Sedimentary deposit refers to a kind of deposit which occurs in layers in sedimentary rocks dominated by fine clastic rocks, and is characterized by the development of massive sulfide-rich ores with banded features. Lead-zinc deposit is the most common and important deposit of this kind. At present, this kind of deposit is also called hydrothermal deposit in China.

In early studies, these deposits were considered as transitional or terminal types of volcanic massive sulfide deposits, because they have the same characteristics as massive and banded metal-rich sulfide ores. Later work proved that there were few or no volcanic rocks in the ore-bearing rock series in some areas, indicating that volcanic jet was not the necessary mechanism for mineralization. In order to highlight the characteristics of host rocks, some people put forward the view of shale deposit. Especially in the mid-20th century, with the discovery and in-depth study of similar deposits in Australia, Canada and Germany, it is not only recognized that such deposits occupy a prominent position in the related metal resources, but also gradually formed a relatively complete new genetic explanation, that is, the basic genetic concept of ore-bearing hot water evolved from the heating of syngenetic water in multi-stage fault basins, and sprayed or sprayed to the seabed through appropriate channels. After determining the mineralized alteration zone of the footwall passage of the ore body, many kinds of hydrothermal sedimentary rocks have been discovered and studied in China, which is of great significance. Generally speaking, it is now considered that the ore-forming fluid of this kind of deposit is not magmatic hydrothermal solution, but hot water generated by underground seepage and syngenetic water evolution. Mineralization is not supergene filling and metasomatism, but has the nature of sedimentary syngenetic rocks, and of course it can also be transformed by supergene changes to varying degrees.

This deposit is not only one of the most important sources of lead and zinc, but also an important or partial source of copper, gold, silver, manganese, barite and fluorite. It is distributed in five regions of the world, including: ① North China, such as Changba, Qiandongshan, Yindongzi-Daxigou, Langshan-Zhaertai Dongsheng Temple, Huogeqi and Hujiayu-Zhongtiaoshan. (2) Northeast Australia, such as MacArthur River, Mount Isa and Brokenshire; ③ Western North America, such as Howard Pass, Sullivan and Tom; (4) Northwest Europe, such as Meggen lead-zinc barite deposit in Germany, La Maiers Berg lead-zinc deposit and Irish silver deposit; ⑤ Southern Africa, including South Africa and Zimbabwe.

The mineralization of jet sedimentary deposits does not necessarily occur at the same time in a structural area, but often belongs to the same era, in which Mesoproterozoic (65.438+700 million ~ 65.438+400 million years) and early and middle Paleozoic (450 million ~ 300 million years) are the most important metallogenic ages. The metallogenic environment is a geosyncline or a basin with passive continental margin controlled by continental margin rift, and the basin was formed in a non-orogenic tectonic event before orogeny. The ore-bearing rock series of this kind of deposit is in transition from continental debris deposition to flysch formation and structural metastable formation, including fine clastic rocks, mudstone and shallow water carbonate rocks. There are obvious paleogeothermal gradient anomalies in the metallogenic areas of jet sedimentary deposits, which are largely caused by the heating of deep (concealed) magma. Many ore deposits are spatially accompanied by synsedimentary faults that are active in ore-forming period, and these faults may be the channels for ore-bearing hydrothermal fluids to rise from sub-basins to the surface. These synsedimentary fault are usually judged by sedimentary facies and sudden change of thickness.

The deposit consists of one or more layered, layered or lenticular sulfide ore bodies with stable horizons. There are many large-scale ore deposits, the thickness of ore bodies can reach tens of meters, and the extension depth ranges from several hundred meters to more than 1 km. Although some ore deposits were strongly reformed by the later structure, the ore bodies were folded and deformed, but they were synchronized with the surrounding rocks and kept the same occurrence. Vein-like or vein-like ore bodies and hydrothermal alteration, such as silicification, pyritization and albitization, can often be seen under or near layered ore bodies. This vein-like alteration mineralization zone is understood as a channel for the rising of ore-forming materials under the basin. The ore-bearing rocks are water-bearing sedimentary rocks such as siltstone, shale and carbonate rocks, while the direct ore-bearing rocks in the ore-bearing strata are often different types of hydrothermal sedimentary rocks, such as siliceous rocks, albite rocks, barite rocks, mafic carbonate rocks and pyrolusite rocks.

