Speciation and Precipitation of Uranium Complexes in Hydrothermal Solutions Related to Granite—type Uranium Deposits

Uranium-bearing hydrothermal solutions during the stage of ore deposition are weakly alkaline and of the Ca^2 -Na^ /HCO3^- -F^- type.UO2(CO3)2^2- and UO2F4^-, are dominant in the hydrothermal solutions with respect to their activity.Wall-rock hydrothermal alterations ,temperature and pressure drop and the reducing capability of rock assemblage (Δeh) led to a decrease in Eh of the hydrothermal solutions and an increase in Eh at which uranium began precipitating.Therefore,the mechanism of uranium precipitation is essentially the reduction of uranium complexes.The granite-type uranium deposits are the most important type of uranium resources in China.Discussions will be made in this paper concerning the hydrothermal speciation and precipitation mech-anisms of uranium complexes in the light of fluid inclusion and geological data from some major de-posits of this type in South China.     

Geochemical Characteristics of the Sandstone—type Uranium Deposits in Northern Sichuan Province

Described in detail in this paper are the geochemical characteristics of the sandstone-type uranium deposits in northern Sichuan Province .Favorable and unfavor-able conditions for the enrichment of uranium are explored on the basis of the elemental abundances and ratios of U,Th and K determined by gamma-ray spectroscopy. Gamma-ray spectroscopic(U, Th and K) and XRF analyses(As, Ba) are helpful to distinguish U-bearing light-colored sandstones from U-barren light-colored ones and red sandstones (red claystone) from light-colored sandstones. Therefore, the favorable target area for prospecting the sandstone-type uranium deposits can be difined in northern Sichuan Province.     

The Geology and Geochemistry of Porphyrite Iron Deposits in the Nanjing_Wuhu Area,Southeast China

For the iron deposits occurring in andesitic volcaic rocks of the Lower Yangtze Area.the genetic model for porphyrite iron deposits was proposed by chinese geologists more than ten years ago on the basis of their detailed studies in the Nanjing-Wuhu Basin.It comprises a set of deposits of different genetic types ranging from late magmatic segregation ,ore-magma injection,pneumato-hydatogenetic replacing and hydrothermal filling as well as sedimentary origin.The deposits are closely connected with the gabbro-diorite porphyrite subvolcanic intrusive bodies both in space and in genesis.Miner4alization and wall-rock alteration are consistent with the history of the magmatic evolution.Geochemical studies on trace elements and S,O,Sr isotopes have proved that the porphyrite iron deposits are of magmatic origin,The proposed model may be applied to iron ores associated with andesitic volcanites,for example,in Chile,Mexico,Pakistan,Turkey,etc.     

Metallogenesis of Devonian—Carboniferous Strata—bound Carbonate—type Uranium Deposits in South China

This paper deais with the geological conditions.mineralization characteristics,genetic types and space-time distribution of the Devonian-Carboniferous strata-bound carbonate-type uranium deposits in South China.These ore deposits are genetically classified as the leaching type and the leaching-hydrothermal superimposed type,These ore deposits are confined mainly to the strata (D2-3,C1)of platform-lagoon carbonate facies.Unique tectonic settings are a vital factor leading to the formation of these uranium deposits.A metallogenetic model for these uranium deposits has been proposed.     

A New Genetic Type of Gold Deposits-Meso-Epithermal Carbonate-Type Gold Deposits as Exemplified by the Baguamiao Suprlarge Gold Deposit

Gold deposits of the meso-epithermal carbonate type were first proposed based on the study of the Baguamiao gold deposit.This new type of gold deposits has many unique characteristics as follows:(1)Obviously strata-bound.The gold deposits are hosted in Middle Devonian turbidite formations;(2)Structrually controlled.Struc-ture is an important factor leading to metallogenesis of this type of gold deposits.The shape and distribution of orebodies are controlled by byittle-ductile shear zones;(3)Multi-stage wall-rock alteration.According to the characteristics of mineral assemblage,gold mineralization can be classified into three stages in association with various wall-rock al-terations.Wall-rock alterations closely genetically related to the gold mineralization are ankerization ,silicification,pyrrhotization and pyritization ;(4)Mineral compositions of the orebodies are mainly pyrrhotite,pyrite,marcasitolite,chalcopyrite,quartz,ankerite,and sericite.Gold mineralization is associated closely in space and time with iron sulfides;(5)Rare elements and REE in ores are low in contents relative to those of the crust.Au content varies from 1.91g/t to 11.15g/t ,averaging 5.5g/t;(6)Studies of sulfur,hydrogen,oxygen and carbon isotopes in main gangue minerals (quartz and ankerite)indicate that fluids and ore-forming materials came from deep-seated sources;(7)Three types of inclusions are recognized in terms of their composition and the vapor amounts of inclusions.The homogenization temperatures of inclusions range from 210℃to 310℃,averaging 230℃,showing that this type of gold deposits belongs to the meso-epithermal type;(8)Metallogenic age of this type of gold deposits is similar to that of the collision between the Yangtze Plate and the North China Plate,indicating that gold deposits of this type are genetically related to continental-margin plate activity.     

