1. Overview
Black rock series type gold deposits are also called Mulongtou type or epimetamorphic clastic rock type gold deposits. They refer to deposits with high organic carbon content (generally ＞0. 5%) strata-bound deposits in epimetamorphic rock series. The epimetamorphic rock series is dominated by clastic rocks, often containing carbonate rocks, siliceous rocks and volcanic rocks, but is dominated by sandstone and slate. Some appear in turbidite series strata, so some people call them turbidite-type gold deposits (Dai Zixi, 2004).
Black rock series type gold deposits are mostly produced in areas where black rock series are developed. Data show that black rock systems are widely distributed around the world, such as in the upper part of the Rife boundary in Siberia, Russia, and at the bottom of the Lower Cambrian in the Lesser Himalayas of India, northern Pakistan, Iran, southern France, Mongolia, southern Australia, and Canada. It is distributed throughout the Upper Permian from England, Finland, Germany to Central Europe, as well as in the Proterozoic and Paleozoic strata in Central Asia. However, this type of gold deposit is only one of many black rock series type deposits, because black rock series is usually rich in organic matter and rich in PGE, Cu, Ni, Mo, Au, U, V, Mn, Fe, Co, Metal elements such as Bi, Cr, and Se can form certain-scale deposits under appropriate conditions.
Since black rock series gold deposits are often produced on a large to ultra-large scale, they have become one of the major gold deposit types with the most industrial value in the world since their discovery. After the discovery of the ultra-giant gold deposit in Murungtau, Uzbekistan, in the 1950s, countries around the world set off an upsurge in searching for black rock series-type gold deposits. A number of black rock series gold deposits have been discovered successively in the South Tianshan region of Uzbekistan, the northern part of Kazakhstan, Zaisan-Junggar, Chuyili, North Tianshan and other regions, as well as the Tianshan region of Kyrgyzstan, such as the Chazha gold deposits in Kazakhstan. Erkula and Bakirchik gold mines, Kumtor and Savayalton gold mines in Kyrgyzstan, and the Sukhoilog gold and platinum group metal deposits in Russia. In the 1990s, the large-scale Savayalton gold deposit (which has the same name as the gold mine in Kyrgyzstan and actually belongs to the same field) was also discovered in Xinjiang, my country. In addition, such deposits have also been discovered in the United States, Australia, Zimbabwe and other countries, but the overall scale is small. Judging from the spatial distribution of the discovered black rock series type gold deposits, this type of deposits are mostly concentrated in Central Asia, distributed in the Central Tianshan and South Tianshan metallogenic belts, and their resource reserves are huge (Table 1). Among them, representative deposits include Muruntau in Uzbekistan, Bakirchik in Kazakhstan, and Kumtor gold mine in Kyrgyzstan.
Table 1 Basic characteristics of the world's major black rock series gold deposits
Continued table
Data source: Liu Chunyong et al., 2007; Yang Fuquan et al., 2005 ; И. Ф. Мигачев et al., 2008
II. Geological Characteristics
1. Basic Characteristics
1) Geotectonic background: black rock series type gold deposit It is mostly produced in back-arc basins, foreland basins and island arc zones, which reflects that the black rock series was in a relatively stable tectonic environment when it was formed. From the perspective of trough structure, black rock series gold deposits are mostly produced in the maogeosyncline, or in the transition zone between the maogeosyncline and the eugeosyncline. Due to the development of magmatic activity and strong crustal activity in the eugeosyncline, it is difficult to form large-scale black rock series type gold deposits.
2) Ore-controlling structure: Black rock series-type gold deposits are obviously controlled by fault structures. Most of the deposits or ore fields are located on suture zones. The deposits produced at the intersection of regional faults are larger in scale, such as Mu Dragon set, Dougiz set, etc. Its ore-controlling faults have compressive ductile shear zone properties and belong to regional tectonic stress compression mineralization. This kind of mineral-bearing ductile shear zone is huge in scale, with a length of tens to thousands of meters. The mineralization in the ductile shear zone is characterized by segmented concentration and local enrichment.
