Abstract: A new technique for the complete dissolution of cassiterite, which makes it possible to determine the age of this mineral by U–Pb isotope dilution (ID-TIMS), has been developed and applied. This method allows a significant improvement in the accuracy of determining the age and expansion of the range of objects that can provide direct information about the age of tin ore deposits. The high-precision age of cassiterite (1539.5 ± 0.9 Ma) from the Pitkäranta tin deposit in the North Ladoga region proves that the formation of the tin ore deposits is genetically associated with the intrusion of rapakivi granites of the Salmi massif.