Re-Os depositional age for Archean carbonaceous slates from the southwestern Superior Province: Challenges and insights

Rhenium-osmium (Re-Os) dating of carbonaceous rocks using H₂SO₄-CrO₃ dissolution provides a powerful new geochronologic tool for Archean terranes. Here we document the oldest Re-Os depositional ages yet reported for carbonaceous sedimentary rocks. Carbonaceous slates are intercalated with graywackes and mafic tuffs of the Joy Lake sequence in the western Wawa subprovince of the Superior Province. Well-developed D₂ cleavage is sub-parallel to bedding. Pyrite occurs in nodules, disseminations, and bedding-parallel stringers. Samples from two drill holes (DH26503 and DH26506) yield isochron ages of 2695 ± 14 Ma and 2684 ± 16 Ma and initial ¹⁸⁷Os/¹⁸⁸Os ratios (Os_i) of 0.15 ± 0.16 and - 0.29 ± 0.20, respectively. The ages, identical within uncertainty, provide the first temporal constraints for the westernmost Wawa subprovince greenstone sequence. The ages are consistent with cessation of mafic/ultramafic volcanism before ca. 2695 Ma in the western Wawa subprovince and D₂ deformation before 2685 Ma farther east. The Joy Lake sequence is likely the temporal equivalent of turbiditic graywackes deposited further east, both sourced from arc volcanoes during collision of the Wawa and Wabigoon arcs along the southern margin of the Superior Province. A chondritic Os_i (0.109 at 2.69 Ga) is expected for Archean shales, but the negative Os_i for DH26506 requires explanation. Re, which is significantly more mobile than Os, may have been released from intercalated high-Re/Os mafic tuffs by local oxidation during recent or Cretaceous exposure to meteoric waters. The adjacent carbonaceous rocks are a ready sink for mobile Re. A small Re addition can shift isochron data points to higher ¹⁸⁷Re/¹⁸⁸Os ratios with minimal slope change. This increases scatter and uncertainty for the Os_i, shifts the isochron to the right, and produces a spuriously low Os_i intercept. If Re addition is recent, the age is not significantly affected. Notably, the slates in DH26506 which yield the negative Os_i are intercalated with relatively permeable and reactive lithic wackes and mafic tuffs, whereas the slates in DH26503 are in contact with less permeable massive intrusive rocks. This process can be detected only if the true Os_i is approximately chondritic, and is more likely where reactive mafic rocks are prevalent-both the case for Archean greenstone belts.