ORIGIN OF ORE DEPOSIT- 



37 



aft ri- the deposition of the sediments on the floor of the sea, or 

 some later period, cither hefore or after these Bedimentarics have 

 become land masses. 



Ore deposits thus formed are divided into two classes: (1) 

 Bedded deposits, in which the ore body conforms with the strata 



FIG. 26. Iron ores of Michigan interbedded with igneous and sedimentary 

 rocks. (After Emmons.) 



in which it lies, as shown in Figs. 25, 26, and 27. (2) Fissure 

 deposits, in which the ore conforms to the fissures and joints of 

 the strata which may cross the sedimentaries at any angle. 

 The former type embraces the most important iron and man- 



FIG. 27. Geological section in the manganese region of north Arkansas. 

 The block bands represent beds of manganese that were deposited in layers 

 alternating with the accompanying rocks. 



ganese formations, while the latter includes the silver, lead and 

 zinc deposits. 



There are three ways of recognizing a metasomatic ore body: 



(1) The absence of symmetrical banding of its vein material. 



(2) The absence of breccias cemented by gangue minerals. (3) 

 Lack of definition between the country rock and the ore body. 



