During granulite-facies metamorphism of metasedimentary rocks by the infiltration of carbonic fluids.the disappearance of hydrated minerals leads to the liberation of aqueous fluids.These fluids arc strongly enriched ...During granulite-facies metamorphism of metasedimentary rocks by the infiltration of carbonic fluids.the disappearance of hydrated minerals leads to the liberation of aqueous fluids.These fluids arc strongly enriched in Fand CI,and a series of Large-lon-Lithophile(LIL) elements and rare metals.resulting in their depletion in granulites.To sum up the fate of these elements,we focus on three domains representing different crustal levels and showing distinct behaviours with respect to these elements.The Lapland metasedimentary granulites illustrate the behaviour of the LI1.E and rare metals during lower crustal metamorphism.There is no change in Ba.moderate loss in Kb,and extreme depletion in Cs,Li,and Sn.F and CI contents are also very low compared to the protoliths or average upper continental crust.Biotite and amphibole breakdown leads to the incorporation of their partitioning into a fluid or a melt.The Tranomaro metasomatized marbles recrystallizing under granulite-facies conditions represent a demonstrative example of fluid transfer from granulite-facies supiacrustals to traps represented by regional scale skarns.Such fluids may be at the origin of the incompatible element enrichment detected in leucosomes of migmatites from St Malo in Brittany(France) and Black Hills in South Dakota.The northern French Massif Centra]provides us with an example of a potential association between incompatible element enrichment of granitic melts and granulite-facies metamorphism.U- and Fenriched fine-grained granites are emplaced along a crustal scale shear zone active during the emplacement within the St Sylvestre peraluminous leucogranitic complex.We propose that during granulite-facies metamorphism dominated by carbonic waves in a deep segment of the continental crust,these shear zones control:(i) the percolation of F-,LILE-,rare metal-rich fluids liberated primarily by the breakdown of biotite;(ii) the enhancement of partial melting by F-rich fluids at intermediate crustal levels with the generation of F-,LILE-,rare metal-rich granitic melts;(iii) their transfer through the crust with protracted fractionation facilitated by their low viscosity due to high F-Li contents;and finally(iv)their emplacement as rare metal intrusions at shallow crust levels.展开更多
The present work aims at identifying Nb-Ta-, Zr-Hf-, REE-, Th-U-bearing two-mica granite from geological, geophysical cross-sections and mineral chemistry studies from three boreholes at G. El Sela shear zone. Microsc...The present work aims at identifying Nb-Ta-, Zr-Hf-, REE-, Th-U-bearing two-mica granite from geological, geophysical cross-sections and mineral chemistry studies from three boreholes at G. El Sela shear zone. Microscopically, the three boreholes are composed mainly of two-mica granite. They are composed of K-feldspar, plagioclase, quartz, biotite and muscovite. Accessories are pyrite, zircon, fluorite, rutile, monazite with Th-U-mineralization identified by scanning electron microscope (SEM) and electron probe-microanalyses (EPMA). Chlorite, muscovite, sericite, kaolinite are secondary minerals. Geochemically, two-mica granite boreholes are A-type granites and peraluminous characteristics. They are enriched in large ion lithophile elements (LILE;Ba, Rb and Sr), high field strength elements (Y, Zr and Nb), and LREE but depleted in HREE with negative Eu anomaly. U-enrichment associated with chloritization, muscovitization, albitization, sericitization, kaolinization and argillization results from convective hydrothermal circulation of fluids through brittle structures along the ENE-WSW main shear zone. The ratios Nb/Ta (7.7 - 17.7) and Zr/Hf (16.9 - 26.4) are relatively enriched in the lighter isovalents Ta and Hf. The accessory minerals observed in the two-mica granites are represented by metallic sulfides (pyrite, arsenopyrite, chalcopyrite, galena and sphalerite), Nb-rutile, Hf-zircon, fluorite, monazite, columbite, betafite, thorite, phosphothorite, uranothorite, brannerite, uraninite, coffinite and pitchblende at G. El Sela shear zone. Uraninite with a low Th content indicates a hydrothermal origin of U-mineralization, Thorite, uranothorite, monazite and zircon is the main uranium bearing minerals of magmatic origin within the enclosing granite. The primary U-mineralization has been observed in two boreholes. In order to illustrate the geophysical signature of El Sela U-mineralization, the radiometric, magnetic, and