3) Ore-hosting rock strata: Black rock series-type gold deposits have layer-controlled characteristics, and the age of ore-bearing strata is mostly concentrated in the Paleozoic Era, followed by the Proterozoic Era, especially the Cambrian-Carboniferous Era in Central Asia. (Department) Mainly. The ore-hosting rock system is a carbonaceous black clastic rock system with turbidity sedimentation characteristics and has experienced shallow metamorphism. The ore-hosting lithology is carbonaceous phyllite (Savayalton, Kumtor), phyllite Mumelite, carbonaceous slate, carbonaceous schist (Chalkula), carbonaceous metamorphic siltstone, metamorphic sandstone.
4) Surrounding rock alteration: The surrounding rock alteration is strong, and the main types include silicification, pyriteization, arsenopyrite (such as Savayalton, Daugiztau), sodium silicification Petrochemical, potassium feldsparification (such as Murongtau, Kumtor), sericitization (such as Daugiztau), carbonation, chloritization (such as Amentaitau), etc.
5) Mineralization stage: The mineralization process of this type of deposit is obviously divided into three main stages, namely, deposition-diagenesis stage, structure-metamorphism stage and intrusion-thermal metamorphism stage. Sedimentary-diagenetic stage, a calm reducing coastal environment, gold and its associated elements enter into the rhythmic layered carbonaceous clay, carbonaceous siltstone and mudstone sediments, forming a gold-rich black rock series; Structural-Metamorphism In the first stage, the deformation and metamorphism in the structural sutures triggered the migration of metamorphic fluids, which activated the gold and migrated along the fold shear zones, precipitating at the structural intersections and geochemical reduction barriers; finally, during the intrusion-thermal metamorphism Under the influence, gold is activated again, migrates along the shear zone together with the gold in hydrothermal fluids, mineralizes in favorable locations, and makes the early-formed ore bodies and associated metasomatic rocks more enriched.
6) Ore body morphology and mineral characteristics: The ore body morphology and structure of black rock series gold deposits are very complex, usually composed of a combination of steeply inclined and gentle large quartz vein zones and thin vein zones. The ore body is large in scale, distributed along the bedding, and of low grade. The sulfide content in the ore is low, with pyrite, arsenopyrite, chalcopyrite, and chertite as the main components, followed by a small amount of sphalerite, galena, and bismuthite. Gangue minerals include quartz, biotite, orthoclase, chlorite, calcite and albite.
7) Relationship with rock masses: Some mining areas expose rock masses or dykes at the same time as the mineralization, and mineralization is closely related to magma intrusion activities (such as Muruntau in Uzbekistan and Dashankou in China) . According to geophysical data, there may be hidden rock masses at a depth of 3 to 5 km underground in a small number of ore fields (Amantaintau and Dogiztau in Uzbekistan) (Yang Fuquan et al., 2005).
2. Geological characteristics of typical deposits
(1) Muruntau Gold Mine in Uzbekistan
The Muruntau Gold Mine is a world-famous extra-large gold deposit. Located in the Kizilkum Desert in western Uzbekistan, it is part of the South Tianshan structural belt, including 3 gold deposits (Murontau, Myotinbay, and Besopantau) and 8 gold mineralized sections. The mining area About 9km2. The mining area is located near the intersection of the complex anticline and the deep fault in terms of regional structure, that is, the intersection of the NW-trending complex anticline and the nearly EW-trending fault (Figure 1). Faults and fold structures are extremely developed in the mining area. To the north, There are nearly vertical deep faults in the EW direction (extending more than 3.7km) in the west, as well as secondary faults in different directions, forming a connected network in the mining area and becoming a channel for the migration of mineral-containing hot water solution. The Mulungtao gold deposit takes the form of a very deep mine building. In addition to gold mineralization in the mining area, tungsten, uranium, and molybdenum mineralization are also found in the depths.
