Bafangshan-Erlihe lead-zinc deposit is located in Fengtai ore concentration area of Devonian gold polymetallic metallogenic belt in the middle Qinling pre-arc basin. The exposed strata in this area are Gudaoling Formation of Middle Devonian (D2g) and Lithologic Member 1 and 2 of Xinghongpu Formation of Upper Devonian (D3x). Gudaoling Formation is fine grained microcrystalline limestone, mixed with silty limestone and argillaceous limestone, and its thickness is greater than 150m. The lower part of the first lithologic member of Xinghongpu Formation (D3x 1) is calcareous phyllite, ankerite phyllite and carbonaceous phyllite, and the upper part is banded thin limestone mixed with calcareous sericite phyllite, and quartz siltstone and chlorite-bearing sericite phyllite are mixed locally, with a thickness of 370 ~ 640 m; The second lithologic member (D3x2) is chlorite sericite phyllite with a thickness of more than 560m.

Folds and faults are developed in this area. The important and obvious fold structure in the mining area is the Jianshan-Bafangshan anticline that runs through the whole area. The axial direction of Bafangshan anticline is nearly east-west, with strike of 103 ~ 120, dip of S, dip angle of 70 ~ 85, wide in the east and narrow in the west. The ridge of anticline is fish-ridged, with the occurrence of 15 ~ 30 ∠ 75 ~ 90 in the north wing and 195 ~ 2 15 ∠ 60 ~ 70 in the south wing. The angle between the two wings is 3 1 ~ 60, which belongs to a relatively closed anticline. The anticline dips from east to west, with dip angle15 ~ 30. This anticline forms a piercing anticline in Jinchangping section, which strictly controls lead-zinc ore bodies. Faults in mining areas can be divided into longitudinal faults and transverse faults. The longitudinal fault is large in scale, roughly parallel to the axis of the anticline, partially filled with time pulses, and mainly developed in the contact zone between limestone and phyllite in the northern wing of the anticline, which is a compression-torsion fault; The transverse fault is perpendicular to the anticline axis and stratum strike, with the fault strike of 0 ~ 40, dip angle w and dip angle of 55 ~ 85. The formation sequence of roots, the presence or absence of dike filling in faults and the damage to ore bodies can be divided into two categories: the first category is equidistant distribution of 300 ~ 400 m, filled with diorite porphyrite dike, with small fault distance and no damage to ore bodies; The second type was formed later than the first type, and the fault distance was large, which destroyed the continuity and integrity of the ore body.

Magmatic rocks in this area are undeveloped, mainly diorite porphyrite veins and granite porphyry veins. The diorite porphyrite veins are controlled by NE-trending faults, and the veins are equally spaced. The length of a single vein is several hundred meters, the thickness is 0.5 ~ 1.0m, and individual veins are 5 ~ 6m. About 2km south of the mining area is the NWW-trending granite porphyry vein belt, which runs through the distribution area of Fengtai Devonian from the intrusion front of Xiba rock mass to the west, with a width of several hundred meters to more than a thousand meters. The intrusion of dike zone provides thermodynamic, structural and material conditions for the formation of lead-zinc deposits.

2. Geological characteristics of the deposit

Bafangshan-Erlihe ore body is located in the saddle of anticline structure and its two wings (the north wing is inverted). The main ore body occurs in siliceous rocks at the junction of Gudaoling Formation and Xinghongpu Formation. Only the piercing anticline formed in Bafangshan denudes the surface, and concealed lead-zinc (copper) ore bodies are formed at the east and west ends. The zoning law of ore-forming elements is that copper is dominant in the west and lead-zinc is dominant in the east (Figure 3-).

Figure 3-9 Geological Schematic Diagram of Bafangshan-Erlihe Lead-zinc Deposit in Fengxian County

The host rocks of this deposit are mainly siliceous rocks. The content of microcrystals in siliceous rocks is more than 80%, and its chemical composition is characterized by high SiO2 _ 2 content, Ti and Mn content close to limestone, al, Fe and Mg content close to phyllite, and the composition content changes greatly. The spatial distribution of siliceous rocks is controlled by anticline structure, its shape is consistent with anticline, and it is also controlled by limestone, and its distribution range is completely consistent with ore body. Most ore bodies occur directly in siliceous rocks; Siliceous rocks come in three colors: black, gray and grayish white. Similar to limestone, but hard, brachiopod fossils and sea lily stem fossils can be seen, sometimes filled with sphalerite.

