PLATINUM-GROUP ELEMENT AND STABLE ISOTOPE GEOCHEMISTRY OF PGM-BEARING TROCTOLITIC ROCKS OF THE DULUTH COMPLEX, MINNESOTA

Platinum-group minerals (PGM), selenides, arsenides, graphite and the common association silicates + sulphides + oxides were identified in the basal troctolites of the Duluth Complex. Platinum-group element (PGE) concentrations (ppb) in the troctolites are as follows: Os (3-15), Ir (0.1-25.1), Ru (1-50), Re (1-13), Pt (25-390), Pd (5-1300). Element ratios (Pt+Pd/Ir+Ru+Os ranging from 2.66 to 61.77 and Pd/Ir from 16 to 245) are in the range of flood basalt related deposits. The PGE/chondrite distribution is similar to most mafic-ultramafic rocks. The overall PGE data define two distinct PGE-chondrite normalized distributions. Delta-S-34 and delta-O-18 data range from +4 to +9 ppm and +2 to +7.3 ppm respectively. A maximum delta-S-34 value of +13 ppm has been documented in a graphite-rich troctolite. The delta-S-34 data lie within a narrow range of values different from those reported for the Cu-Ni mineralized troctolites (without PGM) and similar to the PGE-rich Babbitt zone. This suggests a different mechanism for the origin of the PGM-bearing rocks in comparison with the Cu-Ni mineralized zone. The low delta-O-18 values (<5) are from magnetite-rich troctolitic samples. Most of the platinum-group minerals are associated with hydrous silicates (prehnite, actinolite, chlorite, biotite, serpentine) and secondary sulphides (valleriite, violarite). Altered silicates in the mineralized samples contain high amounts of Cl (e.g. 0.5-1.1 wt% Cl in serpentine).