AN ORE MICROSCOPY STUDY OF THE EAGLE MASSIVE SULFIDE NI-CU DEPOSIT, UPPER MICHIGAN

The Eagle Project is a sulfide-bearing mafic-ultramafic intrusion associated with the 1.1 billion year Midcontinent Rift System. It is located in the early Proterozoic Baraga Basin in northern Michigan and was discovered in 2001 by Kennecott Minerals Company. Based upon its geometry from drill core and geophysical data, it is believed to be a component of a dynamic magma conduit that was connected to a deeper level staging chamber and possibly higher level dikes and sills. Mineralization is found throughout the intrusion and consists of disseminated, semi-massive, and massive sulfides. We conducted an ore microscopy study of 6 core samples (provided by Kennecott Minerals Company) from the same drill hole that intersected all varieties of mineralization. Our goal is to characterize the mineralogy and texture of the different ores and determine whether sulfide immiscibility occurred in-situ, occurred at a higher level and accumulated in the chamber by magmatic segregation, or occurred in a deeper staging chamber and was carried to the Eagle chamber by surging magma from below. The major sulfides at Eagle consist of pyrrhotite, chalcopyrite, and pentlandite. Minor amounts of cubanite, pyrite, and other rare sulfides were also identified. Massive sulfide ore consists of both Cu-rich (60-98% cp, 0-9% pn) and Ni-rich (20% pn, 13% cp) zones. Semi-massive ore is richer in Ni than Cu (8% pn, 5% cp), while disseminated ore is richer in Cu (4% cp, 2% pn). Pyrrhotite ranges from 26-37% with the exception of one Cu-rich sample and the disseminated sample (trace and 4% po respectively). Pentlandite occurs as both early-formed euhedral crystals and as late-stage exsolution lamellae and flame structures in po. Chalcopyrite is euhedral, massive, and can occur as exsolution lamellae in po. Pyrrhotite is generally massive. A conspicuous “loop” texture occurs in one massive sulfide sample and consists of cp in semi-circular vein-like structures within massive po and pn. The paragenetic sequence appears to be pn–po–cp–pn/cp exsolution in the massive sulfide samples, and pn-cp-py in the semi-massive and disseminated samples. The 2 stages of pn crystallization and the apparent difference in paragenetic sequence between the massive sulfide and the semi-massive and disseminated samples may be useful in resolving the immiscibility history.