Weathering Products and Residual Deposits 

Weathering not only disintegrates a rock but by removing the undesirable constituents in it, concentrates the residual useful substance which forms an economic deposit.

The effects of weathering usually do not extend deeper than a couple of meters but occasionally reach 30 to 60 meters. It is most active in tropical and sub-tropical climates. In such climates, rock decay is carried further; leaching is more complete; the silicates are thoroughly broken down and surface water readily removes the silica, thus bringing about the concentration of the residual material which, when useful, forms valuable mineral deposits.

Bauxite is an important example of this kind. Valuable deposits of clay have been formed by removal in solution of the lime in a limestone bed. The removing by weathering of the useless constituents has brought about the concentration of the sparse nickel content in ultra­basic rocks to form valuable deposits of the metal.

Alluvial and eluvial deposits are the results of weather­ing and mechanical concentration. The tough, chemically stable useful metals or minerals disseminated through a rock are freed from their matrix and moving water or air progressively concentrate the released particles due to their higher specific gravity. In other words, nature acts as a gravity separator in which the heavy minerals are separated from the lighter ones. The only difference is that nature takes a much longer time to do the job.

·      Under the action of weathering, rocks and enclosed mineral deposits undergo disintegration and decomposition - soluble parts are removed and insoluble residues accumulate, forming residual mineral deposits.

·      Mechanical disintegration, such as that by frost action, frees minerals and renders them capable of being carried by various agencies, and be sorted according to size and specific gravity.

Residual Concentration:

·      Residual concentration results in the accumulation of valuable minerals when undesired constituents of rocks or mineral deposits are removed during weathering.

·      The concentration is largely due to a decrease in volume effected by surficial chemical weathering.

·      The residues may continue to accumulate till their purity and volume make them of commercial importance.

Mechanical Concentration:

·      Mechanical concentration is the natural gravity separation of heavy from light minerals by means of moving water or wind.

·      The resulting deposits are called placer deposits and their formation involves two stages:

1)   freeing of the stable minerals from their matrix by weathering, and

2)   their concentration.

·      Concentration can occur only if the valuable minerals have:

a)    high specific gravity

b)   chemical resistance to weathering, and

c)    durability (malleability, toughness and hardness).

 Process of Formation of Residual Deposits:

·      The first condition is the availability of rocks or lodes containing valuable minerals, of which the undesired substances are soluble and the desired ones insoluble under surface conditions.

·      The climatic conditions should favour chemical decay.  Tropical & sub-tropical climatic conditions are most favourable.

·      The relief must not be too great, or the valuable minerals will be washed away as soon as they are in the least concentrated.

·      Long continued crustal stability is essential for residues to accumulate in quantity, and the deposits may not be destroyed by erosion.

·      Important deposits include: Iron ores, manganese, bauxite, clays, nickel, phosphate, kyanite, barite, ochers, tin, gold, etc.

·      Given these conditions, a limestone formation with minor iron oxides will slowly be dissolved leaving the insoluble iron oxides as a residue.  As bed after bed of limestone disappears, an overlying mantle of iron ores of sufficient thickness, and grade accumulates to make a workable deposit.

Residual deposits therefore form in two ways:

1)   the residue is simply an accumulation of a mineral that has not changed during the process e.g. iron oxides in banded iron formations, and

2)   the valuable mineral first comes into existence as a result of weathering processes, and then persists and accumulates e.g. the feldspars of a syenite decomposes upon weathering to form bauxite, which persists at the surface while other constituents are removed in solution.

·      Valuable deposits of iron ore, manganese, bauxite, clays, nickel, phosphate, kyanite, barite, ochre, tin, gold and other substances occur as residual concentrations.

 Residual Deposits and their Source Materials:

Iron Concentrations:

1)   Lode deposits of siderite or iron sulfides - these residues are rarely used as iron ores.

2)   Disseminated iron minerals in non-aluminous limestones.

3)   Limestones that have been partly replaced by iron minerals, either before or during the period of weathering.

4)   Basic igneous rocks.

5)   ferruginous siliceous sediments.

Manganese Concentrations:

 1)   Limestones or dolomites low in alumina but containing disseminated syngenetic manganese carbonates and oxides.

2)   Limestones containing disseminated introduced manganese.  Carbonate rocks precipitate manganese under certain conditions.

3)   Manganiferous silicate rocks such as crystalline schists or altered igneous rocks.

4)   Lode deposits of manganese minerals or ores high in manganese e.g. veins, replacement deposits or contact metasomatic deposits (containing rhodochrosite, rhodonite, manganiferous siderite and calcite, spessartite, tephroite, alleghenite, piedmontite, hausmannite, manganosite, etc.).

Bauxite Formation:

Rocks relatively high in aluminum silicates and low in iron and free quartz e.g.

