The Elements of economic geology

Bibliographic data

Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part II. Ore deposits

Collection:: Economics Books

ORES OF IRON 145 and are estimated to amount to 5000 million tons of ore with from 50 to 55 per cent. of iron, and from -2 to +7 per cent. of phosphorus; and the Wabana ores of Bell Island, New- foundland, which are also Ordovician, and their five beds, varying from § to 20 feet in thickness, along a three-mile outcrop, contain, according to A. O. Hayes (G.S. Canada, Mem. 78, 1915), between 2000 and 3000 million tons of ore with about 53 per cent. of iron and ‘85 per cent. of phosphorus and 7000 million tons of lower grade ore. These bedded ores have proved one of the puzzles of sedi- mentary petrology. The banded ironstones, which consist of thin layers of quartz and hematite or magnetite, in the pre-Palzozoic rocks of India, Western Australia, and Africa, and the Itabirites of Brazil, are probably altered representa- tives of this type. The bedded ores are generally associated with iron sili- cates including glauconite, and also chamosite and thuringite which are chloritic hydrosilicates of alumina and iron oxide (FeO). Some of the oolitic grains contain a skeleton of silica that indicates their formation from silicates, and conversion ‘nto ore during the formation of the deposit. These bedded ironstones have been generally formed under shallow water marine conditions. The Clinton and Wabana ores contain marine shells and Bryozoa replaced by Hematite, and the mud-cracks and ripple-marks show that the deposits were Occasionally above water; the deposition may have been under lagoon conditions. Three chief explanations have been offered of these ores. According to the first, the metaso- matic theory (Sorby, Hudleston, and later Hatch and Rastall, Petrol. Sed. Rocks, 1913, pp. 210-12), they are limestones altered to carbonate of iron, and perhaps later to oxide. The second explanation of some of the ore was by saturation and partial replacement of sandstone by carbonate of iron (Judd). These conclusions rested mainly on the oolitic grains and marine shells which were formed as carbonate of lime. These explanations are inadequate as many of the oolitic grains had been altered into hematite before they had reached their present position, and their matrix shows no sign of alteration by infiltration. The third theory is that the ore was formed by precipita- tion on to the sea floor of iron carbonate (which occurs in the Ie

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