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

44 ECONOMIC GEOLOGY and regarded the hot water as derived from the syenite ; but the pneumatolytic minerals are very exceptional, and the water probably acted at a lower temperature. The explanation which on a visit to the field in 1907 appeared to the author best to explain the facts is that the ore was de- posited as a surface sheet of hematite by water that perco- lated through the syenite-porphyry, and that the sheet of ore had been thickened on the under side by secondary replace- ment, probably when the apatite veins and occasional tongues of ore were deposited in the quartz-porphyry. This explanation is consistent with the gradual passage of the ore into the syenite-porphyry, with the sharp separation from the overlying quartz-porphyry, with the phosphoric and practi- cally non-titaniferous nature of the ore, and with the occur- rence of the pebbles of the lode in the base of the overlying quartz-porphyry. Superficial sheets of ore, especially those of organic origin (cf. p. 145), are characterized by a high proportion of phos- phorus just as the igneous ores are characterized by their richness in titanium. The bulk of the Kiruna ores is esti- mated to contain from 1} to 2 per cent. of phosphorus, and this argument for its organic origin has been emphasized by W. H. Herdsman. Deposits of such antiquity in an area which has undergone such changes and earth-movements naturally show marked differences from analogous modern deposits. The apatite has been dissolved and redeposited in streaks in the ore and as veins and replacements in the quartz-porphyry. At the same time some iron has been dissolved and redeposited in places as tongues protruding into the quartz-porphyry. These have been described by Per Geijer as dykes, but that explanation is inconsistent with the pebbles, of which the evidence seems the more weighty. Tur BEDDED IRONSTONES—AQUEOUS PRECIPITATES— About 80 per cent. of the known supplies of iron ore are inter- bedded in sedimentary rocks. They include the British Jurassic ores which have played a leading part in the dis- cussion on these ores owing to their accessibility, early economic importance and variety; the Liassic minette of Lorraine, which is the largest reserve of iron ore in Europe ; the Clinton oolitic hamatites of Ordovician age in Alabama, which occur in a basin about 50 miles by 10 miles in area

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