1 regional geological background
Tanjianshan gold deposit is located in the northern margin of Qaidam block and belongs to the "Canshan fault fold belt in the northern margin of Qaidam". The structural layers in the area are distributed in the northwest direction, and the periphery is bounded by faults (Figure 1).
Figure 1 Diagram of Regional Geological Structure of Tanjianshan Gold Deposit
(According to Feng Chi et al. 1998)
Q- four yuan; J-R- Jurassic-Tertiary (Paleogene); O3Tn—— Qinjianshan Group of Upper Ordovician; Pt2Wd—- Mesoproterozoic Wandonggou Group; Pt1dk-Proterozoic reaches Kenda Ban Group; γ5- Indosinian granite; γ4- Variscan granite; γ3- Caledonian granite; V3- Caledonian basic rocks; σ3- Caledonian ultrabasic rocks. 1- boundary of continental basin; 2- ductile shear zone; 3— Brittle fault zone; 4- presumed fault zone; 5-Tanjianshan Gold Mine
The Proterozoic Zhouda Kenda Ban Group constitutes a regional crustal crystalline basement, which is composed of medium-deep metamorphic rock series. The Mesoproterozoic Wandonggou Group is formed in the continental margin depression trough, and consists of shallow metamorphic carbonaceous argillaceous rocks and magnesium-rich carbonate rocks, in which the black rock series is the gold source bed. The Upper Ordovician Tanjianshan Group was formed in the intracontinental rift environment. It consists of a set of intermediate-basic volcanic rocks, pyroclastic rocks, some terrigenous clastic rocks and carbonate rocks, with a thickness of over 5,000m, and has undergone regional metamorphism in greenschist facies. Late Paleozoic strata are only scattered, mainly marine and transitional facies deposits. Since then, the regional crust has been separated from the marine environment. Due to the repeated opening and closing evolution of the regional crust and the large-scale intracontinental nappe in the later period, the strata in the above different eras are in fault contact. Tanjianshan gold deposit is located in the NW-trending ductile shear zone on the north side of the junction zone between Wandonggou Group and Tanjianshan Group.
2 Geological characteristics of mining area
2. 1 layer
The strata exposed in the mining area are mostly Wandonggou Group, which is a set of carbonaceous argillaceous clastic rocks and magnesium-rich carbonate rocks subjected to regional metamorphism and dynamic metamorphism, and generally distributed in the northwest direction (Figure 2).
The strata of Wandonggou Group are divided into upper and lower rock groups: the lower rock group consists of thick layered dolomite marble and banded dolomite marble, with carbonaceous sericite phyllite interlayer in the upper part; The upper rock stratum is mainly gray-black carbon-bearing "spotted" phyllite schist, and the upper part is crystalline limestone. Carbonaceous phyllite-schist strata in the upper strata are important ore-bearing strata in this area.
According to the regional data, the gold content of Wandonggou Formation is low (upper member 1.04× 10-9, lower member 1.0× 10-9), and the gold content changes little (upper member 1.76, lower member).
2.2 structure
The tectonic activity in this area is strong, which is mainly manifested in stratigraphic folding, rock fragmentation and fault activity.
There are two groups of fold structures: one group is a NW-trending compound syncline structure, the core is mainly the carbonaceous phyllite-schist stratum of the upper rock group of Wandonggou Group, and the two wings are dolomite marble of the lower rock group. With the formation of this syncline, foliation appears in the same direction, and "spots" composed of garnet phenocrysts are formed in phyllite schist. The other group of folds is an interlayer dorsal structure with nearly north-south direction. With the formation of this group of folds, a foliation zone with nearly north-south direction is produced. The NW-trending composite syncline and the syn-foliation belt formed the Eastern Glacial Age, and the near SN- trending interlayer folds and syn-foliation belt formed the Variscan Age. The revived part of the early foliation zone (ductile shear zone), the brittle fracture zone of the late foliation zone, the axial fracture (or cleavage) of the near north-south interlayer fold and the interlayer collapse part (or interlayer sliding zone) (figure 1) are important ore-hosting structures in the mining area.
