The Khetri Copper Belt (KCB) in northwestern India contains several economically important hydrothermal Cu-(Au) deposits. Orebodies in these deposits are locally rich in monazite-(Ce). In this study, in situ U-Pb ages and Sm-Nd isotopic compositions of monazite-(Ce) are used to constrain the timing of mineralization and to better understand the genesis of the Cu-(Au) deposits. Monazite-(Ce) crystals were identified in the Madhan-Kudhan and Kolihan deposits in the KCB. Most monazite-(Ce) crystals in the two deposits show close textural associations with hydrothermal minerals (biotite, chlorite, and sulfide) and have low contents of ThO2 (mostly <2wt.%), indicating a hydrothermal origin. The hydrothermal monazite-(Ce) crystals have U-Pb ages of 833 +/- 5 to 840 +/- 6Ma, which are interpreted to represent the timing of the mineralization in the KCB. Another set of monazite-(Ce) crystals in the Kolihan deposit has relatively high contents of ThO2 (4.72wt.% on average) and commonly shows concentric zonation with respect to Th. They have a weighted average Pb-207/Pb-206 age of 1362 +/- 29Ma, which may record a pre-ore metamorphic event in the KCB. Hydrothermal monazite-(Ce) crystals from the Madhan-Kudhan deposit have epsilon(Nd)(t=835Ma) values ranging from -4.3 to -16.8 (-9.6 on average), indicating that isotopically heterogeneous materials were in the metal sources or along the flow paths of hydrothermal fluids. The ages of Cu-(Au) deposits in the KCB fall within the age range of the regional Ca-Na metasomatism (830 to 850Ma). Combining this temporal association with relevant geochemical and isotopic data, we propose that circulation of hydrothermal fluids caused widespread Ca-Na metasomatism, mobilized certain ore-metals, and thus facilitated the formation of Cu-(Au) deposits.