Figure 1 Traffic Location Map of Dashui Gold Mine
1996-2000, the third geological team of Gansu Bureau of Geology and Mineral Resources conducted an exploration of Dashui Gold Mine, and in 2000 submitted the Geological Report on Detailed Investigation of gerk Gold Mine in Maqu County, Gansu Province. The accumulated proven reserves of C+D gold deposits in Dashui mining area are 37 052 kg, with an average grade of 12.67× 10-6, which belongs to large-scale fine disseminated (Carlin-like) gold deposits.
In 2005, the resource potential of Dashui gold mine was investigated. The ore-forming geological conditions in the mining area are superior and the prospecting potential is huge. It is estimated that the total resources will reach 100 t, which is the largest scale so far.
1 regional metallogenic geological environment
1. 1 geotectonic unit
The geotectonic position of the deposit is located in the southern belt of Qinling-Dabie metallogenic province in Qin-Qi-Kun metallogenic domain, bordering Ganzi-Songpan fold belt in the south and Lueyang-Maqu fault in the south. Its northern part is Xiqingshan uplift belt, and its southern part is Brazil syncline. The deposit is located in the southern margin of Xiqingshan uplift belt.
1.2 regional stratum
The oldest stratum exposed in this area is Silurian, which belongs to lagoon facies-shallow sea facies deposition, and is distributed in the axis of Geer left fold bundle. From the shaft to the south wing, there are Carboniferous, Permian, Triassic and Lower Jurassic in turn. Carboniferous, Permian and Triassic are composed of carbonate rocks of shallow sea facies and carbonate platform respectively. The Lower Jurassic consists of lacustrine clastic rocks, carbonate rocks and continental volcanic rocks. Cretaceous is dominated by clastic rocks with a small amount of carbonate rocks. Among them, the average gold abundance of Triassic limestone is 30× 10-9, which is more than 6 times higher than that of similar rocks, indicating that Triassic strata is a gold-rich rock series in this area.
1.3 regional tectonic framework
The area is bounded by the Onni-QuHalden fault, and the Xiqingshan uplift belt is divided into two secondary structural units, with the Gerqu fold bundle in the north of the fault and the Langmusi fold bundle in the south, which are parallel to each other and extend in a strip shape (Figure 2).
Schematic diagram of the geotectonic structure of the west Qinling Mountains.
(According to Zhao Yanqing et al., 2003)
Qla-Qilian structural belt; NQL-North Qinling tectonic belt; WQL-West Qinling tectonic belt; SG- Songpan-Ganzi structural belt; Yz- Yangtze plate; SF 1- Wushan-Tianshui-Shangdan suture zone; SF2—- Maqu-Nanping-Lueyang suture zone
Faults are developed in the area, mainly in the east-west direction and SN-SE direction, which are composed of three groups of faults: Lueyang-Maqu, Onnyqu Halden and Casarsa-Zhargazuoli, which control the development of geological structures in the area. Among them, Lueyang-Maqu thrust fault group is a group of main faults that control the gold belt. The Lueyang-Maqu fault constitutes an arc structure protruding in the southeast, with the west wing of the arc structure, and the main structural line is northwest or nearly east-west, forming a series of northwest fracture zones along the fault, which has obvious control over gold deposits.
1.4 regional magmatism
The magmatic activity in the area is weak, mainly occurring in the late Indosinian-late Yanshan period, with the characteristics of multi-stage and multi-cycle activity. The spatial distribution is obviously controlled by faults, and the magmatic activity has evolved from neutral to intermediate acid, with weak alkaline and weak alkaline characteristics, belonging to normal series and alumina supersaturated series.
Magmatic activities in this area are dominated by early Yanshanian magmatism, mainly distributed in the northern margin of Dashui-Zhongqu fault, including Zhonggezhana rock mass, Zhongqu rock mass and Geerkuohe rock mass, with the areas of 4.8km2, 0.05km2 and 1.76km2 respectively, which are dendritic and dendritic, and the lithology is granodiorite, granodiorite porphyry, diorite porphyry and syenite porphyry.
1.5 metallogenic unit
The tectonic position of the deposit is located in the south Asia belt of Qinling-Dabie metallogenic province in Qin-Qi-Kun metallogenic domain.
