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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 < 2 wt.%), indicating a hydrothermal origin. The hydrothermal monazite-(Ce) crystals have U–Pb ages of 833 ± 5 to 840 ± 6 Ma, 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 (4.72 wt.% on average) and commonly shows concentric zonation with respect to Th. They have a weighted average 207Pb/206Pb age of 1362 ± 29 Ma, which may record a pre-ore metamorphic event in the KCB. Hydrothermal monazite-(Ce) crystals from the Madhan-Kudhan deposit have εNd(t = 835 Ma) 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 ~ 850 Ma). 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. [ABSTRACT FROM AUTHOR]