Abstract: In recent years, many carbonate interlayers, including lenticular limestones and siderites, have been discovered in the Lower Member of Ningguo Formation of Ordovician in the Jiangnan Area of East China. Controversies occur with regards to the origin of such carbonate rocks. In this paper, the carbonate rocks are studied by means of the petrogeochemistry and C-O isotopes. The Σ(CaO+MgO+LOI) values of these carbonate rocks vary in a relatively narrow range from 95.67% to 89.89%, and the SiO₂ and TFe₂O₃ values are negatively correlated with the Σ(CaO+MgO+LOI) values. However, the values of Al₂O₃, MnO, P₂O₅ and TiO₂, are not significantly correlated with the Σ(CaO+MgO+LOI) values. The Co/Ni values of these carbonate rocks vary within the range from 0.24 to 0.28, much lower than 1.0, and the Mn/Sr ratios change from 0.14 to 0.33, much lower than 10. In the ternary plots of Al-Fe-Mn, (Cu+Co+Ni)×10-Fe-Mn and Ni-Co-Zn as well as the crossplot of LgU-LgTh, all the studied samples fall in the domain of hydrothermal carbonates. The ΣREE values of these carbonate rocks vary between 13.89×10⁻⁶ and 40.73×10⁻⁶, much less than the average ΣREE value of the upper crust (146.37×10⁻⁶) and slightly higher than the ΣREE value of chondrite (about 2.56×10⁻⁶). The δ¹³C_V-PDB, δ¹⁸O_V-PDB and δ¹⁸O_SMOW of these carbonate rocks are 0.33‰～0.67‰, −9.76‰～−10.1‰ and 12.56‰～12.9‰ respectively, which are obviously different from those of Marine and lacustrine limestones. Fault systems are well developed in the area, which may provide channels for the downward infiltration of seawater, of which the mixing effects the hydrothermal solution. As a result, the convective activities between seawater and hydrothermal continuously supply heat and diagenetic ions for the ocean basin. This marine hydrothermal deposition event represents a special genetic mode. It lasted for a long time and occurred in a wide range, depending on the provenance of hydrothermal materials and their properties.