Ordos basin is one of the important oil-bearing basins in the central and western China. Yanshan period is the main period of oil and gas migration and accumulation in the basin. Based on the analysis of sedimentary pattern, structural deformation, magma and thermal events from late Mesozoic to late Jurassic to early Cretaceous, it is considered that the basin was mainly controlled by Tethys tectonic domain before late Jurassic. The influence of the ancient Pacific plate on the basin pattern did not appear until the late Jurassic. Deep mantle creep is the main reason for the change of paleogeothermal field in the basin. It not only leads to the sharp rise of geothermal gradient in the basin, but also has some influence on the shallow crustal deformation around the basin in the later period. It is also an important factor that caused the late Mesozoic difference between Shanxi and Ordos basins, and controlled the migration and accumulation of oil and gas. According to the paleostress field, paleogeothermal field and structural deformation characteristics of Yanshan period, Yanshan period can be considered.
Based on the mineralogy, petrology and geochemistry of Cenozoic basalts and their inclusions in eastern China, it is found that the lithosphere thinning is the main feature in eastern China during Yanshan period, and there is a unique geological phenomenon that the asthenosphere mantle directly contacts with the eastern crust. Most of the ancient lithospheric mantle that should have existed earlier ceased to exist due to subsidence, but the main body of lithospheric mantle was formed in the late Yanshan period and later. The essence of Yanshan movement in eastern China is the thinning of lithosphere, even the disappearance of lithospheric mantle. It is considered that the fuse of lithospheric thinning may be related to the subduction of the eastern ocean plate at that time. The dynamic effect of the direct contact between the asthenosphere mantle and the crust is the strong underplating of the magma plate and the accompanying high-temperature metamorphism and partial melting of the deep crust, which leads to the emplacement and eruption of a huge amount of magma. At the same time, this geodynamic process will bring a lot of mantle materials (including ore-forming materials) into the crust, forming large-scale fluid circulation on the crust scale, thus producing large-scale and sudden huge mineralization. Adakite is a set of intermediate-acid igneous rocks, which is characterized by HREE loss and no negative europium anomaly, indicating that its formation depth is deep and garnet remains in the source area. Many Yanshanian intermediate-acid magmatic rocks in eastern China have similar geochemical characteristics to adakite, but their formation environment has nothing to do with subduction. Therefore, this paper divides adakite into O-type and C-type: O-type adakite is rich in Na, and its genesis is related to plate subduction; C-type adakite is rich in K (mostly sodium and a few potassium), which may be the product of partial melting of granulite in the lower crust caused by basaltic magma invading the thickened bottom of continental crust (> 50 km). C-type adakite is instructive to explain many geological phenomena in Yanshan period in eastern China. Because C-type adakite preserves many imprints of the lower crust, it can also be used to invert the composition of the lower crust for exploration.
Uranium is an important strategic and energy resource that affects the peace and development of the world today. The first two decades of this century are a period of rapid development of nuclear power in China. In order to ensure the sufficient supply and reserve of natural uranium for national energy security, strengthening uranium mineralization research, improving research level and establishing metallogenic model are not only major scientific problems faced by nuclear geologists at present, but also of great practical significance and practical value for guiding the strategic deployment of a new round of uranium resources exploration in China. The classification of uranium deposits in China is used to the classification of host rocks, and volcanic type uranium deposits are one of the main proven industrial types of uranium deposits in China. Although the host rocks are different, the geochemical characteristics of uranium determine that uranium mineralization is essentially * * *. Uranium mineralization is a dynamic process of source-transportation-accumulation, and fluid is the main controlling factor that runs through the formation of the deposit. The interaction between water (fluid) and rocks creates the metallogenic process. In the past, the research on uranium mineralization mostly focused on the induction and deduction of observed basic geological phenomena, or the inference of analysis and test data. In the organic whole of source-transport-accumulation mineralization