Fluorapatite fingerprints on magmatic-hydrothermal fluid evolution of the Lianyunshan Nb-Ta-Sn deposit, NE Hunan (South China)

Abstract

The textures and compositions of apatite grains have been reported for many types of economic and non-economic deposits, but apatites in hydrothermal NbTa (rare-metal) deposits have received scant attention to date. Apatite is common in magmatic–pegmatites and metasedimentary cover rocks of the Lianyunshan Complex (LYSC) of northeastern Hunan Province in South China, potentially providing insights into NbTa metallogenic systems. Here, we present integrated textural, major- and trace-elemental, and in situ U-Pb-Sr isotopic data for hydrothermal apatite grains from the metasedimentary (schist)–magmatic (muscovite monzogranite)–pegmatite systems of the LYSC, to investigate its crystallization and mineralization processes in detail. Four types of fluorapatite (S-ap1 and S-ap2 in schist, G-ap in muscovite monzogranite, and P-ap in pegmatite) are recognized. The first three types crystallized in a magmatic-hydrothermal (transitional-stage) setting at ∼132–129 Ma, whereas the P-ap crystallized during a late-stage hydrothermal NbTa mineralization event at ∼123 Ma. All four types show similar and low Cl content (< 0.01 wt%) and high F content (2.70–4.49 wt%), suggesting their precipitation from a melt derived through partial melting of crustal material rich in F-bearing minerals. Relatively homogeneous (⁸⁷Sr/⁸⁶Sr)_i ratios are shown by the S-ap1 and S-ap2 grains (0.75067 ± 0.00010), contrasting with higher and variable values in the G-ap (0.75242 to 0.75943, mean 0.75610) and P-ap grains (0.75350–0.75776, mean 0.75533). The Sr isotope signatures of the LYSC appear to be inherited from Lengjiaxi Group metasedimentary strata. Our study highlights application of integrated petrographic-geochemical analyses of fluorapatites to trace the nature of ore-forming fluids in hydrothermal NbTa (rare-metal) deposits of metasedimentary–magmatic–pegmatite systems.