Acritarch Fossils, Detrital Zircon Geochronology And Depositional Environment Study Of The Xikeng Formation,Silurian,in Jiurui Metallogenic District,Jiangxi Province

ABSTRACT:Jiurui metallogenic district is one of the most important ore concentration areas in the Middle-Lower Yangtze metallogenic belt of eastern China, where occurred Chenmenshan copper deposit, Wushan copper deposit, Yangjishan gold deposit and many other important Cu-Au polymetallic ore deposits. In this paper, the author carried out a systematic study on the Aritarch fossils, detrital zircon U-Pb ages and depositional environment from two sections in this district (Chenmenshan section and Wangfushan section). At Chenmenshan section, the author found out that the age of the fossils, which comes from the stratum that used to be considered as Early Silurian, is actually Neoproterozoic or Early Cambrian. Combining with the data of the detrital zircon geochronology, we suggest that this stratum should be Early Cambrian. Field observation also revealed that there exists an angular unconformity between Devonian Wutong Formation and this possible Early Cambrian stratum, which is quite different from other place, such as Wangfushan and Zuantianshan, where the contact relationship between Wutong Formation and Xikeng Formation is disconformity. However, at Wangfushan, the so-called Silurian Xikeng Formation is confirmed, and the age for this stratum in the Jiurui district is suggested to be later than Telychian of Silurian epoch. And zircon geochronology study also shows that the time of Wutong Formation should be later than early Middle Devonian. So the beginning time of "Yangtze Uplift" in Jiurui district is thought to be later than Telychian and the ending time is thought to be later than early Middle Devonian.We also carried out some depositional environment work on the samples of Wutong Formation and the Early Cambrian stratum at Chenmenshan, and of Xikeng Formation at Wanfushan. The study methods include lithology identification, CM pattern, grain size analysis and Sahu plot. The result shows that the lithology of samples from Wutong Formation is chiefly coarse-grained sandstone with good sorting and high texture maturity. And the depositional facies of Wutong Formation is thought to be littoral facies. The lithology of samples from the Early Cambrian stratum is chiefly fine-grained sandstone with bad sorting and low texture maturity. And the depositional facies of the Early Cambrian stratum at Chenmenshan is turbidity. While the lithology of samples from Xikeng Fromation at Wangfushan is chiefly coarse-grained sandstone with good sorting and relatively high texture maturity, which depositional facies is thought to be shallow sea facies.