Jinlongshan Gold Mine, Zhen 'an County, Shaanxi Province

Jinlongshan Gold Mine in Zhen 'an County, Shaanxi Province belongs to Miliang Township, Zhen 'an County, on the southern slope of Qinling Mountain, Shaanxi Province. It is a fine disseminated antimony-gold deposit discovered by the Armed Police Gold Force in the Dingma mercury-antimony ore belt in Xunzhen Basin, South Qinling in the late 1980s, and has reached a large scale after exploration.

1 regional metallogenic geological environment

1. 1 geotectonic unit

Jinlongshan gold deposit is located in Indosinian fold belt of South Qinling fold system, and its regional structure is located in the secondary anticlinorium structure on the south side of Zhen 'an-Banyuanzhen fault and the north wing of Jinjiling composite syncline.

1.2 regional stratum

The Paleozoic strata in the area are relatively complete (Figure 1), including Cambrian-Ordovician, Silurian, Devonian, Carboniferous and Permian. There are only Triassic in Mesozoic and Quaternary in Cenozoic.

Map 1 Geological Map of Jinlongshan Gold Ore Field

(According to Gold Command of Chinese People's Armed Police Force 1997)

Q- four yuan; P2L-Longdongchuan Formation of Upper Permian; P2y—— Tietantan Formation of Upper Permian; P 1sh—— Lower Permian Shuixiakou Formation; C 1-2t- Tiechangpu Formation of Middle and Lower Carboniferous; C 1-2S- Sixiakou Formation of Middle and Lower Carboniferous; C 1y—— Yuanjiagou Formation of Lower Carboniferous; D3n—— Nanyangshan Formation of Upper Devonian; D2l— Lengshuihe Formation of Middle Devonian. 1- fracture; 2- Veins and Numbers

Paleozoic is mainly distributed in the periphery of Zhen 'an-Xunyang basin. With the continuous evolution of sedimentary basins, the sedimentary range of Paleozoic strata is also expanding, and the sedimentary center moves northward. South yangshan Formation of Upper Devonian and Yuanjiagou Formation of Lower Carboniferous are developed in the center of sedimentary basin. Triassic is distributed in the middle of the study area and constitutes the core of Jinjiling compound syncline, which is a set of shallow sea clastic rocks and argillaceous carbonate rocks.

1.3 regional tectonic framework

The fold structure in this area is developed, mainly Jinjiling syncline, which is located in the south of Zhen 'an-slate fault, and is the main body of Indosinian fold in Zhen 'an-Xunzi area, extending axially from east to west. The core of the anticline is composed of Triassic, Permian and Carboniferous, its wings and upturned ends are composed of Devonian and Lower Paleozoic, and its wings are secondary folds. The fault structures mainly include Shangnan-Feng Dan fault, Zhen 'an-Banyuanzhen fault, Fengzhen-Yang Shan fault and Shiquan-Ankang-Zhushan fault.

1.4 regional magmatism

No rock mass has been found in the study area, but there is strong magmatic activity in the area. The intrusive rocks are mainly Indosinian granite, mainly distributed in Fengxian-Ningshan area in the west of the study area. There are Variscan and Caledonian granitoids related to copper, lead and zinc mineralization, ultrabasic granitoids related to chromite and adamellite related to rare earth deposits on both sides of the study area.

2 Geological characteristics of mining area

2. 1 ore-bearing strata

Orebodies occur in the Upper Devonian South yangshan Formation (D3n) and the Lower Carboniferous Yuanjiagou Formation (C 1y) in Xunyang Basin, in which the South yangshan Formation is the main ore-bearing stratum of gold deposits, and the ore-bearing rock series generally contains organic matter (Zhang et al., 2000).

Nanyang Mountain Formation is integrated on Lengshuihe Formation. The lithology and lithofacies change greatly in this area. From west to east, it can be roughly divided into three facies stratigraphic sections: carbonaceous argillaceous silty calcareous slate, limestone-siltstone-shale and silty medium-fine limestone of Nanyang Formation, with the same horizons and times.