The ores of spouted sedimentary deposits are characterized by simple sulfide assemblage, including pyrite, sphalerite and galena, followed by pyrrhotite or white iron ore, chalcopyrite and arsenopyrite, and occasionally sulfide minerals. The ore is dominated by massive, banded and layered structures, and the reticulated vein altered ore is fissure-filled, veinlets and breccia, and some ores may have new ore structures due to epigenetic transformation. Jet sedimentary deposits have obvious mineralization zoning. From the jet center outward, Cu-Pb-Zn-SiO _ 2-Baso _ 4 (Fe) and other sulfides, oxides and sulfates form copper-rich nuclei, with widely distributed lead and zinc, with siliceous rocks, barite and sometimes hematite at the edge. Vertical, bottom-up is usually Cu, Zn, Pb-(Ba). The homogenization temperature of mineral gas-liquid inclusions is generally 150 ~ 300℃, and the salinity is generally between 7% ~ 22% (NaCl).

Two. Types of mineral deposits and important examples

In the past, jet sedimentary deposits were mostly divided into shale type and carbonate type according to host rocks. In fact, the host rocks of this kind of deposit include siltstone, siliceous shale, siliceous rock and other hot water sedimentary rocks, and almost all water-borne and hot water sedimentary rocks can be the host rocks of this kind of deposit. Many mineral deposits are composed of fine clastic rocks-shale-carbonate rocks, and ore bodies can occur in different sedimentary rocks at different horizons. According to the types of minerals, jet sedimentary deposits can be divided into jet sedimentary lead-zinc deposits, jet sedimentary polymetallic sulfide deposits, jet sedimentary copper deposits and jet sedimentary pyrite deposits. Some people think that some stratabound silver and gold deposits also belong to the origin of jet deposition.

1. Changba and Lijiagou lead-zinc deposits in Gansu Province

Located in Chengxian County, Longnan District, Gansu Province, it is the largest lead-zinc deposit in the hydrothermal sedimentary metallogenic series of the Devonian polymetallic metallogenic belt dominated by lead and zinc in Qinling. The average grade of lead and zinc in ore is more than 10%, and the total metal content of lead and zinc has exceeded 5 million tons. The deposit includes two ore sections, Changba and Lijiagou (Figure 6-8).

Fig. 6-8 Geological Schematic Diagram of Changba and Lijiagou Lead-zinc Mines in Gansu Province (cited by Qi Sijing et al., 1993)

The ore-bearing stratum is Anjiacha Formation of Middle Devonian, and the original rock is a set of mudstone-fine clastic rock mixed with carbonate rock, which is a transition zone from complex continental clastic rock construction to flysch-like construction in the northern margin of Yangtze Platform. The exposed thickness of the metallogenic area is 3000 m ... Anjiacha Formation is divided into two layers, the lower layer is long dam layer (D2a 1) and the upper layer is Jiaogou layer (D2a2). The long dam layer is a mineral-bearing layer, with mudstone and siltstone in the upper part and carbonate rock in the lower part. The rocks generally experienced greenschist facies, and some of them reached amphibolite facies, becoming biotite quartz schist, biotite garnet tremolite diopside schist, calcite quartz schist and biotite banded marble. The long dam layer from top to bottom is: metamorphic siltstone, schist; Calcite quartz schist; Muddy banded marble; Dolomite; Biotite quartz schist, biotite garnet schist.

According to the ore-bearing rocks, the whole ore-bearing strata are divided into two ore-bearing systems, namely, marble ore-bearing system dominated by calcium and schist ore-bearing system dominated by fine clastic rocks. Hot water sedimentary rocks are developed in both systems, and the mining areas are mainly albite, siliceous rocks and barite rocks, among which tourmaline is as high as 3%, and other hot water forming minerals such as ankerite, barium feldspar, tremolite and diopside. Lead-zinc ore bodies are mainly distributed in calcite quartz schist and marble in layered, layered and lenticular forms, with multiple ore-forming horizons, and their occurrence is consistent with that of surrounding rocks (Figure 6-9). At present, 20 industrial ore bodies have been proved. The main ore bodies are Changba 1 (marble hosting), Changba No.2 (calcite quartz schist hosting), Lijiagou 1, No.2 (marble hosting) and Lijiagou No.3 (schist hosting), of which Changba No.2 (extending over 400 meters and over 600 meters) has a maximum thickness of nearly 400 meters. The total thickness of ore-bearing strata is more than 500m, and the ore bodies are concentrated in 4 ~ 5 horizons (Figure 6-9).