2 Geological characteristics of mining area
2. 1 mining stratum
Most of the mining area is covered by Quaternary eluvial deposits, bedrock is exposed sporadically, and the stratum is single (Table 1). It is mainly composed of the Mareson Group of Middle Triassic, with a small amount of Permian, Jurassic and Cretaceous (Figure 3; Table 1). Ta- 12 is the first lithologic member of the Lower Rock Formation of Middle Triassic, and it is the ore-bearing horizon of gerk gold deposit.
The first member of Lower Rock Formation of Middle Triassic (): The lithology is a set of dolomite, dolomitic limestone and siliceous broken breccia.
The second member () of Lower Rock Formation of Middle Triassic: The lithology is a set of micro limestone, argillaceous limestone and sandy limestone. The lithology of this section is unstable, with great changes along strike and dip, and the shale content is uneven. Most argillaceous limestone or sandy limestone is replaced by massive limestone, which belongs to shallow sea-continental deposition.
Table 1 mining stratum table
Fig. 3 Geological schematic diagram of Dashui gold deposit
Cretaceous; T1-Lower Triassic; T2- Middle Triassic; P- Permian; 1- granodiorite porphyry; 2- granodiorite vein; 3- gold ore body; 4- Unconformity boundary; 5- structural breccia zone; 6- compression-torsion fault; 7— Formation occurrence; 8- granodiorite
2.2 magmatic rocks in mining area
Magmatic activity in the mining area is general. A large Geerkuohe granodiorite porphyry is exposed only in the north of Dashui gold mining area, and granodiorite veins can be seen radially invading the mining area in the south.
2.2. 1 geer closed rock mass
Geerkuohe rock mass occurs in the northern margin of the mining area and intrudes into the limestone stratum of Middle Triassic, with an exposed area of 1.76km2, which is nearly circular. According to the characteristics of rock minerals and petrochemistry, it can be divided into two lithologic zones, namely, early biotite diorite porphyry and late granodiorite porphyry.
1) biotite diorite porphyrite is irregularly distributed in the north, east and west of the intrusive body, and is in intrusive contact with the surrounding rock (limestone). The rocks are grayish green, with porphyritic structure and massive structure, and the matrix has aphanitic-microcrystalline structure. 25% of phenocryst, 0/2% ~15% of plagioclase, 3% ~ 5% of amphibole and 8% of biotite; The matrix consists of 50% ~ 60% siliceous (secondary reaction time), 5% feldspar, 3% calcium and 2% iron oxide. Plagioclase has automorphic-semi-automorphic crystals, and some of them are molten and eroded. The edge of biotite is replaced by carbonate, which is serrated, and amphibole is long column with glass fiber fossils. The accessory minerals are magnetite (1%), apatite, zircon and a small amount of tourmaline.
2) Granodiorite porphyry, distributed in the south-central part of Juegeer xenolith intrusion, is surrounded by marble. In the south, the internal and external contacts have altered phenomena such as hematitization and silicification. The rocks are light green-gray, mottled and huge. The matrix is microcrystal structure, with 20% ~ 25% porphyritic crystals, mainly feldspar (> 12%), and has a twin crystal structure. Biotite (about 6%); Hornblende (> 5%) is a common hornblende with long plate shape and multi-colors. The matrix is mainly chalcedony (60%), chlorite (10%) and veinlet calcite (5%). The accessory minerals are magnetite, apatite, zircon, tourmaline and pyrite.
Granodiorite vein
A large number of granodiorites run through the north-south fault zone or fault outside the Geer broad-rock granodiorite porphyry, and occur in the form of rock branches. The veins are distributed in 0 ~ 10 and 290 ~ 3 10, which are generally consistent with the SN- oriented structure and the distribution direction of gold ore bodies. The vein is100 ~ 700m long and10 ~ 50m wide, forming a large vein in the deposit. The vein shape is very complex, mostly harbor-shaped, banded and convex mirror-shaped, with the characteristics of expansion, contraction and branching, which is the product of homology and heterogeneity with granodiorite porphyry.
The 40Ar/39Ar plateau age of Geerkuohe rock mass is 235.41.3ma, the isochron age is 235.2±2.3Ma (Wang Pingan et al., 2000), and the whole rock K-Ar age is 190.0 ~ 190.6 Ma, Rb. The 40Ar/39Ar plateau age and isochron age of granodiorite veins in gerk gold deposit are 222.5±2.6Ma and 223.0±2.8Ma； respectively. , respectively. The fission track age of apatite in granite ranges from117.9 4.9 Ma to189.4 5.2 Ma (Han Chunming et al., 2004). The above diagenetic and metallogenic ages indicate that the diagenetic age of Dashui gold deposit is from Indosinian to Yanshan, and the mineralization has gone through two stages, that is, the deposit is the product of many tectonic-magmatic activities.
