The genesis of the giant Ag-Cu-Pb-Zn polymetallic mineralization in the northern Lanping basin, Southwest China, remains controversial. To address the sources of metals, a systematic study on Hg isotope compositions was conducted for the Cu-dominated deposit at Baiyangping and the Pb-Zn-dominated deposits at Fulongchang and Liziping. The Cu deposit shows positive Delta Hg-199 signatures (0.14 +/- 0.13 parts per thousand), in contrast to the Delta Hg-199 of the Pb-Zn deposits (- 0.09 +/- 0.06 parts per thousand). As positive Delta Hg-199 values are commonly observed in marine sediments and the Lanping Triassic marine sedimentary rocks show exclusively positive Delta Hg-199 signals (0.03 +/- 0.07 parts per thousand), the Hg in Cu ores was mainly sourced from the Triassic strata. The negative Delta Hg-199 signals observed in the Pb-Zn deposits, typical of terrestrial Hg, agree roughly with those of the Jurassic to Paleocene terrestrial sedimentary rocks (- 0.05 +/- 0.08 parts per thousand), indicating that the terrestrial strata provided the Hg in Pb-Zn ores. Compared to the source rocks, the Cu deposit shows isotopically lighter Hg enrichments (delta Hg-202 = - 2.30 +/- 0.35 parts per thousand), possibly resulting from fractionations induced by Hg2+ sorption, organic complexation, and precipitation of Hg-bearing sulfides. The Pb-Zn deposits show comparable or slightly heavier delta Hg-202 (- 0.56 +/- 0.48 parts per thousand); moreover, delta Hg-202 values of late-stage sulfides are higher than early-stage delta Hg-202 values, suggesting that the delta Hg-202 variation was primarily caused by sulfide precipitation. Thus, Hg isotope data indicate that separate hydrothermal events resulted in Cu and Pb-Zn mineralization. More importantly, this study reveals the great potential of Hg isotopes to discriminate sedimentary sources of metals for low-temperature hydrothermal deposits.