5. 1.2. 1 mineral composition
(1) altered rock type gold deposit
Altered rock type gold deposits are mainly distributed in Niujiaxiaohe, Longquan Station and Nanxiaoyao. The ore is transformed from tectonic rocks by multi-stage and multi-stage hydrothermal alteration, and the ore material composition is complex.
1) silicified pyritized mylonite cataclastic rock. It is the main metallogenic type in this area. The oxidized ore is brownish yellow, grayish black and grayish white, with widespread limonitization and pyrite crystal illusion in some areas. Fresh ore is grayish green-grayish black, light grayish red, grayish white, etc. , locally containing smoky gray silicified venules. The ore structure is broken, veinlet-disseminated and massive; Pyrite, a gold-bearing mineral, is disseminated, veined and massive, mainly distributed in cements and fractures, and the early pyrite contains better gold.
The mineral composition of ore is relatively simple. The metal minerals are mainly pyrite, with a content of 5% ~ 12%, and a small amount of chalcopyrite and galena. Gangue minerals in porphyry are mainly chlorite, silicified quartz, sericite and plagioclase.
Pyrite: According to the optical characteristics, structural characteristics and occurrence forms of minerals, it can be clearly divided into two stages: ① Early pyrite, yellow-white reflection, obvious homogeneity, medium coarse-grained structure, banded, punctate and veinlet structure, mostly distributed along mylonite foliation. The later crushing cracks are often filled with timely silicides and various sulfides. (2) Late pyrite, yellow-white reflection color, homogeneous, fine-grained autotype-semi-autotype grain structure, disseminated structure. The particle size of minerals is generally between 0.02 mm and 0.3 mm, and the particle size is relatively large. During this period, pyrite has obvious metasomatism and cross-action, which is mostly distributed around early minerals. The content of late pyrite is slightly higher than that of early pyrite, which is the main load mineral of gold minerals.
Chalcopyrite: The reflection color is yellow, showing weak heterogeneity, which can be divided into two stages according to the sequence of mineral formation. Early chalcopyrite was born in the crystal gap of early pyrite, which is explained briefly. The late chalcopyrite is mostly distributed in irregular particles, locally disseminated in veinlets or distributed along early pyrite fractures. At this stage, the oxidation circle composed of chalcopyrite is often seen at the edge of chalcopyrite particles.
Galena: The reflection color is bright gray, homogeneous, and the mineral particles are extremely fine, often coexisting with early chalcopyrite, but the content is very small.
Gangue minerals are mainly micro-scale chlorite, followed by sericite and powdery felsic particles and powder, and a small amount of eyeball-shaped potash feldspar, plagioclase debris and late carbonate veinlets. There is no obvious contour boundary between these new mineral particles, and they are irregularly mixed together, showing a good directional distribution, which makes the rock show a banded structure and mylonite surface structure. The late carbonate veinlets are mainly interspersed along the fault in the form of irregular veinlets, and locally superimposed on the later broken rocks in blocks.
Ore texture includes granular texture, cataclastic texture, metasomatic texture, interstitial texture and inclusion texture.
Granular structure: mainly pyrite, chalcopyrite, galena and other metal sulfides, which are distributed in the ore in semi-self-shape-shaped granules.
Broken structure: Pyrite and chalcopyrite formed in the early stage are broken or in the form of breccia and broken spots, and some of them are filled with later metal sulfides or micro-time veins along the broken cracks.
Metasomatic texture: Pyrite and chalcopyrite formed in the late stage often form cracks, cleavage and metasomatic molten minerals along the early stage, so that metasomatic minerals are distributed in the metasomatic minerals in harbor, island or irregular shape.
Interstitial structure: Pyrite, galena or late pyrite is distributed in early pyrite cracks or crystal gaps in interstitial form.
Inclusion structure: Pyrite often contains chalcopyrite and galena, and gold minerals are included as inclusions in early pyrite and silicification.
Common ore structures are: banded structure, breccia structure, vein structure, disseminated structure and massive structure.
