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



and continuously into wolfrana lodes and these again into pure 

 quartz veins. 



When the distribution of these minerals is studied in detail it is 

 seen that the original metallic minerals of the granites and the ore- 

 minerals of the pegmatites, greisens, and quartz lodes are of common 

 genesis. They all arise as products of the crystallization of the 

 differentiated magma. There is, therefore, in this case, no real 

 distinction between magmatic segregations and yein - deposits. 

 Consequently, the classification so prevalent in German textboots 

 into syngenetic and epigenetic deposits here breaks down. The 

 difference is mainly one of time, or as it may be otherwise expressed, 

 a difference of phase. 



The formation of these deposits, then, may be summarized as 

 follows : — 



1st phase: Concentration within the magma of the metallic 

 constituents in combination with the volatile elements, 

 especially fluorine and boron. 

 2nd phase : Separation from the crystallizing granite of the com- 

 pounds thus formed, and escape of the same through fissures. 

 3rd phase : Chemical reactions between the compounds in the 

 escaping gases or solutions, leading to the formation of 

 crystallized ore and gangue minerals. 

 It is, of course, impossible to draw any hard and fast line between 

 these different phases; they are all parts of one continuous process, 

 and in many instances doubtless proceed concurrently. 



Turning now to the mineralogical and chemical side of the question, 

 we find a marked similarity of composition in all parts of the world. 

 Although there are local differences in detail, yet it is possible to 

 enumerate certain minerals of almost universal occurrence in 

 association with wolframite and cassiterite. Of these the most 

 characteristic among the sulphides are arsenopyrite and molybdenite. 

 Chalcopyrite again is common, while blende and galena are more 

 sporadic. Of the non-metallic minerals, tourmaline, topaz, and 

 fluorite are characteristic, showing the presence of the highly 

 volatile and chemically active elements fluorine and boron. 



Special interest attaches to the presence in certain localities only 

 of notable amounts of some of the rarer elements, such as niobium, 

 tantalum, and uranium. The last, for example, is found in Cornwall, 

 while columbite is abundant in Burma and in the extraordinary^ 

 pegmatitic masses of Etta Knob and Ivigtut. At Etta Knob it is 

 accompanied by a striking development of lithium minerals. In 

 Bolivia silver is the most characteristic element. From these and 

 similar considerations it appears possible to subdivide the tin- 

 tungsten occurrences into paragenetic sub-types, as follows : — 



/uranium . . Cornwall. 



Tungsten, tin -j 



molybdenum, arsenic 



tantalum and Burma, Etta Knob, 

 niobium Ivigtut. 



bismuth . . Queensland. 



silver germanium . Bolivia. 