Siliceous rock is a kind of hot water jet rock. Consistent with the collapse space of the anticline and the decompression and expansion structure along the strike and dip of the fault, it is 25 ~ 30 to the east, indicating that it has the characteristics of oblique spraying or transformation from east to west and from bottom to top at 25 ~ 30. Siliceous rocks are the result of early Paleozoic hydrothermal jet basement mineralization invaded by Indosinian-Yanshanian granite and "drawer-type" transformation mineralization from east to west and from bottom to top in Devonian.

The shape of ore body is controlled by anticline. In the Bafangshan ore block in the west, the surface mineralized zone is irregular ring around the core of anticline, with crystalline limestone of Gudaoling Formation in the ring and phyllite of Xinghongpu Formation outside the ring. The profile shows two forms: Figure 3- 10. The ore bodies in Erlihe ore block mainly occur in the saddle and the north inversion wing of the anticline, which are located in the contact area between Xinghongpu Formation and Gudaoling Formation, buried underground, and are crescent-shaped in plane and crescent-shaped and parabolic in section.

At present, there are 465,438+0 large and small ore bodies in this deposit, and the ore bodies are mainly produced in siliceous rocks (Figure 3-65,438+065,438+0), with II-65,438+0 being the largest, followed by II-2. Ⅱ-1orebody is 2345m long, with an average thickness of 6.06m. The maximum extension depth of the orebody in the north wing of the anticline is 560m, and the minimum extension depth is 60m. The average grade of the orebody is 1.33%, and the zinc is 6.06%. Some orebodies are copper orebodies, with an average grade of 0.8 1%, accounting for the total proven. ⅱ-2 ore body is 7 15m long, with an average thickness of 4.55m The west of Line 75 is a copper body with an average copper grade of 0.80% (the highest is 8.47%). To the east of Line 75 is a lead-zinc ore body with an average lead grade of 0.98% and zinc grade of 4.24%, accounting for 2.43% of the total reserves. From the characteristics of ⅱ- 1 and ⅱ-2 ore bodies, it can be seen that ① the two ore bodies are located in the same mineralized siliceous rock, and the cumulative length has reached 3060 meters. According to the anomaly obtained by measuring potential and gradient by borehole charging method, the anomaly reaches line 209 to the east, and the ore body still extends to the east and west. It is estimated that the total length of ore bodies can reach more than 4000m, and there is still more than1000 m. (2) The ore bodies in siliceous rocks at the interface between Gudaoling Formation (D2g) and the lower member of Xinghongpu Formation (D3x 1) account for 95.33% of the total reserves of the deposit; (3) According to the mining exposure, the ore body has the characteristics of sharp extinction and reappearance along the inclined direction, so the ore body still has certain expansion potential to the deep.

Figure 3- Geological Map of Bafangshan-Erlihe Lead-zinc Mine 1 13 Line

The main metal minerals in the deposit are sphalerite, galena and chalcopyrite, followed by pyrite, white iron ore, arsenopyrite, pyrrhotite, tetrahedrite, jamesonite and wheelstone. Gangue minerals are mainly Yingshi, ankerite and calcite.

The ore structure mainly includes crystal structure, metasomatic texture, solid solution separation structure, metamorphic structure and internal structure. Ore structures are mainly disseminated, veinlets, massive, variegated and banded.

The wall rock alteration is undeveloped and weak, and pyrite, pyrrhotite, silicification, sericitization, pyrophyllite and graphitization can be seen in phyllite. There are silicification and fading phenomena in limestone, a large number of calcite veins, graphitization and carbon deposition on vein walls and fractures. Alteration related to mineralization is limited to the upper and lower walls of the ore body within 2m, and the saddle sometimes reaches 5 ~ 10m.

