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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Document type:: Monograph

Structure type:: Chapter

Title:: Part I. Introduction

Collection:: Economics Books

1% ECONOMIC GEOLOGY The ore-forming solution may enter the rocks beside the fissures and deposit its constituents as an impregnation. The lode passes gradually into the country through a band impregnated with metallic minerals, such as the *“ capel ” beside the Cornish tin-lodes, the “ emborroscado beside the pyrite masses in Spain, and the irregular network of veinlets forming a stockwork. Still further impregnation re- places the country rock entirely by a replacement or meta- somatic ore. Such an ore-lode may fade. outward into the country, as in some Rhodesian mines, where ore rich in gold with no trace of the original rock constituents passes through ore in which the felspars can still be recognized, into country with only a slight impregnation of metallic minerals. One extreme development of replacement deposits produces the great pyritic lenticles (cf. Chap. VI) which may be hundreds of feet across, and yet include no fragment of the unaltered country as large as a walnut, though the structure of the original rock may be recognizable by the sheen on a surface of pyrites. it has been considered that ore formation is possible only within a shallow zone, as no spaces can exist where the rocks flow under the pressure. The depth of this zone is being steadily increased from Heim’s estimate in 1878 of 14 miles, and Van Hise’s of 7'4 miles, to Sir Charles Parson's (Nature, 20th October, 1904), of at least 12 miles. His view was confirmed by the experiments of F. D. Adams (Fourn. Geol., xx, 1912, pp. 115, 117) who proved that empty cavities persist in granite under the pressure of IT miles deep, and that cavities filled with water or gas would remain at a still greater depth. Ore formation by the filling of cavities may therefore take place to the depth of at least 12 miles and by replacement to indefinitely greater depths. ORE SEQUENCE IN DEPTH—Crustified deposits often show a succession of different ores and veinstones in a transverse section of the lode. An analogous vertical succession also Occurs owing to zonal variation controlled by temperature. Some lodes have been formed at high temperatures near the source of the ores and plutonic water; others have been formed under cooler conditions near the surface. No known lode includes the full vertical succession of ores. At great depths the change in temperature and pressure is very

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