Near-field high temperature transport: evidence from the genesis of the Osamu Utsumi uranium mine, Poços de Caldas alkaline complex, Brazil

The chemical, isotopic and mineralogical alteration which occurred during primary uranium ore deposition at the breccia pipe-hosted Osamu Utsumi mine, Poços de Caldas, Brazil was studied as a natural analogue for near field radionuclide migration. Chemical and isotopic alteration models were combined with finite difference models of the convective cooling of caldera intrusives. The modeling indicates that the intense chemical, isotopic, and mineralogical alteration of the Osamu Utsumi breccia pipe requires the circulation of > 10⁵ kg/cm² of boiling hydrothermal fluid > 200°C through each square centimeter cross-section of the pipe. This circulation could be driven by heat from a 6 km diameter intrusive extending to 10 km depth. Even with this large amount of circulation concentrated in the permeable breccia pipe, uranium solubilities must be orders of magnitude greater than indicated in the most recent experiments (and more in line with previous estimates) to produce the primary uranium mineralization at the Osamu Utsumi mine.The same models applied to a hypothetical high temperature waste repository show that heat from radioactive decay will produce a hydrothermal circulation system remarkably similar to that studied at the natural analogue site at Poços de Caldas. The depth of fluid convection induced by the hypothetical repository would be 5 to 10 km, the maximum temperature would be 300°C, the lifetime of the high temperature phase would be a few thousand years, and boiling would occur and cause most of the alteration within the hypothetical waste repository. This physical analysis emphasizes the importance of permeability on a 10 × 10 × 10 km scale in controlling the potential amount of circulation through the hypothetical repository.Application of the chemical models successfully used to interpret mineralization and alteration at the Poços de Caldas Osamu Utsumi mine to the hypothetical waste repository shows that even in a worst case scenario (waste implaced in a permeable host rock with no measures taken to inhibit flow though the repository) the amount of hydrothermal alteration in the hypothetical repository will be 0.1% of that in the breccia pipe at Osamu Utsumi. Assuming no barriers to uranium mobility, uranium precipitation above the hypothetical repository would be 0.04 ppm (rather than 40 ppm), hydrothermal alteration 0.03 wt% (rather than 30 wt%), etc.Our analysis indicates that modeled mineralogical alteration is sensitive to the thermodynamic data base used. Prediction of mineralogical alteration (which may be necessary to predict the migration of radionuclides other than uranium, for example) probably cannot be based directly on even very carefully collected laboratory thermodynamic data. Mineralogical complexities of the system, as well as data base uncertainties will require calibration of the thermodynamic framework against mineralogical alteration observed in the laboratory or field.     

U-Th-Pb systematics of zircon inclusions in rock-forming minerals: A study of armoring against isotopic loss using the Sherman Granite of Colorado-Wyoming,USA

Zircon inclusions were separated from the five major rock-forming minerals of the Sherman Granite of southern Wyoming, in order to evaluate the degree of discordance as a possible function of host minerals. U-Th-Pb isotopic ratios were determined for two size fractions of zircon inclusions from each mineral, plus five size fractions from the bulk rock. Isotopic data from the inclusions have more than double the spread of data on a discordia obtained from the bulk sample, thereby yielding better-resolved concordia intercepts. However, isotopic ratios and morphologic characteristics indicate that the Pb/U systematics are complicated by inherited radiogenic lead. Although the data array cannot unequivocally be explained by the armoring process, the proposed methodology has succeeded in identifying groups of zircon with different isotopic characteristics. As such, this technique can be used to decipher complex geologic/isotopic histories and may be a useful addition to routine zircon geochronology.     