The main elements of industrial significance in ores are zinc, lead and copper. In addition, the associated gold, silver, cadmium and mercury can be comprehensively recovered. Other elements such as Ga, In, Ge, Ti and Te are low in content and cannot be used.

3. Geochemical characteristics of the deposit

(1) Trace elements

The contents of transition metal elements copper and zinc in different types of rocks (ores) in Bafangshan-Erlihe lead-zinc deposit are quite different (Table 3-4). The Cu content of lead-zinc mine is much higher than that of ore-bearing siliceous rocks, phyllite and mineralization time-pulse, while the Zn content of lead-zinc mine is higher than that of ore-bearing siliceous rocks and mineralization time-pulse, but lower than that of phyllite, which accords with the characteristics that copper and ore coexist and some silicified phyllite are ore-bearing surrounding rocks. The content of Co in rocks (ores) is between (14 ~ 20) × 10-6, and the content of Ni is between (28 ~ 58) × 10-6, all of which are relatively high, indicating that their material sources are deep.

Figure 3- Typical Ore Characteristics of Bafangshan-Erlihe Lead-zinc Deposit +0 1

Table 3-4 Composition of Trace Elements in Rock (Ore) of Bafangshan-Erlihe Lead-zinc Deposit

Note: The samples were analyzed by ICP-MS of Northwest Nonferrous Metals Mineral Geology Test Center; The unit of gold and silver content is 10-9, and other elements are 10-6.

The content of incompatible element Ba in the rocks (ores) of the Bafangshan-Erlihe lead-zinc deposit is relatively high, ranging from (182 ~11868) ×10-6, which shows the characteristics of hydrothermal deposits in the basin, and high Ba content is a sign of the existence of hydrothermal deposits.

(2) Rare earth elements

The total amount of rare earth elements in the rocks of Bafangshan-Erlihe lead-zinc deposit varies greatly (Table 3-5 and Table 3-6), ranging from (7.04 ~ 206.51) ×10-6, and the total amount of rare earth elements in the ore is (19.38 ~/kloc-). The standardized distribution patterns of rare earth chondrites in rocks (ores) are all right-leaning light rare earth enrichment types (Figure 3- 12), and the ratio of LREE/HREE is greater than 3, indicating that the material sources and geological conditions of the two are almost the same. The distribution pattern of rare earth elements in rocks (ores) is similar to that in seawater, indicating that the water in the thermal fluid from the deep crust is mainly seawater (Wang Ruiting et al., 20 1 1). The vast majority of samples have negative europium anomalies, with δEu values ranging from 0.24 to 0.99, and some samples have positive europium anomalies, which may be related to their high carbonate content or barite-barite feldspar minerals. Generally speaking, Eu2+ replacement of Ca2+ or Ba2+ will lead to positive europium anomaly.

Table 3-5 Rare Earth Element Content of Rock (Ore) in Bafangshan-Erlihe Lead-zinc Deposit (wB/ 10-6)

Note: See Table 3-4 for the properties of the first five samples; Others are based on Wang Ruiting, 2005; Samples 82, 830, 84 1 are ankerite siliceous rocks, and samples 8 18 and 868 are siliceous rocks.

Table 3-6 Rare Earth Element Characteristic Parameters (wB) of Rock (Ore) in Bafangshan-Erlihe Lead-zinc Deposit

Fig. 3- 12 standardized distribution model of rare earth chondrites in rocks and ores of bafangshan-erlihe lead-zinc deposit.

(3) Ore-forming fluid

The hydrogen and oxygen isotope analysis shows that the δ 1 8 osmow of the Bafangshan-Erlihe lead-zinc mine 1 siliceous rock sample is 19.4‰, δ30SiNBs-28 is -0.5 ‰, and the δ 18 osmow of the sample is timely. 1997a), which is consistent with the δ 18OSMOW range of Devonian submarine hydrothermal sedimentary siliceous rocks in Qinling, indicating that siliceous rocks were formed by submarine hydrothermal chemical deposition.

The composition test of fluid inclusions shows that the average concentration of Na+ in ore-forming fluid is 5. 12× 10-6, and the average concentration of Ca2+ is 16.90× 10-6, and Mg2+. na+& gt； Mg ~ 0.44, which belongs to low salinity chloride brine, has the characteristics of infiltration brine, indicating that the ore-forming fluid is submarine hydrothermal.

