In the Saloi mining area, siliceous rocks are produced both between basic lava flows and in volcaniclastic rocks. In the Aktash mining area, siliceous rocks are mainly produced in the upper part of massive ore bodies or in volcaniclastic rocks. The siliceous rock formations are integrated with the strata. The thickness is generally several meters, and the thickest one can reach 4 m. It extends intermittently to nearly 1000 m along the direction.
1. Petrology and petrochemistry of siliceous rocks
The siliceous rocks exposed in the mineral belt can be divided into two categories: light gray with striped or laminated structures Siliceous rock can be called chert rock; purple-red siliceous rock with striped or laminated structure is called jasper rock because the siliceous rock contains a large amount of disseminated iron oxide. In the Aktash mining area, only chert rock and no jasper rock were seen; in the Saloyi mining area, both chert rock and jasper rock were produced. Banded chert rocks all contain a certain amount of sulfide to a greater or lesser extent. The most common sulfide is pyrite and sometimes sphalerite. Sulfides are often concentrated in strips or laminae, and they are interlayered with siliceous strips to form very developed strip structures. Research has proven that there are certain differences in the mineral composition of the siliceous rocks in the two mining areas.
Siliceous rock in Aktash Mining Area: The siliceous rock in this area has a strip-like structure. The strips are formed by the relatively concentrated distribution of pyrite and siliceous, with poor continuity and a bandwidth of 0.2 ~1.3 mm. Microscopic studies have shown that the siliceous strips are composed of quartz and pyrite. Quartz accounts for about 95% of the total volume and is in the form of a heterogeneous granular aggregate with a particle size of 0.05 mm and a granitic crystal structure. Occasionally quartz phenocrysts are produced in fine-grained quartz aggregates, with a particle size of 0.65 mm. Phenocrysts have different shapes, some are round, and some are sharp knife-shaped, with the characteristics of volcanic crystal debris. Fine-grained recrystallized quartz appears in long strips around this phenocryst and grows vertically around the phenocryst. This shows that phenocrysts are formed before quartz recrystallization and cannot be the product of later recrystallization. There is a small amount of disseminated pyrite in the siliceous strips, mostly square or rectangular euhedral crystals with a particle size of 0.05 to 0.25 mm. Pyrite bands are mainly composed of pyrite, quartz and muscovite. Pyrite accounts for about 55% of the total, and is mostly semi-euhedral granular with a particle size of 0.12 to 0.15 mm. There are often a large number of impurity mineral inclusions in single crystals, and microscopic crystals are sometimes seen. In addition, in this type of pyrite, accretionary crystals in diagenetic rings are relatively developed, providing evidence for the syngenetic origin of the deposit. Pyrite is strongly affected by late tectonic fragmentation, and its crushing structure, pressure solution structure and dislocation grooves are well developed. Quartz accounts for about 35% of the total volume of the strip. This kind of quartz produced with pyrite is mostly long plate-shaped crystals, 0.3 mm long and 0.03 mm wide. Long plate-shaped quartz often grows vertically around the pyrite and has a bundle-like structure. Muscovite accounts for about 7% to 10% of the total volume of the zone. It is a plate-shaped euhedral crystal with a length of 0.04-0.75 mm and a width of 0.01-0.07 mm. The mineral is often concentrated in bands along the pyrite bands. Muscovite is affected by strong late-stage alteration, and sometimes completely decomposes and becomes opaque. In addition, there are about 2% zeolite minerals and about 1% chlorite in siliceous rocks, both of which are products of later hydrothermal alteration.
