East Rainy Prospect, Delta River District, Southeast Fairbanks Borough, Alaska, USA
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Geology: The East Rainy prospect consists of an 800-foot-long and as much as 150-foot-wide float train of mineralized olivine melagabbro. Melagabbro rubble blocks lie below a covered contact between Tetelna [Volcanics] volcanic rocks and barren fine-grained gabbro (W.T. Ellis, oral communication, 2001). At one location where the olivine melagabbro appears to be in place, it strikes N85E and dips vertically. The melagabbro weathers chocolate brown, is medium to coarse grained, contains 1 to 2 percent phlogopite, and commonly has 1 to 3 percent sulfide (as much as 10 percent maximum). 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 mineralized melagabbro contained 0.9 percent nickel, 0.82 percent copper, 0.035 percent cobalt, 1,210 parts per billion (ppb) palladium, 800 ppb platinum, and 512 ppb gold. The sample was relatively low in sulfide but contained 3.55 parts per million (0.1 ounce) of platinum group elements (PGE)-plus-gold per ton, along with only 0.9 percent nickel, 0.8 percent copper, and 4.6 percent sulfur. The Rainy ultramafic-mafic complex is a Late Triassic 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 mineralized melagabbro contained 0.9 percent nickel, 0.82 percent copper, 0.035 percent cobalt, 1,210 parts per billion (ppb) palladium, 800 ppb platinum, and 512 ppb gold. A sample analyzed for total platinum group elements (PGE) and gold by ACNC yielded 949.5 ppb platinum, 946 ppb palladium, 198.5 ppb iridium, 642.5 ppb osmium, 395 ppb ruthenium, 97.9 ppb rhodium, and 320.1 ppb gold (W T. Ellis, oral communication, 2001). The sample was relatively low in sulfide but contained 3.55 parts per million (0.1 ounce) of PGE-plus-gold per ton and only 0.9 percent nickel, 0.8 percent copper, and 4.6 percent sulfur. The occurrence is on active claims of Fort Knox Gold Resources.
Age: Late Triassic.
Alteration: The mineralized olivine melagabbro is moderately serpentinized.
Commodities (Major) - Cu, Ni, Pd, Pt; (Minor) - Co, Ir, Os, Rh, Ru
Development Status: None
Deposit Model: Ni-Cu-PGE in differentiated mafic-ultramafic sill.
4 entries listed. 4 valid minerals.
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