Laochang tin-copper mine in Gejiu City, Yunnan Province

The mining area is located in the composite part of NW, SN and EW structural systems and in the Gejiu fault fold bundle at the westernmost end of Caledonian fold system in South China. Its western end is cut off by the Red River Fault, and it is adjacent to sanjiang fold system and Yangtze paraplatform in the north. Triassic and some upper Paleozoic caprock folds are exposed, Indosinian movement ended marine deposition, and Yanshan movement is the most important intracontinental deformation in this area.

Second, the mining area geology

(1) stratum (Table 2- 105)

Table 2- 105 stratigraphic scale of Gejiu area

Gejiu Formation is the main ore-hosting stratum, with great lithologic changes and few fossils. Among them, Malage section is subdivided into four layers, which are ore-bearing layers of Malage tin deposit; Kafang section is subdivided into six layers, which are the ore-bearing layers of Laochang, Kafang and Songshujiao tin-copper deposits.

(2) Structure

Regional fold is an integral part of the arc structure in the northern margin of the ancient land in northern Vietnam. Anticlinorium of Wuzhishan in the northeast is a first-class fold in Gejiu area, with a total length of 40km. Laochang tin-copper mine is located on the northeast secondary Wanzi Street anticline. The east-west and north-south structures are dominated by faults. The former belongs to Nanling structural system, which mainly presents dense ore-hosting fault groups; The latter belongs to Sichuan-Yunnan tectonic system, bounded by Gejiu fault running through the whole region. Granite bedrock is exposed in the west with weak mineralization, and a series of concealed granite strips are exposed in the east. Laochang and other major tin and copper mines are located in the east. The NW-trending structure belongs to sanjiang fold system superimposed composite structure, which is characterized by short-axis folds. It is often the place to control the enrichment of ore bodies and ore belts by compounding and reforming with the NE-trending structure. In a word, Gejiu area is dominated by caprock folds, with gentle occurrence, multi-directional structural superposition and prominent features of fault blocks and domes.

(3) intrusive rocks

Magmatic activity in Gejiu area is very strong, and intrusive rocks are mainly divided into three stages (Figure 2- 156). The main rock mass in the whole area is 10, with an exposed area of 400km2.

Figure 2- Schematic Diagram of Magmatic Rock Distribution in Gejiu Area (Figure 2- 156)

1- porphyritic biotite granite; 2- porphyritic biotite granite; 3- medium grained biotite granite; 4- Fine grained biotite granite; 5— Alkaline syenite; 6- nepheline syenite; 7- Volcanic rocks; 8- gabbro-monzonite; 9— Diabase; 10-metamorphic diabase; 11-Indosinian gabbro; 12- Early Yanshanian granite; 13-Late Yanshanian granite; 14-late Yanshanian syenite; 15-Granite isobath

Jiasha rock body was formed in Indosinian period and is gabbro-monzonite. Longchahe rock mass, which was formed in the early Yanshan period, is the main part of the complex bedrock in the west of Gejiu. Its lithology is basic coarse porphyritic biotite granite with isotopic age of 65438 047 Ma 3 Ma. Due to the inclusion of amphibole and diorite granite in the rock mass, plagioclase is composed of Labrador, intermediate feldspar and plagioclase, and the initial value of strontium isotope is 0.7 102 9, which belongs to the product of mixed granite slurry stage. Granites in the late Yanshanian are the most important. According to its chronological order, it is divided into three stages. Ma Song pluton is a normal porphyritic biotite granite, which was formed in the late and early stage, and its isotopic age is (100 ~103) 2ma. The initial values of strontium isotopes are 0.7 142 8 (Malag) and 0.7088 5 (loose feet), which still belong to the mixed granite with uneven magmatic evolution. Rock bodies such as Shenxianshui, Baishachong, Laochang, Kafang and Xinshan were formed in the middle and late stage, including coarse-grained biotite granite and medium-fine light-colored granite, with isotopic age of (84.41.1) ~ (81.2) Ma. The initial value of strontium isotope is 0.7102 ~ 0.7108; There are no black inclusions in the rock mass; Petrochemistry is characterized by high silica alkali, low aluminum and low magnesium, and the content of No2O is greater than K2O, so it is a granite with high magma homogeneity.

