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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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part II. Ore deposits

Collection:: Economics Books

2 lo8 ECONOMIC GEOLOGY The pre-Palzozoic ore deposits of Northern Rhodesia in- clude at the Rhodesian Broken Hill Mine a large body of complex lead-zinc ore deposited at the intersection of two fissures traversing dolomite and dolomitic schists. There are no adjacent igneous rocks, and the ore must be due to solutions rising from below ; like many pre-Palzozoic ores their composition is varied, including lead, zine, silver, vanadium, arsenic, and phosphorus. The bulk of the ore was deposited by replacement in limestone, and that now being mined is a mass of secondary carbonates, sulphates of lead, and silicate of zinc due to descending solutions. The mine was discovered in 1902, but the difficulties in treating its complex ore and the distance of 1330 miles from its port at Beira delayed development until after 1916. The ores will doubtless pass downward into primary sulphides, which have already been found in bore holes at depths of between 150 and 400 feet. SEDIMENTARY OrES—Lead minerals being soft and soluble are comparatively rare in alluvial deposits, but bedded ores of sedimentary origin are found at Commern, N. of the Eifel in Western Germany. A bed of white Lower Trias sandstone contains concretions up to a quarter of an inch in diameter, of quartz grains cemented by galena or cerussite; the richer ore yielded 2 per cent. of lead, but was profitably mined as the concretions were easily concentrated. The lead was prob- ably present in small alluvial grains that have been dissolved and redeposited as concretions. A conglomerate with a cement of pyrite and galena in the Upper Trias at St. Sebas- tian in the Department of Gard in France, probably also obtained its lead from alluvial grains; its galena cement is analogous to the copper cement in the conglomerates of Michisan. SOURCE OF THE LEAD IN Loprs The lead ores illustrate the independence of lodes of the country rock. In many areas as in Great Britain, the Linares region of Spain, and the Rocky Mountains, the ordin- ary lodes of lead are strikingly similar in composition and essential features whether they occur in granite, slate, sand- LS. J. Speak, Mig. Mag., xxi, 1919, pp. 203-9.

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