This paper presents a systematic study of major and trace elements and Sm-Nd isotopes in leucogranites closely related to uranium mineralization in the Gaudeanmus area, Namibia. The results illustrate that the uranife...This paper presents a systematic study of major and trace elements and Sm-Nd isotopes in leucogranites closely related to uranium mineralization in the Gaudeanmus area, Namibia. The results illustrate that the uraniferous leucogranites possess high SiO2 （68.8wt%-76.0wt%, average 73.1wt%） and K （4.05wt%-7.78wt%, average 5.94wt%） contents, and are sub-alkaline and metaluminous to weakly peraluminous, as reflected by A/CNK values of 0.96-1.07 with an average of 1.01. The leucogranites are rich in light rare earth elements （LREE/HREE = 2.53-7.71; （La/Yb）s = 2.14-10.40）, have moderate Eu depletion and high Rb/Sr ratios （2.03-5.50 with an average of 4.36）; meanwhile, they are enriched in Rb, K, Th, U and Pb, and depleted in Ba, Nb, Ta, and Sr. The tNd（t） values of uraninites range from -14.8 to -16.5, and the two-stage Nd model ages are 2.43-2.56 Ga. Detailed elemental and Sm-Nd isotopic geochemical characteristics suggest that the leucogranites were formed in a post- orogenic extensional environment. The U-rich pre-Damara basement was the main source of uranium during the primary mineralization event, which is disseminated in leucogranites, whereas the uranium mineralization in veins possibly resulted from remobilization of the primary uranium minerals.展开更多
Age calibrated deformation histories established by detailed mapping and dating of key magmatic time markers are correlated across all tectono-metamorphic provinces in the Damara Orogenic System.Correlations across st...Age calibrated deformation histories established by detailed mapping and dating of key magmatic time markers are correlated across all tectono-metamorphic provinces in the Damara Orogenic System.Correlations across structural belts result in an internally consistent deformation framework with evidence of stress field rotations with similar timing,and switches between different deformation events.Horizontal principle compressive stress rotated clockwise ~180°in total during Kaoko Belt evolution,and^135° during Damara Belt evolution.At most stages,stress field variation is progressive and can be attributed to events within the Damara Orogenic System,caused by changes in relative trajectories of the interacting Rio De La Plata,Congo,and Kalahari Cratons.Kaokoan orogenesis occurred earliest and evolved from collision and obduction at ~590 Ma,involving E-W directed shortening,progressing through different transpressional states with ~45° rotation of the stress field to strike-slip shear under NW-SE shortening at ~550-530 Ma.Damaran orogenesis evolved from collision at ~555-550 Ma with NW-SE directed shortening in common with the Kaoko Belt,and subsequently evolved through ~90°rotation of the stress field to NE-SW shortening at ~512-508 Ma.Both Kaoko and Damara orogenic fronts were operating at the same time,with all three cratons being coaxially convergent during the 550-530 Ma period;Rio De La Plata directed SE against the Congo Craton margin,and both together over-riding the Kalahari Craton margin also towards the SE.Progressive stress field rotation was punctuated by rapid and significant switches at ~530-525 Ma,~508 Ma and ~505 Ma.These three events included:(1)Culmination of main phase orogenesis in the Damara Belt,coinciding with maximum burial and peak metamorphism at 530-525 Ma.This occurred at the same time as termination of transpression and initiation of transtensional reactivation of shear zones in the Kaoko Belt.Principle compressive stress switched from NW-SE to NNW-SSE shortening in both Kaoko and Damara Belts at this time.This marks the start of Congo-Kalahari stress field overwhelming the waning Rio De La Plata-Congo stress field,and from this time forward contraction across the Damara Belt generated the stress field governing subsequent low-strain events in the Kaoko Belt.(2)A sudden switch to E-W directed shortening at ~508 Ma is interpreted as a far-field effect imposed on the Damara Orogenic System,most plausibly from arc obduction along the orogenic margin of Gondwana(Ross-Delamerian Orogen).(3)This imposed stress field established a N-S extension direction exploited by decompression melts,switch to vertical shortening,and triggered gravitational collapse and extension of the thermally weakened hot orogen core at ~505 Ma,producing an extensional metamorphic core complex across the Central Zone.展开更多
This paper focuses on the effect of the later hydrotherm on uraniferous leucogranites and the stages of uranium mineralization. Here, we review C-H-O stable isotope, elements and fluid geochemistry of uraniferous leuc...This paper focuses on the effect of the later hydrotherm on uraniferous leucogranites and the stages of uranium mineralization. Here, we review C-H-O stable isotope, elements and fluid geochemistry of uraniferous leucogranites in Gaudeanmus, Namibia. The results show that there is significant increasing amount of rare earth element from non-mineralized to uraniferous leucogra-nites, indicating the synchronization of REE enrichment and uranium mineralization. Uranium enrichment may have close relations with Pb, Th, Co, Ni, REE in this region, so REE and U evidently exist homology. There are at least two stages of uranium mineralization by later hydrothermal alteration: firstly, due to magnatic residual high temperature and low salinity fluid, the temperature of main metallogenetic epoch ranges from 470°C to 530°C, salinity ranges from 3.55% to 9.60% NaCleq, and C, H, O stable isotope is -23‰ - -13.6‰, -53.3‰ - -46.4‰, 7.71‰ - 8.81‰, respectively. Secondly, due to superim-posed hydrothermal fluid, the temperature, salinity, and C, H, O stable isotope is 150°C - 220°C, 4.65% - 19.05% NaCleq, -20.3‰ -?-3.7‰, -64.7‰ - -53.6‰, 1.49‰ - 1.99‰, respectively. The fluid for reformation is derived from postmagmatic fluid, mixed with a number of meteoric water.展开更多
基金supported by the National Natural Science Foundation of China (41602080)Nuclear Energy Development Project from the National Defense Science and Industry Bureau
文摘This paper presents a systematic study of major and trace elements and Sm-Nd isotopes in leucogranites closely related to uranium mineralization in the Gaudeanmus area, Namibia. The results illustrate that the uraniferous leucogranites possess high SiO2 （68.8wt%-76.0wt%, average 73.1wt%） and K （4.05wt%-7.78wt%, average 5.94wt%） contents, and are sub-alkaline and metaluminous to weakly peraluminous, as reflected by A/CNK values of 0.96-1.07 with an average of 1.01. The leucogranites are rich in light rare earth elements （LREE/HREE = 2.53-7.71; （La/Yb）s = 2.14-10.40）, have moderate Eu depletion and high Rb/Sr ratios （2.03-5.50 with an average of 4.36）; meanwhile, they are enriched in Rb, K, Th, U and Pb, and depleted in Ba, Nb, Ta, and Sr. The tNd（t） values of uraninites range from -14.8 to -16.5, and the two-stage Nd model ages are 2.43-2.56 Ga. Detailed elemental and Sm-Nd isotopic geochemical characteristics suggest that the leucogranites were formed in a post- orogenic extensional environment. The U-rich pre-Damara basement was the main source of uranium during the primary mineralization event, which is disseminated in leucogranites, whereas the uranium mineralization in veins possibly resulted from remobilization of the primary uranium minerals.
