The Precambrian rocks of northern Michigan, Wisconsin, and Minnesota have undergone widespread regional metamorphism, with thermal zoning recognizable by progressive mineralogic changes in their on formations and associated rocks. Oxygen isotopic compositions of magnetite, hematite, quartz, and calcite from some of the metamorphosed iron formations have been measured. Mineral pairs such as quartz-hematite and calcite-hematite have isotopic differences that bear consistent relation to inferred thermal gradients in the lower metamorphic zones. By relating the observed isotopic fractionation to the experimentally derived fractionation curve for CaCO₃-H₂O, it is possible to estimate that apparent temperatures of last crystallization reached about 200° C in the chlorite zone, 275° C in the biotite zone, and 350°C in the garnet zone. The indicated temperatures for rocks formed at temperatures above that of the garnet zone do not show a similar consistent pattern; we attribute this largely to retrograde equilib rationduring the cooling period. Temperature estimates for formation of other mineral deposits, based o n isotopic fractionations of iron oxides, quartz, and calcite, include: Iron River, Michigan, oxidized iron formation and post-ore mineralization, 20° to 100° C; Balmat, New York, post-ore supergene mineralization, 90° C; Coeur d’Alene district, Idaho, 200° C; Iron Mountain, Missouri, 280°–365° C; and Iron Springs, Utah, 710° C.