Baiyunshan pluton, formed in the late Yanshan period, is syenite and nepheline syenite, and its Rb-Sr age is 93.3ma±2.4ma, which contradicts the intrusive contact relationship. The age value of K-Ar method is 59.5 ~ 62 Ma, which seems to be Himalayan period. Its initial strontium isotope value is 0.7038 ~ 0.7099, which belongs to the differentiation product of mantle-derived magma series.

From Indosinian basic rocks to late Yanshanian super-acidic rocks (excluding alkaline rocks such as Baiyun Mountain), the evolution of REE characteristics is very regular: from 940.6× 10-6 to 195.52× 10-6, in which Laochang rock mass is 2 18. The content of light rare earth decreased from 905.77× 10-6 to 74.46× 10-6 (old factory). HREE increased from 30. 17× 10-6 to 143.82× 10-6 (old factory): w(LREE)/w(HREE) decreased from 30.02 to 0.52 (old factory). Δ EU value decreased from 0.76 to 0.03 in turn (old factory). It can be seen that Laochang tin-copper mine is the result of high differentiation of granitoids.

Compared with the mixed porphyry granite in early Yanshanian and early Yanshanian, Laochang granite body has many rock differentiation indexes reflecting its highly differentiated characteristics. W (TiO _ 2)/W (Ta) value is 2200 in the early rock mass and 7 ~ 20 in the old factory; The value of f [w (Li+Rb)/w (Sr+Ba)] was 193 in the previous period and 6607 ~ 21563 in the old factory; The felsic index was 8 1.50 in the early stage and 90.45 in the old factory; W(Na2O)/w(K2O) is 0.53 in the early stage and 0.73 in the old factory; The aluminum index (A/CNK) was earlier 1.003, and the old factory 1.043. The ratio of w(F)/w(Cl) is 1.96 in the previous period and 8.26 in the old factory. The W(Rb)/w(Sr) ratio of early rock mass is 0.259 ~ 0.966, and that of the old factory is 2.98 ~ 63.88. The average Snn content of Laochang rock mass is 25× 10-6, and the average F content is 3750× 10-6, which is much higher than that of ordinary granite.

One of the biggest characteristics of granite intrusions in Gejiu area is that the rock mass formed in the later period is mainly concealed in the east of Gejiu fault, and large-scale tin-copper deposits such as Malage, Songshujiao, Laochang and Kafang have been formed from north to south. These rock masses are small and medium-sized, mainly composed of rock plants and rock processes, and the top surface fluctuates greatly, so the contour line of the top surface of granite is specially marked in Figure 2- 156 above.

Three. Copper mine geology

(a) the shape and size of the ore body

The mining area is 5.7km2, and it is a NE-trending compound anticline. The east and west sides are respectively controlled by two NE-trending faults (Huangnidong fault and Aotoushan fault). The north and south sides are respectively sandwiched by two east-west faults (Meiyu thrust fault and Yiku thrust fault). Because the main deposits in the mining area are controlled by concealed granite, the horizontal profile (projection) of a representative tin-copper deposit in the middle underground section (compiled according to the roadway and control data) is shown in Figure 2- 157. In addition to the surface placer tin ore, the following ore bodies appear in the longitudinal section from bottom to top (Figure 2- 158): the deepest part is the ore-bearing skarn or cassiterite sulfide ore body in the granite contact zone; The middle part is interlayer fracture zone and partial vein oxidized ore body; Shallow part is mineralized marble. Tourmaline veinlets are formed in the middle and shallow parts along the process of granite plants. There are 274 large and small ore bodies in the whole region.

