Changes in the melt structure and enrichment of rare metals W-Sn-Nb-Ta in granitic magma: An example from the Xiaru Early Paleozoic granites

A number of Cenozoic high slica leucogranites in the Himalayan orogenic belt are enriched in rare metals ( e. g., Nb, Ta, W, Sn, Be, Li) and have high potential to produce economic rare metal deposits. In addition to the Cenozoic granites, the Early Paleozoic granites are widely distributed and have similar geochemical characteristics to the Cenozoic granites. Whether they also have mineralization potential is an important question to be further explored. The Xiaru Gneiss Dome hosts dominantly Early Paleozoic granitic gneisses intruded by Cenozoic leucogranites and pegmatites. These granitic gneisses show similar geochemical characteristics to those in the Cenozoic leucogranites and can be subdivided into two groups in terms of their enrichment in Sn-W and Nb-Ta. Group -1 granitic gneisses are enriched in W and Sn ( W =5 x 10(-6) similar to 42 x 10(-6), Sn = 12 x 10(-6) similar to 35 x 10(-6)), whereas Group -2 enriched in Nb and Ta ( Nb =23 x10(-6) 108 x10(-6), up to 217 x10(-6) Ta =8 x10(-6) similar to 38 x 10(-6), up to 143 x10(-6)). Compared with Group -1 granitic gneisses, Group -2 ones are characterized by (1) higher Na20 but lower K-2 O, FeOT, TiO2, P2O(5) Sr, Zr; ( 2) slightly enrichment in MREE, but depletion in LREE and HREE with pronounced Eu negative anomalies; and (3) higher Nb, Ta, but lower W, Sn. The systematic relationship of elements suggests that (1) both groups of granite gneisses are the products from more primary magmas by different degrees of fractional crystallization of plagioclase, zircon, mica, and ( 2) Group -2 granites represent the most evolved melts. Both groups of granitic gneisses in the Xiaru Dome have Nb/Ta lower than 5, but Zr/Hf ratios in Group -1 are greater than 20 in contrast to less than 20 in Group -2. Therefore, we suggest that with fractional crystallization of granitic magma, when Zr/Hf ratios drop to 20, the melt structure changes substantially in conjunction with changes in the melt' s composition from potassium -rich to sodium -rich. Such changes further induce changes in the dissolution behaviors and subsequent enrichment of Sn, W, Nb, and Ta in the melts of granitic compositions as the magma differentiation proceeds. Results yielded in this study demonstrate that (1) both the Cenozoic granites and the Early Paleozoic granites within the Himalayan orogenic belt can enrich in rare metal elements, and ( 2) change of melt structure in the evolving granitic magma plays a key control on the enrichment of rare metals in granites.