SULFUR ISOTOPE CHARACTERISTICS OF SULFIDE MINERALS IN THE ARCHEAN AND PALEOPROTEROZOIC BASEMENT ROCKS ASSOCIATED WITH THE MIDCONTINENT RIFT

The source of extraneous sulfur in the magmatic systems associated with the Midcontinent Rift system has been frequently questioned. Since the Midcontinent Rift is known for several economic metalliferous magmatic sulfide deposits such as Eagle, Tamarack and Duluth, there must have been large quantities of extraneous sulfur which was incorporated into the magma for sulfide saturation and for the formation of these deposits. The basement rocks of the Eagle deposit in northern Michigan include Paleoproterozoic metasedimentary rocks of the Marquette Range Supergroup underlain by the Middle to Late Archean gneissic and granitic rocks. Recent exploration at the Baraga Basin area of northern Michigan has revealed significant quantities of sulfide minerals not only in the Paleoproterozoic slaty and schistose rocks of the Baraga Group but also consistently within the Archean gneissic rocks. The principal minerals include pyrite, pyrrhotite and marcasite. These minerals mostly occur as thin veins and small disseminated grains. The large abundances of sulfide minerals in the basement rocks indicate the possibility of other conduit-like magmatic sulfide deposits in the Midcontinent Rift area.

The measured δ³⁴S values in the Eagle and Eagle East deposits span a small range between 0 and 5 ‰ V-CDT. Similar values have been reported for the sulfide deposit of the Tamarack intrusion as well. However, the δ³⁴S values of the sulfide minerals in the siltstone and slaty rocks of the Baraga Group range between 6 and 20 ‰ V-CDT while the δ³⁴S values of the sulfide minerals in the Archean granitic and gneissic rocks range between -11 and 7 ‰ V-CDT. The large observed diversity in sulfur isotope characteristics in the basement rocks of the Midcontinent Rift indicate a large spectrum of sulfur sources and possibly complex processes of sulfur isotope fractionation by low temperature hydrothermal processes.