Yuanjiagou Formation covers the Nanyang Mountain Formation as a whole, and its lithology is gray-dark gray medium-thin layer containing flint zone (block) fine limestone, microcrystalline limestone and granular limestone. These rocks are rich in foraminifera and Odonata benthos.

2.2 Ore-controlling structure

Ore-controlling structures are characterized by regional fold brittle-ductile and ductile-brittle shear deformation structures. The south (F 1) and north (F2) faults in the mining area have become the largest first-class fault structures in this area. F 1 and F2 bend in a wave shape along the strike, with the fault plane dipping N and the dip angle of 70 ~ 80, belonging to high-angle thrust faults, and the Jinlongshan deposit is confined between the two faults. NE-trending and NW-trending faults are conjugate contemporaneous faults, and NW-trending faults are developed with large scale and long time sequence. The strike of NE-trending faults is 40 ~ 60, and the fault plane is steeper than NW-trending. The close relationship between SN- strike fault and mineralization is very obvious at regional and small-medium scale. East-west, northeast, northwest and north-south structural compounding are important structural conditions for the formation of gold deposits. The rice-shaped structural profile and favorable ore-bearing lithologic position formed by them are closely related to the gold mineralization alteration caused by stress concentration, and a series of permeable fracture structures including joints, fractures and cleavage are formed. When these permeable fracture structures are developed in the shear zone favorable for ore-bearing strata, the broken rocks can directly constitute industrial ore bodies.

2.3 surrounding rock alteration

The wall rock alteration is generally simple, and the alteration closely related to antimony gold mineralization is mainly silicification and carbonation, followed by barite, Yili petrochemical, pyrite, arsenopyrite, limonite and kaolin. Mineralization alteration can be divided into three stages: the early stage is characterized by pyritization, arsenopyrite, arsenopyrite and weak silicification; In the middle stage, it is still characterized by pyritization, arsenopyrite and arsenopyrite, but the associated silicification, ankerite and calcite are obvious. The late mineralization alteration is characterized by coarse-grained pyrite mineralization, stibnite mineralization and Cinnabar mineralization, accompanied by strong jasper-like mineralization, calcite mineralization, barite mineralization and a small amount of illite mineralization. Among them, mineralization and alteration in the early and middle stages are closely related to gold mineralization, and arsenic-bearing pyrite and arsenopyrite are the most important gold-bearing minerals.

3 Geological characteristics of ore bodies

3. 1 ore body characteristics

The large industrial ore bodies in Jinlongshan mining area are mainly 1, No.3, No.6 and 16, while others are smaller or only mineralized.

Ore body 1 and its outer edge alteration zone are 380m long and 4m wide. The southwestern part of the ore body is vein-like, with a strike of 40 ~ 60; The northeast of the ore body is lentil-shaped, and the strike is 60 ~ 100. NW-N is generally inclined, with an inclination of 72 ~ 86. The strike and dip of the ore body are gentle, wavy and the overall shape is complex. The actual control length of the ore body is 230m, the horizontal thickness is 1.03 ~ 20.80m, and the average thickness is 6.71m. The maximum exposed elevation of the ore body is 906m, and the lowest elevation controlled by the project is 746m m. The gold grade of the ore body1.04×10-6 ~ 64.26×10-6, with an average of 6.13×/kloc-0-0. Ore bodies are mainly composed of disseminated pyrite-arsenopyrite type gold deposits and disseminated pyrite type gold deposits, and breccia-like antimony gold deposits occur in strongly fractured alteration zones where local ne-trending brittle faults overlap.