In these ore bodies, tin generally reaches industrial value, but the main copper ore is limited to cassiterite sulfide ore bodies in granite contact zone. The vertical zoning of different associated elements is as follows: Sn, Be, W, Nb and Ta are enriched in granite due to greisenization; The granite contact zone is rich in tin and copper. The central oxide ore is rich in tin and lead; Shallow oxidized ore and mineralized marble are rich in lead, silver and tin; Veined ore bodies are rich in beryllium, tungsten, tin, lithium and rubidium. As the most important tin-copper ore body, its shape changes with the ups and downs of granite, the mineralization scale is large, and the thickness and structure of the ore body also change greatly, mainly layered, followed by lenticular, veined, columnar and irregular (Figure 2- 159). Length 100 ~ 500m, width 25 ~ 250m, thickness 6 ~ 16m, local expansion 30m. The average content of Sn is 0.3% ~ 1.2%, and that of rich ore is 2% ~ 3%.

The average Cu is 1. 18%, and the rich ore is 2% ~ 3%. The associated tungsten, bismuth, beryllium, zinc, lead, gallium, indium and silver have comprehensive utilization value. The most important Wanzijie ore block contains 30.26% tungsten, 0.03% bismuth, 0. 127% beryllium, 3.72% zinc, 4.9 1% lead and 50.93× 10-6 silver on average.

(2) Mineral composition of ore

The main metal minerals are arsenopyrite, pyrrhotite, pyrite, chalcopyrite, cassiterite, chalcopyrite, scheelite and natural bismuth.

Gangue minerals include diopside, andradite, almandine, pumice, tremolite, andalusite, fluorite, Yingshi and calcite.

In other tin-based oxidized ore bodies and veinlet ore bodies, the copper content is low, and most of them are combined copper oxide or malachite in hematite and limonite.

(3) Ore texture and structure

The ore is mainly granular and crystalline with irregular structure. Oolitic and pea-shaped structures composed of residual collophanite metasomatized by pyrrhotite were found locally.

The ore structure is mainly dense massive, disseminated, variegated and reticulate.

(d) Symbiosis related to beneficial ingredients.

There are many beneficial components associated with symbiosis in Laochang tin-copper deposit, and different types of ore bodies have different spatial zoning of element combination. According to statistics, copper (and tungsten) ore bodies with industrial value are distributed less than 300m away from granite, tin ore bodies are 200 ~ 600 m, and lead-zinc-silver ore bodies are mostly 500 ~1300 m.

When the Laochang tin-copper deposit forms vertical zonation of veinlet type ore belt along the protruding part of the rock, the changing rules of other element combinations are as follows: Be and W are enriched in the lower part, accompanied by Cu and Mo; The middle part is rich in tin, boron and fluorine; The upper part is rich in lithium, rubidium and cesium, with lead, zinc and silver.

Fig. 2- 157 Schematic diagram of horizontal profile of Laochang tin-copper deposit (1: 20000) Fig. 2- 157 Horizontal profile of Laochang tin-copper deposit (according to Huang Tingran).

1- concealed granite; 2- Sulfide in granite contact zone; 3— Distribution range of interlayer deposition; 4- fracture; 5- syncline; Six anticlines

Figure 2- 158 Schematic Diagram of Vertical Zoning of Various Deposits in the Old Factory Figure 2- 158 Profile of Vertical Zoning of Mineralization in the Old Factory

1- middle and late Yanshanian granite; 2- skarn; 3- Sulfide ore in contact zone; 4- Placer; 5- greisenization; 6- oxidized ore; 7— Tourmaline veinlet type

Figure 2- 159 Schematic diagram of contact orebody of Laochang deposit (according to Gejiu tin mine geology 1984) (after 1984)

1-tin ore body; 2- skarn; 3- felsic rocks; 4- granite; 5- marble; 6-interlayer of marble and limestone dolomite; 7- marble; 8- Tunnel; 9- Drilling

(5) changes in surrounding rock near the mine

There is a certain spatial relationship between the combination of useful elements and the strain of concealed granite, and the alteration zoning also depends on the rock mass. Skarnization of carbonate surrounding rocks is developed within 300 meters from the rock mass; Pyritization, phlogopitization, etc. , 500 m farthest from the rock mass; The transition from coarse-grained marble to fine-grained marble and the widely distributed fading zone can be more than 600m away from the rock mass

Potash feldspar widely exists in granite; There are obvious albitization and light mica outburst at the edge and top of the rock mass, and there are self-metamorphic light alkali feldspar granite with a thickness of several meters to several tens of meters. In some places, greisen-type tin ore bodies are widely distributed (such as 102 1 area).

