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A practical method is presented to estimate rare earth element (REE) concentrations in a magmatic fluid phase in equilibrium with water-saturated granitic melts based on empirical fluid&#8722;melt partition coefficients of REE (<inline-formula> <math display="inline"> <semantics> <mrow> <msubsup> <mi>k</mi> <mi>P</mi> <mrow> <mi>R</mi> <mi>E</mi> <mi>E</mi> </mrow> </msubsup> </mrow> </semantics> </math> </inline-formula>). The values of <inline-formula> <math display="inline"> <semantics> <mrow> <msubsup> <mi>k</mi> <mi>P</mi> <mrow> <mi>R</mi> <mi>E</mi> <mi>E</mi> </mrow> </msubsup> </mrow> </semantics> </math> </inline-formula> can be calculated from a set of new polynomial equations linking to the Cl molality (<inline-formula> <math display="inline"> <semantics> <mrow> <msubsup> <mi>m</mi> <mrow> <mi>C</mi> <mi>l</mi> </mrow> <mi>v</mi> </msubsup> </mrow> </semantics> </math> </inline-formula>) of the magmatic fluid phase associated with granitic melts, which are established via a statistical analysis of the existing experimental dataset. These equations may be applied to the entire pressure range (0.1 to 10.0 kb) within the continental crust. Also, the results indicate that light REEs (LREE) behave differently in magmatic fluids, i.e., either being fluid compatible with higher <inline-formula> <math display="inline"> <semantics> <mrow> <msubsup> <mi>m</mi> <mrow> <mi>C</mi> <mi>l</mi> </mrow> <mi>v</mi> </msubsup> </mrow> </semantics> </math> </inline-formula> or fluid incompatible with lower <inline-formula> <math display="inline"> <semantics> <mrow> <msubsup> <mi>m</mi> <mrow> <mi>C</mi> <mi>l</mi> </mrow> <mi>v</mi> </msubsup> </mrow> </semantics> </math> </inline-formula> values. In contrast, heavy REEs (HREE) are exclusively fluid incompatible, and partition favorably into granitic melts. Consequently, magmatic fluids tend to be rich in LREE relative to HREE, leading to REE fractionation during the evolution of magmatic hydrothermal systems. The maximum <inline-formula> <math display="inline"> <semantics> <mrow> <msubsup> <mi>k</mi> <mi>P</mi> <mrow> <mi>R</mi> <mi>E</mi> <mi>E</mi> </mrow> </msubsup> </mrow> </semantics> </math> </inline-formula> value for each element is predicted and presented in a REE distribution diagram constrained by the threshold value of <inline-formula> <math display="inline"> <semantics> <mrow> <msubsup> <mi>m</mi> <mrow> <mi>C</mi> <mi>l</mi> </mrow> <mi>v</mi> </msubsup> </mrow> </semantics> </math> </inline-formula>. The REE contents of the granitic melt are approximated by whole-rock analysis, so that REE concentrations in the associated magmatic fluid phase would be estimated from the value of <inline-formula> <math display="inline"> <semantics> <mrow> <msubsup> <mi>k</mi> <mi>P</mi> <mrow> <mi>R</mi> <mi>E</mi> <mi>E</mi> </mrow> </msubsup> </mrow> </semantics> </math> </inline-formula> given chemical equilibrium. Two examples are provided, which show the use of this method as a REE tracer to fingerprint the source of ore fluids responsible for the Lake George intrusion-related Au&#8722;Sb deposit in New Brunswick (Canada), and the Bakircay Cu&#8722;Au (&#8722;Mo) porphyry systems in northern Turkey.