Geology and Stable Isotope Geochemistry of the Biwabik Iron Formation, Northern Minnesota

Abstract

The Biwabik Iron Formation of Middle Precambrian age defines a narrow belt of iron-rich strata in northern Minnesota that extends east-northeastward along strike for about 200 km. It consists of various proportions of quartz, magnetite, greenalite, minnesotaite, stilpnomelane, siderite, ankerite, and lesser amounts of hematite, chamosite, talc, calcite, and dolomite; it is assignable to James9 (1954) silicate-carbonate facies of iron-formation. Complex textural relationships imply that there are several generations of some of the minerals, but there is no diagnostic petrographic evidence indicative of the chemical and physical conditions under which they formed.The east end of the Biwabik Iron Formation has been metamorphosed by the Duluth Complex, a large mafic intrusion of Late Precambrian age. Carbonates and phyllosilicates were replaced by grunerite, fayalite, pyroxenes, and garnet. Metamorphism was largely isochemical and characterized chiefly by loss of CO 2 and H 2 O; temperature was the primary intensive control on the observed mineral assemblages.Oxygen and carbon isotope fractionation between minerals in the iron-formation provides evidence of diagenetic and post-diagenetic processes affecting the rocks. Based on the quartz-magnetite geothermometer, it is probable that quartz and magnetite were subjected to extensive oxygen isotopic homogenization at a temperature of approximately 150 degrees C. Further isotopic exchange, possibly during cooling, seems to have occurred in the carbonate minerals. This temperature might equally well characterize high-grade diagenesis or low-grade metamorphism. In contrast, the temperature reached about 650 degrees C (measured by the quartz-magnetite geothermometer) near the basal contact of the Duluth Complex and prograde metamorphism extended outward about 400 m perpendicular to the contact, a distance of about 2 km in outcrop.Like other Precambrian iron-formations, carbonates in the Biwabik Iron Formation are characterized by variable C 13 /C 12 ratios (delta C 13 PDB from --3.7 to --18.6ppm), which are low compared to the ratios of Phanerozoic marine carbonates. Limited sampling shows that the residual organic matter has a uniformly low C 13 /C 12 ratio (delta C 13 = -- 33.1ppm with maximum variations of --1.4 and +2.4ppm), whereas the difference in delta C 13 between carbonate carbon and organic carbon from the same samples ranges from 17.5ppm to 29.5ppm. Possibly, the isotopic composition of the carbonate carbon records diagenetic and post-diagenetic oxidation-reduction reactions of the form:6 Fe 2 O 3 + C(organic, delta C 13 = -33.5ppm) --> Fe 3 O 4 + CO 2 ,with subsequent exchange between C 13 depleted CO 2 and surrounding carbonate minerals.