VLF-EM data as well as radon concentration are included. The magnetic, electrical conductivity and radiometric profiles were produced from detailed ground surveys. The shear zone is characterized by relatively weak levels for both K and eTh, but very high eU anomalies (<3500 ppm), Therefore, the Sela shear zone acts as a good trap for U-mineralization. The Sela Shear zone coincides with positive conductivity anomalies, which are the most prominent features on the respective profiles. The magnetic field over the Sela shear zone is also conspicuous by the sharp contrast which makes with the strong negative signatures of the altered microgranite. The radon distribution map showed the presence of seven high anomalies that are mostly controlled by the structures due to the easy movement of radon through them.展开更多
Southern India and Sri-Lanka are the places where “incipient charnockites”,i.e.the local transformation of amphibolite-facies gneisses into orthopyroxene-bearing,igneous looking charnockites,have been discovered in ...Southern India and Sri-Lanka are the places where “incipient charnockites”,i.e.the local transformation of amphibolite-facies gneisses into orthopyroxene-bearing,igneous looking charnockites,have been discovered in the early sixties.The fact that some incipient charnockites occur along a network of brittle fractures,together with CO2 remnants preserved in mineral inclusions,had called for the role of fluids during charnockite alteration.The present work presents new observations on fluid inclusions and microtextures of incipient charnockites from type localities in southern India.In addition to CO2-rich fluid inclusions in quartz and feldspar,all of the occurrences have disrupted remnants of concentrated aqueous alkali chloride solutions.CO2 inclusions are more abundant in paragneiss (Kerala) than in orthogneiss (Karnataka/Tamil Nadu).The finding of disrupted brine inclusions in the Kabbal charnockite is a key link between closely associated massive charnockites and Closepet Granite,both of which also share the brine remnants.All of the occurrences studied here have feldspar or feldspar-quartz microvein networks along grain boundaries of recrystallized quartz,feldspar and orthopyroxene.These metasomatic veins again indicate the action of alkali-exchanging fluids (i.e.,saline solutions).Feldspar microveins,which have been found in most “massive” charnockites,along with the CO2-rich fluid inclusions,suggest a commonality of incipient charnockite and massive charnockite,both types differing in intensity of interaction with metasomatizing pore fluids.展开更多
The Uranium exploration history of the Central African Republic(CAR)extends on two periods separated by the independence in 1960,of the ex-Ubangi-Chari,a colony of French Equatorial Africa.The uranium research is cont...The Uranium exploration history of the Central African Republic(CAR)extends on two periods separated by the independence in 1960,of the ex-Ubangi-Chari,a colony of French Equatorial Africa.The uranium research is contemporary of the French Atomic Energy Commission(CEA)implementation in this country in 1947 and done by the CEA.There began the first period.The second period started after the 1960 year.Before the independence,the exploration works were realized in two stages from 1947 to 1957 and from 1958 to 1961.The first stage regarded the recognizing of uranium occurrences in the magmatic and pegmatitic formations.For this,three missions were organized in the east of the CAR.The first mission took place from March to July 1947.The second mission was realized from April to June 1949,and the third mission from November to April 1956.The second stage had concerned the uranium research in sedimentary basins by the prospecting Mba?ki Series(January 1958-January 1960)and Fouroumbala Series(August 1959-June 1961).These series offered the chance to CEA to discover the Bakouma uranium deposits.After this discovery in 1965,the falls of prices of the uranium provoked altered stoppings and resumptions of works with the following societies:the Company of uranium ores of Bakouma(URBA)1969;Centralafrican uranium(URCA)in 1975 but dissolved in 1981;in 1989-1991,the Japanese Nuclear Power Corporation(PNC)for a deposit re-examination;URAMIN Centrafrique in 2004-2005 and AREVA Resources Centrafrique in 2007.The last-mentioned had stopped works since 2012.展开更多
