474 GEOLOGY 



ORE-DEPOSITS.^ 



Ore-deposits are but a special phase of the rock-forming processes 

 already discussed. They have pecuUar interest because of their indus- 

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

 profitably extracted, though for convenience the term is used more 

 broadly to include unworkable lean ores and ore material. The metal 

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

 the criterion is solely economic and not petrologic. A gold ore rarely 

 contains more than a very small fraction of one percent, of the precious 

 metal, while high-grade iron ore yields sixty-odd percent, 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 constituent, from 

 the petrologic point of view. 



Concentration. — The essential fact in the formation of ores is the 

 unusual concentration of the metal. There are vast quantities of all 

 the metals disseminated through the rock substance of the earth and 

 even throughout the hydrosphere, but they do not constitute ores 

 because they have no economic value. They become ores when con- 

 centrated in accessible places to a workable richness. 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 metaUic materials through the rocks, (2) their 

 solution by circulating waters (or, rarely, by other means), (3) their 

 transportation in solution to the place of deposit, (4) their precipita- 

 tion in concentrated form, and (5) perhaps their further concentration 

 and purification by subsequent processes. 



Exceptional and doubtful cases. — There are a few cases where ore- 

 deposits are made by volcanic fumes or vapors, but these may be neg- 

 lected here. Formerly, ores were often attributed to vapors supposed 

 to arise from the hot interior, but this mode of origin seems incompatible 

 with physical conditions. Ores have been attributed to water origi- 

 nally contained as steam in lavas, and to waters escaping from the interior 



A comprehensive discussion of the ''Genesis of Ore Deposits" may be found in 

 Vols. XXIII and XXIV of the Trans, of the Am. Inst, of Min. Eng. (also printed 

 with additions in book form by the Institute, 1902), in which Posepny, Emmons, 

 Van Hise, LeConte, Blake, Becker, Ricard, Raymond, Lindgren, Weed, Vogt, Wins- 

 low, Winchell (H. V.), Church, Cazin, Adams, Keyes, Bain, CoUins, Beck, and 

 DeLaunay participated. Various phases of the leading modern views are set forth. 



