THE STURGEON FALLS GABBROIC INTRUSION AND ITS RELATION WITH THE METALLIFEROUS SULFIDE ROCKS OF THE PENOKEAN VOLCANIC BELT

The Penokean Volcanic Belt (PVB), located in Upper Peninsula Michigan, Northern Wisconsin and Northern Minnesota was formed during the Paleoproterozoic accretion of the continent of North America. The region is well-known for several VMS deposits such as Crandon, Flambeau and Back Forty which are hosted by a complex mixture of mafic to felsic, tholeiitic and calc-alkaline volcanic and plutonic igneous rocks. The Sturgeon Falls intrusive body in Dickinson and Menominee Counties of Michigan and Marinette County in Wisconsin is one of several gabbroic plutonic units located in the belt. It lies between the Niagara shear zone in the north and a thrust fault zone in the south and it is primarily composed of gabbro and hornblende gabbro with local occurrences of pyroxenite xenoliths and enclaves. The common minerals in the gabbroic rock include plagioclase, clinopyroxene and hornblende with secondary actinolite, epidote, calcite and quartz. The SiO₂ content ranges between 44.1 and 52.5 wt. % while the MgO content varies between 3.1 and 10.5 wt. %. The pyroxenite unit has average SiO₂ and MgO contents of 40 wt. % and 30 wt. % respectively.

It is envisioned that the plutonic rocks of the PVB participated in the transmission of metalliferous fluids which led to the formation of the VMS deposits in the area. The δ³⁴S values determined from the Back Forty VMS deposit, also in Menominee County range between -2.53 and 3.92 ‰ V-CDT with most values close to 0 ‰. δ³⁴S values determined from other VMS deposits in the PVB such as Crandon, Bend and Flambeau cluster around the 0 ‰ value which is strongly indicative of primary magmatic source of sulfur. Thus, the Sturgeon Falls intrusion and similar contemporaneous mafic to ultramafic intrusive bodies in the PVB can be regarded as prospective targets for primary magmatic sulfide mineral deposits.