As the main ore body of tin-copper mine, the wall rock alteration is undoubtedly skarnization. In the stage of contact metamorphism, anhydrous skarns and water-bearing skarns are widely formed, and tin-bearing magnetite skarns only appear in a few sections, and most of them are directly superimposed as cassiterite-sulfide ore bodies. The early simple skarn composed of anhydrous island chain calcium silicate rarely formed industrial ore bodies; The complex skarn composed of late hydrosilicate is the main host rock of cassiterite-polymetallic sulfide deposits in Gejiu area.

Fluorite mineralization and electrochemistry are important signs of near-ore alteration, and their range can be superimposed on tin-copper ore in skarn zone, and can also migrate far upward with fracture zone, especially different types of tourmaline veins.

Pyritization, phlogopitization, marbling and fading zones can be hundreds of meters away from tin-copper ore bodies, which are also prospecting indicators.

Others, such as topaz, beryl, lepidolite, limonite and manganese soil, have only secondary indicative significance in the above-mentioned main alteration zones.

(vi) Geophysical and geochemical anomalies of the deposit.

Gravity survey has a good effect on judging the scale, occurrence and thickness of concealed granite. Figure 2- 160 is the gravity anomaly map of Gejiu area. The granite foundation in the west area is dominated by low gravity, and the isolines of concealed granite plants such as Laochang and Malage in the east area are relatively complex and have low gravity. In the prominent part of rock foundation and rock plants, the contour slope is gentle and the surrounding slope is steep.

Electrical sounding is very effective for further determining the undulation of granite interface.

Soil survey and primary halo are mainly based on the zoning of ore deposits, and it is more direct to track anomalies of lead, zinc and silver at a long distance and anomalies of tin, tungsten and copper near the mine.

Fig. 2- 160 gravity anomaly in old mining area Tu Tu 2- 160 gravity anomaly in old mining area (according to Li Xinglin) (after Li Xinglin)

The anomaly of heavy sand in water system and soil has a good indication value for the surface display and long-distance transportation of Sn and W.

Verb (abbreviation of verb) metallogenic conditions

(1) stable isotope

(1) sulfur isotope: δ34S of pyrite and galena in Laochang have little change, and the model is between 0 ~ 1‰, and the average value is very close to meteorite sulfur, which fully reflects the characteristics of magmatic genesis.

The δ34S of all tin, copper and silver deposits in eastern Gejiu, including Songjiao and Shuangzhu, which are related to granites in various stages of late Yanshanian, are between-2 ‰ and+8 ‰. Comparatively speaking, the change range of the old factory is the smallest, and the average value is also the closest to the 0 axis. The average δ34S value of ore-forming sulfides related to early Yanshanian or pre-Yanshanian mixed granites in the west of Gejiu is more than 8 ‰, and the range of variation is-1‰~+22‰, indicating that more exogenous sulfur is added.

(2) Hydrogen, oxygen and carbon isotopes: the average δ 18O of the four whole rocks in Laochang is+1.85 ‰, which is different from that of Malage (+1 12 ‰) and Kafang (+)

Hydrogen isotope has not been measured in Laochang mining area, but the δD values of Malage inclusion water-95 ‰ and Kafang inclusion water-77 ‰-95 ‰ which are similar to Longchahe and Shenxianshui rock bodies in the west of Gejiu are generally close to-58.5 ‰-99 ‰.

