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Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation

1
Xinjiang Research Center for Mineral Resource, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China
2
College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
3
Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
4
School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China
5
School of Earth and Planetary Sciences/John de Laeter Centre, Curtin University, Perth 6845, Australia
6
Institute of Mineral Resources Research, China Metallurgical Geology Bureau, Beijing 101300, China
*
Author to whom correspondence should be addressed.
Academic Editors: Massimo D’Antonio and Ilenia Arienzo
Minerals 2022, 12(7), 785; https://doi.org/10.3390/min12070785
Received: 6 May 2022 / Revised: 14 June 2022 / Accepted: 17 June 2022 / Published: 21 June 2022
Alkaline rocks are generally enriched in rare metals (e.g., Nb, Ta, and Zr) and rare earth elements (REE), but the key factors controlling Nb-Ta-REE enrichment remain unclear. The Boziguoer Nb (Ta-Zr-Rb-REE) deposit in Southwest Tianshan (northern margin of Tarim Basin) is China’s largest, with reserves of 0.32 Mt Nb2O5 and 0.02 Mt Ta2O5. It is an alkaline felsic complex 4.45 km in length and 0.5–1.3 km in width, composed of alkalic granite and syenite, which can be subdivided into syenite I and syenite II. The main minerals in each lithofacies are the same (albite, K-feldspar, quartz, arfvedsonite and aegirine). The Nb in the deposit is mainly hosted in pyrochlore supergroup minerals, ubiquitous in alkalic granite and syenite of the Boziguoer deposit. The wide variation in cations (Ca, Na, REE, U, Th) in the A-site further classifies the Boziguoer pyrochlore supergroup minerals as fluornatropyrochlore, fluorcalciopyrochlore and fluorkenopyrochlore. All Boziguoer pyrochlore supergroup minerals are Nb-rich and Ta-poor at the B-site and dominated by F at the Y-site. These cation occurrence illustrate a new mechanism of substitution in the Boziguoer pyrochlore supergroup minerals (2Ca2+ +Ti4+ +4Ta5+ = REE3+ +A-V + 5Nb5+, where A-V is the A-site vacancy). This substitution mechanism is different from that in the pyrochlore supergroup minerals from other rocks such as carbonatite and nepheline syenite, which are dominated by the replacement of Ba (Rb, Sr) with Ca+ Na + A-V. In addition, the substitution of REE (mainly La, Ce) for Ca in the Boziguoer pyrochlore supergroup minerals is likely a result of either REE enrichment or a change in the REE partition coefficient during the evolution of the alkaline magma. Both the pyrochlore supergroup minerals and their host rocks display negative large ion lithophile element (LILE; K, Rb, Sr, and Ba) anomalies, positive high-field-strength element (HFSE) anomalies and light rare earth element (LREE) enrichment with negative Eu anomalies. This is consistent with the crystallization of the pyrochlore supergroup minerals from the magma rather than from hydrothermal fluids, suggesting a magmatic origin. These findings indicate that the mechanisms of pyrochlore supergroup minerals crystallization in alkaline magma may be significantly different from those in carbonatite and nepheline syenite, and that magmatic differentiation processes may have played a role in the enrichment of the Boziguoer deposit by Nb. View Full-Text
Keywords: Boziguoer Nb-Ta-Zr-REE deposit; niobium mineralization; mineral chemistry of pyrochlore; new element substitution mechanism; alkaline felsic rocks; northern margin of Tarim basin Boziguoer Nb-Ta-Zr-REE deposit; niobium mineralization; mineral chemistry of pyrochlore; new element substitution mechanism; alkaline felsic rocks; northern margin of Tarim basin
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MDPI and ACS Style

Sun, Z.; Qin, K.; Mao, Y.; Tang, D.; Wang, F.; Evans, N.J.; Zhou, Q. Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation. Minerals 2022, 12, 785. https://doi.org/10.3390/min12070785

AMA Style

Sun Z, Qin K, Mao Y, Tang D, Wang F, Evans NJ, Zhou Q. Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation. Minerals. 2022; 12(7):785. https://doi.org/10.3390/min12070785

Chicago/Turabian Style

Sun, Zhenghao, Kezhang Qin, Yajing Mao, Dongmei Tang, Fangyue Wang, Noreen J. Evans, and Qifeng Zhou. 2022. "Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation" Minerals 12, no. 7: 785. https://doi.org/10.3390/min12070785

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