Fig. 6-9 Section of Exploration Line 37 in Changba Lead-zinc Mine, Gansu Province (quoted from Qi Sijing et al., 1993) (the legend is the same as Figure 6-8).

The host rock type of Changba Ⅱ ore body is calcite biotite quartz schist. Massive ore occurs in the lower part of layered ore body, containing tourmaline and albite zone; The middle part is dominated by banded ore, which is an alternating zone of metal sulfide, albite and timely; Barite-rich bands appeared at the top. In the footwall of lenticular ore body, chlorite actinolite albitization network vein belt and altered breccia belt appear. The average grade of ore is Zn 12. 15% and Pb 2. 96%. The host rocks of Lijiagou I and II ore bodies are biotite banded marble with an average lead and zinc content of 10%. The occurrence horizon and host rock type of Changba I ore body are the same.

The main ore minerals in Changba deposit are pyrite (with high content in schist ore-bearing system) and sphalerite (mainly marmatite, with the highest iron content of 12). 9%), galena, in addition to more pyrrhotite and a small amount of arsenopyrite, jamesonite, copper minerals are rare. The types of gangue minerals are complex, which are related to the types of host rocks. The main gangue minerals are Yingshi, biotite, calcite, barite and albite. The ore is dominated by massive banded structure, followed by disseminated structure and coarse-grained metamorphic structure. The lead isotopic composition of the ore is very uniform, with a μ value of 9. 15 ~ 9.72. The homogenization temperature of mineral fluid inclusions in ore is between 1 10 ~ 180℃ and the salinity is 19. The fluid is Na-K-Cl type, with high CH4 content and 87Sr /86Sr of 0. 708 ~ 0.

2. Dongshengmiao polymetallic deposit in Inner Mongolia

The deposit occurs in Hangjinhouqi, Bayannaoer City, Inner Mongolia. The ore-bearing strata are Mesoproterozoic Jishugou Formation, Zenglongchang Formation and Agulugou Formation, which are deposited in the rift basin on the northern margin of North China Block. The Langshan-Shihahe ancient island in the central uplift of the continental shelf divides the basin into inner and outer troughs, and the Dongshengmiao deposit occurs in the inner trough. The ore-bearing rock series is composed of shallow-sea clastic rocks-carbonate rocks-carbonaceous mudstone, with shallow-sea clastic rocks in the lower part, including glutenite, timely feldspar sandstone and quartzite (Ji Shu Gou Formation), shallow-water platform carbonate rocks and faulted basin facies in the middle part, and layered limestone, dolomite and carbonaceous mudstone (Zenglongchang Formation) in lithology, and shelf margin faulted basin facies in the upper part, with carbon-rich mudstone in lithology.

The main orebodies of Dongshengmiao deposit occur in silty phyllite and dolomite (pyrite orebody and copper orebody) of Zenglongchang Formation and carbonaceous slate (lead-zinc orebody) of Agulugou Formation. Ore-hosting rocks include water-deposited carbonaceous dolomite, carbonaceous slate and iron dolomite, hydrothermal sedimentary siliceous rocks and albite quartzite. The ore body is layered and quasi-layered, which is consistent with the occurrence of surrounding rock, but it folds synchronously with surrounding rock due to regional deformation (Figure 6- 10).

There are 2 industrial ore bodies 12 in the mining area, including 6 main ore bodies, 9 with a length of1400 ~1700 m. 88 ~ 14. The average thickness is 86 meters. The roof of the ore body is carbonaceous silty slate, and the floor is hydrothermal sedimentary rocks such as albite quartzite, biotite magnetite dolomite and biotite schist containing tourmaline. In addition, there are a large number of hydrothermal sedimentary minerals in the ore body, such as ankerite, chlorite, tourmaline, epidote, tremolite, actinolite and a small amount of albite and barite. Besides biotite and magnetite, there are almandine and epidote belts in the floor rocks. In addition, there are microcline, albite and chlorite veins in the seam. The ore bodies are mainly massive pyrite, accounting for 60% of the total number of ore bodies, followed by zinc ore bodies, lead-zinc ore bodies and a small amount of copper ore bodies. The average grade of lead-zinc ore body is 1.4 1% ~ 4.92%, lead is 0.86% ~ 1.82%, the ratio of zinc to lead is 9/1, and part of it contains copper 0.39% ~ 1.39. The total metal content of zinc and lead in the deposit exceeds 4.5 million tons, and the pyrite reserves exceed 200 million tons. The main ore minerals are pyrite, pyrrhotite and marmatite, followed by chalcopyrite, galena, siderite and magnetite. The ore is mainly schistose structure, followed by disseminated structure, with breccia and reticulate veins.

