Magmatic-hydrothermal evolution of the Donggou porphyry Mo deposit at the southern margin of the North China Craton: Evidence from chemistry of biotite

Abstract

Late Mesozoic granitoids are widespread in the southern margin of the North China Craton (NCC), occurring commonly as both small porphyritic stocks and large batholiths. Most of the Mo deposits are closely associated with small porphyritic bodies. In order to determine the relationship between Mo mineralization and the granitoids, a systematic geochemical study of biotite from the Taishanmiao batholith and the Mo mineralization-associated Donggou porphyry was conducted.Trace element features of biotites indicate a differentiation trend from rocks of the Taishanmiao batholith to those of the Donggou porphyry, as revealed by systematically decreasing K/Rb ratios, Co, Ba, V and Ti, and increasing Cs, Li, Ta and Tl. The contents of Mo also increase with the degree of magmatic differentiation.The compositions of biotite follow a trend towards more magnesium-rich compositions, and mostly plot above the NNO buffer. The Fe3+/Fe2+ values of biotite gradually increase from the Taishanmiao batholith to the Donggou porphyry, indicating the progressive increasing fO2 during magmatic differentiation. The halogen fugacities of magmatic fluids calculated from biotite compositions show a trend of magmatic differentiation. The earlier fluids associated with the Taishanmiao batholith are relatively F-poor with log(fHF/fHCl) < 0, whereas the later fluids derived from the Donggou porphyry are relatively F-rich with log(fHF/fHCl) > 0. The high degree of melt fractionation and progressive increasing of oxygen fugacity is beneficial to concentrate Mo in the residual melt. In addition, later relatively F-rich fluid may be beneficial to extract Mo from the melt, and thus favorable for Mo mineralization.