The siliceous rock in the Saloi mining area: consists of banded sulfide chert rock and banded jasper rock. Banded sulfide chert consists of interbedded sulfide and siliceous bands. Siliceous strips account for more than 70% of the total volume of the whole rock, with bandwidths ranging from 0.2 to 2.1 mm. The main minerals of this strip are quartz and chlorite. Quartz is a cryptocrystalline and microcrystalline aggregate, accounting for about 70% of the total volume of the siliceous strips. The mineral is in the shape of other-shaped granules, with a particle size of 0.02-0.05 mm, and a granitic crystal structure. The chlorite is scaly, with a particle size of 0.02-0.05 mm. Chlorite is often distributed concentratedly, forming strips, and is interbedded with quartz strips to form laminae of siliceous rock. Chlorite accounts for about 30% of the total volume of the siliceous strips, and its content is obviously much higher than that in the siliceous rocks of the Aktash deposit. Sulfide bands account for about 30% of the total volume of the whole rock and are mainly composed of pyrite and sphalerite. Pyrite accounts for 90% of the total volume of sulfide and is semi-hedral crystal with a grain size of 0.5-2.1 mm. This mineral often contains columnar quartz, with column lengths of 0.06 to 0.1 mm. This is a typical low-temperature quartz, common in VHMS-type and Sedex-type deposits (Eldridge, et al., 1983, Hanfa et al., 1997).
The presence of this quartz provides evidence that the deposit was formed under low temperature conditions. Under the influence of later deformation and metamorphism, pyrite was strongly fragmented, forming pressure solution structure and dislocation deformation. Sphalerite accounts for approximately 7% of the total volume of the sulfide bands. Under transmitted light, sphalerite appears light beige, indicating that the mineral is a low-iron variety. Electron probe analysis proves that the iron content in sphalerite is quite low, ranging from 0.5% to 1.6%. This low-iron sphalerite forms at low temperatures. There are often a large number of pyrite particles in sphalerite, which are round, milky-point or rod-shaped. Some of these pyrite particles are distributed irregularly, and some are distributed along sphalerite cleavage or fissure systems. Therefore, pyrite may have two phases: the first phase pyrite (disorderly distributed) may be formed at the same time as sphalerite and exist in the form of micro-inclusions; the second phase pyrite may be formed during the later thermal It was formed during the event and exists in the micro-cracks of sphalerite in a filling-mesotopic manner. Overall, sphalerite and pyrite have a close spatiotemporal relationship, and their formation times may be very close.
In addition, a small amount (about 1%) of chalcopyrite is often produced in banded sulfide chert rocks. This mineral sometimes appears as fine veins of metasomatic pyrite, and sometimes appears as a disseminated distribution in siliceous rocks. In short, chalcopyrite is the product of later mineralization events and is not directly related to the syngenetic origin of sulfide chert rocks.
Striped jasper is dark red or purple-red, with well-developed striped or laminated structures. The strips can basically be divided into three types: strips mainly composed of quartz and chlorite; strips mainly composed of quartz, clinzoisite, epidote, and manganese epidote; Strips dominated by epidote and quartz. All three types of strips contain quartz, albite crystal fragments (accounting for 5% to 10% of the total volume of the whole rock) and authigenic potassium feldspar (Table 4-1). Microscopic studies have proven that the above-mentioned mineral output characteristics are basically similar in different strips.
Quartz is in the shape of other-shaped granules, with a particle size of 0.003 to 0.005 mm. These microcrystalline quartz are sometimes in the form of spherical aggregates, with a diameter of 0.05 to 0.1 mm. Chlorite is a scale-like aggregate with a scale particle size of 0.005 to 0.01 mm. There are three types of zoisite minerals: zoisite, epidote and manganese epidote. In general, there are more epidote in dark bands, and there are more manganese epidote in light bands. Both epidote and manganese epidote are in the shape of other-shaped granules, with particle sizes ranging from 0.02 to 0.035 mm. Individual manganese epidote is columnar, with a column length of 0.2 to 0.35 mm, and particularly significant pleochroism: np is orange-yellow, nm is purple-red, and ng is bright red. This kind of columnar manganese epidote is often distributed along the strips of local hydrothermal reactivation, and clinozois is also relatively developed in this strip. Orthozoisite is one of the common minerals in jasper rocks in this area, and its content can account for about 10% of the total volume of the whole rock. The mineral is irregularly granular, sometimes in the form of long strips. This kind of long strips of clinzoisite often form spherulite aggregates with a perfectly round outline and a spherule diameter of 0.03 mm, uniformly distributed in microcrystalline quartz and epidote aggregates.