Ore body No.6 is distributed on the east side of the ore block. The structural alteration zone of gold deposit is longer than > 1000 m, wider than 3.00 ~ 12.70 m, with strike of 40 ~ 55, NW dip and dip angle of 75 ~ 88. Among them, the controlled length of ore body is 280m, the horizontal thickness is 0.73 ~14.92m, the maximum exposed elevation of ore body is 849.6m, the lowest elevation of engineering control is 750. 1 1 m, the controlled dip angle is 257.60m, and the gold grade of ore body is1.00×/. Orebodies are generally lenticular, with bifurcated edges, and there are often surrounding rock blocks inside. The ore body is mainly composed of disseminated pyrite-arsenopyrite gold ore and disseminated pyrite gold ore, with occasional breccia antimony gold ore.

16 ore body is distributed in the southeast of the ore block and located in the south wing of Jinlong Mountain anticline. The ore body is controlled by near east-west interlayer faults. Fracture length > 300m, strike 90 ~ 105, dip 180 ~ 195, dip 75 ~ 80. The engineering controlled ore body is 98m long, with a horizontal thickness of 2.06~9.32m and an average thickness of 5.00m The maximum exposed elevation of the ore body is 855m, and the lowest elevation of the engineering controlled ore body is 733m. The gold grade of the ore body is1.13×10-6 ~ 52.58×/kloc-.

No.3 ore body is located in the axial part of Jinlongshan anticline, and occurs in the middle and upper lithologic section of Nanyang Mountain Formation, with east-west distribution, mainly controlled by cleavage extension zone on the axial plane of fold. The surface controlled ore bodies are 60m long, with a horizontal thickness of 5 ~ 15m, and gold grade of1.03x10-6 ~ 33.53x10-6, with an average of 4.13×/kloc-0. The ore body is mainly composed of disseminated pyrite-arsenopyrite gold ore and disseminated pyrite gold ore, which is lenticular.

3.2 Ore composition

The main metal minerals in the ore are arsenopyrite, arsenopyrite, stibnite, pyrite and a small amount of cinnabar and chalcopyrite. Gangue minerals include quartz, calcite, ankerite, sericite and dickite. The main gold-bearing minerals are arsenopyrite and arsenopyrite. Gold occurs in submicroscopic (0.0 1.04 ~ 0.075 micron) beads and hammer balls in arsenic-bearing pyrite accretion zone and arsenopyrite particles.

3.3 Ore fabric and metallogenic stage division

The ore structure is mainly automorphic-semi-automorphic-heteromorphic, including metasomatism, solid solution separation and dissolution. The structure is (veinlet) disseminated, breccia, massive and (reticulate) veined.

The mineralization of the deposit can be divided into three metallogenic periods: the pre-enrichment period of syngenetic sedimentary mineralization, the mineralization period of structural superposition-hydrothermal transformation and the secondary enrichment period of supergene oxidation. The metallogenic period of structural superposition-hydrothermal transformation can be further divided into four stages: fold deformation mineralization stage, secondary fold-strain slip cleavage mineralization stage, knee fracture mineralization stage and brittle deformation mineralization stage.

3.4 Ore Composition

Ore is rich in gold, copper, lead, zinc and other elements, and the content varies greatly. Au is mainly positively correlated with Cu, Ag, Bi and Fe, with correlation coefficients of 0.86, 0.60, 0.44 and 0.30 respectively, which is basically consistent with that of pyrite and chalcopyrite.

4 genetic analysis of the deposit

4. 1 Characteristics of mineral inclusions

See table 1, table 2 and table 3 (Zhang Fuxin,1997; Zhao, 1997).

The inclusions are well developed, but most of them are less than 5 microns, generally between 1 ~ 3 μ m, and from gold mineralization, antimony mineralization to carbonation, the inclusions gradually become larger, mainly in liquid phase and gas-liquid phase, and the gas-liquid ratio is between 5% and 25%. The inclusions are spherical, oval, long and irregular. Under the microscope of 40× 10, we can see many gas phases in the form of small black dots jumping in the liquid inclusions.

Generally speaking, the ore-forming fluids are Cl-> F- and Na+> K+, which belong to Cl-na++ type fluids.

