Role of mantle material in the formation of Sn mineralization – Noble gas constraints from the giant Jiepailing Sn-Be-F deposit, Nanling region, South China

Abstract

The Nanling region hosts a major W-Sn ore belt with numerous Late Triassic and Late Jurassic W-Sn deposits that are the final product of crustal melting due to crustal extension and upwelling of mantle material. Less well-known are Late Cretaceous Sn-polymetallic deposits within the Nanling region and their sources. Here, we present U-Pb cassiterite ages and systematic S-Pb-He-Ar isotope data of sulfides from the giant Jiepailing Sn-Be-F deposit. In situ LA-ICP-MS U-Pb dating of cassiterite yields an intercept age of 90.8 ± 0.9 Ma. The Pb isotopic compositions of pyrite (²⁰⁶Pb/²⁰⁴Pb = 18.54–18.75; ²⁰⁷Pb/²⁰⁴Pb = 15.68–15.71; ²⁰⁸Pb/²⁰⁴Pb = 38.84–38.96) are typical for ancient upper crustal rocks. Pyrite and chalcopyrite have overlapping ranges of δ³⁴S values (2.9 ‰–6.2 ‰) that imply S was mainly derived from a magmatic fluid. Fluid inclusions in pyrite have ⁴He and ⁴⁰Ar concentrations of 0.6 to 3.1 (10^-7 cm³ STP/g) and 2.1 to 5.1 (10^-8 cm³ STP/g), respectively, with corresponding ³He/⁴He ratios of 4.0 to 5.7 Ra (where Ra is the ³He/⁴He ratio of air = 1.39 × 10^-6) and ⁴⁰Ar/³⁶Ar ratios of 359.6 to 530.2. The noble gases in the fluid inclusions have a mixed origin from the crust and the mantle, with c. 54 to 75 % He and less than 1 % Ar derived from the mantle. We interpret the mixed geochemical signature to imply that mantle melts provided heat and fluids (and noble gases) for melting of crustal sources that eventually formed the Jiepailing Sn-Be-F deposit. The higher apparent contribution of mantle He reflects that the crustal melt source had very little He, but considerable amounts of Ar. The regional distribution of c. 80–90 Ma old intrusions and Sn mineral deposits in a 1500 km long belt from the Ailao Shan Fault and the Youjiang Basin to the Nanling region implies that the heat source for crustal melting were made available by large scale tectonic processes, including the reactivation of old crustal-scale faults, during the collision of the Gangdese arc and the lithospheric extension above the Paleo-Pacific plate.