Table 4-1 Electron probe analysis results of feldspar andesine in the jasper rock of the Saluoyi deposit (wB/%)
Table 4-2 Electron probe analysis of curode minerals in the Saluoyi deposit Probe analysis results (wB/%)
Cendolite minerals should be products of pseudo-contemporaneous hydrothermal metamorphism, which may be an important feature of hydrothermal sedimentary rocks related to basic volcanism. In submarine hydrothermal mineralization systems, it is a common phenomenon that hydrothermal sediments contain a certain amount of volcanic tuff material. Research under a microscope has proven that the jasper rock does contain quartz and plagioclase crystal fragments. These crystal fragments are angular and sharp-knife-shaped, with a particle size of generally 0.075 to 0.1 mm. Of course, finer tuff substances that are difficult to identify under a microscope are certainly present. These basic volcanic ash materials are rich in Al, Mg, Fe, Ca, and Mn. Under the influence of thermal metamorphism, minerals such as zoisite, epidote, and manganese epidote are formed. Electron probe analysis shows that these codonite minerals are rich in Al, Fe, and Ca, especially manganese epidote is relatively rich in MnO (Table 4-2).
This study conducted a comprehensive petrochemical analysis of banded jasper rocks. Chemical analysis work was performed at the Chemical Analysis Experimental Laboratory of the Institute of Geology, Chinese Academy of Sciences, and the analysis method was X-ray fluorescence spectroscopy.
From the analysis results (Table 4-3), it can be seen that the jasper rocks of the Saloi and Gulugunike deposits are rich in components such as Al, Fe, Ca, Mg, Mn, etc., and the contents of FeOt, MgO, MnO, and CaO are In basic rocks, they are 11.03%, 7.46%, 0.25%, and 9.68%, respectively, while in acidic rocks, they are 3.79%, 0.70%, 0.08%, and 1.63%, respectively, which are 3 to 10 times lower than the former. Therefore, the jasper rock in the study area is rich in components of basic rocks, which is direct evidence that this type of rock was formed in a basic volcanic submarine hydrothermal mineralization system.
In addition, some jasper rocks are particularly rich in Na2O (Table 4-3, sample No. 97SL025). This is because this type of sample contains more plagioclase crystal fragments. Electron probe analysis shows (Table 4-1) that these plagioclase feldspars are all albite, which may be formed by sodium metasomatism in the submarine volcanic hydrothermal system. In addition to albite that exists in the form of crystal fragments, the siliceous rocks in the study area actually contain microcrystalline albite to a greater or lesser extent. These microcrystalline albite may be derived from finer basic volcanic ash material.
Table 4-3 Chemical composition of hydrothermal sedimentary rocks in the Saloyi deposit and the Gulugunike deposit (w B/10-2)
2. Rare earth elements in siliceous rocks Characteristics of elemental composition
The composition of rare earth elements was measured on 4 jasper samples from the Saloi mining area. The measurement work was completed at the Chemical Analysis Experimental Laboratory of the Institute of Geology, Chinese Academy of Sciences, and the measurement method was plasma spectroscopy. It can be seen from the analysis results (Table 4-4) and the chondrite standardized graph (Figure 4-1) that the ΣREE of these four samples are obviously different and can be divided into two groups: the ΣREE of group 1 changes at 156.31×10 -6 ~ 170.36×10-6, which is relatively high; the ΣREE of group 2 varies from 73.55×10-6 to 99.53×10-6, which is relatively low. Studies under a microscope have shown that the former (sample numbers 96008 and 96053) is rich in cordite minerals, which may be the reason for its enrichment of REE. It can also be seen from Table 4-4 and Figure 4-1 that the europium anomalies of the four samples are not obvious, but the cerium is significantly depleted. The lowest δCe is only 0.58. The fractionation of light and heavy rare earths is not strong, and La/Yb varies from 3.90 to 6.10. , the mean value is 4.77.