2.3 Mining Area Structure
Faults are developed in Dashui mining area, and mineralization is obviously controlled by fault structures (Figure 3).
2. 3. 1 NWW- east-west main fault zone
The fault runs through the whole mining area, and generally spreads in the direction of100 ~10. The dip angle is S or SW, and the dip angle is 60 ~ 75, which is tensional and torsional. The biggest feature of this fault is that there is a very wide fault zone with a fault bandwidth of 10~30m ~ 30m, and fault gouge, fault breccia, structural lens and cataclastic rocks are developed.
2.3.2 NE-NE fault
It is mainly developed in the west of the mining area, and it is roughly distributed in the direction of 10 ~ 30. Its fault scale is small, with a length of 600 ~ 1400 m and a dip angle of 60 ~ 75. This group of faults cuts nearly east-west faults, which are tensile and torsional. The fracture zone is filled with a large number of granodiorite veins and calcite veins, and the veins have obvious expansion and contraction, forming many beaded local low-pressure extension spaces, which provide a good place for hydrothermal activity and gold precipitation. A group of pinnate faults in the direction of 100 ~ 1 10 are developed on one side of the SN- strike fault, which intersect with the fault fracture zone at an acute angle and form a stepped ore-controlling structure. In particular, their intersection is the most concentrated and enriched part of the ore body. Therefore, SN- trending faults play a positioning role in the deposit and control the distribution and output of ore bodies.
NW-trending fault
The fault occurrence is 220 ~ 240 ∠ 50 ~ 60, the fault plane is gentle and wavy, there are structural lenses in the fracture zone, and there are tension-torsion joint combinations near the fault, and the mechanical properties are tension-torsion.
Dashui mining area has experienced multi-stage superimposed reconstruction of tectonic activities. The NWW-trending main fault zone in the mining area not only controls the distribution of magmatic rocks, but also is the ore-guiding and ore-controlling structure in this area. The host structure is mainly NWW secondary fault, followed by NNE and NNW faults. In addition, karst structure is also an important ore-hosting structure.
A large number of data show that the main formation period of the east-west structure is the early Yanshan period, which is characterized by the filling of granodiorite in the compressional fault of the east-west structure, dynamic metamorphism and the formation of Dashui-Zhongqu fault, which laid the basic framework of the structure in this area and played a certain control role in mineralization. SN- trending structure cuts the east-west structure, which is the result of the east-west compression at the end of Cretaceous and the reflection of the late Yanshan movement in this area, mainly manifested in the ups and downs of fault blocks.
2.4 surrounding rock alteration
The alteration of surrounding rock in mining area is strictly controlled by faults and fracture zones, and the alteration is characterized by medium-low temperature hydrothermal alteration. The main types of wall rock alteration are silicification, hematitization and carbonation, followed by kaolin, chloritization, sericitization, secondary limonite and a small amount of mercury and arsenic sulfide.
Silicification refers to hydrothermal alteration in which siliceous hydrothermal solution replaces carbonate rocks and intermediate-acid dikes in surrounding rocks, so that their content in surrounding rocks increases, and at the same time, the fabric and mineral composition of original rocks change accordingly. Therefore, silicified alteration does not include siliceous cements of siliceous rocks and breccia, which are directly precipitated by ferrosiliceous hydrothermal solution in open space. Silicification is the most important alteration mineralization in Dashui gold mine area, which runs through the whole mineralization process. There are mainly two stages, the early stage is plane and strip alteration, which has a wide range and high intensity. The main performance is that limestone or dolomitic limestone is replaced by silicified limestone and jasper-like rock with different contents, and the matrix of granodiorite porphyry is mostly replaced by microcrystalline quartz and feldspar chalcedony. In the late stage, it was transformed into linear alteration, and the structural fractures of the early altered mineralized body were filled in the form of calcite-containing timely veinlets or net veins.
Hematitization is a unique type of alteration and mineralization in Dashui gold mining area. There are two main forms of output: one is dust-like or granular dispersed in microcrystalline-fine Shi Ying particles, which is closely related to early silicification; The other is distributed in irregular blocks and short veins along structural fractures.
