38 



ECONOMIC GEOLOGY 



Metasomatic Iron Ores. The ores of iron that owe their origin 

 to chemical action through solution and precipitation are the 

 carbonate (siderite), the oxide (hematite) and the hydrous oxide 

 (limonite). (See Fig. 28.) 



The source of the iron is generally some neighboring intrusive 

 or pyritiferous sedimentary. The iron is dissolved by percolating 

 waters. The solutions may find their way into strata capable of 

 effecting the chemical change suggested, or find their way into 



FIG. 28. Section to illustrate the usual occurrence of iron ore deposits 

 in the Mesabi range. It is below the glacial drift and resting upon quartz- 

 ite. (After Winchell.) 



standing waters where the iron can be extracted and deposited 

 by metasomatic action. 



Pyrite will oxidize readily to melanterite, FeSO 4 ,7H 2 O. Solu- 

 tions bearing calcium carbonate will bring about the reaction 

 through which the iron would be precipitated as the carbonate 

 and the calcium sulphate will be precipitated as gypsum, CaS0 4 ,- 

 2H 2 0, or transported elsewhere. 



FeSO 4 + CaC0 3 = FeC0 3 + CaSO 4 . 



If the percolating waters bear chlorine, the chloride of iron, 

 Fe 2 Cl6, will be formed and calcium carbonate acting upon the 



