Vertical evolution of Ag-Pb-Zn-(Cu)-Mo in porphyry system: A case study from the Laochang deposit, SW China

Abstract

Distal massive sulfide Ag-Pb-Zn mineralization is commonly associated with proximal porphyry Mo-(Cu) systems, but the vertical evolution of Ag-Pb-Zn-(Cu)-Mo distribution within porphyry systems remains poorly understood and documented. The Laochang deposit located in the southeastern Sanjiang metallogenic belt, SW China contains both the massive sulfide Ag-Pb-Zn-(Cu) and porphyry-skarn Mo-(Cu) mineralization (1737 t Ag@222 g/t, 0.51 Mt Pb@5.1 wt%, 0.34 Mt Zn@3.1 wt%, 117, 800 t Mo@0.15 wt%, and 0.1 Mt Cu@0.2 wt%), thus representing an excellent case study. The Laochang deposit is mainly hosted in early Carboniferous volcanic rocks, late Carboniferous to early Permian carbonate rocks, and links with Eocene granite porphyry. Here, we report Rb-Sr age of sphalerite and pyrite from the massive sulfide Ag-Pb-Zn-(Cu) ores, zircon U-Pb age for the granite porphyry, C-O isotopes of hydrothermal calcite, and S-Zn isotopes of sulfide minerals to reveal the vertical distribution of Ag-Pb-Zn-(Cu)-Mo mineralization in porphyry system. The sulfides Rb-Sr age (45.1 ± 3.6 Ma) is similar to the zircon U-Pb age (44.6 ± 0.2 Ma), indicating the Ag-Pb-Zn-(Cu) mineralization has a genetic link with the granite porphyry and formed in the same system as the porphyry-skarn Mo-(Cu) mineralization. Furthermore, the C-O isotopic compositions of the hydrothermal calcite suggest a diminishing contribution of the host rocks and an increasing input of the meteoric water in the hydrothermal fluids from deep to shallow. The S-Zn isotopic compositions of the sulfide minerals indicate a magmatic source of the ore-forming elements. The δ34S values decrease, and the δ66Zn values increase with elevated levels of sampling depth, implying a Rayleigh fractionation process. This means fluids and late-stage sulfides evolved toward isotopically lighter sulfur and heavier zinc isotopes. Hence, we propose that the Laochang deposit is a porphyry system comprising distal Ag-Pb-Zn-(Cu) and proximal Mo-(Cu) mineralization. In addition, the sulfur and zinc isotopic systematics observed in this study provides a new insight into the mineralization processes of the porphyry-distal systems.