Figure 1 Geological map of the Murontau deposit in Uzbekistan (cited from L. J. Drew et al., 1996)
The hydrothermal alteration in the mining area is strong, and the main alteration types are silicification and biotite. chemical, chlorite, potassium feldspar, albite, epidote, carbonatization and mudification, etc. Hydrothermal alteration related to several stages of mineralization is distributed along the northern and southern intersection zones of the Sangerontau-Tamdetau shear zone, and quartz-biotite-potassium feldspar altered metasomatic rocks are developed in the NW-trending structure. In the fissure zone, it is extremely closely related to network vein mineralization. The temperature of fluid inclusions in quartz is 410 to 500°C. CO2 is the main gas phase component, and there are also abundant CH4 and N2.
The Mulungtau gold deposit is generally a large-scale linear-column network vein body with complex structure, which dips eastward along the fold axis. The ore body is layered in section and occurs bedding in the lower part of the variegated Besopan subformation of the Middle Ordovician to Early Silurian (Fig. 1). The ore-bearing variegated Besopanya Formation consists of a set of metamorphic siltstone, sandstone and mudstone. It can be divided into 4 sections according to its age, color and clast particle size. From oldest to youngest, they are bs1, bs2, bs3 and bs4. (Table 2). The thickness of the ore body ranges from tens to hundreds of meters.
In the 2397-2404m section of deep hole СГ-10, an ore body with a thickness of 7m and an average Au grade of 15.2×10-6 and an average Ag grade of 8.5×10-6 was also discovered. The ore bodies in the entire mining area can be divided into two categories. One type is the large vein type. Gold-bearing quartz veins are produced in steeply inclined fissures. The thickness is more than 0.5m, and the maximum thickness can reach more than 20 meters. The length is generally 100 to 300m, and the maximum length is 700m. The average gold grade is 10×10 -6 or above, up to (300~400)×10-6. The gold reserves in this type of ore body can account for 12% to 15% of the total reserves of the deposit. The second type is the network vein type, which is the main type of gold mineralization and consists of gold-bearing quartz veinlets, quartz-sulfide veinlets, quartz-calcite veinlets, quartz-microcline veins, and quartz-tourmaline. The veins are alternately developed. The occurrence of these veinlets ranges from steeply dipping bedding to gently dipping bedding. This type of mineralization has a huge scale, but the gold grade is low, generally (3~5)×10-6.
Table 2 Main stratigraphic units exposed in the southern mountainous area of ??the Muruntau gold mine in Uzbekistan
Data source: L. J. Drew et al., 1996
Gold ores have mixed types , quartzite type, quartz schist type, silicified schist type, fine vein type and large vein type. More than 90% of the gold is natural gold, which is found in quartz in the form of scales, and 7% of the gold is contained in sulfides (pyrite, arsenopyrite). The sulfide content in the ore is low, accounting for only 0. 28% to 3. 4% of the total ore (average 1. 78%). It is mainly pyrite, arsenopyrite, chalcopyrite, and sulfide, followed by Small amounts of sphalerite, galena, and bismuthite. Natural gold particles are very fine and difficult to see with the naked eye. Ag, Pd, and W are also available for recycling in the ore, but the Ag content is not high, and the Au/Ag ratio is about 4 on average. Tungsten is mainly produced in the form of scheelite. Gold is primarily related to silicification and secondarily to sulfide particle size and crystal form. The smaller the particle size of pyrite and arsenopyrite, the higher the gold grade. The average gold content of pentagonal dodecahedral pyrite is (50-60) ×10-6. In the past, gold mineralization was thought to be related to carbonaceous matter, but now it has been found that areas with high carbon content may not necessarily have high gold grade. On the contrary, areas with low carbon content may have high gold grade.
(2) Bakirchik Gold Mine, Kazakhstan
The Bakirchik Mine is located in the structurally complex Late Hercynian collision zone, which involves the Zaisan Fold System Some silico-magnesian fault blocks and disturbed ophiolite fault blocks (Fig. 2). The sedimentary construction in the suture line deformed strongly during the main folding stage, resulting in compressional folds with the long axis in the NW direction, as well as brittle-ductile faults and shear zones. These structures are all cut through by a nearly EW-trending Kizilof thrust fault zone. The adjacent northward gently dipping brittle-ductile fault system is accompanied by lying compression folds, cleavage and fine folds along the bedding, which are mostly found in the floor and middle part of the belt. As a result of compression and folding, distinct sausage structures and sandstone interlayers of lamprophyre dykes are visible. In the Kizilov thrust zone where the structure is relaxed, plagiogranite-granodiorite (C3-P1) intrudes.
