Native elements

Abstract

Native elements that can occur as important placer deposits include gold, platinum and diamond. Placers are metallic mineral deposits that have been formed by mechanical concentration. Most placers occur within a few kilometres of their source, thus most marine placers are restricted to the nearshore zone. Important gold and platinum placers occur off the coast of Alaska (Nelson & Hopkins, 1972; Cronan, 1980) and diamond placers have been located and mined off the coast of south-west Africa (Prentice, quoted in Cronan, 1980). Gold-bearing placers have also been reported off the coast of New South Wales, Australia (Jones & Davies, 1979). Gold usually occurs as rounded flattened grains which are golden yellow in reflected light and have a metallic lustre. Platinum most commonly occurs as worn, rough-textured flattened polygonal plates or irregular grains which are white to aluminium grey in reflected light (Milner, 1962). Both are, of course, opaque in transmitted light. Diamond occurs as clear, octahedral, often slightly rounded grains with very high relief. Grains often contain inclusions and are isotropic. It is possible that diamonds may be found long distances from their primary source due to their toughness and extreme hardness (Emery & Noakes, 1968).