Oxygen isotope fractionation in metamorphosed iron formations of the Lake Superior region and in other iron-rich rocks

The Precambrian rocks of northern Michigan, Wisconsin, and Minnesota have undergone widespread regional metamorphism, with thermal zoning recognizable by progressive mineralogic changes in the Fe formations and associated rocks. Oxygen isotopic compositions of magnetite, hematite, quartz, and calcite from some of the metamorphosed Fe 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 CaCO (sub 3) -H (sub 2) O, it is possible to estimate that apparent temperatures of last crystallization reached about 200 degrees C. in the chlorite zone, 275 degrees C. in the biotite zone, and 350 degrees 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 equilibration during the cooling period. Temperature estimates for formation of other mineral deposits, based on isotopic fractionations of iron oxides, quartz, and calcite, include: Iron River, Michigan, oxidized Fe formation and post-ore mineralization, 20 degrees C to 100 degrees C.; Balmat, New York, post-ore supergene mineralization, 90 degrees C.; Coeur d'Alene district, Idaho, 200 degrees C.; Iron Mountain, Missouri, 280 degrees -365 degrees C.; and Iron Springs, Utah, 710 degrees C.