The geology and geochronology of the Annandagstoppane granite,Western Dronning Maud Land,Antarctica

The Annandagstoppane Granite is exposed at three nunataks in Western Dronning Maud Land, Antarctica. It comprises medium- to coarse-grained granite crosscut by veins of pegmatite and graphic granite and has many S-type characteristics such as containing normative corundum greater than 1.1%, molecular Al₂O₃/(CaO+K₂O+Na₂O) greater than 1.1 and very little zircon. Hydrothermal alteration in the Granite is variably developed and has affected only certain minerals in any phase. R-Sr and Pb whole rock and mineral isotopic data suggest: 1) that Sr isotopes within it were nearly homogenized on a whole rock scale about 2823 Ma ago by this hydrothermal alteration; 2) that the Pb isotopic system was also disturbed at that time, and 3) that the Granite may have been was emplaced sometime during the interval 3115 Ma to 2945 Ma ago. The Granite was probably intruded by the Annandagstoppane Gabbro about 1200 Ma ago, resetting the Rb-Sr system in biotite. The Annandagstoppane Granite may form part of a basement complex to the Proterozoic sedimentary, volcanic and mafic igneous rocks exposed to the east in the Ahlmannryggen and the Borgmassivet. Its chemical composition and geologic history appears to be unique in Antarctica and in the Kaapvaal Craton of Southern Africa, consistent with the possibility that the Annandagstoppane Granite is part of a crustal fragment that joined Antarctica relatively late in the history of that continent.     

Genesis,mineralogy and geochemistry of uranium in the Gortdrum stratiform copper deposit,Ireland

The Gortdrum Cu-Ag-As-Sb-Hg-U orebody occurs in Lower Carboniferous strata on the downthrown side of an ENE transcurrent fault system in Southern County Tipperary, Republic of Ireland. The deposit comprises a linear discordant zone of brecciated carbonates and country rocks that are altered and mineralised. Research has indicated anomalous radioelement concentrations associated with the main copper orebody and in particular with the altered breccias within this zone. The metal association (Cu-Ag-As-Sb-Hg-U) is considered unique for Irish Carboniferous carbonate-hosted copper deposits. However, the low temperature mineral assemblage of early uranium-bearing minerals with later sulphide stages is reminiscent of many vein-type hydrothermal ore deposits. The geochemistry of uranium-enriched rocks indicates that the alteration of the basic dykes was a most significant event in the trapping of uranium within the limestones. Propylitization of the early basic dykes by CO₂-bearing fluids produced the alteration which led to the development of hematite, leucoxene and clay minerals. The reducing environments attendant with such mineralogy created a favourable environment for uranium precipitation. The reduction of U⁺⁶ to U⁺⁴ by oxidation of reduced Fe in dolomitized brecciated limestone occurred prior to the main sulphide mineral sequences. The genesis of uranium in the deposit is linked to radioelement remobilization from uranium-bearing heavy minerals in sediments of upper Devonian-Lower Carboniferous age. The uranium became available to low temperature hydrothermal fluids which carried then deposited the uranium along a transcurrent fault system. The source of the heavy minerals is considered to be the Leinster Granite to the east, by which time in the Carboniferous, unroofing of the main pluton had commenced.Previous address: Department of Geology, Trinity College, Dublin 2, Ireland     

沽源-红山子地区火山岩型铀矿床蚀变特征

本章着重对沽源-红山子地区几个典型铀矿床的近矿围岩蚀变特征、蚀变期次及蚀变带地球化学进行了研究,结果表明：红山子铀矿床与矿化关系密切的围岩发育碱性蚀变,主要包括钠长石化、赤铁矿化和绿泥石化;其它铀矿床与矿化关系密切的围岩则发育酸性蚀变,主要包括水云母化、硅化和萤石化。蚀变带的岩石地球化学分析表明,U与Pb、Zn、Mo的含量呈明显正相关,而与Cu的相关程度较低;蚀变带岩石出现Cd、Sn的含量异常。     

电子探针技术研究粤北龙华山岩体中独居石蚀变晕圈的结构与成分特征

独居石是华南产铀花岗岩中常见的含铀副矿物.龙华山岩体是粤北诸广山复式岩体中一个重要的产铀花岗岩,该岩体的独居石具有蚀变晕圈现象.但是,该岩体中独居石蚀变晕圈的结构和成分特征以及对铀成矿的指示意义尚未开展研究.本文利用电子探针(EPMA)对龙华山岩体的独居石蚀变晕圈开展结构和成分研究.测试结果表明:独居石蚀变晕圈是从内到...     

