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Unnamed Occurrences (ARDF - MH167; east side of North Fork Rainy Creek), Delta River District, Southeast Fairbanks Borough, Alaska, USA

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Location: This record represents an area of several occurrences east of North Fork Rainy Creek at an elevation of 4,250 feet in the NE1/4NE1/4 section 36, T. 18 S., R. 9 E., Fairbanks Meridian. This location is approximated from Foley and others (1989) and corresponds to locality S79 in table 2 of Nokleberg and others, 1991).
Geology: The occurrences consist of several sulfide-bearing, garnet-pyroxene skarns in carbonate beds intruded by the Rainy ultramafic-mafic complex of Late Triassic age and of sulfide-bearing basalt, diorite, gabbro, and serpentinite (Foley, 1992). The mineralized mafic rock and skarn came from rubble near the southern contact of the Rainy complex. The sulfide minerals form disseminations and nearly massive pods of pyrite, pyrrhotite, magnetite, and chalcopyrite and trace amounts of pentlandite, cobaltite, and safflorite. Sample 79CH019A of chromite-bearing olivine cumulate collected by Nokleberg and others (1991) contained more than 5,000 parts per million chromium. A U.S. Bureau of Mines sample of gabbronorite rubble contained 0.9 percent nickel, 0.25 percent copper, 0.02 percent cobalt, 1,070 parts per billion (ppb) palladium, 725 ppb platinum, 300 ppb iridium, 70 ppb rhodium, and 60 ppb ruthenium. Massive sulfide float contains 0.41 percent copper, 0.09 percent cobalt, and 65 ppb palladium (Foley, 1992). Mineralized skarn samples contain as much as 6.2 ppm silver, 85 ppb gold, 0.05 percent cobalt, 2.0 percent copper, and 0.09 percent nickel. 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: A U.S. Bureau of Mines sample of gabbronorite rubble contained 0.9 percent nickel, 0.25 percent copper, 0.02 percent cobalt, 1,070 parts per billion (ppb) palladium, 725 ppb platinum, 300 ppb iridium, 70 ppb rhodium, and 60 ppb ruthenium. Massive sulfide float contains 0.41 percent copper, 0.09 percent cobalt, and 65 ppb palladium (Foley, 1992). Mineralized skarn samples contain as much as 6.2 parts per million (ppm) silver, 85 ppb gold, 0.05 percent cobalt, 2.0 percent copper, and 0.09 percent nickel. A U.S. Geological Survey grab sample contains disseminated chromite in olivine cumulate with more than 5,000 ppm chromium (Nokleberg and others, 1991). The occurrence is on active claims of MAN Resources.
Age: Late Triassic.

Commodities (Major) - Cu, Ni, Pd, Pt; (Minor) - Au, Co, Cr, Ir, Rh, Ru
Development Status: No
Deposit Model: Ni-Cu-PGE in differentiated mafic-ultramafic sill; Disseminated chromite in a l

Mineral List



13 entries listed. 9 valid minerals.

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References

Foley, J.Y., 1992, Ophiolite and other ultramafic metallogenic provinces in Alaska (west of the 141th meridian): U.S. Geological Survey Open-File Report 92-20-B, 55 p. Foley, J.Y., Burns, L.E., Schneider, C.L., and Forbes, R.B., 1989, Preliminary report of platinum group element occurrences in Alaska: Alaska Division of Geological and Geophysical Surveys Public Data File 89-20, 32 p., 1 map sheet, scale 1:2,500,000. Nokleberg, W.J., Lange, I.M., Roback, R.C., Yeend, Warren, and Silva, S.R., 1991, Map showing locations of metalliferous lode and placer mineral occurrences, mineral deposits, prospects, and mines, Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1996-C, 42 p., 1 sheet, scale 1:250,000.

 
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