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ELUVIAL-ALLUVIAL

Placer Deposits

Some of the largest natural specimens of the metal have come from placers, frequently of the eluvial type. From these have come some of the largest nuggets of gold won by man. The 'Welcome Stranger' found by accident in a cart rut in the eluvium near Ballarat, Victoria, Australia weighed 2516 oz troy and the 'Blanch Barkley' also from Victoria weighed 1743 oz.

The Carson Hill nugget found in California topped the scales at 1296 oz. More generally the gold of placers occurs in small-flattened scales or grains averaging a few millimeters in diameter or as moderately finely divided particles known as dust averaging a few tenths of a millimeter in diameter. The fineness of placer gold varies greatly and has been studied by many investigators. A survey of the literature indicates that for most placer deposits the fineness ranges from 500 to 999, that is from electrum to nearly pure metal; most of placer gold is above 850 fine. The other elements in placer gold are mainly silver, copper and iron. Vein gold generally has a fineness that ranges from about 500 to 900. There are innumerable references in the literature that refer to the fact that in any particular district the fineness of the placer gold will usually be higher than that in the veins from which the metal presumably came.

In California the vein gold averages 850 fine whereas the transported placer gold in the Tertiary channels averages 930 to 950. Similarly, the placer gold of Manhattan, Nevada is finer than the lode gold. In the Carolinas and Georgia the placers and eluvial deposits commonly have gold with a fineness greater than 900 whereas the vein gold is generally much less than this value, in places being as low as 500. Distance traveled and size of the placer gold also seem to be factors in the fineness, the further from the source and the smaller the size of the particles the higher the fineness. Certainly, flour gold generally has a high fineness. An average of about 950 as the fineness of the Snake River flour gold. There is much evidence to suggest that most of the gold in gossans and oxidized zones is of greater fineness than that in the primary ores. A worldwide study of the fineness of gold in various environments, concluded that gold in the oxidized zone is nearly always higher in grade than the primary gold, and this difference varies largely according to chemical conditions and the facilities that exist for removal of the silver in solution, the gold being redeposited close by, or merely left enriched by the removal of the silver. Under conditions suitable for taking gold into solution the gold redeposited, if the concentration of silver remains high, will not necessarily be of high fineness. A very high gold fineness is usually the result of oxidation under conditions favorable for the complete removal of silver, e.g., Mt. Morgan, Queensland, and that high fineness is often associated with the oxidation of telluride ores, e.g., Fiji and Cripple Creek, Colorado. Finally, it should be noted that alluvial gold, being derived from the oxidized ore, is of higher quality (fineness) than the average of the vein gold, and shows an increase in value downstream, due to surface refining action, as the size of the grains decreases.

In Yukon Territory and at Yellowknife, Northwest Territories some of the supergene gold is of great fineness (990), whereas the primary gold is in the range 900 to 950. Another feature of placer gold, in the Klondike, is that the outer parts of nuggets commonly have a higher fineness compared with the inner parts. The rim of greater fineness is rarely more than 0.03 mm (30 microns). A rim of relatively pure gold on nuggets seems to be rather general.
The same is true in the gold of the Cariboo of British Columbia; in the Morobe goldfield, New Guinea; in much of the placer gold of Russia, in Rheingold; in much of the placer gold of Yukon; and in placer gold from various parts of the United States. However, the rind of purer gold on nuggets, flakes and dust is not always present. The rim effect has generally been interpreted as the result of solution of silver, but it also may represent the precipitation of gold (and some silver) on gold nuclei. An argument in favor of the latter process is given in a later section on the origin of placers.

It should be noted that the rim effect in placer gold evidently begins during oxidation of the primary deposits, because it is found in gold in the oxidized zones of auriferous deposits and in eluvial deposits. Gold with enriched rims has been observed by a number of investigators in near-surface shallow deposits (mainly Tertiary) in a number of places, and the phenomenon is noticeable in some kuroko ores in Japan. Enriched rims are rare in deep-seated gold deposits. In the oxidized ores and placer deposits the rims are generally much wider and better developed than in the primary deposits.

Placer gold commonly contains many inclusions usually small in size (0.0054 - 0.50 mm). The inclusions as seen in polished sections and under the microprobe include various sulphides, particularly pyrite, galena and chalcopyrite; also arsenopyrite, various sulphosalts, tellurides and quartz, sericite, rutile, etc., most of the minerals normally found in gold deposits. Many of these minerals are probably original constituents of the gold nuggets since they are commonly near the core of the gold particles. Some often appear as though they were the nuclei about which gold precipitated in the oxidized zones of the deposits. These inclusions should receive detailed study during geochemical prospecting surveys since one might be able to divine the exact source of the gold, especially if material from a few primary sources in the district under investigation is available for control purposes.

Other internal features of placer gold have been extensively studied. A long term study of the placer gold of Russia, also note the nearly universal occurrence of high grade rims on gold nuggets and flakes and founds that these have a fine-grained polyhedral texture rather than a laminated one as thought by the earlier investigators. They also noted that much of the placer gold retains the internal granular structure and other microscopic features of the primary deposits of gold, but found that much placer gold, especially that held in placers for long periods of time, exhibits marked internal deformation and recrystallization textures. They also observed the presence of intergranular stringers of gold in nuggets and flakes. These were found in the placer gold and in the gold from oxidized zones of auriferous deposits but not in the gold from the primary ores.

The external form color, luster and other features of placer gold visible to the naked eye are commonly characteristic. The miners and bankers in a placer district can generally tell from which creek the gold originated simply by visual inspection.

Placer gold usually has an entirely different appearance from that found in veins and other deposits. A subdued luster in the placer replaces the high luster of the vein gold; due, it appears, to incipient crystallization on the surface. Some varieties of gold are colored due to a number of reasons.
Black and deep brown gold are commonly due to the presence of coatings of manganese and iron oxides or manganese and iron humates. Alternatively some black and brownish gold is due to a coating of very finely divided colloidal or microcrystalline gold. Whitish and dull grayish varieties of gold owe their color generally to thin coatings of calcium carbonate, colloidal silica or fine-grained (colloidal) clay. In most placers the gold has the characteristic golden color, but in others foreign coatings are quite common. 'Black gold' (ouro preto) is very characteristic of a number of placer districts in Brazil and other tropical areas.

Practically every form of gold exists in placers and the closely associated oxidized zones of gold deposits. The most common forms are scales, plates and nuggets. In addition some placers contain crystals of gold, hopper-shaped crystallized particles, masses having filiform, reticulated and dendritic shapes and films, wire and mossy gold. The growth forms of large nuggets are commonly fairly regular, many looking like nuts or potatoes; other nuggets are highly irregular and gnarled in shape. Some partly enclose vein quartz fragments or rounded quartz and other types of pebbles and mineral fragments.
Some nuggets have unusual growth proturberances that may appear as distorted crystals. A few nuggets have been found that have secondary crystalline outlines.

Various terms are used in the following descriptions to give a semi-quantitative estimation of the size of the gold particles in placers. A 'color' has no exact meaning; it is used by placer miners to refer to a small piece of gold that usually varies from l.5mm upward. Estimation of the value of gold colors in a pan and hence of the value of placer grounds can only be gained by experience since the value varies with the size of the colors, their thickness, and their purity. A good estimate can be made by picking out the colors from a number of pans, weighting them on a small pocket microbalance, and calculating the value per yard.

[ 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


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This document is in the public domain.

March, 2011