The determination of mineral fluid inclusions shows that the cations in the liquid phase of gold-bearing ore-forming fluid are K+> Na+, Au > Sb anion () > (F-+Cl-); The gas phase composition is mainly H2O, followed by CO2, CH4, CO and N2.

From the mineralization stage of gold and antimony to the carbonation stage, the contents of N2 and CO2 in inclusions increased, and a certain amount of O2 was also detected in the carbonation stage, indicating that with the evolution of mineralization, the ore-forming fluid system became more and more open, and more sediments were added to the ore-forming fluid, indicating that the mineralization depth gradually became shallow, and crustal uplift occurred during the mineralization process.

Table 1 characteristics and homogenization temperature of fluid inclusions in Jinlongshan gold belt

Table 2 Composition of fluid inclusions in Jinlongshan gold belt

Note: ① Not counted; ② Ca2+is not calculated; "-"is below the detection limit.

From morning till night, the content of H2O in ore-forming fluid decreased, the content of CO2 increased, and the ratio of H2O/CO2 decreased obviously (Table 3), indicating that the concentration of CO2 gradually increased. According to the principle of chemical equilibrium, the content of sum ion in solution should also be increased. And the characteristics of typical Carlin-type gold deposits (Kerrich et al., 2000; Berger et al., 199 1) and the mineral assemblage sequence of the Jinlongshan gold belt.

Table 3 Gas phase composition of fluid inclusions in Jinlongshan gold belt

Note: the reduction parameter r =(H2+ CO+CH4)/CO2; "-"is below the detection limit.

Table 4 Isotopic compositions of carbon (PDB standard), hydrogen and oxygen (SMOW standard) of minerals and mineral fluid inclusions in Jinlongshan gold belt.

Note: In order to calculate the numerical value, the equations of chronotropic and calcite-water fractionation are: 1000l nα chronotropic-water = 3.42× 106t-2-2.86 (Zhang Ligang, 1985, 200 ~ 500℃); 1000lnα calcite-water = 2.78× 106t-2-2.89 (O 'Neill, 1969, 0 ~ 800℃), others are measured values; * Quoted from et al. (1997), * quoted from Zhao (1997), and the unknown note was detected by Isotope Open Laboratory of Chinese Academy of Geological Sciences. The mass spectrometer model is MAY25 1EM, and the accuracy of carbon and oxygen isotope analysis is 0.2% and that of hydrogen isotope analysis is 2%. "-"is untested.

According to the gold deposits in Jinlongshan and Qiu Ling, the contents of CH4 and C2H6 in the late inclusions are higher than those in the gold-antimony mineralization stage, which may be due to the high content of organic matter in the ore-bearing strata, which was oxidized and decomposed into CH4, C2H6 and CO2 under the action of ore-forming hydrothermal solution, resulting in the higher content of light hydrocarbons in the late inclusions than in the main mineralization stage. Another reason is that the late mineralization temperature is low, which is beneficial to the stable existence of C2H6 and others.

4.2 Physical and chemical conditions

In the gold mineralization stage, the homogeneous temperature range of inclusions is 158 ~ 268℃, concentrated in 180 ~ 220℃, and the salinity is 5.7% ~ 7.85% (Zhang Fuxin et al., 1997). In the stage of antimony mineralization, the homogenization temperature is 120 ~ 277℃, concentrated in 140 ~ 220℃, and the salinity is 8.3% ~ 8.6% (Zhang Fuxin et al., 1997). However, the homogenization temperature in the later stage of mineralization is 8 1 ~ 184℃, which is concentrated between 130 ~ 180℃. It shows that the uniform temperature gradually decreases from the main metallogenic period to the late metallogenic period, and the ore-forming fluid has the characteristics of medium-low temperature and medium-low salinity, which accords with the general characteristics of micro-disseminated gold deposits.

