1 regional geological and metallogenic environment
1. 1 geotectonic unit
Tongling area is located in the south of Dabie-Sulu orogenic belt in the east of China, and belongs to the hedge zone formed by the interaction between Dabie orogenic belt and Yangtze block (Tang Yongcheng et al., 1998). Huangshi Laoshan gold mine is located in the southeast wing of Tongguanshan anticline in the south section of Tongling-Fanchang fault fold belt (figure 1), and together with Tongguanshan copper mine, Jinkouling gold mine and Mashan gold and sulfur mine, it constitutes the famous Tongguanshan gold, copper and sulfur ore field.
Figure 1 Geological Schematic Diagram of Tongguanshan Gold, Copper and Sulfur Ore Field
(modified according to Xue Hu's data, 1983)
T1-Lower Triassic; P2- Upper Permian, p 1- Lower Permian; C2+3- Middle and Upper Carboniferous; D3w- Upper Devonian Wu Tong Formation; South Silurian. 1- quartz diorite; 2- Translation error; 3- Reverse failure; 4- Geological boundary; 5— Ore body
1.2 regional stratum
The strata exposed in this area are Silurian deep-sea sandstone, Devonian river facies and coastal facies sandstone, Carboniferous shallow-sea facies carbonate rock, Permian shallow-sea facies carbonate rock, terrestrial facies carbonate rock, siliceous rock and clastic rock mixed with coal measures, Triassic shallow-sea facies and coastal facies carbonate rock and clastic rock. Carbonate rocks account for about 75%, and Carboniferous Huanglong Formation, Permian Qixia Formation and Dalong Formation are important ore-controlling strata of copper, gold and sulfur deposits in this area.
1.3 regional tectonic framework
The basement structures in this area are mainly east-west faults, and the caprock structures are mainly northeast Indosinian folds and Yanshanian faults. Among them, the Yanshanian fault structure controlled the intrusion of intermediate-acid hypabyssal rocks (diorite, quartz diorite, granite, etc.). ) is related to mineralization in the area. The isotopic age is 1 10 ~ 160 Ma.
1.4 regional magmatism
Magmatic activity is strong in the west of Mashan gold mine in the north of this area. The main intrusive rocks are diorite complex, and the thermal metamorphism of surrounding rocks is strong. The sandstone shale and marble formations of Huanglong Formation and Chuanshan Formation are keratinized, reaching the contact zone of rock mass more than 1000 meters. Alteration includes skarnization (local), dolomitization, pyritization, silicification, tremolite and serpentine. The deposit is 400 ~ 2000 m away from diorite complex, and only diorite porphyrite, diorite porphyrite vein and lamprophyre vein are exposed in the area, and the thermal metamorphism of surrounding rocks is not strong. The mudstone shale of Wu Tong Formation is andalusite and slate-like, and the marble of Huanglong Formation is marble-like.
There are more than 70 kinds of intrusive rocks, most of which are distributed in the east-west magmatic metallogenic belt controlled by Tongling-Nanling deep fault (Changdeng,1991; Wu Cailai et al., 2003), controlled the distribution of main copper, gold (iron) minerals such as Tongguanshan ore field, Shizishan ore field, Xinqiao ore field, Fenghuangshan ore field and Shatanjiao ore field in Tongling block, and a few of them were distributed in Wuguiqiao and Ding Qiao areas on the south side of Tongling block.
The main types of magmatic rocks are: ① pyroxene diorite, distributed in Baimangshan (Chaoshan), Jiaochong and Shujiadian, with a small exposed area; ② Synchronicdiorite is widely distributed along Tongling-Nanling fault, and it is the main magmatic rock in Tongling area. ③ Granodiorite, distributed in Yaoshan and Fenghuangshan. There are also a few quartz diorite, granite veins, diabase veins and lamprophyre veins scattered.
1.5 metallogenic unit
The regional metallogenic units are I-3 Qinqikun metallogenic domain, II-7 Qinling-Dabie metallogenic province and III-28 Tongbai-Dabie metallogenic belt.
2 Geological characteristics of mining area
2. 1 ore-bearing strata
Ore bodies are obviously located on the interface between Wu Tong Formation and Qixia Formation, especially on the interface between Wu Tong Formation and Huanglong Formation, and on the interface between the upper and lower lithologic members of Huanglong Formation. Formation lithology plays an important role in controlling mineralization. Sandstone, shale and mudstone of Wu Tong Formation are inactive in chemical properties and have poor water permeability, which play a good shielding role in the process of mineralization. The carbon contained in it is beneficial to reduction and promotes mineral deposition. This formation generally contains syngenetic pyrite and siderite lenses, which can provide some sources of iron and sulfur. The content of CaO and MgO in Chuanshan Formation of Huanglong is very high and its chemical properties are very active, which is beneficial to mineral metasomatism.
