GEOLOGY, GEOCHEMISTRY, AND PGE POTENTIAL OF MAFIC-ULTRAMAFIC INTRUSIONS IN MINNESOTA, EXCLUDING THE DULUTH COMPLEX

This presentation describes the preliminary results of a 4-year study evaluating the potential for platinum group elements (PGE) and associated metals in mafic to ultramafic intrusions in Minnesota outside the well-studied 1.1 Ga Duluth Complex. The study is a collaboration between the Minnesota Geological Survey and the Natural Resources Research Institute and is funded by the State Legislature on recommendation of the Minnesota Minerals Coordinating Committee.

Exploration for polymetallic deposits, including PGE, has traditionally focused on mafic intrusions of the Mesoproterozoic Duluth Complex for obvious reasons—the complex contains known occurrences of PGE associated with basal copper-nickel deposits that were discovered decades ago. By contrast, this study investigates the many smaller intrusions and Archean layered mafic complexes scattered throughout the remaining four-fifths of the state. Phase 1 of the study, completed in 2003, compiled an extensive inventory of over 150 occurrences of alkalic, mafic, and ultramafic rock bodies exposed in outcrop or identified in drill core. An extensive digital database of geologic, geochemical, petrographic, and geochronologic information was compiled from existing sources and from new analyses and observations. In addition, a small geochronologic program—using 40Ar/39Ar analyses of magmatic hornblende and biotite separates—was conducted on 8 samples to better constrain the general temporal framework of some intrusions. Phase 2 of the study is currently underway and involves more detailed evaluation of the geologic setting, igneous stratigraphy, and geochemistry of about 15 specific intrusions for which there is sufficient drill core information. For one of the best exposed intrusions in the state—the Deer Lake Complex, a sheeted mafic to ultramafic sill complex in north-central Minnesota—detailed geologic mapping and follow-up petrographic and lithogeochemical studies are being conducted. In addition, gravity and aeromagnetic data will be modeled to develop more accurate portrayals of the geology of select intrusions. This presentation will report on the current results of the Phase 2 study and present preliminary conclusions about the potential for their hosting economic PGE deposits.