Re–Os and O isotopic variations in magnetite from the contact zone of the Duluth Complex and the Biwabik Iron Formation, northeastern Minnesota

Abstract

Osmium and oxygen isotopic analyses of magnetite from the South Kawishiwi Intrusion (SKI) of the Duluth Complex and country rocks of the Biwabik Iron Formation (BIF) were undertaken to evaluate the potential importance of country rock assimilation in the genesis of PGE enrichment in the intrusive rocks, and to track the response of the Re–Os isotopic system to contact metamorphism of the iron formation. Oxygen isotopic values of magnetite from contact metamorphosed Biwabik Iron Formation and magnetite-rich oxide assemblages from highly contaminated troctolitic rocks range from 7.0 to 13.7‰, distinctly higher than oxide minerals from less strongly contaminated troctolitic to melatroctolitic rocks of the SKI that range from 2.3 to 6.9‰. 187Re/ 188Os ratios of metamorphosed BIF tend to be much lower than values from low-grade, weakly recrystallized BIF. Only one sample of BIF from the contact aureole shows the type of Os isotope ratios expected as a result of mixing with a chondritic fluid at 1.1 Ga, the time of emplacement of the SKI. Both low- 18O and high- 18O magnetite populations from the SKI are characterized by 187Re/ 188Os values less than 7.2 and Os isotopic ratios that plot above a 1.878 Ga reference isochron for the low-grade BIF samples. Although elevated γ Os values are consistent with the premise that the SKI was contaminated by crustal material, we suggest that the Os isotopic compositions reflect a later lowering of Re/Os ratios related to interaction with a fluid, potentially that which was involved in the locally extensive serpentinization of the melatroctolitic rocks. Oxygen and H isotopic values of serpentine and serpentinized melatroctolites indicate that the fluids responsible for hydrothermal alteration ( T ~ 300 °C) were low in 18O and characterized by a large meteoric water component. Magnetite δ 18O values record less than a 15% approach to equilibrium with the fluid involved in the serpentinization process, suggesting that perturbations to the Os isotopic system under hydrothermal conditions may occur in the absence of significant shifts in magnetite δ 18O values.