Banded structure: In pyritized mylonite cataclastic rocks, newly-born chlorite, sericite and amorphous powder felsic components are all distributed in the ore in a good orientation, making the ore in a banded structure.
Bubble structure: Mylonite cataclasts and granitic cataclasts are breccia-like, which were later filled and cemented by pyrite, quartz and other minerals to form breccia-like structures.
Vein structure: Pyrite veinlets and timely veinlets are filled along ore fractures, forming network veinlets and veinlets.
Disseminated structure: Pyrite is unevenly distributed in the ore in the form of fine particles. According to the degree of mineral aggregation, it can be divided into sparse disseminated structure and dense structure.
Block structure: Pyrite and other minerals are uneven blocks, and there are few ores with this structure in the general survey area.
2) Silicified pyritized sericitized granitic cataclastic rocks. Oxidized ore is yellowish brown and grayish white, fresh ore is grayish white-grayish green, and pyrite is veinlet-like, disseminated and massive distributed in cements and fractures. In time, it is mainly filled along the cracks in the form of veins, and some of them are massive.
The main metal mineral of the ore is pyrite, with a small amount of limonite and chalcopyrite. Non-metallic minerals include sericite, feldspar, quartz and chlorite.
Pyrite: the reflection color is yellow and white, and it is homogeneous, with a fine-grained autotype-semi-autotype granular structure, with a particle size of 0. 1 ~ 0.5 mm, mostly disseminated, and distributed in rock fractures in the form of veinlets or blocks.
Chalcopyrite: The reflection color is yellow, showing a star-shaped structure. The grain size is between 0.5 ~ 1.2 mm, and minerals often occur alone, occasionally distributed along early pyrite fractures, with less content.
Timing: Unequal-grained structure, generally granular aggregate, with a particle size of 0.5 ~ 2 mm
Sericite: microscopic scale-like aggregate, mostly in irregular small blocks, surrounded by timely silicide.
Chlorite: microscopic scale, leaf-like aggregate, flaky green. This mineral is related to the late Ying Shi.
Ore texture and structure: Ore texture includes semi-homogeneous isomorphic granular texture, crushed texture, metasomatic texture and interstitial texture.
Ore structures include veinlets, disseminated structures, massive structures and breccia structures.
3) Pyritized chlorite amphibole schist. A wide range of chlorite-epidote veins run through the cracks in the fracture zone, forming altered minerals dominated by chlorite in the larger fracture zone. At the edge of chlorite vein, especially in mineral microcracks, epidote granular band particles and small star-shaped epidote particles distributed along microcracks are formed in minerals. Under the influence of chlorite-epidote vein, with the addition of calcium component, the original crushed basement minerals, recrystallization time and mineral chips in the fracture zone were replaced and transformed to form polycrystalline plagioclase. On the basis of primary minerals, complete and identical multi-flake twins have appeared, in which the outline shadow of primary mineral particles can still be seen. The composition of plagioclase is plagioclase (No27). Plagioclase particles often appear around or at the end of chlorite-epidote veins in granular or aggregate form, indicating that plagioclase replaces synbiotite and forms plagioclase particles as big as synbiotite particles, with the particle size of 0. 1 ~ 0. 15 mm, and sometimes it can be seen that long epidote begins to form inside synbiotite particles, and then multiple twins and twin plagioclase are formed.
Chlorite: light green-light yellow green, flaky and indigo with abnormal interference color, permeates along the fracture vein, permeates and spreads to the periphery, sometimes in 0.4 mm chunks, and extends in a strip shape along the fracture.
Epidote: light green-light yellow green, granular, with a particle size of 0.005 ~ 0. 1 mm, with high protrusions, often appearing at the edge of chlorite, entering the surrounding rock minerals along micro cracks, forming star-shaped discontinuous lines and particles, accompanied by gas-liquid inclusions. In the vein, epidote aggregates with a length of 0. 1 ~ 0.2 mm and a width of 0.02~0.05 mm can be seen locally. The extension direction is consistent with the vein direction, and its interference color fades, and it evolves into sphene and then pyrite.
