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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Document type:: Monograph

Structure type:: Chapter

Title:: Part I. Introduction

Collection:: Economics Books

2 ECONOMIC GEOLOGY Rhodesia, has replaced (77. LM.E., xxxi, 1906, p. 85) ilmenite. Although the ferro-magnesian minerals probably contain small amounts of other metals besides iron and manganese, it is only where igneous rocks have undergone strong secondary changes that the less common metals occur in workable quantities, These oresare secondary constituents of the rock. Dr. A. Brammall gives reference to literature in favour of the primary occurrence of gold in igneous rocks; but in the two cases he describes the gold is found with vein-quartz and such secondary. minerals as tourmaline (Miner. Mag., xxi, 1926, pp. 15-16). ORE FORMATION IN RELATION To IGNEOUS Rocks—The discovery of many metals in igneous rocks and of these rocks in most important ore-fields led to the lateral secretion theory giving place to the view that the metals in most lodes are derived from particles present as original constitu- ents of igneous rocks. This dependence of ores upon igneous rocks was widely adopted after a paper by the late J. F. Kemp on *“ The Role of Igneous Rocks in the Formation of Veins” (Tr. Amer. {.M.E., xxxi, 1901, pp. 169-08) which is one of the classics of mining geology. Igneous rocks are undoubtedly the source of the iron and manganese in many ores. The theory was extended to the ordinary lode metals— zold, platinum, silver, copper, tin, lead, zinc, nickel, cobalt, stc., of which igneous rocks are either barren, or contain only minute traces that may be secondary. There is no a priori reason why copper should not be a primary con- stituent of ferro-magnesian minerals; but as among igneous rocks it is most often found in diabase, using that term in its English sense, it occurs in altered rather than in fresh rock. Gold is found in diorite and porphyrite, where the rock has been altered to propylite or by the development of chlorite. That unaltered igneous rocks are barren of the ordinary metals is indicated by the vast areas of those rocks that contain no lodes. The interior of granite masses are generally barren in all parts of the world. Scotland includes igneous tocks of all kinds and ages, and they have been exceptionally closely examined. Quartz-veins in them are innumerable ; but lodes are scarce, and the most important in Scotland are not in the vicinity of great igneous intrusions ; thus the lodes at Wanlockhead are in sediments and the few igneous

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