MINERAL CHEMISTRY AS A TOOL TO DETERMINE THE ORIGIN OF A MAFIC PEGMATITE WITHIN THE DULUTH COMPLEX, NORTHERN MINNESOTA

Mafic pegmatites are known to occur within layered mafic intrusions, such as the 1.1 Ga Duluth Complex (DC) of Northern Minnesota. The South Kawishiwi Intrusion (SKI) that occurs within the DC contains two similar troctolitic units that are separated by a pegmatitic unit. Previous petrographic and whole-rock geochemical studies were unable to establish clear petrogenetic relationships between the three units. This study will focus on the geochemistry of plagioclase and olivine phenocrysts collected from each of the units.

In overview, the basal portion of the SKI in our study area is separated into three units, with notable petrographic variations between units. The ultramafic three unit (U3) is the lowermost unit. This unit has an average thickness of 100 ft and is characterized by massive oxide horizons, as well as sulfide and PGE mineralization. A pegmatitic unit (PEG) sporadically overlies the U3 unit, has an average thickness of 95 ft, and is a boundary between a sulfide-rich zone below and sulfide-poor zone above. The ultramafic two unit (U2) overlies the PEG and has an average thickness of 90 ft. This unit is petrographically similar to the U3 with the exception that the U2 lacks massive oxides and sulfide-rich horizons. A plagioclase symplectic texture is rare below the PEG, is common within the PEG, and less common in the U2.

Preliminary mineral chemistry from corresponding rims and cores was obtained from plagioclase and olivine phenocrysts from all units. In the U3, Fo ranges from 67-69 with no distinction between the rim and core; An ranges from 64-80 with all cores being slightly more calcic. In the PEG, Fo ranges from 47-55 with no distinction between core and rim; An ranges from 51-65 with distinctly higher calcic cores. In the U2, Fo ranges from 64-66; and An ranges from 61-81 with the cores being slightly more calcic. Based on this preliminary chemical data, we infer that the relationship between the U3 and U2 is consistent with fractional crystallization. The origin of the PEG is less certain. It may represent a fluidized injection between the U3 and U2 during their formation; or it may represent crystallization in the presence of a fluid phase that developed concurrent with crystallization of the U3 and U2. If the latter is the case, the PEG formed while plagioclase was forming in the U3 and U2 but before olivine was forming.