Nome Offshore Placer deposits

Site Name

Primary: Nome Offshore Placer deposits

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: Alaska
District: Nome

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Land Status

Not available

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Producer
Operation Type: Unknown
Years of Production:
Organization:
Significant:

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Physiography

Not available

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Mineral Deposit Model

Not available

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Orebody

Not available

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Structure

Not available

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Alterations

Not available

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Rocks

Name: Sand and Gravel
Role: Host
Age Type: Host Rock Unit
Age Young: Pleistocene

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Analytical Data

Not available

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Materials

Ore: Arsenopyrite
Ore: Gold
Ore: Ilmenite
Ore: Magnetite
Ore: Pyrite
Gangue: Garnet

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Comments

Comment (Geology): Age = Pliocene and Pleistocene.

Comment (Exploration): Status = Active

Comment (Reserve-Resource): Reserves = A gold resource for this region was calculated by Cecilia A. Bronston (1992; unpublished data, 1993) using a cut-off grade of 300 milligrams of gold per cubic meter. The total identified proven and probable ounces in nine deposits was 585,016 ounces at an average grade of 1,020 milligrams per cubic meter. The best single deposit was identified as Humpy. It has a proven and probable gold resource of 148,239 ounces and a grade of 1,327 milligrams per cubic meter. A total gold resource for the area could approximate 1,000,000 ounces.

Comment (Reference): Primary Reference = Rusanowski, 1991

Comment (Workings): Workings / Exploration = Some attempts were made to exploit offshore gold in the early years of the district, but they were unsuccessful largely because the machinery was poorly conceived or could not withstand the physical conditions. The first serious work began in the 1960's, when Shell Oil Company obtained leases on 2,750 acres offshore and drilled 568 holes (Daly, 1969). In 1967, the U.S. Bureau of Mines R/V Virginia City drilled more than 60 holes using a Becker-type rig. In that year and earlier, bottom samples were collected by the U.S. Geological Survey from several reseach vessels, using a clam-shell type sampler (Nelson and Hopkins, 1972). Shell's leases were acquired by ASARCO, who drilled 500 more holes (Wojcik, 1974). A prospect was identified by ASARCO, but it was subeconomic at the low early 1970's gold prices. The ASARCO leases were ultimately transferred through Inspiration Mining to Western Gold Mining and Exploration Ltd. (WestGold). In 1985, WestGold recovered about 885 ounces of gold by means of a clam-shell bucket mounted on sea-going barge. In 1986, WestGold acquired and rehabilitated the large bucket-line dredge (the BIMA; Rusanowski, 1989 and 1991). WestGold also did extensive environmental work (see, for example, Demlow and others,1989) and continued making improvements in drilling techniques (Bronston, 1989). Significant work on sampling of low-grade offshore materials was done by Clifton and others (1967, 1969).

Comment (Production): Production Notes = WestGold produced a significant amount of gold in its more or less experimental attempt at mining offshore Nome. In addition to their test mining in 1985, WestGold produced about 36,709 ounces of gold in 1987, 35,554 ounces in 1988, 30,882 ounces in 1989, and 15,208 ounces in 1990 (Rusanowski, 1989, 1991).

Comment (Deposit): Model Name = Placer; offshore, relict gravel, submerged beaches.

Comment (Deposit): Other Comments = High-resolution seismic surveys initiated by the U.S. Geological Survey (Tagg and Greene, 1973) and further refined by industry (Graul and others, 1989) were effective in mapping glacial deposits, possible covered beach deposits, and channels.

Comment (Geology): Geologic Description = Gold occurs in offshore placer deposits off the coast of Nome, particularly from a point west of the mouth of the Nome River to near Penny River. From about 1960 to 1991, and especially from 1987 to 1990, there has been significant exploration and gold production. From mean lower low tide, the auriferous deposits extend seaward for about 2 miles. The origin of the deposits is complex and considered to be intimately related to the glacial history of the area (Hopkins and others, 1960). There are potential complications with this interpretation, but these are beyond the scope of this summary.? Some of the offshore gold is derived from the abrasion platform immediately offshore of the modern beach. Gold also occurs in lag deposits formed from weakly auriferous glacial drift and outwash far from the modern beach deposits. The glacial deposits are roughly centered on the projection of Snake River, before it makes an abrubt easterly turn to its present mouth. The offshore marine glacial deposits have been mapped by high-resolution seismic studies, first by Tagg and Greene (1973), then, using refined instruments and techniques, by Graul and others (1989). The gold in the auriferous glacial deposits was at least partly derived from gold-bearing alluvium in the ancestral Snake River. One interpretation is that productive placer deposits at Bangor Creek, Monument Creek, Boulder Creek, Glacier Creek, and Rock Creek were effectively cut off at the flood plain of Snake River by a glacier that occupied the main valley of the ancestral river. Nelson and Hopkins (1972) reported that average offshore glacial till derived from this and possibly other glacial events contained 70 ppb of gold and that glacial outwash contained more. They also found that lag gravel on top of the till contained as much as 2,500 ppb gold and that one-third of their samples of materials enriched by lag processes contained more than 600 ppb gold. The gold in the relict concentrations was as much as several mm in diameter, and averaged about 1 millimeter in diameter and 1 milligram in weight. Probably most of the rich relict gravel is only about 30 centimeters (1 foot) thick.? Investigations carried out by the U.S. Geological Survey and the U.S. Bureau of Mines in the mid- to late 1960's followed extensive exploratio by a minerals division of Shell Oil Company (Daly, 1969). Shell's work was followed by extensive exploration by ASARCO (Wojcik, 1974). Most of the Shell and ASARCO programs were done during the winter, off the seasonal ice shelf at Nome. Later work conducted by WestGold used Becker-type drills that drilled from vessels or off winter ice. Ultimately, more than 3,500 holes were drilled by Shell, ASARCO, and WestGold (Bronston, 1989).? the offshore gold deposits at Nome were mined by WestGold from 1987 to 1990, using a large bucket-line dredge. About 100,000 ounces of gold were recovered. The large dredge (the BIMA) successfully operated in difficult weather, but it was not an effective miner. The dredge diluted rich surface material with lower grade till, it could not mine rich near-shore accumulations of gold, and its bucket line catenary did not allow for the most efficient loading of buckets. Nevertheless, it was an impressive experiment. WestGold also experimented with an undersea miner (Bosse, 1990). There remains a significant offshore resource of gold at Nome that could be mined under suitable economic conditions.