广西苗儿山铀矿田张家铀矿床成矿时代:沥青铀矿微区原位测定

张家铀矿床是苗儿山铀矿田北部的代表性铀矿床之一,对其开展详细的成矿年代学研究不仅对认识区域铀成矿规律十分重要,也对探讨华南花岗岩型热液铀矿床的成矿大地构造背景及动力学机制具有重要意义.在详细的镜下观察基础上,采用电子探针U-Th-Pb化学法、LA-ICP-MS U-Pb同位素方法对脉状沥青铀矿进行了年代学研究.20个点的U-Th-Pb化学年龄为55.3~81.1 Ma,其中19个点的加权平均年龄为71.4±1.9 Ma.根据稀土元素特征的不同,将34个点的LA-ICP-MS U-Pb同位素年龄分为2组,第一组共15个点,其中13个点的206Pb/238U加权平均年龄为69.4±4.9 Ma;第二组共19个,其中16个点的206Pb/238U加权平均年龄为94.1±3.0 Ma.电子探针U-Th-Pb化学法加权平均年龄（71.4±1.9 Ma）与LA-ICP-MS U-Pb同位素法较年轻的一组206Pb/238U加权平均年龄（69.4±4.9 Ma）一致,代表张家铀矿床的成矿时代,LA-ICP-MS U-Pb同位素法较老的一组206Pb/238U加权平均年龄（94.1±3.0 Ma）可能是沥青铀矿受后期改造和/或样品剥蚀过程杂质矿物影响而地质意义不明确.晚中生代期间,古太平洋板块向欧亚大陆俯冲可能是控制华南大规模花岗岩型铀矿床形成的动力学机制,包括张家铀矿床在内的华南花岗岩型铀矿床大规模成矿作用可能受80~50 Ma古太平洋板块对亚洲东部的斜向俯冲动力体制的控制.      

华南诸广山复式岩体中段花岗岩的碱交代蚀变

诸广山复式花岗岩体中段某大型铀矿田中的部分花岗岩广泛发育碱交代蚀变作用。据对３１３个碱交代蚀变花岗岩样品的岩石化学分析资料及岩相学研究结果表明,该地碱交代蚀变是以钠长石化为主的多阶段钾、钠叠加交代（即混合交代）,蚀变强度大多较弱,局部可较强并生成二长石岩、钠长石岩等交代蚀变岩。测得该碱交代蚀变岩石的Ｒｂ－Ｓｒ等时线年龄为１３２．１±５．８Ｍａ,在时序上它与主体侵入的似斑状黑云母花岗岩（Ｒｂ－Ｓｒ等时线成岩年龄为２１５．２±６．３Ｍａ）、补体侵入的细粒少斑黑云母花岗岩（测得Ｒｂ－Ｓｒ等时线成岩年龄为１５５．２±５．４５Ｍａ）构成一连续演化序列,推测碱质可能源自该产铀花岗岩岩浆演化钠转折分异阶段的产物;该蚀变岩属铀成矿前已遭剥蚀的钨、锡矿化矿根相,成为随后工业铀矿化（Ｕ－Ｐｂ法测得成矿年龄为１０３．２±０．６Ｍａ）的有利围岩。     

Comparative Study of Acid and Alkaline Stimulants with Granite in an Enhanced Geothermal System

