USING THE GEOCHEMISTRY OF SULFIDES TO UNDERSTAND THE GENESIS OF TI-RICH FE-OXIDE ULTRAMAFIC INTRUSIONS WITHIN THE DULUTH COMPLEX

The Duluth Complex formed 1.1 Ga as a part of the Midcontinent Rift System when a series of massive mafic intrusions were emplaced in northeastern Minnesota. The rock types within this complex include a series of gabbroic, troctolitic, and anorthositic intrusions which protruded into the Archean and Paleoproterozoic footwall. This footwall comprises the metasedimentary rocks of the Biwabik Iron Formation and the Virginia Formation. Twelve Fe-Ti oxide-bearing ultramafic intrusions (OUIs) are located along the western margin of the Duluth Complex near the footwall. These OUIs were first identified in geophysical studies, but their genesis is unknown. In this study, we investigate and compare sulfide mineralogy and geochemistry between sulfide minerals from the OUIs and other sulfide occurrences in the Duluth Complex to better understand how they formed. Prior to this study, no compositional data were available for sulfides within any OUI. We analyzed samples from drill cores from two OUIs: Longnose, located within the Partridge River Intrusion, and Titac, located in the Western Margin Intrusion. Using reflected light microscopy, scanning electron microscopy, and energy dispersive spectroscopy, we identified a range of sulfide minerals. With electron probe microanalysis, we quantitatively determined the major and minor elemental composition of the sulfides. Lastly, we utilized laser ablation-inductively coupled plasma-mass spectroscopy to determine the trace element composition of the sulfides. Our findings demonstrate that Titac contains 1-5 modal percent sulfide while Longnose only contains about 1 modal percent. Titac and Longnose are both dominated by chalcopyrite, with trace bornite, sphalerite, and millerite. Titac also contains pyrite and pentlandite, whereas chalcocite is only found in Longnose. By comparing the trace element composition of sulfides from the OUIs with other sulfide occurrences in the Duluth Complex, such as the disseminated Cu-Ni mineralization found at the footwall contact, we will be able to determine a possible parental sulfide melt composition, and ultimately whether or not similar processes were involved in the emplacement of these OUIs.