Essay on Sedimentary processes of ore-formation

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As we know the combined action of sharp temperature fluctuations, wind, water freezing in rock crevices and plant roots penetrating into the rockmass" decomposes great blocks which gradually split into chunks, then into smaller fragments and are finally reduced to detritus and sand, in addition to mechanical destruction, primary minerals are subjected to chemical alteration, of which water is the principal agent. The products of weathering are transported by various geological agents and finally the transported detritus gets deposited in various condi­tions of aqueous medium, to give rise to sedimentary formations. Five important processes are associated with the sedimentary mineralisation, viz.

1. Residual concentration.

2. Mechanical concentration.

3. Oxidation and Supergene Enrichment.

4. Remobilisation by meteoric circulating water.

5. Sedimentation.

1. Residual concentration:

The term 'residual concentration indicates the concentration of ores as residue. Residue left as insitu after weathering, followed by transportation sometimes give rise to valuable ore deposits. These are the insoluble products of rock weathering, the process which removes the undesired constituents of rocks or minerals. The residue may continue to accumulate until their purity and volume make them commercially important.

Of the three modes of weathering, the chemical mode of weathering is of paramount significance for the formation of residual deposits.

Factors Affecting the Residual- Concentration:

(i) Climate:

The climatic conditions determine the course and intensity of rock-decomposition. Dittler has shown that a temperature above 20°C favours the process of chemical decomposition of bed rocks where Si02 goes into solution. The hot climate of the tropics creates the condition for residual concentration. Besides, an average amount of regular precipitation is most able for the formation of a mantle waste and subsequently residual concentration.

(ii) The presence of rocks or lodes containing valuable minerals of which the undesired constituents are soluble and the desired substances are generally insoluble under surface condition.

(iii) Long continued crustal stability is essential in order that dues may accumulate in quantity and that the residues may not be destroyed by erosion.

(iv) Existence of proper drainage is an important factor.

(v) There should be availability of reagents to bring about the breaking down of silicates and solution of silica.

(vi) Local relief:

The optimum conditions for the formation of residual deposits are provided by the existence of an average hill- country topography that ensures percolation of the meteoric preci­pitation down to the water table.

The relief must not be very great or the valuable residue will be washed away as rapidly as formed.

(vii) Since it takes much time to form mature, thick and well developed residual deposits, the acid rocks because of their relatively high alumina-content are the suitable rocks to produce residual deposits.

Under these conditions, for example, the feldspars of syenite or granite decomposes upon weathering to form 'bauxite', which persists at the surface, while other constituents are removed.

Hematite, manganese ore, bauxite, residual clays, ochres, kyanite deposits in India are some of the examples of residual concentration. Residual deposits are usually associated with laterite.

2. Mechanical concentration:

It is the process of natural gravity separation of heavy from light minerals by means of moving water, or air by which the heavier minerals become concentrated into deposits called 'Placer-deposits'.

As we know, during the process of weathering the resistant minerals like-quartz, gold, platinum, magnetite, cassiterite, ilmenite, garnet etc. are set free in individual grains. Erosion now steps in and the detritus is swept down the slopes and into the water channels. Mechanical separation in running water or along sea or lake beaches sorts the detritus according to the specific gravity and the size of grains. The heaviest particles tend to collect in the lower part of the asserted detritus, the quartz grains are carried further, the minute and easily moved scales of clayey substances are ultimately deposited as sedimentary beds ; the colloids are coagulated by the electrolytes in the sea water.

Thus, two stages are there in the formation of placer deposits:

(i) The freeing by weathering of the stable minerals from their matrix, and

(ii) Their transportation and concentration.

Requirements for mechanical concentration:

(a) The valuable minerals should be of high specific gravity.

(b) They should be chemically resistant to weathering.

(c) They should be of adequate durability.

(d) There should be a continuous supply of placer minerals for concentration.

In the formation of placer deposits, nature employs in her own leisurely way the processes of crushing and concentration.

Factors Affecting Formation of Placer Deposits:

(i) Specific gravity of minerals.

(ii) Specific surface of the particles.

(iii) Shape of the particles.

(iv) The ability of a body of flowing water to transport the particles and the viscosity of the transporting medium.

Types of placer deposits:

(a) Eluvial placers:

Placer deposits along hill slopes are formed due to weathering and erosion of the country rocks contain­ing low-grade deposits of the desired materials and are known as eluvial placers.

(b) Deluvial placers:

When the weathered and disintegrated material is shifted down hills deluvial (scree or talus) placers are formed.

(c) Proluvial (colluvial) placers:

Accumulation of the material at the foot of a slope can lead to the development of proluvial placers.

(d) Alluvial placers:

Running water is the most important agency in the formation of alluvial placers. Irregularities on the floor of the channel in the form of natural barriers or riffles encourage deposition of placer deposits. Besides, at the meander­ing of the river and at the confluence of tributaries, alluvial placers are formed.

(e) Aeolian placers:

These are because of wind action, by which the lighter sand particles are blown away leaving behind a mass of coarser detritus containing valuable minerals.

(f) Beach placers:

These are formed along the shores of lakes seas and oceans, mainly by the wave action.

These are essentially four factors which have much significance in the formation of placer deposits, they are:

(i) Geomorphological factors.

(ii) Climatic factors.

(iii) Hydrographic factors which is associated with the river action and the deposits at the meandering of the river and the junction between the tributaries.

