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

Usage license:: Get license information via the feedback formular.

Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part II. Ore deposits

Collection:: Economics Books

RA ECONOMIC GEOLOGY which have been intruded by an andesite-porphyry that grades into quartz-diorite. This intrusion was fractured and invaded by a superheated boric acid solution which altered the rock into a mosaic of quartz, tourmaline, sericite, and rutile, and deposited large quartz-veins containing sulphides and magnetite. While the andesitic diorite was still charged with superheated water it was invaded by dykes of alkaline dacite-porphyry. The consequent rise of temperature, perhaps combined with release of pressure owing to fracturing, exploded the superheated water and blew out a crater 3000 feet in diameter and 6000 feet deep. This crater was occupied by a lake and gradually filled by the Braden Tuff being washed into it. Solutions rose along fractures around the crater, and deposited quartz-veins with tourmaline and pyrite, and large bodies of ore containing about I per cent. of copper. The less permeable tuffs re- ceived lower-grade ores with 4 per cent. of copper. An alkaline porphyry breccia was upthrust between the tuffs and the crater wall, and tourmaline was deposited both in this breccia and the Braden Tuff. As the volcanic activity waned, the rising solutions were cooler and did not carry boric acid; they deposited bornite and other sulphides, sulpharsenites, sulphates, carbonates, and tungstates. Later still cooler solutions deposited chalcopyrite and bornite, with quartz and gypsum, which in some of the cavities grew into crystals 10 feet long. After the volcano had become wholly extinct, descending meteoric waters leached the ores from the surface to the depth of 150-300 feet and redeposited the copper as secondary enrichments of chalcocite. The Braden Mine therefore illustrates all stages from pneumato- lytic and contact ores to ordinary quartz-pyrite veins and secondary enrichments. This association of the different types is due to volcanic action being local and intense; in most mining fields the subterranean conditions vary less quickly. Lopes or Cornwarr—>Solutions that acted at less depth and at a lower temperature than those that produced the pneumatolytic ores and had silicic acid as the predominant acid, have formed the quartz-copper lodes, which were for long the chief source of copper. The lodes of Cornwall and Devon show a transition downward into pneumatolytic tin

Access restriction

Copyright

fullscreen: The Elements of economic geology

Monograph

Chapter

Table of contents

Download

Monograph

Chapter

This page

Image fragment

Citation links

Citation recommendation