The Bakurchik mining area has large gold reserves, including 7 carbon deposits including Bakurchik, Bolshevik, "Deep Valley", "Middle", Chalobai, "Cold Spring" and Sarbas. Gold-bearing sulfide deposits. The rocks in the mining area are Carboniferous marine, neritic and terrestrial terrigenous clastic sedimentary rocks (Bakrchik black shale layer) and carbonaceous siltstone-mudstone containing contemporaneous gold-sulfide mineralization. Some gold-bearing layers. Its gold grade is an order of magnitude higher than the background value. The gold grade in the Bakirchi black shale layer reaches 100-150 mg/t; the organic carbon content is 0. 2% to 1. 5%-2%. In Bakirchi The carbon pitch lens in the gram layer actually reaches 20. 5% ~ 54. 1%. Tuff layers and porphyry flows are sometimes found in siltstone-sandstone sedimentary rocks. The Carboniferous terrigenous sedimentary rocks are individual rock strains and massive dyke intrusions of plagioclase granite porphyry and diorite porphyry, which together form NW-trending and nearly EW-trending dike groups.
The outstanding feature of the genesis of this deposit is that the contemporaneous gold in the black shale rock series was activated during the structural deformation and metamorphism of the ore-hosting rocks. It has experienced three main mineralization stages: sedimentation-diagenesis, structure-metamorphism and intrusion-thermal metamorphism.
The proven depth of mineralization in the Bakirchik deposit has reached 1 to 1. 5km, the ore-bearing structure has been geophysically explored to a depth of 3km, and the average gold grade is 9. 4g/t. < /p>
Mineralization in the Bakelchik mining area is clearly controlled by structures. The carbonaceous gold-bearing sulfide-type ores are all located in the Kizilov deformation zone at the intersection of the shear zones formed during the main folding period. Mineralization occurs in the form of semi-conformable laminated and banded veins, all with gold-bearing sulfide mineralization. The gold-bearing veins dip northward at an angle of 35° to 40° and extend downward along the Kizilov thrust fault.
The vertical zoning of geochemical element combinations is obvious. The near surface layer is Hg-Sb-Ag combination, while the deep layer is Mo-Bi-W-Be combination (Figure 3). The elements (As, Pb, Mo) and the ratio (As/P) can be used to distinguish the lower ore zone, near ore zone and upper ore zone. Au/ (P, Cu, Pb, Mo) has obvious banding.
Pyrite and arsenopyrite are the main ore minerals. Near the surface, the mineral assemblage also includes stibnite and occasionally marcasite, chalcopyrite, pyrrhotite, and galena. The mineral genesis assemblage is divided into 4 generations: ① Pyrite (Ⅰ) - pyrite - marcasite; ② Gold ( Ⅰ) - pyrite ( Ⅱ) - arsenopyrite; ③ Gold ( Ⅱ) - Sphalerite - Galena - Chalcopyrite; ④ Gold (Ⅲ) - Stibnite - Marcasite - Sulfarsenic copper ore. Gold in sulfide ores is mainly wrapped in arsenopyrite and marcasite, appearing in the form of very small (0. 1 ~ 5 μm) drop-like inclusions, dendrites and granules. Mingjin is found in the cracks of sulfides and is associated with stibnite. Finely dispersed natural gold is produced in 3 forms: follicle-like aggregates, lumps and granules. The fineness of gold is high (95% ~ 98%). Follicular gold is often associated with green nickelite (NiO), and metallic solid phase AuNi2 is also common. The deposit also contains ultrafine natural gold, which occurs in both colloidal and ionic phases.
