Guangxi Debao Qinjia copper-tin deposit

1. Geotectonic unit

The deposit is located on the west wing of the Qinjia isoaxial anticline (or dome) on the southern edge of the Youjiang fold belt of the South China fold system. Three sets of structural layers are exposed in the area: geosyncline type sedimentation (∈), platform type sedimentation (D-T1) and flysch-like sedimentation (T2), which have experienced three evolutionary stages: geosyncline, platform and platform activation.

II. Geology of the mining area

The Qinjia copper-tin deposit occurs in the Cambrian system that intrudes into the outer contact zone of the northern edge of the Qinjia granite in the center of the Qinjia dome. The geological structure is relatively complex. complex.

(1) Stratigraphy

Fig.2-166 Geological map of Qinjia copper-tin ore district,Debao area< /p>

1—Devonian; 2—Second to eighth segment of Cambrian; 3—First to second segment of Cambrian; 4—Caledonian granite; 5—Yanshanian basement 6—Unconformity boundary of sedimentary rock; 7—Stratigraphic boundary; 8—Normal fault; 9—Fault of unknown nature; 10—Copper-tin ore body; 11—Ore section number; 12—Stratigraphic inclination and dip angle; 13 —Granite body

The exposed strata in the mining area include the Lower Paleozoic Cambrian System and the Upper Paleozoic Devonian System (Figure 2-166). The Cambrian geosynclinal sedimentary clastic rock intercalated with carbonate flysch is distributed in the core of the Qinjia Dome. Under the influence of granite intrusion, it suffered varying degrees of metamorphism. It controls the distribution of copper-tin deposits and pyrite, iron ore, and barite deposits. Within the scope of the Qinjia mining area, the Cambrian system is divided into eight lithological sections and 18 layers. Related to the copper-tin deposits are six layers 3, 4, 5, 7, 8, and 9, composed of mudstone, limestone, and argillaceous lime. Rocks such as rock, calcareous shale and muddy sedimentary rock are metamorphosed into marble, skarn and hornfels under the influence of intrusions.

The Devonian system includes the Lianhuashan Formation, Nagaoling Formation, Yujiang Formation, Tangding Formation, Nabiao Formation and Dongganling Formation from bottom to top. They are all platform-type deposits and are not closely related to this deposit. The Lianhuashan Formation is in unconformable contact with the underlying Cambrian system.

(2) Mining area structure

The mining area is located in the Cambrian monoclinic strata beside the Heishui River fault, and faults are extremely developed in the mining area (Figure 2-167). There are four groups of faults in the area, namely SW, SE, SN, and EW directions. The SE and SN two groups are compression-torsional, the EW group is tensile fractures, and the SW direction is tensile first and then compressive-torsional. The two groups of SW and SE direction played a role in guiding the ore during the mineralization process, but the SW trending fault cut through the Cambrian strata that were beneficial to the mineralization during the compression and torsion activity after the mineralization, causing damage to the mineralization. Destructive effect (Figure 2-168).

Fig.2-167 Geological profile of exploratory line 50 in Qinjia copper-tin ore district

1—Middle Devonian; 2—Lower Devonian; 3—Cambrian 8 to 18 layers; 4—Ore bodies in Cambrian 9 layers; 5—Cambrian 5 to 7 layers; 6—Cambrian 3 to 4 layers: 7—Caledonian granite; 8—limestone; 9—mudstone, siltstone mudstone; 10—bottom conglomerate; 1l—metamorphic sandstone; 12—hornfels and spotted hornfels ; 13—Main ore bodies in layers 5 and 7; 14—Lentular ore bodies in layers 3, 4, and 9; 15—Granite; 16—Stratigraphic unconformity; 17—Faults; 18—Boreholes

< p>Fig.2-168 Sketch section showing control of Caledonian fracture on distribution of Cu-Snore bodies in Qinjia copper-tin ore bodies in Qinjia copper-tin ore district

1—Middle Devonian limestone; 2—Lower Devonian bottom conglomerate and mudstone; 3—Cambrian metamorphic sandstone, hornfels, skarn, and marble; 4—Caledonian granite; 5—Lower Devonian limestone; 6—Caledonian (before rock mass intrusion) faults; 7—Cambrian 5 and 7 layers that are favorable for mineralization

(3) Intrusive rocks

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The intrusive rocks exposed in the mining area include Caledonian biotite granite, Yanshanian quartz porphyry, diabase, etc.

