The deposit was discovered by the second detachment of the first geological team of the former Gansu Provincial Bureau of Geology and Mineral Resources in the area of1June 19901∶ 50,000, and the general survey and evaluation was undertaken by the seventh detachment of the first geological team of the Gansu Provincial Bureau of Geology and Mineral Resources in191~1June 1994. In 2003, the First Exploration Institute of Gansu Province conducted a new round of investigation and obtained abundant data.
1 metallogenic geological background
Chaijiazhuang gold deposit is located in the western Qinling orogenic belt and the northern Qinling Caledonian fold belt. The North Qinling fold belt is a compound fold belt formed on the basis of the North China block. It was formed in Proterozoic, and received marine volcanic-clastic deposits in the early Caledonian, folded and orogened in the late Caledonian, and underwent strong transformation in the Variscan and Indosinian-Yanshan periods (Huo et al., 1995).
The exposed strata in this area include Paleoproterozoic Qinling Group, Lower Paleozoic Li Ziyuan Group, Devonian System and Cretaceous System. Among them, the moderately metamorphic greenschist series dominated by basic-intermediate acid volcanic rocks in Li Ziyuan Group is the most widely distributed and has high gold content, which is the main source bed of gold deposits in this area. There was magmatic activity in Caledonian-Yanshan period, and the acid intrusive rocks in Indosinian-Yanshan period were the most developed, which was closely related to gold mineralization (Li et al., 2006).
Regional fault structures are developed, with SN- trending regional large faults and NW-trending secondary faults as the main structural lines, superimposed with NE-trending and nearly EW-trending late faults, forming an intricate basic framework of fault structures intertwined in different periods and directions. The unique tectonic environment and multi-stage tectonic magmatism provide superior geological conditions for the formation of hydrothermal endogenetic minerals.
2 Geological survey of mining area
The ore-bearing stratum is the third lithologic member of the Lower Paleozoic Li Ziyuan Group (Gansu Provincial Bureau of Geology and Mineral Resources, 1997), which can be divided into three layers according to its rock assemblage: the lower layer is light gray fractured amphibolite with the thickness >134m; The middle layer is quartz schist mixed with amphibole schist, and the thickness is > 257 m; The upper layer is plagioclase amphibolite schist mixed with garnet quartz schist and marble, and the thickness is > > 2 10m. All layers are in overall contact (Figure 1) (Yin Xianming et al., 2000). The abundance of gold in the formation is 58× 10-9 ~ 65× 10-9. Among them, the plagioclase amphibolite schist formed by metamorphism of intermediate-basic volcanic rocks in the lower part has a gold content as high as 98× 10-9, which is an important source bed and main host rock.
The overall structural form of the mining area is a compound monoclinal structure with NW dip, and secondary small folds are developed. Fault structures are developed, which can be roughly divided into three groups: first, the NNW-trending faults are active in multiple stages, mainly in compression and torsion in the early stage and in tension and torsion in the late stage, characterized by ductile-brittle deformation, with strong physical and chemical action of tectonic plates in the belt, hydrothermal alteration, diorite fine-grained veins and gold-bearing quartz veins distributed, which are ore-controlling structures; The second is NE-trending faults, which are generally tensional and torsional in the early stage and compressive and torsional in the later stage, with obvious multi-stage activity and parallel distribution in space, often forming relatively dense compression zones. The rocks in this zone often change in different degrees and develop gold-bearing quartz veins, which are the main ore-controlling and ore-hosting structures in this area. Third, the east-west fault is a post-mining fault, which has destructive effect on ore bodies.
During Indosinian period, Chaijiazhuang adamellite occurred in irregular harbor-like rocks with an exposed area of 45km2. It belongs to S-type hypabyssal granite with shallow erosion. The wall rock alteration is fully developed, and the alteration bandwidth is tens of meters. The average content of gold in rock mass is 5. 13× 10-9, and the internal contact zone is as high as 56× 10-9. Gold-bearing quartz veins are mostly distributed within 2m of the outer contact zone.
Veins are developed, and timely veins are important gold-bearing veins in this area. Diorite fine-grained veins are closely associated with gold-bearing quartz veins, and are closely related to gold mineralization in ore belt ⅳ, and gold bodies are formed in areas with strong alteration. In LD403, IYM 1, it can be seen that diorite veins cut through gold-bearing quartz veins, and most of them are filled along the late fault zone near east-west, destroying gold ore bodies. Granite pegmatite dike, diorite porphyrite dike and lamprophyre dike have no obvious relationship with gold mineralization (Wuhan Institute of Geology, 1985).