Sandstone type deposits are the most common type of uranium deposits in the world.A large variety of sub-types have been defined,based either on the morphology of the deposits(e.g.,tabular,roll front,etc),or on the se...Sandstone type deposits are the most common type of uranium deposits in the world.A large variety of sub-types have been defined,based either on the morphology of the deposits(e.g.,tabular,roll front,etc),or on the sedimentological setting(e.g.,paleovalley,paleochannel,unconformity),or on tectonic or lithologic controls(e.g.,tectonolithologic,mafic dykes/sills),or still on a variety of others characteristics(phreatic oxidation type,interlayer permeable type,multi-element stratabound infiltrational,solution front limb deposit,humate type,etc.),reflecting the diversity of the characteristics of these deposits,but making it difficult to have a clear overview of these deposits.Moreover,uranium deposits occurring in the same sedimentological setting(e.g.,paleochannel),presenting similar morphologies(e.g.,tabular),may result from different genetic mechanisms and thus can be misleading for exploration strategies.The aim of the present paper is to propose a new view on sandstone-related uranium deposits combining both genetic and descriptive criteria.The dual view is indeed of primordial importance because all the critical characteristics of each deposit type,not limited to the morphology/geometry of the ore bodies and their relationships with depositional environments of the sandstone,have to be taken into account to propose a comprehensive classification of uranium deposits.In this respect,several key ore-forming processes,like the physical-chemical characteristics of the mineralizing fluid,have to be used to integrate genetic aspects in the classification.Although a succession of concentration steps,potentially temporally-disconnected,are involved in the genesis of some uranium mineralization,the classification here proposed will focus on the main mechanisms responsible for the formation and/or the location of ore deposits.The objective of this paper is also to propose a robust and widely usable terminology to define and categorize sandstone uranium deposits,considering the diversity of their origin and morphologies,and will be primarily based on the temperature of the mineralizing fluid considered as having played the critical role in the transportation of the uranium,starting from synsedimentary uranium deposits to those related to higher temperature fluids.展开更多
基金supported by the French National Research Agency through the national program "Investissements d'avenir" the reference ANR-10-LABX-21-RESSOURCES21
文摘During granulite-facies metamorphism of metasedimentary rocks by the infiltration of carbonic fluids.the disappearance of hydrated minerals leads to the liberation of aqueous fluids.These fluids arc strongly enriched in Fand CI,and a series of Large-lon-Lithophile(LIL) elements and rare metals.resulting in their depletion in granulites.To sum up the fate of these elements,we focus on three domains representing different crustal levels and showing distinct behaviours with respect to these elements.The Lapland metasedimentary granulites illustrate the behaviour of the LI1.E and rare metals during lower crustal metamorphism.There is no change in Ba.moderate loss in Kb,and extreme depletion in Cs,Li,and Sn.F and CI contents are also very low compared to the protoliths or average upper continental crust.Biotite and amphibole breakdown leads to the incorporation of their partitioning into a fluid or a melt.The Tranomaro metasomatized marbles recrystallizing under granulite-facies conditions represent a demonstrative example of fluid transfer from granulite-facies supiacrustals to traps represented by regional scale skarns.Such fluids may be at the origin of the incompatible element enrichment detected in leucosomes of migmatites from St Malo in Brittany(France) and Black Hills in South Dakota.The northern French Massif Centra]provides us with an example of a potential association between incompatible element enrichment of granitic melts and granulite-facies metamorphism.U- and Fenriched fine-grained granites are emplaced along a crustal scale shear zone active during the emplacement within the St Sylvestre peraluminous leucogranitic complex.We propose that during granulite-facies metamorphism dominated by carbonic waves in a deep segment of the continental crust,these shear zones control:(i) the percolation of F-,LILE-,rare metal-rich fluids liberated primarily by the breakdown of biotite;(ii) the enhancement of partial melting by F-rich fluids at intermediate crustal levels with the generation of F-,LILE-,rare metal-rich granitic melts;(iii) their transfer through the crust with protracted fractionation facilitated by their low viscosity due to high F-Li contents;and finally(iv)their emplacement as rare metal intrusions at shallow crust levels.