1)   nephline syenite.

2)   Limestones or clays in limestones

3)   Clastic sediments derived from Archaean rocks

4)   Basalts

5)   Clay alluvium

6)   Feldspathic sandstones

Clay Formation:

The source rocks are crystalline rocks and silicic granular rocks rich in feldspars and low in iron minerals such as:

1)   Granites and gneisses

2)   Basic ignous rocks

3)   Feldspar rich pegmatites

4)   Syenites

5)   Limestones

6)   Shales

7)   Sericitized igneous rocks

 Process of Formation by Mechanical Concentration:

When mineral grains of different density are moved by flowing water, the less dense grains will be most rapidly moved, and a separation of high-density and low-density grains can be effected. Mineral deposits formed as a result of gravity separation based on density are called placer deposits.

For effective concentration, placer minerals must not only have a high density (greater than about 3.3 grams per cubic centimetre), they must also possess a high degree of chemical resistance to dissolution or reaction with surface water and be mechanically durable. The common sulfide ore minerals do not form placers, because they rapidly oxidize and break down. Ore minerals having suitable properties for forming placers are the oxides cassiterite (tin), chromite (chromium), columbite (niobium), ilmenite and rutile (titanium), magnetite (iron), monazite and xenotime (rare-earth metals), and zircon (zirconium). In addition, native gold and platinum have been mined from placers, and several gemstone minerals--in particular, diamond, ruby, and sapphire--also concentrate in placers.

Placer deposits are formed as a result of the concentration of valuable minerals in depositions of the detritus developing during the disintegration and redeposition of the materials of rocks and minerals.  Their development is linked with the physical and chemical weathering of both rocks and minerals..

Placer deposits are formed as a result of the concentration of valuable minerals in depositions of the detritus developing during the disintegration and redeposition of the materials of rocks and minerals.  Their development is linked with the physical and chemical weathering of both rocks and minerals.

The placers formed on the site of the destruction of the primary sources are known as Eluvial.  When the weathered and disintegrated material is shifted downhill Scree or Deluvial (talus) placers are formed.  Accumulation of the material at the foot of a slope can lead to the development of a Proluvial (colluvial) placer.  When clastic and weathered material is carried away by streams, it serves as the source of Stream (River) or Alluvial placers.  Beach or Lateral placers are built up along the shores of lakes, seas and oceans.  In addition, Glacial placers can develop as a result of glacial activity and Eolian placers as a result of the action of wind.

 ·      Placer minerals that can be mechanically concentrated are gold, platinum, tin-stone, magnetite, chromite, ilmenite, rutile, native copper, gemstones, zircon, monazite, phosphate and quicksilver (rare).

·      Depending upon the agency involved, placers are of three types:

1)   Eluvial placers - form upon hill slopes, concentration of valuable minerals by gravity

2)   Stream or alluvial placers - concentration of valuable minerals by running water

3)   Eolian placers - concentration of valuable minerals by wind action

Eluvial Palcers:

These may be considered an intermediate or embryonic stage in the formation of stream or beach placers.  They are formed, without stream action, upon hill slopes from materials released from weathered lodes that outcrop above them.  The heavier, resistant minerals collect below the outcrops; the lighter nonresistant products of decay are dissolved or swept downhill by train wash or are blown away by wind.  This brings about a partial concentration by reduction in volume, a process that continues with continued down slope creed.   Fairly rich lodes necessary to yield workable deposits by this incomplete concentration.  Important deposits formed in this manner are of gold and tin, minor deposits include manganese, tungsten, kyanite, barite and gemstones.

Stream or Alluvial Placers:

Flowing water is the most effective separator of heavy from light material, and stream placers are the most important type of placid deposits.   They have even the greatest quantity of placid gold, in stone, Platinum, and precious stones.  Likely places of accumulation of heavy material along a stream channel are:

i.      the middle reaches of streams

ii.     where streams flow through polished canyon floors

iii.   where they enter valleys of gentler gradient

iv.   convex side of meanders

v.    where streams cross highly inclined or vertically layered hard and soft rocks

vi.   at the confluence of two streams, at the mouth of the more sluggish one

The minerals that make up placer deposits may be derived from:

a)    Commercial deposits, such as gold veins, and

b)   Non-commercial lodes such as gold stringers or veinlets of cassiterite.

Eolian Placers:

Wind instead of water may act as the agent of concentration and give rise to placer deposits.   This can occur only in arid regions.   These are the least important of all placer deposits.   It is reported that some eolian gold placers have been formed in the Australian deserts from the disintegration of gold quartz lodes.   The light decomposed materials have been blown away; the heavy gold particles, freed from their matrix, remained behind.   Similar concentration has taken place El Arco, in Lower California, Mexico.

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