Ore genesis of Kongxigou and Nanmushu Zn-Pb deposits hosted in Neoproterozoic carbonates, Yangtze Block, SW China: Constraints from sulfide chemistry, fluid inclusions, and in situ S-Pb isotope analyses

Abstract

The recently discovered Kongxigou and Nanmushu Zn-Pb deposits are hosted in the Neoproterozoic Dengying Formation carbonates in the Mayuan district of Shaanxi Province, SW China, the northern margin of Yangtze Block. The deposits show a three-period mineralization history: (1) pre-ore sedimentary and diagenetic, (2) main-ore hydrothermal mineralization (stages I, II, and III), and (3) post-ore supergene. Multiple sulfide generations are distinguished, including two generations for sphalerite, e.g., Sp(a) from the stage I and Sp(b) from the stage II; and three generations for pyrite, e.g., Py(s) from the diagenetic period, Py(a) from the stage I and Py(b) from the stage II. The sedimentary and diagenetic period of the Kongxigou and Nanmushu deposits are characterized by Py(s) with high positive δ34S values (+16.2 to +19.9‰). The hydrothermal Zn-Pb sulfide precipitation stages are characterized by positive δ34S values (+11.4 to +23.9‰) that were likely caused by thermochemical sulfate reduction (TSR) of seawater or evaporitic sourced sulfates. In situ δ34S characteristics from the hydrothermal stage I to stage II record different sulfur sources. The ore-forming fluids show features of moderate-high temperature (137 °C–329 °C), high salinity (can reach up to 27.7 wt% NaCl + CaCl2 equivalent) with a Na-Ca- (Mg) -Cl dominant composition, hydrocarbon-rich (e.g., CH4 and bitumen), which are interpreted as basin brines. In situ lead isotope analyses of hydrothermal stage sulfides show 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios of 37.865–37.979, 15.635–15.661 and 17.885–18.101, respectively, which suggest a mixing of lead from the basement and the host rocks. We suggest the ore genesis of Kongxigou and Nanmushu deposits differs from the typical MVT Zn-Pb deposit models, and propose a new mixing model of basin brine accumulation and Zn-Pb sulfide precipitation in the hydrothermal stages.