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

40 ECONOMIC GEOLOGY sharply bounded by a fault plane with an impermeable band of clay. The hanging wall was an irregular ill-defined passage into gneiss (Fig. 44). The ore included masses of gneiss in their original positions, showing that the ore had been formed around them by replacement. The Gellivaara ores belong to the Lake Superior type, for they are due to descending water which had become charged with iron—the abundant biotite in the gneiss is the obvious source—and deposited it in masses where further descent was blocked by an im- permeable layer. The ore is a valuable non-titaniferous magnetite. In the Adirondack Mountains is another occurrence of non-titaniferous magnetite in pre-Paleozoic gneiss, schists, limestones, and plutonic rocks, which has been regarded as an igneous segregation. The ore is in lenses or pod-shaped F16. 44.—IroN ORE Deposits aT GELLIVAARA, The ore is in biotite-gneiss resting on a sheet of pug formed by a fault (F); the hanging wall is irregular; blocks of gneiss occur in the ore, in situ. bodies bent in harmony with the folds in the country rocks. The association of the magnetite with quartz, fluorite, apatite, and pegmatite, and its occurrence in any of the local rocks except the basic igneous rocks—in which igneous ores would be most likely—render probable a hydrothermal origin, as advocated by the author in 1924 (Trans. Faraday Soc., xx, P. 454) and by H. J. Alling (Econ. Geol., xx, 1925, pp. 335-63), who has shown that the magnetite is of three distinct ages, that some of it, as in the granite (see Fig. 45), is of late origin, and has replaced quartz and felspar. He concludes (ibid., p. 363) that these ores are * magmatic-replacement deposits due to aqueo-igneous magnetite-rich solutions de- rived from a differentiating granitic magma." Middle Sweden—The iron-fields of middle Sweden, where Swedish iron and metallurgists earned their high reputation, are in metamorphosed pre-Palzozoic rocks. The rocks and ore are traversed by granite-pegmatite dykes which are also

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