2.2 magmatic rocks in mining area
There are no large igneous rocks exposed in the mining area, only some intermediate-acid dikes intrude along NW-trending extensional faults and NE-trending interlayer faults. According to the drilling data, there is a concealed quartz diorite body at the southern end of the mining area.
2.3 Ore-controlling structure
The structure in the area is dominated by Tongguanshan anticline, and Huangshi Laoshan Gold Mine is located in the southeast wing of the anticline and began to tilt to the northeast. The occurrence of the upper wing stratum is normal, and the SE dip angle is 35 ~ 55; The middle and deep parts become steep to almost vertical, with an inclination angle of 65 ~ 90; The deep occurrence is inverted, with NW dip angle of 60 ~ 80, and the inversion elevation gradually deepens from south to north. The main faults are NE-trending compressive faults and NW-trending tension-torsion faults, which are the main rock-controlling and ore-controlling structures in the area. The main ore-hosting structure in this area is the interlayer fracture between the Devonian-Permian stratigraphic interface and the upper and lower lithologic members of Huanglong Formation.
2.4 surrounding rock alteration
The thermal metamorphism of surrounding rocks is not strong. The mudstone shale of Wu Tong Formation is andalusite and slate, and the Huanglong Formation is marble. Alteration includes dolomitization, pyritization and sericitization.
3 Geological characteristics of the deposit
3. 1 deposit (entity) characteristics
The main ore body of the deposit is layered in profile and banded in plane, with a length of about1200m, northeast strike and nearly vertical dip angle, and the occurrence of the ore body is stable, with the maximum horizontal thickness of 23.2m and an average of 5. 1 m.. The thickness of ore body is stable along strike and dip, and the grade changes relatively. The upper part of the ore body is gold-bearing limonite, and the lower part is gold-bearing limonite. According to the fact that pyrite remains in gold-bearing limonite body, it shows that gold-bearing limonite is transformed from gold-bearing limonite.
3.2 Ore composition
The ore composition of this deposit has different characteristics in different ore zones. According to the relative proportion and content of sulfide and limonite in the ore, the main ore body of the deposit can be divided into three zones from top to bottom: the upper part is oxidation zone, which is mainly composed of limonite+time+clay; The middle part is a semi-oxidized zone, which is mainly composed of limonite+pyrite+clay+bischofite; The lower part is the primary zone, which is mainly composed of pyrite and colloidal pyrite. Secondary metal minerals include pyrrhotite, arsenopyrite, galena, sphalerite, white iron ore and chalcopyrite. Gangue minerals are mainly timely and calcite.
The oxidation zones in the upper part of the deposit are all limonite-type gold deposits, which can be subdivided into three types according to iron content: ① gold-bearing limonite-type, with iron content of 25%, accounting for 64%; ② Gold-bearing iron clay mixed with limonite with iron content 15% ~ 25%, accounting for 27%; ③ Gold-bearing iron clay mixed with gravel, with iron content of 6% ~ 15%, accounting for 9%. It can be seen from the proportion that gold-bearing limonite is absolutely dominant.
3.3 Ore fabric and metallogenic stage division
3.3. 1 ore fabric
The ore structure is mainly semi-shaped-shaped granular, and the ore structure is mainly massive; Followed by flake, reticulate vein and disseminated.
3.3.2 Division of Metallogenic Stages
The study shows that the deposit has gone through three metallogenic stages in its formation and evolution.
The first stage occurred in Carboniferous, and it was submarine jet mineralization. Mainly gold-bearing iron ore deposits. In this period, the gold content of pyrite ranges from 0.09× 10-6 to 56.40× 10-6, and most of it ranges from 0.3× 10-6 to10-6, and silver. The lead isotope composition of layered gold-bearing bodies formed by mineralization in this period is uniform, and its 206 Pb/204Pb is between18.21~18.399, with a variation of < 0.9 1%. The range of 207Pb/204Pb is 15.537 ~ 15.625, with a variation of < 0.57%. The range of 208Pb/204Pb is 38.236 ~ 38.576, with a change of < 0.89%; It shows that the source of metal substances is stable, reflecting that the underlying layer is the single supplier of ore-forming materials under the condition of deep convection of seawater. The δ34S of sulfide minerals is between 0.14 ‰ and 7.30 ‰, indicating that the sulfur source is stable and is the source of sulfate reduction in seawater. The timely δ 18O value in sulfide ore is between 20.867‰~ 2 1.559‰. The gold-bearing sulfide ore bodies formed in this metallogenic period are thick and widely distributed, which laid a rich material foundation for the formation of this deposit. Its physical and chemical parameters: temperature 160 ~ 330℃ (timely homogenization method, pyrite, pyrrhotite, galena, sphalerite and arsenopyrite explosion method), sulfur fugacity ()10-1~ 65438+. The oxygen fugacity () is10-36 ~10-41.5, and the pH is 5.9 ~ 6.3.
