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A relationship exists between hydrothermal processes where heat, pressure and time form minerals and fluvial deposits where eluvial, and alluvial (principally river transport) processes aggregate materials in commercially viable deposits. We could say that the mineral factory is the hydrothermal deposition of minerals within rocks (along with other similar processes) while fluvial processes, both eluvial and alluvial, are accumulating or transporting these products elsewhere.

Placers: Eluvial, Alluvial and Fluvial.

<em.These terms apply to surface deposits that result in the accumulation of heavy minerals that are not easily altered further. Mineral veins or lode deposits formed at depth and long ago weather. crumble and accumulate on the surface over time due to the action of atmospheric agents . Once freed from bounding solid rock material the heavy minerals are uprooted, transported and sorted by the action of flowing water. The valuable portions are classified as two distinct types: eluvial and alluvial.

Eluvial

Eluvial is the term applied to the mineral material found in the immediate vicinity of the parent rock. The free gold found on the slopes adjacent to the outcrop of a gold vein is an eluvial deposit. Auriferous veins or deposits, when exposed to the elements are subject to attack by the processes of weathering–the physical and chemical disintegration of rock at or near the surface. Except for their external appearance and physical size eluvium (very small to large pieces of rock) does not differ much from the parent rocks themselves. Still, eluvium subsequently undergoes transformation; part disappears through aqueous solution but the more resistant debris particles begin to slowly migrate due to the processes of erosion. Eluvium may be considered as the initial phase in the process of creating river deposits, beaches, and other forms of transported detritus. An eluvial deposit is formed without significant transport (save the actions of gravity),

eluvial gold

Eluvial Gold, details

where heavier, more weather resistant minerals accumulate below an outcrop and the lighter, less resistant products of decomposition are dissolved or are washed away. This process of concentration continues down slope from point of origin until, eventually, the lower elevation, somewhat more concentrated mineral mass is entrained and carried into the channels of streams and the mightiest of rivers.

Alluvial or Fluvial Deposits

Minerals carried by running water, either in pure or chemically combined form, continue to move downstream until current conditions and velocity cause them to be deposited in or along the banks of a stream. River deposits of precious gold and associated debris are called placers and are one of the most important sources of this metal. Placer gold deposits have produced the largest amounts of gold in the history of the metal and account for a considerable portion of total world-wide gold production today.

alluvial-gold-dust-particles

Alluvial gold dust particles, taken from Motagua River in Guatemala

Fresh, moving water is the most effective separator of light and heavy materials. While river water always flows in one direction it does not flow uniformly. Currents and eddies within river channels vary greatly in velocity; change greatly from one part of the channel to another, from one season to the next and, overall, from one year or decade to another. As a general rule, however, velocities are less on the gently flowing inner portions of river bends, and except during periods of high flood are not favorable to the accumulation of heavier materials. Velocity and ability to transport material is greater in mid channel of a river, and the water usually flows more quickly near the outside bank of a river bend. While other factors, such as mid channel bar deposits and stream bed roughness, also impact the ability of the stream to move heavier material these higher velocity locations offer the most promising placer formation conditions.

Site specific conditions also come into play. At the confluence of two rivers, where one stream loses velocity as it meets another, gravel bars are formed. This is a favorable place for the formation of placers, as are steeply dipping hard rock layers that project upwards into the stream bed followed immediately downstream by softer, easily eroded materials that can form depressions. Such environments offer an excellent opportunity for the deposition of heavy minerals. If a river happens to flow across an ore bearing vein, for example, the eroded minerals will concentrate and be found in deposits that extend immediately downstream.

The formation of placers requires well “leveled” rivers, where a balance is achieved between erosion, transport, and deposition (Lindgren.1933 in V.Lopez.1981). Lindgren considered moderate gradients (approximately 10 meters per kilometer) as a likely optimum for concentration. An increase in gradient, a sudden decrease in bed load of the river, or an increase in current velocity causing down-cutting of the channel and thus the formation of bordering terraces or elevated gravel bars in the valley offers additional prospects for possible exploitation. Repetition of this cycle of deposition and down-cutting can form more than one series of such terrace features. While the top of the terraces representing the earlier valley floor may partially consist of landslides or non-fluvial material, terrace gravels will again be moved laterally toward the river and subject to a second concentration. In the absence of lateral drag or movement to the present stream channel the banks or terraces themselves frequently offer placer gold extraction possibilities.

where-to-find-gold-alluvial-gold

In areas affected by subsidence subsequent burial may have covered preexisting auriferous placer gravels to a depth of several meters or more. Such deposits are also potentially important though their exploitation may be difficult when there is excessive over-burden or groundwater is present.

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