Piaoac granites exposed in the Cao Bang region, northern Vietnam, are S-type granite, which are associated with W-Sn-Mo-Be-F mineralization. Zircon U-Pb ages, major and trace elements, mineral chemical and Hf isotopic...Piaoac granites exposed in the Cao Bang region, northern Vietnam, are S-type granite, which are associated with W-Sn-Mo-Be-F mineralization. Zircon U-Pb ages, major and trace elements, mineral chemical and Hf isotopic compositions of the W-Sn-bearing granites from the Piaoac District have been investigated in detail. LA-ICP-MS U-Pb dating of zircon grains from these granites yielded ages of 82.5±2.3 and 82±1.8 Ma, representing an episode of Late Cretaceous magmatic event. These granites are characterized by high peraluminous and have typical S-type geochemical signatures with high SiO_2(72.37 wt.%–73.07 wt.%), high A/CNK values(1.61–1.65) and Al_2O_3(14.4 wt.%–15 wt.%). They are enriched in Rb, U, K, Th, Ta and Pb and display pronounced negative Ba, Sr, Nb, Ti and Eu(Eu/Eu*=0.19–0.24) anomalies. The high degree of fractional crystallization is characterized by low Rb, Sr, Ba and Eu concentrations with high ratios of La/Sm and Eu/Eu*. Zircon grains show εHf(t) values from-9.69 to-0.9 and the corresponding TDM2 range from 1.2 to 1.7 Ga, indicating that these granites could be derived from the Proterozoic basement rocks with minor input from mantle-derived magmas. The calculation of Fe^(3+) and Fe^(2+) of biotites indicates a low oxygen fugacity condition(log fO_2 ranging from 10-17 to 10-18 bars, below MH), which is favorable for the W-Sn mineralization. Tungsten and tin have been enriched in granitic magmas through fractionation, and low oxygen fugacity conditions have promoted the accumulation and transportation of W-Sn in the hydrothermal fluids, leading to deposition of mineral phases. The geochemical data suggest that Piaoac granites formed in an extensional setting related with the Late Cretaceous magmatism occurring large-scale lithospheric extensional in South China Block.展开更多
Uranium exploration especially in currently non-producing countries like Nigeria possesses high economic prospect. This study investigates a new uranium prospect in Mika, Northeastern Nigeria. The Mika uranium mineral...Uranium exploration especially in currently non-producing countries like Nigeria possesses high economic prospect. This study investigates a new uranium prospect in Mika, Northeastern Nigeria. The Mika uranium mineralization is located in Mika, Taraba State. Two lodes were identified and additional nine (9) trenches were added. The main lode in the west extends about 10 m, ore vein strikes 348°, inclination 42°and the strike of the lode 306°with average uranium content of 18%. The eastern trench is about 8 × 4 × 6 m which exposes a veinlet of pitchblende. From the petrography, one can deduce that the granitic host rock has suffered deformation resulting in crushing of quartz crystals and stretching of plagioclase. The uraninite and chalcedony in the late phase filled up the fractures along the crystal grain boundaries as veinlets. The laboratory gamma ray analysis of the samples showed that the secondary uranium content is 0.1%, while the primary uranium ore grade is 1.5%. Radiometric in situ measurements showed that Th and K ranges from 47.3 - 3654 ppm and 4.26% - 6.26% respectively. From the survey, a strong radiometric zone extends 800 × 35 m in an NW-SE direction and has highest radiation content of 1200 cpm against the background count rate of 30 cpm. Generally, the uranium concentrations in the ores in Mika area range from 0.03% - 0.12%. Since only the surface occurrences have been explored, the study area is a good prospect for future development when properly explored. The North-eastern Nigeria has been roughly explored by Nigeria Uranium Mining Company (NUMCO) in 1980, but no formal exploration had been followed after that, especially near the site of this article. The company (ACE Mines, Ltd) of the first author was among the few uranium mining companies approved in the list of Nigerian Geological Survey agency of Abuja in Nigeria. The authors determined the coordinates of the site in their first survey which formed the prospect area in the licence.展开更多
