Geology, geochemistry and genesis of Keyue: A newly discovered Pb-Zn-Sb-Ag polymetallic deposit associated with magmatic center in Southern Tibet, China

摘要

The Keyue Pb-Zn-Sb-Ag polymetallic deposit is located within the North Himalayan Metallogenic Belt (NHMB). This study aims to clarify whether the Keyue deposit has close association in time and space with magmatism, and discuss the significance for further exploration. Orebodies are hosted by the Lower Jurassic Ridang Formation black carbonaceous slate and controlled by the NE-trending faults. Keyue is characterized by multiple stages of hydrothermal veins, including Mn-Fe Carbonate-sulfide (I), quartz-sulfide-sulfosalt mineral (II) and quartz-calcite-stibnite veins (III). Hydrothermal minerals contain six types of fluid inclusions, including H2O-rich bi-phase aqueous (type I), pure CO2 (type II), CO2-rich H2O-CO2-NaCl tri-phase (type III), H2O-rich H2O-CO2-NaCI tri-phase (type IV), halite-bearing tri-phase (type V) and halite-bearing H2O-CO2-NaCl multiphase inclusions (type VI). All six types of inclusions were observed in stage II quartz, whereas only types I and III inclusions in stage I siderite and sphalerite and type I inclusions in stage III quartz and calcite. Type I inclusions in stage I display total homogenization temperatures (T-h,T- total) ranging from 250.1 to 289.3 degrees C, with salinities of 9.6-16.3 wt% NaCleq. Types I, III, IV and V inclusions in stage II yield T-h,T- total of 175.5-282.7 degrees C, 188.8-286.3 degrees C, 214.3-287.5 degrees C and 198.3-266.0 degrees C, with corresponding salinities of 4.0-21.5 wt% NaCleq, 0.6-2.9 wt% NaCleq, 0.0-11.0 wt% NaCleq and 30.1-33.2 wt% NaCleq, respectively. Type I inclusions in stage III have T-h,T- total values of 172.3-240.7 degrees C and salinities of 3.1-12.6 wt% NaCleq. Fluid phase separation occurred in stages I and II, resulting in CO2 and H2S release, pH increase and rapid Pb-Zn-Ag precipitation, whereas mixing of magmatic fluids and meteoric water in stage III caused the cooling, dilution and stibnite precipitation. Fluid inclusion analysis and C-H-O isotopic compositions indicate that the initial hydrothermal fluids were magmatic water, with increasing input of meteoric water with time. The delta S-34(CDT) values of sulfides from three stages are similar and range from 4.8 to 10.3 % (average 7.7 parts per thousand), showing characteristics of mixture of magmatic and sedimentary sulfur. The Pb isotopic compositions of sulfides are closely consistent with those of Cenozoic leucogranite and Precambrian metamorphic crystalline basement in the NHMB, suggesting that the ore-forming metals were sourced from the hidden causative Himalayan leucogranite formed by the decompression melting of metamorphic crystalline domains. Combining evidence from geochronology, fluid inclusions, isotopes suggest that the Keyue Pb-Zn-Sb-Ag and Cuonadong W-Sn-Be mineralization at different locations around the Cuonadong Dome constitute an integrated magmatic-hydrothermal system, with the distal end-member Pb-Zn-Sb-Ag veins centered on the Cuonadong leucogranites-related W-Sn-Be mineralization. Pegmatite-skarn-type W-Sn-Be polymetallic mineralization may occur at deeper sites beneath the Keyue area, and the distal Pb-Zn-Sb-Ag veins controlled by faults within strata also need more attention around these leucogranite domes in the NHMB.