Study On Water-Rock Interaction And Its Relation With Uranium Metallogenesis

Uranium is an important resource of strategy and energy which influences the peace and development in present world. Nuclear power in our country is rapidly developing during the first 20 years of this century. In order to ensure adequate supply and reserve of natural uranium, enhancing study of uranium metallogenesis, increasing research level and establishing metallogenic modle are not only great scientific problem that nuclear geologists encounter now, but also have important realistic significance and practical value to guide strategical planning of a new round uranium source exploration in China.Uranium deposits in China are used to classified according to host rocks. Volcanic type uranium deposit is one of main industrial types of uranium deposits which are discovered in China. Although host rocks are different, it is basically common that property of uranium geochemistry determines uranium metallogenesis which is a dynamics process of source-migration—accumulation. Fluid is the chief control factor through the whole process of deposit formation. Metallogenic process results from water (fluid) -rock interaction. The previous study on uranium metallogenesis was mainly induction and deduction of basic geological phenomena observed, or excessively emphasis on extrapolation of test and measure data, accumulation was often study object in organic integrity of "source-migration-accumulation" metallogenic process, systematic evolution and dynamics process are merely studied. Conclusions on many problems are obtained on phenomenology which are controversy, based on test data and space position.This paper chooses Xiangshan orefield, the largest and richest volcanic type uranium orefield in China to classical research. The paper is guided under systematic scientific thought, emphasizes evolution and dynamics process analysis of source, migration and accumulation which are key factors in metallogenic system, and puts forward concept of metallogenic hydrothermal solution in post volcanic magma. Metallogenesis in Xiangshan orefield is the product of metallogenic hydrothermal solution system evolution in post volcanic magma, uranium metallogenic process acompanies with activity of hydrothermal solution system in post volcanic magma period. The paper discusses regional geological background and geological characteristics of the orefield, confers sources of metallogenic substances and fluids, discusses flow system and way of underground water in Xiangshan volcanic basin at metallogenic stage. Dynamics process of metallogenic fluid system formation and evolution is analysed on the basis of systematic study on metallogenic substance enrichment, metallogenic fluid movement and metallogenic substance accumulation. Uranium metallogenic model is finally established for Xiangshan orefield, direction for further exploration is reviewed for the orefield.1. Uranium metallogengsis in Xiangshan orefield is product of post volcanic magma constrainted by regional tectonic environment evolutionUranium metallogenesis in Xiangshan orefield is a stage product of hydrothermal solution (fluid)-rock interaction dynamics process in hydrothermal solution system of post volcanic magma at regional geological background and specific geological tectonic environment of Xiangshan volcanic basin. Metallogenic substance-energy field is supplied by volcanic magmatism at earth dynamic background controlled by extension. The research indicates that large volcanism is followed by uranium metallogenesis with time span of 50 Ma. Metallogenesis is a relatively continuous process, but main metallogenesis in the orefield can be divided into two stages. The early stage formed uranium mineralization of alkalic sodium metasomatic type with little time difference between rock and ore, the late stage formed acidic fluorite-hydromica type of mineralization with large time difference between rock and ore. In space, uranium mineralization in early stage is mainly located in granite-porphyry and its both inner and outer contact zones in north orefield, uranium mineralization in late stage is chiefly located into structures and their composites of volcanic rocks in west orefield. Therefore, high-intensity metallogenesis of different stages in Xiangshan orefield correspondingly occurred in same time-space domain, was the objective product of hydrothermal solution system evolution of post volcanic magma which has close relation with regional tectonic environment.2. Discussion of source of metallogenic substances in Xiangshan orefieldSource of metallogenic substances in Xiangshan orefield is always a controversial problem. For long time, analysis of uranium source was focused on accumulation without analysis of migration process and historic evolution of regional metallogenic substance. Various test and measurement data from "accumulation" are induce source of metallogenic substances. On tha basis of temporal and spatial distribution characteristics of regional metallogenic substances, the paper analyzes indication significance of uranium source, discusses metallogenic substance source in the orefield with application of research results of isotope and trace element geochemistry. The conclusions are following:(1) Distribution characteyistics of uranium content in regional strata and variety characteristics of substance migration in geochemistry process show that regional uranium source beds consist of Lower Cambrian.