Mineralogical constraints on Nb-Ta fractionation in Early Cretaceous A-type granites from the Suzhou pluton, SE China

Abstract

The Suzhou A-type granite pluton in Jiangsu Province, South China, hosts the first Nb-Ta deposit discovered in China. Three granite phases are exposed in the field: porphyritic amphibole- and biotite-bearing granite (Am-BG), medium-grained porphyritic biotite granite (MBG), and fine-grained porphyritic biotite granite (FBG-SZ). The granites have high Nb-Ta contents, yet only FBG-SZ contains minor columbite. Besides, fine-grained porphyritic biotite granite (FBG-DH) and fine-grained zinnwaldite-albite granite (ZAG) are exposed in nearby drill holes. Granites from the boreholes are enriched in Nb-Ta with preliminary inferred grades of 0.008–0.062 wt% Ta2O5 and 0.056–0.157 wt% Nb2O5. Here, we report whole-rock geochemical data, textural and chemical analyses of various Nb–Ta-bearing minerals, and zircon and columbite U-Pb ages for the Suzhou granites. The granitic rocks are metaluminous to weakly peraluminous, which have evolved through progressive fractionation. The granites have high Nb contents (up to 1700 ppm) and a wide range of Nb/Ta ratios (1–154). Niobium and tantalum have an affinity with Ti-bearing host minerals including silicates (e.g. amphibole, titanite and micas) and oxides (e.g. ilmenite, rutile and columbite). The main host minerals for Nb in the Am-BG and MBG are annite, magnetite, ilmenite, titanite ± amphibole (in the Am-BG) ± rutile (in the MBG), whereas in the FBG (including -SZ and -DH) and ZAG the main host minerals for Nb are Li-Fe micas (lithian annite and zinnwaldite), columbite-group minerals (CGMs), ±ilmenorutile and magnetite (in the FBG), and ±tapiolite (in the ZAG). With decreasing whole-rock TiO2 contents, the Nb/Ta ratios first increase gradually and then decrease abruptly, recording fractionation of various Nb-Ta-bearing minerals. Based on columbite U-Pb dating, Nb-Ta mineralization in the FBG occurred at 123 ± 1–121 ± 1 Ma, and is thus slightly younger than the crystallization age of the granites (126 ± 1–125 ± 1 Ma). The metaluminous-peraluminous, low-P (PLP)- type rare-metal granites of the Suzhou pluton, are compositionally and likely genetically different from typically strongly peraluminous Nb-Ta granites in South China. Fractionation at high temperature, high alkalinity and fluid enrichment were important factors for the extreme Nb enrichment in the Suzhou granites. The Suzhou pluton forms one of several metaluminous-peraluminous A-type granites with recognized Nb-Ta mineralization within the Jiangnan Orogen/metallogenic belt, and we thus suggest that Early Cretaceous Nb-Ta mineralization associated with PLP-type rare-metal granites should be the subject of further exploration.