(4) Sulfur isotope

The results of sulfur isotope analysis of main sulfides in Bafangshan-Erlihe lead-zinc deposit show that (Northwest Nonferrous Geological Exploration Bureau 7 17 Corps, 1993), sulfur isotope δ34S of sulfides are all positive, ranging from 2‰~ 12‰, with an average of 9.79‰, which belongs to heavy sulfur enrichment type. The sulfur isotopic composition of sulfide is different from magmatic hydrothermal deposits and biogenic sulfur sources, which may reflect the mixed source of sulfate reducing sulfur in seawater and sulfur in deep formation syngenetic thermal fluid.

(5) lead isotope

The 206Pb/204Pb of sulfide in Bafangshan-Erlihe lead-zinc deposit is17.78 ~18, and 207pb/204pb is15.46 ~15.8/kloc-. Table 3-7). The composition of four lead isotopes is stable. Combined with the analysis of lead isotopic composition and age data of other lead-zinc deposits in Fengtai ore concentration area, it is considered that ore lead belongs to normal lead with single-stage evolution. The Pb-Pb isotope model is between 395 Ma and 584 Ma, which is earlier than the Middle Devonian (384Ma), indicating that lead may mainly come from the underlying basement strata or the old denudation area of Devonian sedimentary basin, that is to say, the deep source of lead is very likely, but it is not excluded that the lead-zinc ore body was influenced by hydrothermal transformation after mineralization of Xiba rock mass in Indosinian period.

4. Genesis and metallogenic regularity of the deposit

Although the lead-zinc deposits in Fengtai Basin are similar to those in Xicheng Basin, Gansu Province, for example, the lead-zinc deposits in both areas are located between carbonate rocks and phyllite of Middle Devonian, the lead-zinc deposits in Xicheng Basin are located in Xihanshui Formation (D2x), and the lead-zinc deposits in Fengtai Basin are located between Gudaoling limestone (D2g) and Xinghongpu Formation (D3x) phyllite, but there are also many differences. Most of the lead-zinc deposits in Xicheng basin are located in. The ore-bearing formation is an interbedded zone of clastic rocks and carbonate rocks, while the lead-zinc deposits in Fengtai Basin are mainly produced in carbonate sedimentary closed depressions (Wang Hengdeng, 1996), and the ore-bearing surrounding rocks are mainly silicified limestone and siliceous rocks. The Indosinian-Yanshanian ore bodies basically underwent hydrothermal transformation, and the mineralization changed to some extent. Bafangshan-Erlihe lead-zinc mine is different from Changba lead-zinc mine in West Qinling. Changba lead-zinc mine belongs to SEDEX type, while Bafangshan-Erlihe lead-zinc mine, although hydrothermal activity is the most important mineralization in the same sedimentary period, some ores in the deposit are hot water filling metasomatic type (vein or layered ore bodies in limestone below the main ore-bearing bed), and the ore bodies are mainly controlled by anticline structure. According to the geological and geochemical characteristics of Bafangshan-Erlihe lead-zinc deposit, it is considered that the deposit belongs to hydrothermal sedimentary transformation type.

Table 3-7 Lead Isotopic Composition of Ore in Bafangshan-Erlihe Lead-zinc Deposit

Note: Wang equals,1996; 7 17 Corps of Northwest Nonferrous Geological Exploration Bureau,1993; Wang Junfa et al., 199 1.

Metallogenic laws can be summarized as follows:

1) episodic jet stage of hot water activity controls the ore-bearing strata of layered lead-zinc deposits in the corresponding sedimentary basins. Different ore-bearing strata reflect different stages of hydrothermal deposition. It is preliminarily considered that there are at least two main hydrothermal active episodes in the Devonian basin of Fengtai, which correspond to the hydrothermal sedimentary lead-zinc deposits in the following two horizons: ① the ore-bearing strata near the ①D2g-D2x interface, that is, the known large and medium-sized deposits, such as Qiandongshan and Bafangshan-Erlihe; ② Ore-bearing strata near the ②D3x-D3j interface, namely Magou, Donggou and Jianzigou to the north of Weiziping.