Chloritization, sericitization and kaolinitization are the main dark minerals, feldspar phenocrysts and matrix components in granodiorite porphyry, and chlorite and sericite are formed by hydrothermal metasomatism.
The alteration closely related to gold mineralization is silicification and hematitization, and all major gold ore bodies occur in the strongest range of silicification, hematite (brown) mineralization and reticular calcite. Silicification, alteration of hematitization and calcite run through the whole hydrothermal period, and gold mineralization is formed on the basis of superposition and enrichment of multi-stage mineralized hydrothermal solutions. The areas with the strongest silicification and hematitization are also the areas with the richest gold mineralization. The widely developed fine-mesh vein carbonation and coarse-grained pegmatite calcite vein indicate the existence of large-scale ore-forming fluid activity, which is an important symbol for finding deep concealed ore bodies.
3 Geological characteristics of ore deposits (bodies)
3. 1 ore body characteristics
At present, 127 gold ore body has been circled in Dashui gold mine area. Ore bodies mainly occur in limestone, dolomitic limestone and dolomite of the Middle Triassic Maresong Formation, followed by Yanshanian granodiorite veins and the inner and outer contact zones of Jurassic conglomerate strata, with the characteristics of subsection enrichment. The ore body strikes nearly east-west, northwest and north-south, and the dip angle of the ore body is steep, ranging from 45 to 80. The shape of the ore body is complex, with irregular branches (formed following several groups of faults), layered, lenticular, cystic, tubular and veined, and has the characteristics of expansion and contraction, branch compound and pinch-out.
The ore body is strictly controlled by fault structure and paleokarst, and the boundary between ore body and surrounding rock is obvious. The length of ore body is 20 ~ 220m, the maximum length is 280m, the extension of ore body is controlled at 20 ~ 330m, the distribution elevation of ore body is 3342～3854m, and the thickness is 0.84 ~ 41.19m. The grade of gold is generally1.0×10-6 ~ 29.36×10-6, and the average grade in some areas is 8.52× 10-6. The variation coefficient of ore body thickness is 82% ~ 134%, and the variation coefficient of grade is 38% ~ 160%.
Although there are many ore bodies in Dashui mining area, the main reserves are concentrated in several large ore bodies such as Au7, AU20-1A.
3.2 Ore composition
According to the microscopic observation of a large number of rock mineral slices, combined with artificial heavy sand and electron probe, it shows that the mineral assemblage of Dashui gold deposit is relatively simple. The main minerals of metal ores are hematite, pyrite, natural gold and limonite, with traces of cinnabar, realgar, orpiment, stibnite, magnetite, scheelite and rhodochrosite, with the total content not exceeding 5%. Non-metallic minerals mainly include chalcedony, microcrystalline-spar Shi Ying and calcite, followed by dolomite, chlorite, sericite and barite.
Synchronism is the main component mineral of all kinds of ores, which can be divided into aphanitic-microcrystalline chalcedony, fine-grained synchronism, veinlets or reticular coarse-grained synchronism.
Hematite is the main metal mineral and characteristic mineral in Dashui Gold Mine. There are two completely different views on the genesis of hematite. Yan Shenghao and others (2000) believe that there are two forms of occurrence. One is dispersed in microcrystalline-sparsitic particles or calcite in the form of dust or particles, which is found in various siliceous rocks and metasomatic ores, and hematite precipitates with timely particles. The second kind of hematite is distributed in irregular blocks, short veins, belts or shells along structural fractures and their intersecting parts or holes, which is formed by filling and precipitating iron-rich ore-forming hydrothermal solution along structural fractures in an open oxidation environment. This shows that the siliceous rock ore rich in hematite in Dashui Gold Mine is not caused by secondary oxidation in the supergene environment, but by primary genesis. Li Hongyang and others (2007) believe that hematite and limonite, which are widely distributed in shallow oxidized minerals in Dashui Gold Mine, are caused by supergene oxidation of sulfides such as fine dense disseminated pyrite and reticulated pyrite.
According to mineral composition, mineralized protolith and structure, the ores of Dashui gold deposit can be divided into five types: hematitization silicified carbonate rock type, metasomatic jasper rock type, hematitization silicified granodiorite type, layered, banded siliceous rock type and breccia type. Among them, the first three are the main ore types, especially the silicified carbonate rocks in hematitization are the most widely distributed. However, layered and banded siliceous rocks and breccia minerals are less distributed.
