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Alluvial Deposits

The simplified classification of placers adopted here follows essentially the scheme suggested by Lindgren (1933) and others based mainly on the geological location and the tectonic history (Table 3).

The formation of alluvial placers depends upon many interacting physical and chemical factors, our general knowledge of which is fairly well established. However, our understanding of many of the chemical and physical details of the accumulation of gold in alluvial placers is still remarkably poor, especially with respect to the formation of nuggets. There are also many differences of opinion as to why gold is concentrated on or near bedrock in alluvial placers.

The physical and chemical factors involved in the formation of alluvial gold placers are:
(1) adequate source of primary gold,
(2) favorable oxidation processes for its release,
(3) favorable concentration sites, all of which depend essentially on the tectonic environment, which controls the dynamics of streams and rivers.

1. Nature of the oxidation processes in the primary gold-bearing deposits: several factors are involved, including climatic and groundwater conditions and the nature of the primary gold, gangue and wall rocks. The nature of the primary gold and the chemical system operating during oxidation of the deposits are two critical factors in the formation of alluvial placers.

The size of the primary gold particles is a major consideration. Where the particles of native gold in a primary deposit exceed 100 µ there appears to be relatively little solution or flotation of the gold in the natural waters (physical transport0. During oxidation processes the particles tend to go directly into the eluvium and creep from there into the alluvial placers. They may, however, accrete gold from the soil solutions and stream waters as they proceed on their journey.

In some districts, however, where the primary gold is extremely finely divided (<50 µ) or is a microscopic or lattice constituent of auriferous pyrite, arsenopyrite, tellurides, etc., alluvial placers are not formed to any extent even though other conditions are favorable. One striking example of this is the paucity of alluvial placers associated with the great Witwatersrand deposits. In such districts the gold does not seem to aggregate to form large particles. On the contrary it appears to be dispersed as flour gold or is dissolved and dispersed throughout the ground and surface water systems. In other districts the reverse situation exists, and alluvial deposits abound even though the gold is finely divided or a constituent of pyrite, etc. in the primary deposits. In these the gold accretes mainly in the oxidized zones and eluvial deposits to form relatively large particles, which ultimately find their way into the alluvial placers. To explain the difference in behavior between the two extremes is not simple because of the complex chemical systems involved.

One important factor appears to be the purity of the finely divided primary gold. If its silver and/or copper content is high the gold appears to be more readily dissolved. This is understandable from etching procedures in microscopic work on gold - pure gold etches with great difficulty whereas alloyed gold is readily etched. Once dissolved the gold may be dispersed or reprecipitated (accreted) depending on the chemical system, especially on the Eh and pH of the environment and the presence of various colloids and precipitants. The relatively pure, finely divided gold, on the other hand, enters the erosion cycle chemically unaffected and is mechanically dispersed far and wide. Finely divided gold contained within pyrite and arsenopyrite and lattice gold in these minerals and in tellurides, aurostibite, etc. tends to accrete and form large particles, wires, etc. for reasons that are obscure but which may involve the solvent effects of thiosulphates, cyanides and ferric sulphate, interaction of the dissolved gold with natural reductants and various colloidal reactions. The surface characteristics of finely divided gold also seem to be important in its mechanical transport in water and probably also in its dissolution in oxidizing solutions. Some finely divided gold particles appear to have a relatively small contact angle due probably to films of organic substances, which permit air bubbles to attach themselves readily, a feature that promotes their easy flotation downstream. In the case of the Witwatersrand gold the film in some cases may be the hydrocarbon (thucholite) with which the gold is intimately associated. Films of all kinds, including both organic and inorganic substances, commonly inhibit the solution of finely divided gold in natural solutions. The most common inhibitors appear to be the various hydrous iron and manganese oxides and silica.

2. Nature of the bedrock or false bottoms: since alluvial gold is invariably found on or near the bedrock surface or on false bottoms the characteristics of these substrates are commonly decisive in the formation of alluvial placers. Few 'pay streaks' or 'runs of gold' occur on smooth bedrock bottoms for obvious reasons. The most favorable bedrocks are those that form natural riffles perpendicular to the stream or river course as in a gold sluice. Alternating thin beds of soft schist and hard quartzite or slate and quartzite are particularly favorable. It is a surprising fact however, that in some placer areas bedrock riffles parallel to the direction of the stream flow are more effective in trapping the gold than those crossing it at a large angle. Limestone bedrocks that are pitted and pinnacled provide particularly good settling places for gold, as do potholes and nests of boulders in some placer streams. Partly weathered schists and gneisses in which the leaves of the schist and bands of the gneiss are slightly separated are also ideal rocks for the concentration of alluvial gold. The little nuggets, flakes and specks of gold dust work their way into such rocks often to depths of 3 m. or more. Similar phenomena are noted where the bedrocks are sheared, fractured or intensely shattered. Limestones in karst terrain are also good traps for gold, the metal having worked its way down into the rock to depths of 10 or 13 m. in some placer areas. Mining of these zones of enriched bedrock is common in many alluvial gold districts.

As for false bottoms, indurated clay and hardpan layers appear to be particularly favorable, although rough gravels and sands cemented by limonite, wad or carbonate provide suitable surfaces for the accumulation of gold in some districts. Compacted sands with rough surfaces are favorable in others. Some placer areas are characterized by stacked pay streaks due to alternating bands of clay, compacted sand, etc. in the sedimentary section.

3. Stream and river dynamics: this subject is complex involving hydrodynamics, sedimentation phenomena, geomorphology and isostasy (uplift and subsidence), subjects that can be treated only in a general manner here.

It is assumed that the reader is familiar with the general rudiments of the principles of sedimentation of falling bodies in static water. Briefly, the controlling factors involved are mainly the differences in specific gravity, size and shape of particles. It is axiomatic that of two spheres of the same weight but of different size, the smaller, with its lesser surface area, and hence lesser friction to water, sinks more rapidly in a static water medium. This is part of the reason why gold as small nuggets is commonly associated with quartz pebbles 2.5 cm. or more in diameter. Furthermore, the shape of particles is a factor - a spherical mineral has less surface area than a platy mineral of the same weight and hence sinks more rapidly.

Finally, during jigging of bodies of variable specific gravity in water, those with the higher specific gravity sink to the lowest level. During this motion, round, shotty particles of gold will also find the bottom quicker than scaly or spongy particles.

We are, however, not generally dealing with static bodies of water in the formation of placers but with bodies of water that move and exhibit turbulent flow. The principal parameters in stream and river dynamics are the gradient and the volume of water. The gradient is determined by many secondary factors in an area but fundamentally by isostasy, which involves uplift (or depression) of one segment of the earth with respect to the other. The volume of water delivered to a stream or river, likewise, depends on many secondary factors involving the runoff and groundwater systems but primarily on the rainfall of the district. In a district with a pluvial climate once a gradient has been established stream and river systems develop that erode the land and yield a pattern of gulches, canyons and valleys.

[ 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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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011