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

6 ECONOMIC GEOLOGY liquid through the action of a pre-existing solvent: a molten material is liquid owing to high temperature. The problem is therefore whether the ore-forming solutions are given off from some deep layer within the earth as water vapour, which, after its condensation, dissolves metallic constituents from the ore-zone; or whether ore solutions are given off Sy the molten rock as a dense fluid composed of silicates and silica, and containing water and volatile constituents, Whether the ore-forming solution began as water or as residual molten matter squeezed out of the consolidating igneous rock is necessarily uncertain. The decision depends on the general balance of evidence given by the primary deep-seated ores; and the author feels that the ores due to hydrothermal action are more abundant and important than those formed as intrusive dykes of molten magma. Igneous ores were defined by Kemp as * excessively basic development of fused and cooling magmas” (Ore Deposits U.S., 1900, p. 59), but such ores are few and, except for chromite, are commercially unimportant. The aqueous ores include all deposits from ordinary solutions (exclusive, i.e. of solid solutions and molten magmas), and as most of these ores are due to the cooling of hot solutions, they are mainly hydrothermal. Lope Deposirion—HyparogeNEsIs, PropyrITIzATION, AND PNEUMATOLYSIS—The chief metallic lodes are deposited along great fault fissures ; and the materials depend primarily on the solutions that flow through the fissures.” The simplest process (preumato-hydrolysis or hydatogenesis) is the action of superheated water, which attacks the felspars, removes alkalis and lime, and re-deposits the other constituents as grains of quartz, with often some secondary felspar and such minerals as zoisite. The pyroxenes and amphiboles are broken up into granular mixtures of epidote, zoisite, and chlorite. The resultant rock under microscopic examination is a fine-grained granular mosaic in which no trace of the original structure is left; but the outlines of the original crystals and the structure of the rock may often be recognized in hand specimens and in examination under the micro- scope in ordinary light. Igneous rocks thus changed have an unusual lustre and were described by von Richthofen as bropylite ; the change was explained by Judd (0.5.6.5.

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