The geochemical and textural characteristics of magnetite in the Bangbule skarn Pb-Zn deposit, Tibet

• There are five stages of magnetite and reveal the evolution of the magnetite. • The hydrothermal fluid of Bangbule deposit is multi-stage, and mineralization is mainly related to the first stage hydrothermal fluid. The Bangbule deposit is one large skarn-type Pb-Zn deposit newly discovered in the Nyainqentanglha metallogenic belt of Tibet, western China. Combining the texture and geochemistry of magnetite, we distinguished five stages of magnetite. Mt-Ⅰ is euhedral and often replacing early actinolite; Mt-Ⅱ has a triple junction texture and is replaced by hematite at the edge; Mt-Ⅲ is porous and anhedral, most of which formed earlier than sulfide; Mt-Ⅳ is characterized by core-rim texture and oscillating zoning; Mt-V usually cuts early quartz and calcite as veins. The composition of magnetite in Bangbule is comparatively abundant in Al, Cr, Mg, Mn, and Zn, but poor in Sn, indicates that it is hydrothermal skarn originated. Mt-Ⅰ contains high Ti and V content which is related to the early hydrothermal fluid generated during the intrusion of quartz porphyry, and is distinguished by high Mg, Al, Mn, Cr and In. The 120° triple junction texture of Mt-Ⅱ and the lower V indicates the result of fluid-assisted recrystallization and the temperature of hydrothermal fluid decreases and the oxygen fugacity increase. Mt-Ⅲ has lower Cu and Zn content, and its textural that are thought due to the dissolution-reprecipitation process, and its formation closely related to mineralization. Mt-Ⅳ contains the lowest content of vanadium and titanium in the five stages of magnetite, and Mt-Ⅳ-rim has higher Si, Mg, Mn and Al contents than Mt-Ⅳ-core which most likely resulted from crystal formation during diverse fluid compositions. Mt-V contains high contents of Ti and V usually occurs as a vein, indicating that it was formed in the late hydrothermal solution. The geochemistry of the magnetite shows the temperature decreases from Mt-Ⅰ to Mt-IV in turn, but suddenly increase in temperature from Mt-IV to Mt-V, indicating the existence of multiple fluid pulses throughout the skarn system. The early fluid was a high-temperature fluid containing a large amount of lead and zinc. And it became a medium-temperature, salinity and relatively high- f O 2 fluid simultaneously with large amount of metal sulfides began to precipitate. The second pulse of relatively high temperature was released in the early quartz sulfide stage. The research demonstrates that the evolution trend of trace elements in magnetite can effectively constrain the evolution of the fluid. Furthermore, we have discovered that multiple influxes of magmatic hydrothermal fluid in Bangbule deposit and the second pulse of fluid is not related to mineralization.