A PRELIMINARY ANALYSIS OF STRATIGRAPHIC COMPOSITIONAL VARIATIONS IN THE ARCHEAN OTTER CREEK IGNEOUS COMPLEX, NW IOWA

The Otter Creek Igneous Complex is a buried layered mafic intrusion located in Northwest Iowa. It was discovered in 1963 when cores were drilled into a magnetic anomaly near Matlock, IA. The Otter Creek intrusion has been dated at 2870 ± 100 Ma (Sm-Nd) and 2710 ± 8 Ma (U-Pb zircon) making it similar in age to the Stillwater Complex in Montana. Other than geochronology, limited work has been done on the Otter Creek intrusion. Compositional characterization is hampered by high levels of hydrothermal alteration and serpentinization that likely formed during prolonged exposure in the late Proterozoic. With the exception of Cr-spinel, preservation of primary igneous phases (olivine, pyroxene, and plagioclase) is very limited. Recently the USGS has been tasked with identifying and characterizing potential strategic critical mineral resources throughout the country, which has renewed the focus on unique intrusions like Otter Creek. A previous study asserted that the Otter Creek intrusion has been tilted, uplifted, and partially eroded, with a composite stratigraphy that is approximately 700 m thick. The lower units form an ultramafic series (bronzitite, harzburgite, and dunite) that is overlain by a banded series (gabbro and anorthosite). In-situ compositional analyses on the archived cores at the Iowa Geologic Survey were carried out using a handheld portable x-ray fluorescence instrument to screen for compositional variations in mineral chemistry, and to identify whether any large-scale trends are developed through the stratigraphy. Analyses were taken approximately every two feet starting from the bottom of the core until significant alteration was observed. Five of the 12 cores were analyzed to develop a representative geochemical background for each of the lithologies previously identified. A total of 24 representative samples have been collected for major and trace element analysis by XRF and ICP-MS and mineral chemistry with EMP. These new compositional data will be presented at the meeting with early stage interpretations.