Pitchblende Mineralization in the Precambrian Plateau of Finnmarksvidda, Northern Norway
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Abstract Pitchblende, hematite, and chlorite are formed in a large carbonate breccia consisting partly of greenstones and greenschists, partly of albite-rich rocks of several kinds. One of the latter, a coarse-grained albite »syenite» contains the uranium mineralization, while hematite is common in the whole breccia. The genesis of the pitchblende mineralization is discussed in relation to the main petrogenic processes of the orogeny: the granitization, the carbonatization, and the albite rock formation. The close spatial relationship of pitchblende and albite »syenite» indicates a close genetic relationship.Keywords:
Uraninite
Breccia
Mesothermal
Uraninite
Fluorite
Gangue
Mesothermal
Ore genesis
Hypogene
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The Longshoushan Metallogenic Belt (northwestern China) is known for its word-class Jinchuan Ni-Cu sulfide (Pt) deposit and is also an important uranium metallogenic belt. The Jiling uranium deposit in this belt is a typical Na-metasomatic uranium deposit, which rarely occurs in China. Mineralization in the Jiling uranium deposit is hosted in granitoids that have suffered a Na-metasomatic alteration. There are three kinds of uranium minerals, including uraninite, pitchblende, and coffinite in the Jiling uranium deposit. Pitchblende is the predominant uranium mineral. Integrating the mineralogy and geochemistry of uranium minerals, and in situ electron microprobe analyzer (EMPA) U-Th-Pb chemical dating, we aimed to unravel the age and nature of the mineralization, to decipher the characteristics of the hydrothermal alteration and the U mineralization process. Based on the microtextural features and compositional variations, primary uraninite was altered to uraninite A and B, and fresh pitchblende was altered to pitchblende A and B. The best-preserved uraninite crystals displayed a euhedral-shape with high Pb and low SiO2, CaO, FeO, and Al2O3 contents, and was interpreted as primary uraninite. The EMPA U-Th-Pb chemical ages revealed that uraninite may have formed at 435.9 ± 3.3 Ma. High ThO2 + ΣREE2O3 + Y2O3 contents illustrated that the best preserved uraninite crystallized at a high temperature. Altered pitchblende A showed a relatively brighter gray color in backscattered electron (BSE) images and with a lower SiO2 content than B. Three analysis spots of the fresh pitchblende showed low contents of ΣSiO2 + CaO, indicating no obvious alteration. EMPA U-Th-Pb chemical dating gave a mean chemical age of 361 Ma. The low Th + ΣREE2O3 contents indicated that this pitchblende formed at a relatively low temperature. According to the different characteristics of occurrence and chemical composition, the coffinite in the Jiling uranium deposit can be divided into coffinite A and B, respectively. The compositional variation of the fresh and altered uraninite and pitchblende indicated that both uraninite and pitchblende underwent at least two discrete hydrothermal fluid alterations. The U mineralization was divided into two stages; uraninite was formed at a high temperature and possibly from a magmatic-hydrothermal fluid during ore stage I. Then, pitchblende was formed at a low temperature, during ore stage II. According to the petrographic observations and their chemical compositions, coffinite A and B resulted from the alterations of uraninite and pitchblende, respectively.
Uraninite
EMPA
Uranium ore
Metasomatism
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As shown in the accompanying paper by Tuttle and Bowen, two kinds of albite exist. They are distinguished by optical properties and differences in X-ray powder patterns. The generally known "normal" albite (called "low" albite in this paper) is one type, the other (here called "high" albite) can be obtained by heat treatment of "low" albite or by direct synthesis. This investigation deals with some of the "low-high" albite relations. X-ray investigations combined with optical measurements have been carried out. The optical measurements are similar to those reported by Tuttle and Bowen. Structural differences as revealed by the X-ray photographs are discussed. The lattice of "low" albite differs from that of "high" albite in that (010)(100) changes from 90°30' to 88°15' and (010)(001) from 86°20' to 86°0'. This lattice difference is accompanied by changes in intensities of the X-ray reflections. The structural differences, as far as they are expressed as intensity differences of the X-ray reflections, are of the same order of magnitude as the differences in microcline and orthoclase.
Orthoclase
Microcline
Lattice (music)
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Abstract Pitchblende, hematite, and chlorite are formed in a large carbonate breccia consisting partly of greenstones and greenschists, partly of albite-rich rocks of several kinds. One of the latter, a coarse-grained albite »syenite» contains the uranium mineralization, while hematite is common in the whole breccia. The genesis of the pitchblende mineralization is discussed in relation to the main petrogenic processes of the orogeny: the granitization, the carbonatization, and the albite rock formation. The close spatial relationship of pitchblende and albite »syenite» indicates a close genetic relationship.
Uraninite
Breccia
Mesothermal
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Summary The mode of occurrence of uraninite, pitchblende and coffinite at five localities in south-west England is described. Chemical and isotopic analyses of concentrates containing these minerals provide the following 206 Pb/ 238 U ages (My.): uraninite, South Crofty, Cornwall, 277 ± 10; uraninite, Geevor, Cornwall, 223 ± 5; pitcbblende-coffinite, King's Wood, Devon, 206 ± 5; pitchblende, Wheal Bray, Cornwall, 165 ± 4; pitchblende, South Terras, Cornwall, 474 ± 2; coffinite, Geevor, Cornwall, c. 45 (chemical analysis only). These results are compatible with geological and mineralogical evidence. Consideration of these and other results indicates that there were at least three periods of uranium mineralization in south-west England, at c. 290 My., c. 225 My., and c. 50 My. Other intermediate ages may also be valid but further work is required to substantiate these.
Uraninite
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