The main features of the Bakelchik deposit are: ①The gold-bearing rhythmically layered carbonaceous clay and carbonaceous silt mudstone contain high contemporaneous gold grades (10 to 150mg/t) , the organic matter content is high (1% ~ 10%), and the spherical berry granular pyrite content is also high; ② Strong folds occur in the structural sutures, plastic deformation occurs in the shear zones and the sheared rocks, and in the dislocation Multi-stage deformation occurs at the intersection of the zones and a hybrid structure is formed; ③ In the shear zone and dislocation zone, chlorite-albite, shungite-sericite and sericite-phlogopite-carbonate metasomatic rocks are widely developed .
Figure 3 Geochemical zoning map of the cross section of the Bakirchik gold mine in Kazakhstan (cited from S. Zh. Daukeev et al., 2004)
(3) Kyrgyzstan library Kumtor Gold Deposit
The Kumtor Gold Mine is the largest gold deposit in Kyrgyzstan. The Kumtor Gold Mine is located about 50km south of Lake Issyk-Kul in Kyrgyzstan, and about 100km away from the China-Kyrgyzstan border in a straight line. The deposit is distributed in a narrow strip 15km long and 0. 1 to 0. 4km wide. Its NW and SE boundaries are defined by faults, and its SW and NE boundaries are unclear due to Quaternary and glacial cover. The deposit has reserves of 360t, resources of 545t, and an average grade of 4. 49 × 10-6.
The Kumtor gold mine is located in the Central Tianshan structural belt, with the Nikolaev Line on the north side. The oldest strata exposed in the mining area are Paleoproterozoic metamorphic rocks, and are intruded by Riffian granite. The upper Riffian boundary Kashkasu Formation angular unconformity covers it, and is composed of conglomerate, metasandstone, basalt-rhyolite The rock is composed of bimodal volcanic rocks. The Wendeian Jetim Formation, which is parallel unconformably overlying the Kashkasu Formation in the Upper Rife Boundary, is the ore-bearing country rock. It is composed of slightly metamorphosed carbonaceous flysch rock and is 0. 8 to 1. 0km. It can be further divided into 3 subgroups. The lithology includes carbonaceous phyllite and slate, interbedded with conglomerate and siltstone, sandstone, etc.
Above the ore-bearing rock series are Cambrian-Lower Ordovician chert slate, dolomite and limestone, among which the carbonaceous chert has platinum, uranium and vanadium mineralization. The Middle Devonian-Lower Carboniferous red sandstone and limestone angularly unconformably cover the basement, which is the ore-bearing horizon of the strata-controlled lead-zinc deposits in the area.
The structure of the mining area is a narrow strip of early Hercynian nappe. Faults include overthrust faults and reverse faults, which play an important role in mineralization. The mineralized zone is distributed along the Kumtor thrust fault, 10km long, and tilts to the southeast with an inclination angle of 30° to 50°. The upper wall is the ore-bearing green slate of the Wende Group, and the footwall is the Early Paleozoic limestone, chert and carbonaceous rocks. The fault width is 100-250m, with structural mélange and limonization.
Intrusive rocks are not developed in the area. There are two wall-shaped granite bodies that intruded into the Riffeyian sandstone. They are very small in scale and may be the product of the Riffeyian in the Neoproterozoic. Geophysical survey shows that there is a hidden intrusion 3 to 5km northwest of the mining area.
The ore body in the mining area is strictly limited to the structural zone. The mineralization is divided into the South Mineral Zone, the North Mineral Zone, the Northeast Mineral Zone and the Fine Vein Mineral Zone. The ore belt is 500 to 1000m long, 25 to 100m thick, and 300 to 1000m deep. The mineralization within the mineral zone is quartz veinlets and quartz network veins. The higher the pyrite content, the higher the gold grade. Mineralized rocks include scheelite-containing pyrite-albite-carbonate rock type, pyrite-potassium feldspar-carbonate rock type, and brecciated pyrite-carbonate rock type.
There are about 100 kinds of minerals in the mining area. The main metal minerals are natural gold, pyrite, hematite and scheelite; the main gangue minerals are quartz, sericite, potash feldspar and albite. , adolite, calcite, dolomite, ankerite, siderite and barite. Most gold is found in the fissures and pores of pyrite.