(1) The Caledonian biotite granite, also known as the Qinjia pluton (γ3), has an obvious contact relationship with the Cambrian strata. The rock mass is in the form of a rock mass and is in wavy contact with the surrounding rock. , tilted to all sides. The rock mass is of aluminum supersaturated type. In the rock mass, Sn, Nb, Ta, and B are high in the edge phase, and Cu, As, Li, and F are high in the transition phase. Cu, Sn, As, Bi and other mineralizing elements are 6 to 10 times more abundant than Caledonian granites. The isotope age determined by the Rb-Sr method is 526 Ma (Yichang Institute of Geosciences, 1982, 5), which belongs to the Caledonian granite. According to the Chappell and White criteria, the rock mass is "S" type granite, which is directly genetically related to the copper-tin deposit.

(2) Quartz porphyry and diabase are distributed in the southwest of the mining area. The isotope age of the diabase measured by the K-Ar method is 115-184 Ma, which is a product of the Yanshan period. The age of the zircon in the quartz porphyry measured by the U-Pb method is 90-102 Ma, which should also belong to the Yanshan period.

3. Ore deposit geology

(1) In the mining area, there are two types of ore bodies in the outer contact zone and the fault zone. The former occurs in the third layer of calcareous shale intercalated with argillaceous limestone; the fourth layer of mudstone calcareous shale, mudstone and limestone lenses; the fifth layer of limestone and argillaceous limestone intercalated with calcium shale; the seventh layer of mudstone, limestone, and argillaceous limestone interbeds or lenses; the eighth layer of mudstone, calcareous shale, or argillaceous limestone lenses; the ninth layer of calcareous shale interbedded There are layers such as limestone and argillaceous limestone, but the fifth and seventh layers are the main ones, and the rock layers have all been skarnized. The ore bodies are layered, lens-shaped, and lentil-shaped in the layering; the ore bodies in the fault zone are vein-shaped and bead-shaped. The ore body tilts NW and NE, with an inclination angle of 20° to 50°. ***27 ore bodies, each ore body is 750~50m long, 20~800m long incline, 1~5m thick, generally 2.5~4m, Cu grade is 0.42%~1.64%, Sn is 0.2%~0.79%, Fe is 34%～43%. The proven reserves of ore bodies in layers 5 and 7 account for 96.17% and 95.3% of the total copper and tin reserves in the mining area, respectively.

(2) Ore mineral composition

According to the natural combination of minerals, primary ores can be divided into: actinolite, skarn, copper-tin ore, garnet There are 5 categories including Karyan copper-tin ore, magnetite skarn copper-tin ore, calcite-quartz vein copper-tin ore, and massive sulfide ore. Among them, actinolite skarn copper-tin ore is the most widely distributed and is the main type in the mining area. The second is garnet skarn copper-tin ore and magnetite skarn copper-tin ore. Both are common ores in mining areas. They are mixed with the former and have no obvious boundary. However, magnetite skarn copper-tin ore is Ore is often located in the upper part or top of the ore body, and in some places it gradually changes upward into a simple magnetite body. Calcite-quartz vein copper-tin ore is relatively rare. It mostly appears as veins of varying sizes, interspersed among the above three ores. Massive sulfide ores can occasionally be seen, mainly distributed in places where fractures intersect and are densely packed, and are mostly produced in the form of irregular clumps.

The oxidation zone is composed of oxidized copper-tin ore that is oxidized from the original ore. The tin in the ore is richer and easier to select than the original ore. It is the main target for tin mining in the mining area.

There are 37 kinds of ore minerals. The main minerals are chalcopyrite, cassiterite, magnetite, arsenopyrite, pyrite, etc.; the secondary minerals include pyrrhotite and sphalerite. , marcasite, tin sulfide ore, bismuthite, tin ore, chalcocite, hematite, chalcopyrite, azurite, etc.; non-metallic minerals include actinolite, common hornblende, diopside, Andradite, quartz, feldspar, chlorite, fushanite and clay minerals.

(3) Ore structure

The structure of primary ore includes: metasomatic erosion structure, euhedral granular structure, rim, inclusion and filling structure, etc. Oxidized ores have colloidal, radial and fibrous structures.