It is preliminarily considered that the ore-forming fluid before the main sulfide stage is mainly magmatic water, and the atmospheric precipitation is less than1/5; The main ore-forming fluid in sulfide stage and carbonate stage is mainly atmospheric precipitation, especially in carbonate stage, accounting for about 3/4. The carbon isotope of Laochang tin-copper deposit has not been determined. The CO2δ 13C values of inclusions measured in Longchahe rock mass in western Gejiu are -5.0 ‰ ~-28.2 ‰. It is preliminarily considered that this shows the addition of deep carbon and organic carbon in sedimentary surrounding rock. The hydrothermal mineralization stage, skarnization and marble in Laochang mining area are also likely to join decarbonization.

(3) Lead isotope: galena in Laochang mining area is 206Pb/207Pb 1. 188, which is close to the ratio of other mining areas in the east 1. 18; 206Pb/204Pb= 18.55, which is consistent with other mining areas in the east 18.43 ~ 18.67; 207Pb/204Pb= 15.62, which is similar to other mining areas in the east (15.08 ~ 15.67). 208Pb/204Pb=39.46, which is similar to 39.98 ~ 40.29 in other eastern mining areas. The above data show that the lead isotopic composition is quite uniform, reflecting that the magma source and ore-forming materials all came from the deep crust and experienced high melting and homogenization. The lead isotope μ value is 8.8 ~ 9.0, which is also the characteristic of lead in the crust.

(2) Temperature measurement of inclusions and characteristics of accessory minerals.

The accessory minerals of Laochang rock mass are monazite and xenotime, which have the characteristics of S-type granite and form tin deposits. The accessory minerals in Ma Song rock mass in eastern Gejiu are chalcopyrite, sphene, magnetite and apatite, which have the characteristics of I-type granite and also form tin ore. Except for diagenetic age, basic degree of rock mass and evolution stage, the difference between them may be that, as Yao (1964) pointed out, the rock mass in Laochang mining area invaded into the open environment, and cassiterite ore series developed in large quantities except cassiterite sulfide. However, the rock mass in Songshujiao mining area invaded into a relatively closed environment, and only cassiterite sulfide ore bodies were formed. The example of Gejiu shows that S-type granite and I-type granite can coexist, and I-type granite can also have tin-forming specificity.

The temperature of Laochang rock mass measured by inclusion fracture method is 6 15 ~ 655℃. According to the melting experiment of Zeng et al. (1984) and the calculation of the thickness of the stratum cover, the Laochang tin-copper granite was formed at a depth of 2.5 ~ 4 km, the pressure condition was 1500 ~ 3000 kg/cm2 and the initial melting temperature was 624 ~ 63 1℃.

The homogenization temperature of fluid inclusions measured by Dai (1996) is 40 1℃. The explosion temperature of pyrite in Laochang is 386℃. The homogeneous temperature of sphalerite (measured from loose feet) is 180 ~ 257℃, with an average of 220℃; The homogenization temperature of calcite inclusions is 204 ~ 2 17℃.

According to stonework (1986), when pyrrhotite is generated, the sulfur fugacity fs2 = (8.6×10-7 ~ 6×10-5) pa and α Fe = 0.55 ~ 0.65; When marmatite is formed, the sulfur fugacity fs2 = (1.0×10-4 ~ 3.6×10-5) pa.

No salt-bearing minerals were found in the inclusions, indicating that the solution was in a state of salinity unsaturation. The contents of alkaline components (K+, Na+, Mg2+, Al3+, etc.) in the inclusions are low. ) and rich in acidic components (CO2, H2O, CH4, N2, HF-, etc. ), and the composition is relatively simple.

(3) Analysis of metallogenic conditions

(1) The lower member of Gejiu Formation (Kafang Member) is the most important ore-bearing horizon. In primary ore, 90% of tin, 96% of copper and 4 4% of lead are stored in Kafang ore block.

Carbonate rocks in Gejiu Formation are divided into four types according to chemical composition, in which tin and copper are mainly enriched in calcareous carbonate rocks and calcareous magnesium carbonate rocks; Lead is mainly concentrated in magnesium carbonate rocks; Copper is mainly concentrated in silicon-containing aluminum carbonate rocks. Many layers of carbonate rocks containing gypsum (salt) evaporated salt were found in Gejiu Formation, and their gypsum fading, dolomite fading, gypsum dissolved breccia, cryptophyta carbonate rocks, carbon-rich and bioclastic limestone are very favorable for ore hosting. The more layers of lithology interaction, the abrupt interface of different lithology and the change of surrounding rock physical properties are often the places where mineralization intensity increases.

