The Biyang Depression,lying in the eastern Nanxiang Basin,has a dustpan-shape with faults developed to the southwest and southeast.The Paleogene Hetaoyuan Formation in this depression hosts one of the oldest soda-deposits,the Anpeng deposit.These sodium carbonate-bearing strata consist of lutites,muddy dolomites,and bedded soda-deposits,which are dominated by nahcolite with little trona,mirabilite,and halite.In the top Unit 3 and lower Unit 2 of Hetaoyuan Formation where the sodium carbonates are concentrated,dolomitic shale,muddy carbonate,and sodium carbonate can be separated based on their distinct Al2O3,Ti O2,Na2O,Mg O+Ca O,and LOI(loss on ignition)contents.By using Na2O/(Mg O+Ca O)ratio as an indicator for the relative concentration of Na carbonates to Mg-Ca carbonates,the muddy dolomites can be classified into two groups.One group with Na2O/(Mg O+Ca O)<0.6 exhibits a positive correlation between this ratio and Al2O3+Ti O2 content,an proxy for terrigenous clastic input and the other group with Na2O/(Mg O+Ca O)>0.6 presents a negative correlation.As the clastic proportion decreases,the former indicates that it tends to form pure Mg-Ca carbonates reflecting evaporation less than or equal to replenishment for the lake water in a relative humid climate,and the latter reveals a trend to form sodium carbonates with lake water’s evaporation more than replenishment in a relatively dry climate.From muddy dolomite to sodium carbonate samples,with Na2O/(Mg O+Ca O)ratio increasing,the Al normalized Zr,Ti,La,K,Rb,and Ba tend to be enriched and the post-Archean Average Shale(PAAS)normalized positive Eu anomaly becomes more obvious.Integrated with the high B contents and the report of searlesite in the bedded sodium carbonates and the regional extensional tectonic regime,the above geochemical characteristics were interpreted to signify hydrothermal contribution to forming these soda-deposits through the deep fault systems of basin margins. The Biyang Depression, lying in the eastern Nanxiang Basin, has a dustpan-shape with faults developed to the southwest and southeast. The Paleogene Hetaoyuan Formation in this depression hosts one of the oldest soda-deposits, the Anpeng deposit. The sodium carbonate-bearing strata consist of lutites, muddy dolomites, and bedded soda-deposits, which are dominated by nahcolite with little trona, mirabilite, and halite. the top Unit 3 and lower Unit 2 of Hetaoyuan Formation where the sodium carbonates are concentrated, dolomitic shale, muddy carbonate, and sodium carbonate can be separated based on their distinct Al2O3, Ti O2, Na2O, MgO + CaO, and LOI (loss on ignition) contents.By using Na2O / (MgO + CaO) ratio as an indicator for the relative concentration of Na carbonates to Mg-Ca carbonates, the muddy dolomites can be classified into two groups. One group with Na2O / (MgO + CaO) <0.6 exhibits a positive correlation between this ratio and Al2O3 + Ti O2 content , an proxy for terrigenous clastic input and the oth er group with Na2O / (MgO + CaO)> 0.6 presents a negative correlation. As the clastic proportion decreases, the former indicates that it tends to form pure Mg-Ca carbonates, reflecting less than or equal to replenishment for the lake water in a relative humid climate, and the latter reveals a trend to form sodium carbonates with lake water’s evaporation more than replenishment in a relatively dry climate. Flood muddy dolomite to sodium carbonate samples, with Na2O / (MgO + CaO) ratio increasing, the Al normalized Zr, Ti, La, K, Rb, and Ba tend to be enriched and the post-Archean Average Shale (PAAS) normalized positive Eu anomaly becomes more distinct. Integrated with the high B contents and the report of searlesite in the bedded sodium carbonates and the regional extensional tectonic regime, the above geochemical characteristics were interpreted to signify hydrothermal contribution to forming these soda-deposits through the deep fault systems of basin margins.