A detailed research on allanite in weathered sphere from the Xiajialing REE deposit in Xiangshan, Jiangxi, and its metallogenic significance.

The Xiajialing REE deposit is located in Xiangshan volcanic basin, Jiangxi Province. It is a weathered elution -deposited REE deposit. Among the layers of the weathering profile, the content of REEs in spherical weathering layer is the highest. In order to explore the enrichment law of rare earth elements in the spherical weathering layer, the microscope, the scanning electron microscope and the electronic microprobe were used in this study. The main REE minerals of the weathered sphere in the spherical weathering layer is focused on. It is found that after the REE-rich phase transformation of the allanite, an extremely complex compositional ring had developed, and many REE-rich "phases" had been derived. According to the content difference of silicon, lanthanum, cerium and other main elements, It can be preliminary divided into: the REE-rich phase with high silicon and high lanthanum ( SiO2: 11.51% similar to 19. 77%, La2O3, : 12. 69% similar to 17. 54%, Ce2O3, : 6. 80% similar to 8.49%, Sigma REE2O3, : 46. 14% similar to 61. 34%), the REE-rich phase with low silicon and high lanthanum ( SiO2 : 0. 58% similar to 6. 78%, La2O3, : 21. 09% similar to 24. 64%, Ce2O3, : 4. 45% similar to 6. 34%, Sigma REE2O3, : 68. 96% similar to 81. 52%) and the REE-rich phase with low silicon and high cerium ( SiO2: 0.45% similar to 1. 79%, La2O3 : 12. 89% similar to 19. 16%, Ce2O3, : 20. 66% similar to 26. 59%, Sigma REE2O3, : 67.65% similar to 77. 6%). These phases have large composition ranges and poor composition uniformity. After analysis, it is considered that this REE-rich feature may be partly due to the radioactive decay of thorium, which made Th-rich allanite undergo amorphization and volume expansion, and the expansive allanite was easier for the fluid to pass through. The a decay can also ionize the interior of minerals, which may adsorb charged REE complexes, resulting in the enrichment of REEs. The formation of different REE-rich phases may be due to the different types of REEs leached from the upper part at different stages of weathering. The REE-rich phase with low silicon and high cerium will be destroyed by fluid to supply REEs to the lower part for REE mineralization at last.