The Elements of economic geology

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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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Document type:: Monograph

Structure type:: Chapter

Title:: Part II. Ore deposits

Collection:: Economics Books

142 ECONOMIC GEOLOGY The difficulty of determining the nature of these ores is increased by the extreme alteration of the country rocks. The general relation of the ores indicates that they are re- placement deposits formed under varying but comparatively superficial conditions. The simplest type is ore in lime- stone or dolomite, remote from igneous rock; the form of these ore-bodies recalls that of the replacement ore in the limestones of the N.-W. of England. In a second and common type the ores lie in hollows with an impermeable base, like those in the pre-Pal®ozoic rocks of Lake Superior, due to the stoppage of descending solutions. The ore of the third type is in wide sheets conformable with the country and resting on leptite or halleflinta; this ore may have been deposited on the surface of the igneous rock as a ferruginous crust; the iron was probably leached from underlying rocks, and precipitated on the surface, and may have been supple- mented by bog iron ore deposited in pools and enlarged by solutions during the metamorphism of the area and the intrusion of the granite dykes. The non-titaniferous nature of the ores is opposed to their igneous origin. Their chemical composition and dis- tribution are consistent with the formation of some by the processes which deposited the iron ores of Lake Superior and the NW. of England, and of others by the processes which form iron crusts in arid lands. 3. ANCIENT SURFACE SHEETS, KiIRUNA—The rich magnetite mines at Kirunavaara in Swedish Lapland, despite the re- mote position N. of the Arctic Circle (68° N., 20° E., a little S. of Lake Tornea), are of commercial importance from the high grade of the ore and the easy mining, and are of special geological interest from the widespread belief in their ig- neous formation. Léfstrand in 1891 suggested that the magnetite was a segregation, as at Taberg. That view be- came untenable when it was found that the rocks on the two sides of the ore differ in age and character. The foot- wall is syenite-porphyry of which the upper part shows volcanic structures. Above the ore lie quartz-porphyry lavas interbedded with tuffs and sediments. The igneous origin of the ore has been re-introduced in three forms, According to R. A. Daly (1915), the magnetite solidified in the quartz-porphyry and sank to its base. According to O. Stutzer (New. Fahrb., xxiv, 1907) the ore is an iron oxide

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