Assessment of Prospecting Potentiality for Superlarge Continental Volcanic Rock—Type Uranium Deposits in China

The superlarge continental volcanic rock-type uranium deposits,which were discovered abroad long ago,have not ye been reported up to now in China.This is an important problem that needs to be urgently solved by uranium geologists at present.In this paper,on the basis of analyzing the metallogenic settings and geological conditions of the superlarge continental volcanic rock-type uranium deposits discovered in the world along with the metallogenic characteristics of those of the same type in China,the space-time distribution patterns of continental volcanics and the metallogenic potential of main tectono-volcanic belts in China are discussed,and a synthetic conclusion has been drawn that there is a possibility to discover the superlarge continental volcanic rock-type uranium deposits in China.Moreover,it is evidenced that the Ganhang,Nanling,Yanliao,Da Hinggan Ling and other tectono-volcanic belts possess favorable geological conditions for the formation of ssuperlarge ore deposits of the continental volcanic rock type.The intersecting and overlapping locations of the aforementioned main belts with other tectono-volcanic(-intrusive)belts are the most potential areas where the superlarge continental volcanic rock-type uranium deposits would be found.     

Multistage Enrichment of the Sawafuqi Uranium Deposit: New Insights into Sandstone-hosted Uranium Deposits in the Intramontane Basins of Tian Shan, China

Meso-Cenozoic intracontinental orogenic processes in the Tian Shan orogenic belt have significant effect on the sandstone-hosted uranium deposits in the intramontane basins and those adjacent to the orogen. The Sawafuqi uranium deposit, which is located in the South Tian Shan orogenic belt, is investigated to reveal the relationships between uranium mineralization and orogenies. Recent exploration results show that the Sawafuqi uranium deposit has tabular, stratiform, quasi-stratiform, and lens-like orebodies and various geological characteristics different from typical interlayer oxidation zone sandstone-hosted uranium deposits. Systematic studies of ore samples from the Sawafuqi uranium deposit using a variety of techniques, including thin section observation, α-track radiograph, electron microprobe and scanning electron microscope, suggest that uranium mineralization is closely related to pyrite and organic matter. Mineralization-related alterations in the host rocks are mainly silicification and argillation including kaolinite, illite(and illite-smectite mixed layer) and chlorite. Tree stages of mineralization were identified in the Sawafuqi uranium deposit:(i) uranium-bearing detritus and synsedimentary initial pre-enrichment;(ii) interlayer oxidization zone uranium mineralization; and(iii) vein-type uranium mineralization. The synsedimentary uranium pre-enrichment represents an early uranium enrichment in the Sawafuqi uranium deposit, and interlayer oxidation zone uranium mineralization formed the main orebodies, which are superimposed by the vein-type uranium mineralization. Combining the results of this study with previous studies on the Meso-Cenozoic orogenies of South Tian Shan, it is proposed that the synsedimentary uranium pre-enrichment of the Sawafuqi uranium deposit was caused by Triassic Tian Shan uplift, and the interlayer oxidation zone uranium mineralization occurred during the Eocence-Oligocene period, when tectonism was relatively quiet, whereas the vein-type uranium mineralization took place in relation to the strong orogeny of South Tian Shan since Miocene.     