文摘The present work aims at identifying Nb-Ta-, Zr-Hf-, REE-, Th-U-bearing two-mica granite from geological, geophysical cross-sections and mineral chemistry studies from three boreholes at G. El Sela shear zone. Microscopically, the three boreholes are composed mainly of two-mica granite. They are composed of K-feldspar, plagioclase, quartz, biotite and muscovite. Accessories are pyrite, zircon, fluorite, rutile, monazite with Th-U-mineralization identified by scanning electron microscope (SEM) and electron probe-microanalyses (EPMA). Chlorite, muscovite, sericite, kaolinite are secondary minerals. Geochemically, two-mica granite boreholes are A-type granites and peraluminous characteristics. They are enriched in large ion lithophile elements (LILE;Ba, Rb and Sr), high field strength elements (Y, Zr and Nb), and LREE but depleted in HREE with negative Eu anomaly. U-enrichment associated with chloritization, muscovitization, albitization, sericitization, kaolinization and argillization results from convective hydrothermal circulation of fluids through brittle structures along the ENE-WSW main shear zone. The ratios Nb/Ta (7.7 - 17.7) and Zr/Hf (16.9 - 26.4) are relatively enriched in the lighter isovalents Ta and Hf. The accessory minerals observed in the two-mica granites are represented by metallic sulfides (pyrite, arsenopyrite, chalcopyrite, galena and sphalerite), Nb-rutile, Hf-zircon, fluorite, monazite, columbite, betafite, thorite, phosphothorite, uranothorite, brannerite, uraninite, coffinite and pitchblende at G. El Sela shear zone. Uraninite with a low Th content indicates a hydrothermal origin of U-mineralization, Thorite, uranothorite, monazite and zircon is the main uranium bearing minerals of magmatic origin within the enclosing granite. The primary U-mineralization has been observed in two boreholes. In order to illustrate the geophysical signature of El Sela U-mineralization, the radiometric, magnetic, and VLF-EM data as well as radon concentration are included. The magnetic, electrical conductivity and radiometric profiles were produced from detailed ground surveys. The shear zone is characterized by relatively weak levels for both K and eTh, but very high eU anomalies (<3500 ppm), Therefore, the Sela shear zone acts as a good trap for U-mineralization. The Sela Shear zone coincides with positive conductivity anomalies, which are the most prominent features on the respective profiles. The magnetic field over the Sela shear zone is also conspicuous by the sharp contrast which makes with the strong negative signatures of the altered microgranite. The radon distribution map showed the presence of seven high anomalies that are mostly controlled by the structures due to the easy movement of radon through them.
文摘Southern India and Sri-Lanka are the places where “incipient charnockites”,i.e.the local transformation of amphibolite-facies gneisses into orthopyroxene-bearing,igneous looking charnockites,have been discovered in the early sixties.The fact that some incipient charnockites occur along a network of brittle fractures,together with CO2 remnants preserved in mineral inclusions,had called for the role of fluids during charnockite alteration.The present work presents new observations on fluid inclusions and microtextures of incipient charnockites from type localities in southern India.In addition to CO2-rich fluid inclusions in quartz and feldspar,all of the occurrences have disrupted remnants of concentrated aqueous alkali chloride solutions.CO2 inclusions are more abundant in paragneiss (Kerala) than in orthogneiss (Karnataka/Tamil Nadu).The finding of disrupted brine inclusions in the Kabbal charnockite is a key link between closely associated massive charnockites and Closepet Granite,both of which also share the brine remnants.All of the occurrences studied here have feldspar or feldspar-quartz microvein networks along grain boundaries of recrystallized quartz,feldspar and orthopyroxene.These metasomatic veins again indicate the action of alkali-exchanging fluids (i.e.,saline solutions).Feldspar microveins,which have been found in most “massive” charnockites,along with the CO2-rich fluid inclusions,suggest a commonality of incipient charnockite and massive charnockite,both types differing in intensity of interaction with metasomatizing pore fluids.