Figure 1 Geological Map of Chaijiazhuang Gold Mine (soil geochemical anomaly)
1 —— Upper layer of the lower strata of the Lower Paleozoic Li Ziyuan Group; 2- the middle layer of the lower strata of the Lower Paleozoic Li Ziyuan Group; 3- The lower stratum of the lower strata of the Lower Paleozoic Li Ziyuan Group; 4- adamellite; 5- diorite vein; 6- quartz diorite vein; 7- diorite porphyrite vein; 8- Timely pulse; 9- ore belt; 10- failure; 1 1- comprehensive anomaly
3 Geological characteristics of the deposit
3. 1 ore belt and ore body characteristics
Four gold mineralization zones have been found in Chaijiazhuang Gold Mine (Figure 1), and the 1 1 gold ore body is circled.
The I, II and III gold mineralization belts are located in the south of the mining area, controlled by a group of NE-trending brittle faults, which are parallel to NE and tend to NW with an inclination of 52 ~ 7 1. Mineralized rocks are single-fractured gold-bearing quartz veins, mostly occurring in the fault zone in the form of single vein and lens, controlled locally by a group of secondary fault planes, and many veins occur in parallel (Figure 2), with obvious expansion, contraction and pinch-out. The boundary between the vein and the surrounding rock is clear, and there is often a layer of fault mud with a thickness of 1 ~ 5 cm on the contact surface between them. The surrounding rocks near the mine are mostly amphibolite schists with weak silicification, chloritization and carbonation alteration.
The ⅳ ore belt is located in the north of the mining area, controlled by the NNW brittle-ductile fracture, with strike of 345 ~ 350, surface dip of NEE, deep dip of SWW, and dip angle of 65 ~ 70. The ore-bearing rocks are composed of fractured gold-bearing quartz veins and nearby pyrite-sericite altered rocks, and locally altered diorite fine-grained rocks also constitute gold ore bodies. The roof rock is cataclastic amphibolite schist, the floor rock is diorite fine-grained rock, and the interlayer rock is sericite schist (Figure 3). The surrounding rocks are strongly altered, including sericitization, silicification and pyritization.
Gold ore bodies are mainly vein-shaped, followed by lenticular, mainly occurring in I, III and IV ore belts, with a length of 15 ~ 380m, a thickness of 0.27~2.70m and a controlled extension of 45 ~ 125m. The gold grade is 3.91×10-6 ~ 35.90×10-6, the highest grade of a single sample is 208.64× 10-6, and the average grade of the deposit is 20.70× 10-6.
3.2 Gold-bearing quartz vein characteristics
Time-dependent pulse is the most important gold-bearing vein in this area, and its formation is mostly controlled by ductile shear zone, which is mainly syntectonic time-dependent pulse and mostly spreads in northeast direction. Twenty-seven timely veins with a certain scale have been found in the mining area. Among them, 1 1 ore belt Ⅰ, 2 ore belt Ⅱ, 1 ore belt Ⅲ and 13 ore belt Ⅳ. According to its characteristics and gold content, it can be roughly divided into two types, namely, milky white time pulse and smoky gray crack time pulse. The distribution of the former has no obvious law, and the gold mineralization is weak; The latter is generally controlled by faults and mostly constitutes gold ore bodies.
The quartz vein type gold deposit is the main ore type of the deposit, and the light gray-smoky gray quartz is also one of the main gold-bearing minerals. The gold content of timely single mineral analysis is 4.22× 10-6.
Fig. 2 I1orebody 1755m middle section schematic diagram.
Past-amphibolite schist; 1- gold ore body and its number; 2- timely pulse; 3- lamprophyre vein; 4- Measuring Reverse Fault
Fig. 3ⅳ 1 orebody 1936m middle section schematic diagram.
1-plagioclase amphibolite schist; 2- diorite fine-grained rocks; 3- sericite; 4— Normal fault of measurement; 5— Measured reverse fault; 6— Gold ore body and number; 7- time-pulse type gold deposit; 8— Altered rock type gold deposit
3.3 Ore characteristics
The natural types of ores are simple, mainly time-pulse type gold deposits, followed by altered rock type gold deposits.
