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

ORES OF MANGANESE AND CHROMIUM 131 with so many veins of chromite that it has a stratified aspect. The adjacent serpentine is intensely broken and slicken- sided. The ore band yields about 7 per cent. of chromic oxide, but nodular expansions of the veins contain 50 per cent. Some veins of an acid rock with vesuvianite project from the ore into the serpentine. The ore was formed after the consolidation of the peridotite of which the ser- pentine is the altered representative, and the chromite veins were deposited in a band crushed by earth-movements! (Cirkel, Chrome Irom Ore, Quebec; Canada Dep. Mines, 1909). Chromite occurs in Beluchistan, Mysore, etc., also in ser- pentine in veins which are presumably later than the rock. In California, at Little Castle Creek, the chrome ore is in a serpentine formed from pyroxenite; the lower part of the rock is too poor to be mined, and the ore is just below the outcrop, as if concentrated by secondary enrichment. As chromite is a primary constituent of basic rocks, its ores have been often regarded as igneous segregations. This origin appears probable for some cases. Thus in the gneiss of Maryland an intrusive peridotite, which contains no chromite but -5 per cent. of chromic oxide, is surrounded by an irregular sheath of chromite which is attributed to its concentration in the quickly cooled margin of the magma (Pratt and Lewis, N. Carolina G.S., i, 1905, pp. 370, 372). Nevertheless the ores of most economic importance, whatever may have been the original source of their chromite, are secondary ; _they are found in veins and nodules that were formed after the country had been altered to serpentine, and even after that rock had been fractured. The slicken-siding of the lenses and masses of ore shows their association with earth-move- ments, which by crushing the rock rendered possible the con- lentration of its scattered grains of chromite. The price of chromium ore is based on the percentage of chromic oxide (Cr,0,). Ore shipped from Southern Rho- desia containing 52 to 54 per cent. has been sold at the port of shipment since 1914 at from 43s. to 89s. per ton; poorer ore is used as a refractory material for lining furnaces for smelting iron. ! Dresser considers that the dislocations were later than the segre- gation of the chromite though recognizing the vesuvianite as pneuma- tolytic (G.S. Canada, Mem. 22, 1913, Pp. 74-95).

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