Paleomagnetic dating of the transformation of oolitic goethite to hematite in iron ore

We show that the oolitic hematite ores of Birmingham, Alabama, carry a remanence direction stable enough to have survived major folding. Nevertheless, this remanence was very likely acquired in the Pennsylvanian, about 130 Ma after the ore's Early Silurian deposition. This long delay is understandable if we accept the common hypothesis that the ore originated as oolitic goethite. We would then not expect acquisition of a stable remanence until deep burial in the Pennsylvanian raised the temperature to the ~80 °C probably needed to transform coarse goethite to hematite in water. Although the paleomagnetism of oolitic hematite may be of limited value in defining primary paleopoles, it may allow dating of when the ore's temperature was first raised to ~80 °C, perhaps aiding studies of sedimentary basin evolution and oil formation. Our results also support using caution in interpreting the paleomagnetism of more common red beds, emphasizing that a positive fold test is not necessarily evidence of primary remanence and that burial history may control the timing of remanence acquisition.