abstracts: geology I.V.I 



ores. One of the most significant variations with reference to the origin 



of the ore is in the relative abundance of greenalite rocks and siderite. 



The average original iron content of the iron-bearing formations, thai 



is, of the siderite and greenalite phases, exclusive of interbedded slati 

 as shown by all available analyses, is 24.8 per cent and of the ferruginous 

 cherts and jaspers, from which there has been but little leaching of silica, 

 26.33 per cent. The amphibole-magnetite phases of the formations show 

 approximately the same percentage. The average iron content of the 

 formations, as shown by all available analyses, different phases, including 

 the ores, being taken in proportion to their abundance, is 38 per cent. 

 A comparison of this figure with 24.8 per cent for the orginal siderite and 

 greenalite and 26.33 per cent for the ferruginous cherts and jaspers from 

 which silica has not been removed shows what secondary concentration 

 has accomplished. It is possible, however, that the ores have in part 

 been derived from the richer portions of the iron-bearing formations. So 

 far as this is true, the concentration has been less than these figures 

 indicate. The iron ores form but a very small part of the rocks of the 

 iron-bearing formations. 



The following theses bear on the genesis of the iron ores: (1) These 

 ores are altered parts of chemically deposited sedimentary formations 

 that originally consisted mainly of cherty iron carbonate and greenalite. 

 (2) A few of the iron-ore deposits represent originally rich layers in which 

 secondary concentration has made only minor changes. (3) In by far 

 the greater number of deposits, including all the larger ones, secondary- 

 concentration has been the essential means of enriching iron-formation 

 layers to ores. (4) The conditions of sedimentation of the iron forma- 

 tion may be roughly outlined. (5) The weathering and erosion of bed- 

 rock surfaces of average composition is inadequate as a source of the 

 materials of the iron-bearing sediments; these materials have been de- 

 rived largely from basic igneous rocks. (6) Some of the sedimentation 

 accompanied or immediately followed each introduction of pre-Cambrian 

 basic igneous rocks into the outer zone of the earth and some took place 

 later under ordinary weathering conditions. (7) The chemistry of de- 

 position under such conditions may be approximated and the original 

 phases of the sedimentary iron-bearing formations may be synthesized 

 in the laboratory. (8) The subsequent oxidation of the iron-bearing 

 formations, the transfer of iron salts, and the leaching of silica by agents 

 carried in meteoric waters have concentrated the ores in all but an in 

 nificant portion of the deposits now mined. (9) This second concen- 

 tration has been localized by structural and topographic conditions. 