The wall rock alteration related to mineralization is very developed, mainly including silicification, sericitization, pyrite carbonation, potassium feldsparization, albite and quartz potassium feldsparization. Quartz is carbonated after mineralization.
Research on fluid inclusions shows that more than 90% of most inclusions are gaseous CO2, and two-phase inclusions are rare. The mineralization homogeneous temperature is 270-240℃, and the quartz carbonate vein formation temperature is 230-160℃. The pH of the ore-forming fluid is less than 7-8, and the oxygen fugacity is between -32 and -47. The mineralization age was determined to be 200-280 Ma based on lead isotope age.
3. Origin of Ore Deposits and Prospecting Signs
1. Origin of Ore Deposits
There has been controversy over the origin of black rock series gold deposits. Take the Mulungtau gold deposit as an example. This deposit is the earliest black rock series type gold deposit discovered in the world. In the decades after its discovery, there have been constant debates about its origin. In summary, there are three main views, namely Hydrothermal origin model, crust-mantle hydrothermal metasomatism origin model and metamorphism-hydrothermal transformation (syngenetic-epigenetic theory) model.
1) Hydrothermal origin model: This model believes that gold-bearing quartz veins were formed by multiple hydrothermal processes, which are related to magma intrusion activities, and gold is not directly metasomatized from the surrounding rocks, but It may be caused by hydrothermal fluids in the early stages of endogenous mineralization.
2) Crust-mantle hydrothermal metasomatism model: This model is proposed on the basis of new concepts such as magma diapirs, mantle and intracrustal metasomatism. It is believed that the late stage of black rock series type gold deposits The mineralization obviously has the characteristics of mantle plume mineralization, which may be related to the local thermal surge mineralization in the ductile shear zone.
3) Metamorphic-hydrothermal transformation model (also known as syngenetic-epigenetic theory): This model believes that gold comes from initial deposition, and then undergoes sedimentation, regional metamorphism, dynamic metamorphism and hydrothermal alteration. , gold is redistributed and enriched within the rock formation, thus forming a network deposit. This model combines the two mineralization stages of syngenetic deposition and epigenetic superposition to a certain extent, and has gained more and more recognition.
Although there is still debate about the origin of the deposit, most people believe that the evidence for the metamorphic hydrothermal model is the strongest. According to this model, gold mineralization in black rock series-type gold deposits is formed in three stages: sedimentation-diagenesis stage, structure-metamorphism stage and intrusion-thermal metamorphism stage. The ore-bearing fluids mainly come from the high-temperature metamorphism and granitization zone under the mine.
2. Prospecting signs
(1) Geological prospecting signs
1) Black rock formation signs: The shore formed in a tranquil restoration environment Shallow marine facies, carbonaceous fine clastic rock-carbonate rock construction is the target, focusing on looking for carbonaceous fine clastic rock rather than carbonate rock. The stratigraphic age of the black rock series is mainly Paleozoic.
2) Geotectonic units: Mainly include back-arc basins, continental margin basins, foreland basins, continental margin active zones and geotroughs without volcanic rocks, and volcanic rocks within the structural units, especially in the middle Acidic intrusive rocks are not developed.
3) Ductile shear structure: The ductile shear structure in the mining area plays a decisive role in mineralization. It not only plays a role in guiding ore, but also plays a role in holding ore. Especially in the zone where the brittle and ductile transitions occur in multiple stages, it is most beneficial to mineralization and is a key location for ore prospecting. Almost all gold deposits in the Southwest Tianshan and Chuyili-North Tianshan gold mineralization provinces are controlled by shear zones. For example, the Au, As and Au/As anomalies in southern Tamdetau, Uzbekistan have a very good spatial correspondence with the shear zone (Fig. 4).
4) Alteration signs: The ore-bearing rocks are characterized by regional low-temperature dynamic metamorphic hydrothermal processes with strong deformation and weak metamorphism. The signs are obvious pyritization, sericitization, and chloritization. , carbonation, silicification and weak graphitization. During the rock construction, the carbonate rock has hardly undergone metamorphism and basically maintains the characteristics of the original rock, but there is deformation. In some areas, organic carbon or asphaltene films are also seen on the carbonate rock layer.
