Lannigou Gold Deposit, Zhenfeng County, Guizhou Province

Lannigou Gold Mine in Zhenfeng County, Guizhou Province is located in Lannigou, Shaping Township, about 40 kilometers southeast of Zhenfeng County, southwest Guizhou Province. 1984, the Institute of Geophysical and Geochemical Exploration of Guizhou Geological Exploration Bureau carried out1∶ 200,000 geochemical river sediment survey and gold prospecting in southwest Guizhou, and circled 127 gold anomaly. During the period of 1986, the regional investigation institute of Guizhou Geological Exploration Bureau found the Lannigou arsenic mining area during the verification of1∶ 50,000 harmonic geochemical anomalies in Fan Luo area, and carried out sampling analysis of gold content for geological verification. 1987 to 1988, the geological survey of the gold professional team in Guizhou 1 17 was carried out simultaneously with the soil geochemical survey in this area1:10000. At 1990, the Huangchanggou mine section completed the drilling of 3000m and the roadway of 1400m, and roughly calculated the new reserves of 7 t. Through1:1000,1:1000 large-scale geological mapping, trench exploration, pit exploration and drilling exposure, a number of gold-bearing fault zones were newly discovered, Lintan gold mine was confirmed and Anbao gold mine was discovered. 199 1 ~ 1993, * * drilling completed 12 000m, tunnel 7500m, from 1994 to 1996, Yaojiatian, Shizhu water well and tunnel were discovered one after another. 1997 further identified the occurrence law of Lannigou gold deposit and found the ore-rich pillar.

Figure 1 Regional Structural Location of Lannigou Gold Mine (A) and Geological Schematic Diagram of the Mining Area (B)

(1) Revision according to the Southwest Guizhou Structural Research Group 1992; B According to Luo Xiaohuan, 1993, modified)

1- platform carbonate rock; 2- Basin clastic rocks; 3- gold ore body; 4- nappe fault; 5- Fault; 6— Compressive and torsional faults and their number; 7— Shear faults and their number; 8- Fault of unknown nature; 9— Ore-controlling fault zones and their number; 10-axis of anticline; 11-the axis of the inverted anticline; 12- syncline axis

Geological background of 1

The structural position of Lannigou gold deposit belongs to the southwest margin of Yangtze platform, and this area (the northern margin of Youjiang basin) has experienced the evolution from passive continental margin to foreland basin from early Triassic to middle Triassic. The mining area is located in the southwest of Banchang thrust nappe structure, at the nose bulge of the east wing of Laizishan anticline (Figure 1A). The main ore body is strictly controlled by NE-trending F 1 fault and NWW-trending F3 fault (Figure 1B). The total length of the fracture zone is close to 1000m, the width is generally 5 ~ 15m, and the maximum width can reach more than 50 m. The main ore-bearing stratum is the Bianyang Formation of Latin Stage in Middle Triassic, and the original rock is thin-medium-thick layered siltstone.

Controlled by fault fracture zone, ore bodies are mostly plate-shaped. Take the F3 fault as an example, its overall strike is 290, and its dip angle is 65 ~ 85. Within the range of 520 meters controlled by existing projects, the width of fracture zone and mineralization alteration do not seem to weaken from top to bottom.

2 Geological characteristics of mining area

2. 1 mining stratum

The exposed strata in the mining area are Quaternary (Q), Middle Triassic Xinyuan Formation (T2x), Lower Triassic Luolou Formation (T 1l) and Upper Permian Wujiaping Formation (P2w). The stratum is generally distributed in the northeast direction. Influenced by Huangjiapingzi short-axis anticline, the eastern strata are inclined to NEE direction, and the western strata are inclined to SWW direction. Due to the influence of structure, the occurrence of strata on the axis has changed and distorted greatly.