Fault structures are divided into three groups: NW, NE and nearly EW (Figure 2). Among them, the NNE (and near SN- trending) faults are small in scale, mostly axial faults on fold wings or interlayer strike-slip faults, which are important ore-controlling and ore-hosting faults in the area.
Fig. 2 Geological schematic diagram of Tanjianshan gold mining area
(According to Cui Yanhe et al., 2000)
Q- four yuan; 1- carbonaceous phyllite schist; 2- marble; 3- plagioclase granite porphyry; 4- dike; 5- ore body; 6- Fault; 7— axis of anticline; 8— syncline axis; 9-CD part (position in Figure 3)
2.3 magmatic rocks
The magmatic rocks in the mining area are mainly Variscan intermediate-acid intrusive rocks, and the rock types include plagioclase granite porphyry, granite porphyry, granite fine grain, plagioclase porphyry or diorite fine grain, Yunhuangyan and so on. The plagioclase granite porphyry occurs as a small intrusive body, which is the exposed part of the plagioclase granite porphyry in Tanjianshan mining area. Most dikes have been reconstructed, and some have been bent and shrunk due to tectonic movement. Vein gold ore bodies are formed after alteration and mineralization of vein rocks.
According to the regional data, the average content of ore-forming elements in variscan intrusive rocks in the mining area is lower than Clark value, and the content has little change. Only plagioclase granite porphyry has relatively high gold content (2.54× 10-9 on average) and correspondingly high arsenic content (9.65× 10-6). From basic to neutral to weakly alkaline, the contents of gold and arsenic increase synchronously in turn, reflecting that the intermediate-acid rocks in the plagioclase granite porphyry in the mining area are likely to constitute the source area of gold.
3 Geological characteristics of the deposit
The main industrial ore bodies are distributed in NNE direction, and a few are distributed in NW direction. The ore bodies are mostly layered, veined and lenticular (Figure 2 and Figure 3), and have no obvious boundary with altered surrounding rocks, showing a gradual transition relationship.
Fig. 3 C-D profile of tanjianshan gold mine
(According to Cui Yanhe et al., 2000)
1- Quaternary diluvium; 2- carbonaceous phyllite; 3- dolomite marble; 4- granite porphyry; 5- gold ore body; 6. Inferring gold ore bodies; 7- drilling; 8- Puncture the pulse
Ores are mainly structural altered rocks, which can be divided into altered carbonaceous phyllite-schist type and altered dikes type according to the different altered protoliths. Ore minerals mainly include natural gold, silver-gold ore, arsenopyrite and arsenic-bearing pyrite. Gangue minerals mainly include quartz, sericite, chlorite, epidote and dolomite.
The ore structure is mainly self-shaped-semi-self-shaped cube, pentagonal dodecahedron granular, ring edge, ring belt and sieve inclusion structure. Ore structures are mainly disseminated, eyeball-shaped lumps and veinlets-reticulate veins.
The alteration of the surrounding rock of the deposit is mainly pyritization, silicification, sericitization and a little carbonation of the host rock (carbonaceous phyllite-schist), and the dark minerals in the surrounding rock are mostly chloritization and epidote. There is a gradual transition relationship among ore bodies, altered rocks and unaltered surrounding rocks.
3. 1 ore-controlling shear zone
The regional NW ductile shear bandwidth is about 1km, which is superimposed on the carbonaceous argillaceous rocks in the mining area, covering the distribution of all known ore bodies and occurrences. The NE ductile shear bandwidth is about 500m, sandwiched in the regional NW ductile shear zone, which is the product of the progressive evolution of the regional NW ductile shear zone. Due to the many left-handed and right-handed shear activities of NW ductile shear zone, ne ductile shear zone in the mining area has experienced many intense compressions, contractions and expansions, which provides favorable conditions for the circulation and convergence of mineralized fluids and controls the distribution of main ore bodies in the mining area (Figure 4).
