Magmatic-hydrothermal fluid evolution of the tin-polymetallic metallogenic systems from the Weilasituo ore district, Northeast China

The large Weilasituo Sn-polymetallic deposit is a recent exploration discovery in the southern Great Xing'an Range, northeast China. The ore cluster area shows horizontal mineralization zoning, from the inner granite body outward, consisting of high-T Sn-W-Li mineralization, middle-T Cu-Zn mineralization and peripheral low-T Pb-Zn-Ag mineralization. However, the intrinsic genetic relationship between Sn-W-Li mineralization and peripheral vein-type Pb-Zn-Ag-Cu mineralization, the formation mechanism and the deep geological background are still insufficiently understood. Here, we use fluid inclusions, trace elements concentrations in quartz and sphalerite, and H-O isotope studies to determine the genetic mechanism and establish a metallogenic model. Fluid inclusion microthermometry and Laser Raman spectroscopic analysis results demonstrates that the aqueous ore-forming fluids evolved from low-medium salinity, medium-high temperature to low salinity, low-medium temperature fluids. Laser Raman spectroscopic analysis shows that CH4 is ubiquitous in fluid inclusions of all ore stages. Early ore fluids have delta O-18(H2O (v-SMOW)) values from + 5.5 to + 6.2 parts per thousand and delta D values of approximately - 67 parts per thousand, concordant with a magmatic origin. However, the late ore fluids shifted toward lower delta O-18(H2O (v-SMOW)) (as low as 0.3 parts per thousand) and delta D values (similar to - 136 parts per thousand), suggesting mixing between external fluids derived from the wall rocks and a contribution from meteoric water. Ti-in-quartz thermometry indicates a magmatic crystallization temperature of around 700 degrees C at a pressure of 1.5 kbar for the magmatic ore stage. Cathodoluminescence (CL) imaging and trace element analysis of quartz from a hydrothermal vug highlight at least three growth episodes that relate to different fluid pulses; each episode begins with CL-bright, Al-Li-rich quartz, and ends with CL-dark quartz with low Al and Li contents. Quartz from Episode 1 formed from early Sn-(Zn)-rich fluids which were likely derived from the quartz porphyry. Quartz from episodes 2 and 3 formed from Zn-(Sn)-Cu-rich fluid. The early magmatic fluid is characterized by low fS(2). The SO2 produced by magma degassing reacted with heated water to form SO42-, causing the shift from low fS(2) to high fS(2). The SO42- generated was converted to S2- by mixing with CH4-rich, Fe and Zn-bearing external fluid which led to late-stage alteration and dissolution of micas in vein walls, thus promoting crystallization of pyrrhotite, Fe-rich sphalerite and chalcopyrite and inhibiting the precipitation of anhydrite. This study shows that ore formation encompassed multiple episodes involving steadily evolved fluids, and that the addition of external fluids plays an important role in the formation of the later Cu-Zn and Ag-Pb-Zn mineralization in the Weilasituo ore district.