3. Hujiayu and Bizigou copper mines in Zhongtiaoshan.

Yuanqu County is located in the south of Shanxi Province. The deposit occurs in the marginal rift zone of Paleoproterozoic intracontinental rift environment, and the intracontinental rift developed on the basis of Archean ancient land in North China block. Archean metamorphic volcanic rocks are widespread in mining areas, and the new Archean in mining areas is a set of metamorphic ultrapotassic volcanic rocks. Metamorphic bimodal volcanic rocks composed of andalusite biotite schist, amphibole biotite schist, rhyolite and rhyolite tuff constitute the main body of metamorphic core complex. Above Archaean is a set of thick terrigenous sedimentary metamorphic rock series accumulated in the marginal rift zone in the Paleoproterozoic intracontinental rift environment, that is, the Lower Proterozoic middle schist group, which is composed of quartzite, calcareous schist, phlogopite-bearing dolomite marble, black schist and andalusite dolomite marble. The ore-bearing strata are mainly black schist formation of Bizigou Formation in Zhongtiao Group, and the lithology can be divided into two types: water schist, gray slate, garnet sericite schist and a small amount of marble. The latter includes striped pyrophyllite, phlogopite timely dolomite marble, timely albite and variable breccia (Sun Haitian et al., 1990).

Figure 6- 10 Vertical Profile of Dongshengmiao Deposit in Inner Mongolia (quoted from Zhai Yusheng et al., 2003)

The output of industrial ore bodies is strictly controlled by strata and lithology, which is basically consistent with the spatial distribution of hydrothermal sedimentary rocks. The main body is layered, quasi-layered, lenticular, integrated with surrounding rocks, with clear boundaries, and a few ore bodies are veined. The main ore minerals in the ore are pyrite, chalcopyrite and pyrrhotite, and the gangue minerals are mainly timely, dolomite, albite, phlogopite and tourmaline. The copper grade of ore is 1% ~ 3%, and the deposit reserves are large.

Three. mineralize

In the spreading center of modern ocean floor, rift zone and its vicinity, many vents are formed by submarine hydrothermal activity, and deep hydrothermal solution is sprayed to the seabed from the vents, and metal-containing deposits (ooze) are formed on the modern ocean floor through hot water deposition, which is understood as jet deposition. The types of deposits described in this section are similar to those in modern submarine hydrothermal deposits, mainly occurring in extensional rift tectonic environments such as intracontinental/intercontinental rift or Ola Valley, and located in rift-controlled craton or its marginal depression sedimentary basin; However, there is no typical oceanic crust in a similar basin formed by jet deposition, which is one of the obvious differences from volcanic massive sulfide deposits. The metallogenic ages of jet sedimentary deposits are mainly Mesoproterozoic and early and middle Paleozoic. Mesoproterozoic was at the turning point of continental division and supercontinent formation in the process of earth evolution. Early and Middle Paleozoic is the initial period of cold continent formation after supercontinent rupture, and the stress field of continental margin changes from tension to compression. In such a special stage of crustal development and special crustal thermal state, it is easy to form not only rift channels directly to the seabed, but also hot water convection system in the crust, which is the basic background for the formation of submarine jet sedimentary deposits.

According to the main characteristics, tectonic environment and the basic background of crustal evolution at that time, most scholars now think that it was formed by submarine jet deposition.

1. Formation of ore-forming fluids

The fluids forming such deposits are mainly formation water (construction water) and circulating seawater from the basin, and the composition is mainly chloride-rich solution. In the sedimentary basin, with the increase of buried depth, temperature and rock pressure, the sediments are gradually compacted and dehydrated, and the salinity of pore water increases. When the temperature rises to 90℃, the sediments are compacted continuously, and the expansive clay is transformed into non-expansive clay, such as montmorillonite into illite. At this time, metal substances will begin to be extracted from minerals by fluid action and enter the contemporaneous aqueous solution. When the temperature reaches 130℃, further pressure dehydration. On the one hand, the discharged syngenetic water is greatly increased, and more metals are extracted and activated by the fluid and enter the syngenetic fluid. Under the normal geothermal gradient heating condition, the initial ore-bearing hydrothermal solution and ore-forming fluid are formed. Recent studies show that the mineralization of deep magmatic water with sub-basin origin cannot be ruled out.

