B. H. Rastall — The Genesis of Tungsten Ores. 195 



hiibnerite, MnWO^. These two molecules may mix in any pro- 

 portion. The varieties rich in manganese are perhaps the more 

 common. Since every possible gradation of composition is known, 

 it has been proposed by Messrs. Hess and Schaller ^ that arbitrary 

 divisions shall be made as follows : all varieties with not more than 

 20 per cent of the manganese molecule are called ferberite and those 

 with not more than 20 per cent of the iron molecule hiibnerite, 

 while intermediate varieties are called wolframite. In the following 

 pages, however, the names wolfram or wolframite are used in the 

 miner's sense to include all varieties of iron and manganese 

 tungstates. 



The tetragonal tungstates include scheelite, CaWO^, and stolzite, 

 PbWOj.. Of these the former is by far the more common and 

 important. A new mineral from Northern Queensland, chillagite, is 

 of interest, since it is an isomorphous mixture of the stolzite 

 molecule, PbWO^, and the wulfenite molecule, PbMoO^, thus 

 emphasizing the paragenetic connexion which undoubtedly exists 

 between tungsten and molybdenum. The close mineralogical 

 association between these two elements will appear later. Some 

 varieties of both wolframite and scheelite contain a little copper, but 

 this does not seem to be of much significance. 



At this point it becomes necessary as a matter of convenience to 

 anticipate somewhat and to state that the tungsten deposits can be 

 most satisfactorily described under four headings, as follows : — 



(a) Primary wolframite ores with cassiterite. 



(b) Primary wolframite ores without cassiterite. 



(c) Primary scheelite ores. 



{d) Secondary (detrital) tungsten deposits. 

 The justification for this classification will appear in later sections of 

 this paper. 



Paet I: Wolframite Oees with Cassiterite. 



The tungsten ore deposits of the British Isles have lately been 

 described very fully in a special memoir of the Geological Survey, - 

 hence it is unnecessary to enter into a large amount of detail 

 concerning them. In Cornwall the principal ore is wolframite, in 

 Cumberland scheelite. The primary wolframite ores of Cornwall are 

 of great interest from the theoretical point of view, since they afford 

 an admirable example of the wolframite-cassiterite association. The 

 primary ores of Cornwall are clearly of Avhat is generally known as 

 pneumatolytic origin, being associated with other minerals and rock- 

 types characteristic of this phase of igneous action. It will, 

 however, be pointed out in the general discussion of the genesis of 

 these ores in a later section that the term pneumatolysis has been 

 somewhat overworked in this connexion, and as commonly employed 

 it really comprises two quite distinct classes of phenomena, or rather 

 phases of igneous activity. 



1 " Colorado Ferberite and the Wolframite Series " : Bull. 583, U.S. Geol. 

 Surv., 1914, p. 37. 



^ Special Reports on the Mineral Besources of Great Britain, vol. i : 

 Tungsten and Mqnganese Ores (Mem. Geol. Surv.), 1915. 



