ECONOMICALLY IMPORTANT GEOLOGICAL CHARACTERISTICS OF MARINE PLACERS

Location and mineral content are the main economic criteria by which existing marine placers are assessed. Situation involves water depth, sea and weather conditions, and mining vessel accessibility. Content is expressed in terms of grade and volume (or area). A placer's size reflects that of the primary or intermediate source(s), whether terrestial or submarine, its proximity, coastal topography, and the mineral(s) involved.

The profitability of mineral extraction is determined by a placer's geological characteristics. Continuity of both grade and physical form, the thickness of any overburden, and the deposit's outline in plan and section contribute to the conclusion. Additional features are the vertical grade profile, the lithology and uniformity, or otherwise, of the bedrock surface, the average, and range of, particle size of the mineral and its disaggregated host sediments, accessory minerals, and the geotechnical particulars of each component sediment requiring excavation.

These attributes result from a placer's manner of formation which must be considered, not in submerged isolation but as a whole, from the mineral's source to its destination - from primary to secondary mineralization sites under specific climatic conditions. Each mineral has been variably subjected to a permutation of processes: liberation, transport to, within and from, the ocean, dispersal, disintegration and sorting, deposition and precipitation, concentration, and preservation. Multiple changes in relative sea level, in places combined with coastal faulting, have initiated repetition of each process, accentuated the effects, and modified the earlier ones.

Each mineral has behaved differently, depending on its effective specific gravity and resistance to wear, under tropical to arctic conditions, from the Cretaceous to the Quaternary. Examples of marine placers explored and mined for diamonds, industrial minerals, cassiterite, and precious metals illustrate contrasting formational histories with demonstrated determining influence on their important geological characteristics.