based on this, it can be considered that the sulfur isotope variation range of the deposit is narrow, and the sulfur isotope composition is mainly heavy sulfur, which is close to that of meteorites, indicating that the sulfur isotope of the deposit comes from primary sulfur and has no obvious isotope fractionation effect, reflecting that its ore-forming material source is deep.
Table 2-22 Sulfur Isotope Composition of Dongzigou Ag-Cu Deposit
According to the variation law of sulfur isotope values in different minerals, pyrite has the highest sulfur isotope value, while galena has the lowest sulfur isotope value, which evolves from pyrite to chalcopyrite to tetrahedrite to galena. The evolution order of δ34S is1.74 →1.10 → 0.75 →-5.2, and. This decline law accords with the basic law of sulfur isotope fractionation in the mineral growth process of the deposit, which shows that sulfur isotope fractionation has reached equilibrium, so it can be considered that the sulfur isotope of the deposit comes from deep-source sulfur that has undergone obvious isotope fractionation.
2. Lead isotope
The determination of 14 metal sulfide samples (Table 2-23) shows that the ore 206Pb/204Pb is 15.443 ~ 16.73, with an average value of15.79; 207Pb/204Pb is 14.857 ~ 15.485, with an average of15.19; 208Pb/204Pb is 35. 18 ~ 37.246, with an average of 35.86. The lead isotope of the deposit has the normal lead characteristics of single-stage evolution. Put the lead isotope ratio on the corresponding map (Figure 2-30), except for two samples, all the other points fall between the normal lead curve mantle and the evolution line of the lower crust, indicating that the lead isotope source should be mainly in the deep part of the earth, with a small amount of crust-derived materials added.
Table 2-23 Lead Isotopic Composition of Dongzigou Ag-Cu Deposit
Figure 2-30 Lead Isotope Evolution Diagram of Dongzigou Silver (Copper, Gold) Deposit
3. Composition characteristics of mineral inclusions
It can be seen from Table 2-23 that the ratios of Na/K, Na/Mg, F/Cl and CO2/H2O in the inclusions of the deposit are 0.83 ~ 2.08, 0.82 ~ 0.95, 0.23 and 0. 15 ~ 0.24, respectively. All the above numerical characteristics indicate that the hydrothermal source of the deposit is neither typical magmatic hydrothermal solution nor typical hot brine. Table 2-23 also shows that the anions in hydrothermal fluids are mainly Cl- and F-, and the contents of F- and Cl- are relatively high, which also reflects the deep source characteristics of ore-forming fluids.
According to Lin's research (199 1), the gas phase composition of inclusions in gold and silver deposits is magmatic hydrothermal, and CO2/H2O is less than 0.5. According to the research of Fan et al. (198 1), Na+/k+< 2, Na+/(Ca2++Mg2+) > 4 is a typical magmatic hydrothermal solution, and 2 < Na+/k+<10,65438+. Compared with the analysis results of the deposit (Table 2-24), the ore-forming hydrothermal solution mainly comes from magma, mixed with atmospheric precipitation and rain. Because the samples are all taken from autonomous ore bodies, they only reflect the hydrothermal source of mineralization.
Table 2-24 Gas-liquid composition analysis results of inclusions in Dongzigou silver (copper, gold) deposit
4. Hydrogen and oxygen isotope characteristics
See Table 2-25 for the characteristics of hydrogen and oxygen isotopic compositions of timely inclusions in ores. As can be seen from Table 2-25, the δ 18OH2O in the mining area is 4.24 ~ 6.86, and δDH2O is -73.2 ~-67.5. Kuroda et al. in Japan (1978) thought that δ 18OH2O was 4.5 ~ 7.0; When δDH2O is-100 ~-60, the water in hydrothermal solution is mantle magma water. Comparing the results of hydrogen and oxygen isotopes in this area, it shows that the water in the ore-forming hydrothermal solution of this deposit is mainly mantle magma water.
Table 2-25 Hydrogen, Oxygen and Carbon Isotope Characteristics of Dongzigou Silver (Copper, Gold) Ore
According to the geochemical characteristics of Dongzigou silver-copper deposit, the ore-forming materials of Dongzigou silver-copper deposit should mainly come from the deep part of the earth. With the activity of mantle branch structure in eastern Hebei, Huangyaguan fault revived, which led to the detachment of sandstone in Changzhougou Formation in its upper wall, and formed a secondary detachment zone above the main detachment zone. Infiltration from the deep along the secondary detachment zone or interlayer weak zone-structural expansion zone, forming a mesothermal hydrothermal deposit obviously controlled by the structure. Ore-forming fluids mainly come from deep magmatic water, and some of them are mixed with water.