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References

Reference (Deposit): Bosse, P.J., 1990, Development of innovative underwater mining techniques: Western Gold Exploration and Mining, Ltd., presented at Underwater Mining Conference, Vancouver, B.C., Canada.

Reference (Deposit): Bronston, C.A., 1992, Ore reserve calculation procedures: Nova Natural Resources, unpublished report.

Reference (Deposit): Bronston, M.A., 1989, Offshore placer drilling technology--A case study from Nome, Alaska: Mining Engineering, v. 42, no. 1, p. 26-31.

Reference (Deposit): Graul, M., Bronston, M.A., and Williams, C., 1989, High-resolution seismic exploration for gold: Offshore Technology Conference, 21st , Houston, Texas, Paper OTC-5941, p. 579-592.

Reference (Deposit): Metcalfe, J.B., and Tuck, Ralph, 1942, Placer gold deposits of the Nome district, Alaska: Report for U.S. Smelting, Refining, and Mining Co., 175 p.

Reference (Deposit): Hopkins, D.M., MacNeil, F.S. and Leopold, E.B., 1960, The coastal plain at Nome, Alaska, A late Cenozoic type section for the Bering Sea region, in Chronology and climatology of the Quaternary: International Geological Congress, 21st, Copenhagen , Proceedings, Part 4, p. 46-57.

Reference (Deposit): U.S. Bureau of Mines, 1967, Sample drilling; seafloor heavy metals placer deposit of Alaska's Nome Beach, The Bureau of Mines 1967 offshore campaign: U.S. Bureau of Mines unpublished report.

Reference (Deposit): Clifton, H.E., Hubert, Arthur, and Phillips, R.L., 1967, Marine sediment sample preparation for analysis for low concentrations of fine gold: U.S. Geological Survey Circular 545, 11 p.

Reference (Deposit): Clifton, H.E., Hunter, R.E., Swanson, F.J., and Phillips, R.L., 1969, Sample size and meaningful gold analysis: U.S. Geological Survey Professional Paper 625-C, 17 p.

Reference (Deposit): Daly, A.F., 1969, Off-the-ice placer prospecting for gold: Offshore Technology Conference, Paper 1029, OTC Transactions, p. 277-284.

Reference (Deposit): Nelson, C.H., and Hopkins, D.M., 1972, Sedimentary processes and distribution of particulate gold in the northern Bering Sea: U.S. Geological Survey Professional Paper 689, 27 p., 1 plate.

Reference (Deposit): Tagg, A.R., and Greene, H.G., 1973, High resolution seismic survey of an offshore area near Nome, Alaska: U.S. Geological Survey Professional Paper 795-A, p. A1-A23.

Reference (Deposit): Wojcik, J.R., 1974, Summary of the Nome project: ASARCO technical report, 85 p.

Reference (Deposit): Rusanowski, P.C., 1989, Nome offshore placer project - Issues and answers - A three year perspective , in Placer mining in today's world: Proceedings of the 11st annnual conference on placer mining, Fairbanks, Alaska, March 29-April 2, 1989, p. 4-10.

Reference (Deposit): Rusanowski, P.C., 1991, Nome offshore placer project -- A model for resource extraction projects in Alaska, in Alluvial Mining, Proceedings of conference on Alluvial Mining, London, UK, November 11-13, 1991: Institution of Mining and Metallurgy, Elsevier Applied Science for the Institute for Mining and Metallurgy, p. 587-601.

Reference (Deposit): Demlow, T.C., Bosse, P.J., and Rusanowski, P.C., 1989, Bucketline dredge disposal system turbdity modelling, in Magoon, O.T., Converse, Hugh, Miner, Dallas, and others, eds., Proceedings of the sixth Symposium on Coastal and Ocean Management: American Society of Civil Engineers, v. 3, p. 2955-2966.

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