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 II. Ore deposits

Collection:: Economics Books

ORES OF LEAD, ZINC, AND SILVER 97 sandstone are barren in slate, which being softer may be crushed and not form an open ‘fissure ; ‘the conditions are reversed where the sandstone resisted fracture, and the slate is fissured. In some fields, as at Przibram in Czecho-Slovakia, the lodes are productive both in sandstone and slate. The usual veinstones are quartz, calcite, barite, and fluorite. Fluorite often occurs only in the upper levels, as superheated Steam prevents its formation. The pneumatolytic minerals tourmaline and cassiterite are exceptional; but boric acid has in places formed axinite. The deep-seated origin of the primary lodes is shown by their lead being of a higher atomic weight and specific gravity than the uranium lead of igneous rocks. Primary and secondary lead oresare often closely associated. In secondary ores the zinc and lead are usually separated Owing to their different solubilities. The secondary ores are often banded or radial with large crystals, whereas those In primary deposits are usually small and granular. The Primary lodes often go deep; they have been worked, for example, down to 1900 feet in the Isle of Man, to 1800 feet In Cornwall, to 1700 feet in Shropshire, to 1800 feet at Wan- lockhead, to 2500 feet at the Ceeur d'Alene in Idaho, to 3000 feet in Clausthal, and to 3600 feet at Przibram. The lodes are usually a few feet thick, but are often widened by replacement of the walls. Igneous rocks are absent from most lead-fields, and if present, they appear to have had no Influence on the mineralization; in Derbyshire they are known ag toadstone, a corruption of the German todstein or deadstone, which indicates their barrenness and even nfavourable effect on the lodes. Primary lead lodes are mostly of medium geological age. They have been formed at lower temperatures and nearer the surface than lodes of copper and tin, but deeper than ores of mercury. Lead lodes occur in pre-Palzozoic rocks, at Broken Hill in Australia, the Northampton field in Western Australia, Northern Rhodesia, British Columbia, Eastern Canada, the Eastern United States, and in Scotland ; but the lodes may be much younger than the country rock. Some of the ores are associated with Kainozoic volcanic activity, as at Hauraki in New Zealand. The great ma- Jority of primary lead ores were formed during the ~

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