Abstract: Great progress has been made in recent years in prospecting for Pb-Zn ore deposits in western Hunan and eastern Guizhou provinces. There are more than two hundred Pb-Zn ore deposits hosted in the Cambrian and Ordovician strata. The authors carried out a systematic analysis of the carbon and oxygen isotopic compositions of carbonate wall rocks and hydrothermal calcites formed at the ore-forming stage. The results show that carbonate wall rocks have relatively homogeneous [δ]13CPDB values (ranging from -1.16‰ to 1.70‰, 0.51‰ on average) and [δ]18OSMOW values (ranging from 18.56‰ to 22.42‰, 21.04‰ on average); in contrast, hydrothermal calcite has lower [δ]13CPDB value (ranging from -5.80‰ to 0.42‰, -1.18‰ on average), and significantly declined [δ]18OSMOW values (ranging from 12.96‰ to 23.05‰, 18.36‰ on average). In the [δ]13CPDB vs [δ]18OSMOW diagram, the wall rocks are plotted in the marine carbonate rock area, and hydrothermal calcites are between marine carbonate rock and upper mantle. In combination with previous results, it is concluded that C of the ore-forming fluids was mainly derived from carbonate wall rocks, sulfur from sulfates in the sedimentary strata, and Pb and Zn from the rocks of lower Cambrian Niutitang Formation. The ore-forming fluids were higher salinity and lower temperature basin brines. Precipitation of ore minerals and hydrothermal calcites probably resulted from extensive fluid-rock interaction between ore-forming fluids and carbonate wall rocks in combination with corresponding decreasing temperature.