NATURE 



281 



THURSDAY, DECEMBER 13, 1917. 



GOLD-BEARING CONGLOMERATES OF 

 SOUTH AFRICA. 



The Banket: A Study of the Auriferous Con- 

 glomerates of the Witwatersrand and the Asso- 

 ciated Rocks. By Prof. R. B. Young. Pp. xv 

 + 125. (London: Gurney and Jackson, 1917.) 

 Price 8s. 6d. net. 



THE British Empire is not uniformly fortunate 

 in its natural resources of the useful and 

 precious metals. The ore-supplies of some of 

 these, e.g. copper, aluminium, platinum, and mer- 

 cury, as at present proved, are either deficient or 

 entirely wanting". In such cases partial or com- 

 plete dependence upon foreign sources is un- 

 avoidable, unless and until greater supplies are 

 discovered. In other cases, although the known 

 ore-resources are plentiful, the domestic output of 

 metal has hitherto been inadequate owing to a de- 

 plorable lack of metallurgical enterprise within 

 the Empire — raw materials, or partly smelted 

 products, of British origin being exported to 

 foreign countries and reduced to metal by foreign 

 skill and labour. Zinc, lead, nickel, and until 

 recently tungsten, may be cited as notorious ex- 

 amples, and for supplies of these metals British 

 consumers have been placed in an unwarrantable 

 {XJsition of insecurity. In a few cases, however, 

 e.g. tin and gold, not only are the ores abundant, 

 but they are smelted entirely within the Empire, 

 and furnish metal sufficient, in normal times, both 

 for Imperial consumption and for export. Cases 

 of this kind should and could be more numerous, 

 and that they will be, in the future, is already in- 

 dicated by the birth and remarkable growth of a 

 British tungsten industry since the inception of 

 the war, and by the steps that are now being 

 taken to ensure a greatly augmented Imperial 

 production of nickel, zinc, lead, and iron. 



In its gold resources the Empire is particularly 

 fortunate. Of the world's current yearly output 

 of 720,000 kilos, of new gold, 450,000 kilos., or 

 more than 62 p>er cent., are of British origin. Of 

 this amount 283,000 kilos. , or 63 per cent, of the 

 British production and 39 per cent, of the world's, 

 are derived from the Transvaal. The remarkable 

 "reefs" of conglomerate or "banket," from which 

 the gold of the Witwatersrand is obtained, possess, 

 therefore, a unique economic importance. They 

 have a considerable scientific interest also, and 

 it is with this aspect of them that Prof. Young 

 deals in the publication now under review. The 

 book summarises the results of researches which 

 have been made by many investigators during 

 the thirty years of gold-mining activity on the 

 Rand. The information it contains is already 

 familiar to many mining geologists, but has not 

 hitherto been available in the pages of a single 

 small volume, and in this conveniently epitomised 

 form it will be widely welcomed. 



The subject-matter is presented in five chapters. 

 In the first the geology of the auriferous region 

 NO. 251 1, VOL. 100] 



of South Africa is briefly described, attention 

 being confined to the Swaziland, Witwatersrand, 

 and Transvaal Systems, in which alone gold oc- 

 (urrences of economic Importance have been found. 

 The most important of these, the Witwatersrand 

 System, with its three highly productive " banket 

 reefs," is shown to be a complex of sedimentary 

 rocks, derived from the waste of the underlying 

 Swaziland System, and of igneous rocks intruded 

 into them in the form of dykes and sills. 



In the second chapter the author deals with the 

 original constituents of the gold-bearing con- 

 glomerates. Among the pebbles, vein-quartz 

 almost exclusively preponderates over quartzite, 

 quartz porphyry, schist, and other rocks. In 

 the sandy matrix also quartz is in large excess over 

 the other minerals recorded — zircon, tourmaline, 

 chromite, diamond, iridosmine, etc. — which occur 

 only in minute proportions. The remarkable 

 absence of magnetite and ilmenite grains is be- 

 lieved to be due to their replacement by pyrite 

 and other substances. The iridosmine, the genesis 

 of which has been much debated, is shown con- 

 clusively to be a detrital constituent, and, with 

 the chromite which so often accompanies it, is 

 attributed to the disintegration of ultra-basic 

 igneous rocks of the Swaziland System. It is of 

 interest to note that in modern alluvials which 

 carry this mineral detrital gold is invariably found 

 with it, and the fact that in these ancient pebble- 

 beds also gold accompanies it — although not in 

 the alluvial condition — is one that must be borne 

 in mind when the genesis of the gold in the 

 "banket" is being considered. 



In the third chapter the secondary minerals of 

 the "banket" are described. These are especially 

 numerous, and include quartz, chloritoid, chlorite, 

 sericite, tourmaline, rutile, anatase, calcite, dolo- 

 mite, carbon, pyrite, and gold. Several of them 

 are strongly suggestive of hydrothermal action, 

 and are doubtless rightly ascribed to the opera- 

 tion of magmatic waters emanating from the 

 igneous intrusions. Particularly interesting are 

 the carbon and pyrite, and the vexed question of 

 their origin is discussed at some length. The 

 rounded forms often shown by them, which have 

 caused them to be regarded as original con- 

 stituents, are referred to concretionary growth 

 and to the pseudomorphous replacement of water- 

 worn pebbles or grains. The gold never shows 

 detrital form, its boundaries being either crystal- 

 line or very irregular. Moreover, it is often so 

 intimatelv associated with certain of these un- 

 doubtedly secondary minerals, especially the two 

 just mentioned, that in its present condition, at 

 any rate, a secondary origin must be ascribed 

 to it. 



The fourth chapter deals with the sedimentary 

 and igneous rocks with which the " bankets " are 

 associated. .Secondary mineralisation is shown to 

 have affected not only the gold-l>earing con- 

 glomerates, but also the sediments in which they 

 are enclosed, and in many cases a definite con- 

 nection can be established between this mineralisa- 

 tion and the igneous intrusions. 



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