The second stage occurred in Yanshan period, and it was gold-bearing hydrothermal superimposed mineralization. The study shows that there are reticulate veins, veinlets and pyrite veins in some layered primary ore bodies, and some primary ores show signs of hydrothermal superposition transformation. The pyrite in the vein contains 3.8× 10-6 gold, 9.7×10-6 silver and 234.5× 10-6 copper. According to the data of Tongling 8 12 team, in lead-zinc ore bodies,
The third stage mainly occurred in Cenozoic, and it was weathering and leaching mineralization. Under the comprehensive action of climate, groundwater, structure and landform, gold-bearing sulfide is continuously oxidized and decomposed, sulfur is lost and gold is enriched. The comparative study of ores shows that after the gold-bearing iron ore body is transformed into gold-bearing limonite ore body, the gold enrichment is 1 ~ 5 times. The elements rich in gold are silver, copper, lead, zinc, antimony, nickel, manganese, selenium, iron and rare earth. According to the data of Tongling 8 12 team, the average contents of copper, lead, zinc and iron in some ore blocks of the main ore body of the deposit are 0. 17%, 0.46%, 0.47% and 4 1.4% respectively, which can be comprehensively recovered. According to the research of Wang Ende, the total content of rare earth in primary ore bodies is generally less than 10× 10-6, while the total content of rare earth in weathered and oxidized ore bodies is generally 50× 10-6 ~ 400× 10-6, which can be comprehensively utilized. The mineralization in this period is of great significance. Without mineralization in this period, it is impossible to form an independent gold deposit.
3.4 Weathering characteristics of ore
The weathering characteristics of ores are mainly manifested in the oxidation zone. According to the weathering and leaching intensity, the oxidation zone of the deposit can be further divided into: ① strong oxidation sub-zone; ② Lymphatic sub-band; ③ Secondary oxidation enrichment sub-zone.
The ores in the strongly oxidized sub-zone are composed of clay, timely and ferromanganese. The mineral assemblage of pyrolusite, pyrolusite and hematite, as well as honeycomb and stalactite structure, are the main signs of this sub-zone.
The main feature of the leaching subregion is the appearance of a large number of soil-like, porous and loose goethite and goethite.
The main feature of the secondary oxidation enrichment zone is the remarkable enrichment of gold, and the gold content is generally > 5× 10-6, and the highest is 130× 10-6.
3.5 Ore Composition
See table 1 for the analysis results of single minerals of gold, arsenic, cobalt and nickel of pyrite in the ore body. The Co content in pyrite is between1.6×10-6 ~176.6×10-6, which varies greatly, and most of them are between 3.4×10-6 and 42.6×/kloc. The nickel content ranges from 4.75×10-6 to 43.4×10-6, with a relatively small change. The Co/Ni value of pyrite in ore body is 66% < 1, 34% > 1. It can also be seen from the table 1 that the arsenic content of pyrite is relatively high, which indicates that pyrite is deposited by jet rather than biochemistry, because the arsenic content of pyrite deposited normally is generally low, while the arsenic content of pyrite related to submarine volcano or jet is generally high.
Table 1 List of Trace Elements in Pyrite w(B)/ 10-6
Note: The sample numbers are arranged from north to south; The data were analyzed by Institute of Geology, East China Geological Survey.
4 genetic analysis of the deposit
4. 1 physical and chemical conditions
According to the results of Xue Jianhuan's temperature measurement of corresponding minerals, calcite, siderite and metallic minerals (pyrite, galena, sphalerite and arsenopyrite) in primary ore bodies, we think that the temperature of jet fluid is about 330 ~ 1 10℃.