2. Convective circulation of fluid

After the ore-bearing hydrothermal solution is formed, it can form a convection circulation system with the infiltrated seawater in this special area of the transitional crust, and the ore-bearing hydrothermal solution enriches the ore-forming materials in the convection circulation. The basic elements of hot water convection cycle follow Reynolds equation:

Basic mineral deposit science

Description: The formula includes permeability (k), fluid expansion coefficient (α), gravitational acceleration (g), thickness of permeable layer (h) and temperature of top and bottom plate of permeable layer (δt), that is, temperature gradient, fluid viscosity (v) and thermal diffusivity of saturated medium (Km). Permeability and temperature gradient are the basic parameters of convection. Therefore, on the basis of summarizing a lot of predecessors' knowledge, M. J. Russell put forward the model of "jet sedimentary lead-zinc deposit formed by deepening hot water convection chamber during crustal extension". Solomon (1976) described in detail the circulation model of hydrothermal solution and seawater in the submarine convection pool. After calculation, the diameter of this convection system is about 10 km and the depth is 4 ~ 5 km. Therefore, the rocks through which the hydrothermal solution flows can provide a large amount of metallic substances. Russell (1978) pointed out that the early Paleozoic convection system under the Carboniferous basin in central Ireland was 35 ~ 50 km in diameter and 14 km in depth. Therefore, a large zinc deposit in Nawan, Ireland, can be formed only by hydrothermal solution flowing through 1% zinc in the source rock.

Hydrothermal convection circulation system has different extraction ability for different ore-forming elements in different stages of its evolution, so ore-forming fluids in different stages form different ore-forming characteristics and types. Most researchers believe that lead-zinc ore was formed in the main period of its evolution, when the enthalpy in the system is the highest, and the pH and oxygen fugacity are suitable for extracting lead and zinc from rocks, so a large flow of lead-zinc ore-forming fluid may be formed. This may be the main reason why lead-zinc deposits dominate this kind of deposits in the world.

The convection circulation of ore-bearing hydrothermal solution needs heat source, which is obvious in the unusually high heat flow area of the deposit. Many evidences show that the regional abnormal geothermal gradient is related to deep magma chamber activity and mantle hot spots. In other words, it is the mantle hotspot that provides enough heat energy for the convection and circulation of a large number of syngenetic water (formation water) and seawater.

3. Ejection of ore-forming fluid and precipitation of metal sulfide

Sufficient thermal energy can make the ore-bearing hydrothermal solution circulated by convection in the crust discharge to the seabed along a certain syngenetic structure, and hot water deposition and mineralization will occur on the surface and near the surface. The drainage channel is usually a syngenetic fault cutting the convection pool, which has been active during mineralization. The activity of these faults is one of the most important mechanisms for the sudden ejection of ore-bearing hydrothermal solution to the surface. The basement fault zone usually divides the earth's crust into many blocks, which sink at different speeds, resulting in the spouted sedimentary deposits located in the basin or uplift margin.

Generally, after the ore-bearing hydrothermal solution enters the fractured channel, its physical and chemical conditions will change rapidly, such as temperature drop, pressure drop, fO2 and pH value increase, and because it is mixed with the infiltrated seawater, it may provide rich H2S, HS- and S2-, which will make the ore-forming fluid unload before reaching the seabed, precipitate into minerals in the channel, accompanied by brecciation of the main rock, and form reticular vein alteration mineralization. If the overlying water body is deep, a large hydrostatic pressure is formed at the nozzle, and a large amount of ore-bearing hydrothermal solution gushes out, so that the channel system is relatively smooth, and the hydrothermal solution in the channel will not boil, but will always overflow the seabed to produce hot water deposition.

