Alluvial placer deposits

ALLUVIAL PLACER DEPOSITS (EN: alluvial placer; DE: Alluvialseifen; FR: gite alluvionnaire, gisement alluvionnaire; ES: placer aluvial; RU: аллювиальные россыпи) are the industrial accumulations of the grains of the useful minerals in the clastic depositions of the riverbed facies of the alluvium of the permanent and temporary water flows; they emerge as the consequence of the destruction and erosion of the rocks, of the primary deposits, and of the intermediate collectors.

The typical grain composition of the alluvial placer deposits
Useful mineral Fractions, millimetres
less 0.25 0.25-1 1-4 more 4
Gold1 5 15 55 25
Gold2 75 25 - -
Diamond1 - 5 60 35
Cassiterite 20 40 25 15
1 Predominantly large, 2 Small grains

Most of the alluvial placer deposits are monomineral (for example, the alluvial placer deposits of the gold, platinum, tin, diamonds), but the polymineral ones may be found often (for example, gold-platinum, tin-tungsten, titanium-zirconium, tantalum-niobium). They are common mainly in the mountainous regions, and are formed in association with the erosion cycles during the phases of the deepening (the deep erosion) and extension (the lateral erosion) of the valley. The physical properties of the useful minerals (density, hardness, size of the grains) determine the major differences in the structure of the alluvial placer deposits. The useful minerals of the greatest density (1.5-2.1 • 10^4 kilograms per cubic metre), namely, the osmiridium, glandular platinum, gold, are concentrated in the near-bedrock part of the alluvium, sometimes penetrating through the fractures into the bedrock down to the depth of 1-1.5 metres, and form the placer deposits with the thickness from the tens of centimetres to several metres, which are overlain by the unproductive depositions, which are exceeding the placer deposits by the thickness. The minerals of the medium density (6-8 • 10^3 kilograms per cubic metre), namely, the cassiterite, wolframite, columbite, tantalite, form the placer deposits with the thickness of several metres, which are confined to the lower half of the riverbed alluvium. The minerals of the low density (3.5-5 • 10^3 kilograms per cubic metre), namely, the ilmenite, zircon, monazite, diamond, and other precious and semi-precious (ornamental) stones, are distributed throughout all the riverbed alluvium, forming the placer deposits with the thickness of 10-12 metres or more. The alluvial placer deposits have the different grain composition of the useful minerals. The large grains of the minerals of any density, and also the small particles of the high density, deposit themselves at the foundation of the riverbed alluvium; the small grains (sometimes even of the minerals of the high density) distribute themselves in the upper layers of the alluvium, increasing the thickness of the alluvial placer deposits, which are confined to the bedrock. Because of this, the alluvial placer deposits have the different grain composition (table).

The granulometric composition of the alluvial placer deposits ranges from the coarse-grained to the significantly sandy (5-10% small boulders, 30-80% pebbles, 10-40% gravel, 10-30% sand, 5-10% silt, 1-5% clay).

Geomorphological situation of the alluvial placer deposits

In accordance with the geomorphological conditions, depending on the neotectonic movements, there are distinguished the alluvial placer deposits, which are situated at the different altitude levels: within the limits of the uplifts, namely, the watershed, terraced, valley, riverbed, nasal ones; in the depressions, namely, the buried, terraced, and valley ones; and in the accumulating strata on the false bedrocks, namely, the hanging ones (Figure 1).

There dominate within the uplifts the shallow-embedded placer deposits (with the depth of less than 15 metres), there within the valleys dominate the deep-embedded (with the depth of 20-300 metres, more often of less than 100 metres); the hanging placer deposits are embedded at the shallow depth. The initial embedment of the alluvial placer deposits may be disturbed by the subsequent epigenetic geological processes, namely, by the multiplicative and disjunctive tectonic deformations, by the fluvial and marine erosion, by the gouging by the glacier.

There are located in the valleys of the modern fluvial network the alluvial placer deposits mainly of the Quaternary (Anthropogenic) age, in the valleys of the ancient fluvial network the Neogene, Paleogene, and Mesozoic ones. There are also known the more ancient placer deposits, before and including the Proterozoic age.

The alluvial placer deposits are usually ribbon-like in the horizontal plane projection, longitudinal with respect to the direction of the valley, but there may be found sometimes in the mature valleys the diagonal and transverse ones, which are the extension of the alluvial placer deposits of the modern tributaries, or are the horizontal projection of the primary sources and small paleo-tributaries, which were converted into the phase of the expansion of the bottom of the valley; the placer deposits in the young incising valleys repeat the pattern of the meandering riverbed. There may be often found the lenticular (less often of the irregular shape), isometric, nested placer deposits. The length of the alluvial placer deposits is from the few hundred metres to the several tens kilometres, the width from the several tens metres to the several hundred metres, sometimes several kilometers. The alluvial placer deposits with the high level of the industrial content of the (TiO2) are characterized by the relatively uniform distribution of the useful components; the alluvial deposits with the low content of the gold, platinum, diamonds are characterized by the non-uniform distribution of the useful components, with the tendency of the increase of the irregularity with the increase of the sizes of the valleys (Figure 2).

The distribution of the minerals in the alluvial placer deposits

The alluvial placer deposits are explored by the drilled wells, exploring shafts, mine shafts with the shaft insets, trenches. The situation of the exploratory workings depends on the morphology of the placer deposits: they use the linear network of the workings during the exploration of the ribbon-like placer deposits, the rectangular network of the workings during the exploration of the lenticular deposits, the square-rhombic network of the workings during the exploration by the isometric and reworked epigenetic processes of the alluvial placer deposits. The density of the exploration network is directly proportional to the rate of the variability of the parameters of the deposit according to their morphology and distribution of the valuable minerals. There complicate the operation the mining-technical conditions, namely, the increase of the depth of the embedment, the narrowing of the bottom of the valley (100 metres and less), the existence in the composition of the large boulders (larger than 1 metre), the increase of the content of the clay (more than 5%), the increase of the share of the small classes of the useful minerals (less than 0.5 millimetres), the penetration of the grains of the useful minerals through the fractures into the bedrock, which has been formed by the rocks of the increased strength, the frozen condition of the shallow-embedded placer deposits, and the watering of the deep-embedded placer deposits.

Among the various types of the placer deposits, the alluvial placer deposits play the leading role for the extraction of the gold, platinum, tin, tungsten, the subordinate role for the extraction of the precious and semi-precious (ornamental) stones, the negligible role for the extraction of the titanium, zirconium, tantalum, and niobium. There belong to the best known alluvial placer deposits the gold-bearing placer deposits of Kolyma and Chukotka (USSR), Alaska and California (USA), the diamond alluvial placer deposits in the South Africa. See the "Placer deposits" article about the development of the alluvial placer deposits.