(2) Magmatic rock bearing uranium has the following characteristics: uranium content in anatexis type granite than syntexis type granite in the same tectonic region; uranium content is higher in Caledonian and later granites; Mesozoic intermediate-acidic volcanic rock has higher uranium content than intermediate-basic volcanic rock. It is shown that uranium content in magmatic rock has close relation with source of petrogenic substance. Melting of regional uranium source beds is root reason resulting in uranium content difference of magmatic rocks.(3) Intermediate-acidic volcanic rock in Xiangshan is formed by anatexis of deep continental crust substances which has higher uranium content than average uranium abundance of regional continental crust, is also higher than uranium content of Sinian and Post middle-cambrian strata.Combining with distribution characteristics of uranium content of regional magmatic rock, it is considered that hybrid of regional uranium bearing strata (∈₁) result in high uranium content of Xiangshan volcanic rock. Therefore, Xiangshan volcanic basin is the accumulation area for metallogenic substances, regional uranium source beds are basical source of metallogenic substances, volcanic magmatism is the accumulation process of metallogenic substances, volcanic magma and later hydrothermal solution are migration media of metallogenic substances.(4) Geochemical characteristics of trace elements in both rock and ore show that schist of basement and rhyodacite provide some portions of metallogenic substance source during uranium metallogenesis.3. Determination of metallogenic solution source in Xiangshan orefieldCompositions of H and O isotopes in metallogenic solution are important basis for determination of metallogenic solution source. The paper recognized that fluid inclusion is a "fossil" of metallogenic solution. Isotopic composition of metallogenic solution is influenced by water-rock exchange, temperature of exchange and isotopic composition of rocks, etc. Therefore, determination of metallogenic solution source is based on basic property of fluid inclusion, combined with metallogenic geology and geochemical characteristics, is not simply data correlation.(1) Chemical composition, salinite, temperature and pressure of metallogenic fluid varies at different temporal and spatial stage in Xiangshan orefield. Metallogenic fluid in early stage had relative high temperature, high pressure, high salinity, while temperature, pressure and salinity were relative low in metallogenic fluid in late stage.(2)δ¹⁸O_H2O in solution has tendence of decline from pre-mineralization stage to post-mineralization stage in Xiangshan orefield. This phenomenon is caused by portion increase of meteoric water. Solution isotopic composition of H and O in metallogenic stage can be divided into two groups: one group has about -60%δD, the other -80%δD. The former is corresponding to fluorite-hydromica type mineralization; the later corresponding to sodium metasomatic type mineralization.(3) In triangular diagram of rainwater,seawater and magmatic water, H and O istopic compositions of metallogenic solution are between magmatic water regian and rainwater line, are on alignment representing magmatic water of igneous rock and rainwater. Isotopic composition of rainwater end-member in early metallogenic solution is the average of H and O isotopic composition of rainwater, while rainwater end-member of late metallogenic solution is H and 0 isotopic composition of Mesozoic rainwater. This shows that metallogenic solution of both early and late stages are mix of magmatic water and rainwater, but it does not mean that circulation of epigenic water produced by rainwater did not directly enter into metallogenic solution, it improves that rainwater portion existed in metallogenic solution.