Stable isotope geochemistry of organic matter alteration in Animikie Basin sediments within the thermal aureole of the Duluth Ccomplex

We have determined total organic carbon (TOC), total carbonate carbon (TCC), and δ¹³C values for TOC in samples from the Early Proterozoic Animikie Basin sediments (Virginia and Biwabik Iron Formations) that have been variably metamorphosed by the Duluth Complex. TOC values for the variably metamorphosed Virginia Formation show no systematic variation with distance from the intrusion (TOC from 0.06 to 2.54%). In the Biwabik Iron Formation, samples outside the thermal aureole of the intrusion have TOC generally below 1% compared to 1.7-6.2% for samples within the thermal aureole. This enrichment may reflect extensive remobilization of organic carbon within the thermal aureole. However, δ¹³C_org values measured for both the Virginia and Biwabik Iron Formations within the thermal aureole are not significantly different from those outside the thermal aureole. It thus appears that, in the Iron Formation, the thermal shock caused by the intrusion induced migration of labile organic carbon that was isotopically similar to the residual kerogen. The relatively low TCC in samples within the thermal aureole of the intrusion appear to indicate decarbonation of the Iron Formation also occurred.