The role of hydrothermal alteration in tungsten mineralization at the Dahutang tungsten deposit, South China

Abstract

The giant Dahutang tungsten deposit has total reserves of more than 1.31 Mt of WO3 with a scheelite/wolframite ratio of ∼1 and is mainly hosted by the Neoproterozoic Jiuling granodiorite batholith (∼820 Ma). The deposit is characterized by four types of alteration, including biotite alteration, phyllic alteration, greisenization and silicification. Whole-rock geochemical analyses showed that the elements Ti, Ni, V, Sc, and Lu exhibited immobility during the four alteration processes. The mobile element geochemistry effectively differentiated among four distinct hydrothermal alteration styles. During the biotite mineralization, there were mass gains in Al2O3, Fe2O3, MnO, MgO, K2O, P2O5, and W and depletions in SiO2, CaO, and Na2O. The phyllic alteration exhibited mass gains in SiO2, Fe2O3, MgO, and W and depletions in CaO, Na2O, and K2O, but Al2O3, MnO, and P2O5 were immobile. The weak greisenization exhibited mass gains in SiO2, Fe2O3, K2O, P2O5 and W and depletions in Na2O, MgO, and CaO, whereas Al2O3 and MnO remained immobile. The silicification exhibited mass gains in SiO2 and W and depletions in Al2O3, Fe2O3, MgO, CaO, Na2O, and K2O, but MnO exhibited immobility. These alterations were related to at least three major hydrothermal fluid systems. Firstly, a hydrothermal fluid caused biotitization zones and Fe + Mn ± W mineralization (mostly biotite) at temperatures ranging from 560 to 450 °C, and the magmatic hydrothermal fluids were derived from the Jurassic porphyritic biotite granite (∼150 Ma) and characterized by alkaline, oxidized, moderate-pressure, and low-salinity features. Secondly, a hydrothermal fluid was formed by the mixing of magmatic fluid with meteoric water and was responsible for the phyllic/weak greisenization alteration zones at temperatures ranging from 440 to 450 °C and characterized by alkaline, weakly oxidized, moderate-pressure, and low-salinity features. Thirdly, a hydrothermal fluid caused greisenization and silicification zones at temperatures ranging from 440 to 160 °C, which were characterized by acidic, reduced, high-pressure, and moderate- to low-salinity features, derived from the Cretaceous fine-grained biotite granite (∼144 Ma), and associated with the main W mineralization at Dahutang.The characteristics of Fe-enriched biotite in the biotitized and greisenized rock – decreasing temperature, high pressure, and high log(fH2O/fHCl) – could facilitate the formation of scheelite deposits, but the high F suppressed it. The decreasing temperature and f(O2) and the Fe-Mn released from biotite contributed to the formation and precipitation of wolframite. During alkaline alteration, biotite and apatite acted as storage places for Fe-Mn and Ca, which were subsequently released by acidic alteration to form scheelite and wolframite. There was no enough Ca was released from biotitized rock for the acidic alteration to provide the WO42− to form scheelite. Meanwhile, Fe was added, and Mn was released to form the scheelite + wolframite deposit. Based on all the results, we have developed a genetic model for tungsten mineralization including superimposed alteration processes (alkaline overprinted by acidic alteration), which led to the gain and loss of elements, and corresponding to two magmatic events, the Jurassic porphyritic biotite granite and the Cretaceous fine-grained biotite granite. The superposition of alterations played an important role in the mineralization of the Dahutang giant tungsten deposit.