Abstract

The Babbitt Cu-Ni sulfide deposit, located in the Duluth Complex of Minnesota, contains associated low-grade platinum-group element (PGE) mineralization. Interelement correlations show that for both massive and disseminated ore zones, Rh, Ru, Os, Re, and Ir are positively correlated to S abundance and suggest that they have behaved as immobile elements. However, Pt, Pd, and Au are strongly decoupled from the Rh-Ru-Os-Re-Ir group and show no significant correlation with S, Cu, or Ni. A PGE-enriched zone (up to 3,000 ppb Pt and 1,100 ppb Pd) is located laterally adjacent to the highest grade Cu-Ni ore. The PGE-enriched zone is characterized by abundant bornite, lower sulfide delta 34 S values (-1.4 to +8.9ppm) than those from other sulfide-bearing zones at Babbitt, and oxide assemblages that include Cr-bearing hercynite-and magnetite-rich spinels, as well as ilmenite. Sperrylite (PtAs 2 ) is the only discrete PGE mineral identified in the Babbitt deposit. Synchrotron X-ray fluorescence microprobe analyses indicate that some rocks with a high-bulk PGE content are characterized by ppm quantities (16-132) of Pd in cubanite, chalcopyrite, maucherite, and mackinawire. Although Cl contents of biotite and apatite from mineralized units range from 0.10 to 0.71 and 0.87 to 1.43 wt percent, respectively, no direct relationship between PGE enrichment and Cl content of volatile-rich phases has been observed. Geochemical data suggest that both magmatic and hydrothermal processes have been important in the localization of PGE at the Babbitt deposit. The distribution of Rh, Ru, Os, Re, and Ir reflects magmatic concentration processes, whereas Pt, Pd, and Au have been locally mobilized and redistributed during hydrothermal alteration.

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