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

+ Q ECONOMIC GEOLOGY Section A. PRIMARY ORES I. Gorp-QuarTz Fissurg LopEs—CALIFORNIA, BALLARAT, MysorE—The character of these lodes varies so greatly with the nature of the rock fissures that they fall into four divisions, those in older sedimentary rocks, in pre-Palzozoic gneisses and schists, in the younger volcanic rocks, and pneumatolytic lodes. The standard gold-quartz lodes are those in the Sierra Nevada of California. ~The Sierra Nevada consists of pre- Palaeozoic schists and gneiss, covered by slates, sandstones, and igneous rocks, which range in age from early Palzozoic to Lower Cretaceous. At the end of the Jurassic or begin- ning of the Cretaceous the country was invaded by massifs of granodiorite with offshoots of augite-porphyrite, diabase, and serpentine, and was uplifted by faults, beside which quartz was deposited in fissures and fractures and by re- placement of slate and limestone. Gold and gold-bearing pyrites were deposited in the quartz. The date of their intro- duction is shown by the placer gold in some conglomerates, which are at latest early Cretaceous. The quartz-veins form a series of belts, which extend 700 miles in length by from 20 to 60 miles in width. The greatest of these belts is * the Mother Lode of California,” which trends N.W. for [12 miles through Mariposa, Calaveras, and Eldorado, localities famous from the writings of Bret Harte. The “ Mother Lode” is a mineralized belt with innumerable irregular discontinuous veins, generally in slates near grano- diorite, and sometimes along the contact; but they also beeur in the granodiorite or at considerable distances from it, or in serpentine. The distribution of the gold is irregular; of two parallel adjacent veins one may be barren and the other rich. Pure white or * buck-quartz ” is generally barren. Quartz banded with thin lines of micas, such as mariposite (a potash-mica coloured by chromite), and ros- coelite (a mica with 25 per cent. of vanadium oxide) is usually richer. The other common veinstones are calcite and dolo- mite, and, in Places, barite. The metallic minerals are sulphides, chiefly pyrites, with a little galena and blende. The gold usually occurs free in pyrites, The mines have been worked to depths of about 4000 feet ;

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