3.3 Ore fabric and metallogenic stage division
The ore structure mainly includes automorphic-semi-automorphic-allomorphic granular, metasomatic residual, microcrystalline-fine crystal, mottled, colloidal, oolitic (pea-shaped); Ore structures mainly include dense massive, banded, disseminated and veinlets disseminated, veinlets and reticulated veins, fractured breccia, pores (holes) and honeycombs.
Yan Shenghao et al. (2000) divided the ore-forming process into hydrothermal period and secondary weathering period according to the macroscopic and microscopic interpenetration of hydrothermal veins and their associated mineral assemblages. The hydrothermal stage can be divided into five metallogenic stages: ① the microcrystalline isochron-hematite stage, which is an important metallogenic stage of gold and an early surface alteration mineralization of ore-bearing hydrothermal solution; ② The microcrystal isochron-hematite-calcite stage is the filling and deposition stage of ore-bearing hydrothermal solution, which is the main metallogenic stage; ③ The time-calcite stage, in which time-calcite veinlets or net veins are formed, accompanied by a small amount of sulfides such as Cinnabar, realgar, orpiment and stibnite, is a secondary ore-forming stage; ④ In coarse calcite stage, white and variegated coarse calcite and a small amount of siderite are formed; ⑤ banded comb calcite stage.
The secondary weathering period is mainly characterized by the formation of karst laterite by primary limestone in weathering environment, and a small amount of limonite can be seen in primary limestone.
4 genetic analysis of the deposit
4. 1 Characteristics of fluid inclusions
Fluid inclusions in Dashui gold deposit are irregular rhombus, strip, circle, rice grain and irregular long tube. From microscopic observation, it can be seen that fluid inclusions in calcite are well developed and have different shapes. The coexistence of inclusions with different shapes indicates that the dashui deposit may have suffered from the late thermodynamic action during its formation.
The size of primary fluid inclusions in calcite is 3 ~ 90μ m, mostly concentrated in 10 ~ 25μ m, while the smaller inclusions are mainly single-phase inclusions and partial gas-liquid inclusions, especially secondary inclusions. The chronological grain size of Dashui gold deposit is very small (0.05 ~ 0. 1 mm), most of which contain impurities, dust iron and mud, and inclusions are not developed. Jasper-like rocks or siliceous rocks are mainly composed of cryptocrysts and microcrystals, in which fluid inclusions are very small. The inclusions in calcite are mostly colorless and transparent, and the gas-liquid boundary is very clear. A few liquid components in inclusions are dark gray, and most gas components are gray and gray-black. The gas-liquid ratio of inclusions is different, most of them are 5% ~ 10%, and only a few are less than or equal to 5%.
4.2 Physical and chemical conditions
The homogeneous temperature range of inclusions measured by Yan Shenghao et al. (2000) is 105 ~ 386℃, and the peak value is 150 ~ 200℃. Han Chunming et al. (2004) measured that the uniform temperature variation range of ore-forming fluid is 100 ~ 400℃, and the peak value range of ore-forming temperature is 150 ~ 200℃. Judging from the results of temperature measurement, the deposit belongs to a medium-low temperature hydrothermal deposit. The metallogenic pressure measured by Han Chunming et al. (2004) is between 40.50 ~ 10 1.30 MPa, which is relatively large. According to the average pressure growth rate of 0 ~ 35 km in the earth is 28.5× 106Pa/km, it can be inferred that the mineralization depth of Dashui gold mine area is between 1.40 ~ 3.55 km. Combined with the ore-bearing surrounding rock plagiogranite and limestone, it is considered that the deposit was formed in the middle and shallow part.
The salinity ranges from 2.70% to 9.10%, with an average of 4.88%. The salinity of ore-forming fluids is mostly between 2% and 5%, which indicates that the salinity of ore-forming fluids in this mining area is very low. According to the P (pressure) -T (temperature) -D (density) diagram of NaCl-H2O system compiled by Rhodes (1980), Han Chunming et al. (2004) calculated that the ore-forming fluid density of Dashui gold deposit was 0.875 ~ 970 g/cm3, and Yan Shenghao et al. (2000) calculated that the fluid density was 0.8655.
