508 tyler. SEDIMENTARY IRON DEPOSITS [Ch. 28 



Mechanical 



The mechanical transportation of iron-bearing minerals by traction 

 or suspension may be attended by considerable abrasion and decom- 

 position before final deposition. This tends to concentrate the more 

 resistant minerals, such as magnetite and ilmenite in the form of placer 

 sand deposits, whereas the less resistant iron-bearing minerals are 

 deposited with the silts, clays, and chemical sediments. 



Colloid 



Gruner (1922, pp. 407-460) and Moore and Maynard (1929, pp. 

 272-303, 365-402, 506-527) state that iron is transported by present 

 streams mainly in the form of ferric oxide hydrosol stabilized by or- 

 ganic matter. Although the average iron content of river waters in 

 North America is less than 1 part per million, there are many streams 

 and lakes that show a higher concentration. Moore and Maynard 

 (1929, p. 300) have shown experimentally that as much as 36 parts per 

 million of ferric oxide may be held in colloidal solution by 16 parts per 

 million of organic matter. 



Solution 



Iron may be transported in solution as ferrous bicarbonate, sulphate, 

 or chloride, or as salts of organic acids, provided that the environment 

 is one in which there is a deficiency of oxygen. Ground waters often 

 provide this environment. In moist regions where there is an abun- 

 dance of decaying organic matter, the ground water removes and 

 transports iron as the bicarbonate. If sulphides are present, ground 

 water is likely to transport iron in the form of ferrous sulphate. When 

 the ground waters emerge at the surface, the ferrous salts are oxidized, 

 and transportation of the iron is principally in the form of colloids. 

 It is possible that in the geologic past the atmosphere may have been 

 largely composed of carbon dioxide and deficient in oxygen. This 

 would allow extensive transportation of iron bicarbonate over long 

 distances. 



DEPOSITION OF IRON 



Mechanical 



Mechanically transported iron-bearing minerals are deposited when 

 the velocity of the transporting medium drops to a critical level, which 

 is specific for the size, shape, and specific gravity of the particle con- 

 cerned. Incomplete sorting usually leads to the deposition of the heavy 



