Work

Genesis of Pb–Zn deposits in the Zhen’an–Xunyang basin, south Qinling: Constraints from Rb–Sr geochronology and H–O–S–Pb isotopes

Year: 2025

Type: article

Abstract: • The Pb-Zn deposits of the Zhen’an-Xunyang Basin experienced extensive modification during the Late Indosinian-Early Yanshanian period. • H-O-S-Pb isotopes indicate a significant magmatic influence on the modification processes. The Zhen’an–Xunyang Basin (ZXB), situated within the southern Qinling orogenic belt, is a rifted basin hosting significant Pb–Zn–Hg–Sb–Au deposits. The Pb–Zn deposits represent the primary economic resources, are predominantly hosted in Silurian silty and carbonaceous phyllites along the basin’s southern margin. However, the genetic mechanisms of these deposits remain controversial, particularly concerning the influence of modifications on the preservation of the Pb–Zn mineralization. Consequently, constraining the sources of ore-forming materials and the timing of mineralization is crucial for resolving these genetic ambiguities. To address these limitations, this study investigated representative deposits, including Sirengou, Guanzigou, Baijiapo, and Honghuaping. Our comprehensive methodology involved in-situ trace element analysis of sphalerite, fluid inclusion H-O isotope analysis of ore-stage quartz and sphalerite, and in-situ Rb–Sr geochronology of sericite. Rb–Sr isochron dating of sericite from the Sirengou and Honghuaping deposits yielded ages of 207 ± 17 Ma and 181 ± 16 Ma, respectively. These ages, falling within the Late Indosinian to Early Yanshanian period, are significantly younger than the Silurian host rocks, indicating hydrothermal overprinting events rather than the timing of primary syngenetic mineralization. Sulfur isotope analyses of Pb–Zn ores from the Sirengou and Renjiagou areas yielded δ 34 S values ranging from 0 ‰ to + 5 ‰, consistent with a magmatic sulfur source. In contrast, the associated wall rocks exhibit considerably higher δ 34 S values (generally >+15 ‰). Furthermore, δ 34 S values tend to increase with increasing distance from the Sirengou–Renjiagou area, suggesting a greater contribution from sedimentary sulfur, likely reflecting mixing between magmatic and sedimentary sulfur sources. This finding is consistent with the trace element assemblage characteristics, which reveal pronounced variations in trace element compositions within the Sirengou Pb-Zn deposit. In contrast, the Pb–Zn mineralization on both flanks of the Sirengou area exhibits relatively weaker variations. H-O isotope analyses of mineralization-stage quartz yielded δD V-SMOW values ranging from −129 ‰ to −73.6 ‰ and δ 18 O H2O values ranging from 8.3 ‰ to 13.5 ‰, indicating the involvement of both magmatic and organic fluids. In contrast, sphalerite exhibits significantly lower δ 18 O values (–2.7 ‰ to 5.1 ‰), pointing to the involvement of meteoric water during mineralization. Pb isotope compositions of the ores plot predominantly within the orogenic Pb field, contrasting with the regional background of radiogenic Pb, which implies the input of deep-sourced lead. In summary, the Pb–Zn mineralization in the Zhen’an–Xunyang Basin represents a superimposed metallogenic process. This involved the overprinting and modification of pre-existing syngenetic SEDEX-type deposits by subsequent magmatic–hydrothermal activity.

Source: Ore Geology Reviews

DOI: 10.1016/j.oregeorev.2025.106941