Abstract

The Duluth Complex, a plutonic component of the midcontinent rift system, has potential for deposits of Cu-Ni sulfides, Ti-V-Cr oxides, and platinum-group elements. A major obstacle to exploration in much of the complex has been the lack of reliable geologic mapping because of the cover of glacial deposits.Unfiltered and derivative-enhanced gravity and aeromagnetic data were used to interpret the geology of the poorly exposed central part of the Duluth Complex. This study confirms that the complex consists of multiple intrusions of anorthositic, troctolitic, and gabbroic rocks. This configuration contrasts with the well-stratified structures of other large mafic complexes and is consistent with emplacement by multiple feeders through a highly faulted crust.The Duluth Complex in the southern part of the study area is dominated by gabbroic and troctolitic rocks, whereas in the northern part it is dominated by anorthositic series rocks. A negative gravity saddle over this large anorthositic mass reflects a decrease in density and possibly in thickness of the complex. Strong gravity and magnetic signatures delineate a large belt of diabasic rocks and layered mafic cumulates emplaced into the anorthositic and volcanic rocks along the southeastern part of the study area. Mafic intrusive rocks appear to dominate this belt, although granitic and volcanic rocks occur locally. Most of these intrusive rocks are probably similar to the hypabyssal and plutonic rocks of the Beaver Bay Complex. Several features delineated by this study have significant potential for economic mineralization.

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