RE-OS ISOTOPIC CHARACTERISTICS OF PGE MINERALIZATION IN THE BIRCH LAKE AREA, SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX, MN

    The Birch Lake prospect is located along the northwest edge of the South Kawishiwi Intrusion within the 1.1 Ga Duluth Complex.  Within the Birch Lake area, localized concentrations of PGE (8-9 ppm Pt and Pd) are found, generally associated with layers of semi-massive to massive Fe-Ti-Cr oxides contained within a troctolitic sequence.  Sulfide concentrations are low (<2%), though variable in the PGE-rich zones.  Melatroctolites and other oxide-bearing sequences are strongly serpentinized.  An investigation of Re/Os isotopic systematics was initiated to evaluate the possible importance of country rock contamination in the genesis of PGE mineralization.  Osmium isotopic analyses were performed on oxide minerals separated from massive to semi-massive oxide layers and oxide rich melatroctilites.  Previous d¹⁸O analyses of massive oxides indicate variable degrees of contamination with oxides derived from the Biwabik Iron Formation, the immediate footwall country rock.  d¹⁸O values vary from 3.8 per mil to 5.6 per mil (magmatic) to 11.4 per mil (contaminated).     ¹⁸⁷Os/¹⁸⁸Os values of the oxide separates range from 0.135 to 1.41, indicating a high degree of crustal contamination within the troctolitic units containing the PGE enrichment.  γOs values of the oxides range from 165 to 456.  Previous analyses of Duluth Complex rocks located south of Birch Lake indicate 1% to 3% bulk contamination by material derived from the adjacent Virginia Formation.  Preliminary results from the Birch Lake samples suggest a similar level of contamination from the Biwabik Iron Formation as has been described from the Virginia Formation.  These levels of bulk contamination suggest that up to 40% of the osmium in the rocks may be of crustal origin.  This presents an interesting possibility for the source of platinum and palladium as well.  The decoupling of some of the d¹⁸O and Re/Os isotopic data suggests that hydrothermal alteration involving an external fluid may have been involved in the subsolidus addition of crustal rhenium and osmium to the igneous rocks.  Temperatures of this process must have been below those at which oxygen isotopic exchange between fluid and oxide minerals would have occurred.