3.3. 1 mineral composition
The metal minerals in the ore are mainly pyrite and chalcopyrite, followed by galena (found in artificial heavy sand and IYM 1-2 tunnel), and a small amount of sphalerite, magnetite, arsenopyrite, chalcocite, celestite and malachite. Gangue minerals are mainly chronological, followed by sericite and chlorite, and a small amount of feldspar, kaolinite and calcite.
1) Pyrite: light yellow, yellow, grayish yellow, mainly semi-autotype crystals, followed by semi-autotype-autotype cubic crystals. Most of the cracks in pyrite are filled with chalcopyrite, chalcocite, celestite and other minerals, and a small amount of pyrite is oxidized into limonite around it. Pyrite is unevenly distributed in the ore, and locally appears as massive, banded and irregular veinlets. The particle size of pyrite varies greatly and its thickness is uneven. Most of the large pyrite particles are crushed. Pyrite is closely associated with gold minerals and is an important gold-bearing mineral.
2) Timely: light gray, gray, smoky gray, irregular shape, and irregular particle aggregate. The particle size is generally between 0.4-2mm, and the maximum is10.5mm. Due to the late tectonic action, it is strongly broken and forms fine gravel, which has been completely recrystallized. Under the action of late secondary stress, cracks and fissures are produced and filled by late metal sulfide and calcite veinlets. There are some pyrite and chalcopyrite inclusions closely related to gold minerals. Single mineral analysis shows that the gold content is 4.22× 10-6, which is one of the gold-bearing minerals.
3) The mineral composition of gold in the ore is relatively simple, mainly silver-gold ore, followed by natural gold. Gold minerals are golden yellow, mainly in the form of breccia and plate, followed by branches, circles, leaves and hairs. The grain size is 0.005 ~ 0. 18 mm, with an average of 0.027 mm, in which the fine gold < < 0.037mm accounts for 7 1.84%. The occurrence state of gold is that fracture gold accounts for 46.38%, intergranular gold accounts for 43.00%, and wrapped gold only accounts for 10.62%. Fracture gold and intergranular gold mainly occur in cracks and intergranular of pyrite and chalcopyrite, and the wrapped gold is mostly wrapped by limonite, chalcopyrite and timely.
3.3.2 Ore texture and structure
Time-related vein minerals have isomorphic, semi-isomorphic and heteromorphous unequal grain structures, and metasomatism, interpenetration, dissolution, inclusion and fragmentation structures are common. Altered rock type gold deposits have scaly granular crystal structure; Uneven disseminated, veinlets, massive and breccia structures are the main structures of primary ores in this area, and honeycomb structures are common in oxidized ores.
3.3.3 Contents and changes of beneficial ingredients
The main ore-forming element Au content is generally/kloc-0 /×10-6 ~ 50×10-6. Up to 208.64× 10-6. Among them, the gold content of time-pulse-type ore is relatively high, generally > 20× 10-6, and it is also common to be higher than 100× 10-6, while the gold content of altered rock-type ore is generally1×10-6 ~. In space, with the alternating appearance of two kinds of ores, the content of gold changes dramatically, while in the same ore, the content of gold is relatively stable, and the coefficient of change is generally less than 80%. Associated silver content is 8.12×10-6 ~ 26.12×10-6, copper content is 0.22% ~ 0.94%, which can be comprehensively recovered, and other elements are very low.
3.3.4 Metallogenic period and stage
The isotopic age of Chaijiazhuang granite body is198 ~ 206 Ma (biotite measured by K-Ar method), and the mineralization is late. The NE-trending fault closely related to ore bodies passes through granite and strata before Cretaceous, and multi-level dikes are developed in the fault zone. It is speculated that the metallogenic age of Chaijiazhuang gold deposit is from the end of Indosinian to Yanshanian.
According to the symbiotic combination of mineralization and minerals and their relationship, gold mineralization in mining areas can be divided into two stages and six stages.
In the first stage, metamorphic hydrothermal mineralization took place in the Caledonian Caledonian. On the basis of the initial enrichment caused by the regional metamorphic hydrothermal solution, the gold was reactivated and migrated to the favorable structural position by superimposing the tectonic hydrothermal solution, and the sericite-Yingshi-pyrite-native gold combination was formed by metasomatism. Altered rock type gold deposit is the first stage of gold mineralization in this area.
