Metals pre–enrichment during the syn–sedimentary period of Mengxing Pb–Zn Deposit, western Yunnan, SW China: Evidence on LA–ICP–MS trace elements and S isotopes of lamellar sphalerite

Abstract

The Mengxing Pb–Zn deposit, which contains up to 1.02 Mt of Pb + Zn metal with an average Pb + Zn grade of ca. 12.13 %, is a representative of sedimentary–reworked deposit in western Yunnan. The orebodies, which are strata–bound and veined, occur in the Silurian Shangrenheqiao formation. The mineralization process has been divided into sedimentary stage and hydrothermal stage. Previous research has primarily focused on the economic ores enriched during the hydrothermal stage. However, the formation of syn–sedimentary ore, responsible for metal pre–enrichment, has been largely ignored. To solve the issue above, the in–situ trace elements and mapping, as well as in–situ S isotopes analyses, were conducted on lamellar sphalerite ore, which retains the characteristics of the syn–sedimentary ore. Analysis of the trace elements revealed that the sphalerite at Mengxing are rich in Ge, Cd, As, Sb and poor in Fe, Mn, In, Sn and Co overall, suggesting a metallogenic environment of medium–low temperature. The LA–ICP–MS trace elemental mapping graph indicated that colloidal sphalerite exhibits the obvious oscillation belts. The dark zonal band of sphalerite is more enriched in Fe, Mn, Cu, Ge, As, Ag, Sb, and Pb, and depleted in Ga, Cd, and Sn, compared to the light zonal band, suggesting the various formation temperature. Based on the equation of the first principal component (PC1*) relating trace elements to mineral formation temperature, it is estimated that ore in the sedimentary stage occurred at relatively lower temperatures (174.74℃ – 203.18℃), while that of the hydrothermal stage occurred at higher temperatures (209.78℃ – 240.18℃). The S isotopic composition of sphalerite indicated a significant distinction between the sedimentary stage (δ34SVCDT = 13.29 ‰ – 37.06 ‰, with an average value of 27.97 ‰) and the hydrothermal stage (δ34SVCDT = −6.45 ‰ – 31.57 ‰, with an average of 9.01 ‰). It is hypothesized that sulfur during the early stage originated primarily from thermochemical sulfate reduction (TSR) of seawater sulfate, while sulfur during the hydrothermal stage was likely influenced by hydrothermal fluid from the deep. In conclusion, the early lamellar ores formed during the Silurian period exhibit characteristics consistent with SEDEX type mineralization, leading to initial metal pre–enrichment, followed by further enrichment of ore grade by superimposed fluid during the hydrothermal stage.