CU-NI±PGE MINERAL DEPOSITS ASSOCIATED WITH THE DULUTH COMPLEX, NORTHEASTERN MINNESOTA

Large resources of low-grade copper-nickel sulfide ore that locally contain anomalous PGE concentrations are well documented by drilling in the basal zones of the Partridge River and South Kawishiwi intrusions. At least nine subeconomic deposits have been delineated in the basal 100 to 300 meters of both intrusions. The mineralization consists predominantly of disseminated sulfides that collectively constitute over 4.4 billion tons of material averaging 0.66% Cu and 0.20% Ni. In 1985, a geochemical evaluation of portions of a drill hole, from the Birch Lake area, found significant values of 9 parts per million (ppm) combined Pt and Pd. Additional drill holes were sampled and analyzed for PGEs throughout the Duluth Complex, and as a result, significant PGEs were found at several more of the Cu-Ni deposits.

Some of the PGE-enriched zones are “stratabound” in that they are correlative with certain units of the igneous stratigraphy. For example, four stratabound horizons, each containing generally 1.0 ppm Pd, have been documented at the Dunka Road deposit and appear to be related to magma mixing. Still other PGE-enriched zones appear to be related to either localized structural conditions and/or combinations of stratigraphy and structure. The PGE-enriched horizon at Birch Lake appears to be related to several factors that include: 1. close proximity to a vent; 2. magma mixing; and 3. localized modification of the PGE content by a later hydrothermal event.

The potential of developing the Cu-Ni deposits using hydrometallurgical techniques has once again sparked renewed interest in the Duluth Complex. PolyMet Mining Corporation has acquired the Dunka Road deposit (NorthMet deposit) and plans to use its patented PlatSol technique to recover Cu, Ni, Co, and PGE. Teck Cominco has leased the Babbitt deposit (Mesaba deposit) and plans to use its patented Cominco Engineering Services Laboratory (CESL) process to recover the same metals. Franconia Minerals Corporation has applied for permits to put an exploratory shaft in the Birch Lake area to obtain material for hydrometallurgical testing. If it can be proven that these “hydromet” processes are feasible and economical, the next phase (the “permitting era”) in developing the low-grade Cu-Ni±PGE deposits of Minnesota could begin in the near future.