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 II. Ore deposits

Collection:: Economics Books

[12 ECONOMIC GEOLOGY diabase had become cold and been fractured. At the Beaver Mine, e.g. the veins are almost in the middle of the diabase sill, so that it must have been solid, and relatively cool throughout. The association of the veins with faults is established throughout the field (cf. Whitman, California Univ. Public., xiii, 1022, pp. 263-5, 209) ; many of them have a throw of only a few feet, but they formed impermeable sheets, which blocked the drainage, like the crushed bands with the silver veins of Annaberg. The diabase was the toughest rock in the area, and when the country was folded, shearing took place along the margins of the sill producing many small compression fractures, and planes of slipping. The veins at Cobalt, as at Annaberg, in spite of their great richness therefore have a limited range in depth, as the fis- sures were formed beside horizontal or gently inclined shearing planes, and not as great vertical fractures. Some of the greater faults, such as Cobalt Lake Fault with a throw of 500 feet, must be deep-seated and may have served as channels for solutions from below. They were at first comparatively cool and deposited calcite veins, which filled any fissures whatever their inclination. Subsequently nickel and cobalt sulphides and arsenides were brought from a greater depth and were deposited mainly in the steeper fissures, because the high gas pressure forced the solutions along the most vertical course. New cracks were formed in the old veins, and native silver deposited after the sulphides, arsenides, and antimonides. According to C. R. Van Hise (¥. Can. M.1., x, 1007, p- 53), S. F. Emmons (Types of Ore Dep., 1911, p. 151), J. M. Bell (Econ. Geol., xviii, 1923, p. 604), and E. S. Bastin (Econ. Geol., xii, 1917, pp. 225-8), the rich silver ores are secondary. Some authorities hold that the silver was derived from an upward extension of the lodes; but the arguments by W. L. Whitehead (Econ. Geol., xv, 1920, pp. 127-30) against this view appear conclusive. The native silver though secondary in origin, was probably, as urged by E. S. Bastin, introduced by magmatic waters as acid sulphides during the last stage of mineralization. The deep- seated origin of the ores has been maintained by J. B. Tyrrell (Tr. I.M.E., xxxv, 1908, pp. 494-5), and by Spurr (Eng. and Min. Journ. Press, cxvi, 1923, pp. 709-12), who describes the lodes as *f veindikes.”’

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