Ore Deposits Formed by Sedimentation
The process of sedimentation results in the formation of some important mineral deposits viz., iron, manganese, copper, phosphate, coal, oil shale, carbonates, cement rock, diatomaceous earths, bentonite, fullerís earth magnesite, sulfur and uranium-vanadium deposits.
The essential conditions for the formation of sedimentary deposits are:
adequate source of material
of the solution by solution or other processes
of the material to the site of deposition, and
deposition of material in sedimentary basins.
Compaction, alteration or other chemical changes may follow deposition.
Source of Material:
Materials of sedimentary deposits have been derived chiefly from:
and oxidation of former mineral deposits viz., iron, manganese, copper.
some material constituting sedimentary deposits may have passed through
an intermediate organic stage.
The earthís crust contains on average 5.6% Fe. This means that beneath the surface, upto a depth of 30 km, the earthís crust contains about 30 million tons/km2 of iron. Of this, only 0.0001% is concentrated in commercial deposits. Iron in sedimentary rocks comes from the iron bearing minerals of igneous rocks such as hornblende, pyroxene and mica, from the iron-bearing minerals of sedimentary and metamorphic rocks, and from the red coloring matter of sedimentary rocks. Manganese of sedimentary deposits hs been derived from the weathering of Mn-bearing minerals in rocks, former sedimentary concentrations and lode deposits of manganese. Mn makes up about 0.095% of the earthís crust, there being 50 times as much iron as manganese. There are over 200 minerals containing manganese as an essential constituent.
The source of sedimentary phosphate is phosphorous bearing minerals , among which apatite is the most common. Some phosphorous is also derives from the weathering of collophanite and dahllite in sedimentary rocks. Constituents of sedimentary carbonates viz., industrial limestone, dolomite and magnesite are derived from the sea or saline waters to which they are largely supplied by rock weathering. Constituents of clay deposits, bentonite and fullerís earth originate in rock weathering.
Solution of material constituting sedimentary deposits goes on during weathering. This is true of Fe, Mg, P, CO3, Cu, and some rare metals, but not of clays. The chief solvents are carbonated waters, humic and other organic acids and sulfate solutions.
These are very effective solvents of limestones, iron. Manganese and phosphorous. Vast quantities of metals are transported as carbonates by carbonated waters. Ferrous iron is soluble whereas ferric iron offers resistance. To undergo solution, ferric iron must first b converted to the ferrous state. Organic matter aids such a conversion. The Precambrian iron ores were probably transported as ferrous bicarbonate solutions or in the colloidal state.
Humic and other Organic Acids:
These are the decomposition products of vegetation and are considered excellent solvents. Weak organic acids dissolve large quantities and are the most effective of all solvents. It has been noticed that iron is not carried as bicarbonate in surface waters rich in organic matter.
These are excellent solvents of Fe and Mn but rarely
abundant enough to effect large scale solution and transportation.
FeS2 + 7O + H2O ŗ FeSO4 + H2SO4.
Most of the materials (except coal) are transported by rivers and
subsurface waters. Most often these substances reach the sea, but some are
arrested en route and are deposited in inland water bodies or basins.
The dissolved substance remain in solution so long as the solution does
not undergo any appreciable physical or chemical change.
Some or all of the iron and manganese may be lost if the solution
traverses limestone country. If Fe
and Mn escape these hazards, they may be transported to bogs, lakes, playas or
the sea, where their concentration and deposition takes place.
Deposition of material that forms sedimentary mineral deposits can take place in one of the following manner:
The manner of deposition depends upon:
nature of solvent
The pH and Eh (redox) conditions, eg. in the sea or swampy basins.
Deposition from solutions depends upon the environmental relations of chemical sediments in normal sea water.
Separation of Manganese & Iron:
Separation of manganese and iron occurs if precipitation is taking place from carbonate solutions. This happens because manganese carbonate is more stable in carbonate solutions than iron carbonates, and is hence carried further than the latter. The separation of iron and manganese in an oxidizing environment takes place because the iron oxides precipitate at a lower oxidation potential than the comparable manganese compounds at any given pH. Similarly under fixed Eh, iron starts precipitating as an oxide at a lower pH than manganese. In a neutral environment both iron and manganese may precipitate together as carbonates.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)