Little is known about the range or controls on the molybdenum isotopic composition of low-medium temperature hydrothermal ore-forming systems.We present molybdenum isotope data from 12 hydrothermal syndepositional silicalite and carbonaceous slate samples from the Dajiangping pyrite deposit in western Guangdong Province,South China.The δ 97/95Mo values from Orebody III range from ?0.02‰ to 0.29‰,with an average of 0.18‰.In contrast,the composition values from Orebody IV display a larger variation from ?0.70‰ to 0.62‰.However,the five samples from the main ore bed all show strong negative values.This indicates that the significant variation in molybdenum isotopic composition supports different hydrothermal ore-forming metallogenesis and a metallogenic environment between the two orebodies.Orebody III is likely to have been deposited from submarine exhalative hydrothermal fluids under a relatively strong reducing environment and Orebody IV may have also been influenced by hydrothermal superimposition in a more oxidized disequilibrium condition.In addition,the δ 97/95Mo values of Orebody IV are clearly negative,together with the values increasing stratigraphically upward in the ore beds,suggesting that the metallogenic environment of Orebody IV is different from the open oceanic systems.There might be dynamic fractionation in this restricted environment.Therefore,the molybdenum isotope can be used as an effective tracer for the ore-forming fluid and metallogenic environment.
SONG ShiMingHU KaiWEN HanJieZHANG YuXuLI KuangFAN HaiFeng
Over the past decade, as one of nontraditional stable isotopes, Mo isotope has developed rapidly and now become an important geochemical proxy to trace paleo-oceanic and atmospheric evolution through geological history. In this paper, Early Cambrian formations in southern China are investigated. The results indicate that δ 97/95Mo values of Early Cambrian seawater may have been larger than 1.4‰, values that are close to those of the modern ocean. It was also found that the variations in Mo isotope composition in samples from two sections (Huangjiawan and Gezhongwu in Guizhou) were closely related to changes in redox conditions during sedimentary processes. Combining our results with existing data, a preliminary model for the evolution of seawater Mo isotope composition through geological history was provided. It indicated that Mo isotopic variations were generally consistent with the evolution of atmospheric oxygen.
