Abstract: Laojinchang gold deposit is one of the most representative medium-sized magmatic hydrothermal gold deposits formed at medium-low temperature in the southern Beishan metallogenic belt. Based on the cross cutting relationships of the different auriferous veins, mineral paragenesis, ore texture and structure, its mineralization stages can be divided into quartz-pyrite stage(Ⅰ), quartz-arsenian pyrite-arsenopyrite stage(Ⅱ), quartz-pyrite-polysulfide stage(Ⅲ), and quartz-calcite stage(Ⅳ). In this paper, the element concentration and composition in the gold-bearing minerals of different ore-forming stages were analyzed by using the electron microprobe analyzer (EMPA). The pyrite of stage I is primarily coarse-grained (0.50-1.50 mm) euhedral cube, with low content of As, Au, and a small amount of fine-grained anhedral asenopyrite. The arsenian pyrites of stage II are surrounded by a large number of arsenopyrites, and the arsenian pyrites are dominantly cubic and pentagonal dodecahedron, rich in As and Au with particle size of 0.30-1.00 mm. Stage II is the most intensive period of mineralization, and the main formation period of arsenopyrites. The arsenopyrites in this stage are primarily appeared as rhombic-columnar, columnar, and radiate-columnar aggregates, rich in S but depleted of As. The pyrites in stage Ⅲ commonly occur as veins of pyrite-chalcopyrite-sphalerite mineral paragenesis, appeared as long strip, with high content of S, Cu, Zn, Au but low content of Fe and As. The mineralization in stage Ⅳ is relatively weak with a small amount of fine-grained anhedral pyrites and asenopyrites. The δ³⁴S_V-CDT values of pyrite or and asenopyrite range from -3.8‰ to -2.9‰ (average -3.3‰) in stage Ⅰ, the δ³⁴S_V-CDT values of pyrite and asenopyrite range from -4.7‰ to 2.6‰ (average -3.3‰) in stage Ⅱ, and the δ³⁴S_V-CDT values of pyrite and sphalerite range from -1.9‰ to 1.0‰ (average 0.1‰) in stage Ⅲ; which suggests a mantle-derived magmatic sulfur provenance, and contaminated by sedimentary sulfur at the late stage, as indicated by in-situ sulfur isotope compositions. Based on the previous study, it is proposed that the ore-forming fluid was evolved from S-rich and As-poor fluids to As-rich and S-poor fluids during the mineralization. In stage Ⅰ, the ore-forming system was in neutral and stable environment with abundant sulfur. In the main stage(II), the ore forming fluid was rich in As, poor in S,and high in oxygen fugacity, and the As-rich fluids was injected into hydrothermal system due to the leaching-infiltration of meteoric water, which led to the formation of the Au-As complexes, and the possible precipitation and accumulation in appropriate place. The ore forming system was characterized by variety of metallogenic elements, rich in S, poor in As, and weak reduction in stage Ⅲ, and Au might enter the pyrite lattice in the form of [Au(HS)₂]^- or [AuS]^- complexes.

Key words: ore-forming stage, gold-bearing mineral, EPMA, sulfur isotope, Laojinchang gold deposit, Beishan orogenic belt