Arctic Prospect, Shungnak District, Northwest Arctic Borough, Alaska, USA
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Geology: Arctic is one of several volcanogenic deposits in the Ambler schist belt along the south flank of the Brooks Range. The deposits may be part of a rifted continental margin (Schmidt, 1981, p. 548). Arctic is a syngenetic deposit hosted in a thick sequence of Devonian or Mississippian, low to medium-grade metamorphosed basaltic and rhyolitic rocks, submarine ash flow tuffs, volcaniclastic and minor plutonic rocks, and pelitic, carbonaceous and calcareous sedimentary rocks, known as the Ambler schist belt. These rocks are part of a large fold structure termed the Kalurivik arch. A Devonian or Mississippian age of mineralization is based both on fossil evidence and U-Pb radiometric dating (Hitzman and others, 1986, p.1592-1618). The Arctic polymetallic, stratabound, volcanogenic deposit consists of tabular masses of banded massive and disseminated sulfides, one foot to more than 55 feet thick, composed of 20 percent to 90 percent pyrite, chalcopyrite and sphalerite, along with lesser amounts of pyrrhotite, chalcocite, bornite, galena, tennantite-tetrahedrite, arsenopyrite and stibnite (Schmidt, 1988). The sulfides are enclosed in calcareous talcose to quartzose lenses within a metavolcanic (rhyolitic) unit. The mineralized area is 3,000 feet by 2,200 feet in area and about 270 feet thick. The massive sulfide occurrences are covered by a small gossan cap 9 to 15 feet deep. Kennecott completed 70 drill holes in the Arctic deposit in the 1970's and defined an inferred resource of 36.3 million tons grading 4.0 percent copper, 5.5 percent zinc, 0.8 percent lead, 0.7 gram of gold per ton, and 54.9 grams of silver per ton, or 8.0 percent copper equivalent. (Eakins and others, 1985, p. 6). NovaGold Resources, Inc. (2007, Ambler) reached an earn-in agreement with Kennecott Exploration Company and Kennecott Arctic Company (subsidiaries of Rio Tinto PLC) in March 2004. The agreement covers a 35,000 acre area in the Ambler district. Initial exploration by NovaGold in 2004 focused on the Arctic deposit and included new descriptions of existing core, structural geology studies, and the drilling of 11 infill holes totaling 9,768 feet (2,978 meters). These holes confirmed previous drill results and helped refine the 3-D geologic model for the Arctic deposit. Some 2004 drill intercepts reported by NovaGold include: 1) hole AR04-80 with 6.5 meters (11.5 feet) grading 3.36 percent copper, 0.91 gram of gold per ton, 1.90 percent lead, and 7.93 percent zinc; 2) hole AR04-86 with 12.5 m (41 feet) grading 3.76 percent copper, 0.91 gram of gold per ton, 52.4 grams of silver per ton, 0.58 percent lead, and 6.01 percent zinc, or an 8.0 percent copper equivalent; 3) hole AR04-87 with 7.4 m (24 feet) grading 9.65 percent copper, 0.73 grams of gold per ton, 108.2 grams of silver per ton, 1.64 percent lead, and 10.35 percent zinc, or 16.9 percent copper equivalent. As of April 2007, NovaGold Resources Inc. is actively exploring the deposit (NovaGold Resources, Inc., 2007, Ambler). In 2005, NovaGold drilled about 3,000 meters of core hole and carried out district-scale surface geology, geochemistry, and geophysical surveys. In 2006, NovaGold drilled 12 holes to test geophysical anomalies near the Arctic deposit and outlined additional mineralization. They also carried out extensive detailed surface geologic mapping and geochemical surveying. NovaGold continues to cite the 1970's resource figures for the size of the deposit but a new resource assessment based on their recent work is underway and should be completed in 2007.
Workings: Kennecott completed 70 drill holes in the Arctic deposit and defined an inferred resource of 36.3 million tons of ore in the 1970s. NovaGold Resources, Inc. reached an earn-in agreement with Kennecott Exploration Company and Kennecott Arctic Company (subsidiaries of Rio Tinto PLC) in March 2004. The agreement covers a 35,000 acre area in the Ambler district. Initial exploration by NovaGold in 2004 focused on the Arctic deposit and included new descriptions of existing core, structural geology studies, and the drilling of 11 infill holes totaling 9,768 feet (2,978 meters). NovaGold's exploration in 2005 included district-scale surface geology, geochemistry, and geophysical surveys and up to about 3,000 meters (9,800 feet) of core drilling on the Arctic deposit. In 2006, NovaGold drilled 12 holes to test geophysical anomalies near the Arctic deposit and outlined additional mineralization. They also carried out extensive detailed surface geologic mapping and geochemical surveying.
