Origin of the Luping Pb deposit in the Beiya area, Yunnan Province, SW China: Constraints from geology, isotope geochemistry and geochronology

Abstract

The Luping Pb deposit, a newly discovered Pb–(Fe–Au–Ag–Zn) polymetallic deposit close to the Cenozoic Beiya porphyry-related Au–(Fe–Cu–Pb–Zn–Ag) deposit, is located in the central Sanjiang (three rivers: Jinshajiang, Lancangjiang and Nujiang) region (western Yangtze Block) in southwestern China. Ore bodies of the Luping deposit are mainly stratiform type, host in the Middle Triassic Beiya Formation limestone and Lower Triassic Qingtianbao Formation sandstone, and occur along NNW-trending bedding-plane faults within axis of Beiya–Qinghe syncline. Three ore types, namely the Pb–Au sulfide, Fe–Au, and Au ores, can be discerned regarding their ore metal association with reserves of 2.7million tonnes (Mt) (average grade: 3.06wt.% Pb, 0.66g/t Au, 41g/t Ag, and 0.82wt.% Zn), 3Mt (average grade: 30wt.% Fe, 0.96g/t Au and 31g/t Ag), and 0.67Mt (average grade: 2.16g/t Au and 13g/t Ag), respectively. Pb–Au sulfide ores contain mainly galena, siderite, native gold, sphalerite, and pyrite, with minor chalcopyrite, pyrrhotite and arsenopyrite, and gangue minerals such as quartz, calcite and dolomite. Sulfide ore textures include mainly brecciated, banded and vein, and minor massive and disseminated. LA-ICP-MS zircon U–Pb dating of alkali-rich quartz syenite porphyry from drill holes yielded a Late Eocene (Priabonian) age (36.2±0.4Ma), broadly coeval to the zircon U–Pb ages (ca. 38Ma–31Ma) of alkaline porphyries and molybdenite Re–Os age (36.9±0.3Ma) from the Beiya deposit near Luping. Calcite separates have δ13CPDB and δ18OSMOW values ranging from −5.8‰ to −5.1‰ (average: −5.6‰) and +13.2‰ to +14.7‰ (mean: +13.6‰), respectively, suggesting association with the mantle. The sulfides have a narrow range of δ34SCDT values (+0.8‰–+2.8‰, mean +1.8‰), which is similar to that of mantle-derived S (0±3‰). 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of the sulfides range from 18.62 to 18.69, 15.66 to 15.70 and 38.92 to 39.08, respectively, similar to those of the porphyry whole-rock (206Pb/204Pb 36 Ma=18.61–18.71, 207Pb/204Pb 36 Ma=15.64–15.77 and 208Pb/204Pb 36 Ma=38.88–39.32). Ore deposit geology and C–O–S–Pb isotopic data suggest that the sources of metals and ore-forming fluids were likely to be derived from the alkali-rich porphyries in the Beiya area. We propose that the Luping deposit is best classified as an epithermal Pb polymetallic deposit associated with the Beiya alkaline porphyries (ca. 36Ma–31Ma). Similar to other deposits related to the Cenozoic alkaline porphyries in the Jinshajiang–Red River (Ailaoshan) metallogenic belt, the Luping deposit may have been formed in the India–Eurasia post-collisional setting, when partial melting occurred in the Triassic stagnant slab that thickened the lower crust and the upper mantle, producing the alkaline magmatism and its associated mineralization.