CHARACTERIZATION OF PRECIOUS METAL MINERAL OCCURRENCES IN THE NorthMet DEPOSIT OF THE PARTRIDGE RIVER INTRUSION, DULUTH COMPLEX, MINNESOTA

The NorthMet deposit is a Cu-Ni-PGE magmatic sulfide ore body located along the northwestern margin of the Partridge River Intrusion (PRI), which is part of the 1.1 Ga Duluth Complex. PolyMet Mining Company is currently seeking a permit to develop an open pit mine at the site, which is about 7 miles south the town of Babbitt, MN. During pilot-plant test runs by PolyMet, approximately 75% of the total mass of precious metals (75% is the average recovery of Pd, Pt, and Au) known to exist from assay data were recovered; total sulfide recovery was 90%.

As the 75% precious metal recovery would predict, NorthMet precious metal minerals (PMM) primarily occur in association with sulfide minerals. Of the 346 PMM investigated in this study, 267 (77%) were hosted by sulfide minerals (mostly chalcopyrite and pentlandite), either as inclusions or at sulfide grain boundaries. The remaining 23% (79) of PMM were found in a variety of primary silicates, secondary silicates, and apatite. Forty-eight percent of sulfide-hosted PGM are located at sulfide grain boundaries, 52% occur as inclusions in sulfide. The conclusions of this study are:

1) The lower recovery rate of precious metals relative to base metals in NorthMet ores is largely due to the textural and mineralogical occurrence of Au-Ag minerals. Furthermore, most Au-Ag minerals do not appear to be strongly attached to and intergrown with sulfide host minerals either as inclusions or at grain boundaries and are likely lost to tailings prior to introduction to sulfide flotation systems.

2) Platinum group minerals (PGM) have a strong sulfide association: at least 90% are hosted in sulfide minerals. PGM occur in secondary silicates (7%) and in association with apatite (3%) in contact with, or close to sulfides. PGM primarily occur at sulfide grain boundaries in sulfide halo textures, usually in plagioclase. PGM do not occur as inclusions in primary silicate minerals.

3) An orthomagmatic model is invoked to explain the genesis of NorthMet ores. The strong sulfide association of PGM, the indirect sulfide association of Au-Ag, the general paucity of hydrous, secondary minerals in PGM-bearing sulfide halo textures, and the mostly well-preserved primary igneous textures indicate that magmatic processes formed the NorthMet ore body.