368 R. H. Rastall — The Genesis of Tungsten Ores. 



respect both, wolfram and scheelite behave like stream-tin, gold, and 

 platinum. In fact, the properties of wolfram are so like those of 

 cassiterite tliat their separation by mechanical processes is very 

 difficult, and it was not till the introduction of magnetic separation 

 that this difficulty was overcome. 



Geologically the secondary tungsten deposits are so simple and 

 straightforward that it seems unnecessary to describe any individual 

 examples, while 'the practical details of their exploitation, con- 

 centration, and after-treatment do not fall within the scope of this 

 paper. Essentially they consist mainly of breccias, gravels, and 

 sands, formed to a large extent by water-action, and occasionally 

 resulting from the effect of dry denudation in regions of small 

 rainfall. The briefest possible reference may also be made to the 

 '■'■ Head" of Bodmin Moor and other parts of Cornwall, so admirably 

 described by Mr. Barrow.^ In this connexion it may also be 

 mentioned that of late years it has been found profitable in many 

 instances to work over the old tin-dumps for wolfram, which was 

 thrown away as worthless by the earlier miners. 



Part V ; Conclusions. 



In the foregoing pages an attempt has been made to give a general 

 account of the mode of occurrence and mineral paragenesis of the 

 tungsten ores. Attention has been paid chiefly to the theoretical 

 side of the subject, Avith a view to elucidating as far as is possible 

 the genesis of the ores and their relation to the associated minerals 

 and rocks. 



Taking first the wolframite deposits, it is found that these occur 

 most commonly along with cassiterite ; other minerals also accom- 

 panying them in nearly all cases are arsenopyrite and molybdenite. 

 The gangue minerals nearly always include some that are char- 

 acteristic of the pegmatitic or pneumatolytic dykes or veins. 

 Furthermore, it appears that this general association of tungsten, 

 tin, molybdenum, and arsenic may be further subdivided on the basis 

 of the rarer metallic elements present into subtypes or local metallo- 

 genetic provinces, such as the uranium province of Cornwall, the 

 tantalum-raobinm provinces of Burma and Dakota, and so forth. 

 Another major subdivision is afforded by the tungsten-tin-silver- 

 germanium group of Bolivia. From the evidence brought forward it 

 may be regarded as established that the tungsten-tin deposits are 

 derived in all cases from granitic magmas. Wolfram and cassiterite 

 are found as original minerals in granite, being direct products of 

 the crystallization of the magma; as constituents of pegmatite dykes 

 within and in direct continuity with the granite, while wolfram is 

 also found in quartz veins, so-called, which are continuations in 

 space of pegmatite dykes. Hence there is no real distinction 

 between a pegmatite dyke and a quartz vein, and the study of these 

 lodes lends strong support to the idea that many of the larger quartz 

 veins are in fact formed by crystallization of the last residues of 

 an acid magma. The separation of these residues is, of course, 



1 Quart. Journ. Geol. See, vol. Ixiv, p. 384, 1908. 



