Geochronology and Geochemical Characteristics of Lower-Middle Triassic Clay Rock and Their Significances for Prospecting Clay-Type Lithium Deposit

Ma Shengchao; Wang Denghong; Sun Yan; Li Chao; Zhong Hairen

Abstract：The Lower-Middle Triassic boundary clay rocks(LMTBC,for short),related to volcanic material eruption and deposition,are rich in lithium and widespread in Chongqing,Guangxi,Guizhou of southwest China.It is of great economic significance to evaluate the ore-forming potential of sedimentary clay-type lithium deposit and confirm the prospecting direction in LMTBC.Based on the method of chronology and geochemistry,MC-ICP-MS zircon U-Pb data indicates that the formation ages of LMTBC from Wenquan,Xiejiawan,Fuchu,Maluojing are 248.6±0.8Ma,242.5±0.7Ma,246.1±0.6Ma,248.1±0.8Ma,and they all are close to the Lower-Middle Triassic boundary age(247.2Ma).The geochemical data indicate that LMTBC is enriched in Li, K2O,high of MgO and depleted in Na2O,and displays high incompatible elements(Rb,W,Sn,Bi,Th,and U)abundances and low compatible element(V,Cr,Co and Ni)abundances.The Li shows positive correlation with the incompatible elements(Sn,Ta,and Ga),which is similar to rhyolitic-dacitic rocks.All above data suggest that LMTBC is the products of the clay-alteration from the Triassic multiple-stages volcanic eruption-deposit materials.Considering the spatio-temporal relationship geochemical characteristics of LMTBC and rhyolitic-dacitic igneous rocks in southwest China,it is suggested that the tectonic setting of volcanic activities is possibly the subduction and collision of Yangtze plate and Cathaysia plate,or the hydrothermal-sedimentary activities of subsequent Emei great igneous province.The inheritance component of the volcanic sedimentary materials is the key factor for Li anomalies of LMTBC,the ions adsorption effect of clay minerals may be also important;the superposition action of lithium rich brine caused by the evaporation hardly caused the large-scale lithium-rich LMTBC.Accordinlgly,it is suggested that the clay layers of LMTBC volcanic eruption-sedimentary origin are the ideal bedding for discovering the clay-type lithium deposits in southwest China.The contemporary ancient crater basins and similar paleogeomorphology are the favorable preservation of eruption sediments,and the important prospecting area for clay-type lithium deposits.