Genesis of the Dongzigou Pb−Zn deposit in the southeast margin of Sichuan Basin: Evidence from fluid inclusions, Rb−Sr and C−H−O isotopes

This paper is the result of mineral exploration engineering.

Objective The Dongzigou lead−zinc deposit is located in the Sangmuchang lead−zinc metallogenic belt on the southeastern margin of the Sichuan Basin, and it is a representative lead−zinc deposit rich in organic matter within this metallogenic belt. In this paper, by investigating the properties and evolution process of the metallogenic fluid of this deposit and combining with the Rb−Sr dating results of sphalerite, a lead−zinc metallogenic model will be established, which will further improve the metallogenesis of lead−zinc resources rich in organic matter around the basin.

Methods Based on detailed studies on basic geological characteristics and mineralogy, this paper conducts petrographic observations of fluid inclusions, microthermometric measurements of fluid inclusions and Raman spectroscopy tests. Combined with the characteristics of C−H−O isotope compositions and the isochron ages of Rb−Sr isotopes of sphalerite, it clarifies the lead−zinc metallogenic process and establishes the metallogenic model.

Results Petrography of fluid inclusions shows that fluid inclusions in the main metallogenic stage are mainly liquid−rich inclusions (LV). Combined with laser Raman analysis, it indicates that the metallogenic fluid belongs to the NaCl−H₂O system. The results of microthermometry demonstrate that the metallogenic fluid is generally of medium−low temperature, medium−low salinity and medium−low density. The characteristics of C−H−O isotope compositions suggest that the metallogenic fluid is closely related to marine carbonate rocks. In the early stage, the metallogenic fluid is mainly formation water, and in the middle and late stages, meteoric water is mixed in. Moreover, carbonate rock dissolution occurred during the metallogenic process. The isochron diagram of ⁸⁷Rb/⁸⁶Sr−⁸⁷Sr/⁸⁶Sr isotopes of sphalerite shows that the isochron age of the Dongzigou deposit is (135.2±6.1) Ma with MSWD = 1.10.

Conclusions The lead−zinc precipitation mechanism of this deposit is dominated by the non−isothermal mixing of ore−forming fluids and meteoric water. The Yanshanian tectonic uplift not only provided favorable ore−conducting and ore−hosting spaces for the deposit, but also drove the migration of ore−forming fluids and their mixing with meteoric water. This process further reduced the solubility of lead and zinc in the fluids, thereby facilitating the precipitation and enrichment of lead−zinc minerals.