LA-ICP-MS trace element geochemistry and sulfur isotopic compositions of sphalerite from Jiama deposit: Implications for its genesis and mineralization processes

Abstract

The Jiama copper polymetallic deposit is a typical representative of the post-collision porphyry copper deposits in the eastern segment of the Gangdese metallogenic belt in China. Currently, the occurrence mechanism of the trace elements in sphalerite and the genesis of the distal lead–zinc mineralization are not clear in the Jiama deposit. Sphalerites, the primary ore minerals in the Jiama distal lead–zinc mineralization, have rich colors and are hosted in skarn (Sp1), hornfels (Sp2), and manto (Sp3) orebodies. In this study, we utilized laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and in-situ sulfur isotope analysis to investigate the genesis of sphalerite and reveal the distal lead–zinc mineralization processes. Sp1 had high Fe and Cd contents, Sp2 had low Mn and Ga contents, and Sp3 had high Cu and Co contents. The trace elements in sphalerite (e.g., Fe, Mn, Cd, Co, Ga, In, and Cu) primarily existed in the form of isomorphic substitution. The substitution mechanisms mainly included (Fe, Mn, Cd) ²⁺ ↔ Zn²⁺, (Cu⁺, Ag⁺) + Ga³⁺ ↔ 2Zn²⁺, and Cu⁺ + In³⁺ ↔ 2Zn²⁺. The sphalerite showed a diverse array of colors; based on these colors, the sphalerites were classified into three categories: brown with high contents of Fe, Cd, Mn, and Ga (Type 1); yellow with Ge and Cu enrichment (Type 2); and dark blue with high Ag concentrations (Type 3). The sulfur in the sphalerite in the Jiama deposit was derived from magma; this sphalerite generally formed under medium-to-high temperatures and moderate sulfur fugacity conditions. As the ore-forming fluids moved away from the hydrothermal center, the temperature and sulfur fugacity of the sphalerite formation gradually decreased, accompanied by a reduction in the Fe and Cd content of sphalerite. Thus, sphalerite can be utilized as one of the prospecting indicators for identifying the fluid source of porphyry mineralization systems.