Rainy Breccia Occurrence, Delta River Mining District, Southeast Fairbanks Census Area, Alaska, USAi

Regional Level Types
Rainy Breccia Occurrence	Occurrence
Delta River Mining District	Mining District
Southeast Fairbanks Census Area	Census Area
Alaska	State
USA	Country

Location: The Rainy Breccia occurrence is located at an elevation of 5,700 feet on the divide between North Fork Rainy Creek and Ann Creek. It is in the SW1/4SE1/4 section 19, T. 18 S., R. 10 E., Fairbanks Meridian.
Geology: The Rainy Breccia occurrence contains as much as 6 percent coarse-grained sulfides in a melagabbronorite matrix that hosts subrounded dunite clasts. The unit is several hundred feet long and approximately 50 feet thick. The breccia strikes S65E and dips 75N and is just below the upper (northern) contact of the Rainy ultramafic-mafic intrusive complex of Late Triassic age (W.T. Ellis, oral communication, 2001). The disseminated sulfide minerals are predominantly pyrrhotite and magnetite and 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. The Rainy ultramafic-mafic complex is a steeply north dipping tablular intrusion of dunite, peridotite, and gabbro that varies from less than 100 feet thick to more than 6,000 feet thick and extends for more than 12 miles in length (W.T. Ellis, oral communication, 2001). Discontinuous marginal gabbro extends along most of the southern (lower) contact and is less continuous along the northern (upper) contact. The complex intrudes the Slana Spur Formation of Pennsylvanian age (Nokleberg and others, 1991). Magmatic mineralization is synchronous with emplacement of the Rainy complex, which is part of a 120-mile-long belt of mafic-ultramafic and associated rocks in the east-central Alaska Range.
Workings: Exploration in this area has been by American Copper and Nickel Company (ACNC) working with Fort Knox Gold Resources, Inc. Exploration from 1995 through 1998 included rock sampling, airborne and ground geophysical surveys, and completion of one diamond drill hole (W.T. Ellis, oral communication, 2001). This occurrence was discovered and staked by ACNC in 1995. A grab sample of the sulfide-bearing breccia contained 0.12 percent nickel, 0.15 percent copper, 0.01 percent cobalt, 26 part per billion (ppb) palladium, and 25 ppb platinum. A moderate airborne electromagnetic anomaly coincides with the east-southeast-trending breccia zone (W.T. Ellis, oral communication, 2001). The occurrence is on active claims of Fort Knox Gold Resources.
Age: Late Triassic.
Alteration: Irregular serpentinized zones occur along the breccia contact.

Commodities (Major) - Cu, Ni, Pd, Pt; (Minor) - Co
Development Status: None
Deposit Model: Ni-Cu-PGE in differentiated mafic-ultramafic sill.

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Link to USGS - Alaska:	MH156

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