The primary ore structures include dense blocks, lumps, stars, strips, veins and breccias. Oxidized ores have structures such as crust-like, grape-like, grid-like, and hole-like structures.

(4) Chemical composition of ores

The chemical compositions of various ores in the ore body in the mining area are shown in Table 2-106.

The concentration of Cu in all kinds of ores is very high, and the grade of Cu in garnet, skarn copper-tin ore is the highest. Tin is most abundant only in the magnetite skarn copper-tin ore. Gold, silver and other elements are associated with all kinds of ores.

The main harmful component in the ore is As, with an average content of 0.37% and some individuals reaching 16.031%. It exists in the form of arsenopyrite, mainly in quartz veins and ores with high siliceous content.

Table 2-106 Chemical composition of different-type ores in different ore bodies in the mining area

Gold and silver are ×10-6

The relationship between the main components in the ore is:

(1) The mineralization range of tin in the ore body is larger than that of copper. The content of tin in the copper ore body is higher, up to 0.1 %~0.2%, while the copper content in the tin ore body is very small.

(2) The Cu and Sn banding phenomenon is not obvious in the horizontal direction.

(3) There is no correlation between the associated gold in the ore and the copper content in the copper ore, but the correlation with the Ag element is obvious, with a correlation coefficient of 0.63 to 0.95.

(4) Sn and Cu grades change greatly in tendency. There is no close relationship between grade and thickness.

(5) Surrounding rock alteration

The main surrounding rock alterations include:

(1) Skarnization: mainly distributed in rock mass and cold Near the Wu system contact zone (within 1 to 2km from the rock mass). According to the mineral combination, it is divided into simple skarns such as garnet, diopside and fushanite, and complex skarn composed of garnet, actinolite, ordinary hornblende, fushanite, epidote, diopside, etc. Cayan. The former has very little mineralization, while the latter is the main mineralized rock of the copper-tin ore body in the area. In most complex skarn bodies, it is itself a copper-tin ore body.

(2) Marbling and carbonation: distributed in Cambrian carbonate rocks with pure quality and large single-layer thickness in the outer contact zone of the rock, in the form of lenses and strips Shape, alternately produced in the same layer with skarn and skarnized hornfels. It is not mineralized itself, but they are often developed near copper-tin ore bodies or areas with strong mineralization, and are the outer zones of skarnization.

(3) Hornfeltization: It is common in Cambrian muddy rocks near the Qinjia Dome. The hornfels in the mining area can be divided into three different phase zones, namely, biotite or biotite-containing hornfels zones, clinzoisite, tremolite hornfels zones, and potassium feldspar or potash-containing feldspar hornfels zones.

(4) Silicification: Mainly distributed in rocks with high silicon content in rock bodies and Cambrian original rocks. Silicification is obvious near the ore body. The stronger the silicification, the richer the copper-tin ore grade.

(5) Potashization or sodiumification: Distributed in the contact zone outside the rock, areas with strong potassium and sodiumification near the Cambrian skarn are often well mineralized.

(6) Pyrrhotite mineralization: It is distributed in the Cambrian system in the contact zone outside the ore body, especially in the biotite hornfels zone and the clinopterite hornfels zone.

IV. Mineralization conditions

(1) Sulfur isotopes

The characteristics of stable isotopes of sulfur measured for different types of ores, surrounding rocks, and rock masses are as follows :

(1) The sulfur isotope δ34S in the ore ranges from -10‰ to +7.4‰, most of which are -1‰ to +1‰, the average is 0.85‰, the deviation is 2.4‰, and δ34S=0 ‰～＋0.2‰ is the tower effect centered. It reflects that the sulfur isotopes are relatively uniform and the fractionation is not significant.

(2) The δ34S value in pyrite is generally higher than that of chalcopyrite and arsenopyrite, which indicates that it is consistent with the sulfur isotope balance.

(3) The distribution range of δ34S values ??is closely related to the distance from the rock mass. For example, in layers 4 to 7 near the rock mass, δ34S is mostly concentrated in -0.2‰~+1‰, similar to the rock mass. However, the δ34S values ??of layers 14 and 17 slightly away from the rock mass are scattered and far away from zero, ranging from -2‰ to +6‰; in the fault zone, δ34S tends to extremely negative values, which is -26.7‰. ~—31.9‰. These characteristics indicate that the mineralization occurred under high-temperature conditions (consistent with the temperature measurement data), and also indirectly indicate that the heat source of the mineralization comes from Qinjia granite. The sulfur in mineralization does not necessarily come from the rock mass, but a considerable part is supplied by the ore-forming rock layer itself and nearby surrounding rocks. The sulfur in the fault zone is obviously neither homologous nor contemporaneous with the sulfur of mineralization. It may be produced by bacterial sulfur under low temperature conditions after mineralization.