The second stage is Indosinian-Yanshanian magmatic hydrothermal mineralization. With the occurrence of large-scale acid magmatic intrusion, abundant post-magmatic hydrothermal solution is mixed with some meteoric water and metamorphic water to form sufficient ore-forming fluid, which carries a large amount of ore-forming materials to the favorable parts of the structure for filling and mineralization, and forms time-related vein-type rich ore in the area (Wang Youwen et al., 1985). According to the mineral association, this period can be divided into five metallogenic stages:
The first mineralization stage is pyrite-isochron stage: it is mainly composed of ivory isochron with a small amount of pyrite and gold minerals. Pyrite is embedded in the form of coarse cubic natural crystals.
The second metallogenic stage is gold-quartz-pyrite stage, which is mainly composed of medium-coarse pyrite and quartz. Pyrite mostly exists in the form of semi-authigenic granular crystals, which is a vein-like massive aggregate superimposed on the previous stage and is the secondary ore-forming stage of gold in this area.
The third stage is gold-quartz-chalcopyrite-pyrite stage, which is mainly composed of medium-fine grained heteromorphic pyrite, fumed quartz and a small amount of chalcopyrite. Pyrite is fragmented and coexists with chalcopyrite, arsenopyrite, silver-gold ore and natural gold, which is the main metallogenic stage of gold. The main mineral assemblage is gold-yingshi-chalcopyrite-pyrite.
The fourth mineralization stage is gold-quartz-polymetallic sulfide stage, which consists of fine-grained grayish-white quartz, medium-coarse galena and a small amount of fine-grained deformed pyrite, and is often enriched locally to form massive high-lead gold deposits. There are many early fine pyrite and smoked breccia in galena particles.
Silver-gold deposits mostly occur in galena particles in the form of irregular particles, which is another important metallogenic stage of gold (Yang Gensheng, 2007).
4 genesis of ore deposit
4. Geochemical characteristics of1
Regional geochemical characteristics of1.1
According to the spectral analysis of bedrock (Golden Fuji, 1994), the abundance values of main elements in Li Ziyuan Group are 0. 138× 10-6, Pb is 20.5× 10-6 and As is 3.97×/kloc-0. Among them, the average gold content of intermediate-basic volcanic rocks is 6.4× 10-9, and the gold content of Chaijiazhuang granite is 5. 13× 10-9, both higher than the Clark value of the crust.
4. 1.2 Geochemical characteristics of the mining area
Distribution characteristics of (1)Au element in strata and rock mass of mining area
According to the geochemical profile (bedrock spectrum analysis) data of the mining area 1∶2000, the average abundance of Au in the lower strata of Li Ziyuan Group is 17× 10-9, which is more than four times the Clark value of the crust, and I ore belt is located in it. In the middle layer of the lower strata, the average abundance of Au is 58× 10-9, which is more than 14 times of Clark value, and there are ⅱ, ⅲ and ⅳ ore belts. In the upper layer of the lower strata, the average abundance of Au is 96× 10-9, which is 24 times of the Clark value of the crust. This stratum is far away from all gold belts, and no gold body has been found yet.
The average abundance of gold in Chaijiazhuang rock mass is 56× 10-9, which is 4 times of Clark value in the crust.
From the above geochemical characteristics of gold, it can be seen that both Chaijiazhuang rock mass and Li Ziyuan Group volcanic rocks in the mining area have experienced the initial enrichment of gold, which is higher than the regional value of the same element, and become the high background zone of Au, which has the basic conditions for providing ore sources.
(2) Distribution characteristics of trace elements in ore bodies and surrounding rocks.
1) Distribution characteristics of trace elements: See table 1 for distribution characteristics of trace elements in mining area. As can be seen from the table, the contents of trace elements in various rocks are basically close to or slightly higher than Clark value, but they are obviously enriched in the corresponding veins, followed by structural breccia, intermediate-basic dikes and volcanic rocks, especially Ag, Cu, Mo and other elements, showing the correlation with gold mineralization.
