ABSTRACT
The gold first mined in 1900 on the present-day beach of Nome, Alaska, owed its existence to a combination of factors. Nearby, primary gold deposits had been eroded by glaciation, and the products were redeposited as terminal and side moraines. The glacial debris and contained, particulate gold were subjected to repeated faulting along a climatically exposed shoreline, with changing sea levels. Gold collected on submerged, buried paleobeaches and abrasion platforms cut into, and around, the moraines above bedrock. It was also concentrated in more widespread, thin, lag gravels. Glacial lithologies, mostly till, with locally high gold grades, now extend on the seabed for nearly 5 km offshore in water depths of 20 m and less.
Since its discovery, several imaginative, but short-lived attempts have succeeded in recovering a little gold from the seabed deposit. In 1986 a U.S. company, WestGold, imported a 33-ft3 bucket-ladder dredge, the world’s largest, to mine reserves estimated from earlier drilling by other companies. The Bima had previously been operated as a cassiterite producer in very different, Indonesian waters for which it had been designed.
In Alaska, the sub-Arctic climate restricted the mining season to six months, and the operating conditions were extremely difficult for a conventional floating dredge. Very hard digging, combined with a long swell, restricted the excavation rate to less than half that anticipated. Environmental constraints and permitting procedures affected the project. High unit costs and the inability to extract ore selectively added to the problems. A decreasing gold price required a higher cutoff grade, which seriously fragmented the reserves. Time and financial constraints prevented additional drillhole sampling at the necessary density, and serious mining dilution was unavoidable. Recovery averaged 106% of that expected from reserve estimation, but varied widely. Maintenance of a complete metal balance from seabed to final production was essential for performance monitoring and correction, reserve reconciliation, and production planning.
In 1989 full-scale, on-site trials demonstrated the economic applicability of a track-mounted, highly selective mining system deployed on the seabed from an anchored barge, the beach, or sea-ice. With no dilution, very high recovered grades were attainable. Only the addition of a bucket-wheel was required to achieve the necessary digging rate.
The operation was terminated in 1990 and the leases surrendered. From 1987 to 1990 inclusive, the Bima’s production totaled 118,078 fine oz. (3,673 kg), with an average recovered grade of 824 mg/m3. There has been no further offshore mining activity at Nome. Exploration for marine, placer gold off the coasts of other countries has not succeeded in identifying any economic deposits.
