DISTINCTIVE GEOPHYSICAL ANOMALIES OVER NORTHEAST IOWA: WHAT DO THEY REPRESENT FOR PROTEROZOIC MAGMATISM AND MINERAL RESOURCES?

Large gravity and magnetic highs over northeast Iowa are interpreted to reflect a buried intrusive complex composed of mafic/ultramafic rocks, the northeast Iowa Intrusive Complex (NEIIC), intruding Yavapai Province (1.7-1.8 Ga) rocks. The complex’s age is unproven, although it has been considered to be Keweenawan (~1.1 Ga). Because only four boreholes reach the complex, which is covered by 200-600 m of Paleozoic sedimentary rocks, geophysical methods are critical to developing a better understanding of the nature and mineral resource potential of the NEIIC. Geophysical anomalies include 15-40 km diameter ring- and horseshoe-shaped magnetic highs that correspond with gravity highs. These anomalies have previously been interpreted to reflect dense mafic/ultramafic lopolithic intrusions with relatively weak central magnetizations. Recent collection of high-resolution airborne magnetic and gravity gradient data clarifies the geophysical signature of one of these interpreted plutons near the town of Decorah, IA. There, ring-shaped magnetic and gravity gradient highs coincide with a simple gravity high, and gabbro of unknown age (assumed Proterozoic). Modeling shows that these observations are consistent with the previous interpretations of intrusion geometry and properties. Two types of intrusive systems are known to produce similar geophysical anomalies. The first is alkaline ring complexes, with examples including the Keweenawan Coldwell and Killala Lake Complexes in Ontario. The second is 1.3-1.6 Ga anorogenic intrusive complexes, known to exist in a swath from the southwest U.S. through eastern Canada and into the Baltic Shield. Examples include the Kiglapait and Hettasch intrusions in Labrador, and the Ahvenisto Complex in Finland. Both types of intrusive systems appear in the broader NEIIC region: Keweenawan alkaline intrusions occur along the northern part of the Midcontinent Rift System in Ontario, and the NEIIC lies adjacent to the rift’s southwestern arm. The NEIIC also lies along the swath of anorogenic magmatism extending across North America. Both intrusive systems imply notable economic potential, as the Keweenawan Coldwell Complex hosts a PGE-Cu deposit, and the Mesoproterozoic Labrador examples are similar to the intrusion that hosts the Voisey’s Bay magmatic nickel deposit.