After the ore-bearing hot water solution is discharged to the seabed, due to its high density, it flows along the surface in the form of density current from the jet center and can maintain its original chemical composition for a long time, so the ore-bearing hot water can be mineralized in a far range from the jet center. Generally, in the low-lying areas of the basin, a brine pool is formed first, and then mixed with seawater with the rapid decrease of temperature, and the reduced sulfur increases, and a large number of ore minerals precipitate and gather at the bottom of the basin, forming layered mineralization. The hydrostatic pressure (water depth) in the hot water active area will determine whether there is boiling and sulfide precipitation near the spout when the ore-bearing hydrothermal solution overflows. Whether the seabed in the hot water active area is a slope, basin or depression determines whether the ore-bearing hydrothermal solution flows again. Therefore, under the above different conditions, jet sedimentary deposits have different characteristics. For example, some deposits have two types of mineralization: reticular vein alteration mineralization and layered mineralization; Others only have layered mineralization, but it is difficult to see reticular vein alteration mineralization in mining areas; Some coal floor has obvious alteration, while others have no obvious alteration.

In the process of hot water deposition and mineralization, metal sulfide will precipitate according to its solubility, environmental temperature of hot water deposition, Eh, pH value and distance from hot water activity center. Chalcopyrite is the sulfide with the lowest solubility. With the decrease of hydrothermal temperature, it first precipitates, often in waterways or near water crossings, showing reticular veins, breccia and altered mineralization. Then sphalerite and galena precipitate. Therefore, such deposits often have obvious zoning of minerals and elements. Vertically, copper is at the bottom, followed by zinc and lead. In the horizontal direction, copper is confined to the core of zoning sequence, surrounded by lead and zinc, and then iron. The ores in the horizontal zoning deposits usually have a good banded structure, each banded represents a hydrothermal ejection activity and hydrothermal deposition, while the horizontal zoning in each layer reflects the gradual gradient changes of physical and chemical conditions such as hydrothermal temperature far away from the hydrothermal activity center.

The jet sedimentary metallogenic model (Figure 6- 1 1) summarizes the main metallogenic mechanism and characteristics of this kind of deposit: during the burial process, the porous sediments in the basin are compacted to discharge pore water and interlayer water (syngenetic water), heated by (often highly abnormal) geothermal gradient, and the acidity and salinity increase, so that the metal substances in the stratigraphic column can be extracted to form ore-bearing hot water solution. Driven by thermal power, the ore-bearing hydrothermal solution circulates in the crustal convection pool with the infiltrated seawater, and more ore-forming materials are extracted from the rocks flowing through it. In areas where geothermal anomalies and tectonic activities are extremely high, ore-forming fluids are discharged from sedimentary strata along the contemporaneous faults of the cutting convection system, and deposit and mineralize in low-lying areas of the surface. Fluid can be ejected once along the fault to form a layer of ore body, or it can be ejected many times to form a multi-layer ore body. The metallogenic model clearly shows the spatial relationship and genetic relationship between the intersecting network vein metallogenic system (metasomatic filling) in the river channel and the layered metallogenic system (deposition) in the basin (brine pool), as well as the zonation of ore-forming materials in the deposit. Many examples of ore deposits show that hydrothermal channels and sedimentary brine pools can have various relationships in space, and some are far apart, such as MacArthur River deposit. Some are located at one side of brine pool, as shown in Figure 6- 1 1, such as Silvermins deposit; Others are located directly below the brine pool, such as Rammelsberg deposit. No matter how far away the rivers are from the basin, they are a unified metallogenic system, and layered mineralization is closely related to the alteration mineralization of reticulated veins, rather than caused by two different mineralization.

Fig. 6-1/jet sedimentary metallogenic model (according to F. W. Iydon,1983; D.E. large size, 198 1)

Four, the main points of exploration and evaluation of mineral deposits

Jet sedimentary deposits are significantly different from water-borne sedimentary deposits and volcanic massive sulfide deposits not only in characteristics but also in formation conditions.

Such deposits are controlled by multistage basins (Figure 6- 12). The first-class basin is a continental margin rift trough or an inland rift basin. These basins are hundreds to thousands of kilometers long and 100 kilometers wide, and are characterized by extremely thick clastic-argillaceous deposits or carbonate deposits (several kilometers). Huge thick sedimentary columns are the basic conditions for the formation of large-scale hydrothermal convection system and the possible occurrence of hydrothermal deposition in metallogenic belts.