基金a largely self-funded 1TAR projectsupported by ARC grants A00103456+1 种基金DP0210178 to Prof.David GrayNSF grants EAR0440188 and EAR0738874 to Prof.David Foster
文摘Age calibrated deformation histories established by detailed mapping and dating of key magmatic time markers are correlated across all tectono-metamorphic provinces in the Damara Orogenic System.Correlations across structural belts result in an internally consistent deformation framework with evidence of stress field rotations with similar timing,and switches between different deformation events.Horizontal principle compressive stress rotated clockwise ~180°in total during Kaoko Belt evolution,and^135° during Damara Belt evolution.At most stages,stress field variation is progressive and can be attributed to events within the Damara Orogenic System,caused by changes in relative trajectories of the interacting Rio De La Plata,Congo,and Kalahari Cratons.Kaokoan orogenesis occurred earliest and evolved from collision and obduction at ~590 Ma,involving E-W directed shortening,progressing through different transpressional states with ~45° rotation of the stress field to strike-slip shear under NW-SE shortening at ~550-530 Ma.Damaran orogenesis evolved from collision at ~555-550 Ma with NW-SE directed shortening in common with the Kaoko Belt,and subsequently evolved through ~90°rotation of the stress field to NE-SW shortening at ~512-508 Ma.Both Kaoko and Damara orogenic fronts were operating at the same time,with all three cratons being coaxially convergent during the 550-530 Ma period;Rio De La Plata directed SE against the Congo Craton margin,and both together over-riding the Kalahari Craton margin also towards the SE.Progressive stress field rotation was punctuated by rapid and significant switches at ~530-525 Ma,~508 Ma and ~505 Ma.These three events included:(1)Culmination of main phase orogenesis in the Damara Belt,coinciding with maximum burial and peak metamorphism at 530-525 Ma.This occurred at the same time as termination of transpression and initiation of transtensional reactivation of shear zones in the Kaoko Belt.Principle compressive stress switched from NW-SE to NNW-SSE shortening in both Kaoko and Damara Belts at this time.This marks the start of Congo-Kalahari stress field overwhelming the waning Rio De La Plata-Congo stress field,and from this time forward contraction across the Damara Belt generated the stress field governing subsequent low-strain events in the Kaoko Belt.(2)A sudden switch to E-W directed shortening at ~508 Ma is interpreted as a far-field effect imposed on the Damara Orogenic System,most plausibly from arc obduction along the orogenic margin of Gondwana(Ross-Delamerian Orogen).(3)This imposed stress field established a N-S extension direction exploited by decompression melts,switch to vertical shortening,and triggered gravitational collapse and extension of the thermally weakened hot orogen core at ~505 Ma,producing an extensional metamorphic core complex across the Central Zone.
文摘This paper focuses on the effect of the later hydrotherm on uraniferous leucogranites and the stages of uranium mineralization. Here, we review C-H-O stable isotope, elements and fluid geochemistry of uraniferous leucogranites in Gaudeanmus, Namibia. The results show that there is significant increasing amount of rare earth element from non-mineralized to uraniferous leucogra-nites, indicating the synchronization of REE enrichment and uranium mineralization. Uranium enrichment may have close relations with Pb, Th, Co, Ni, REE in this region, so REE and U evidently exist homology. There are at least two stages of uranium mineralization by later hydrothermal alteration: firstly, due to magnatic residual high temperature and low salinity fluid, the temperature of main metallogenetic epoch ranges from 470°C to 530°C, salinity ranges from 3.55% to 9.60% NaCleq, and C, H, O stable isotope is -23‰ - -13.6‰, -53.3‰ - -46.4‰, 7.71‰ - 8.81‰, respectively. Secondly, due to superim-posed hydrothermal fluid, the temperature, salinity, and C, H, O stable isotope is 150°C - 220°C, 4.65% - 19.05% NaCleq, -20.3‰ -?-3.7‰, -64.7‰ - -53.6‰, 1.49‰ - 1.99‰, respectively. The fluid for reformation is derived from postmagmatic fluid, mixed with a number of meteoric water.
