Occurrence of cadmium and ore genesis in the Bijiashan lead-zinc deposit: Evidence from LA−ICP−MS trace elements and in-situ sulfur isotope of sulfide

The Bijiashan deposit is the first lead−zinc deposit discovered in the Xicheng ore field in the West Qinling Orogenic Belt (WQOB), with reserves of 1.2 million tons of lead and zinc, and grades of 1.86% and 5.42%, respectively. This deposit is characterised by the presence of two distinct types of orebodies: limestone−type and phyllite−type, which are located within limestone and phyllite formations, respectively. The mineral assemblages present in both ore types include sphalerite, galena, pyrite, and chalcopyrite. Previous investigations have primarily concentrated on the geological characteristics, petrography, isotope geochemistry, and fluid inclusions associated with the deposit; however, the occurrence of cadmium (Cd) is still uncertain. In this study, laser ablation inductively coupled plasma mass spectrometry (LA−ICP−MS) was employed to analyse various sulfide minerals (sphalerite, galena, pyrite, and chalcopyrite) from both ore types. The analytical results indicate that the average Cd content in sphalerite from the limestone−type and phyllite−type ores is 2076×10⁻⁶ and 1695×10⁻⁶, respectively, exceeding the general industry standard of 1000×10⁻⁶. Conversely, the concentrations of gallium (Ga), germanium (Ge), selenium (Se), indium (In), and thallium (Tl) in galena, pyrite, and chalcopyrite from both ore types are below the mineral industry standard of 100×10⁻⁶. Based on petrographic observations and LA−ICP−MS analyses, it is proposed that there are no independent Cd minerals present in the Bijiashan lead−zinc deposit. Instead, Cd is primarily incorporated into sphalerite through the substitution mechanism of Cd²⁺ ↔ Zn²⁺, with a minor substitution pathway of Cd²⁺ + Fe²⁺ ↔ 2Zn²⁺. The estimated formation temperature for sphalerite in the limestone type and phyllite type ores are 198–254°C (mean of 227°C) and 203–245°C (mean of 230°C), respectively. These findings are consistent with previous studies utilising fluid inclusion thermometry, suggesting a moderate temperature for ore−forming mineralisation. In−situ sulfur isotope analysis reveals average δ³⁴S values of 13.64‰ and 17.72‰ for the respective ore types, indicating a marine sulfate source. Drawing upon the evidence from LA−ICP−MS trace element data of sphalerite, petrography, geological characteristics of the deposit, isotope geochemistry, and metallogenic temperature, we propose that the Bijiashan lead−zinc deposit is classified as a sedimentary−exhalative (SEDEX) deposit.