IGNEOUS STRATIGRAPHY, SULFIDE MINERALOGY, AND PRECIOUS METAL DISTRIBUTION OF THE SERPENTINE CU-NI DEPOSIT, DULUTH COMPLEX, MINNESOTA

The Serpentine Cu-Ni deposit is located ~7km southeast of Babbitt, MN. It is hosted within the South Kawishiwi intrusion, which forms part of the 1.1 Ga Duluth Complex. The South Kawishiwi intrusion consists of layered mafic and ultramafic rocks. The footwall rocks in the Serpentine area are comprised of metasedimentary rocks and banded iron ores of the Paleoproterozoic Virginia and Biwabik Iron Formations.

The well established igneous stratigraphy of the South Kawishiwi intrusion is not recognizable in the area of the deposit. This is possibly related to the fact that Serpentine is centered on the major regional Grano fault, which is believed to have been instrumental in controlling the location of a magmatic feeder for the Bathtub intrusion as well as the Local Boy massive sulfide deposit to the south. Drill core logging in the Serpentine area has identified a heterogeneous mix of troctolitic rocks with minor norites and ultramafics as well as a basement high. The basement high is host to most of the known massive sulfide and is thought to be a fault bounded uplifted block. The delineation of an igneous stratigraphy in the area would help to test the history of faulting and emplacement. Initial investigation in the Serpentine area suggests that an ultramafic sequence may be used as a stratigraphic marker.

The occurrence of base metal sulfides is widespread within the Serpentine deposit. Core logging and petrography have identified a number of Cu-Fe-Ni sulfides including pentlandite, pyrrhotite, chalcopyrite, and cubanite. Geochemical analyses show that platinum, palladium, and gold are often associated with the base metal sulfides, with some of the highest concentrations reached in intersections of massive sulfides within the basement high. Preliminary sulfur isotopic analyses on the massive sulfides yielded values of +12 to +16 delta 34S, suggesting that a significant portion of the sulfur is derived from the Virginia Formation.

The Serpentine deposit is one of many Cu-Ni deposits within the Duluth Complex, but its position relative to a major structure, presence of massive sulfide, and location near the border of two large mafic intrusions make it unique. The detailed study of this deposit could help further constrain processes of mineralization in the Duluth Complex and their possible relationship to magmatic feeders.