process, "accumulation" is often the main research object, lacking the study of system evolution and dynamic process, and various phenomenological understandings have been obtained on many issues, such as only analyzing test data and only positioning in space. This paper chooses Xiangshan ore field, the largest and richest volcanic uranium ore field in China, for legislative research. Guided by the thought of system science, this paper emphatically analyzes the evolution and dynamic process of important elements that interact and depend on each other in the metallogenic system, and puts forward the concept of post-magmatic hydrothermal system. It is considered that uranium mineralization in Xiangshan ore field is the product of the evolution of hydrothermal system after magmatic period, and the uranium mineralization process is accompanied by the activity of hydrothermal system after magmatic period. This paper discusses the regional geological background and geological characteristics of the ore field. The source of ore-forming materials and ore-forming solution are discussed. The groundwater flow system and flow form in Xiangshan volcanic basin during mineralization period are discussed. Based on the study of ore-forming material enrichment, ore-forming fluid migration and ore-forming material accumulation system in hydrothermal system after volcanic magmatism, the dynamic process of formation and evolution of ore-forming fluid system is analyzed. Finally, the uranium metallogenic model of Xiangshan ore field is established, and the prospecting direction of the ore field is reviewed. 1. Uranium mineralization in Xiangshan ore field is the product of post-volcanic magmatic period controlled by the evolution of regional tectonic environment. Uranium mineralization in Xiangshan ore field is the product of a stage (period) in the dynamic process of hydrothermal (fluid)-rock interaction in the post-volcanic magmatic hydrothermal system under the regional geological background and specific geological structural environment of Xiangshan volcanic basin. Volcanic magmatism under the geodynamic background of extension or transition to extension provides a material energy field for mineralization. The study shows that uranium mineralization began after large-scale volcanism, and the time span is as long as 50Ma. Mineralization is a relatively continuous process in time. However, the main metallogenic periods of ore fields can be divided into early and late stages. Alkaline sodium metasomatic uranium mineralization was formed in the early stage, and the time difference between ore and rock was small. Acid fluorite-hydromica type uranium mineralization was formed in the late stage, and the time difference between ore and rock is large. In space, the early uranium mineralization mainly occurs in the granite porphyry in the north of the ore field and its internal and external contact zone, and the late uranium mineralization mainly occurs in various structures and their composite parts in the volcanic rocks in the west of the ore field. It can be seen that the high-intensity mineralization of Xiangshan ore field in different periods occurred in a certain time and space, which is the objective product of the evolution of hydrothermal system after volcanic magmatism and closely related to the regional tectonic environment. 2. Discussion on the source of ore-forming materials in Xiangshan ore field The source of ore-forming materials in Xiangshan ore field has always been a controversial issue. For a long time, in the process of uranium source analysis, people often focus on the "subsidence" area, lacking the analysis of the migration process and historical evolution of regional ore-forming materials, and infer the source of minerals from various test data in the "subsidence" area. Based on the temporal and spatial distribution characteristics of regional ore-forming materials, this paper analyzes its indicative significance to uranium source, and discusses the source of ore-forming materials in the ore field by using the geochemical research results of isotopes and trace elements, and draws the following conclusions: (1) The distribution characteristics of uranium content in regional strata and the characteristics of material migration and change in geochemical process show that the early Cambrian strata constitute the regional uranium source bed. (2) The uranium content of magmatic rocks has the following characteristics: in the same tectonic movement, the uranium content of remelted granite in South China is higher than that of syntectic granite; Caledonian and later granites have high uranium-like content; The uranium content of Mesozoic intermediate-acid volcanic rocks is higher than that of intermediate-basic volcanic rocks. It can be seen that the uranium content of magmatic rocks is closely related to the source of diagenetic materials, and the melting of regional