The North Rainy prospect is in the Late Triassic, Rainy ultramafic-mafic complex that intrudes the Slana Spur Formation of Pennsylvanian and Permian age. The Rainy complex is up to 5,000 feet thick, dips shallowly north and is mainly serpentinized dunite with subordinate peridotite, pyroxenite, and gabbro (Nokleberg and others, 1992 [Open-File]; Nokleberg and others, 1992 [MF]; Bittenbender and others, 2007). In this area, the base of the complex is a layered gabbro as much as 1,500 feet thick.

The rocks at this prospect are variably serpentinized dunite near fine-grained gabbro. Near the hanging wall contact of the complex, there is a well-developed breccia zone at a dunite-gabbro contact that is locally mineralized. The breccia consists of angular dunite clasts of various sizes and at least one large inclusion of hornfelsed sedimentary rocks in a fine-grained gabbro matrix.

From 1995 to 1999, the area was explored by American Copper and Nickel Company (ACNC) working with Fort Knox Gold Resources, Inc. in their regional search for copper-nickel-PGE mineralization. Their work in the area included rock sampling, and airborne and ground geophysical surveys W.T. Ellis, oral communication, 2001). They found the North Rainy prospect in 1998 and carried out a large-loop geophysical survey. over it. There were active claims on the prospect to at least 2002. It was examined and sampled by the Bureau of Land Management in the early 2000s.

The mineralization consists of 2 to 4 percent sulfides, mainly pyrrhotite, with chalcopyrite, and pentlandite (?), disseminated in gabbro, (W.T. Ellis, oral communication, 2001). Float of sulfide-bearing dunite, gossan, and copper-stained serpentinite occur in a few places along the breccia contact. Rock samples contained as much as 0.18 percent nickel, 0.63 percent copper, 0.02 percent cobalt, 76 parts per billion (ppb) palladium, and 80 ppb platinum.