(2) The parent granite of Laochang tin-copper mineralization is the rock mass with the latest emplacement stage, the highest degree of differentiation and typical characteristics of crust source remelting. The rock mass is acidic, alkali-rich, calcium-magnesium-poor, rich in diabase and high in beneficial elements background value, which is very favorable for mineralization. Concealed granite is not thickly covered, but its denudation degree is low, which can preserve various types of deposits. The undulating shape of the top surface of the rock plant is very important for the control and enrichment of ore bodies. Figure 2- 16 1 compares the prominent morphology of concealed granite with the distribution law of tin-copper ore bodies, which intuitively reflects the close relationship between them. Among them, a series of NE-trending multi-peak dorsal secondary uplift zones (Gongwangshan-Juhuashan-Lanshe Cave, Yu Sun Dam -05, 403- 102 1, 4 14 1 uplift, etc. ) is one of the most important conditions for the accumulation of Laochang tin-copper deposit.

(3) The enrichment of tin, copper and other resources in Laochang deposit not only benefits from the combination of Nanling East-West Belt, Sichuan-Yunnan North-South Belt and Sanjiang Tethys Belt, but also is superimposed by Cathaysian or Pacific Rim Belt in Northeast China, and various stress fields are intertwined and balanced, resulting in the favorable configuration of short-axis folds, domes, interlayer slippage and secondary faults in this area, forming various ore-hosting structures.

(4) Very thick impure carbonate rocks were emplaced by medium-deep granite, resulting in extensive contact metamorphism and hydrothermal alteration halo. The most abundant copper and tin minerals are diopside and calcium-iron-aluminum garnet skarn formed by three carbonate rocks: silicon-aluminum, calcium and calcium-magnesium.

The rise and transformation of water-rich volatile solution leads to the close symbiosis between water-bearing skarn and cassiterite sulfide. There are many hydrothermal alterations in Laochang mining area, especially electrochemistry, fluorite, andalusite, greisenization, lepidolite, silicification, carbonation and fading. It is not only a plane distribution, but also a typical vertical zoning, which is a sign of orderly evolution of mineralization.

(5) According to the characteristics of various primary ore bodies in Laochang, the ore-forming stages are divided into: contact metamorphic silicate stage, in which skarn containing scheelite is only formed, and the ore-forming temperature is about 400℃; In the oxidation stage, greisen type, wolframite chronological vein type, tourmaline veinlet type, beryllium-bearing granite and other ore bodies are formed, and the metallogenic temperature is 350 ~ 380℃. The sulfide stage is the metallogenic period of tin, copper and polymetallic in this area, and the metallogenic temperature is 260 ~ 350℃. Mineralization in carbonate stage is weak, mainly polymetallic and low-grade tin ore, and the metallogenic temperature is 200 ~ 260℃ or lower.

(6) The diversity of ore-forming elements also reflects a feature of Laochang deposit. In addition to forming independent minerals or isomorphic images with rare, rare earth and dispersed elements such as lithium, beryllium, niobium, tantalum, rubidium, cesium and boron, a wide variety of rare minerals are also produced. For example: beryl, jamesonite, hydrotalcite, chalcopyrite, boron-calcium-tin, hydrotalcite, stibnite and so on. Some minerals are rare in China. In addition, the undeveloped sulfide minerals are also the characteristics of this area.

(7) It is not difficult to see that Laochang tin-copper mine is a typical cassiterite sulfide deposit related to granite. However, there are still some places that need further study in the cause of formation. One is the superposition of other reasons (desliming, salinization, karst or others) for interlayer oxide ore, and the other is the initial enrichment of Triassic semi-closed basin deposits in Gejiu area.

Figure 2- 16 1 schematic diagram of the spatial relationship between the shapes of Laochang granite (a) and cassiterite-sulfide deposit (b) and cassiterite-sulfide ore body (below).