D2g-D2x interface is the main ore-bearing horizon, which is determined by the main episodes of hot water activity. Hot water activities are concentrated in the transition stage from carbonate rocks to fine clastic rocks in the filling sequence of extensional basins. The internal reason may be that the middle and late Devonian transition period is the strongest period of energy exchange between the surface and deep crust, which is conducive to the occurrence of hot water activities.

In addition, in a certain mining area, the occurrence of lead-zinc deposits is strictly controlled by "isochronous surface", not always controlled by a certain lithologic interface or a certain lithologic layer, especially in the area where sedimentary phase transition occurs.

2) The main ore-controlling structures of lead-zinc deposits in this area are syngenetic fault structures that provide hot water migration and local sedimentary basin structures that are rich in integrated minerals, including two types of syngenetic faults (main syngenetic faults that control sedimentary phase transition and inherited faults that fill NNE dikes in carbonate strata) and three types of hydrothermal sedimentary basins (structural depressions formed by syngenetic faults, barrier secondary sedimentary basins near syngenetic faults and synsedimentary anticlines composed of geochemical barriers and reefs that adsorb minerals).

5. Metallogenic age

Determining the metallogenic age of lead-zinc deposits in Fengtai ore concentration area has always been a difficult problem in deposit research. Predecessors have determined the lead-lead isotopic age of the Bafangshan-Erlihe lead-zinc mine, ranging from 395 to 584 Ma. The original lead isochron age of the four samples is 400Ma (Wang Hengdeng, 1996), indicating that the ore-forming materials mainly came from Devonian, not the real ore-forming period. The ore body of Bafangshan-Erlihe lead-zinc mine is mainly controlled by anticline structure. The ore body occurs in the core, saddle or inverted wing of anticline, and a large number of metasomatic structures are developed in the ore, indicating that the ore body has undergone hydrothermal transformation after the tectonic-magmatic period.

In this work, samples of mineralized silicalite, granite porphyry and diorite porphyry near Bafangshan-Erlihe lead-zinc deposit were collected, and zircon U-Pb isotopic dating was studied. The test was completed in the State Key Laboratory of Geology and Mineral Resources of China Geo-University (Wuhan). In-situ U-Pb isotopic analysis of zircon was carried out by La-ICP-MS, and the results are shown in Table 3-8.

It can be seen from Table 3-8 and Figure 3- 13 that zircon in mineralized siliceous rocks has obvious characteristics of detrital zircon, which is well rounded and has small particles, which is consistent with the dispersed phase of age peak described later. The diorite porphyrite zircon in Erlihe mining area has obvious magmatic zircon characteristics, most of which are long columnar and needle-like with banded structure, reflecting the rapid cooling process during the formation of dikes. A small amount of zircon has the characteristics of circular detrital zircon, which reflects that magma captured zircon in surrounding rock.

Figure 3- 13 Zircon cathodoluminescence images of mineralized siliceous rocks (upper row) and diorite porphyrite (lower row) in Bafangshan-Erlihe mining area.

From Figure 3- 14, it can be seen that the zircon crystal form of granite porphyry near Bafangshan-Erlihe mining area is good, which is characterized by tabular and columnar magmatic zircon, but most of it has been altered and appears black on the CL image, indicating that there is a certain alignment date.

Table 3-8 Zircon U-Pb Isotopic Dating Results of Siliceous Rock, Granite Porphyry and Diorite Porphyry in Bafangshan-Erlihe Lead-zinc Deposit

sequential

Note: Li 1-02 ~ Li 1- 18 is siliceous rock; Li3-0 1 ~ li3- 15 is granite porphyry; Li2-0 1 ~ Li2-20 is diorite porphyrite. The samples were tested in situ and analyzed by La-ICP-MS in the State Key Laboratory of Geology and Mineral Resources of China Geo University (Wuhan). Pb* is radioactive. The influence of.

