Genesis and Mineralization Process of the Lanuoma Sediment-Hosted Pb–Zn Deposit, Sanjiang Metallogenic Belt, Southwestern China: Constraints from Zn, Pb, and S Isotopes

The Lanuoma sediment-hosted lead–zinc (Pb–Zn) deposit, situated in the central part of the Sanjiang base metal metallogenic belt (SMB) within the Changdu Basin, is hosted by Triassic Bolila Formation limestone. The source of metals and sulfur (S), as well as the ore-forming processes for the deposits in this belt, are contentious. To constrain the metal and sulfur sources and to define the ore-forming mechanism, we analyzed Zn, Pb, and S isotopes of sphalerite and robinsonite, as well as Zn isotopes of the host limestone and the metamorphic basement. Sphalerite shows homogeneous δ66Zn values (−0.31‰ to −0.12‰; mean = −0.20‰). The calculated δ66Zn of the ore-forming fluid (~0.00‰) matches that of the Triassic limestone, indicating a sedimentary Zn source (δ66Zn = −0.11‰ to −0.09‰; average 0.00‰). Robinsonite displays a wider δ66Zn range (−0.22‰ to 0.44‰), reflecting a mixture of sedimentary and metamorphic sources (δ66Zn = 0.12‰ to 0.42‰; average 0.22‰). Lead isotopes of sphalerite are uniform (206Pb/204Pb = 19.041–19.079) and indicative of a sedimentary rock source, whereas robinsonite shows wide variation (206Pb/204Pb = 19.070–19.156) and linear trends between low- and high-radiogenic end-members, indicating mixed Pb sources from sedimentary rocks and metamorphic basement. Sulfur isotopic compositions of sulfides (δ34S = −1.4‰ to 2.6‰; mean = −0.1‰) cluster near 0‰, consistent with a deep magmatic origin. A strong linear correlation between 206Pb/204Pb and δ66Zn, coupled with a lack of correlation between both 206Pb/204Pb and δ34S and δ66Zn and δ34S in the sulfides, indicates that Pb and Zn were derived from a common metal source, whereas sulfur originated from a distinct reservoir. Combined with previously published fluid inclusion, rare earth element, and multi-isotopic constraints, these results suggest that Pb–Zn mineralization at Lanuoma was controlled by a mixing between metal-rich basinal brines and sulfur-rich deep-sourced fluids, leading to sulfide precipitation dominated by open-space filling. This study provides new insights into the genesis and mineralization process of sediment-hosted Pb–Zn deposits in the Sanjiang metallogenic belt.