2. 1. 1 new source group

The lithology of this layer is mainly gray thin-medium thick mudstone, calcareous mudstone, argillaceous siltstone and silty mudstone, with a small amount of marl and carbonaceous shale locally, with a thickness of > 300m m, and the mudstone is in a scale-like structure with directional arrangement and developed horizontal bedding. Siltstone has a granular structure of silty sand mixed with fine sandstone to varying degrees. The composition is mainly chronological, followed by siliceous rock fragments and feldspar, with a content of 70% ~ 80%. Cement is mainly hydromica clay minerals, followed by calcium and silica, with a content of 20% ~ 30%. The cementation type is mainly pore cementation. Submerged structure, horizontal bedding, oblique bedding and cyclotron bedding are common, which are one of the ore-bearing strata in this area and are in integral contact with the underlying Luolou Formation.

2. 1.2 Lower Triassic Luolou Formation

According to lithology, it can be divided into two sections: the second section (T 1l2) is gray medium-thick layered gravelly limestone, marl mixed with thin dolomite limestone and argillaceous limestone, and the rock is dense and massive. Gravel fragments in gravelly limestone vary greatly, and are mostly flat, with poor roundness and poor sorting. Gravel fragments are dense microcrystalline limestone fragments. The top is red thin marl, containing a small amount of mudstone and 200 meters thick, which is one of the main ore-bearing strata in this area. The first section (T 1l 1) is mainly composed of gray thin-layer striated marl and argillaceous banded limestone, with a small amount of dark gray thin-layer mudstone. Limestone, dense massive structure, horizontal bedding development, thickness of 190 m, and overall contact with the underlying stratum.

2. 1.3 Wujiaping Formation (P2w)

The oldest strata exposed in this area are located in the core of anticline, which are thick-layered flint limestone and biolimestone, and the exposed thickness is > 300m m.

2.2 Mining area structure

The area is dominated by NNE structures, mainly including Huangjiapingzi short-axis anticline and F3 thrust nappe fault. A number of thrust faults with high axial angles, such as F28, F4, F5 and F7, are developed under the F3 nappe in the northwest wing of the anticline. The strike of the fault zone is 2000-4000m long, with an inclination of 280-300 and an inclination of 68-80. The fault width is several meters to more than 30 meters, which is the most important ore-controlling fault in this area. The southeast wing of the anticline is the F 10 fault, which strikes ne, is more than 2,000 meters long, and tends to NW with a dip angle of 72. There is slight mineralization along this fault. The F 1 fault in the middle of the mining area is NW-trending, which is a late fault and cuts the NE-trending ore-controlling fault.

F3 thrust nappe fault is located in the west of the mining area, moving from west to east, with large scale, gentle dip angle and undulating profile. A layer of silicified breccia with stable extension is formed along the fault zone, with a thickness of 10 ~ 35m, generally about 25m.

The F 1 reverse fault is located in the middle of the mining area, with a dip of 220 and an inclination of 75, with a length of > > 4500m. When cutting NE and NE structures, the fracture zone changes in the range of 10~80m ~ 80m, which damages the ore body.

The F8 fault is roughly parallel to the F 1 fault, crosscuts the Huangjiapingzi anticline, and meets F7 and F5 in the Au-4 ore body. It is about 500 meters long, with fracture tendency NE and dip angle of 76. The fault width is 2 ~ 10m, and it reaches tens of meters at the intersection with F7. Tectonic rocks are developed in the fault zone.

3 Geological characteristics of ore bodies

3. 1 Ore body shape and output characteristics

Except for No.7 ore body, 1 ~ 5 ore bodies are all controlled by steeply inclined fracture zone, and the occurrence of ore bodies is consistent with the ore-controlling structure, with an inclination angle of 60 ~ 85. Among them, 1 ~ 4 ore bodies have roughly the same strike and are mainly distributed in the NNE thrust fault zone. No.5 ore body strikes NW with a dip angle of 50, with small strike extension, but large extension, large thickness and high grade.

The scale of gold ore bodies in this area is small, with the thickness of 1 ~ 15~80m, generally 2 ~ 3m, and the extension of 15~80m, but the mineralization is continuous, and the shape of ore bodies is generally lenticular, followed by wedge.

