THE NORTHEAST IOWA INTRUSIVE COMPLEX: A MAGMATIC CONUNDRUM RELATED TO THE MIDCONTINENT RIFT SYSTEM

The Keweenawan-age Midcontinent Rift System (MRS) has been the focus of decades of research for its enigmatic geologic history and its wealth of economic minerals. The latter has been concentrated in the Lake Superior region where the MRS is exposed at or near the land surface. Copper-nickel sulfide and platinum group element deposits have been identified along the north shore in Ontario (Coldwell Complex) and along the western shore in Minnesota (Duluth Complex). These magmatic deposits are related to the MRS and are geophysically distinct, with high amplitude magnetic anomalies and associated gravity highs (Drenth et al., 2015 and references therein).

Since 2012, the U.S. Geological Survey (USGS) has conducted two major high resolution geophysical surveys of northeastern Iowa and southeastern Minnesota in an attempt to better understand the nature of the Precambrian basement geology concealed beneath at least 1,000 feet of Paleozoic sedimentary rocks. The surveys, both magnetic and gravity, have succeeded in refining the area previously identified as the Northeast Iowa Plutonic Complex (Anderson, 2006), now called the Northeast Iowa Intrusive Complex (NEIIC). The NEIIC has an aerial extent of over 6,000 mi² consisting of several large ring/horseshoe shaped anomalies and associated linear features. Characterizing these features using geophysics has been done, yet with a limited number of boreholes that reach the NEIIC, accurate lithologic and geochronologic data has remained elusive. One iron exploration core drilled in 1963 intersected a dike extending northeast from the main part of the NEIIC and encountered more than 700 feet of ultramafic olivine-plagioclase cumulate. The Iowa Geological Survey (IGS) and the University of Iowa Department of Earth and Environmental Sciences are using pXRF and micro probe analyses to isolate datable minerals. Preliminary results show there are distinct zones within the core that have elevated uranium and zirconium, typically in the form of baddeleyite or zirconolite. If these zones are deemed viable, samples will be sent to the USGS for geochronologic analyses. Obtaining a reliable age date from the core could provide the missing piece to the NEIIC puzzle and answer the question of whether it is in fact related to the MRS and other economic mineral deposits in the region.