Geochronology, geochemistry, and mineral chemistry of the Lingshan-Huangshan complex, South China: Insights into Nb and Ta enrichment

Abstract

The Huangshan Nb-Ta deposit, located at the northern segment of the Qin-Hang metallogenic belt of South China, is closely associated with the Lingshan-Huangshan complex. As a large Nb-Ta deposit, with the Nb-Ta mineralization hosted by the late two phases of the Lingshan-Huangshan complex, the factors that led to Nb and Ta enrichment in the Huangshan deposit remain unclear. Therefore, we conducted a comprehensive investigation of petrography, geochronology, whole-rock geochemistry, and mineral compositions (zircon and mica) of the Lingshan-Huangshan complex. The Lingshan-Huangshan complex comprises the Lingshan coarse-grained porphyritic amphibole-biotite granite, the Huangshan coarse-grained biotite granite, the fine-grained albite-biotite granite, and the pegmatite from the center outwards. Zircon U-Pb dating shows that the different phases within this complex formed at 133.4–132.7 Ma. They are characterized by high SiO2 (66.46–79.11 wt%), total alkali (7.38–10.39 wt%) contents, moderate A/CNK values (average of 1.01), and high Ga/Al and Zr + Nb + Ce + Y values, presenting geochemical affinities to A-type granites. Combined with the pre-existing Nd-Hf isotopic data, they originated from the Proterozoic crust of South China with some input of juvenile crustal or mantle-derived materials. The whole-rock Al2O3, TiO2, and Fe2O3 contents and Nb/Ta and Zr/Hf ratios decrease with increasing SiO2 contents from the Huangshan coarse-grained biotite granite, the fine-grained albite-biotite granite to the pegmatite. These variations, together with the increasing Hf and Th + U + Y in zircon and increasing Rb, Nb, and Ta in mica, indicate that these phases (especially the late two) underwent highly fractional crystallization. The Lingshan coarse-grained porphyritic amphibole-biotite granite shares part of the whole-rock geochemical variations with the other phases, but its Nb/Ta ratios varied discordantly, which may be attributed to the solidified residual crystal-rich mush. The increasing F and Li in micas from the early to late phases of the Lingshan-Huangshan complex reflect enrichment of F and Li content during the magmatic evolution process, which is conducive to enhancing the solubilities of Nb and Ta in the melt. Accordingly, we suggest that the Nb-Ta enrichment in the Huangshan deposit is closely associated with highly fractionated magma and elevated volatile elements F and Li in melts.