5) On the surface of the upper part of the ore body, yellow-brown iron staining formed by oxidation and jarosite forming cataclastic iron cap bands are common. This is because the veins are characterized by irregular gold-bearing quartz thick veins and fine quartz veins containing pyrite and iron dye, and the metallic minerals are limonite, pyrite, jarosite and iron dye. feature.
Figure 4 Relationship between Au, As and Au/As anomalies and shear zones in southern Tamdetau, Uzbekistan (quoted from L. J. Drew et al., 1996)
6) This is an area where complex gold-bearing quartz veins, quartz-sulfide veins and network veins develop. These veins often undergo varying degrees of metamorphism and strong deformation. Quartz vein type mineralization generally does not form a separate large deposit, and large gold reserves are only related to gold-sulfide type mineralization. The shallow gold-bearing quartz vein mineralization is a sign of the deep gold-sulfide ore body.
(2) Geophysical prospecting signs
1) Low gravity anomaly characteristics: reduced gravity field and increased fracture degree, combination of faults, schistosity zones and fracture zones, and Silicification, etc. For example, the large-scale metasomatic altered rocks in the Mulungtau deposit and the altered rocks developed in the ore-bearing fault zones can cause a reduction in the gravity field, which is reflected in medium-scale gravity measurements. Figure 5 shows the corresponding relationship between the gravity field value and the ore body calculated based on the density difference between the surrounding rock and the ore body during high-precision gravity measurement. In addition, low values ??of the gravity field also exist on the deep granites, longitudinal compression structural zones and deep faults where the NE and NW directions converge in the Mulungtau ore field.
Fig. 5 The relationship between the calculated gravity field value (a) and the ore body (b) of the Murontau section in Uzbekistan (cited from Г. Н. Голищенко et al., 2007)
2) Low-resistance and high-polarization induced electric anomaly: Because the ore-bearing rocks are mostly carbonaceous or rich in metallic minerals (pyrite) in local areas, they show low anomalies or weak anomalies. Such anomalies can be used as indirect ore prospecting signs and can be measured using aeromagnetic measurements.
3) Because ore-hosting rock formations generally have low magnetic susceptibility, low magnetic field characteristics can be used as an auxiliary prospecting mark.
(3) Geochemical prospecting signs
1) Abnormal elements such as Au, Sb, As are the most effective prospecting signs. Kyrgyzstan's Sarbulak and Savayalton, as well as China's Dashankou deposits were discovered based on gold geochemical anomalies. The Muruntau gold mine in Uzbekistan was discovered during the verification of As anomalies. Research shows that geogas measurements and active metal measurements have obtained better Au anomalies on the Mulungtau ore body (Figure 6).
2) Geochemical anomalies of base metals (Cu, Pb, Zn) are more effective than those of iron group elements (Ni, Co, Cr), which are often associated with non-ore-forming diagenetic sulfide aggregates accompanying.
3) Common Au geochemical anomalies in rock samples.
4) The geochemical zoning is obvious: the near surface is mostly composed of Hg, Sb, and As combinations, and the deep parts are mostly composed of Mo, Bi, W, and Be combinations (Figure 3).
5) There are abnormal fluid inclusions containing NH4+ N2.
Figure 6 Au distribution on the Mulontau section of Uzbekistan (quoted from Wang Xuequiu et al., 2000)
(4) Remote sensing prospecting signs
Remote sensing technology control Prospecting signs such as mineral strata, structures, rock masses and alteration zones play a unique role. Zhang Ruijiang (2007) used simulated true color synthetic ETM image interpretation technology to conduct a comparative study of this type of deposits at home and abroad, and pointed out that the carbon-containing ore-controlling strata of this type of deposits mostly appear gray-black and white in the image, and the ore-controlling toughness The shear zone structure is generally gray-black and presents an intermittent linear image. The alteration type, mainly limonization, appears as brown-yellow discontinuous bands or block shadows. The deep structure can be predicted based on the ring structure. The hidden rock mass indirectly determines the ore-controlling rock mass.
(Tang Jinrong Jin Xi)