Pyrite: opaque mineral, associated with chlorite and epidote. When paragenetic with chlorite, the particle size of pyrite is slightly larger, and when paragenetic with epidote, the particle size of pyrite is slightly smaller, and fine pyrite is mostly concentrated in veinlets.
(2) Time-pulse type gold deposits.
The gold body of Dongying quartz vein type gold deposit belongs to this type. The ore is sericitized limonite with chronological vein, reddish brown, light gray, light brown, fine-grained texture, broken block structure and block structure. The ore is mainly composed of fine silicified quartz, sericite and a small amount of limonite. Silicification, sericite and limonite are developed in the ore.
5. 1.2.2 Chemical composition of ore
In this study, the chemical composition of ore was analyzed and tested. Completed by the laboratory of Shandong Academy of Geological Sciences, the testing basis is DZG2002- 199 1, GB/T 14506-93, and the testing environment is 26℃ and the relative humidity is 68%. The instruments and equipment are Hitachi -557 dual-wavelength spectrophotometer and IRIS IntrepidⅱII full-spectrum direct-reading plasma emission spectrometer. From the chemical composition analysis results of main ore types (Table 5-5), it can be seen that the contents of K2O and Na2O in ores in this area are relatively high, especially altered amphibolite schist. The increase of K2O and Na2O content may be related to potassium and sodium, which is consistent with the observation results of mineralization and alteration.
The trace elements Au and Ag in the ore show an obvious upward trend and a positive correlation, indicating that they have similar metallogenic characteristics (Table 5-6).
Table 5-5 Chemical Composition of Main Ore Types
Table 5-6 Contents of Trace Elements in Main Ore Types
5. 1.2.3 alteration and alteration zoning
There are mylonite structural cataclasts, mylonitized structural schists, quartz diorite mylonite, granite mylonite and granite structural cataclasts in the belt, and some sections are filled with siliceous belts or chronological veins. Alterations include silicification, sericitization, chloritization, epidote, limonite, pyrite, chalcopyrite and lead-zinc mineralization. Among them, the alteration of chloritization and epidote is common, the silicification and sericitization are uneven and strong locally, the carbonation is mostly filled with veinlets, and the gold ore bodies are formed in areas with strong alteration and mineralization and strong ductile deformation.
Microscopic identification features: rocks have undergone many deformation and metamorphism, which can be divided into cataclastic diagenesis, penetration of silicified veins, formation of the second brittle fracture zone, penetration of calcite veins and formation (mineralization) of sericite veins in chronological order.
1) Formation of cataclastic rock: During the formation of cataclastic rock, the original rock is divided into two parts: crushed base and cuttings (ore cuttings), and the ratio of the two parts is about 2∶3.
Crushed base: The timely particles with the particle size of 0.003~0.005 mm are lumpy, which are recrystallized in most cases and merged into 0.0 1 ~ 0.2 mm recrystallization timely particles, whose shape is irregular due to the original shape of the gravel, and the outline of the gravel can still be seen inside the particles.
Mine cuttings and rock cuttings: they are mainly composed of time, and their particle sizes vary greatly. They can be divided into two particle fractions, the smaller one is 0.05~0.2 mm, and the larger one is 0.5 ~ 1 mm, which are cut into irregular particles and strips. Most of the ore cuttings are irregular, and the edges are torn into pieces by the crushing zone. There are usually several small broken zones inside the ore cuttings.
2) Penetration of silicified pulse. The width of silicified veins in the fracture zone is 0.05 ~ 65438±0.5mm, which is narrow and bifurcated. The silicified veins are composed of Yanshi and pyrite, with a particle size of 0. 1 ~ 0.5 mm, which are alternately arranged in granular aggregates or strips with certain directionality, and the veins are parallel or oblique. Pyrite is distributed near the vein center, with a particle size of 0.03~0.5 mm, and usually exists as automorphic crystals or polycrystals. Time-dependent silicide can enter the interior of the cuttings along lateral branch cracks, forming a strip-shaped time-dependent structure different from the arrangement direction of the cuttings. Under the influence of silicification, strip-shaped chronotropic particles with different sizes are formed in the broken matrix, which have certain directionality but are obviously different from the original chronotropic particles in the broken matrix.