3.2 Ore-bearing strata
Due to strong shear deformation, the original bedding in the mining area has been replaced, and the horizon is equivalent to the black rock series in the middle of Wandonggou Group, mainly carbonaceous mylonite schist, which is located in the strong strain domain of the mining area. The chemical composition of the rock is similar to that of hydromica clay rock, with reduced carbon content of 1.53%, gold content of13.6×10-9 ~ 32.0×10-9, and arsenic content of17.68×/.
4 genesis of ore deposit
4. 1 Geochemical characteristics of the deposit
4. 1. 1 Geochemical characteristics of ore-forming elements
Gold in altered phyllite, schist and similar ores in Tanjianshan gold deposit is closely and positively related to arsenic (RAU- As = 0.9437) and antimony (RAU- Sb = 0.9082). Arsenic and molybdenum (Ras-Mo = 0.9945), antimony and molybdenum (RSB-Mo = 0.9364) are closely and positively correlated. Molybdenum is closely and positively related to arsenic and antimony. Selenium is closely and positively correlated with tellurium (rse-te = 0.875 3). The correlation matrix of ore-forming elements is shown in table 1 (Yu Fengchi et al., 1998).
Schematic diagram of geological structure of Tanjianshan gold deposit.
(According to Feng Chi et al. 1998)
Ph- carbonaceous mylonite schist; M B- microcrystalline marble; Sch-mica quartz schist; γοπ4- Late Variscan plagioclase granite porphyry; γ3- Caledonian gabbro. 1- ductile shear zone; 2— Brittle shear zone; 3— Ore body
Table 1 correlation matrix of ore-forming elements of altered phyllite and schist (including some ores) in Tanjianshan gold deposit
Note: Relevant critical value: r 0.05 = 0.755R0.0 1=0.875.
It can be seen from Table 2 that there is a close positive correlation between Au and Mo (RMO-Au = 0.7974) and Ag (RAU-Ag = 0.4693) in the Variscan intrusive rocks, which further shows that Au(Ag) and Mo are derived from magmatic hydrothermal solution. In addition, there is a close positive correlation between W and Cu and Pb (RW-Cu = 0.6275, RW-Pb = 0.5239), which indicates that they have a common geochemical migration pattern, and there is a close positive correlation between As, Sb and Hg (Ras-Sb = 0.9799, Ras-Hg = 0.9574, RSB-).
4. 1.2 fluid inclusion
The timely fluid inclusions in this mining area include gas inclusions, gas-liquid inclusions, liquid inclusions and CO2 inclusions. The size of inclusions is mostly between 5 ~10 μ m. The average homogenization temperature of fluid inclusions is 232℃ and the explosion temperature of pyrite is 278℃. Therefore, it is judged that the metallogenic temperature should be 232 ~ 278℃, which is relatively low, which is consistent with the low temperature condition represented by the combination of Au mineralizer elements As, Sb and Hg. The salinity is 0.16% ~ 8.9%; The estimated metallogenic pressure is12.16 ~ 23.438+0pa; The estimated mineralization depth is 0.6 ~ 1.0 km, belonging to epithermal gold deposit, which is consistent with the hydrothermal mineralization characteristics of vein rocks and porphyry magma in Tanjianshan gold deposit.
Table 2 Correlation matrix of ore-forming elements of variscan intrusive rocks (including some vein ores) in Tanjianshan Gold Mine
Note: Critical value of correlation: r 0.05 = 0.460R0.0 1=0.590. According to Feng Chi et al. 1998.
The ore-forming fluid is mainly composed of H2O (about 95%), and the gas phase is mainly composed of CO2, followed by CO and CH4. The ion combination of ore-forming fluid is type. The oxygen fugacity value () of ore-forming fluid varies from-34 to-37, which has the characteristics of reducing environment.