4.3 Isotopic geochemical markers
4.3. 1 sulfur isotope
The variation range of δ34S value in the deposit is narrow (-1.8 ‰ ~ 4. 1 ‰), with an amplitude of 5.9‰ and an average of 2.4‰, which has obvious tower distribution characteristics (Figure 4), with 2 ‰ ~ 3 ‰ as the tower peak, reflecting the deep source characteristics of sulfur.
Fig. 4 histogram solution of sulfur isotope composition in Dashui gold deposit
(According to Han Chunming et al., 2004)
Carbon and oxygen isotopes
The carbon and oxygen isotopes of carbonate were analyzed by 100% phosphoric acid method and determined by MAT25 1 EM mass spectrometer. δ 13C is based on PDB, and δ 18O is based on PDB and SMOW, respectively, and the analysis accuracy is 0.20 ‰. The δ 13CPDB of limestone is between-1.2 ‰ and+3.4 ‰, with an average of 0.83 ‰; δ 18OPDB is -2 1.6 ‰ ~-5.4 ‰, with an average value of-13.29‰ (Han Chunming et al., 2004).
Hydrogen and oxygen isotopes
According to the research results of oxygen isotope and hydrogen isotope of calcite 10 in Dashui gold deposit by Han Chunming et al. (2004), the δ 18OSMOW of calcite minerals is 6.63‰~ 19.42‰. According to the fluid inclusion data of the same sample and O'Neil( 1969) isotope, these data are put into the figure, and three early calcite samples are put into the magmatic water range, and four samples are located near the magmatic water range. Late calcite samples all fall between magmatic water and atmospheric precipitation. This trend shows that the ore-forming fluid is mainly magmatic water in the early stage of mineralization and mixed with atmospheric precipitation in the later stage, which is completely consistent with the information given by δD (Figure 5). In fig. 5, some samples are close to the atmospheric precipitation line, while others are far away from the atmospheric precipitation line. This "δ 18O drift" phenomenon reflects the oxygen isotope exchange between atmospheric precipitation and rocks to varying degrees. According to Taylor (1979), when isotope exchange occurs between water and rock, there is the following formula: W/R (water/rock) = (δ end rock-δ initial rock)/(δ initial water-δ end water), where "end" means after exchange and "initial" means before exchange. At the initial stage of mineralization, the ore-forming fluid was magmatic water, with the initial δ water of 9. 1‰ and the initial δ rock of 20‰, which can be calculated as the W/R (water/rock) ratio of Dashui mining area of about 0.75 ~ 1.00. Therefore, although magmatic water occupies a certain proportion in ore-forming fluids, its proportion is not high. The ore-forming fluid in the later stage is mainly exchanged atmospheric precipitation, which is consistent with the δ 18O-δD diagram.
4.4 Geochemical Characteristics of Rare Earth Elements
Systematic sampling and rare earth element test analysis were carried out on geological bodies such as siliceous rocks, dikes, hydrothermal calcite, ore-bearing strata and magmatic rocks (dikes) in the mining area. Comprehensive analysis of these data can get the following understanding.
1) the total amount of rare earth elements in different geological bodies such as magmatic rocks, limestone, various ores and hydrothermal calcite in Dashui gold mining area is obviously different. Rock mass σσREE is the highest, ranging from148.32 to 352.79×10-6, with an average value of 186.56× 10-6. The total amount of rare earth elements in dikes ranges from100.21~170.05×10-6, with an average value of 127.32× 10-6. The lowest σσREE in limestone is 1.97× 10-6. However, various altered mineralized rocks and hydrothermal calcite σσREE are between limestone and magmatic rocks.
Fig. 5 diagram of hydrogen and oxygen isotope composition of ore-forming fluid
(According to Han Chunming et al., 2004)
2) The ratios of σ lree/σ hree, (La/Yb)N and (Ce/Yb)N of all samples are much larger than 1, and they are light rare earth-enriched, with the distribution curve on the right (Figure 6). The composition of rare earth is similar to that of alkaline-alkaline magmatic rocks rich in light rare earth.
Fig. 6 Distribution pattern of rare earth elements in Dashui Gold Mine
(According to Yan Shenghao et al., 2000)
3) From limestone to silicified limestone ore to siliceous rock ore, σ REE, σLREE and σHREE all increase significantly with the increase of alteration mineralization: σ REE ranges from1.97 → 20.77 → 54.46×10-6; σσLREE ranges from1.73→17.59→ 49.09×10-6; σσHREE varies from 0.24 to 3.18 to 5.37 mg/g, and the σσREE of hydrothermal calcite is also significantly higher than that of limestone. All these fully show that the mineralization process is accompanied by the introduction of a large number of rare earth elements.