文摘The Uranium exploration history of the Central African Republic(CAR)extends on two periods separated by the independence in 1960,of the ex-Ubangi-Chari,a colony of French Equatorial Africa.The uranium research is contemporary of the French Atomic Energy Commission(CEA)implementation in this country in 1947 and done by the CEA.There began the first period.The second period started after the 1960 year.Before the independence,the exploration works were realized in two stages from 1947 to 1957 and from 1958 to 1961.The first stage regarded the recognizing of uranium occurrences in the magmatic and pegmatitic formations.For this,three missions were organized in the east of the CAR.The first mission took place from March to July 1947.The second mission was realized from April to June 1949,and the third mission from November to April 1956.The second stage had concerned the uranium research in sedimentary basins by the prospecting Mba?ki Series(January 1958-January 1960)and Fouroumbala Series(August 1959-June 1961).These series offered the chance to CEA to discover the Bakouma uranium deposits.After this discovery in 1965,the falls of prices of the uranium provoked altered stoppings and resumptions of works with the following societies:the Company of uranium ores of Bakouma(URBA)1969;Centralafrican uranium(URCA)in 1975 but dissolved in 1981;in 1989-1991,the Japanese Nuclear Power Corporation(PNC)for a deposit re-examination;URAMIN Centrafrique in 2004-2005 and AREVA Resources Centrafrique in 2007.The last-mentioned had stopped works since 2012.
基金financial support and providing access to their properties.Patrice Bruneton is warmly thanked for a thorought revision of the manuscript.This paper is a contribution to the IGCP project 675“Comparative analysis of mineralization of Sandstone-type U deposits”。
文摘Sandstone type deposits are the most common type of uranium deposits in the world.A large variety of sub-types have been defined,based either on the morphology of the deposits(e.g.,tabular,roll front,etc),or on the sedimentological setting(e.g.,paleovalley,paleochannel,unconformity),or on tectonic or lithologic controls(e.g.,tectonolithologic,mafic dykes/sills),or still on a variety of others characteristics(phreatic oxidation type,interlayer permeable type,multi-element stratabound infiltrational,solution front limb deposit,humate type,etc.),reflecting the diversity of the characteristics of these deposits,but making it difficult to have a clear overview of these deposits.Moreover,uranium deposits occurring in the same sedimentological setting(e.g.,paleochannel),presenting similar morphologies(e.g.,tabular),may result from different genetic mechanisms and thus can be misleading for exploration strategies.The aim of the present paper is to propose a new view on sandstone-related uranium deposits combining both genetic and descriptive criteria.The dual view is indeed of primordial importance because all the critical characteristics of each deposit type,not limited to the morphology/geometry of the ore bodies and their relationships with depositional environments of the sandstone,have to be taken into account to propose a comprehensive classification of uranium deposits.In this respect,several key ore-forming processes,like the physical-chemical characteristics of the mineralizing fluid,have to be used to integrate genetic aspects in the classification.Although a succession of concentration steps,potentially temporally-disconnected,are involved in the genesis of some uranium mineralization,the classification here proposed will focus on the main mechanisms responsible for the formation and/or the location of ore deposits.The objective of this paper is also to propose a robust and widely usable terminology to define and categorize sandstone uranium deposits,considering the diversity of their origin and morphologies,and will be primarily based on the temperature of the mineralizing fluid considered as having played the critical role in the transportation of the uranium,starting from synsedimentary uranium deposits to those related to higher temperature fluids.