(4) Basic property of metallogenic fluid and H,O isotopic compositions of metallogenic solution show that metallogenic fluid sources of mineralization types in different stages are different. Metallogenic solution source in early stage is hydrothermal solution of post volcanic magma, its rainwater portion source is rocks including Proterozoic, Palaeozoic and Mesozoic meteoric water, therefore its rainwater end-member is average isotopic composition of rainwater. Metallogenic solutionsource is mix of hydrothermal solution of primary magma generation Zone-high position magma chamber-volcanic construction system and meteoric water, water-gas solution condensed due to temperature and pressure decrease, portion ratio of meteoric water may be obviously larger than metallogenic solution of early stage because rainwater end-memberi is Mesozoic rainwater isotopic composition.4. Analysis of metallogenic substance migration processMetallogenic substance migration process is analyzed by study on REE geochemical characteristics, quantitative calculation of substance migration in altered rock and geochemical simulation calculation of water-rock interaction, combining with source of metallogenic substance and solution.(1) REE geochemical characteristics indicate that in the upward-risen way magma formed by anatexis of deep older continental materials did not only mix with uranium bearingstrata, but also had crystallization of high intensive differentiation which results in uranium migration from magma to latest hydrothermal solution. Magma and the later.hydrithermal solution are media of uranium. Uranium content in preliminary magma of Xiangshan volcanic rock is obviously higher than in magma of magmatic chamber. This shows that uranium transfers from magma to gas and liquid during magmatic evolution.(2) Quantitative calculation result of substance migration in altered rock demonstrates that quantity of inflow and outflow of metallogenic elements varies little. This supports the analyse conclusion of metallogenic substance source, also indicates that volcanic magma and later hydrothermal solution are media of metallogenic substance, which migrates in magmatic dissolution.(3) REE association characteristics correlation of rock and ore in Xiangshan volcanic basin shows that interaction of basement schist and rhyodacite with hydrothermal solution of post magma can provide some uranium for metallogenic hydrothermal solution with rich CO₂ gas in post volcanic magma. Geochemical simulation calculation result of water-rock interaction at temperature condition of metallogenesis also shows that fluid with rich CO₂ gas conduces to uranium migration from volcanic rock and metamorphic rock.5. Epigenic groundwater enters into metallogenic solution in "turbulence" movement way during metallogenic stageTectohydrogeological frame has not basically changed in Xiangshan area since metallogenic stage. With application of basic theory of gravity penetrating groundwater flow, groundwater flow field originated from meteoric water and controlled by palaeotopography at metallogenic stage in Xiangshan area is specified, groundwater flow net at metallogenic stage in typical section is also described.What kind of movement way did epigenic groundwater driven by gravity potential enter into metallogenic fluid with relative high temperature and high presure? There was no explanation for it. Can meteoric water driven by palaeotopographic potential enter into metallogenic fluid in convectional movement way?The paper calculates differentiation of the palaeotopography according to pressure value of metallogenic fluid and pressure value of gas-liquid inclusion, the result is contradictory to the palaeotopography at metallogenic stage. So it is conferred that movement way in which epigenic groundwater enters into metallogenic solution at metallogenic stage is "turbulence" movement driven by topographic potential, magmatic residual heat and gradients of temperature and pressure of high temperature and high pressure fluid. In fact, inclusions with different temperature and pressure valves coexist together in a deep and very small room in Julongan deposit of Xiangshan orefield. This also provides evidence that epigenic groundwater enters into metallogenic solution in "turbulence" movement.6. Uranium metallogenesis in Xiangshan orefield results from systematic evolution of metallogenic hydrothermal solution in post volcanic magma stageIn this paper, concept of metallogenic hydrothermal solution system in post volcanic magma is put forward on the basis of above researches. Uranium metallogenesis with a period of 50 Ma in Xiangshan orefield result from evolution and activity of hydrothermal solution system in post volcanic magma and in close relation with evolution of regional tectonic environment.