STRUCTURES AND CHARACTERISTICS OF TUNGSTEN DEPOSITS. 229 



Fissure veins. 



The modes of occurrence of wolfram and cassiterite already 

 described are, with the exception of the grei- 

 General features. geng ^ th(j p agaye pegmatites, of- more theo- 



retical interest than economic importance. Leaving out of cal- 

 culation the surface deposits, quartz veins furnish the great bulk 

 of the concen rates produced in Tavoy. Mineral-bearing quartz veins 

 are found either in the granite, penetrating its contact with the 

 sedimentary rocks, or enclosed within the sedimentary rock i them- 

 selves at no 'great distance from the granite. A glance at the 

 geological map will show that all producing mines are situat <i on the. 

 granite or are not far removed from it. Exceptional cases do occur 

 of bearing veins which appear to be further away from the granite 

 than this statement would seem to warrant, but investigation shows 

 that in such instances there is reason to suspect the near presence of 

 underlying granite not yet exposed by denudation. 



Every major granits intrusion in the dis net lnu in some part 

 or other its associated o/e-bearing veins. Very often huge expanses 

 of granite are found where such do not exist now. The reasons 

 for this are discussed later, bnt where favourable conditions remain 

 and where exploration has taken place, there the veins occur. 



It is only natural that a large number of veins, widely distri- 

 buted over a vast area, formed intermittently by processes which 

 may have varied slightly from place to place, subjected to more 

 or less severe changes, dynamical and chemical, and denuded to 

 different levels later, should exhibit changes of structure to-day. 

 Yet there are certain generic characteristics apparent in them which 

 may be classified as follows, with the reservation that exceptions to 

 each case may exist. 



(1) The veins are formed by the infilling of fissures; 



(2) they often occur in parallel groups ; 



(3) they often form series of overlapping lenses ; 



(4) the lenses themselves are often irregular ; they thin out and 



then thicken again, or split up and reunite; 



(5) they generallv posses clean and well-defined wall-*; 



(G) alteration of the granite walls to greisen is practically uni- 

 versal ; 



(7) the quartz of which they are formed is usually dense, 

 milky white and very compact ; 