南岭两类铀矿床的地质地球化学特征及成因探讨

Occurring in the northern part of the Narling region, the deposits under investigation are multimetal sulfide uranium deposits of hydrothermal. (medium to low temperature)origin, which can be divided into two broad types: quartz sulfide and carbonate sulfide.Evidence shows that uranium mineralization is genetically connected with Early Yenshanian granite, quartz porphyry and granodiorite. The author suggests that the regional source-beds may be the main provider Of uranium for such sulfide-type U-deposits (the source-beds include the old metasedimantary rocks, especially those of the Cambrian and Carboniferous). The mechanism of uranium mineralization can be explained as follows: In the process of migmatization, U became highly activated and took its way into granitic magma. In response to the evolution of late orogenic cycle, U was concentrated into the younger small rocks masses, transported in the form of U ^6-F-carbonate complex and uranyl-silicate complex, or complex anions, and finally deposited in favourable structural settings. In the course of U transportation, the interaction took place between wall rocks and hydrothermal solutions containing uranium, leading to the escape of CO2 with decreasing temperatures and pressures. Under such circumstances, the complexes were decomposed and, at the same time, U^ 6 was reduced into U^ 4, forming pitchblende. Meanwhile, owing to strong silicification, pyritization, hematitization, carbonization and fluoritization, mesothermal and meso-epithermal sulfide-type U-deposits of multimetals were formed.     

The Discovery of Natural Native Uranium and Its Significance

This study analyzed the composition and uranium valence of pitchblendes sampled from the hydrothermal Guidong and Zhuguang uranium deposits of the middle Nanling metallogenic belt, Southern China using X-ray photoelectron spectroscopy (XPS). A revolutionary discovery is that the uranium not only exists in the forms of tetravalent and hexavalent uranium oxides, but also occurs in the form of native uranium. This is the first discovery of the existence of native uranium in nature. It greatly helps to reveal the origin of hydrothermal mineralization of uranium, and also has great significance for studying the thermal energy, formation and evolution of the earth.     

Uranium Provinces in China

Three uranium provinces are recognized in China, the Southeast China uranium province, the Northeast China-Inner Mongolia uranium province and the Northwest China (Xinjiang) uranium province. The latter two promise good potential for uranium resources and are major exploration target areas in recent years. There are two major types of uranium deposits: the Phanerozoic hydrothermal type (vein type) and the Meso-Cenozoic sandstone type in different proportions in the three uranium provinces. The most important reason or prerequisite for the formation of these uranium provinces is that Precambrian uranium-enriched old basement or its broken parts (median massifs) exists or once existed in these regions, and underwent strong tectonomagmatic activation during Phanerozoic time. Uranium was mobilized from the old basement and migrated upwards to the upper structural level together with the acidic magma originating from anatexis and the primary fluids, which were then mixed with meteoric water and resulted in t     

双桥—新路地区铀矿成矿条件分析

在前人研究的基础上,以铀源条件、构造条件、含矿主岩、围岩蚀变等热液型铀矿的主要控制因素为研究内容,较详细地总结和分析了大桥坞矿床(671)、白虎岩矿床(670)和杨梅湾矿床(621)及部分铀矿化点的成矿特征和控矿规律。发现各矿床(点)的含矿围岩虽然不尽相同,但围岩蚀变类型相近且构造控矿特征尤为显著,以此为依据构建了研究区典型铀矿床成矿模式(热液型),为后期借助模式找矿、预测评价等奠定了基础。     

Catagenesis of terrigenous rocks in the neoproterozoic intracratonic sedimentary basins and its effect on the formation of unconformity-type uranium mineralization

Mineral transformation of host rocks and localization of orebodies at the unconformity-type uranium deposits are considered for the Karku deposit in the northern Ladoga region. It is shown that the great depth of uranium mineral formation and the peculiar composition of host rocks, along with temperature and chemistry of fluids, played a critical role in variation of lithostatic and fluid pressure, porosity, and permeability. The compaction of quartz sandstone and gravelstone, which are typical host rocks at unconformity-type deposits, the development of microstylolithic sutures, conformal structures, pressure solution and deposition of quartz in free pores gave rise to the closure or constraint of pore space and to increase in pore pressure of fluids in the deep part of the Riphean troughs with approaching lithostatic loading. A transitional zone between hydrostatic and lithostatic pressure controlled localization of orebodies and was decisive for uranium mineral formation. This zone coincided with the Riphean-Paleoproterozoic unconformity and sank somewhat into the crystalline basement. Below this transitional zone, the intergranular fluid was under a pressure that was close to the pressure on solid phases, i.e., P _tot ≈ P _fl. The reliability of this phenomenon is confirmed by cessation of pressure solution-redeposition of quartz and distinct deceleration of dehydration of hydrous minerals. As is shown for the Karku deposit, the highly hydrated clay minerals of the illite-smectite series are widespread in its subore portion and lacking at the supraore levels along with termination of quartz regeneration. It is suggested that a zone of superhigh fluid pressure in deep parts of sedimentary basins constrains localization of uranium orebodies by structural and stratigraphic unconformity between Riphean and Paleoproterozoic rocks. It is stated that altered wall rocks at the unconformity-type uranium deposits cannot be identified with products of hydrothermal phyllic and argillic alteration of host rocks at the medium- and low-temperature endogenic deposits. The main distinctions consist in lack of wall-rock metasomatic reaction zoning and acid-alkaline evolution of solutions. All transformations of host rocks should be classified as diverse manifestations of deep catagenesis of sedimentary sequences and buried regolith. The carbon and oxygen isotopic compositions of calcite from host rocks at the Karku deposit are far from those of magmatic and hydrothermal carbonates. They are characterized by a high δ¹⁸O = +17 to +25‰ and a high dispersion of δ¹³C = −1.5 to almost −15‰. No granitoid magmatism is known in the regions, where the unconformity-type uranium deposits occur. Therefore, the rocks of the crystalline basement are the most probable source of uranium, which precipitated on the reductive barrier as a product of interaction with bitumen contained in the Riphean basal beds.     

