STABLE ISOTOPIC STUDIES 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 Duluth Complex.  The Birch Lake area contains relatively high, localized concentrations of PGE (8-9 ppm Pt and Pd).  The PGE mineralization is 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 PGE zones, and usually take the form of small, disseminated blebs.  Melatroctolites and other oxide-bearing sequences are strongly serpentinized.  The origin of the PGE remains uncertain; the relative effects of primary magmatic processes versus secondary, low temperature hydrothermal effects are under investigation.  Assimilation of adjacent Biwabik Iron Formation (BIF) has been suggested to account for some of the oxide enrichments in the PGE-rich zones.     We have undertaken a stable isotopic and mineralogical study to aid in the determination of the origin of the PGE enrichment.  The d ³⁴S values of the disseminated sulfides within the PGE-rich zones generally range between 1.8 and 3 per mil (VCDT); values that are close to those of mantle derived sulfur (0 +/- 2 per mil).  The d ³⁴S values are higher where portions of recrystallized BIF are found as xenoliths.  d ¹⁸O values for the oxides in these xenoliths range from 6.9 to 10.2 per mil (VSMOW).  These values are considerably higher than the d ¹⁸O values of the oxide melatroctolites and massive oxides, 3.9 to 6.2 per mil (VSMOW).  Serpentinized melatroctolites have anomalously low signatures, 2.5 to 4.2 per mil (VSMOW) produced in response to the meteoric water dominated low temperature hydrothermal processes that affected the area.  Oxide d ¹⁸O values are not strongly suggestive of the assimilation of oxides from the BIF.  Although assimilation of BIF does not appear to be an important process in the PGE enrichment at Birch Lake, hydrothermal alteration is extensive and may have mobilized and concentrated PGEs.