Rare earth elements and carbon-oxygen isotopes of calcite from the Tongjiachong Cu deposit, South China: Implications for fluid source and mineral precipitation

•Calcite is a major gangue mineral at the Tongjiachong Cu deposit in South China.•The Tongjiachong ore-fluids may have sourced mainly from organic-rich formation water.•Calcite and Cu sulfides precipitated under low temperature and suboxic-anoxic conditions.•Water-rock (W/R) reaction was the key to calcite deposition.•Temperature drop and W/R reaction likely drove the Cu sulfide mineralization.