3.2 Ore characteristics

Ores can be divided into two types: one is the ore produced in marl, with micrite structure, banded and massive structure, and the main ore minerals are timely and calcite (accounting for 90%); Followed by clay minerals, containing a small amount of calcite veins, timely veinlets and limonite (oxidized from pyrite), with the highest gold content of 5.8× 10-6, generally1.5×10-6 ~ 4.5×10-6, and arsenic. Antimony < 150× 10-6 has a low grade and is relatively stable. Gold occurs in joints and fractures of silicified marl in the form of fine free gold, which belongs to the main ore type (1, No.4 orebody). Secondly, the ore produced in argillaceous siltstone has argillaceous sandy structure and layered and massive structure. Ore minerals are mainly timely (accounting for 70%), followed by clay minerals (accounting for 20%), sericite and limonite, which belong to pore cementation. The fillers are mainly clay minerals and embedded iron, and the highest gold content is 15× 10-6.

In addition, there are gold deposits in weathered laterite, such as No.7 ore body. Due to strong oxidation, its grade becomes rich, with the highest gold content of 8× 10-6.

3.3 Change type

The alteration of mineralized surrounding rocks is mostly distributed in a belt along the fracture zone, mainly silicification and pyritization, followed by carbonation, kaolin and organic carbonization.

Silicification is divided into three stages: in the first stage, it is weak, mainly small timely particles accompanied by chalcedony, dirty crystals and extremely low transparency; In the second stage, silicification is strong, marked by the formation of fine-grained timely, timely veinlets and reticular timely veinlets, with dyeing phenomenon and low transparency; In the third stage, the seasonal pulse is thick, with large particles (up to 2.3 cm), pure quality and good transparency.

Pyritization, accompanied by the first silicification in the early stage, is granular and irregular. In the middle stage, it is semi-self-shaped, shaped and granular. The crystal forms are mainly pentagonal dodecahedron and octahedron, and the crystallinity is not high. Most of them are oxidized to limonite by naked eyes, which appears as disseminated, concentrated in small sections such as joints and cracks in the fracture zone and its vicinity. Pyrite formed by alteration at this stage is the main gold-bearing mineral. In the later stage, the pyrite particle size is coarse, usually cubic, and the pyrite particle size is fine, generally 0. 1 ~ 0.4 mm, and the gold-bearing iron ore mostly exists in the form of semi-autogenous fine particles and fine particles.

Carbonation, mainly ankerite and calcite in the early and late stages, with little content, generally veinlets.

Kaolinization, mainly characterized by pink vein kaolin, is extremely rare.

Organic carbonization is a major feature of alteration in this area, developed in ore bodies, mainly disseminated along joints and small cracks, and formed by biological carbonization during hydrothermal process in closed environment.

4 Geochemical characteristics

4. 1 Macros and trace elements

The main elements in the ore are obviously rich in silicon, while the surrounding rocks are rich in calcium and magnesium. The DOP value of ore (pyrite degree of iron) is obviously higher than that of surrounding rock, in which the DOP value of ore is 0.25 ~ 0.93, while that of surrounding rock is < 0. 190.

The change of some trace elements has a good correlation with gold mineralization and is often regarded as an indicator element of gold mineralization. Gold and arsenic, antimony, mercury, thallium and uranium constitute an important combination of ore-forming elements, especially arsenic, antimony and mercury often coincide with gold halo. A systematic comparison of trace elements such As As, Sb, Hg, Au, Ni and V in two tunnels at an altitude of 720m and 560m shows the following characteristics.

See table 1 for the average element content of shale and sandstone provided by (mineralized) shale and (mineralized) siltstone in 1 area and Tu Liqian. The contents of arsenic, antimony and mercury in gold-bearing ores and surrounding rocks near Lannigou mining area are unusually high. Ni and V show weak abnormality (high or low) or no abnormality. The corresponding comparative study of trace elements in mudstone and siltstone of Triassic Banna Formation in this area also shows similar results.

