ABSTRACT

The late Triassic Baolun gold deposit hosted by Silurian phyllites is a large-scale gold deposit in Hainan Island, South China. In order to discuss the source of ore-forming material and deposit type, H-O isotope analyses of fluid inclusion, He-Ar isotopic and S isotopic studies of pyrite have been analyzed from auriferous quartz veins. The oxygen and hydrogen isotopic compositions (δ¹⁸O = 4.1–6.5‰, δD = –77.7 to –60.5‰) of ore-forming fluid, which are incompatible with metamorphic fluid, coincide with the characteristics of magmatic fluid. The He-Ar isotopic compositions from pyrite imply that the metallogenic material is derived from a crustal component and a mantle component. The δ³⁴S values of pyrite and pyrrhotite from mineralized quartz veins range from 2.3‰ to 6.3‰ and 1.5‰ to 7.1‰ respectively, which are lower than δ³⁴S values of pyrite from ore-hosting rocks and other gold deposits in Hainan Island. The values are higher than that derived from the mantle and are similar to magmatic rocks in nearby, which suggest that the sulfur is mainly derived from magmatic rocks of Hainan Island. Hence, the ore-forming fluid is primarily the product of magmatic differentiation, but the contribution of meteoric water could not be excluded. Based on the close spatial and temporal relationship with the Jianfnegling granite pluton (ca. 240 Ma), which are alkali granites containing mantle components, the Baolun gold deposit is proposed to be an intrusion-related gold deposit instead of presupposed orogenic gold deposit. Based on structures, ore-hosting rocks, and isotope characteristics, the Baolun gold deposit was identified as a sediment-hosted intrusion-related gold deposit. Combined with contemporary Gezhen-type gold deposits (ca. 228 Ma) that are a series of similar gold deposits hosted by the Baoban Group along the Gezhen ductile shear zone in Hainan Island, it is concluded that an identical gold metallogenic event occurred in Hainan Island during middle-late Triassic.