Table 4-4 Rare earth element content (w B/10-6) in the hydrothermal sedimentary rocks of the Saluoyi deposit and the Gulugunike deposit (w B/10-6)
Figure 4-1 Saluo Rare earth element patterns of jasper from the Yi deposit and the Gurugunnik deposit
Chemical sediments precipitated by modern seawater, such as Fe-Mn sediments near mid-ocean ridges, accurately reflect the relative depletion of seawater Characteristics of Eu and Ce. Foer (1977) argued that chemical sediments, particularly banded ferruginous formations, are the best materials for tracing the evolution of rare earth elements in depositional environments over time. As a typical hydrothermal sedimentary rock, iron-containing structures have the following characteristics in their rare earth element composition over the evolution of geological history: First, from the Archaean to the present, the total REE content of various types of iron-containing structures has been low (Fryer 1983; Graf, Jr., 1978; Fleet, 1984), except of course those samples rich in apatite, monazite or xenotime, as these minerals are particularly enriched in REE; second, they are not found in Archean iron-bearing formations There are obvious anomalies in Ce, and examples of two anomalies, enrichment and depletion, have been observed since the Mesoproterozoic and Neoproterozoic. There is a significant Ce depletion in modern ocean ridge metallic sediments; thirdly, the effects of Eu anomalies on different types of Chemical precipitates are different. For example, Algoma-type iron structures, regardless of whether they were produced in the Archean or Paleozoic, generally have positive Eu anomalies, and their REE patterns are very similar; while Lake Superior-type iron structures generally do not have positive Eu anomalies, but almost every Among the test samples from various regions, there are always individual samples showing weak Eu anomalies. Negative Eu anomalies are common in modern ocean ridge metal sediments; fourth, the fractionation of rare earth elements in iron-containing structures during the Archean Era is not obvious, with the La/Yb ratio being as low as 3.5. However, since the Proterozoic, the fractionation of light and heavy rare earth elements has tended to be Enhanced, the La/Yb ratio of sediments at the top of the East Pacific Rise is 6.1 to 6.3 (Fleet, 1984). But in general, this ratio is still lower than that of terrestrial sediments of the same era. For example, the La/Yb ratio of modern deep-sea soft mud is 11 (Mclennan and Taylor, 1980).
It should be pointed out in particular that in the evolution of geological history, the obvious regularity of Ce and Eu anomalies in iron-containing structures is related to the evolution of the earth's crust and seawater itself. During the Archean and Paleoproterozoic, due to intense volcanic activity, a large amount of strong reducing thermal fluids were discharged into the ocean. Therefore, the seawater at that time was rich in Eu2+, and all types of chemical sediments had significant positive Eu anomalies. Since the Mesoproterozoic Era, as seawater has become oxidized, Eu2+ and Ce3+ have been oxidized into Eu3+ and Ce4+ respectively and separated from other rare earth elements. Therefore, since the Mesoproterozoic and in some areas since the Paleoproterozoic, various chemical sediments have been significantly depleted of Eu and Ce, especially characterized by metallic sediments at modern mid-ocean ridges (Eryer, 1979). Of course, under special conditions, due to the presence of strong reducing thermal fluids in the local environment, some chemical sediments have positive Eu anomalies, such as the sulfide ores and near-source fumarites in the Broken Hill deposit (Lottermoser, et al., 1989), Paleozoic Algoma-type iron formations and some massive sulfide ores (Graf, Jr., 1977, 1978). In particular, research on thermal fluids and related chemical sediments in hydrothermal systems under different geological environments in the past ten years has proven that at the vents of submarine hot springs, when the amount of seawater mixed in is small, the thermal fluids and their chemical sediments are significantly It is rich in light rare earths and has strong positive europium anomalies (Michael, et al., 1983, 1989; Timothy, et al., 1990, Table 4-5).
Table 4-5 Some characteristic parameters of rare earth elements in high-temperature hydrothermal fluids and their chemical sediments in different geological environments
It can be seen from the above that the ΣREE of jasper in the study area is higher than that of ordinary hydrothermal fluids The ΣREE of siliceous rocks is relatively high (Hanfa et al., 1997), which may be caused by a certain amount of volcanic tuff material contained in this type of rock. As Fryer (1983) pointed out, the rare earth element content of hydrothermal sediments is generally very low. Therefore, any meaningful addition of terrestrial material or volcanic debris will seriously affect the abundance of REEs and the interpretation of their data. Nonetheless, the cerium and europium anomalies in the jasper rocks in the study area are very similar to modern ocean ridge metal deposits, indicating that they were deposited in water bodies but are related to submarine hydrothermal systems. In addition, similar to various types of iron-bearing structures, jasper rocks in this area are characterized by relatively richer heavy rare earth elements than clastic sedimentary rocks of the same era. This is because heavy rare earth complexes have greater stability, so they stay in the water longer than light rare earths. As a result, seawater is relatively richer in heavy rare earths than marine clastic sediments of the same period, such as Pacific seawater and modern deep-sea soft soils. The La/Yb ratios of mud and North American shale are 5.41, 11 and 10.6 respectively. The average La/Yb ratio of jasper in the study area is 4.77, which is not only lower than modern deep-sea soft mud, but also lower than the average of 40 North American shales, proving that they are not ordinary terrestrial sediments, but are formed with the participation of seawater. Hydrothermal sedimentary rocks formed by chemical precipitation.
