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

16 ECONOMIC GEOLOGY STAKE, ALASKA, AND Porcurine—In some goldfields the solutions, in the absence of fissures, percolated through irregular pores and spaces, and have thus produced impreg- nations of gold ore, usually with pyrite. These ores are usually low grade and have to be treated in bulk. Such impregnations occur in Rhodesia (Tv. 1.M.E., 1906, XXXI, P- 87) as mineralized schists along faults in the Gaika Mine, in diorite dykes in the Ayrshire Mine, in crushed granite, and in a complex of quartz-veins in a shattered mass of Banded Ironstones at the Wanderer Mine. These impregna- tions vary from rock with a sprinkling of auriferous pyrite to ore-bodies which have completely replaced the country rock. The Homestake Mine at Lead in the Black Hills of South Dakota is famous for its vast bodies of low-grade ore in pre-Paleozoic rocks that were laid down as sandstones, dolomitic limestones, and clays, and have been altered by regional metamorphism into gneiss, garnetiferous-mica- schists, cummingtonite {an amphibole) and chloritic schists, quartzite, and crystalline limestone, The schists were bent into crowded and overturned folds. In the Keewenawan Period the area was intruded by amphibolite dykes, and worn down to a plain which was covered unconformably by Cam- brian quartzites. The field was invaded by Eocene rhyolites and phonolites ; a dyke of rhyolite was forced along the crest of the Homestake anticline, with minor dykes in all directions, Later the ground was again disturbed by earth-movements which fractured these dykes and formed along them layers of pug. The ore-bodies occur in the Homestake Formation, which consisted originally of magnesian limestone. The schists near the Eocene rhyolites are traversed by many “verticals,” or thin seams of pyrites, pyrrhotite, and quartz. According to J..D, Irving, the ores are pre-Cambrian, and provided the placer gold in the neighbouring Cambrian conglomerate. According to Hosted and Wright (Eng. and Min. Fourn. Press, cxv, 1923, pp. 793 and 842) the ores are of Eocene age, and the gold in the Cambrian conglomerates is not alluvial, but was introduced by infiltration, The pre-Cambrian age of the Homestake ore is maintained by S. Paige (U.S.G.S. Bull., 765, 1924, p. 42), who supports Irving's view and considers that the Kainozoic gold. some of

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