As can be seen from Figure 3- 15, zircon U-Pb age peaks of mineralized siliceous rocks in Bafangshan-Erlihe mining area are scattered, with 400Ma, 600Ma, 800Ma and 1000 Ma, even 1800 Ma, which has obvious characteristics of detrital zircon age. These age values can not represent the formation age of siliceous rocks or lead-zinc deposits, but they have the same characteristics as the age spectrum of Yangtze plate. However, the characteristics of many detrital zircons in siliceous rocks reflect that the formation environment of mineralized siliceous rocks may be a coastal submarine depression.

Fig. 3- 14 cathodoluminescence image of zircon from granite porphyry in bafangshan-erlihe lead-zinc mine area

Fig. 3- 15 zircon U-Pb harmonic curve of mineralized siliceous rocks in bafangshan-erlihe lead-zinc deposit

It can be seen from Figure 3- 16 that the zircon U-Pb harmonic age of diorite porphyrite in Bafangshan-Erlihe mining area is (214 2) Ma, which is the product of late Indosinian tectonic movement. The relatively small weighted mean variance (MSWD)(0.34) indicates that the geochemical error is small, and the geochemical significance of this age value is clear. At the same time, from Figure 3- 16, we can see a few age values greater than 2 14Ma, such as 240Ma, 420Ma and 440Ma, which reflect that zircon in the surrounding rock material was captured during magma emplacement. The zircon U-Pb harmonic age of granite porphyry near Bafangshan-Erlihe mining area is (217.9 4.5) Ma, in which the weighted average variance is 5.7, which is slightly larger than diorite porphyry, but it still has geochemical significance. This age value is close to 2 14Ma of diorite porphyry, indicating that granite porphyry is also the product of late Indosinian tectonic movement.

Fig. 3- 16 zircon U-Pb harmonic curves of diorite porphyry (left) and nearby granite porphyry (right) in bafangshan-erlihe lead-zinc mine area.

The above data further confirmed that the Bafangshan-Erlihe lead-zinc deposit did undergo the tectonic-magmatic hydrothermal transformation in Indosinian period, indicating that the main metallogenic period of the Bafangshan-Erlihe lead-zinc deposit was Indosinian period, not Hercynian period, and Hercynian period was the main metallogenic period of the lead-zinc deposit.

6. Metallogenic model

Bafangshan-Erlihe lead-zinc deposit is a typical lead-zinc deposit in Fengtai ore concentration area. Lead-zinc ore bodies are mainly saddle-shaped or layered in siliceous rocks between limestone of Gudaoling Formation of Middle Devonian and phyllite of Xinghongpu Formation. Since 1980s, researchers have been paying close attention to this deposit. A lot of research work has been carried out in the aspects of metallogenic geological environment, metallogenic ore-controlling law, deposit genesis and deposit types, and many important research results have been obtained. However, the genetic model of the deposit is still controversial. One view is that it is hydrothermal jet deposition (SEDEX) or deposition-reconstruction (Zhang Fuxin et al.,1988; Qi Sijing et al.,1993; Wang Jilei et al.,1996; Wang Dengping,1996; Xue, Fang et al., 2000; Wang Ruiting et al., 2007a, 2011); Another view emphasizes that the deposit is supergene, and that the lead-zinc deposit in Fengtai ore concentration area is the product of supergene structure-hydrothermal process (Wang Yitian et al., 2009).

According to the geological background, geochemical characteristics, metallogenic regularity, genesis and metallogenic age of the Bafangshan-Erlihe lead-zinc deposit, it is considered that the deposit belongs to the type of jet deposition-structure-magma strong transformation. The metallogenic model is summarized as follows (Figure 3- 17):

Devonian fault basin (1) formation and submarine hot water jet deposition stage

The lead isotopic age of the lead-zinc deposit in Fengtai ore concentration area is 438 ~ 476 Ma, and that of the Bafangshan-Erlihe lead-zinc deposit is 455Ma, which is roughly equivalent to the Ordovician (500 ~ 440 Ma) to Silurian (440 ~ 475 Ma). This is consistent with the isotopic age of Qinling-type lead-zinc deposits, but the isotopic age of ore is far from the formation age of ore-bearing strata, and the age of ore-bearing strata of several deposits is obviously different, indicating that hydrothermal jet sedimentary mineralization mainly occurred in the formation period of early Paleozoic rift basin, accompanied by island arc marine volcanic eruption (flow) mineralization (inferred from the ore-bearing background and mineralization and mineralization characteristics of volcanic rocks in the northern Fengdan Group and Caotangou Group). Early Paleozoic tectonism, hot water jet and marine volcanic mineralization laid the basement structural framework and metallogenic scope of Devonian sedimentary basin in Fengtai-Taiyuan ore concentration area.

