Characteristics Of Barite And Its Paleoenvironmental Significance Of Lower Silurian Longmaxi Formation In Chengkou And Wuxi Area In The Southern Margin Of Daba Mountain

Barite concretions and bands are widely distributed in black shale-chert intervals in the Lower Silurian Longmaxi Formation in the study area(Chengkou and Wuxi).But the formation mechanism of barite has not been studied in Chengkou and Wuxi area.Accordingly,Barite in the mingzhong section in Chengkou and Xujiaba section in Wuxi are selected to discuss.The petrology and mineralogy of barite concretions and bands were studied by means of thin section,scanning electron microscopy and XRD.And on this basis,Combined with Strontium isotope characteristics of barite and geochemical characteristics of black mudstone,siliceous mudstone and chert in the Lower Silurian Longmaxi Formation in the study area,Formation environment and mechanism are analyzed.The study will provide important information for understanding the marine environment and diagenetic conditions of Early Silurian(Rhuddanian)in the northern part of Sichuan basin.Volcanic rock(bentonite and crystal tuff),black shale,calcareous sandstone,chert and barite comprise theWufeng–Longmaxi successions in the study area.Barite(Barium mineral)can be divided into four class: barite concretions,barite containing chert bands,barytodolomite bands and barite-gypsum Laminae.And the morphology of barite can be divided into six class: Submicron spheroidal crystal,coarser crystal,lamellar crystal,rhombus crystal,dendritic crystal structure and dissolved barite crystal.The 87Sr/86 Sr ratio(0.7087-0.7093)of barite concretions are significantly higher than those of the Early Silurian seawater(0.7078).And lower than 87Sr/86 Sr of surrounding rock.The results of 87Sr/86 Sr ratio show that barite is not directly formed by hydrothermal fluid and seawater,and the 87Sr/86 Sr ratio of barite concretions is not sufficient to retrieve the source of deep fluid in the region,and The source of strontium(barium)is closely related to seawater.The study of the main elements of black shale,calcareous sandstone and chert in the study area show that they were formed in the background of continental margin basin,and chert were formed by biological sources.The study of trace elements show that chert were formed in the anoxic seawater environment and the hydrothermal fluid had little effect.The rare earth element also reflects the hot water activity is not strong.The biogenic barium content of chert is very high,which indicates that the organisms had enrichment effect on barium.In addition,the energy spectrum analysis shows that the crystal tuff is quite high in barium content.Therefore,volcanic material also provides barium source for barite(barium mineral)formation.The Study suggests that the bands contain a large amount of barite veins,which were formed later than the deposition of Longmaxi formation,and barite vein should be deep hydrothermal formation by extensional tectonic.Besides,barite deposits precipitated from the enclosed porewater in sedimentcolumn,which evolved from the penecontemporaneous seawater with weak interaction with the hostfine-grained siliciclastic sediments and highly-depleted sulfate in response to prolonged strong bacterialsulfate reduction without necessary renewal.The minerals composed of nodules and bands are barite,carbonate,gypsum,edingtonite and pyrite,which were material response of methane percolation event.With the continuous burial of sediments,SO42-was consumed by microbial bacteria,SO42-in pore water had Exhausted when buried to a certain depth.CH4 and Ba2+ upward diffused by decomposition of organic matter and dissolution of biogenic barium.When Ba2+ diffused out the sulfate depletion zone,barite were formed in the barite front.Sulfate reduction coupled with anaerobic oxidation of methane(AOM)in the sediments were beneficial to form pyrite and some authigenic carbonate minerals.With the continuous decomposition of organic matter,methane gas can be input into the sea water.And CO2 formed above the redox interface.If the input is large enough,it would cause the greenhouse effect which cause Sea level rise.Therefore,This study can provide important implication and constrain forunderstanding the oceanic barium and sulfur cycling and early diagenetic process in the early Slurian.