Genesis of the Maoling gold deposit in the Liaodong Peninsula:Constraints from a combined fluid inclusion,C-H-O-S-Pb-He-Ar isotopic and geochronological studies

摘要

The large tonnage Maoling gold deposit(25 t@3.2 g/t)is located in the southwest Liaodong Peninsula,North China Craton.The deposit is hosted in the Paleoproterozoic metamorphic rocks.Four stages of mineralization were identified in the deposit:(stageⅠ)quartz-arsenopyrite±pyrite,(stageⅡ)quartz-goldarsenopyrite-pyrrhotite,(stageⅢ)quartz-gold-polymetallic sulfide,and(stageⅣ)quartz-calcitepyrrhotite.In this paper,we present fluid inclusion,C-H-O-S-Pb-He-Ar isotope data,zircon U-Pb,and gold-bearing sulfide(i.e.arsenopyrite and pyrrhotite)Rb-Sr age of the Maoling gold deposit to constrain its genesis and ore-forming mechanism.Three types of fluid inclusions were distinguished in quartzbearing veins,including liquid-rich two-phase(WL type),gas-rich two-phase(GL type),and daughter mineral-bearing fluid inclusions(S type).Fluid inclusions data show that the homogenization at temperatures 197 to 372°C for stageⅠ,126 to 319°C for stageⅡ,119 to 189°C for stageⅢ,and 115 to 183°C for stageⅣ,with corresponding salinities of 3.7 to 22.6 wt.%,4.7 to 23.2 wt.%,5.3 to 23.2 wt.%,and 1.7 to14.9 wt.%Na Cl equiv.,respectively.Fluid boiling was the critical factor controlling the gold and associated sulfide precipitation at Maoling.Hydrogen and oxygen stable isotopic analyses for quartz yielded δ^(18)O=-5.0‰ to 9.8‰ and δD=-133.5‰ to-77.0‰.Carbon stable isotopic analyses for calcite and ankerite yielded δ^(13)C=-2.3‰to-1.2‰ and O=7.9‰ to 14.1‰.The C-H-O isotope data show that the oreforming fluids were originated from magmatic water with meteoric water input during mineralization.Hydrothermal inclusions in arsenopyrite have ^(3)He/^(4)He ratios of 0.002 Ra to 0.054 Ra,and ^(40)Ar/^(36)Ar rations of 1225 to 3930,indicating that the ore-forming fluids were dominantly derived from crustal sources almost no mantle input.Sulfur isotopic values of Maoling fine-grained granite range from6.‰1 to 9.8‰,with a mean of 7.7‰,δ^(34)S values of arsenopyrite from the mineralized phyllite(host rock)range from 8.9‰ to 10.6‰,with a mean of 10.0‰,by contrast,δ^(34)S values of sulfides from ore vary between 4.3‰and 10.6‰,with a mean of 6.8‰,suggesting that sulfur was mainly originated from both the host rock and magma.Lead radioactive isotopic analyses for sulfides yielded^(206)Pb/^(204)Pb=15.830–17.103,^(207)Pb/^(204)Pb=13.397–15.548,^(208)Pb/^(204)Pb=35.478–36.683,and for Maoling fine-grained granite yielded ^(206)Pb/^(204)Pb=18.757–19.053,^(207)Pb/^(204)Pb=15.596–15.612,and ^(208)Pb/^(204)Pb=38.184–39.309,also suggesting that the ore-forming materials were mainly originated from the host rocks and magma.Zircon U-Pb dating demonstrates that the Maoling fine-grained granite was emplaced at 192.7±1.8 Ma,and the host rock(mineralized phyllite)was emplaced at some time after2065.0±27.0 Ma.Arsenopyrite and pyrrhotite give Rb–Sr isochron age of 188.7±4.5 Ma,indicating that both magmatism and mineralization occurred during the Early Jurassic.Geochronological and geochemical data,together with the regional geological history,indicate that Early Jurassic magmatism and mineralization of the Maoling gold deposit occurred during the subducting Paleo-Pacific Plate beneath Eurasia,and the Maoling gold deposit is of the intrusion-related gold deposit type.