(2) the sedimentary stage of Devonian ore-bearing strata

In Devonian, Fengtai basin was in a relatively stable marine sedimentary period, forming Devonian ore-bearing strata. At the same time, in different sedimentary stages, the corresponding magmatic activity and the inherited submarine hot water jet made the basement fault continue to move and spread to the Upper Devonian, which laid the foundation for the subsequent structural and magmatic transformation and mineralization.

Fig. 3- 17 schematic diagram of metallogenic model of bafangshan-erlihe lead-zinc deposit

(3) The Indosinian period changed the metallogenic stage.

From Devonian to Triassic, due to strong orogeny, a complex combination of symmetrical folds, faults, syntectonic magmatic rocks and other metallogenic structures was formed, which laid the overall metallogenic structural framework of the Devonian system in Fengtai. NWW is transformed into a contemporaneous fault and a thrust nappe fault, which extends from the deep basement to the upper caprock, and the divergence is weakened in the contact zone between limestone and phyllite in the inverted wing of the anticline, and disappears in the decompression and expansion part of the saddle and wing of the anticline.

With the Indosinian tectonic and magmatic activities, the ore-forming materials in the basement and syngenetic fault zone were strongly activated, migrated from bottom to top and from east to west along the magmatic intrusion direction, and the ore-forming fluids were enriched in the saddle of Bafangshan-Erlihe anticline and the "sheath-like" expansion structure (bedding slip fault) space connected with the deep structure in the north wing. The uplift zone of Laochang-Dujiahe fault in the northeast of the west and Sanlihe-Baiyanggou fault in the east have certain uplifting and blocking effects on the NWW-trending metallogenic structure, which makes the Bafangshan-Erlihe anticline form a closed-semi-closed ore-accumulating structure to the east of Sanlihe-Baiyanggou fault and form mineralization.

7. Exploration model

(1) geological markers

The contact zone between limestone of Gudaoling Formation and phyllite of Xinghongpu Formation, or the reef subfacies and depression subfacies in the transition from carbonate platform to clastic rock in shallow sea basin are lithofacies indicators for prospecting. Various ore-bearing siliceous rocks are developed in these areas. The drilling data of Bafangshan-Erlihe lead-zinc deposit are sorted out and the database is established. It is found that the main ore bodies are almost all in siliceous rocks, and the boundary between siliceous rocks and surrounding rocks is clear. Therefore, it is considered that siliceous rocks are the most important indicator of prospecting along the structure in this area.

The expansion space of imbricate anticline formed by thrust nappe is the main structural part of mineralization and prospecting, that is, the "sheath-like" structure formed by saddle, steep wing or inverted wing of anticline dipping eastward.

(2) Geochemical signs

Planarly, the primary halo anomalies such as lead, zinc, copper, silver, antimony and arsenic are closely related to lead-zinc ore bodies. The combination of vertical and downward anomalous elements is Sb→Zn→As→Ag→Cu→Pb→ Hg. In addition, mercury is a good indicator for finding concealed deposits.

(3) Geophysical prospecting marks Comprehensive geophysical anomalies are indirect marks for finding blind ore bodies and concealed ore bodies. For shallow-buried ore bodies, high-power induced polarization method can reflect the effect well; For deep-buried ore bodies, CSAMT has an ideal effect. These methods can also judge the strike distribution of deep limestone and concealed anticline structures.

(4) signs of mineralization and alteration

Silicification, carbonation, pyritization, barite, electrochemical petrochemical and other surrounding rock alteration development sites are relatively direct prospecting indicators; Synchronous calcite veins are developed in and near lead-zinc deposits, and synchronous calcite veins and diorite porphyrite veins are also important prospecting indicators.