Pb isotope and sequential extraction constraints on metal sources of Mississippi Valley−type Zn-Pb deposits in the Lanping fold-thrust belt, Eastern Himalayan-Tibetan orogen (Southwest China)

Metals for Mississippi Valley−type Zn-Pb mineralization were previously considered to be leached from regional shallow sedimentary strata and basement rocks. However, determining the specific metal source is challenging because of the complex fluid interaction histories in Mississippi Valley−type ore fields. To better constrain their metal sources, we conducted a comprehensive study of the Lanping ore field of Southwest China. This ore field hosts the world-class Jinding Mississippi Valley−type deposit and Baiyangping Mississippi Valley−type ore district. The latter comprises numerous smaller deposits. All these deposits formed during the Cenozoic Era and are situated within the Cenozoic fold-thrust belt of the Eastern Himalayan-Tibetan orogenic belt. Sequential extraction of regional sedimentary strata (Permian to Eocene) targeted four fractions (carbonate, Fe-Mn oxide, organic and reductive, and residual silicate). This study, combined with leaching experiments on Triassic magmatic basement rocks and Cenozoic high-K plutons, demonstrates that magmatic rocks constitute the key metal sources, particularly through feldspar dissolution. Pb isotope data indicate that most fractions extracted from sedimentary rocks deviate from the linear Pb isotope trend observed in the Jinding sulfides. The lower intercept of this trend aligns with lower Cretaceous Jingxing Formation muddy siltstone, while the upper intercept extends to middle Triassic dacite K-feldspar. Model ages and linear mixing models indicate that Triassic felsic−mafic magmatic rocks were significant radiogenic Pb-bearing metal sources for Jinding, while lower Cretaceous siltstone or an end member 213 Ma or younger provided common Pb-bearing metals. The Pb isotopes of Baiyangping sulfides partially overlap with the gabbro, biotite monzogranite, and granite porphyry of Cenozoic high-K plutons and the Eocene Baoxiangsi Formation’s argillaceous fine sandstone fractions. These results indicate that the voluminous Triassic felsic−mafic rocks, which served as the basement for the entire Mesozoic Lanping Basin and rolled into the Cenozoic fold-thrust belt, have undergone large-scale fluid-rock reactions with the regional basin brines, providing a plentiful supply of Zn-Pb metals for regional Mississippi Valley−type deposits. In contrast, Cenozoic high-K plutons may intrude into more restricted spatial domains and might have undergone smaller-scale fluid-rock interactions, providing less Zn-Pb metals. This explains why a world-class Zn-Pb deposit formed at Jinding, while only small-scale Zn-Pb deposits developed at the Baiyangping ore district. This study illustrates that metal sources for Mississippi Valley−type Zn-Pb mineralization commonly display composite provenance characteristics, integrating contributions from sedimentary successions and basement rocks. In addition, it underscores the importance of metal release through interactions between fluids and magmatic rocks for the formation of large-scale Zn-Pb mineralization and demonstrates the utility of Pb isotope model ages in evaluating metal sources.