(iv) Tectonic factors, which is associated with the rejuvenation of the base level of local and general erosion, creating conditions for recurrent cycles of erosional activity for development of alluvial placers.

3. Oxidation and supergene enrichment:

This process is called as 'Infiltration' deposits also. This involves weathering and leaching of the upper parts of a mineral deposit (zone of oxidation) and re-deposition of the ore-minerals at lower levels (zone of secondary or supergene enrichment).

The portion of the ore-body lying above the water-table is described as the zone of oxidation, since within this zone the ore- minerals forming the deposit may be oxidised readily in presence of air and water. By the reaction of the surface water containing free oxygen with the ore a solvent is formed. This solvent is very reactive and is helpful to oxidise the whole of the rock up to the water-table.

Because of oxidation, there is a break-up in iron-sulphides (e g, pyrite) giving rise to ferrous and ferric sulphates, sulphuric acid, sulphur and ferric hydroxide. Of these products ferric sulphate and sulphuric acid act as solvents and cause oxidation and solution of other ore minerals like galena, sphalerite, chalcopyrite, chalcocite etc.

Thus there are two main chemical changes within the zone of oxidation:

(i) The oxidation, solution and removal of the valuable minerals.

(ii) The transformation, in-situ of metallic minerals into oxidised compounds.

Since oxygen has no action on gold as well as other insoluble minerals like cassiterite, wolframite etc., these minerals remain in the outcrop without any change and are enriched upon contraction of volume. Much of the soluble minerals are naturally removed by running water. Newly formed compounds are precipitated due to reactions between solutions, by hydrolysis, by coagulations, as well as by reaction between the solutions and solids.

Factors controlling oxidation:

(a) Water table:

Since oxidation takes place above the level of water table, the position of water table affects oxidation to a greater extent.

(b) Low-rate of erosion.

(c) Warm-humid climate with evenly distributed rainfall favours oxidation.

(d) To some extent the chemical composition of the rocks and the associated structural features also control oxidation.

Two types of deposits are mainly formed because of oxida­tion:

I. Above water table, there is an oxidised deposit.

II. Deposits of supergene enrichment, formed below the water, table. In the case of the 1st type, the deposits are formed because of evaporation, saturation, and reaction between solutions bringing about their precipitation.

In the IInd type, the valuable minerals present in the oxidised zone, gets leached down because of oxidation. They begin to deposit .in the secondary zone to make this zone enriched with ore, which is due to the fact that when the oxidised solution with minerals trickled -down to the secondary zone which is below the water table; there is no available oxygen and there they undergo deposition as secondary ores. The portion of the pre-existing ore-body, in which supergene sulphides are precipitated, is known as the zone of supergene- enrichment.

In the zone of supergene-enrichment the precipitation of the metallic sulphides is affected following Schurmann's law in the following order-silver, gold, copper, bismuth, lead, zinc, nickel, cobalt, iron etc. according to increasing solubility.

Gossan:

Oxidation, solution and consequent downward move­ment of the valuable minerals lead to the concentration of useless residual materials and some of the desiccated products of oxidation upon the surface, where the ore-body had its outcrop and these together form a hard mantle known as gossan or cap-rock.

The gossan is made up principally of limonite, gangue minerals and some of the oxidised products of the ore minerals. Sometimes, false gossans are, however, produced as a result of precipitation of extraneous ferruginous solutions upon the exposed surfaces of the country-rocks.

But in the majority of cases, gossans supply many decipherable inferences as to the size, character and mineral contents, of the hidden ore deposits. Therefore gossans are considered as sign boards of oxidised as well as enriched zones beneath the surface.

Thus the process of oxidation and secondary enrichment produces the following:

(a) Gossan.

(b) Zone of oxidation and leaching.

(c) Secondary zone of enrichment.

(d)Zone of unriched primary ore etc.

4. Process of remobilisation by circulating meteoric water:

This process involves the solution of materials dispersed in the adjacent rocks by ground water and their subsequent concentration under suitable physico-chemical conditions.

This process operates at moderate depth to shallow depths, under moderate pressure and low temperature (.ranging between 0°C to 100°C). These deposits are formed by ground water, and occur mainly in sedimentary rocks like-limestones, sandstones and shales, in the presence of some reducing agents.

The occurrence of some manganese deposits in the Keonjhar District of Odisha are believed to have formed by this process where by manganese at one level has been remobilised to the other and its subsequent concentration at that level. Other examples are-phos­phites, barite, celestite, pyrite, flint etc.

5. Sedimentation:

The process of sedimentation gives rise to following types of deposits:

(i) Evaporation deposits:

These are formed by evaporation of surface waters giving rise to deposits of salt, anhydrite, gypsum, borates, nitrates etc.

(ii) Chemically deposited materials, which is due to reactions between the solutions in bodies of surface water, are sometimes resulted in the process of sedimentation. Calcareous, ferruginous, manganiferous deposits are usually due to such processes.

(iii) Organic deposits:

Accumulation of organic matter whether due to chemical reactions or due to as such transportation and deposition by the geologic-agents give rise to deposits like bog iron ore, coal, phosphatic, siliceous as well as calcareous deposits of chalk, fossiliferous limestones etc.

The mechanically deposited materials like placer deposits are also due to sedimentary processes. The diamondiferous-conglome- rates as occur in the Vindhyan system of India are .the examples of such deposits.

Thus 'sedimentation' -gives rise to deposits both through Mechanical means as well as through chemical reactions and also through organic-chemical actions (i.e., by bio-mechanical+bio­chemical actions).


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