330 BROWN & HERON GEOLOGY AND ORE DEPOSITS OF TAVOY. 



(2) Pegmatites carrying wolfram and cassiterite exist and some 



of them pass laterally into quartz veins with the same 

 metallic minerals. The inference drawn from this is 

 that veins in which the pegmatitic origin is not so clear, 

 may have been formed under conditions approximating 

 closely to those under which the pegmatites themselves 

 arose. 



(3) In so far as it may be correct to regard the pegmatite- 



aplite family of rocks as differentiation products of 

 • granites, it seems reasonable to look upon their metal- 

 bearing contents as segregations from acid magmas to 

 an equal extent. 



(4) In cases wnere the pegmatitic origin of veins is obscure 



they may have been formed by later hydrothermal 

 phases of the same processes. 



Admitting the existence of pegmatites and pegmatitic veins 

 on one hand and those of an intimately connected hydrothermal phase 

 on the other, a place must be found for pneumatolysis somewhere, 

 for it is inconceivable that the gases of the magma took absolutely 

 no part in the vein formation. The very common occurrence of 

 greisen bands carrying wolfram or cassiterite or both, bordering 

 veins in granite, or even existing without any vein matter at all, 

 may be due to this cause. In addition to the potent action of 

 superheated steam, the constant association of wolfram with pyrite 

 and other sulphides raises the question as to whether the places of 

 boron and fluorine in the typical pneumatolytic processes may not 

 have been taken by sulphur and arsenic. 



J. Coac/in Brown, 1917. — In a popular paper, published in 1918, 

 these views were put forward again. The granite was held res- 

 ponsible for the mineralization, the wolfram, cassiterite, molybdenite, 

 bismuth and other metalliferous minerals are believed to have come 

 from it, and the veins to have been formed during a short period of 

 metallization towards the end of the igneous epoch. Many of the 

 quartz veins were formed in the same manner as pegmatites, both 

 quartz and water being of direct igneous origin. Towards the 

 end of the pegmatitic period, an intense hydrothermal phase super- 

 vened and some of the veins were doubtless formed then. ' 



1 Coggin Brown (4), pp. 65-70. 