4.3 Isotopic geochemical markers

4.3. 1 hydrogen, oxygen and carbon isotopes

Mineral δ 18O is concentrated in the range of 16.5 ‰ ~ 25.5 ‰ and enriched in 18O, which is different from sedimentary rock δ 18O (5 ‰ ~ 25 ‰; Wei Juying et al., 1988). From the characteristics of δ 18O of Jinlongshan deposit, it can be seen that there is a decreasing trend from the gold mineralization stage (24.3 ‰ ~ 25.9 ‰) to the carbonation stage (16.5 ‰ ~ 17.9 ‰), and the temperature of the ore-forming process is decreasing, so the δ/kloc-0 of the fluid. From morning till night, the δD value of fluid inclusions gradually increases (Table 4). The average δD of fluid in the main metallogenic period of Jinlongshan deposit is -87‰ and -65 ‰ in the later period. Qiu Ling deposit changed from -83‰ to-69 ‰; These δD values fluctuated around the local Mesozoic (J-K) atmospheric precipitation δ d (δ d =-88 ‰) (Zhang Ligang, 1985). On the δD-δ 18O diagram of ore-forming fluid, the projection point of ore-forming fluid in the study area is located in the magmatic water area or on the left and right sides and near the metamorphic underwater boundary. The projection points of most samples obviously drift to the right relative to the atmospheric precipitation line, indicating that the fluid has been fully exchanged with or from the surrounding rock, thus enriching δ 18O, and also indicating that the ore-forming fluid may have evolved from sedimentary construction water derived from atmospheric precipitation (Table 4).

From morning till night, ore-forming fluids often evolve into atmospheric precipitation. The distribution of projection points on δD-δ 18O map of Jinlongshan gold belt is similar to that of Devonian Baguamiao gold deposit (timely inclusion) in this area, with δD and δ 18O values of-17.9 ‰ ~-53.5 ‰ and-3.07 ‰ respectively. 1994)) and double gold deposits (the δD and δ 18O of inclusions in Yingshi, calcite and ankerite are -62.2 ‰ ~- 13 1.9 ‰ and 8.31‰/kloc, respectively.

In addition, it can be found that the average δD of fluid inclusions is -98.0‰(4 samples), -78.6‰(6 samples) and -32.6‰( 1 sample) (Shuo Fan Chengdeng, 1994), which gradually increases. The average values of δ 18O are 13.9‰, 10.8‰ and -7.63‰, respectively, which decrease in turn, and this trend is consistent with the Jinlongshan gold belt.

These similar features may be related to the fact that they have the same tectonic background, which leads to similar fluid evolution processes.

Sulfur isotope

The δ34S value of pyrite in sedimentary rocks of ore-bearing rock series is-4.23×10-3 ~+0.73×10-3, with a range of 4.96× 10-3, which is close to meteorite sulfur and tends to be negative, indicating that it has a sulfur source from volcanic hydrothermal solution and was slightly reduced by organisms. The δ34S value of hydrothermal sulfide in ore is11.05×10-3 ~19.76×10-3, and the range is 8.7/kloc-0 /×1.

Lead isotope

The lead isotopes of pyrite in ore-bearing rock series vary greatly, with 206Pb/204Pb being 18.058% ~ 18.478% and 208Pb/204Pb being 15.523% ~ 15.843%.

4.4 metallogenic age

40Ar/39Ar age analysis shows that the gold mineralization age is 65438±035.5Ma, indicating that the deposit was formed in the Early Cretaceous.

According to the above analysis, it is preliminarily considered that the Jinlongshan gold deposit is veinlet disseminated type.

refer to

Liu Xinhui, Liu Shuang, Ethan, etc. 2000. Regularity of ore-forming fluid storage and transportation and regional gold prospecting direction in Jinlongshan-Qiu Ling gold deposit. Geological exploration series, (increase): 10 ~ 14.

Zhang Jing, Chen Yanjing, et al. 2002. Geochemistry of ore-forming fluids in Carlin-type gold belt in Jinlong Mountain, Shaanxi Province. Geology of mineral deposits, 2 1 (3): 283 ~ 29 1.

(Author Zhang Yanchun)