uranium source beds is the fundamental reason for the difference of uranium content in magmatic rocks. (3) The intermediate-acid volcanic rocks in Xiangshan are formed by deep melting of continental crust materials, and their uranium content is higher than the average uranium abundance of regional continental crust and the uranium content of sedimentary strata in Xiangshan area after Sinian and Middle Cambrian. Combined with the distribution characteristics of uranium content in regional magmatic rocks, it is considered that the contamination of regional uranium-rich horizons (∈_ 1) leads to the high uranium content in Xiangshan volcanic rocks. Based on this, Xiangshan volcanic basin is the "sink" area of ore-forming materials, the regional uranium-rich layer is the most fundamental source of ore-forming materials, the volcanic magma activity process is the accumulation process of ore-forming materials, and volcanic magma and late hydrothermal solution are the carriers of ore-forming materials migration. (4) The geochemical characteristics of trace elements in rocks and ores show that basement schist and rhyolite dacite provided some ore-forming materials during uranium mineralization. 3. Judging the source of ore-forming solution in Xiangshan ore field is an important basis for judging the source of ore-forming solution. In this paper, it is recognized that fluid inclusions are "fossils" of ore-forming solutions, and the isotopic composition of ore-forming solutions will be influenced by water-rock exchange during evolution, temperature and pressure conditions during exchange, and isotopic composition of rocks. Therefore, on the basis of understanding the basic characteristics of fluid inclusions and combining with the geological and geochemical characteristics of mineralization, the source of ore-forming solution can be judged, rather than simply comparing with data. (1) The chemical composition, salinity, temperature and pressure values of ore-forming fluids in different time and space stages in Xiangshan ore field are also different. The early ore-forming fluid was relatively high temperature, high pressure and high salinity, while the late ore-forming fluid was relatively low temperature, high pressure and low salinity. (2) The δ~( 18)O(H2O) of the solution in Xiangshan ore field shows a downward trend, which can be explained by more and more precipitation components. The hydrogen and oxygen isotopic composition of ore-forming solution can be divided into two groups. One group's δD is about -60‰, and the other group's δD is about -80‰. The former corresponds to fluorite-hydromica mineralization and the latter to sodium metasomatism mineralization. (3) In the triangle diagram of rainwater, seawater and magmatic water, the hydrogen and oxygen isotopic composition of the ore-forming solution is located between the magmatic water area and the rain line, on the connecting line representing the magmatic water and rainwater components of ore-bearing igneous rocks. The rainwater end-member isotopic composition of early ore-forming solution is the average hydrogen and oxygen isotopic composition of rainwater, while the rainwater end-member of late ore-forming solution is the hydrogen and oxygen isotopic composition of Mesozoic rainwater. Obviously, the early and late ore-forming solutions are a mixture of magmatic water and rainwater, but this does not mean that the exogenous water circulation caused by rainwater directly enters the ore-forming solution, but only proves that there is rainwater in the ore-forming solution. (4) The basic characteristics of ore-forming fluids and the hydrogen and oxygen isotopic composition of ore-forming solution indicate that the ore-forming fluids come from different ore-forming stages. Based on the metallogenic geological characteristics of the ore field, it is considered that the source of the early metallogenic solution is mainly the post-magmatic hydrothermal solution, and its rainwater component comes from the melting of rocks containing Proterozoic, Paleozoic and Mesozoic sediments during volcanic magmatism and enters the magma, so its rainwater terminal shows the average isotopic composition of rainwater; The source of late-stage ore-forming solution is the post-magmatic hydrothermal solution of protomagmatic belt-high magma chamber-volcanic tectonic system. Due to the decrease of temperature and pressure and the mixing of condensed water vapor solution and precipitation, the proportion of precipitation may be obviously larger than that of early-stage ore-forming solution. Therefore, the rainwater endmembers are composed of isotopes of Mesozoic rainwater. 