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 GOLD 53 foot of the volcano. This ore contains about 6 dwt. per ton of gold and 2% per cent. of copper; the mine which in ts early days contained the purest of recorded native gold ended as a low-grade copper mine yielding gold as a bye- product. Section C. ALLUVIAL GOLDFIELDS Pracers—Alluvial deposits or placers (cf. p. 32) and their altered representatives are especially important in gold mining, though the same methods are applied to piers Containing tin and platinum. The first economic process 0 working low-grade placers was by hydraulic sluicing which Was developed in California. Water is impounded in a high level reservoir and brought to the alluvial deposit by a lead or trench; it falls through an iron pipe to a nozzle, which directs it against the gold-bearing material. A ‘* Giant nozzle weighing 2000 1b. can control a stream with a head of 500 feet; the water leaps from it with a velocity of some- times 2 miles a minute, and in a jet so strong that a stick may be broken across it as over a metal bar. The jet digs into the gravel, and washes the material along a chain of sluice boxes, on the floor of which the gold is caught in between ridges or “rifles,” or on canvas, or by mercury. The chain of sluice-boxes may be from 100 feet to miles in length. This process is most effective in working gravels about 80 feet thick ; if the beds are thinner, time is lost in frequent movements of the nozzle ; if the beds are thicker the material falls in unmanageable masses and for safety the nozzle has to be too far from the cliff for the jet to have its full excavating power. Thicker beds have therefore to be worked in two layers. The cost of hydraulic sluicing is low, and in Cali- fornia is from 13d. to 6d. per cubic yard. Hydraulic sluicing has been successfully used in alluvial fields. in most parts of the world. Dredging is usually the cheapest method of working a low-grade placer in river valleys. The first device was a floating timber platform, on to which gravel from a river bed was shovelled by a man standing in the water, or by 2 Spoon dredge worked from a boat. Another hand system Was to drop a bucket on to the river bed and haul it ashore. In 1882 McQueen, a New Zealand miner who has been

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