The Enhanced Geothermal System (EGS) is an artificial geothermal system that aims to economically extract heat from hot dry rock (HDR) through the creation of an artificial geothermal reservoir. Chemical stimulation is thought to be an effective method to create fracture networks and open existing fractures in hot dry rocks by injecting chemical agents into the reservoir to dissolve the minerals. Granite is a common type of hot dry rock. In this paper, a series of chemical stimulation experiments were implemented using acid and alkaline agents under high temperature and pressure conditions that mimic the environment of formation. Granite rock samples used in the experiments are collected from the potential EGS reservoir in the Matouying area, Hebei, China. Laboratory experimental results show that the corrosion ratio per unit area of rock is 3.2% in static acid chemical experiments and 0.51% in static alkaline chemical experiments. The permeability of the core is increased by 1.62 times in dynamic acid chemical experiments and 2.45 times in dynamic alkaline chemical experiments. A scanning electron microscope analysis of the core illustrates that secondary minerals, such as chlorite, spherical silica, and montmorillonite, were formed, due to acid-rock interaction with plagioclase being precipitated by alkaline-rock interactions. Masking agents in alkaline chemical agents can slightly reduce the degree of plagioclase formation. A chemical simulation model was built using TOUGHREACT, the mineral dissolution and associated ion concentration variation being reproduced by this reactive transport model.     

Geochemical fixation of rare earth elements into secondary minerals in sandstones beneath a natural fission reactor at Bangombé, Gabon

To study geochemical processes for migration and fixation of fissiogenic rare earth elements (REE) in association with uranium dissolution, in situ isotopic analyses using an ion microprobe were performed on U- and REE-bearing secondary minerals, such as coffinite, françoisite, uraniferous goethite, and uraninite found in a sandstone layer 30 to 110 cm beneath a natural fission reactor at Bangombé, Gabon. Phosphate minerals such as phosphatian coffinite and françoisite with depleted ²³⁵U (²³⁵U/²³⁸U = 0.00609 to 0.00638) contained large amount of fissiogenic light REE, while micro-sized uraninite grains in a solid bitumen aggregate have normal U isotopic values (²³⁵U/²³⁸U = 0.00725) and small amount of fissiogenic REE components. The proportions of fissiogenic and non-fissiogenic REE components in four samples from the core of BAX03 vary in depth ranging from 30 cm to 130 cm beneath the reactor, which suggests mixing between fissiogenic isotopes from the reactor and non-fissiogenic isotopes from original minerals in the sandstone. Significant chemical fractionation was observed between Ce and the other REE in the secondary minerals, which shows evidence of an oxidizing atmosphere during their formation. Pb-isotopic analyses of individual minerals do not directly provide chronological information because of the disturbance of U-Pb decay system due to recent geologic alteration. However, systematic Pb-isotopic results from all of the minerals reveal the mobilization of fissiogenic isotopes, Pb and U from the reactor in association with dolerite dyke intrusion ∼0.798 Ga ago and the formation of the secondary minerals by mixing event between 2.05 Ga-old original minerals and reactor materials due to recent alteration.     

Constraints on incipient charnockite formation from zircon geochronology and rare earth element characteristics

We present high spatial resolution ion-microprobe U–Th–Pb geochronology and rare earth element (REE) data combined with cathodoluminescence (CL) and back-scattered electron (BSE) imaging for complex zircons in incipient charnockites from Söndrum, SW Sweden. Examination of closely paired samples across the dehydration zone demonstrates that incipient charnockite formation at Söndrum is a zircon-forming process. We determined the age of the dehydration event (i.e. charnockitisation) to 1,397 ± 4 Ma (2σ, MSWD = 1.7). This is the first successful attempt to date incipient charnockite formation using U–Pb systematics of zircon. Internal structure, chemical and isotopic characteristics of zircon indicate that the granitic pegmatite in the core of the incipient charnockite is a melting zone. Commonly observed bulk rock HREE depletion in incipient charnockites is not caused by zircon dissolution but by involvement of garnet as a reactant in the dehydration reactions. Moreover, REE patterns of the newly formed zircon are HREE-enriched, indicating non-concurrent growth and suggesting that the degree of charnockite depletion in HREE might be controlled by the volume of newly formed zircon.     