2) Characteristics of soil anomalies: Nine comprehensive anomalies were identified in the soil survey in the mining area, which are generally composed of elements such as Au, Ag, Cu and Hg. The center of concentration is obvious, and the anomalies are well nested, with inner, middle and outer zones. The grade of gold is generally 36× 10-9 ~ 80× 10-9. Most of the anomalies are round or oval, mainly distributed in the northwest direction, which is consistent with the trend of known ore belts. The comprehensive anomalies of AP-4, AP-5 and AP-6 correspond well with known ore bodies.
Table 1 Trace Element Content w(B)/ 10-6 in Chaijiazhuang Gold Mine Rock
Note: Spectral analysis of the first laboratory of Gansu Bureau of Geology and Mineral Resources, 1992.
4. 1.3 Isotopic geochemical characteristics of ore bodies
In order to further study the mineralization, samples of sulfur isotope (pyrite-type gold deposit), hydrogen isotope and oxygen isotope (gold-bearing quartz vein) which can represent the characteristics of the deposit were collected in ore belts I, III and IV, respectively, and the determination results showed clear information about fluid mineralization.
(1) sulfur isotope
According to Zheng et al.' s research (2000), the variation of sulfur isotopes in different provenances: δ34S in iron meteorites is 0.0 ‰ ~ 0.6 ‰, δ34S in granite is-13.4 ‰ ~ 26.7 ‰, δ34S in metamorphic rocks is -20‰ ~+20 ‰, and δ34S in seawater sulfate is very high.
See Table 2 for the determination results of sulfur stable isotope samples in the mining area. As can be seen from the table, the variation range of δ34S is 4.90 ‰ ~ 7.82 ‰, with an average of 5.90‰ and a range of 3.320‰. The variation range of δ34S is narrow, and they are all positive values. Its value is higher than mantle-derived δ34S, but close to granite, indicating that the source of sulfur is related to granite.
Table 2 Sulfur Isotope Determination Results of Chaijiazhuang Gold Mine
(2) Oxygen isotope
Generally speaking, the δ 18O of seawater is 0‰ (variation < 1 ‰), the δ 18O of atmospheric precipitation is -54 ‰ ~ 3 1 ‰ (average -4‰), and the δ 18O of metamorphic water is 5.
Six oxygen isotope samples were taken from gold-bearing quartz veins, and the determination results are shown in Table 3. Δ δ18o varies from 9.52‰~ 1 1.63‰, with an average value of 10.54‰ and a variation range of 3.1‰, with small variation range and hydrothermal characteristics. The δ 18O value is close to that of granitic rocks, indicating that the source of ore-forming fluid is closely related to Chaijiazhuang granite, and there are also some meteoric water and metamorphic hydrothermal solution.
Table 3 Determination Results of Oxygen Stable Isotope in Chaijiazhuang Gold Mine
Note: Analysis of Yu Fuji of Guangzhou Institute of New Technology, Chinese Academy of Sciences.
(3) Hydrogen isotope
Four hydrogen isotope samples were taken from gold ore bodies, and the determination results are shown in Table 4. δD varies from -85 ‰ to -99 ‰. The average value is -9 1.75%, and the interval is-14%. Comparing the δD values of typical world deposit fluids listed in Table 6, it shows that the mineralization is involved in atmospheric precipitation, and a small amount of magmatic water and metamorphic water are mixed.
Table 4 Table of δD Content in Chaijiazhuang Gold Mine and Rock Mass
Note: Analysis of Yu Fuji of Guangzhou Institute of New Technology, Chinese Academy of Sciences.
4.2 Characteristics of Inclusions
Take a timely inclusion temperature and a salt sample at the north and south ends of the ⅳ ore belt. The former is close to the rock mass, with small inclusions (5 ~ 1 1 micron), gas-liquid ratio of 5. 15, formation temperature of 154 ~ 2 18℃, with an average of 65438+. The latter is far away from the rock mass, with large inclusions (5 ~ 20μ m), formation temperature (134 ~ 200℃, average 165.3℃) and salinity of 3.5% ~ 6.7%, average 5.8%. The results of temperature and salt measurement show that mineralization is related to rock intrusion.