4. 1.3 stable isotope
The sulfur isotopic composition δ34S of the deposit varies from 3‰~ 10‰, with a few negative values. This sulfur isotope composition shows that it has undergone a relatively homogeneous process and has the characteristics of magmatic sulfur. Because its δ34S value deviates from meteorite sulfur relatively positively, it has the characteristics of crust-derived magma sulfur. The distribution range of sulfur isotope composition value mainly overlaps with granite, indicating that sulfur mainly comes from granite slurry. The δ34S of some samples is negative, indicating that the sulfur of metamorphic rocks (or sedimentary rocks) was added when the magmatic hydrothermal solution invaded the carbonaceous phyllite-schist mineralization.
The range of carbon isotope (δ 13CPDB) is-12.9‰~ 3.2‰, which is within the range of carbon oxides in magma, and its source is the same as magmatic rocks (mainly intrusive porphyries) in the mining area. The projection scatter of fluid inclusion water in Yingshi mining area mostly falls at the intersection of metamorphic water and magmatic water and its vicinity. Therefore, it is considered that the source of fluid water is the mixture of magmatic water and metamorphic water, and a small amount of Tianshui may be added.
The scatter point of lead isotope projection of carbonaceous schist in mining area is far from the normal lead isotope evolution curve, which has abnormal lead characteristics, which is related to the fact that the original rock was rich in radioactive isotopes 238U and 236U when carbonaceous schist was formed, so the lead isotope composition is rich in radioactive lead. The lead isotope of diorite porphyrite in the mining area falls in the abnormal lead area close to the normal lead evolution curve, indicating that it was polluted by formation lead during emplacement. The lead isotope projection scatters of intrusive rocks (quartz diorite, plagioclase granite porphyry and gabbro) around the mining area are located on the normal lead evolution curve; From schist-type ore, altered surrounding rock pyrite, vein pyrite and schist-type ore pyrite to diorite porphyrite ore and pyrite in diorite porphyrite ore, the lead isotope projection point is far from the lead isotope projection area of carbonaceous schist and close to the projection area of intrusive rocks around the mining area in the normal lead isotope evolution curve, indicating that the lead isotope in the ore includes both abnormal lead in carbonaceous schist and normal lead homologous to intrusive rocks. In addition, the lead isotopic composition of pyrite is closer to magmatic lead than its parent ore. Considering the close symbiosis between gold and pyrite in the deposit, it further shows that gold mainly comes from magmatic hydrothermal solution.
5 genesis and metallogenic mechanism of the deposit
5. 1 source of ore-forming materials
As mentioned above, the water in the ore-forming fluid of this deposit is a mixture of metamorphic water and variscan magmatic water; The sulfur in the ore is mainly granite slurry sulfur mixed with metamorphic sulfur; Magmatic carbon is the main carbon in ore; Lead in ore is a mixture of magmatic lead and metamorphic lead; In Variscan magmatic rocks, the contents of Au and As increase synchronously from basic to acidic and then to alkaline. The correlation between elements shows that under the action of As and Sb mineralizers, Au and Mo migrate along ductile-brittle structural fractures and deposit in carbonaceous phyllite, that is, Au mainly comes from Variscan intrusive rocks.
5.2 Types of ore-forming fluids and ore-forming conditions
The ore-forming fluid of Tanjianshan gold deposit is Na+-K+(Ca2+)-Cl-SO2-4-CO2-H2O type under weak reduction conditions. The metallogenic temperature mainly ranges from 232℃ to 278℃. The salinity of the fluid is 0.16% ~ 8.90%; The metallogenic pressure is12.16 ~ 23.25438+0 MPa. Basically, it belongs to the shallow to moderate temperature liquid type.