Characterization of fresh and altered montroseite [V,Fe]OOH. A discussion of alteration processes

Fresh natural montroseite of the Akouta uranium deposit (Niger) and its alteration products are characterized by reflectance measurement, X-ray diffraction pattern, chemical composition, Raman and infrared spectroscopies. Microprobe analysis allows to distinguish Fe-free and Fe-bearing montroseite. Valence measurements show mainly V^III vanadium in fresh mineral. Iron is recognized as Fe^II with the charge deficit compensated by V^IV. A first alteration step is characterized by V^III to V^IV oxidation, a second one by the Fe^II to Fe^III oxidation. These oxidation steps are compensated by dehydrogenation inferred from water content measurements. Behaviour of hydrogen atoms during this alteration process is discussed from the I.R. spectroscopic data. Finally an evolution mechanism of the crystal structure is proposed.     

Geochemical,isotopic, and geochronlologic constraints on the formation of the Eagle Point basement-hosted uranium deposit,Athabasca Basin,Saskatchewan, Canada and recent remobilization of primary uraninite in secondary structures

The Athabasca Basin hosts many world-class unconformity-related uranium deposits. Recently, uranium reserves for the Eagle Point basement-hosted deposit have increased with the discovery of new mineralized zones within secondary structures. A paragenetic study of Eagle Point reveals the presence of three temporally distinct alteration stages: a pre-Athabasca alteration, a main alteration and mineralization comprised of three substages, and a post-main alteration and mineralization stage that culminated in remobilization of uraninite from primary to secondary structures. The pre-Athabasca alteration stage consists of minor amounts of clinochlore, followed by dolomite and calcite alteration in the hanging wall of major fault zones and kaolinitization of plagioclase and K-feldspar caused by surface weathering. The main alteration and uranium mineralization stage is related to three temporally distinct substages, all of which were produced by isotopically similar fluids. A major early alteration substage characterized by muscovite alteration and by precipitation Ca–Sr–LREE-rich aluminum phosphate-sulfate minerals, both from basinal fluids at temperatures around 240°C prior to 1,600 Ma. The mineralization substage involved uraninite and hematite precipitated in primary structures. The late alteration substage consists of dravite, uranophane-beta veins, calcite veins, and sudoite alteration from Mg–Ca-rich chemically modified basinal fluids with temperatures around 180°C. The post-main alteration and mineralization stage is characterized by remobilization of main stage uraninite from primary to secondary structures at a minimum age of ca. 535 Ma. U–Pb resetting events recorded on primary and remobilized uraninites are coincident with fluid flow induced by distal orogenies, remobilizing radiogenic Pb to a distance of at least 225 m above the mineralized zones.     

Chemical alteration in the micro weathering environment within a spheroidally-weathered anorthosite boulder

A chemical and mineralogical study of the innermost rindlets of a spheroidally weathered boulder from the Wichita Mountains, Oklahoma details incipient alteration of minerals along solution channels. Alteration products in plagioclase areas are highly aluminous, whereas ferruginous residuum is commonly found in olivine alteration zones. Alteration of plagioclase along solution channels results in sharp weathering contacts as contrasted with wider zones of “limonite” bordering solution channels in olivine. In certain instances, cores of fresh olivine are surrounded by limonite indicating that diffusion may play a role in this type of alteration.Mobility of sesquioxides is evidenced by aluminum-rich debris adjacent to mafic materials in solution channels and iron-bearing material in channels cutting through plagioclase. Not resolved is whether the movement of sesquioxides in solution channels is by transport of colloidal particulates or by diffusion of dissolved species.Calcic plagioclase alters to calcite along the innermost solution channel. It is postulated that solution, exterior to the boulder and undersaturated with respect to calcite, enters the rind system via solution channels. Cut off from the CO₂ reservoir of the atmosphere by the narrow confines of this solution channel, the solution reacts with the bytownite to precipitate calcite.Apart from the occurrence of calcite, no crystalline weathering products were found in the innermost rindlet samples. A poorly crystallized illite was identified in the outermost rindlet. These findings suggest that initial alteration products on weathering rinds are amorphous and that, with time, these products attain a degree of identifiable crystallinity.     