Xue Jianhuan measured the salinity of timely inclusions. The salinity w(NaCl)% of two-phase inclusions is 0.9 ~ 26.3, with an average value of 10.2, and the salinity w(NaCl)% of multiphase inclusions is 30.2 ~ 410.7, with an average value of 36.5, indicating that the fluid is a high salinity solution.
Sulfur fugacity () is10-11-16; The oxygen fugacity () is10-36 ~10-41.5, and the pH is 5.9 ~ 6.3.
4.2 Isotope Geochemistry
4.2. 1 lead isotope
The lead isotopic composition is relatively uniform, and the range of 206Pb/204Pb is18.221~18.399, with a variation of < 0.95438+0%. The range of 207Pb/204Pb is 15.537 ~ 15.625, with a variation of < 0.57%. The range of 208Pb/204Pb is 38.236 ~ 38.576, with a change of < 0.89%; It shows that the source of metal substances is stable, reflecting that the underlying layer is a single supplier of ore-forming materials under the condition of deep convection of seawater (Table 2).
Table 2 Lead Isotopic Characteristics of Laoshan Gold Deposit in Huangshi
Note: The data of serial number 1 ~ 2 were analyzed by Tianjin Institute of Metallurgical Geology; The data of serial numbers 3 ~ 7 were analyzed by Yichang Institute of Geology and Mineral Resources. Parameter used in calculation = 9.307? ，b0= 10.294？ The age of the earth is t = t=4.55× 109a.
Oxygen isotope
The oxygen isotopic composition δ 18O in the ore body is between 20.867 ‰ and 21.559 ‰, and the oxygen isotopic composition of the flint in various iron-bearing formations related to the modern submarine hot spring system is 17% ~ 22%. The similarity shows that Yingshi is a submarine hot spring deposit.
Sulfur isotope
The δ34S of pyrite in the stratum is negative, and its variation range is between-13. 1 ‰ ~-29.20 ‰, which is the result of biochemical action. The δ34S values of sulfides in ore bodies are all positive, ranging from 0.14 ‰ to 7.3 ‰, most of which are 3.15 ‰ to 7.30 ‰, with an average value of 5.00‰, with concentrated distribution, narrow variation range and obvious tower effect. The average value of sulfur isotope δ34S of pyrite in ore body is 4.99‰. Because pyrite is the main ore body, and sulfate minerals such as gypsum and barite are not found, it shows that oxygen fugacity is low and sulfur is mainly reduced sulfur during mineralization, so the average δ34S of pyrite is 4.99‰. Sangster (1976) pointed out that the sulfur isotopic composition of massive sulfide deposits related to sedimentary rocks differs from that of contemporary seawater sulfates by about 13.9‰.
4.3 metallogenic age
Carboniferous submarine jet sedimentary mineralization laid a rich material foundation for the formation of the deposit. During Mesozoic Yanshan period, the gold grade of layered gold-bearing massive sulfide ore bodies was improved by tectonic-magmatism and ore-bearing hydrothermal superposition. Under the comprehensive action of structure, climate, groundwater and topography in Cenozoic, the primary gold-bearing sulfide was oxidized and decomposed, and gold was dissociated and migrated, gathering in the middle and lower parts of oxidation zone and semi-oxidation zone, forming an independent gold deposit with industrial significance today.
The K-Ar isotopic age of the rock mass is 65438±043Ma, and the lead model age of the ore is 65438±035Ma.
4.4 Genesis of the deposit
1978, Xu Keqin first proposed that the Carboniferous layered sulfide deposits in the middle and lower reaches of the Yangtze River were sedimentary or submarine volcanic deposits. Later, Gu Lianxing (1984, 1986) supported the above viewpoint through a detailed study of some Carboniferous layered deposits in the middle and lower reaches of the Yangtze River, and put forward that the Mashan gold deposit in Anhui Province is a submarine hot spring (that is, a jet deposit), and Wang Wenbin et al. (65438
refer to
He Jinxiang, Yu Guozhen, Zhu Yalin, et al. 1994a. Geological characteristics and genesis of Huangshi Laoshan gold deposit in Tongling, Anhui Province. Geology of the deposit,13 (3): 201~ 211.
He Jinxiang, Zhu Yalin, Yu Guozhen, et al. Study on genesis of primary gold-bearing iron ore body in Laoshan Tiemao gold deposit, Huangshi. Geology and exploration, 30 (1): 33 ~ 37.
Jiang Qisheng, Liu Dongzhou. 2006. Deep ore-hosting potential prediction of Huangshi Zishan gold mine in Tongling. Anhui Geology,16 (3):194 ~196
(Author Zhang Yanchun)