(1) Enrichment process of metallogenic substance in Xiangshan volcanic basin consists of 3 stages: the first stage occurred when deep continental materials formed primary magma; the second stage occurred at full evolution and relief of magma and upper magmatic chamber; the third stage resulted from fluid (water) -rock interaction of several or dozens Ma in post volcanic magma. Metallogenic substance basis of Xiangshan orefield was established in the first stage. The second stage was the prelude to metallogenesis, uranium released by full evolution of magma can directly provide metallogenic substance for the early uranium metallogenesis in Xiangshan; in the third stage ,water-rock interaction in evolution process of post volcanic magma caused uranium from basement strata and rhyodacite in Xiangshan volcanic basin further enriching in metallogenic fluid.(2) Metallogenic fluid migration main force of hydrothermal solution activity in post volcanic magma in Xiangshan volcanic basin is driven by thermo which came from upper magma chamber where energy from primary magma generation was refilled. Based on comprehensive analysis of characteristics of alteration zone, orebody spatial distribution and forms, thermal drive is considered to force fluid into upwelling , motion direction of metallogenic fluid is from down to up, volcanic collapse structure and fault structure interconnecting with volcanic basement structure are main channel for metallogenic fluid migration, temperature gradient is chief driving force for metallogenic fluid migration. The space for mineralised material precipitation in Xiangshan orefield is provided by dense fractures in sides of a fault cracked by water pressure generated by fluid pressure.(3) Evolution of metallogenic fluid system after large volcaniosm in Xiangshan volcanic basin, the latest hydrothermal solution in magmatic evolution interacts with rocks to form hydromicazation and albitization. While rock chemically changes solution composition varies, alkali in solution increases to combine Fe³⁺ with OH^-, so rock becomes red to form U-hematite type ore with little time difference between ore and rock. Then activity of hydrothermal solution with mantle-derived gas composition in post volcanic magma enhances, the hydrothermal solution migrates upward along fault channel, water pressure cracks. Acid-alkali separation results in alteration zoning of upper acid and lower alkali. U-fluorite type and U-sulfide type ore formed by metallogenic fluid with rich fluor acid in relative low temperature and late time of metallogenesis in west orefield. So interaction of rock-fluid (water) which is close relation with volcanic magmatism facilitates evolution of metallogenic fluid in post volcanic magma, and produces process of uranium metallogenesis.7. Establishment of uranium metallogenic model and discussion of further exploration direction in Xiangshan orefieldMetallogenic model is established and further exploration direction is discussed in Xiangshan orefield on basis of metallogenic geological characteristics of the orefield, source and enrichment of metallogenic substance in metallogenic process, systematic evolution and migration of metallogenic fluid, migration mode and precipitation mechanism of metallogenic substance, etc.The metallogenic model emphasizes:â‘ Valcanic diagenetic process in Xiangshan accompanies enrichment process of metallogenic substances;â‘¡Early uranium metallogenic fluid is formed by interaction of rock with hydrothermal solution of post magmatism, metallogenic substances are provided by hydrothemal solution of post magmatism;â‘¢Late uranium metallogenic fluid contains composition of Mesozoic meteoric water, fluid (water) -rock interaction promotes evolution of metallogenic fluid, basement schist and rhyodacite also provide some uranium source for metallogenesis;â‘£High-angle fault and its lateral zones of dense fractures are not only migration passage of metallogenic fluid, but also room for precipitation of metallogenic substances;⑤Uranium precipitation in Xiangshan orefield is coupling result of metallogenic mechanism of fluid cooling, concentrating and mixing, etc.Through deep analysis of metallogenic process, north and west of Xiangshsn orefield are considered as important targets for exploration in the futune. Main efforts and deep exploration are focused on early mineralization controlled by granite-porphyry and its both inner and outer contact zones in north of Xiangshan orefield. Main efforts are focused on late uranium mineralization locating within all kinds of structures and their conjuctions. The spatial trinity of alteration field, structure and "turbulence" is important area for exploration. Moreover, according to acid-alkali separation caused by permeation effect of metallogenic fluid, this paper puts forwart that exploration should be strengthened in inner and outer contact zones of granite-porphyry in west of xiangshan orefield to look for early uranium mineralization, similar to north of Xiangshan orefield. The exploration shall go further in depth for acidic uranium mineralization positions discovered in volcanic rock.