Understanding the role of footwall sulfur, ore mineralogy, and igneous stratigraphy in the formation of the Serpentine magmatic Cu-Ni deposit, Duluth Complex, northeastern Minnesota

Abstract

The Serpentine deposit represents one of a number of large undeveloped magmatic Cu-Ni deposits of the 1.1 Ga Duluth Complex in northeastern Minnesota. The deposit comprises a historic resource of 393 million metric tonnes grading 0.41percent Cu and 0.14percent Ni. In comparison to other magmatic Cu-Ni deposits of the Duluth Complex, the Serpentine deposits stands out due to the occurrence of disseminated sulfide zones and lenses of massive sulfides. Many of the massive sulfide intersections occur close to the basal contact of the mafic to ultramafic intrusion hosting the deposit, the South Kawishiwi intrusion, and the underlying metasedimentary rocks of the Virginia Formation. However, massive sulfides have also been recognized within the metasedimentary rocks below the contact. The present study reconstructs the emplacement history of a portion of the South Kawishiwi intrusion and the unique conditions that led to the formation and preservation of massive sulfide lenses at the Serpentine deposit. Based on extensive drill core logging and field mapping, key marker units within the igneous rocks of the South Kawishiwi intrusion were recognized and correlated across the deposit area along with the metasedimentary strata of the Paleoproterozoic Biwabik Iron and Virginia Formations. Three-dimensional visualization of the deposit geology provides firm constraints on the relative timing of the massive sulfide formation at Serpentine, which occurred early in the emplacement history of the South Kawishiwi intrusion. Field work coupled with petrographic investigations revealed that the South Kawishiwi intrusion dominantly consists of plagioclase plus or minus olivine cumulates containing variable amounts of interstitial clinopyroxene, biotite, sulfide, and iron oxide minerals. In addition to these troctolitic rocks and minor norite, a series of olivine-rich cumulates were identified in a number of adjacent drill holes. The olivine-rich rocks display systematic variations in olivine composition from north to south, implying that the melt which produced this rock type evolved over very short horizontal distances away from a magmatic feeder. Petrographic observations identified a zoning of Cu-sulfides vertically within this portion of the South Kawishiwi intrusion. In the upper portions of the intrusion, furthest from the basal contact with the metasedimentary rocks of the Virginia Formation, chalcopyrite was found to be the dominant Cu-bearing sulfide mineral. As the basal contact is approached, cubanite becomes dominant and is often the only Cu-bearing sulfide mineral present within the massive sulfides Following a trend similar to the zoning of Cu-bearing sulfide minerals, S isotopic ratios of pyrrhotite, chalcopyrite, and cubanite show a trend of becoming isotopically heavier near the basal contact with the metasedimentary rocks of the Virginia Formation. Sulfur isotopic ratios of sulfides collected from igneous rocks and massive sulfides range from plus 7.5 to plus 21.0, with the overwhelming majority of samples having ratios between plus 13 to plus 16. The results are consistent with the metasedimentary rocks of the Virginia Formation providing the majority of the S within the deposit. This S was presumably liberated from the metasedimentary rocks during contact metamorphism, which results in a conversion of pyrite to pyrrhotite and a loss of S. Whole-rock geochemical investigations identified that the metal tenors of all the collected igneous samples and massive sulfides are quite variable. Samples with low whole-rock S values often display the highest tenors. In contrast, massive sulfide samples have the lowest metal tenors out of the samples analyzed and often display metal ratios that differ from those of the overlying igneous rocks containing disseminated sulfides. These observations suggest that Petrographic observations identified a zoning of Cu-sulfides vertically within this portion of the South Kawishiwi intrusion. In the upper portions of the intrusion, furthest from the basal contact with the metasedimentary rocks of the Virginia Formation, chalcopyrite was found to be the dominant Cu-bearing sulfide mineral. As the basal contact is approached, cubanite becomes dominant and is often the only Cu-bearing sulfide mineral present within the massive sulfides. Following a trend similar to the zoning of Cu-bearing sulfide minerals, S isotopic ratios of pyrrhotite, chalcopyrite, and cubanite show a trend of becoming isotopically heavier near the basal contact with the metasedimentary rocks of the Virginia Formation. Sulfur isotopic ratios of sulfides collected from igneous rocks and massive sulfides range from plus 7.5 to plus 21.0 , with the overwhelming majority of samples having ratios between plus 13 to plus 16 . The results are consistent with the metasedimentary rocks of the Virginia Formation providing the majority of the S within the deposit. This S was presumably liberated from the metasedimentary rocks during contact metamorphism, which results in a conversion of pyrite to pyrrhotite and a loss of S. Whole-rock geochemical investigations identified that the metal tenors of all the collected igneous samples and massive sulfides are quite variable. Samples with low whole-rock S values often display the highest tenors. In contrast, massive sulfide samples have the lowest metal tenors out of the samples analyzed and often display metal ratios that differ from those of the overlying igneous rocks containing disseminated sulfides. These observations suggest that the massive sulfide intervals could not have simply formed by accumulation of disseminated sulfides. The decreased tenor within the massive sulfides suggests that a massive influx of external S is required to explain the negative correlation of metal tenor and whole-rock S. The presence of massive sulfides within the Virginia Formation requires the existence of a local magma feeder, which supplied enough magma that reached sulfide saturation to explain the metal contents of the massive sulfides. The results of the present study also highlight the role of faulting in the Serpentine area. The Grano fault is believed to be a magmatic conduit in areas where it is intersected by fault structures at high angles to its strike. This geometry serves to focus magma within discrete conduits. This reconstruction of the structural history of the Serpentine area has significant implications for exploration. Identifying large fault structures that are intersected at high angles to the strike by additional fault structures are favorable areas because they can serve as magmatic conduits potentially hosting high tenor massive sulfides.