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Geology Guest Speaker - Jack Gibbons '07

"Re-emergence of the Ni-Cu-PGE deposits of the Duluth Complex: Facilitated by analytical, hydrometallurgical, and geologic advances"

Date: Monday, April 23rd, 2012

Time: 4:30 pm

Duration: 1 hour, 30 minutes

Location: Mudd 73

Contact: Ellen Haberoth, x4407

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The discovery and exploration for the Ni-Cu-PGE deposits of the Duluth Complex may be divided into five periods: 1) Initial discovery and limited exploration work (1948-1966); 2) MDNR began leasing state mineral rights to exploration companies (1966-1974); 3) MDNR stopped leasing state mineral rights to exploration companies due to the state mandate for a regional EIS (1974-1982); 4) Exploration companies idled their projects due to lower metal prices  and their inability to market concentrates.  During this idle period, two technological advances increased the economic viability of the deposits.  Platinum group elements (PGE) analyses became less expensive and more accurate.  Systematic reanalysis of historic samples for PGE revealed significant amounts of unrealized PGE mineralization.  Hydrometallurgical advances offered an attractive, i.e. significant recovery of metals without sulfur emissions, alternative to smelting (1982-1997); and 5) Exploration companies were enticed by higher Ni, Cu, and PGE prices to return to the Duluth Complex (1997-present).

            Besides the PGE analytical and hydrometallurgical advances, geologic insights — gained primarily through geologic mapping, geochemical studies, and geophysical work — have both greatly constrained the genesis of these deposits facilitated the successful return of exploration companies to the Duluth Complex.  Geologic mapping by Dean Peterson (then with UM Duluth and now with Duluth Metals) revealed that the Nickel Lake Macrodike (NLM) fed the South Kawishiwi Intrusion (SKI).  Geologic mapping and drill core logging by Dean Peterson and other Duluth Metals’ geologists revealed that the SKI magma had a very high state of crystallinity, which resulted in the disseminated nature of the mineralization.  Drill core logging by Duluth Metals’ geologists revealed the presence and geometry of higher grade magma channels, thermal erosional features in the footwall contact.  Sulfur isotope work by Ed Ripley and his colleagues at the University of Indiana revealed that the SKI magma ingested large amounts of S-rich Virginia Formation, which was the critical event to prompt S-saturation of the SKI magma.  Anisotropy of magnetic susceptibility work by Josh Feinberg and his colleagues at the Institute for Rock Magnetism at the University of Minnesota confirmed that magma flowed from the NLM into the main chamber of the SKI.  Together, these insights allowed for more accurate prediction of mineralization and now provide conceptualization tools for future exploration targets away from known mineralization.

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