200 • Marine Minerals: Exploring Our New Ocean Frontier 



1906, about 4.5 million ounces of alluvial gold had 

 been mined from a 55-square-mile area. Early 

 miners recognized that the Nome gold placers were 

 formed by wave action and that additional depos- 

 its, formed when sea levels were lower, should be 

 found in the adjacent offshore area (figure 5-20). 



Two U.S. companies, ASARCO and Shell Oil 

 Co., sampled offshore deposits near Nome in 1964 

 and recovered alluvial gold. By 1969, proven re- 

 serves offshore of approximately 100 million cubic 

 yards of ore had been established. The rights to 

 these reserves were acquired in 1985 by Inspira- 

 tion Resources, which then began a pilot mining 

 and testing program. This program was followed 

 by mining tests with a bucket ladder dredge in 1986. 

 All operations to date have taken place within 3 

 miles of shore in waters under the jurisdiction of 

 the State of Alaska, although gold resources have 

 been identified out to about 10 miles. The future 

 offshore gold mining operation is examined in this 

 scenario, based on a number of assumptions. 



Operational and Geological Characteristics. — 



Nome is a small town near the Arctic Circle on Nor- 

 ton Sound, a large shallow bay open to the west. 

 Water depths in the bay do not exceed 100 feet. 

 Ten miles offshore water is only 60 feet deep. Gold- 

 bearing sediments are a maximum of 30 feet thick 

 and consist of bedded sands, gravels, and clays 

 alternating with occasional beds of cobbles and 

 boulders. These sediments were sampled from the 

 ice out to about 1 ^ miles from the coast. Gold has 

 been found further offshore, but reserves have not 

 yet been fully delineated. Current mining sites are 

 located less than 1 J4 miles offshore in water depths 

 averaging 30 feet and in formations 6 to 30 feet 

 thick. 



Only between June and October is Norton 

 Sound ice-free and accessible to floating vessels. 

 During the winter, thick pack ice forms over the 

 Sound. Waves reportedly do not exceed periods of 

 7 seconds, but occasional sea-swells with longer 

 periods may come from the west or southwest. Pre- 

 dominant winds are from the north and northeast. 

 Currents and longshore drift are westward. Maxi- 

 mum tides are 6 feet. 



Mining Technology. — The Bima, a bucket lad- 

 der dredge built in 1979 for mining tin offshore In- 

 donesia, was selected to mine the offshore gold 



placers. The Bima was brought to Nome in July 

 1986 for preliminary tests. It was modified in Seatde 

 and is scheduled to begin operation in July 1987. 

 The Bima was designed and built abroad as a sea- 

 going mining vessel. Its hull is 361 feet long, 98 

 feet wide, and 21 feet deep. The entire vessel is of 

 steel construction and weighs about 15,000 short 

 tons, including the dredging ladder and machin- 

 ery. Freeboard is 10 feet and draft 15 feet with the 

 ladder retracted. 



The Bima is not self-propelled. It must be moved 

 to and from the mining site by a tugboat. On site, 

 the dredge is kept in position by five mooring lines 

 attached to 7-ton Danforth anchors. This anchor- 

 ing arrangement allows the dredge to swing 600 feet 

 from side to side and to advance while digging. The 

 anchors are positioned and moved by a special aux- 

 iliary vessel. 



A 15,000-horsepower diesel-electric powerplant 

 is used to operate the bucketline, the ore process- 

 ing plant, the anchor winches, and the auxiliary 

 systems. There is fuel storage on board for 2 J'^ 

 months of operation. 



The Bima's dredge ladder and bucketline were 

 originally designed to operate in 150 feet of water. 

 This scenario assumes that the dredge ladder has 

 been shortened, so that the dredge is able to mine 

 from 25 to 100 feet below the water line at the rate 

 of 33 cubic yards per minute or approximately 

 2,000 cubic yards per hour. The Bima was designed 

 to enable the mass of the ladder and bucketline to 

 be decoupled from the motions of the hull by an 

 automated system of hydraulic and air cylinders 

 that act like very large springs. This feature keeps 

 the buckets digging against the dredging face on 

 the seabed while the hull may be heaving or pitch- 

 ing due to the motions of passing waves. During 

 the trials of the Bima in Norton Sound from July 

 to October 1986, it was not necessary to activate 

 the system. 



At-Sea Processing. — The Bima is equipped with 

 a gravity processing plant to make a gold concen- 

 trate at the mining site. The throughput capacity 

 of the plant is 2,000 cubic yards per hour. The plant 

 consists of two parallel inclined rotary trommels 18 

 feet in diameter and 60 feet long. After removal 

 of any large 'boulders, ore brought up by the dredge 

 bucket slides down the trommels under the spray 



