RE-OS DATING OF C-RICH SLATE FROM THE WESTERN WAWA SUBPROVINCE, MINNESOTA

Re-Os dating of carbonaceous rocks using H₂SO₄-CrO₃ dissolution provides a powerful geochronologic tool for dating potentially ore-bearing sedimentary sections, especially in Archean terranes. C-rich slates in the Joy Lake sequence provide Re-Os ages of ~2.7 Ga, the oldest Re-Os ages yet reported for C-rich sedimentary rocks. The Joy Lake supracrustal sequence (western Wawa subprovince, Superior province, north-central Minnesota) consists of volcanic and sedimentary rocks with intercalated mafic tuff lenses, intruded by layered peridotite-pyroxenite-gabbro sills of the Deer Lake complex. Poor exposure and lack of geochronologic data for this sequence hinder construction of a geologic framework for the Wawa supracrustal rocks, compromising correlation with other sedimentary sections in the Superior province.

Carbonaceous slates of the Joy Lake sequence have well-developed cleavage subparallel to bedding and contain nodules, disseminations, and bedding-parallel stringers of pyrite. Samples from two drill holes (DH26503 and DH26506) yield isochron ages of 2695 ± 14 and 2684 ± 16 Ma and initial ¹⁸⁷Os/¹⁸⁸Os (Os_i) of 0.15 ± 0.16 and -0.29 ± 0.20, respectively. The ages, identical within uncertainty, are consistent with ages for Timiskaming-type sequences in the Wawa (~2688 to 2695 Ma) and Abitibi subprovinces (~2677 to 2680 Ma), that represent late-orogenic deposits and likely postdate the Joy Lake sequence.

A chondritic Os_i (0.109 at 2.69 Ga) is expected in Archean shales, but the negative Os_i for DH26506 requires explanation. Re is remobilized more easily than Os and may have been released from intercalated high Re/Os mafic and ultramafic volcanic rocks during recent exposure and local oxidation of the section. The C-rich rocks are a ready sink for mobile Re. This could shift isochron data points to higher ¹⁸⁷Re/¹⁸⁸Os with minimal slope change, but with slightly increased scatter and uncertainty for the Osi. The slight shift of the isochron to the right caused by increased ¹⁸⁷Re/¹⁸⁸Os ratios produces a spuriously low or negative Os_i intercept, and requires addition of only 0.9% to 2.4% Re relative to the Re concentration of DH26506 drill core samples. If the Re addition is recent, the age is not affected. Notably, the C-rich slates in DH26506 are in direct contact with mafic and ultramafic volcanic tuffs.