Deposition and remobilization of uranium in the North Baikal region: Evidence from the U-Pb isotopic systems of uranium ores

Based on the study of local volumes of minerals, including their microsampling and subsequent analysis of Pb/Pb and U/Pb isotope ratios with the classic methods of isotopic dilution and thermoionization mass spectrometry (TIMS), U-Pb and Pb-Pb isotopic datings of minerals were carried out in uranium ores from deposits in the Akitkan and Nechera-Nichatka ore districts (North Baikal region). Reliable evidence in favor of the Middle Devonian (384 ± 8 Ma) remobilization of Paleoproterozoic primary uranium concentrations and the redeposition of uranium as pitchblende 2 has been obtained for the first time for ores of the Akitkan district. The Paleoproterozoic (1832 ± 13 Ma) age of uraninite mineralization and the timing of the latest (377 ± 5 Ma) transformation of uranium ores at the Chepok deposit (Nechera-Nichatka district) are substantiated.     

相山铀矿田成矿流体特征:来自微量、稀土元素地球化学证据

相山铀矿田的成矿流体性质和来源存在争议,为进一步探讨相山铀矿田成矿流体的性质和来源,本文对相山铀矿田西部的居隆庵铀矿床和北部的沙洲铀矿床中的新鲜围岩、蚀变围岩及矿石的微量、稀土元素含量及其变化进行了研究。结果显示:在含较多热液成因萤石的居隆庵铀矿床中,从新鲜围岩到蚀变围岩到矿石,Zr、Hf含量先降低再升高;而在含少量热液萤石的沙洲铀矿床中,新鲜围岩、蚀变围岩和矿石的Zr、Hf含量基本一致。鉴于富F流体易汲取岩石中的Zr、Hf,因此,这两个矿床中不同类型样品Zr、Hf含量的不同变化趋势,可能与居隆庵铀矿床的成矿流体富F、而沙洲铀矿床的成矿流体相对贫F有关。这两个铀矿床中矿石的稀土配分曲线与其各自的新鲜及蚀变围岩的稀土配分曲线形态相似但又存在差异,说明每个矿床的新鲜围岩、蚀变围岩和矿石之间的稀土元素既具有继承性、又受到不同性质的流体的影响。居隆庵铀矿床中矿石显示Eu负异常,可能主要是继承了围岩的Eu负异常;沙洲铀矿床中矿石Eu显示弱负异常至弱正异常的特征,可能与围岩中斜长石因热液蚀变作用而释放出的Eu的进入流体有关。基于新鲜围岩、蚀变围岩及矿石的U和REE研究,推断居隆庵铀矿床成矿流体中U和REE均以F的络合物形式迁移;但沙洲铀矿床中铀矿石品位较低,可能是与流体中相对贫F有关。     

Behavior of isotopic systematics during deformation and metamorphism: a Hf,Nd and Sr isotopic study of mylonitized granite

A combined compositional and Hf, Nd and Sr isotopic study was performed on a suite of samples of progressively deformed granite from a mylonite zone in the Harquahala Mountains, western Arizona, to evaluate the effects of deformation and metamorphism on the isotopic systematics of typical continental crustal rocks. The 1.4 Ga Harquahala Granite was deformed during Mesozoic thrusting along the Harquahala thrust. Granite in the resulting 60 m wide shear zone ranges from protomylonite to ultramylonite. In most of these mylonites, the protolith is not megascopically recognizable, and can be discerned only by the progressive transition to undeformed granite. Isotopic analyses of Hf, Nd and Sr from the shear zone document the immobility of the Hf and Nd isotopic systems relative to that of the Sr isotopic system during deformation. The Rb–Sr isotopic data show considerable scatter on an isochron plot, exhibiting both gains and losses of Rb and Sr from the whole-rock systems. In contrast, the Sm–Nd and Lu–Hf isotopic systematics are mostly well behaved on isochron diagrams, plotting mostly in tight clusters or along 1.4 Ga isochrons. These results show that while the Sr isotopic system in crustal rocks is quite susceptible to later tectonic disturbance, both Hf and Nd isotopic systems can provide reliable model age information in continental crustal terranes even when the rocks have been subjected to low to medium grades of deformation and metamorphism.     