The abundance of As, Sb and Hg is generally proportional to the abundance of Au, and the concentration order of each element in the ore is δ Hg > δ Au > δ As > δ Sb (table 1).

2) The mineralization degree of siltstone is obviously higher than that of shale, whether it is surrounding rock or ore body; Compared with surrounding rocks, siltstone has the highest degree of mineralization, especially in the ratio of Hg, Au and Sb (enrichment coefficient).

3) The abundance of As, Sb and Hg in deep samples (560m above sea level) is generally higher than that in shallow samples (720m above sea level), and there is little difference between the surrounding rocks of the roof and floor of ore bodies at the same altitude.

4) Vanadium and nickel are not associated elements of gold mineralization in Lannigou mining area.

Table 1 Trace Element Contents in Ores and Surrounding Rocks of Lannigou Gold Mine and Their Relationship with Comparative Values of Standard Sedimentary Rocks w(B)/ 10-6

Note: The average value is on the horizontal line, and the ratio of the average value to the corresponding element content of similar sedimentary rocks provided by Tu Liqian and others (196 1) is below the horizontal line. The standard abundance of gold is 0.005× 10-6, and the lower limit of gold content in mineralized samples is 0.5× 10-6. (According to Jong Li et al., 1995)

4.2 Carbon, oxygen isotopes and rare earth elements

Most of the carbon isotopes (δ 13CPDB) in the ore are-1.3 ‰ ~ 0, and the carbon isotopes in the surrounding rocks are -39.3 ‰ ~-2 ‰. Rare earth elements in all samples of Lannigou Gold Mine keep the basic characteristics of sedimentary rocks. The total amount of rare earths (excluding yttrium) is between109×10-6 ~108×10-6, which is rich in rare earths and generally has negative Eu anomalies. The REE distribution pattern of mudstone and mineralized mudstone is similar, and it is also similar to the North American shale combination sample. Siltstone and mineralized siltstone have obviously different distribution patterns, and the negative anomaly of Eu in the ore is significant, and heavy rare earths are relatively enriched. Heavy rare earth elements show obvious differentiation. The gold mineralization intensity and δEu, σ LREE/σ HREE values all show that the floor ≥ roof ≥ ore body. The samples of rare earth elements in the mining area of Banna Formation in Middle Triassic have obvious high values σ ree, σ lree/σ hree and δCe.

5 genesis of ore deposit

Restricted by the metallogenic age and based on the coupling conditions of metallogenic structure and ore-forming fluid, the metallogenic model is as follows: Youjiang basin cracking-back-arc basin stage (D2-T 1), initial source rock (layer) formed, and contemporaneous fault F7 was active; In the foreland basin stage of Youjiang Basin (T2), the basin tectonic water deposited by thick turbidite was pumped out from the source bed and gradually evolved into ore-bearing fluid. During the compressional orogeny (T3) in Youjiang Basin, F7 was reversed into a reverse fault, and together with the NW-trending reverse faults formed by orogeny (such as F3, F 14) and its supporting strike-slip faults (such as F2, F 12), a ore-guiding network system was formed. F5 and its upper wall T2xm4-3 mudstone * * * form a good structural closed circle, which leads to the ore-forming fluid mainly moving in the hydrothermal migration network dominated by contemporaneous faults. On the one hand, the folds formed during the orogenic period re-folded in the backward compression stage after the collision in Youjiang Basin (J 1), forming ne-trending superimposed folds; On the other hand, with the distribution of tectonic stress on the F _ 2-F _ 3 nearly X-shaped fault system, F _ 3 moves to the right and forwards, forming a decompression expansion zone in the extension area of the intersection of F _ 2 and F _ 3, and ore-bearing fluid enters the decompression expansion space to precipitate and form a super-large gold deposit. It is considered that the deposit is a post-sedimentary low-temperature hydrothermal deposit related to basin fluids and a product of post-collision orogeny and mineralization (Chen Maohong, 2007).

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(Li,, writing)