Barium isotopes reveal the role of deep magmatic fluids in magmatic-hydrothermal evolution and tin enrichment in granites

Abstract

Although deep magmatic fluids are important for the shallow magmatic-hydrothermal evolution and transport/enrichment of ore-forming elements in transcrustal magmatic system, it is still difficult to identify the influences of such fluids by using conventional geochemical indicators. Here we report Ba isotope compositions for granites from the Jurassic Qitianling batholith in the Nanling Range, South China that hosts several large tin deposits. This composite batholith consists of three stages of granites with an age range of ∼15 Ma and sharp contacts between each other, which suggest an underlying long-lived crystal mush-dominated transcrustal magmatic system. The small variation of δ138/134Ba in the first-stage less evolved granites (−0.24–0.37‰) indicates that fractional crystallization in deep crystal mush of this system does not cause significant Ba isotope fractionation. In contrary, the latter-two stage highly differentiated granites show more variable and overall lower δ138/134Ba (−1.79–0.14‰), which cannot be explained by K-feldspar-controlled fractional crystallization in shallow crystal mush. Instead, the distinctively low Ba contents (<100 μg/g) and magmatic-hydrothermal evolution features of the latter-two stage granites suggest that their low δ138/134Ba are due to modification by magmatic fluids. Further modeling demonstrates that exsolved fluids from deep crystal mush can explain the light Ba isotope compositions of the latter-two stage granites. Because these ascending deep magmatic fluids can efficiently scavenge Sn and other fluid-mobile elements then transport to shallow level, we propose that these fluids could provide critical materials for the Sn-polymetallic mineralization.