4. Based on the geochemical characteristics of rare earth elements, quantitative calculation of material migration in altered rocks and geochemical simulation calculation of water-rock interaction, combined with the source of ore-forming materials and ore-forming solutions, the migration process of ore-forming materials, which was relatively weak in previous studies, was analyzed. The geochemical characteristics of (1) rare earth elements show that the magma formed by deep paleocontinental crust materials is not only contaminated by uranium-rich strata, but also highly fractionated and crystallized, which leads to the migration of uranium melted and accumulated in magma to hydrothermal solution at the end of magma evolution, and magma and hydrothermal solution at the later stage are the carriers of uranium. The uranium content in the original magma of Xiangshan volcanic rock is obviously higher than that in the late magma chamber intergranular magma, which also shows that uranium was transferred from magma to gas and liquid during the evolution of magma. (2) The quantitative calculation results of material migration in altered rocks show that the quality changes of ore-forming elements brought in and brought out are not significant, which not only supports the analysis conclusion of the source of ore-forming materials, but also shows that volcanic magma and hydrothermal solution evolved after the period are the carriers of ore-forming materials, that is, the ore-forming materials are migrated through magma melting. (3) By comparing the trace element combination characteristics of rocks and ores in Xiangshan volcanic basin, it is considered that the interaction between basement schist and rhyolitic dacite and post-magmatic hydrothermal solution may prompt them to provide some uranium for post-magmatic hydrothermal solution rich in CO2 gas. The geochemical simulation results of water-rock interaction at metallogenic temperature also show that the fluid rich in CO2 gas is beneficial to uranium migration in volcanic rocks and metamorphic rocks. 5. Since the exogenous groundwater entered the ore-forming solution in the form of "turbulence" during the ore-forming period, the tectonic-hydrogeological pattern in Xiangshan area has not changed fundamentally. Based on the basic theory of gravity cross-layer groundwater flow, the groundwater flow field caused by atmospheric precipitation in Xiangshan area during mineralization period is described, and the groundwater flow network of typical sections during mineralization period is delineated. The predecessors did not explain how the exogenous groundwater driven by gravity potential entered the ore-forming fluid with relatively high temperature and pressure. Based on the pressure values of ore-forming fluids and vapor-liquid inclusions, this paper estimates whether precipitation driven by paleogeographic situation can enter the paleogeographic differentiation of ore-forming fluids in the form of convection, and the results are contrary to the paleogeographic situation during the metallogenic period. It can be inferred that the movement form of exogenous groundwater entering the ore-forming hydrothermal solution during the ore-forming period is a "turbulent" movement driven by the ground situation, the residual heat of magma and the temperature and pressure gradient of high temperature and high pressure fluid. In fact, the existence of inclusions with different temperature and pressure values in the deep narrow space of Julongan deposit in Xiangshan ore field also provides evidence for exogenous groundwater to enter the metallogenic solution in the form of "turbulence". 6. The evolution of hydrothermal system after magmatic period in Xiangshan ore field is helpful to uranium mineralization. Based on the above research, this paper puts forward the concept of post-magmatic hydrothermal system, and holds that the activity and evolution of post-magmatic hydrothermal system closely related to the evolution of regional tectonic environment contributed to the 50Ma uranium mineralization in Xiangshan ore field. The enrichment process of ore-forming materials in Xiangshan volcanic basin (1) includes three stages: the first stage occurs when the deep materials in the continental crust form primitive magma; The second stage mainly occurred in the high-level magma chamber and the stage of magma full evolution and release; The third stage is mainly caused by fluid (water)-rock interaction several to tens of millions years after volcanic magmatism. The first stage laid a metallogenic foundation for the formation of Xiangshan uranium ore field, and the second stage was a prelude to mineralization. The uranium released by the full evolution of magma can directly provide ore-forming materials for the early uranium mineralization in Xiangshan. The water-rock interaction in the evolution of hydrothermal system after the third magmatic period promoted the further enrichment of uranium in the basement strata and rhyolitic dacite in Xiangshan Basin as ore-forming fluid. (2) Ore-forming fluids migrated to Xiangshan volcanic basin. The main motive force of hydrothermal activity after the volcanic magmatic period is thermal drive, which