A- contour map of concealed granite; B- distribution map of ore bodies in contact zone; 1-tin ore body; 2- tin-copper ore body; 3- tungsten ore body; 4— The outline of the protruding top of the concealed granite; 5— Granite depression zone

Fig. 2- 162 Regional metallogenic model of Sn-W-Pb-Zn-Ag deposit in southeastern Yunnan (according to the post-Luo Junlie).

1- Early Yanshanian granite; 2- the first stage granite in the late Yanshan period; Late Yanshanian 3-Ⅱ granite; 4- Himalayan alkaline rocks; 5- sulfide ore bodies in skarn; 6— Layered and veined oxidized ore bodies and sulfide ore bodies; 7- reticular ore body; 8— Orebodies in altered granite (porphyry)

Six, deposit model and prospecting criteria

(A) the deposit model

Southeast Yunnan is one of the famous polymetallic metallogenic areas of tin, tungsten, copper and silver in China, belonging to Yanshanian granite related metallogenic series. In order to compare the similarities and differences between Laochang tin-copper deposit and other deposits, the regional metallogenic model summarized by Luo Junlie (195) (Figure 2- 162, Figure 2- 163) and 10 deposit types are cited here. Among them, Niushipo type is a metamorphic granite porphyry tin-tungsten deposit; Loose-foot type is a tin deposit dominated by skarn type in contact zone; Mala format is a tin deposit in close contact with tubular oxide veins. The card room type is a layered copper-tin deposit in the intersection zone of contact zone and diabase; Shuangzhu type is a vein polymetallic tin-silver deposit controlled by faults far away from the contact zone; Laochang type is the most developed vertical zoning, including greisen type, cassiterite sulfide type, interlayer oxide ore and veinlet type tin-copper ore; Nanyang field type is stratabound skarn scheelite deposit; Hojusan type is a surface skarn copper-tungsten deposit; Bainiuchang type is hot water sedimentary superimposed granite.

Figure 2- 163 Metallogenic model of six types of tin deposits in Gejiu area Figure 2- 163 Metallurgical model of six types of tin deposits in Gejiu area (after Luo Junlie).

1- granite; 2- granite porphyry; 3- Metamorphic diabase; 4-Magnetized ore bodies in skarn; 5- oxidized ore body; 6- Fault zone and oxidized ore body; 7— Tin-bearing dolomite around the veins of cassiterite and tourmaline; 8— Zoning of mineralized elements; 9-molten slurry rich in volatiles at the top; 10- molten slurry rich in oxide silicate; 1 1- early sulfide-rich molten slurry; 12 —— molten slurry rich in sulfide in the late stage.

Silver polymetallic deposits regenerated by high temperature hydrothermal solution in rocks; Dulong type is a stratabound tin polymetallic deposit, which may have the initial enrichment of volcanic deposits. The first six deposit types form a metallogenic sub-series in Gejiu area alone.

(2) Prospecting signs

It can be clearly seen from the above metallogenic conditions and deposit characteristics that the prospecting criteria can be summarized as follows:

(1) Late Yanshanian multi-stage and highly emplaced rock plants and rock processes, especially highly differentiated acidic and super-acidic rocks;

(2) Areas where various structural systems and multi-directional structural components are combined, especially areas where domes, short-axis folds, interlayer peeling and secondary faults are favorable;

(3) Gejiu Formation (especially Kafang Formation) is a region with thick impure carbonate rock series and multi-layer phase transition interfaces such as gypsum salinization, cryptoalgae, bioclastic and carbon-rich;

(4) The ups and downs on the top surface of granite body, such as back-shaped protruding zone, cavity and rock tongue;

(5) Granite is self-metamorphic, skarnized, and various hydrothermal alteration zones are developed;

(6) The areas with complex mineralization assemblages but orderly distribution;

(7) Low gravity, electrical sounding anomaly, tin-tungsten-copper polymetallic prospecting anomaly, heavy sand anomaly and other display areas;

(8) Ancient excavations, slag and other relics.