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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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part I. Introduction

Collection:: Economics Books

29 ECONOMIC GEOLOGY mountain folding. The material of the ore-zone and bary- sphere may be regarded as igneous in the sense that they are holocrystalline and are formed of minerals that consolidated under high temperatures and pressures ; but metallic ores do not appear to be primary constituents of the ordinary igneous rocks of the earth's crust. Even the deepest seated plutonic rocks have received the metals in their lodes—except such as iron, manganese, and aluminium— rom an underlying layer. PLuTtonic, JUVENILE, AND MAGMATIC WaTERS—The normal influence of igneous rocks in ore formation is indirect. Their intrusion produces fractures and fissures along which solu- tions can pass, and they raise the temperature so that the gas pressure forces the water upward. The main intro- duction of metals into the upper layers in the lithosphere, in addition to its normal constituents, is by the ascent of deep-seated water. This water is known as plutonic from its deep origin, or as magmatic after its derivation from igneous magmas, or as Juvenile (Suess) since it is making its first appearance on the earth's surface. A large quantity of this water is constantly arising from the interior (cf. Chap. XIX, Pp. 220-2). It must begin its ascent through minute spaces. The liquid cavities in quartz often occur in lines passing from one crystal to the next, showing that the entrance of the liquid was after the consolidation of the quartz. This movement must be very slow. At depths of 6 to 12 miles the temperature is probably above 687° F,, the critical point of water, which at it can exist only as steam. Below 30 or 40 miles, the temperature would be above 3600°F., and water would be dissociated into its constituent gases, which could combine only in a zone cooler than their dissociation point. Water after its formation would gradually work its way upward into a zone where fissures and cracks would enable it to ascend more freely, That fractures happen at great depths in the crust is shown by the evidence of earthquakes which result from deep- seated disruptions. Ruptured rock surfaces at great depths would remain in close contact, but not too close for the passage of films of superheated water. Ruptures must occur below the level where plutonic water begins its ascent in innumerable tiny trickles through thin spaces or scattered pores. This

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