Laochang is one of the main ore deposits in the world-class Gejiu polymetallic Sn district, Southwest China. Skarn Cu ore is an important mineralization type in the Laochang deposit. In this study, in situ trace element compositions of chalcopyrite and sphalerite and S isotope compositions of sulfide minerals (arsenopyrite, chalcopyrite, pyrrhotite and sphalerite) of the skarn Cu ore in the Laochang deposit are investigated. The results show that the studied chalcopyrite is enriched in Zn, Ag, In and Sn, and is depleted in Co, Ni, Ga, Ge, As, Cd, Sb, Bi and Pb. The sphalerite is enriched in Fe, Co, Cu, Ag, Cd, In and Bi and depleted in Ni, Ga, Ge, As, Sn, Sb, Tl and Pb. The high Fe, Cd and Co contents of the sphalerite indicate the simple substitution mechanism Zn²⁺↔ (Fe²⁺, Cd²⁺, Co²⁺). The S isotope compositions of arsenopyrite, chalcopyrite, pyrrhotite and sphalerite fall into a narrow range of − 2.9‰ to + 1.4‰, indicating a magmatic source of the sulfur. The high Sn contents (6.20 ~ 3339 μg/g) and moderate Cd/Zn ratio (0.009 ~ 0.023) of the chalcopyrite and the high In (271 ~ 723 μg/g) contents and Zn/Cd ratio (120 ~ 172) of the sphalerite imply that the skarn Cu mineralization in the Laochang deposit was influenced by high temperature, magmatic-hydrothermal fluids.