Ore Deposits Formed by Evaporation 

Evaporation is an important mineral forming process, which supplies valuable materials used by the housewife, farmer, builder, chemist, engineer, manufacturer and even birds, beasts and plants.  Great sections of the oceans may be cut off during slow oscillations of land and sea and be gradually evaporated to yield deposits of gypsum, common salt and potash.  Ground waters reaching arid surfaces leave behind valuable minerals upon evaporation.  Lakes may disappear under arid conditions to form playas. Mineral deposits resulting from the concentration and crystallization by evaporation from aqueous solutions are called Evaporites.

 

Conditions of Formation:

Evaporation proceeds most rapidly in warm humid climates.  Evaporation of bodies of saline water leads to concentration of soluble salts, and when supersaturation is reached, the salts are precipitated.  Deposition of minerals by evaporation depends on supersaturation, which in turn depends upon other factors, chief of which are:

a)    temperature

b)   pressure

c)    depositional environment, and

d)   seasonal & climatic changes

Types of Evaporites: There are two types of evaporite deposits - marine, which can also be described as ocean deposits, and non-marine, which are found in standing bodies of water such as lakes.     

 

Marine Evaporites - Deposition from Oceanic Waters:

The salts of oceanic waters are mainly obtained from the weathering of terrestrial rocks.  Rain water carries soluble salts from continental areas to the oceans, evaporates, leaving behind the salts, to return to the continents as more rain.  Small mounts are contributed by submarine volcanism, and by solution from oceanic rocks.  The total amount of salts in the ocean is estimated to be 21.8 million km3, enough to form  60 m thick layer over the ocean bottom.  Of this common salt would constitute 47.5 m, MgCl2 5.8 m, MgSO4 3.9 m, CaSO4 2.3 m and the remaining salts 0.6 m.  The rivers of the world are estimated to contribute 4 billion tons of salt to the ocean annually.  Ocean water also contains gold, silver, base metals, manganese, aluminum, vanadium, nickel, cobalt as well as iodine, fluorine, phosphorous, uranium, arsenic, lithium, rubidium, cesium, barium and strontium.  Of these, iodine concentrates in sea weeds, copper in shellfish, manganese, copper, nickel and other metals in nodules which alone have become concentrated into potentially commercial deposits.  To attain the necessary conditions to induce precipitation, bodies of sea water must become isolated from the ocean in places where evaporation exceeds inflow.  Such isolation may be effected by:

a)    formation of barrier reefs

b)   cutoffs near coasts where sills or reefs isolate sinking inland basins

c)    formation of sand bars

 

If the original body of water contained 100 km3 of water and this were to be concentrated to 50 km3, the iron oxide and calcium carbonate present would be precipitated.  The water would still contain 3500 million tons of salt of which 2700 million tons would be common salt. If evaporated to 20 km3, gypsum would be precipitated.  When the volume reaches about 10 km3, common salt would be deposited. Subsequent evaporation would bring about deposition of magnesium sulfate and chloride followed by the bittern salts.

 

Non-Marine Evaporites - Deposition from Salt Lakes:

 The deposits formed from the evaporation of salt lakes are similar to those obtained from ocean water because salt lakes contain the same salts as the ocean, but generally in greater proportions.  The relatively small size of lakes makes them more responsive to climatic changes resulting in greater fluctuations of deposition.  Evaporites formed during periods of dessication may be redissolved during periods of subsequent expansion.  Since lakes constantly receive new supplies of fresh water, salts and sediments, their deposits are generally thin bedded alternations of impure salts and clays.  On salt playas, desert winds distribute sands and silt upon which later salts may be deposited during subsequent lake periods. Examples of deposits from salt lakes are:

a)    Basin and Rang Province, USA

b)   Salton area of the Imperial Valley, California

c)    Salt pans throughout Central Asia and northern Africa

 

Deposition from Groundwater:

Evaporation of groundwater is universal and in arid regions the evaporites may accumulate as long as the climate remains dry.  Groundwater contains salts similar to those of the ocean and salt lakes but their concentration is low and the proportion of individual salts may vary according to the character of the soil, bedrock, topography and climate.  Calcium carbonate is almost always present; magnesium, sodium, potassium, iron and manganese compounds are common.  Silica, phosphorous and locally boron and iodine are relatively abundant.  Deposition ensues when evaporation occurs at or near the surface, or  in caves.  If the site of evaporation is fed by fresh supplies of groundwater, extensive deposits may eventually result.       Evaporation of groundwater will proceed most rapidly where it is supplied relatively close to the surface viz., valley bottoms, slopes where hills and valleys merge, and long hill slopes interrupted by gentler or reverse grades.  Deposits of economic importance are:

a)    nitrate salts with iodine     

b)   boron

c)    calcium and sodium carbonate

d)   common salt

e)    Gauberís salt

f)     soda

g)    epsom salts

h)    borax

 Artificial brines are obtained by pumping water down wells drilled into rock salt beds, and the brines so formed are the main source of common salt production in the US.  Potash is also obtained in this manner.

 

Deposition from Hot Springs:

Substances contained in hot-spring waters build up deposits around their orifices.  A few of theses are of commercial importance, the others are scenically beautiful. Their deposition is not always the result of evaporation alone, microscopic organisms help deposit some substances, and the escape of carbon dioxide under reduced surface pressure also causes deposition. The chief substances deposited in this manner are calcium carbonate, in the form of tufa, travertine, or calcareous sinter; silica in the form of siliceous sinter or geyserite; iron oxide in the form of ocher; and manganese dioxide in the form of wad. Many other non-metallic and metallic substances are deposited from hot springs but in relatively small amounts.

Important Minerals found in Evaporites:

Sea water is the prime source of minerals formed by evaporation.  Sea water consists of about 3.45% of dissolved salts, of which 99.9% consists of only seven ions.  These are: Na+ (30.61), Mg2+ (3.69), Ca2+ (1.16), K+ (1.10), Cl- (55.04), SO42- (7.68) and HCO3- (0.41).  About 45 other elements whose concentration is known in sea water occur as trace minerals in evaporites. Evaporites often show a repeated sequence of minerals, indicating cyclic conditions with a mineralogy determined by solubility. The sequence in which minerals in marine evaporites are deposited is: calcite, gypsum, anhydrite, halite, polyhalite, and lastly potassium and magnesium salts such as sylvite, carnallite, kainite, and kieserite; anhydrite and halite dominate.

Following are the important minerals that form in marine evaporites:

Chlorides:  Halite (NaCl),  sylvite (KCl), carnallite (KMgCl3.6H2O), langbeinite (K2Mg2(SO4)3 ), polyhalite (K2Ca2Mg(SO4)6.H2O), kainite (KMg(SO4)Cl.3H2O)

Sulfates:  Anhydrite (CaSO4), gypsum (CaSO4.2H2O), kieserite (MgSO4.H2O)

Carbonetes:  Dolomite (CaMg(CO3)2 ), Calcite (CaCO3)Magnesite (MgCO3)


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