Petrogenesis and metallogenic significance of multistage granites in Shimensi tungsten polymetallic deposit, Dahutang giant ore field, South China

Abstract

The Shimensi tungsten polymetallic deposit, situated in the Dahutang ore field, South China, is one of the largest tungsten deposits in the world, with an estimated WO3 reserve of 0.74 million tons. Coarse-grained porphyritic biotite granite (CPBG), fine-grained porphyritic biotite granite (FPBG), fine-grained biotite granite (FBG) and biotite granite porphyry (BGP) are all ore-related, but their diagenetic relationships and contributions to W-Cu-Mo mineralization are still in dispute. LA-ICP-MS monazite U-Pb dating of the CPBG, FPBG, FBG and BGP yield emplacement ages of 147.9 ± 1.1 Ma, 146.4 ± 1.1 Ma, 138.6 ± 0.98 Ma and 142.8 ± 1.7 Ma, respectively. Whole-rock geochemical results indicate that the four granites should be classified as S-type granites, but BGP has distinct features transitional between S- and I-type granites. They were possibly generated by partial melting of upper crustal pelites and basic volcanic rocks with different proportion from the Neoproterozoic Shuangqiaoshan Group in the source. Proportional variation in the magmatic source (clay and basic basalts) induces the change of geochemical compositions of the Shimensi granites. Geochemical characteristics suggest that they were derived from two magma chambers (the CPBG, FPBG and FBG vs. the BGP) and experienced different evolutionary processes and different degree of magmatic differentiation during magmatic evolution. Chondrite-normalized REE patterns for the four granites display low total REE contents, variable and strongly enriched LREE relative to HREE and medium-strong negative Eu anomalies. They are enriched in Rb, Th, U, Ta and depleted in Ba, Nb, Sr, P, Ti. Biotites are iron-rich and aluminum-poor, and can be classified as ferro-biotite (CPBG, FPBG and FBG) and siderophyllite (BGP). The partial melting of tungsten-rich metasediments of the Shuangqiaoshan Group and high degree of fractional crystallization led to enrichment in tungsten in the magma suites. Oxygen fugacities of the CPBG and FPBG declined from early (most above the NNO buffers) to late stages of fractional crystallization (between the NNO and QFM buffers) because of the higher degree of magmatic differentiation in the late stages. In the early stages of fractionation, tungsten accumulated in the residual melts rather than partitioning into accessory minerals. In the late stages, lower oxygen fugacities and high fluorine contents promoted the removal of tungsten from the residual magma into reduced hydrothermal fluids. On the other hand, the FBG and BGP remained constant (above the NNO buffers) over the entire process of crystallization owning to the stable degree of magmatic differentiation, promoting retention of tungsten in the melt and resulting in low grade tungsten mineralization. Tungsten mineralization in the Shimensi deposit is greatly controlled by the redox states of the associated magma. The two porphyritic granites (the CPBG and FPBG) are most likely the main contributors of tungsten, while the FBG and BGP are mainly responsible for copper and molybdenum in the Shimensi deposit. Prolonged multiphase magmatism and prolonged W-Cu-Mo mineralization play important roles in the formation of Shimensi large tungsten polymetallic deposit.