3. Evidence of the origin of siliceous rock fumarole deposition
A large amount of actual data and field geological relationships have been accumulated to prove that Precambrian iron formation is related to massive sulfide deposits Various types of siliceous rocks, feldspar rocks and tourmaline rocks are hydrothermal sedimentary rocks. On this background, an analogy method is used to study the origin of hydrothermal sedimentary rocks.
Judging from the petrology of siliceous rock itself, it has a very developed sedimentary strip structure, which is obviously not formed by metasomatism. However, the mineral's poor crystallization, fine grain size, and characteristic suture structure mean that it has not suffered strong metamorphism, and it cannot be formed by metamorphism of rocks such as sandstone. Especially in siliceous rocks, a colloidal structure composed of microcrystalline quartz is often seen. The diameter of a single colloidal particle is 0.05 to 0.1 mm, and it is mostly round or oval in shape. This colloidal structure is also common in siliceous rocks associated with massive sulfide deposits in the Selwyn Basin, Canada, and in siliceous fumarites in the Mount Windsor volcanic belt in Australia (Duhig et al., 1992). In addition, opal with this colloidal structure has also been seen in barite-opal rocks formed by modern hydrothermal sedimentation at 1990 m underwater in the Lau Basin in northeastern Australia (Bertine, et al., 1975).
Research shows that this structure is formed by colloid deposition, indicating that siliceous rock is the direct product of chemical precipitation of SiO2.
Silicate rocks may be of biological or volcanic origin. However, the following geological facts are very important: ① The distribution of siliceous rocks in geological history was the largest in the Precambrian, and gradually decreased in the Phanerozoic; ② From a quantitative point of view, siliceous rocks are mainly produced in geosynclinal areas , and most of them do not contain or rarely contain siliceous biological shells; ③ Siliceous rocks in the geosyncline area are often directly located on underwater eruptive lava (Carrison, 1974) or directly overlying hot brine sediments (massive sulfide, ocher, etc.) (Bernard, 1982; Robertson, et al., 1974); ④ This kind of siliceous rock is often associated with black shale containing pyrite, and is "incompatible" with carbonate rock ( Siever, 1962; Хворова, 1977); ⑤ Especially for the siliceous rocks in the geosyncline area, although they sometimes contain siliceous biological shells, these siliceous organisms are incidental components that have nothing to do with the origin of the chert rocks, just because they are rich there. The aqueous environment of silica is a good place for their preservation (Robertson, et al., 1974). The above facts show that sedimentary structures related to siliceous rocks were formed in closed deep-water limited basins, which only accepted a small amount or no clastic materials. Therefore, the chemical composition of seawater is different from that of normal seawater, that is, there is abnormal supply of SiO2 that makes siliceous Biological shells are preserved and the pH is low, allowing planktonic calcareous biological remains to dissolve (Siever, 1962). Therefore, the formation of siliceous rocks is closely related to volcanic activity, and biogenesis is not the main factor. Especially in some deposits, such as the Dachang tin-polymetallic deposit in China, the massive sulfide deposits in Sullivan, Canada, and Broken Hill, Australia, the tourmaline laminae in the siliceous rocks have the characteristics of fugitive origin. It is very developed, which is not available in biogenic siliceous rocks. In addition, existing data show that the chemical compositions of siliceous rocks of different origin types are different. As can be seen from Table 4-6, the percentage contents of TiO2, Al2O3, K2O, Na2O, and MgO are generally low in biogenic siliceous rocks, and are common in siliceous rocks related to volcanism or submarine hydrothermal systems. High, this difference is particularly useful for identifying the origin of siliceous rocks. For the sake of clarity, the relevant information is shown in Figures 4-2 and 4-3. It can be seen from the projection map of Al2O3-TiO2 that siliceous rocks of biogenic origin and siliceous rocks of volcanic or seafloor thermal brine origin are clearly divided into two areas. At the same time, in the siliceous rock area of ??biogenic origin, the projection