MAGMATIC AND HYDROTHERMAL PGE MINERALIZATION OF THE BIRCH LAKE CU-NI-PGE DEPOSIT IN THE SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX, NORTHEAST MINNESOTA

The Birch Lake Cu-Ni-PGE Deposit is located 12 miles south of Ely, MN in the South Kawishiwi Intrusion (SKI) of the Duluth Complex (DC). The SKI is one of two layered mafic intrusions along the basal contact of the DC to host sub-economic Cu-Ni-PGE deposits. Mineralization is dominantly hosted by the U3 layer, the lower-most of three ultramafic-troctolite packages characterized as a zone of alternating ultramafic (picrite-peridotite) and troctolite horizons with lenses and pods of oxide-bearing (>5%) ultramafic and/or massive oxide. For this study, samples from 4 drill holes transecting the Birch Lake Deposit were obtained from the Natural Resource Research Institute (NRRI) located in Duluth, MN. EMPA and detailed petrography was used to locate PGE-bearing minerals, averaging 6-9 um in diameter. The purpose of this study is to locate, describe, and characterize the microenvironment of and mechanism by which PGE mineralization occurs, in turn, aiding exploration in locating other deposits and guiding metallurgists in improving recovery techniques.

PGEs occur most often as various Pd minerals with associated Pt, Os, Ir, Ru, Au, Ag, Te, Bi minerals. The PGMs occur in one of the following microenvironments: 1) In "halos" or "skirts" residing most commonly in anorthite-enriched zones in primary plagioclase around either interstitial sulfide (dominantly cpy), interstitial sulfide and silicate (dominantly cpy, cpx, and hydro silicates (amphibole and biotite)), or silicate (dominantly cpx or hydro silicate). 2) In poikilitic anorthite-rich plagioclase (An 75-An 95) and clinopyroxene (Wo 30-Wo 50) with PGEs sometimes residing in disseminated cpy or hydro silicate pockets, but no association with a "halo" or "skirt". 3) In interstitial sulfides or silicates that include hydro silicates, cpy, cpx, an interstitial sulfide that has a "Swiss cheese" texture, calcite, or in symplectite (commonly plagioclase and amphibole). 4) Remobilized PGEs that occur in chlorite, serpentine, or secondary magnetite.

The relative roles of (a) one-stage crystallization, (b) introduction of sulfur, and (c) hydrothermal processes continue to be debated in the origin of these types of deposits. Our data suggest that both primary magmatic and remobilizing (deuteric?) processes control PGE mineralization in the Birch Lake Deposit.