Earth Science Frontiers ›› 2023, Vol. 30 ›› Issue (5): 115-133.DOI: 10.13745/j.esf.sf.2023.5.11

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Geochemical analysis of apatite and columbite-group minerals of beryl-columbite pegmatites in Ningshan, southern Qinling orogen, China

NIE Xiao1,2(), CHEN Lei3, GUO Xianqing1, YU Tao1, WANG Zongqi4   

  1. 1. Chinese Academy of Geological Sciences, Beijing 100037, China
    2. China Deep Exploration Center, Chinese Academy of Geological Sciences, Beijing 100037, China
    3. MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
    4. MNR Mineral Exploration Technical Guidance Center, Beijing 100037, China
  • Received:2022-11-30 Revised:2022-12-15 Online:2023-09-25 Published:2023-10-20

Abstract:

The Ningshan pegmatite district is locate at the outer contact zone on the south side of the Ningshan granitoid batholith. It is one of the three major pegmatite districts in the Qinling orogenic belt with numerous Rb-Be-Nb-Ta pegmatite deposits. This study investigates the mineralogy of beryl-columbite pegmatites by scanning electron microscope (SEM) and electron probe microanalyzer (EPMA) methods, and determines the U-Pb ages for columbite and uraninite. The apatite (Ap) and columbite-group minerals (CGM) of the Ningshan pegmatites can each be divided into two types according to their chemical composition and mineral structural characteristics. Type I apatite (Ap-I) has relatively low Mn (MnO, 0.02%-2.27%)/Cl (0%-0.06%) contents, while type II (Ap-II) has high Mn (MnO, 14.51%-19.12%)/Cl (0.12%-0.16%) contents. Type I columbite-group minerals (CGM-I) is classified as columbite-(Fe) and type II (CGM-II) as columbite-(Fe) to tantalite-(Fe) on the classification diagram. Combining mineral composition and metasomatism characteristics, we consider that type I minerals are formed by magmatic fractional crystallization, while type II are related closely to fluid activities. The U-Pb age for CGM-I ((206.3±1.5) Ma) represents the emplacement age of pegmatites; while in situ Nd isotopic analysis suggests that Ap-I (εNd, -4.3 to -2.5) is genetically associated with Late-Triassic quartz monzonite and monzonite granite in the Ningshan batholith. The above results suggest that the Ningshan beryl-columbite pegmatites experienced multi-stage fluid activities. The U-Pb age proximity between secondary uraninite ((199.2±1.5) Ma) and CGM-I implies that early-stage fluid activities are closely related to magma evolution-likely to the exsolution of residual pegmatite-forming fluids during the magmatic to hydrothermal transition. Whilst a mixing of late-stage fluids with host-rock derived external fluids is indicated by the local high residual-Mn content in CGM-II and the high Mn/Cl contents in Ap-II.

Key words: apatite, columbite group mineral, fluid activity, beryl-columbite pegmatite, Ningshan pegmatite district

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