3) Formation of the second brittle fracture zone. After the formation of silicified veins, there are widely distributed cracks, which make the whole rock undergo extrusion deformation and fracture, and form a strong strip extrusion extinction phenomenon in mineral chips and silicified particles. The mineral debris between the two fracture zones forms wide deformation stripes arranged in parallel shear. Tensile and torsional microcracks in different directions are formed in the original rock, resulting in dislocation and bending deformation of plagioclase lamellae, and fracture zones with a width of 0.05~0.2 mm are formed on both sides of silicified veins, inside and outside mineral debris and inside the early fracture zone. The fracture zone is mainly composed of aggregate strips with particle size of 0.005 ~ 0.0 1 mm, which are angular.
4) Penetration of calcite vein. Single-component calcite vein penetrates, with a width of 0.5 ~ 1.5 mm and a calcite grain size of 0.2 ~ 1.5 mm, showing metamorphic structure. There are obvious rhombohedral twins in calcite, which spread along the main vein to the micro-cracks and boundaries in the surrounding cuttings, forming calcite veinlets and star-shaped calcite particles (particle size is 0.0 1 ~). When the metasomatic strength of calcite is not strong, it can form calcite aphanitic aggregates, and the optical characteristics of calcite are not obvious at this time. Under the influence of calcite vein, plagioclase (composed of plagioclase) is formed at the vein edge, especially at the intersection of vein end and calcite vein, which accounts for timely cuttings, or aggregates to form plagioclase particles (size is 0.05 ~ 1 mm) on the basis of broken matrix, but the outline of primary minerals still exists inside.
5) sericite veins and pyritization. After the formation of calcite vein, another hydrothermal alteration occurred, which was mainly manifested by the formation of potash feldspar and sericite vein and pyrite vein. The activity of this hydrothermal vein is controlled by the micro-fracture zone, which has strong permeability and appears in the early mineral particles or their contact boundaries, but its distribution is not universal, local alteration is strong, and some areas are not obvious. Potash feldspar appears at the edge of vein, especially at the junction between the end of vein and vein. Due to the increase of potassium composition, a potassium belt with a width of 0.05 ~ 0.1mm is formed at the edge of plagioclase. Or enter plagioclase cracks in the form of potassium metasomatism strips, metasomatism forms lattice twin plagioclase or partially replaces plagioclase, forming composite particles of potassium feldspar and plagioclase, and there are flaky plagioclase residual twins in potassium feldspar; Or the replacement of timely ore cuttings forms potash feldspar or timely-potash feldspar composite particles. The metasomatic potassium feldspar particles can sometimes reach 65438±0.5mm；; ; Sericite-chlorite veins run through micro-cracks, in which sericite is the main one, and chlorite only appears in small blocks locally. After the sericite vein is formed, the mineralization phenomenon with pyrite as a single vein is formed. Pyrite forms a star-shaped grain belt along micro-cracks and together with gas-liquid inclusions, sometimes forming dense pyrite veinlets, which is most obvious at the boundary between main minerals and potassium feldspar particles.
Pyrite sericitization is an important alteration in the deposit, and its reaction formula is:
Tectonic evolution and mineralization of Yishu fault zone in Shandong Province
Tectonic evolution and mineralization of Yishu fault zone in Shandong Province
After sericitization of chrysotile, the tectonic movement is mainly tensional, and a large amount of Tianshui is added. At this time, the ore-forming fluid evolved from alkaline to acidic → neutral, from oxidizing environment to reducing environment, and the temperature and pressure became lower, which led to the precipitation of ore-forming components, and its mineralization was accompanied by silicification.
Carbonization is the derivative product of sericitization and silicification of pyrite. During sericitization, a large amount of CaO is precipitated, which combines with carbonate in mineralized fluid to form carbonate minerals, and its reaction formula is:
Tectonic evolution and mineralization of Yishu fault zone in Shandong Province
In a word, the rocks in this area have undergone many deformation and metamorphism, which can be divided into cataclastic diagenesis, penetration of silicified veins, formation of the second brittle fracture zone, penetration of calcite veins and formation (mineralization) of potassium feldspar and sericite veins in chronological order. Potassium and sericitization deserve further study.