5.3 Treatment Forms and Precipitation Conditions of Gold
5.3. 1 Au processing form
The experimental data of T. M. Seward( 1973) show that in alkaline solution, Au(HS)2s 2- complex is the main form of Au, while in neutral solution, mainly Au(HS)2- complex may exist in acidic solution. The experimental results also show that the solubility of sulfur-containing gold complexes in solution is the largest in neutral to weakly alkaline environment. C A Wood et al. (1987) pointed out that gold is a soft metal ion, so it can form the main complex Au(HS)2- with soft disulfide ions in sulfur-containing solution. D M Shenberger and H L Barnes (1989) studied the solubility of gold in water-soluble sulfide solution at a given temperature (150 ~ 350℃), and thought that Au(HS)2- was the most stable and could migrate as Au(HS)2- in hydrothermal solution. T. M. Seward( 1973) also pointed out that due to the geochemical relationship between Au, As and Sb, there may also be arsenic-sulfur and antimony-sulfur complexes in epithermal deposits, namely Au(AsS2)0, Au(AsS3)2- and Au(Sb2S4). Boyle (1984) pointed out that the strong polarization of As and Sb and the strong deformation of Au+ greatly improved the stability of the complex formed by As, Sb and Au.
Pyrite in Tanjianshan gold deposit is an important gold-bearing mineral with high content. In addition, gold in gold ore dominated by altered carbonaceous phyllite is closely and positively related to arsenic and antimony. Therefore, it is considered that the main migration forms of gold in Tanjianshan gold deposit are Au(HS)2-, Au(AsS2)0, Au(AsS3)2- and Au(Sb2S4).
5.3.2 gold precipitation conditions
Because the precipitation of minerals is the opposite process to its dissolution, and the precipitation of minerals in ore-forming fluid is based on the saturation and stability of ore-forming elements and their complex ligands (such as HS-). The factors that lead to the decomposition, solubility reduction and precipitation of sulfur-containing gold complex are as follows: ① The solubility of sulfur-containing gold complex in fluid is reduced by temperature reduction, forcing gold to precipitate. The decrease of temperature in the process of mineralization can be thought to be caused by the heat exchange between ore-forming hot fluid and cold surrounding rock during the upward migration along the fault caused by pressure difference. ② Opening of ore-forming fluid system. The opening of the system will inevitably bring out the components of S, As and Sb in the fluid, and then the complex formed by Au, S, As and Sb will decompose and Au will be precipitated. The opening of the system also makes the water in the system where Au's complex of sulfur, arsenic and antimony is located penetrate into the surrounding rock, which changes the surrounding rock, and the concentration of the complex rises above the saturation state, thus precipitating Au. In addition, the opening of the system will also lead to the mixing of water in metamorphic rocks and change the pH value of the system. The opening of the system will also lead to the pressure drop, and these changes will cause the decomposition and precipitation of Au complex.
5.4 Determination of metallogenic age
6.5438+0 geological foundation
All ore bodies in Tanjianshan Gold Mine occur in carbonaceous phyllite and schist in the upper rock group of Wandonggou Group. Besides carbonaceous phyllite and schist, there are also Variscan intermediate-acid dikes (a few are basic dikes) which have been destroyed and altered by structure. The former experienced metamorphism (late Caledonian-early Variscan), deformation (China Variscan) and magmatic hydrothermal mineralization in the formation of regional NW schist belt, while the latter mainly experienced deformation in the formation of NNE schist belt and magmatic hydrothermal mineralization in the late Variscan.
The location of the gold deposit is controlled by the large-scale interlayer traction fold structure formed by the lower strata of the upper rock formation of Wandonggou Group in Tanjianshan gold mining area. The shape, occurrence and distribution of ore bodies and sporadic ore bodies in the main ore belt are controlled by interlayer fractures, foliation zones, NNE and NW ductile-brittle fracture zones, post-axial cleavage zones and near-North-South fracture zones formed by post-resurrection. It is preliminarily considered that the formation period of ore-controlling structures (middle Variscan? ) is the lower time limit for the formation of the deposit.