In primary basins, extensional structures in the same period often form secondary basins (grabens) and uplifts (horsts). This kind of secondary basin is usually discontinuous, and its horizontal scale is usually tens of kilometers, so it is called secondary basin. The existence of secondary basins can be reflected by the abrupt change of sedimentary facies and thickness (Figure 6- 12).

There are often obvious linear structural zones (fault active zone, basement weak zone, reef growth zone, uplift edge, etc.) near the edge of the above-mentioned primary or secondary basins. ) is a good channel for deep ore-forming fluids to rise to the surface (usually the seabed). Therefore, jet sedimentary deposits are formed at the edges of many primary or secondary basins. For example, Megan deposit in Germany is located on the fault zone at the northern edge of the western Huali trough, and Tome deposit in Canada is located near the active edge of the secondary basin at Macmillan Pass. Tertiary basin is a relatively small depression or depression with a horizontal scale of several hundred meters to several thousand meters. The stagnation of water bodies in the basin is reduced. They are the "brine pool" into which ore-bearing hydrothermal solution flows and the deposition sites of layered mineralized bodies.

Many jet sedimentary deposits are surrounded by faults on one side or both sides, and these faults have been active since the contemporaneous sedimentary period. According to the analysis of lithofacies and thickness changes of ore-bearing sedimentary rocks on both sides of the fault, there are obviously slump breccia, and there are obvious syngenetic deformation structures in sedimentary rocks. These faults are synsedimentary faults, which together with growth faults control the settlement of Tertiary basin, and are probably part of the marginal fault system of primary or secondary basin, so they are the channels for ore-bearing hydrothermal solution to spray to the surface.

Fig. 6- 12 conceptual model for controlling the formation of jet sedimentary deposits in multistage basins (according to D. E. Large, 198 1)

In addition, the following indicators of mineralization can be used as exploration indicators for jet sedimentary deposits.

(1) Layered barite, siliceous rocks, albite rocks, iron oxide, iron magnesium carbonate rocks and other hot water deposits are important components of some exhalative sedimentary deposits, which are mainly laminated and occur on or around layered sulfides. Sometimes barite and metal sulfide form a thin interlayer, and some of them are contained in the ore in the form of aggregates. Studies by W. Gewoz et al. (1974) and D. Stopur (1979) all show that barium in barite is sprayed to the seabed by the same hydrothermal solution together with metallic substances. Therefore, hot water sedimentary rocks such as layered barite in regional sedimentary rock series can be used as important exploration indicators of jet sedimentary deposits.

Layered siliceous rocks, jasper-like rocks, etc. It is common in jet sedimentary sediments, and extremely fine particles (or aphanitic) form rhythm (stratification) with other sediments in time. The mineral characteristics, oxygen-silicon isotope geochemistry and trace element composition of rocks show that they are deposited by SiO2 _ 2 in the gel-like hydrothermal solution sprayed to the surface. In addition, hydrothermal sedimentary rocks, such as pyroxenite, albite and iron carbonate, are closely related to metal sulfides in jet sedimentary deposits. Exploration practice shows that any one of the above hydrothermal sedimentary rocks is a possible sign of finding jet sedimentary deposits.

(2) Special asymmetric wall rock alteration. Alteration is a remarkable sign of hydrothermal activity. In jet sedimentary deposits, alteration is accompanied by network veins and breccia mineralization, and there are usually silicification, ferrosilicon carbonation (siliceous ankerite), electrochemistry and chloritization (such as Sullivan deposit). Most of them are distributed in the rock of the deposit floor in the "tubular alteration zone". This alteration reflects the metasomatism of hydrothermal solution to surrounding rocks in the ascending channel, and some people think it is the result of hydrothermal solution metasomatism in the underlying ground fracture zone after the formation of layered deposits. The layered mineralization of some deposits is generally not accompanied by alteration, and sometimes there is weak silicification in the chassis. Obviously, the alteration characteristics of this kind of deposits are obviously different from those of ordinary hydrothermal deposits, and they are special. The regional "tubular alteration zone" is an important sign to judge the hydrothermal activity of jet sedimentary deposits.

(3) Mineralization zoning. As mentioned above, the zoning of elements and minerals in submarine jet sedimentary deposits is both horizontal and vertical. According to the distribution law of copper, lead, zinc, manganese and iron in this area, the active center of hot water and its distribution range can be determined, the geochemical halo and mineralization center zone around potential sulfide deposits can be identified, and the location of the deposits can be determined.