5. 1.2.4 Gold-bearing minerals and their characteristics
Characteristics of (1) gold-bearing minerals
Mineralogical characteristics of 1) gold-bearing minerals. Gold-bearing minerals are mainly pyrite and veined quartz. Pyrite: light yellow, medium reflectivity, semi-autotype-autotype granular aggregate, with two occurrences. One is related to calcite veins and occurs in the contact zone between calcite veins and surrounding rocks. Pyrite metasomatic crystallization with large particle size; One is related to sphalerite and exists in microcracks of rocks. Sphalerite: dark gray, weakly reflective, low hardness, irregularly distributed along micro-cracks, coexisting with pyrite, and sometimes remaining or wrapped in pyrite.
Early pyrite sphalerite veins were mainly distributed irregularly and intermittently along the fracture surface of surrounding rock. A group of cleavage planes with veins of15 ~ 20 can be seen in the hand sample, and there are star-shaped sphalerite veinlets and pyrite with irregular banded distribution in the cleavage plane. The particle size of sphalerite is 0.005 ~ 0. 1 mm, which is generally arranged in a strip shape, and individual particles are slender, filiform, fine-grained, drop-shaped and beaded. Pyrite is located between sphalerite zones, where sphalerite rarely occurs, or at the end of sphalerite micro-veins, with a particle size of 0. 15 mm, and sometimes pyrite aggregate zones are formed along the cross section, with sphalerite inclusions remaining in the zones, which are in the form of round drops. The wrapped sphalerite particles still maintain the primary sphalerite along the direction of fracture arrangement, which is in direct contact or metasomatic contact. In the surrounding rocks and breccia, the particle size of pyrite is small, ranging from 0.02 mm to 0.3 mm, and the degree of automorphism is slightly poor, ranging from 0.05 mm. The content ratio of pyrite to sphalerite here is about 8∶ 1.
On the edge of the calcite vein, there are autogenous chronotropic crystals growing inward perpendicular to the vein wall, the size of which is 0. 15 ~ 0.5 mm. There is a mineralized alteration zone with a width of 4 ~ 5 mm at the contact with the surrounding rock, and the inside of the mineralized alteration zone is a pyritization zone, which is basically composed of pyrite. Compared with authigenic pyrite, metasomatic microcrystals are 2 ~ 3 mm wide, with particle size of 1 ~ 1.5 mm, which is clean as a whole, and there are irregular metasomatic residues in pyrite, indicating that pyrite is formed by metasomatic crystallization. A silicified zone with a width of 2 mm and pyrite particle size of 0.005~0.6 mm is formed on the outer side, which is a semi-automorphic-automorphic particle produced in dispersed form. In the calcite vein, there are original rock fragments with the size of 65438±0.5 ~ 6mm. Pyrite with a larger particle size of 1 ~ 2 mm is enriched at the edge of the fragment, while pyrite with a smaller particle size of 0.005~0.5 mm is enriched inside the fragment.
The mineral composition of gold ore is relatively simple. The metal minerals are mainly pyrite, with a content of 5% ~ 15%, and a small amount of chalcopyrite and galena. According to the optical characteristics, structural characteristics and occurrence forms of minerals, pyrite can be clearly divided into two stages. In the early stage, pyrite has a yellow-white reflection color, which is homogeneous, with medium-coarse crushing structure, banded, punctate and veinlet-like structure. In the later stage, crushed cracks are often filled with timely and various sulfides. The reflection color of late pyrite is yellow and white, which is homogeneous, with medium-fine self-shape-semi-self-shape grain structure and disseminated structure. In this period, pyrite has obvious metasomatism and staggered phenomenon to early pyrite, which is mostly distributed around early minerals, and early pyrite is the main load mineral of gold minerals [163]. Electron probe analysis of pyrite shows that gold is closely related to pyrite.
