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 I. Introduction

Collection:: Economics Books

THE FORMATION OF DEPOSITS 19 rocks are older than the ores. Some of the most extensive reas of igneous rocks, such as the Deccan, Equatorial Africa, Iceland, the volcanic islands of the Atlantic, are Practically free from lodes, Where, however, igneous rocks are traversed by deep-seated tension faults, as in the Rocky Mountains, they contain important ore-fields. } The distribution of ores in igneous rocks is usually indepen- dent of the nature of the rock. Different parts of one igneous tock may have different ores, as at the Butte copper field, Montana. Special rocks have, it is true, been regarded as attended by particular ores, such as norite by nickel; but various igneous rocks have been called norite apparently because nickel occurs with them. Granodiorite was regarded as the plutonic rock most intimately connected with gold, before it was realized that the granite of the petrographer is relatively scarce. Platinum is often found with serpentine ; but only a most Optimistic prospector would expect platinum wherever serpentine occurs, and as serpentine is an altered rock the platinum oe may have been formed during the secondary changes. Many ore-fields have no igneous rocks yet the gold-quartz of their lodes, as at Warrandyte in Vic- toria, may be indistinguishable from that of a lode beside a dyke. "As ores of different kinds exist in one igneous rock, and ores of the same kind in different igneous rocks, the source of most ores is outside the rock in which they occur. ~ Tur ORE-2ONE—The source of the ores appears to lie IN a zone deeper than that of the ordinary igneous rocks (Gregory, “Ore Deposits and Distribution in Depth,” Ir, R.Inst., 1906, p. 9). The most certain fact about the in- terior of the earth is its high specific gravity which is probably due to its large proportion of metals. If the specific gravity of the earth increased evenly from the surface to the centre, the rise in specific gravity would be so slow that rock heavy With meta] would not occur sufficiently near the surface to feed the lodes, But the innermost core of the earth (cf. p. 16) is probably lighter than the nickel-iron around it, and the barysphere is doubtless surrounded by an ore-zone composed of mixed silicates and metallic minerals (Fig, 8). The surface of the barysphere is probably irregular and Peaks rise from it into the lithosphere and upraise the over- lying ore-zone to a level at which they feed the lodes,

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