基金supported by the National Key R & D Program of China (No. 2016YFC0600404)the National Natural Science Foundation of China (Nos. 41673040 and 41611540339)
文摘Piaoac granites exposed in the Cao Bang region, northern Vietnam, are S-type granite, which are associated with W-Sn-Mo-Be-F mineralization. Zircon U-Pb ages, major and trace elements, mineral chemical and Hf isotopic compositions of the W-Sn-bearing granites from the Piaoac District have been investigated in detail. LA-ICP-MS U-Pb dating of zircon grains from these granites yielded ages of 82.5±2.3 and 82±1.8 Ma, representing an episode of Late Cretaceous magmatic event. These granites are characterized by high peraluminous and have typical S-type geochemical signatures with high SiO_2(72.37 wt.%–73.07 wt.%), high A/CNK values(1.61–1.65) and Al_2O_3(14.4 wt.%–15 wt.%). They are enriched in Rb, U, K, Th, Ta and Pb and display pronounced negative Ba, Sr, Nb, Ti and Eu(Eu/Eu*=0.19–0.24) anomalies. The high degree of fractional crystallization is characterized by low Rb, Sr, Ba and Eu concentrations with high ratios of La/Sm and Eu/Eu*. Zircon grains show εHf(t) values from-9.69 to-0.9 and the corresponding TDM2 range from 1.2 to 1.7 Ga, indicating that these granites could be derived from the Proterozoic basement rocks with minor input from mantle-derived magmas. The calculation of Fe^(3+) and Fe^(2+) of biotites indicates a low oxygen fugacity condition(log fO_2 ranging from 10-17 to 10-18 bars, below MH), which is favorable for the W-Sn mineralization. Tungsten and tin have been enriched in granitic magmas through fractionation, and low oxygen fugacity conditions have promoted the accumulation and transportation of W-Sn in the hydrothermal fluids, leading to deposition of mineral phases. The geochemical data suggest that Piaoac granites formed in an extensional setting related with the Late Cretaceous magmatism occurring large-scale lithospheric extensional in South China Block.
文摘Uranium exploration especially in currently non-producing countries like Nigeria possesses high economic prospect. This study investigates a new uranium prospect in Mika, Northeastern Nigeria. The Mika uranium mineralization is located in Mika, Taraba State. Two lodes were identified and additional nine (9) trenches were added. The main lode in the west extends about 10 m, ore vein strikes 348°, inclination 42°and the strike of the lode 306°with average uranium content of 18%. The eastern trench is about 8 × 4 × 6 m which exposes a veinlet of pitchblende. From the petrography, one can deduce that the granitic host rock has suffered deformation resulting in crushing of quartz crystals and stretching of plagioclase. The uraninite and chalcedony in the late phase filled up the fractures along the crystal grain boundaries as veinlets. The laboratory gamma ray analysis of the samples showed that the secondary uranium content is 0.1%, while the primary uranium ore grade is 1.5%. Radiometric in situ measurements showed that Th and K ranges from 47.3 - 3654 ppm and 4.26% - 6.26% respectively. From the survey, a strong radiometric zone extends 800 × 35 m in an NW-SE direction and has highest radiation content of 1200 cpm against the background count rate of 30 cpm. Generally, the uranium concentrations in the ores in Mika area range from 0.03% - 0.12%. Since only the surface occurrences have been explored, the study area is a good prospect for future development when properly explored. The North-eastern Nigeria has been roughly explored by Nigeria Uranium Mining Company (NUMCO) in 1980, but no formal exploration had been followed after that, especially near the site of this article. The company (ACE Mines, Ltd) of the first author was among the few uranium mining companies approved in the list of Nigerian Geological Survey agency of Abuja in Nigeria. The authors determined the coordinates of the site in their first survey which formed the prospect area in the licence.