points are scattered and there is no correlation between Al2O3 and TiO2; while in the siliceous rock area of ??volcanic and submarine thermal brine origin, the projection points are distributed in a band, and there is no correlation between Al2O3 and TiO2 The correlation coefficient between them is 0.6390, showing a positive correlation. Geochemical studies show that there is a positive correlation between Al2O3 and TiO2 for chemical sedimentary rocks that are fully decomposed (Han Fa et al., 1983). The pattern revealed in Figure 4-2 is consistent with this principle, and it provides another geochemical method for identifying the origin of siliceous rocks. When the analysis results of the siliceous rock samples from the Aktas-Saloi area are projected on Figure 4-2, the four data points all fall in the siliceous rock area of ??volcanic or thermal brine origin, indicating that the siliceous rock is of genesis. Related to hot brine activity. On the w (Al2O3)-w (K2O+Na2O) diagram, the two siliceous rocks of different origins are also clearly divided into two areas. Similarly, the projection points of the samples in the study area also fall within the siliceous rock area originating from volcanoes or submarine thermal brines (Figure 4-3).
Figure 4-2 Relationship diagram of w (TiO2)-w (Al2O3) in siliceous rocks of different origins
Figure 4-3 w (K2O+Na2O) in siliceous rocks of different origins )-w (Al2O3) relationship diagram
Table 4-6 Chemical composition of siliceous rocks of different origins (wB/%)
4. Regarding the source of silicon
To explore the source of silicon, a key question to discuss first is whether submarine volcanic activity or hot brine systems can provide enough material to form hydrothermal sedimentary rocks. In particular, whether this system can provide enough SiO2 to form siliceous rock is the key, because siliceous rock is the most abundant and widely distributed rock among hydrothermal sedimentary rocks.
In addition, at 20°C, amorphous SiO2 must reach 119.26×10-6 before it can be deposited by chemical precipitation from normal seawater, while there is only 4×10-6 amorphous SiO2 in normal seawater. Below we briefly illustrate this issue using some information on modern geothermal systems and submarine thermal brine systems.
There is no volcanic activity in the modern terrestrial geotropical zone of the Himalayas in Tibet, but hot water explosions and geyser activities are very intense. The research results of 277 water samples in this area show that SiO2 is an important component of hot spring water. Its average content is mainly (32~100)×10-6, and those greater than 100×10-6 account for 17% of the total number of samples. In hot springs rich in SiO2, when the hot water sprays onto the surface, the SiO2 turns into a gel state and is deposited by chemical precipitation, forming silica terraces and silica mounds. Some Sihua mounds are as high as 50 m, and some are even 400-500 m higher than the valley. These SiO2 precipitates are often in the form of dense blocks and strip structures (Comprehensive Investigation Team of the Tibetan Plateau of the Chinese Academy of Sciences, 1981). Although the above data comes from the continental hot spring system, it at least shows that the hot spring system can indeed provide a large amount of SiO2 and form a considerable scale of siliceous rock. A large amount of data has been accumulated on modern submarine volcanic activity and thermal brine systems: near the Surtsey volcanic activity area, the SiO2 concentration suddenly increased three times (Stefansson, 1966); near the volcanic fumarogenic activity area of ??Deception Island in Antarctica, Si in the water and Mn concentrations reached 50700mg/L and 2420mg/L respectively (Elderfield, 1972); the SiO2 content in the hot water of the Banu Wuhu submarine volcanic vent is 10 times higher than that of nearby seawater, and the oxide precipitation of iron and manganese is obvious (Zelenow, 1964); The SiO2 content in the brine of the Red Sea hydrothermal system (A-II abyss) is 64.19×10-6, which is 16 times that of normal seawater (Emery, et al., 1969); in the 21°N hot spring system of the Pacific Ocean, near the vent The brine contains SiO2 as high as 1291×10-6 (Rosenbauer, et al., 1983). The main components of the fluid ejected from the white smoke chamber here are barite and amorphous SiO2. Research has proven that the chemical precipitates formed from these brines do contain significant amounts of amorphous SiO2. For example, in the sediments of