Hydrothermal uranium deposits and sulfur isotopes

Research on sulfur isotopes in hydrothermal uranium deposits with acid alterations shed much light on the genetic aspects of hydrothermal uranium deposits. Based on the studies of uranium deposits of different genesis, it is concluded that σ³⁴S of Sulfides in hydrothermal uranium deposits derived from residual magma is within the range of +2‰ ?2.6‰, approximately the same as meteorite sulfur. δ³⁴S of Sulfides in polygenetic hydrothermal uranium deposits is slightly lighter than meteorite sulfur and varies over a restricted range (6.7‰), averaging ?10.15‰. Two intervals can be recognized with respect to sulfur isotopic compositions in palingenetic hydrothermal uranium deposits. δ³⁴S of sulfides formed in diagenesis, autometamorphism and hypothermal stages is similar to meteorite sulfur. On the other hand, at the stage starting from the alteration of uranium mineralization to the formation o uranium deposits and postmineralization the average δ³⁴S is -7.89‰, with a wider range of δ³⁴S variation (13.7‰), which can be attributed to the enrichment of δ³⁴S in palingenetic hydrothermal solutions.     

The outlook for the discovery of new types of economic uranium deposits in the Kodar-Udokan zone of the Transbaikal territory in Russia

On the basis of detailed fieldwork and analytical procedures (microprobe, X-ray diffraction, chemical analysis, etc.), ores and altered wall-rock rocks of the Khadatkanda and Etyrko deposits in the Kodar-Udokan mineragenic zone were studied. Economic concentrations of gold and some other accompanying metals have been established in ore of the Khadatkanda uranium deposit in the Syul’ban uranium district. REE-U lodes with brannerite-uraninite have been found at the Etyrko Fe-Ti-V deposit related to the Chinei layered pluton in the Udokan mining district. The outlook for the discovery of new types of economic uranium deposits is substantiated with respect to the known hydrothermal uranium ore objects and nontraditional setting related to the layered mafic-ultramafic intrusions.     

藏东“三江”地区热液型铀矿成矿条件与找矿方向

文章从大地构造背景与基底成熟度、铀源条件、地层与岩性、岩体成因类型、岩体期次与空间配置、火山岩特征、控矿构造、围岩蚀变等方面对藏东"三江"地区热液型铀矿的成矿条件进行了分析,指出藏东"三江"地区存在形成热液型铀矿的基本地质条件,但总体成矿条件不是十分优越。在找矿方向上,应以寻找花岗岩内、外接触带型铀矿为主攻目标类型。     

Anomalous Lead Isotopic Composition of Galena and Age of Altered Uranium Minerals: a Case study of Chauli Deposits,Chatkal–Qurama District,Uzbekistan

