The Rainy Breccia prospect is near the north margin of the Late Triassic, Rainy Creek ultramafic-mafic complex that intrudes volcanic and sedimentary rocks of the Tetelna Volcanics and the Slana Spur Formations, both of Pennsylvanian and Permian age (Stout, 1976, Nokleberg and others, 1992 [MF]; 1992 [Open-File]; Bittenbender and others, 2007). The complex is up to 5,000 feet thick, dips shallowly north, and can be traced for more than 15 miles. It is mainly serpentinized dunite with subordinate peridotite, pyroxenite, and gabbro. The base of the complex is a layered gabbro as much as 1,500 feet thick.

The prospect was discovered by the American Copper and Nickel Company (ACNC) in 1995 and later examined and sampled by the Bureau of Land Management (Bittenbender and others, 2007). As described by W.T. Ellis (oral communication, 2001), the Rainy Breccia prospect is in breccia that consists of subrounded clasts of dunite in a dark-colored gabbronorite matrix. The unit is several hundred feet long and approximately 50 feet thick. The breccia strikes S65E and dips 75N. The mineralization in the matrix consists of up to 6 percent disseminated pyrrhotite and magnetite, with traces of chalcopyrite and pentlandite as inclusions in the pyrrhotite. A grab sample of the sulfide-bearing breccia contained 0.12 percent nickel, 0.15 percent copper, 0.01 percent cobalt, 26 parts per billion (ppb) palladium, and 25 ppb platinum.