Age: Devonian-Mississippian, the age of the host rocks.
Alteration: Chlorite-rich rocks in footwall and surrounding the sulfides form an alteration zone containing a complex assemblage of barium fluorphlogopite, talc, Mg-chlorite, barite, phengite, quartz, and calcite (Schmidt, 1988).
Reserves: The indicated reserves of 36.3 million tons grading 4.0 percent copper, 5.5 percent zinc, 1 percent lead, 1.6 ounces of silver per ton and 0.03 ounces of gold per ton, have long been documented based on the Kennecott work in the 1970's (Eakins and others, 1985, p. 6). The same 'inferred resources' are currently (April 2007) being reported by NovaGold Gold Resources, Inc. who are now drilling the prospect and carrying out geologic and geochemical studies to define additional resources. A new resource assessment based on their recent work is underway and should be completed in 2007.
Commodities (Major) - Ag, Au, Cu, Pb, Zn; (Minor) - Sb
Development Status: No
Deposit Model: Kuroko massive sulfide (Cox and Singer, 1986; model 28a)
13 entries listed. 13 valid minerals.
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Bottge, R.G., 1975, Impact of a natural gas pipeline on mineral and energy development in Alaska: U.S. Bureau of Mines Open-File Report 20-75, 177 p. Bundtzen, T.K., Swainbank, R.C., Clough, A.H., Henning, M.W., and Charlie, K.M., 1996, Alaska's mineral industry, 1995: Alaska Division of Geological and Geophysical Surveys Special Report 50, 72 p. Degenhart, C.E., Griffis, R.J., McQuat, J.F., and Bigelow, C.G., 1978, Mineral studies of the western Brooks Range performed under contract to the U.S. Bureau of Mines, Contract #JO155089: U.S. Bureau of Mines Open-File Report 103-78, 529 p., 11 sheets. Dillon, J.T., Pessel, G.H., Chen, J.H., and Veach, N.C., 1979, Tectonic and economic significance of late Devonian and late Proterozoic U-Pb zircon ages from Brooks Range, Alaska: in Short Notes on Alaskan Geology, 1978: Alaska Division of Geological and Geophysical Surveys Geologic Report 63, p. 36-41. Eakins, G.R., Bundtzen, T.K., Lueck, L.L. Green, C.B., Gallagher, J.L., and Robinson, M.S., 1985, Alaska mineral industry, 1984: Alaska Division of Geological and Geophysical Surveys Special Report 38, 57 p. Grybeck, D.J., and Nokleberg, W.J., 1979, Metallogeny of the Brooks Range, Alaska, in Johnson, K.M., and Williams, J.R., Jr., The United States Geological Survey in Alaska--Accomplishments during 1978: U.S. Geological Survey Circular 804-B, p. B19-B22. Hitzman, M.W., Profett, J.M., Jr., Schmidt, J.M., and Smith, T.E., 1986, Geology and mineralization of the Ambler district, northwestern Alaska: Economic Geology, v. 81, p. 1592-1618. Mayfield, C.F., and Grybeck, D., 1978, Mineral occurrences and resource map of the Ambler River quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-120-I, 1 sheet, scale 1:250,000. NovaGold Resources, Inc., 2007 (Ambler): http://www.novagold.net/s/Ambler.asp (as of March 2007). Schmidt, J.M., 1981, Volcanogenic massive sulfide deposition in a rifted continental margin--The Arctic Camp deposit, southwestern Brooks Range, Alaska [abs.]: Geological Society of America Annual Meeting, Program with Abstracts, v. 13, no. 7, p. 548. Schmidt, J.M., 1983, Geology and geochemistry of the Arctic prospect, Ambler district, Alaska: Stanford, Stanford University, Ph.D. dissertation, 253 p. Schmidt, J.M., 1986, Stratigraphic setting and mineralogy of the Arctic volcanogenic massive sulfide prospect, Ambler District, Alaska: Economic Geology and the Bulletin of the Society of Economic Geologists, vol.81, no.7, pp.1619-1643. Schmidt, J.M., 1988, Mineral and whole-rock compositions of seawater-dominated hydrothermal alteration at the Arctic volcanogenic massive sulfide prospect, Alaska: Economic Geology, v. 83, p. 822-842. Wiltse, M.A., 1975, Geology of the Arctic Camp prospect, Ambler River quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys, Open-File Report AOF60, 43 p., 1 sheet, scale 1:12,000.