The enrichment of lead isotopic composition of nonuranium minerals, in the first place galena in ²⁰⁶Pb and ²⁰⁷Pb, as compared to common lead is a remarkable feature of uranium deposits. The study of such lead isotopic composition anomalous in ²⁰⁶Pb and ²⁰⁷Pb in uranium minerals provides an opportunity for not only identification of superimposed processes resulting in transformation of uranium ores during deposit history but also calculation of age of these processes under certain model assumptions. Galena from the Chauli deposit in the Chatkal–Qurama district, Uzbekistan, a typical representative of hydrothermal uranium deposits associated with domains of Phanerozoic continental volcanism, has been examined with the highprecision (±0.02%) MC-ICP-MS method. Twenty microsamples of galena were taken from polished sections. Six of them are galena hosted in carbonate adjacent to pitchblende spherulites or filling thin veinlets (approximately 60 μm) cutting pitchblende. Isotopically anomalous lead with ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb values reaching 20.462 and 15.743, respectively, has been found in these six microsamples in contrast to another fourteen in which the Pb–Pb characteristics are consistent with common lead. On the basis of these data and with account for the 292 ± 2 Ma age for the Chauli deposit, the age of epigenetic transformation of uranium ores of this deposit has been estimated. During this process, radiogenic lead partly lost from pitchblende was captured into galena. The obtained date is 170 Ma. In the Chatkal–Qurama district, these epigenetic processes are apparently caused by the interaction of uranium minerals with activated underground water under tectonic activity and relief transformation, which took place from the post-Permian (i.e., after the Chauli formation) to the Jurassic period.     

Chemical factors controlling the solubility of uraninite and their significance in the genesis of unconformity-related uranium deposits

A quantitative evaluation of the solubility of uraninite (UO₂) in aqueous solutions under hydrothermal conditions was made using previously reported thermodynamic data, so as to inquire into the controlling factors for Canadian unconformity-type ore mineralization as observed in the Athabasca uranium field. The results of solubility calculations suggest that uranyl carbonate complexes, such as UO₂CO ₃ ^o , UO₂(CO₃) ₂ ^2- and UO₂(CO₃) ₃ ^4- , predominate under relatively oxidizing and slightly acidic-alkaline conditions and that the uranyl chloride complex, UO₂Cl⁺ is dominant under acidic conditions. These features are predicted at temperatures up to 200 °C over reasonable ranges of CO₂ pressure (Pco₂) and salinity. Consequently, the physico-chemical parameters, such as oxygen activity (ao₂), and pH are regarded as the most important factors controlling uraninite solubility. Judging from the paragenetic sequences observed in most unconformity-type uranium deposits in the Athabasca district, appreciable decreases in the above variables are postulated to have occurred in the stage of principal uranium deposition. Such changes would be due to fluid-mixing phenomenon accompanied by the diagenetic-hydrothermal activity (Hoeve and Quirt 1987).     

铀的地球化学性质与成矿——以华南铀成矿省为例

铀是强不相容元素,随着岩浆演化而不断富集,在岩浆演化末期受结构氧增加影响进入独居石、磷钇矿等副矿物中。岩浆演化通常无法直接形成达到工业品位的铀矿床。铀是对氧逸度敏感的变价元素。在表生风化过程中岩体(层)中的铀被氧化为UO_2~(2+)而极易溶解进入水体中,并可在还原环境沉淀而富集成矿,氧化还原界面是找矿的理想选区。大气水可通过断裂构造系统进入一定深度,并受热源作用形成高氧逸度的热液而萃取出岩体(层)中的铀在还原位置沉淀富集形成矿床。新元古代氧化事件以及Marinoan冰期结束使得表生风化过程中更多的U进入水体;而寒武纪生命大爆发,易在沉积盆地底部形成还原环境,有利于U的沉淀富集。受上述三方面因素控制,在华南形成了广泛分布的富铀黑色页岩层,并被之后的沉积物覆盖,成为华南各型铀矿床的铀源层。印支期构造运动使部分富铀黑色页岩层发生部分熔融形成了富铀的S型花岗岩,该类岩石亦是之后铀成矿作用的铀源岩。燕山运动后期华南发生伸展构造背景下的岩浆热事件为以大气水为主的高氧逸度热液的形成并作用于铀源岩(层)提供了有利条件,促使华南各类型铀矿床开始在白垩纪集中形成。     

西秦岭降扎地区金、铀矿床年代学对比研究

西秦岭降扎地区寒武、志留系中的金、铀矿床,是我国相当重要的碳硅泥岩型矿床。根据金矿床中岩石和矿石异常铅模式年龄、氩-氩同位素年龄和钾-氩同位素年龄等,获得了一批有益的年代学信息。金矿床的各种同位素年龄数据主要有2组:(1)242-186Ma;(2)137-47Ma.结合区域地质发展史、成矿热液脉与岩脉的穿切关系以及矿床产出的其它宏观特征等,确定金矿床的成矿时代为137-47Ma,与区域铀矿床形成的主要时代(117-55Ma)相比,二者成矿时间大体相同。