云南西盟阿莫锡矿成矿物质来源

通过对氧、硫同位素组成、矿物包裹体均一温度的测定,以及稀土配分型式、花岗岩中Sn含量增减变化的分析结果表明,西盟阿莫锡矿成矿物质主要来源于下伏的隐伏花岗岩岩体及围岩中———永龙组下段片岩及大黑山组上段变粒岩。花岗岩浆期后的含矿流体沿断裂迁移、充填、交代,在此过程中,由于压力锐减,发生流体沸腾,气液分离,酸碱分异,锡石沉淀;随着温度的下降,流体化学性质转化,出现不同蚀变和不同矿物组合的多个成矿阶段,而不同断裂类型,又控制着矿体的形态和规模。     

辽宁赛马碱性岩体异性石化学成分特征及其蚀变组合对碱性岩浆-热液演化的指示意义

辽宁赛马碱性岩体早年因产铀矿而闻名,该岩体主要由响岩、霞石正长岩和异霞正长岩组成,其中铀、锆和稀土等元素矿化主要集中于异霞正长岩岩浆阶段。异性石是异霞正长岩中特征的锆-稀土矿物,主要分为两期,晚期异性石表现出更加富集Nb、REE等高场强元素的特点。早期异性石经历了一系列的热液蚀变,根据蚀变强弱程度,蚀变矿物组合可分为:(1)异性石+钠锆石+霓石±钠沸石;(2)异性石+钠锆石+锆石+钠沸石±霓石;(3)异性石假晶,假晶主要由残余异性石+钠锆石+锆石+钠沸石+霓石+钾长石+铈硅磷灰石组成。相比于岩浆锆石,蚀变组合中次生锆石具有富Ca、Al、Fe的特点,与异性石本身化学成分和流体性质密切相关。通过对异性石及其蚀变组合的精细矿物学研究,我们得知假晶的形成可能是异性石"溶解-再沉淀"的结果,致使假晶形成的流体至少包括:(1)占主导的富Na(±K)、Al、F的自交代流体;(2)少量晚期富Ca流体。假晶中次生锆石和铈硅磷灰石的结晶说明了Zr和REE等高场强元素的热液活动性,自交代碱性流体和富Ca流体在此过程中起到"搬运"和"提纯"的作用,这对认识碱性岩稀有、稀土成矿机制具有重要的指示意义。     

Geology, petrography and geochemistry of rhyolite-hosted uranium mineralization at Mohar, Shivpuri district, Madhya Pradesh

In the westernmost part of the Bundelkhand Granitoid Complex (BGC), a mesa structure represents a unique outlier, surrounded by brecciated granite and filled with Vindhyan sedimentary rocks locally known as the Dhala Formation near Mohar village of Shivpuri district, Madhya Pradesh. Uranium mineralisation located in the area is mostly associated with rhyolite of peralkaline to peraluminous in nature, that has a high average uranium concentration (30 ppm). The mineralization is in or adjacent to caldera and is hydrothermal vein-type. Radioactivity is mainly due to coffinite with limited radioactivity due to U-Ti complex, uranium adsorbed in clay and labile uranium along fracture. Coffinite occurs in association with pyrite and chalcopyrite or chlorite with presence of fluorite. Features such as chloritisation, clay formation and sulfide mineralisation manifest hydrothermal alteration. Chemical analysis indicates the aluminous nature of the rock and their high K₂O/Na₂O (3.81–12.84) ratios are suggestive of predominance of potash feldspar over sodic. The alteration index varies from 49.88–92.40, which, reflects high intensity of hydrothermal alteration. Chlorite-carbonatepyrite index (CCPI), a measure of the intensity of replacement of sodic feldspars and glass by sericite, chlorite, carbonate, and pyrite associated with hydrothermal alteration proximal to the ore bodies varies from 3.84–49.66. On the basis of core study, geochemistry and mineralogy, it is envisaged that epigenetic hydrothermal solutions were responsible for concentration of uranium as coffinite, radioactive carbonaceous matter and adsorbed uranium phases in rhyolite with sulfide confined to weak planes.