Tracing fluid exsolution and hydrothermal alteration signature of the Mufushan Nb-Ta-(Li-Be-Cs) deposit, South China: An apatite perspective

Abstract

The appearance of hydrous magmas and the following separation of volatile-rich fluids through hydrothermal alteration are intricately linked to forming granitic rare-metal deposits, the principal source of worldwide Li, Be, Nb, Ta, and Cs production. The lack of mineralogical information from the developing magmatic-hydrothermal system has, however, prevented a thorough comprehension of these processes. Apatite occurs as an accessory mineral in the metasedimentary (schist)–magmatic (muscovite monzogranite)–pegmatite (ore-free/ore-bearing pegmatite) rocks in Mufushan Complex (MFSC) rare-metal ore field of the northeastern Hunan, South China, potentially providing insights into Nb-Ta-(Li-Be-Cs) mineralization. To demonstrate that apatite can potentially record the magmatic-hydrothermal evolution of metasedimentary-magmatic-pegmatite systems, this study presents a combined textural and geochemical study of apatite from the MFSC granitic pegmatite-type rare-metal mineralization. The MFSC apatite textures and compositions have changed (i.e., post-crystallization alteration) since it first crystallized. Apatite from the schist shows homogeneous rim or homogeneous textures with crack or inclusion (S-ap1) and patch core (S-ap2), indicative of a magmatic-hydrothermal origin. Apatite from the muscovite monzogranite (G-ap) displays altered and distinctive core-rim textures, with visible voids, mineral inclusions, and cracks, suggestive of overprinting of early-magmatism texture by hydrothermal fluid. However, compared to the S-ap1, S-ap2, and G-ap, the pegmatite apatite shows more complicated textures, i.e., P-ap1: homogeneous bright and dark areas, and P-ap2: replacement texture involving alteration rim, growth zonation, patchy, and complex zoning patterns. P-ap1 underwent early magmatism and weaker post-hydrothermal overprinting, P-ap2 reflects a magmatic-hydrothermal product. S-ap1 and S-ap2 yield lower intercept ages of 130.6 ± 1.8 Ma and 128.4 ± 3.8 Ma, respectively, which are consistent with the transitional age of magmatic-hydrothermal metallogenic environment in northeastern Hunan. The G-ap and P-ap1 yield older ages of 136.3 ± 2.8 Ma and 141.3 ± 6.7 Ma, respectively, which are corresponding to the age of magmatic early-stage (Nb-Ta)-mineralization within their uncertainty in the northeastern Hunan. The Sr isotopic composition of apatite provide evidence for the provenance of the MFSC batholith in the rare-metal metallogenesis of the Lengjiaxi Group. Therefore, we hypothesize that apatite in granitic rare-metal deposits within metasedimentary-magmatic-pegmatite systems might be employed as a viable proxy to explore fluid exsolution and hydrothermal alteration signature concerning its textures and geochemical fingerprints. Supplementary material: https://doi.org/10.6084/m9.figshare.c.7038699