4) From surrounding rock to ore, the characteristic parameters such as σ lree/σ hree, (La/Yb)N and (Ce/Yb)N change obviously, that is, from limestone to ore gradually increases, while from dike to ore gradually decreases.
5) From the distribution curve of rare earth elements and δEu and δCe values, magmatic rocks and dikes have obvious weak-moderate negative Eu anomalies, with δEu values ranging from 0.58 to 0.85, with an average value of 0.67; Ce anomaly is not obvious, and δCe values are 0.78 and 0.85 respectively. Limestone has obvious Eu positive anomaly and Ce negative anomaly, and δEu and δCe values are 1.23 and 0.49 respectively. The δEu and δCe values of vein ore are 0.69 and 0.82, respectively, while those of siliceous rock ore are 0.69 and 0.84, and those of silicified limestone ore are 0.89 and 0.84. Therefore, the REE distribution curves and δEu and δCe values of various ores are similar or close to those of magmatic rocks (Figure 6), but quite different from those of limestone, indicating that the source of ore-forming materials is related to magmatic activity.
4.5 diagenetic and metallogenic age
The 40Ar/39Ar plateau age of Geerkuohe rock mass in the north of Dashui Gold Mine is 235.41.3ma, the isochron age is 235.2±2.3Ma (Wang Pingan et al., 2000), and the whole rock K-Ar age is190.0 ~190.6ma. The 40Ar/39Ar plateau age and isochron age of granodiorite veins in Dashui gold deposit are 222.5±2.6Ma and 223.0±2.8Ma；, respectively. , respectively. The fission track age range of apatite in granite in Dashui area is117.9 4.9 Ma ~189.4 5.2 Ma (33 pieces). The above diagenetic and metallogenic ages indicate that the diagenetic age of Dashui gold deposit is from Indosinian to Yanshan, and the mineralization has gone through two stages, that is, the deposit is the product of many tectonic-magmatic activities.
4.6 deposit type
Dashui gold deposit is a hydrothermal deposit formed in shallow-ultra shallow oxidation environment. The carbon, silicon, oxygen and hydrogen isotopic characteristics of hydrothermal calcite, siliceous rock ore, silicified limestone, protolith and magmatic rock show that carbon in ore-forming materials is deep magmatic carbon transformed by oxidation, silicon is the source of deep magma or magmatic hot water, and ore-forming fluid is also characterized by mixing magmatic water and construction water.
The REE distribution curves and δEu and δCe values of various ores and hydrothermal calcite are similar to those of alkaline-weakly alkaline magmatic rocks rich in light rare earths, but very different from those of limestone. Ore-forming fluids are rich in rare earths, especially light rare earth elements, and the source of ore-forming materials is related to magmatic activity. Magmatic intrusion and the rise of ore-forming fluid may be a series of products of the evolution of the same structure-magma-thermal fluid system during the Yanshanian intracontinental orogeny.
refer to
Han Chunming, Yuan Wanming, Yu Fusheng et al. 2004. Geochemical characteristics of Dashui gold deposit in Maqu, Gansu Province. Journal of Geology, 25 (2): 127 ~ 132.
Li Hongyang, Li Yingjie, Yuan Wanming et al. 2007. Mineral geochemical characteristics of Dashui diorite gold deposit in Gansu Province. Geology and exploration, 43 (4): 4 1 ~ 45.
Sun Baoping, Pan Zhenxing. 2007. Geological characteristics and prospecting criteria of Dashui gold mine area in gerk. Inner Mongolia Petrochemical Company, (1):128 ~130.
Yan Shenghao, Wang, Gao Lan, et al. Geological characteristics and genesis of dashui gold deposit. Geology of the deposit,19 (2):126 ~136.
Yan Shenghao, Wang, Gao Lan, et al. Study on stable isotopes and REE geochemistry of Dashui gold deposit. Geology of mineral deposits, 19 (1): 37 ~ 45.
Zhao Yanqing, Ye Dejin. 2003. Mineralization characteristics of granite in Dashui gold deposit, West Qinling. Modern Geology,17 (2):151~156.
(Author Li Wenliang)