comes from the high magma chamber and obtains the energy supplement of the original magmatic belt. According to the comprehensive analysis of the spatial output characteristics and morphological characteristics of the alteration zone and ore body in Xiangshan ore field, it is considered that the thermal drive makes the fluid flow upward, that is, the migration direction of the ore-forming fluid is from bottom to top, and the volcanic collapse structure and fault structure which are interconnected with the volcanic basement structure are the main channels for the migration of the ore-forming fluid, and the temperature gradient is the main driving force for the migration of the ore-forming fluid. In the center of fluid activity (where fluid activity is strong), the water pressure generated by fluid pressure breaks, forming a dense fracture zone on the structural side. (3) Evolution of ore-forming fluid system After regular volcanism in Xiangshan volcanic basin, hydrothermal solution interacted with rocks at the end of magmatic evolution, resulting in hydromica and albitization in the early stage of mineralization. At the same time, the composition of the solution changed, and the alkalinity of the solution increased, which led to the merger of Fe3+ and OH-, which made the rock turn red, thus forming a uranium-hematite type with small time difference between ore and rock. After that, the post-magmatic hydrothermal activity of volcanic magma containing mantle-derived gas, controlled by the original magmatic belt-high-position magma chamber-volcanic genesis construction system, increased, and it moved in the direction of decompression (upward) along the structural channel, resulting in hydraulic fracturing. In the process of interaction between hydrothermal solution and rock after volcanic magmatism, acid-base separation formed alkaline alteration zoning under the upper acidity, so the ores formed by fluorine-rich acidic ore-forming fluids with late mineralization time and low mineralization temperature were found in the current exploration depth in the west of the ore field, namely uranium-fluorite type and uranium-sulfide type ores. It can be seen that the fluid (water)-rock interaction related to Xiangshan volcanic magmatism promoted the evolution of ore-forming fluids after volcanic magmatism and created the uranium mineralization process. 7. Establishment of uranium metallogenic model in Xiangshan ore field and comments on deep prospecting direction. According to the metallogenic geological characteristics of the ore field, the metallogenic model of Xiangshan ore field is established by comprehensively considering the source and enrichment process of ore-forming materials, the evolution of ore-forming fluid system, the migration of ore-forming fluids, the migration form and precipitation mechanism of ore-forming materials, and the direction of further prospecting is reviewed. The metallogenic model emphasizes that: ① the diagenetic process of Xiangshan volcanic rocks is accompanied by the enrichment process of ore-forming materials; ② The early uranium ore-forming fluid evolved from the interaction between post-magmatic hydrothermal solution and rock, and the ore-forming materials were mainly provided by post-magmatic hydrothermal solution: ③ The late uranium ore-forming fluid contained Mesozoic atmospheric precipitation, and the fluid (water)-rock interaction promoted the evolution of the ore-forming fluid, and basement metamorphic rocks and rhyolitic dacite also provided some uranium sources for mineralization; (4) Steep-dip faults and fracture-dense zones on both sides are not only the migration channels of ore-forming fluids, but also the places where minerals are deposited; ⑤ Uranium precipitation in Xiangshan ore field is mainly the result of coupling of ore-forming mechanisms such as fluid cooling, concentration and mixing. Through in-depth analysis of the uranium mineralization process in the ore field, it is considered that the north and west are still important targets for future exploration. The main direction in the north is to find early mineralization controlled by granite porphyry and its internal and external contact zones, and the exploration depth is increased. In the west, the main direction is to find the late uranium mineralization of all levels of structures and their composite parts in volcanic rocks, and the trinity area of alteration field, structure and turbulent space domain is an important exploration area; In addition, according to the acid-base separation caused by the seepage of ore-forming fluid, this paper puts forward that the exploration of the inner and outer contact zones of granite porphyry in the western part of the ore field should be strengthened, similar to the early uranium mineralization in the northern part of the ore field, and the identified acidic uranium mineralization parts in volcanic rocks can be deeply explored.