(2) Oxygen isotopes

Results of the determination of oxygen isotopes of magnetite and quartz mineral pairs produced in the ore and the whole rock mass (Figure 2-169) . It can be seen from Figure 2-169:

(1) Qinjia granite should be an "S" type granite, which is formed by the remelting of basement clastic sedimentary rocks. The magnetite and quartz in the ore body both show the characteristics of ground source ores (sedimentary metamorphism);

(2) The calculation results of the oxygen isotopes of magnetite-quartz minerals in the ore show that the ore-forming temperature is at 279~382℃, indicating that the mineralization occurred under high-medium temperature conditions, and the sulfur isotopes homogenized with the original contemporaneous deposition, making the δ34S value tend to zero, which is similar to meteorite sulfur;

Figure 2- 169 Oxygen isotope of ore bodies and rock masses (18O distribution map) Fig.2-169 Composition of Oxygen isotope in ores and rocks

Sample distribution location; - 18O distribution range of ore bodies and minerals from different sources (according to Isotope geological data (volume 2, Zhang Ligang, Oifeil et al.)

(3) Oxygen isotope research shows that the ore-forming solution water is metamorphic water, not magmatic water (Yang Jimin, Yan Chengxian et al., 1984). It further proves that assimilation and metasomatism occurred with sedimentary rocks during the mineralization process, and minerals with sedimentary origin or atmospheric precipitation participated in the mineralization. This indicates that the minerals are derived from multiple sources.

(3) Ore-forming temperature

The temperature of the ore body was measured by the homogenization method of calcite and quartz mineral inclusions, and the explosion method and magnetic measurement of the magnetite and cassiterite minerals were carried out. Calculation of oxygen isotope geological temperature of iron ore and quartz primary minerals, etc. The ore-forming temperature obtained by several temperature measurement methods is 200-450°C, which belongs to the high-temperature to medium-temperature stage, indicating that the Qinjia copper-tin deposit belongs to the category of hydrothermal deposits. Among them, studies on the composition of cassiterite inclusions believe that cassiterite mineralization temperature range is large, from high temperature to medium temperature, and belongs to the post-skarn stage, so it is mostly filled in the crystal lattice and cracks of skarn minerals, and is not only used by other It is wrapped with metal minerals, and at the same time it is wrapped with quartz, magnetite and other minerals. The low temperature of magnetite mineralization may be related to the fact that it is mainly metamorphosed from original sedimentary siderite.

(4) Ore-forming stages and understanding of origin

The formation of ore minerals in this deposit is divided into five stages, namely the early skarn stage, the late skarn stage, and the magnetite stage. Cassiterite stage (oxide) stage, sulfide stage, carbonate stage. In the first and second stages, the formation temperature is 350~550°C, mainly Fe3+, with high fo2, and mainly forms some silicates and some oxides.

The temperature in the third and fourth stages is mainly Fe2+ at 350~700℃, with low fo2, and the main formation is sulfide and partial oxide silicate dissolution; the temperature in the fifth stage is <200℃, mainly carbonate and partial sulfide are formed. substances and very few oxides.

Through the above analysis, the formation of copper-tin deposits in this area mainly experienced:

(1) Intrusion of the Caledonian Qinjia "S" type granite. The rock mass is rich in mineralizing elements such as Sn, Cu, Bi, Be, B, As, Nb, and Ta. It is basically consistent with the ore-forming elements and trace elements contained in the ore. It is an intrusive body that is beneficial to tin ore mineralization and may provide part of the mineral-forming minerals for this deposit;

(2) Qinjia Rock The intrusion of the body caused the Cambrian and other rocks in the area to undergo contact metasomatism, forming garnet skarn copper-tin ore, actinolite skarn copper-tin ore, and magnetite skarn copper-tin ore. The formation of skarn formed by contact metasomatism proceeds simultaneously with the formation of tin-copper ore, indicating that the mineralization is dominant in the formation stage of skarn;

(3) The intrusion and intrusion of the Qinjia rock body The magmatic hydrothermal action after the intrusion further concentrated the sedimentary elements such as Fe, S, Sn, Cu, Au, Ag, Ca, Mg, etc. that were originally rich in the bedrock, forming the mineralization stage of the contact metasomatism (i.e., the early and late stages). Skarn formation class) provides part of the minerals.

