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ABSTRACT: Marine Placer Deposits and Applicable Mining Technology

J. Robert Woolsey

Marine placers generally consist of deposits of heavy minerals of potential economic value. Commodities of interest are typically Au, Pt, Sn, Ti, Cr, and certain rare earths. Habitats of occurrence commonly relate to drowned strand lines and channels. Direct projections of relationships and characteristics to onshore placers, proximate or otherwise, may be obscure, largely because of differential post-depositional processes, i.e., subaerial vs. marine.

Appropriate placer mining technology must relate to critical geologic characteristics of both overburden and ore sections, the full understanding of which requires a comprehensive geological study. Factors such as mineralogy, sorting, and size distribution of the full sediment range, from clays on the one extreme to boulders on the other, provide critical information for broad determination of conceptual systems and detailed engineering.

Environmental factors are equally critical in systems selection and engineering. To date, no placer mining system of site-specific design has been put into operation in the higher latitude seas. The conventional bucket-ladder dredge of onshore tin mining fame in Southeast Asia was readily adapted for offshore operations in adjacent calm tropic seas, but the design is inappropriate in consideration of safety and

operating efficiency in the rigorous working conditions of the high latitudes.

New concepts are under study by the Marine Minerals Technology Center directed at increased efficiency and a wider latitude of marine mining operations. One concept is a hybrid suction system fitted with a continuous moving grizzly ladder-chain for screening and removing oversize from the intake and cutting face and dual side cutter-wheels for bi-directional lateral cutting. The concept provides a workable mining system for site-specific designs on low-profile surface-mounted vessels or under-sea/ice bottom walkers/crawlers capable of working under sea conditions presently considered extreme. The concept also provides a workable low-cost system for retro-fitting conventional suction dredges for certain mining situations in underdeveloped countries with particular economic priorities a d constraints.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990