HIGH-RESOLUTION, MULTI-METHOD GEOPHYSICAL IMAGING OF A PORTION OF THE NORTHEAST IOWA INTRUSIVE COMPLEX

Numerous large amplitude regional aeromagnetic anomalies and ground gravity highs over northeast Iowa and southeast Minnesota suggest the presence of a buried intrusive complex made up of mafic/ultramafic rocks. This complex is known as the northeast Iowa Intrusive Complex (NE IIC). The NE IIC lies along the eastern margin of the Midcontinent Rift System (MRS) and occupies a minimum estimated area of 17,000 square kilometers, making it comparable in size to the Duluth Complex. Country rocks are thought to be accreted island arc terranes of the Paleoproterozoic Yavapai Province (1.8-1.7 Ga), implying at least a somewhat younger age for the NE IIC. While not yet directly dated, these considerations suggest that a Keweenawan (MRS) age for some or all of the NE IIC is possible and imply significant potential for undiscovered Ni-Cu-PGE deposits. The NE IIC may also include Mesoproterozoic intrusions like the Wolf River Batholith in Wisconsin, or alkaline Cambrian plutons. Only four boreholes are known to reach the complex, which is covered by 200-500 meters of sedimentary rocks and sediments. Geophysical methods are critical to developing a better understanding of the nature and resource potential of the NE IIC. A high-resolution, multi-method geophysical mapping program was initiated in 2012 as a collaborative effort between the U.S. Geological Survey Mineral Resources Program, the Iowa Geological and Water Survey, and the Minnesota Geological Survey. An initial airborne data collection campaign in the region of Decorah, Iowa, included magnetic, gravity gradient (AGG), and time-domain electromagnetic (TDEM) data along flight lines spaced 400 m apart. Geophysical data show numerous magnetic anomalies that are paired with AGG highs, indicating widespread strongly magnetized and dense rocks of likely mafic composition. In the Decorah region, a prominent horseshoe-shaped, 15 kilometer diameter magnetic- and gravity-field high is correlated with the occurrence of basement rocks that have been described as gabbro and troctolite, suggesting a ring-shaped anomaly source with similarities to MRS alkaline complexes. A Yavapai age layered(?) metagabbro pluton is suspected to produce complex magnetic and gravity anomalies with different forms than the other basement rocks nearby.