Isotopic chronology
Plagioclase granite porphyry is the largest intermediate-acid rock mass in the mining area, which has invaded the NNE fold structural belt in the mining area and formed slightly later than the NNE schistosity belt. The gold ore bodies were formed during the alteration of diorite porphyrite, chrysolite and fine-grained dikes, so the formation age of these dikes more accurately represents the lower limit of the formation time of the deposit. Granite porphyry passes through plagioclase fine-grained dike, so the formation age of granite porphyry represents the upper limit of the formation time of the deposit. The K-Ar age of plagioclase granite porphyry is 209.4×106 ~ 309.87×106ma, and the Rb-Sr age is 330.03× 106Ma. Considering that the intense tectonic activity in this area may lead to the loss of Ar in the later samples, the K-Ar age is 330. The age of Yunhuangyan (K-Ar age is 288.9× 106Ma), which constitutes the vein gold deposit, is the smallest, so its formation age is set as the lower limit of metallogenic age. K-Ar age of granite porphyry ranges from 234.4×106 to 275.6×106ma, with the highest value of 275.9ma.. The K-Ar age of pyrite sericitized diorite porphyry is 294.29× 106Ma, while the K-Ar age of altered granite porphyry gold deposit is 268.94× 106Ma, which indicates that alteration is earlier than mineralization, and further proves that the Tanjianshan gold deposit was formed in the late Variscan period. After the formation of the deposit, there is still magmatic activity in the mining area, which shows the intrusion of diorite porphyrite in Shi Ying in Yanshan period.
5.5 Genetic mechanism of the deposit
There are two main views on the genetic mechanism of Tanjianshan gold deposit.
Tanjianshan gold deposit is located in the central and western part of the northern margin of Qaidam basin, from late Caledonian to early Variscan, and under the action of regional near-north-south compressive stress, a NW-trending compound syncline structure and a syntonic foliation zone were formed. In the early Variscan period, the compressive stress direction changed to NW-SE direction, and NNE-oriented interlayer fold structure and foliation zone distributed in the same direction were formed in Tanjianshan area. At the same time, the plagioclase granite porphyry was emplaced, and the derived fine-grained dikes were emplaced along the NNE-trending interlayer slip fault, the fold axis cleavage zone and the revived NNE-trending and NNE-trending faults. In the late Variscan period, the structure moved further, and fine-grained dikes and strata folded synchronously. After magmatic stage, hydrothermal solution brought Au, As, Sb, Pb, S, C, H2O and other components into the NNE brittle fracture zone, and exchanged heat and components with surrounding rocks such as high permeability carbonaceous phyllite schist. With the change of physical and chemical conditions (temperature decreasing, Eh increasing, pressure decreasing, etc. ), the gold-sulfur-arsenic-antimony complex decomposes and gold precipitates to form ore.
Tanjianshan Gold Mine is located in the northern margin of Qaidam Basin. Its gold mineralization is synchronous with shear deformation and metamorphism, and it has undergone the evolution of ductile-brittle shear deformation and ore-forming stage. This is a typical syntectonic altered rock type gold deposit (in Fengchi, 1998).
refer to
Cui Yanhe, Zhang Dequan, Li Daxin et al. 2000. Geology, geochemistry and genetic mechanism of Tanjianshan gold deposit in Qinghai Province. Geology of mineral deposits, 9 (3): 211~ 221.
Wei, Yu Fengchi. 1999. Tectonic evolution and genesis of Tanjianshan gold deposit in Qinghai Province. Journal of Xi Institute of Technology, 2 1 (4): 62 ~ 67.
Yu Fengchi, Ma, Wei, et al. Geological characteristics of Tanjianshan gold deposit in Qinghai and its geological significance of lead isotopic composition. Geology and geochemistry, (2): 9 ~ 18.
Yu Fengchi, Wei, Sun Jidong, etc. Metallogenic model of Tanjianshan gold deposit in Qinghai Province. Journal of Xi 'an Institute of Technology, 20 (1): 19 ~ 22.
Yu Fengchi, Wei, Sun Jidong, et al. Ore composition and ore-forming material source of Tanjianshan gold deposit in Qinghai Province. Journal of Xi Institute of Technology, 2 1 (4): 57 ~ 62.
(Li,, writing)
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4. Zhu Yuxi
5. Zhu Chongqi
6. Zhu Chizhen
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8. Zhu Maoran
9. Zhu Chengyu
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