The study of Niujiaxiaohe, Longquan Station and Nanxiaoyao gold deposits shows that pyrite is irregular, broken and loose in structure, and the gold content is high in those with developed fractures. The stress phenomenon is obvious, and the gold content is high in bending and stretching; Fine particles accompanied by other polymetallic minerals with high gold content; Pyrite has compact structure, strong metallic luster and hardness, complete octahedral, pentahedral and cubic crystal forms, and low gold content.
The above gold-bearing characteristics of pyrite are similar to those studied by Chen Guangyuan et al. [167] in Jiaodong gold mine, indicating that although gold ore and pyrite are related, pyrite is more conducive to gold enrichment after crystallization, or early pyrite is not conducive to the formation of gold ore, which is similar to the gold-bearing characteristics of pyrite in Yishu fault zone.
2) The change of gold content in pyrite. EPMA analysis of pyrite shows that the highest gold content in pyrite is 1 100× 10-6, the lowest is 5× 10-6, and the general gold content is130×10-6 ~.
Table 5-7 Electron Probe Analysis Results of Pyrite Composition
sequential
(2) Characteristics of gold minerals
1) The natural form of gold. Through optical thin section identification, the useful minerals in the ore in this area are mainly natural gold and silver-gold ore. According to the observation and statistics of 45 kinds of gold minerals under microscope, the gold minerals are bright golden yellow, homogeneous, with pits on the surface, and the particle size is between 0.003 and 0.18 mm. The mineral morphology is mainly angular, followed by long horn, dendritic, branched and linear (Figure 5-3). Gold minerals mainly occur in early pyrite, followed by silicification time. Its occurrence is mainly fissure gold, followed by interstitial gold and a small amount of inclusion gold. According to the statistics of 45 kinds of gold minerals seen in the photos, in the early pyrite, fissure gold accounted for 58%, crystal gap gold accounted for 20%, inclusion gold only accounted for 7%, and crystal gap gold accounted for 16% (Table 5-8).
It can be clearly seen from the above statistical results that gold minerals mainly exist in the state of fissure gold and interstitial gold, but contain less gold, accounting for only about 7%.
The data analysis of artificial heavy sand samples of pyrite sericitized granite cataclastic rocks (the sample weighs 4 kg) shows that 63 gold particles are found in this sample, which are brownish golden yellow with metallic luster, with oolitic protrusions and irregular pits on the surface, mostly flaky and dendritic in shape, followed by granular. The particle sizes are 0.20mm× 0.10mm× 0.07mm (1granule), 0.33 mm×0. 16 mm×0.07 mm(2 granules) and 0.20mm× 0.13mm (respectively). 0.13mm× 0.13mm× 0.10mm (5 capsules), 0.13mm× 0.10mm× 0.03mm (9 capsules), 0.06.
Figure 5-3 Main Features of Natural Gold
Table 5-8 List of Characteristics of Gold Minerals
2) The fineness of gold. This time, 20 samples from 5 representative borehole cores were sliced by the State Key Laboratory of Mineral Deposits of Nanjing University, and the fineness of gold was analyzed by electron probe. Other samples were analyzed by the laboratory of Shandong Institute of Geology. According to the results of electron probe analysis of gold minerals in the area (Table 5-9), the Au content in gold minerals is 75.242% ~ 90.609%, and the average fineness of gold minerals is 865,438+06.
Table 5-9 Electron Probe Analysis Results of Gold Minerals in Longquan Station Gold Mine, Yishui
ZK-030 1, taken from ZK0 1 hole 53 m, is the sample with the most gold particles. The fineness of gold above shows that gold and arsenic are not mutually generated, but are positively related to silver. The gold mineral is silver-gold ore containing tellurium (Table 5- 10), and copper, lead and zinc sulfides are common.
Generally speaking, the results of electron probe analysis show that the ore-forming materials in the area come from the deep, which is consistent with the conclusion of isotope analysis (see the special discussion in related parts later).
Table 5- 10 Analysis Results of Antimony Silver Ore and Antimony Silver Ore Composition by Electron Probe