Offshore Occurrences of Heavy-Mineral Placers, Northwest Galicia, Spain

Abstract

ABSTRACT Results of offshore water-jet sediment sampling demonstrate the potential for interstadial placer deposits of industrial minerals in the shore-face to inner-shelf of Cape Ortegal, NW Spain. Placer deposition is interpreted to have occurred during glacial interstadials since the Last Glacial Maximum, and deposits sampled at 24 to 42 m. BSL were formed during the Younger Dryas, and subsequently preserved as a result of very rapid rising sea-levels. The sediments are almost entirely terrestrially derived with a particularly high heavy mineral content (Sp. Gr. >2.9), of more than 75% in all samples. Reworked garnet from earlier-sedimentary deposits constitutes an important fraction of the placer deposits identified but is generally absent in other sediments. Indications of the presence of +5 m-thick paleo-placer deposits with sections include: Placer garnet + spinel grades increase with depth in those sediments at 24 to 42 m BSL to the ocean-floor. The d50 of sediments with background grades decrease with depth while those of placer deposits do not display any significant trend. Garnets in non-mineralized sediments are almost entirely angular, whereas in placer enriched sediments they vary in form from well-rounded to angular. Some grains in the latter display surface morphologies typical of reworked sediment while none of the former do. Sediments deposited during glacial interstadials at depths of up to 130 m BSL may represent important, undeveloped exploration targets for offshore placer deposits of industrial minerals. Industrial mineral exploration should extend the depths of investigations to those corresponding to major interstadial periods.