The transported material accumulates in the valleys of rivers, in lakes or on sea beaches, and the resulting deposits are accord ingly classed as alluvial, lacustrine and marine gravels ("placers"). A large proportion of the world's gold has in the past been won from them. On account of its high specific gravity the metal is concentrated in the coarse gravels and among the boulders at the bottom of the placers, the most valuable accumulations being often actually on the bed-rock itself. If the latter happens to con sist of steeply-dipping schists or slates, the upturned edges of the latter act as natural riffles, or bars, which catch and retain the gold particles. Accumulation also takes place on what are known as "false bottoms," which are beds of clay or sand cemented by iron ("pan") and alternate with the beds of gravel.
Placer gold is usually associated with a heavy black sand con sisting of magnetite, ilmenite and haematite, together with chro mite, garnet, zircon, spinel and other heavy resistant minerals; but obviously the particular association is determined by the nature of the parent rock. The character of the gold is variable : it oc curs in flat scales and flakes, in rounded particles, and as irregu larly-shaped grains and nuggets bearing evidence of much attri tion. In size it varies from the finest dust to nuggets weighing thousands of ounces. It is probable that these large nuggets have increased in size since they were first deposited. The fact that auriferous pyrites are found replacing the woody fibre of tree stems in the alluvial drifts of Victoria and California shows that under favourable conditions an enrichment through chemical action (that is, solution and recrystallization) can take place. The richest gravels are formed by a re-sorting of earlier aurifer ous gravels, terraces of the latter being often situated several hundred feet above the workable deposits.
Placers occur in the river systems of every part of the world; but the greatest amount of gold has been won from the Recent and Pleistocene gravels of California, Alaska, Australia and Si beria. The older gravels are often deeply buried under a thick cover ("over-burden") of clay, soil, peat and moss, which is sometimes permanently frozen, as in the tundras of Siberia and Alaska; while in Australia and California the ancient river sys tems are concealed by later flows of lava, their auriferous gravels being then known as "deep leads."
Auriferous beach deposits or marine placers are formed on cer tain coasts where the conditions are favourable for the separa tion, by surf action, of the gold and heavy minerals from the sand and lighter stones. Such deposits occur in New Zealand, on the beaches of Oregon, Alaska (Nome and Cape Yagtag), Chile, and Nova Scotia. Usually the gold occurs in a black sand con sisting of magnetite, ilmenite and haematite ; but these mineral associations are not constant : for instance, while in New Zealand the black sands are so rich in iron that it has been seriously pro posed to work them for that metal, in the Cape Yagtag deposit the iron-ores are absent, their place being taken by garnet.
The auriferous conglomerates (locally termed "banket") of the Witwatersrand in the Transvaal, which have produced so large a proportion of the world's gold since their discovery in 1886, are considered by some to be ancient placers. Against this view, and in favour of a secondary origin for the gold, it might be urged that the latter shows no evidence of detrital origin, occurring, as it does, in crystalline particles and minute flakes in close associa tion with pyrites. Moreover, the rest of the cement consists of crystalline quartz which has evidently been introduced, like the pyrites, subsequently to the deposition of the gravels. The only unmistakably primary constituents, besides the quartz pebbles, are diamonds, chromite and zircon, while the following minerals of secondary origin occur in the cement : quartz, chlorite, chlori toid, pyrites, marcasite, pyrrhotite, galena and blende.
If it be conceded that the gold, derived from the denudation of quartz veins in the schists and granite of the Primitive System, was deposited simultaneously with the quartz pebbles of the grav els, it must, on the other hand, be admitted that the original allu vial gold has been completely dissolved and re-precipitated.
Platinum is occasionally associated with gold in the residual and alluvial gravels of California, British Columbia, Brazil, Colombia and Borneo.
In districts where granite prevails the residual and river gravels often contain cassiterite, and such stanniferous gravels form the source of the bulk of the tin production of the world. They occur in Cornwall, the Malay States, Australia, Tasmania and in South Africa (Swaziland and the Transvaal).