(4) The sulfur isotope in the ore is close to zero, and the sulfur isotope in the mineral far away from the rock mass is far from zero. The δ34S value is inversely proportional to the distance from the rock mass, indicating the mineralization and ore-forming heat. The liquid originates from rock mass. The oxygen isotope characteristics of magnetite, quartz, cassiterite and other minerals indicate that the magma source of the minerals dominates, while mixed magma sources and contemporaneous sedimentary sources also occupy a certain position. This also shows that contact with metasomatic metamorphism and magma heat The addition of formation minerals and hydrothermal minerals during the fluid activity plays a certain role in the formation of mineral deposits.

There have always been different understandings of the origin of this deposit. In the mining area exploration report submitted in 1976, it was stated that this deposit is a "post-magmatic hydrothermal metasomatism skarn type" deposit. In the article "Rediscussing the Formation Conditions of Guangxi's Stratum-controlled Tin Deposits", Xie Baiqi proposed that this deposit is a "sedimentary metamorphic hydrothermal superimposed transformation deposit." And "Geological Research on the Debao Qinjia Copper-tin Deposit" edited by Yang Jimin and Yan Chengxian "The monograph on mineral deposits (1984) proposed a new understanding of "sedimentary metamorphic-magmatic hydrothermal deposits". However, in terms of the current understanding of this deposit, the "contact metasomatic copper-tin deposit" has been recognized by the majority of scholars.

(5) Prospecting signs

Based on the mineralization conditions and characteristics of this deposit, the following prospecting signs are summarized.

1. Stratigraphic Markers

The calcareous mudstone and calcareous shale interbedded with marl or limestone lenses from the 3rd to 9th layers of the Cambrian system are produced in an interbedded manner. The horizon is the ore-bearing horizon of the mineral deposit. Once the formation has obvious thin layers and micro-layers interbedded with different lithologies, it will be more beneficial to the formation of ore bodies.

2. The Caledonian granite body and its contact zone

That is, the contact zone between the granite body and specific layers of the Cambrian (i.e., layers 3 to 9) is the location mark of the ore body , specifically shown in:

Fig.2-170 Sketch section of exploratory line 107 showing contact of granite and host rock and location of ore bodies

1—Lower Devonian; 2—Cambrian; 3—Caledonian granite; 4—Biotite granite; 5—Cambrian 3rd to 4th Layers; 6—The 5th to 9th layers of the Cambrian; 7—The 10th to 18th layers of the Cambrian; 8—Sand and mudstone; 9—Copper-tin ore bodies in the 5th and 7th layers (below is the 10th to 18th layers 5 layers, the upper layer is the 7th layer); 10 - Devonian bottom conglomerate; 11 - fault; 12 - drilling location

(1) Ore bodies occur in the contact zone away from the rock mass Within the range of tens to 200m;

(2) On the plane, the rock mass protrudes toward the front edge and both sides of the surrounding rock (such as rock tongues, rock branches, rock strains), and the contact boundary between the rock mass and In the serpentine-shaped section, the ore bodies are most concentrated in the concave pockets with steep and gentle turns (Figure 2-170).

3. Structure is a prerequisite for the location of ore bodies and controls the distribution of ore bodies

The Qinjia rock mass intrudes into the core of the Qinjia dome, and the rock mass as a whole moves to the north and east In the Cambrian strata on the upper part of the inclined side, SW and SE faults are developed. The intersection of these two sets of faults is the specific positioning condition of the ore body and is also the most favorable area for ore prospecting.

4. Comprehensive geochemical anomalies delineated in river sediments

There is a high background and high anomaly area on the 1:200000 geochemical scan map, and the element combination of geochemical anomalies is Cu , Sn, Mo, Pb, Zn, Cr, Ni, V, each element is exceptionally well matched, with obvious concentration center and high strength, which is an obvious sign of finding this type of deposit.