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%. In a sulfide flotation beneficiation process, it is assumed that precious metals are contained within sulfide minerals as small (micron-sized) platinum group minerals (PGM), as Au-Ag minerals, or in solid solution. The 75% recovery implies that some precious metal mineral (PMM) phases may not be hosted by sulfide minerals. This study seeks to characterize the mineralogical and textural occurrences of PMM in the NorthMet ore feed and concentrates. This information is not only of importance to the beneficiation of NorthMet ores, but also to the understanding of the metallogenesis of PGE-Au in magmatic sulfide deposits. The energy dispersive spectrometer-equipped scanning electron microscope at University Minnesota Duluth was used in backscatter electron composition mode (BEC) to conduct detailed compositional scanning of polished thin sections to locate PMM.
As the 75% precious metal recovery would predict, NorthMet 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.
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, of which 55% are hosted by silicates and apatite. 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 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. Sulfide boundary PGM are often intimately intergrown with adjacent 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.
University of Minnesota M.S. thesis. August 2011. Major: Geological sciences. Advisors: James Miller, Penelope Morton, Richard Patelke. 1 computer file (PDF); viii, 155 pages, appendices A-B.
Cervin, Daniel O..
Characterization of precious metal mineral occurrences in the NorthMet deposit of the Partridge River Intrusion, Duluth complex, Minnesota, USA..
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