Genesis of the Pulang polymetallic mineralization system in Yunnan Province, China: Insights from in situ trace element and S isotope analyses of pyrite

Abstract

The Pulang deposit in the “Sanjiang” area of Yunan Province hosts ~4.8 × 10⁶ tons of Cu and abundant associated metals such as Mo, Au, Ag, Pb and Zn, constituting a polymetallic porphyry system. Nevertheless, the multistage and multispace mineralization processes of these metals and the associated physicochemical evolution controlling their precipitation are still unclear. In this study, pyrite was selected from different stages and ore blocks to conduct in situ trace element, mapping (LA–ICP–MS) and S isotope analyses, to address the above issues.

The results indicate systematic variations in trace element distributions within the pyrite at Pulang, which are correlated with mineralization types (Mo, Cu, PbZn), alteration types (potassic, phyllic, prophylitic), and spatial locations (South, North, East Pulang). These patterns reflect integrated fluctuations in fluid temperature, oxygen fugacity (fO₂), pH, and salinity during fluid evolution processes including boiling, mixing, overprinting, and fluid–rock interactions. Mapping images reveal at least three hydrothermal events in the Cu mineralization stage: event 1 (Co–Ni–Se enrichment), event 2 (CoAs enrichment), and event 3 (Cu–Ag–Sb–Au–Pb–Zn–Bi–Mo enrichment), with the latter exhibiting a genetic linkage to the precipitation of both Cu and Mo. The PbZn stage records two superimposed hydrothermal events: an early Co–Sb–Te–Ni–Au-enriched event 1, overprinted by a later Cu–As–Mo–Ag–Pb–Bi–dominant hydrothermal pulse (event 2) associated with Pb and Zn precipitation. The presence of trace elements and fluctuanting δ³⁴S values (−2.64 to 4.16 ‰) indicate that the successive precipitation of Mo and Cu in South Pulang was triggered by the efficient cooling of fluids, whereas the precipitation of Cu in East Pulang was associated with decreases in both temperature and fO₂, as a result of fluid–rock interactions during the eastward migration of fluid. In contrast, in North Pulang, the decreasing Se/Ge, Co/Ni and Co/As ratios imply that the significant involvement of meteoric water during the northward migration of fluid was also responsible for the precipitation of Cu, Pb and Zn.