PETROLOGICAL AND GEOCHEMICAL EVALUATION OF THE STURGEON FALLS IGNEOUS BODY AND ITS RELATIONSHIP WITH THE PENOKEAN OROGENIC BELT

The Sturgeon Falls Igneous Body (SFIB) is a mafic to ultramafic intrusion located along the Michigan-Wisconsin border just south of the town of Norway, MI. The SFIB is located at the eastern end of the Penokean Volcanic Belt (PVB)-host to several known VMS deposits such as Flambeau and Back Forty. The SFIB is bounded to the north by the Niagara Shear Zone, which separates the Pembine Wausau Terrane from the Superior Craton (Schulz and Cannon, 2007), and to the south by an unnamed thrust fault zone. To the north the SFIB is bordered by the Michigamme Formation, composed mostly of metamorphosed sedimentary rocks, and to the south by the Quinnesec Formation, which is composed of metamorphosed tholeiitic basalt, basaltic to andesitic pillow lavas and massive flows (Sims and Schulz, 1993). The goal of this project is to prepare a geologic map of the SFIB and to determine the relationship of the SFIB and the surrounding rock units. Geochemical and petrological data was used to evaluate the potential of the SFIB as a host for a magmatic sulfide mineral deposit. The SFIB has been divided into three separate rock units: metagabbro, serpentinite, and norite/gabbronorite. The metagabbro shows a large range in grain size. Much of the pyroxene has been altered to amphibole. Locally the gabbro is serpentinized. The serpentinite unit is distinguished from metagabbro in the field by heavy serpentinization with poor to moderate foliation, and is often associated with quartz and/or calcite veins. Norite/gabbronorite is distinguished by the presence of plagioclase and orthopyroxene with little alteration of pyroxene to amphibole.

The origin of the SFIB is controversial. Traditionally the SFIB, has been called a sill. However, the large inferred volume of the SFIB is inconsistent that interpretation. We believe there are two possibilities for the origin of the SFIB. The first possibility is that the SFIB and the rocks in the adjacent Quinnesec Formation represent parts of a Proterozoic ophiolite sequence (Schulz, 2003). The second possibility is that the SFIB formed as a magma storage chamber for sub-marine volcanoes in the PVB, which may have been responsible for the origin of the VMS deposits in the area.