the "A-Ⅱ" abyss of the Red Sea, the iron smectite phase, amorphous goethite phase, sulfide phase and hydromanganite phase contain SiO2 of 24.4%, 8.7%, 24.7% and 7.5% respectively. , among which the only mineral that has been determined to contain SiO2 is iron-montmorillonite, therefore, all other phases contain a large amount of amorphous SiO2 (Bischoff, 1969). There is also a large amount of amorphous SiO2 in the sediments of the "Black Smoke Chamber" at 21°N in the Pacific Ocean. Some of these amorphous SiO2 are spherical on the sulfide surface, and some are produced in thin layers interlayered with the sulfide (Haymon , et al., 1981). The barite-opal rocks found in the Lau Basin in northeastern Australia are modern hydrothermal sediments rich in SiO2. Marine geological research has found that submarine hot spring systems are always located in small, localized deep-water basins. For example, the "A-II" abyss of the Red Sea is 14 km long, 5 km wide, and 2170 m deep. The "black smoke chamber" at 21°N in the Pacific Ocean is distributed in a long and narrow basin that is 6.2 km long, only 0.2-0.5 km wide, and has a water depth of 2600 m. This ensures that the hot brine discharged to the seabed does not mix with the ocean water in the vast sea, which is conducive to the formation of various chemical sediments. In fact, this geological environment is consistent with the formation environment of ancient massive sulfide deposits.
Many researchers have paid great attention to the impact of submarine volcanic activity or thermal brine systems on seawater composition and its evolution in geological history. Table 4-7 shows the data on certain components brought into the ocean by the hot brine discharged to the seabed every year and the rivers flowing into the ocean (Honnorez, 1983). It is not difficult to see that the amounts of brine and the main components brought into the ocean by rivers are almost equal, among which brine brings more Mn, Li, and Rb to the ocean.
According to calculations based on the modern mid-ocean ridge thermal brine convection system, all seawater may circulate through ocean ridge convection within 5 to 11 Ma, and the ocean floor may be renewed once within 200 Ma. Therefore, no rocks older than the Jurassic have been found on the ocean floor. . These data show that even if the crustal movement is relatively stable today, the impact of the submarine hydrothermal system on the composition of seawater still plays a "dividing world" role. If we take into account that in the early days of the earth's formation, submarine volcanic activity was more intense, then the material brought to the ocean by the submarine hydrothermal circulation system was even more considerable. Studies of rare earth element geochemistry have provided evidence for this. As mentioned before, from the Archean and Paleoproterozoic to the Mesoproterozoic and Phanerozoic, the anomalies of Eu and Ce in various iron-containing buildings have changed with obvious regularity. This change proves that a large amount of strong reducing thermal fluid was poured to the seafloor through submarine hydrothermal vent systems in the Archean and Paleoproterozoic (Fryer et al., 1979). This thermal fluid must contain large amounts of SiO2, from which siliceous rocks precipitate. It is not difficult to understand that the distribution of siliceous rocks in geological history mainly appeared in the Precambrian. Of course, with the development of geological history, submarine volcanic activity gradually weakened, and the chemical deposits related to it also decreased accordingly, so that no considerable scale of such chemical deposits has been found in modern ocean basins. Obviously, the principle of "discussing the past with the present" cannot be applied mechanically here. Based on this understanding, by combining the geological environment of the Aktash-Saloi metallogenic belt, the ore-bearing structures, and the petrological and geochemical characteristics of the main ore-hosting rocks, we believe that the siliceous rocks in this area It is formed by syngenetic deposition/diagenesis from the seafloor thermal brine circulation system. In the layers dominated by hydrothermal sedimentary rocks, a small amount of terrigenous mud and volcanic ash materials are mixed in to form laminated epidote rocks. This is not an abnormal phenomenon, but an inevitable result of objective geological processes.
Table 4-7 Comparison of seafloor thermal brine and ocean components brought by rivers (mol/a)