QUARTZ-MAGNETITE OXYGEN ISOTOPE THERMOMETRY OF THE BIWABIK IRON FORMATION

The Biwabik iron-formation of northern Minnesota underwent contact metamorphism by the intrusion of the Duluth gabbro in the Late Precambrian. The eastern 20 km of the Biwabik is well exposed with nearly continuous outcrops in mines, making it ideal for a quartz-magnetite oxygen fractionation study.

Samples were collected over the eastern 20 km of the Biwabik. The Duluth gabbro contact is sub-parallel to the Biwabik. The Biwabik dips 45-30º, southward under the gabbro at the north-eastern end closest to the contact and shallows out to 5º within a few km.

Quartz and magnetite were analyzed for d¹⁸O by laser-fluorination. The quartz samples were ~0.5-1.5 mg and the magnetite were ~1.5-2.5 mg in size. The Δ¹⁸O (qtz-mgt) values vary systematically, decreasing towards the contact with the Duluth gabbro.

Temperatures calculated from the Δ¹⁸O (qtz-mgt) (Chiba et al., 1989) decrease from 685º C at the contact to 270º C at >3 km (normal to the contact). This trend is similar to that of Perry and Bonnichsen (1966), but temperatures are higher at the contact. For this study, outcrop tests were formed by collecting multiple samples from each outcrop location. At the high temperatures, laser analyses cluster tightly, with a total range of 0.5 per mil. There was more variation in temperature at lower grades: 465-270º C.

This variability may be attributed to three things: uncertainty in the true distance (in 3-D), difficulty to attain pure mineral separates, and greater temperature sensitivity at lower grades. True distance is hard to estimate since the contact of the Duluth dips variably from 10º to as much as 60º SE, with the average being about 20-30º SE. Also, the dip directly over the samples may have been different; as inferred from the sub-parallel (to the Duluth gabbro contact) isograds of French (1968). Quartz separates were purified with HCl. It is harder to remove quartz from fine grain magnetite separates at lowest grades. However, any quartz contamination in magnetite (£ 5%) would raise the apparent temperature, not lower them.

Future work will use oxygen isotope temperatures as a calibration for iron isotope fractionation.