4.3 Genesis of the deposit
To sum up, it can be seen from the change of δ34S that its value is higher than that of mantle source δ34S, but close to granite, indicating that the source of sulfur is related to granite. δ 18O is 9.52‰~ 1 1.63‰, which is close to granitic rocks, indicating that the source of ore-forming fluid is closely related to Chaijiazhuang granite, and some meteoric water and metamorphic hydrothermal solution are also involved. From the δD value, it shows that atmospheric precipitation participated in mineralization, and a small amount of magmatic water mixed with metamorphic water; In Figure 4, all samples fall within the range of rainwater-hydrothermal water, indicating that the ore-forming fluid is a mixed source body; The mineralization temperature and salinity are close to the corresponding values of Chaijiazhuang rock mass. Generally speaking, ore-forming fluid mainly comes from magmatic hydrothermal solution, followed by atmospheric precipitation, with the participation of metamorphic hydrothermal solution.
Fig. 4 δD-δ 18O diagram of chaijiazhuang gold mine.
(According to Feng Yimin 1995)
Combined with the geological characteristics of the deposit, that is, Li Ziyuan Group and Chaijiazhuang rock mass have high gold abundance values and are source beds (bodies); The deposit is located in the thermal halo zone of rock mass, and there are multi-level dikes in the mining area, which indicates that magmatic activity is relatively strong and provides heat energy for mineralization. Although strata, regional metamorphism, multi-stage tectonic activities and atmospheric precipitation are all involved in mineralization, the ore-forming materials and hydrothermal solution of the deposit mainly come from magmatic activities, mainly magmatic mineralization (Duan Yongmin, 2006).
Zhang, Sun Jidong (1994) and Jin Fushi (1994) made a preliminary study on the characteristics of the deposit in the early stage of exploration. Du (1999) considered that basic-intermediate-acid volcanic rocks are the necessary conditions for mineralization after studying the sedimentary formation characteristics and ore-bearing property of the group. Song, Feng Yimin and others (1996) think that the formation age of Chaijiazhuang granite body (Variscan period) is earlier than that of gold mineralization period (Indosinian-Yanshan period), so the rock body can only provide some ore-forming materials, but not heat source. Duan Yongmin (2006) considered that magmatic hydrothermal activity is the dominant factor of mineralization and the most important factor among many ore-controlling factors through the analysis of its geochemical characteristics on the basis of previous research results.
5 prospecting signs and prospecting direction
The contact zone of rock mass and the relatively closed mineral liquid circulation and sedimentary tectonic environment in this area are important conditions for mineralization. There are obvious prospecting signs in the area, and the prospecting signs and prospecting directions mainly include the following aspects.
1) Li Ziyuan Group and its Indosinian-Yanshanian intermediate-acid intrusive rocks are regional indicators for prospecting.
2) The NE-trending and NNW-trending faults in the contact zone outside the rock body are the structural signs of prospecting.
3) The outcrop of gold-bearing quartz vein is the most direct prospecting indicator, which is mainly characterized by smoky gray, fractured, honeycomb and grid structure, and contains metal sulfide. The type and content of metal sulfide is a direct indicator of the degree of mineralization enrichment, and those rich in irregular fine-grained pyrite, chalcopyrite and galena have the best mineralization.
4) Signs of wall rock alteration: sericitization, silicification, pyritization, chalcopyrite mineralization and galena mineralization are positively related to gold mineralization, mainly occurring in ore bodies and wall rocks near the mine, and rapidly weakening away from the ore bodies. Chloritization and carbonation are widely distributed in surrounding rocks, but they are strong in ore-bearing fault zones, which are signs of hydrothermal activity.
5) Mineralogical indicators: The main minerals of the ore are Yingshi, pyrite and chalcopyrite, all of which have typomorphic characteristics indicating gold mineralization.
6) The comprehensive anomalies of gold, silver, copper, lead, arsenic and mercury, which are dominated by gold, are geochemical indicators for prospecting. The scale and intensity of anomalies are obviously related to the intensity of gold mineralization and the zoning trend of elements: leading edge elements Hg and As and near-ore indicator elements Au, Ag, Cu and Pb.
7) The formation of ore deposits in this area is controlled by Fengdan Group, Indosinian-Yanshanian acid intrusive rocks and fault structures, and the main prospecting direction is NE-NNW fault within 0 ~ 2 km of the above-mentioned rock external contact zone; Secondly, although it is far from the rock mass, it develops NW-trending faults and intermediate-acid dikes; In the distribution area of Fengdan Group without magmatic activity, altered rock-type gold deposits may also exist in the NW-trending faults with strong alteration.
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(Author Zhang Yanchun)