68 GEOLOGY 



ORE-DEPOSITS 



Ore-deposits are but a special result of the processes already 

 discussed. They have peculiar interest because of their industrial 

 value. An ore is a rock that contains a metal that can be profitably 

 extracted, though for convenience the term is used more broadly 

 to include unworkable lean bodies of ore material. The metal need 

 not preponderate or form any fixed percentage of the whole, for 

 the criterion is solely economic. A gold ore rarely contains more 

 than a very small fraction of one per cent of the precious metal, 

 while high-grade iron ore yields sixty-odd per cent of the metal. 

 In iron ore, the metallic oxide or carbonate makes up nearly the 

 whole rock; in gold ore, the metal is the merest incidental con- 

 stituent from the petrologic point of view. 



All the metals are disseminated through the rock substance of 

 the earth and even throughout the hydrosphere, but they become 

 ores only when concentrated in accessible places to a workable 

 richness. Concentration of the metal is therefore the essential 

 fact in the formation of ores. The degree of concentration required 

 is measured by the value of the metal. The essential elements for 

 consideration are, therefore, (1) the original distribution of the 

 metallic materials through the rocks, (2) their solution by circu- 

 lating waters (or, rarely, by other means), (3) their transportation 

 in solution to the place of deposit, (4) their precipitation in con- 

 centrated form, and (5) perhaps their further concentration and 

 purification by subsequent processes. 



The few cases w r here ore-deposits are made by volcanic fumes 

 or vapors may be neglected here, and only the more common 

 phases of ore-deposition will be considered. 



Original distribution of material. The original distribution 

 of ore material through the primitive rocks is beyond the ken of 

 present science, for even the nature of the true primitive rocks is 

 unknown. For present purposes it is sufficient to regard all rocks 

 concerned in ore-deposition as either igneous or sedimentary, and 

 to inquire, first, how far ordinary igneous and sedimentary 

 processes contribute to the segregation of ore material, and second, 

 what the subsequent processes of local concentration are. 



