ELUVIAL-ALLUVIAL

Placer Deposits

The literature on placers is voluminous. Works containing discussions of general principles and descriptions of known placer districts include those by Liversidge (1893), Weatherbe (1907), McConnell (1907), Crane (1908), Tyrrell (1912), Longridge (1913), MacKay (1921), Jonston and Uglow (1926), Raeburn and Milner (1927), Park (1927), Idriess (1933), Boericke (1933), Lindgren (1933), Gardner and Johnson (1934-35), Fisher (1935), Crampton (1937), Bilibin (1938), Mertie (1940), Averill (1946), Grutterink (1950), Bateman (1950), Shilo (1956, 1960), Gorbunov (1959), Griffith (1960), Harrison (1962), Barkovskaya (1963), Trofimov (1964), Fayzullin (1968), Ivensov et al. (1969), Romanovitz et al. (1970), West (1971), Wells (1973) and Fricker (1976) contain up-to-date deposit descriptions and summaries of the procedures for prospecting, sampling, evaluation, and working of placers. It need hardly be emphasized here that the processes involved in the concentration of gold or diamonds in placers are extremely complex, and the conditions that control them so variable that even under the most favorable circumstances observable, it is generally not possible to estimate the potential of a placer even roughly without detailed sampling, either by pitting, test holes, or deep cuts or trenches.

I thank very much in advance all past and present authors, prospectors, miners and scholars that provided the information that I use or quote in the present essay on eluvial and alluvial deposits.

Placer deposits provided early man with the first samples of gold and since that time have accounted for a large production of the metal. If we include the Witwatersrand and other quartz-pebble conglomerates as fossil placers or modified fossil placers, then the placer type of deposit has provided more than two-thirds of man's store of gold.

Before proceeding further certain terms with respect to placers should be defined.

The term 'placer' is evidently of Spanish derivation and was used by the early Spanish miners in both North and South America as a name for gold deposits found in the sands and gravels of streams. Originally, it seems to have meant 'sand bank' or 'a place in a stream where gold was deposited'. While many other terms have been coined for deposits in weathered residuum and alluvium none is quite as succinct and expressive as 'placer'.

The terminology of the zone or stratum containing an economic concentration of gold in eluvial and alluvial placers is varied. We shall use the miner's term 'pay streak', which is commonly used in Canada and the United States. Other English terms in use include 'pay gravel', 'pay sand', 'pay dirt', 'pay wash', 'pay channel', 'pay lead', 'run of gold', 'gutter' and 'wash dirt'.

The tenor of pay streaks or of placer gold gravels and sands, in general, is referred to by the value (in ounces, grams, pennyweights, or in any unit of currency) per cubic yard or meter, per running length (foot or meter) of channel, per surface unite of cross-section, or per unit of surface (square foot or meter); also occasionally in bonanzas by dollars or some other unit of currency per pan. Note that placer deposits can be worked whose gold content is as low as 0.1 ppm.

The pay streaks of placer deposits may rest on or near bedrock or on some stratum above bedrock. The bedrock in placer deposits is commonly referred to as the 'true bottom', although the term is little used today. When the streaks rest on a well-defined stratum of sand, gravel, or clay above the bedrock they are said to be on a 'false bottom'.

Placers have been variously categorized, but here we shall use a simple nomenclature based upon whether the placers are formed by concentration of gold in situ over or in the immediate vicinity of primary deposits, namely 'residual' or 'eluvial placers', or by agencies that have concentrated the gold in the near vicinity or at some distance from the primary source. In the latter category we recognize 'alluvial', 'beach' and 'aeolian placers'. The terms 'saprolite' or 'saprolitic placer' were formerly used for certain types of eluvial placers, mainly in the eastern United States.

Eluvial, alluvial, beach and aeolian placers may become buried after their formation and are sometimes referred to as 'buried placers'.

These placers may be buried under:

(1) volcanic deposits as in California and Australia;

(2) glacial deposits as in Canada and Russia;

(3) talus and other slope deposits;

(4) aeolian deposits as in Australia;

(5) alluvial sands and gravels;

(6) marine and lacustrine deposits.

[ Placer Deposits 1 2 3 Eluvial 5 6 Alluvial 8 9 10 Examples 12 13 14 15 ]

Maps of alluvial gold deposits in: California, Western Canada, Eastern Canada, Russia, World
Maps of primary gold deposits in: Precambrian, Paleozoic, Mesozoic, Cenozoic Rocks
Diamonds: Large and Famous Properties Geology and Mining Diamond Cutting

Rafal Swiecki, geological engineer email contact
February, 2006

This document is in the public domain.