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 I. Introduction

Collection:: Economics Books

24 ECONOMIC GEOLOGY superseded, and the ores are attributed to water given off from the interior of the earth. This water is generally described as magmatic, but that term is ambiguous and is used for ore deposits with three different meanings. Harker (Vat. Hist. Igneous Rocks, 1909, p. 31) defines a magma as “molten rock material.” “The original molten rock matter is conveniently termed magma,” says Tyrrell (Prin- ciples Petrol., 1926, p. 46); Lindgren (Tr. Amer. I.M.E., Ixxiv, 1927, p. 77) gives a definition in the language of physical chemistry which means the same, as he remarks {1bid., p. 76), magma is *‘ the material from which all our igneous rocks have consolidated.” In accordance with this petrographic use of the term many ores are described as magmatic, being attributed to the direct consolidation of molten rock material. R. H. Rastall (Geol. Met. Dep., 1923, P. 109) entitles one group of ores “ magmatic segregations ; ores actually separated from igneous magmas by crystal lization ; ” and W. Lindgren devotes a chapter (Min Dep., 1013, pp. 735-72) to ‘‘ Mineral Deposits formed by Concen- tration in Molten Magmas.” The magnetites of Taberg, of Lapland, and the Adirondacks (cf. p. 140), the nickel ores of Sudbury and South Africa (cf. pp. 114-19), and the tin of South Africa (Recknagel. T7. G. Soc. S. Afr., xii, 1910, p. 128), have been represented as magmatic in this sense of the term ; but in the account of the ores in the following chapters it is claimed that the ores thus formed are few and of little present commercial importance. The second set of ores called magmatic are due to water of magmatic origin. This use of the term was begun by J.H.L. Vogt (Z. prakt. G., 1804, p. 381), who extended it to emanations that are given off after the consolidation of the molten material and are discharged through hot springs, fumaroles, and solfataras. Many authors have followed Vogt’s lead and accepted as magmatic all ores due to water of magmatic origin; thus W. H. Goodchild (Ming. Mag. 1918, xviii and xix) includes even the Banket of the Rand, and truly says ‘they are ores of endless variety " (ibid. xviii, p. 135). According to this usage many beds of tufa, L Cf. Gregory, ©“ Magmatic Ores,” 7¥, Faraday Soc., xx, 1925, pp. 149-38.

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