CHAPTER V. 



THE ORDER OF DEPOSITION OF THE METALLIC VEIN MINERALS 

 AND ITS ECONOMIC BEARING. 



Examination of a very largo number of mixed intergrowths 

 of oassiterite and wolfram from Tavoy district have convinced 

 us that in the great majority of cases the wolfram is older than the 

 tin ore. Dr. Morrow Campbell states : — " I have examined care- 

 fully the mixed wolfram cassiterito ores from most of the mines 

 of the district and in the vast majority of cases there is no shadow 

 of doubt that wolfram was deposited before tin. I have scum many 

 hundreds of samples of cassiterite filling the spaces between wolfram 

 crystals, enclosing wolfram and carrying the impression of its 

 striations, several where it has filled cracks in wolfram and two 

 excellent examples of tin ore crystals actually formed on surfaces 

 of wolfram." 1 



Molybdenite appears to have been the first ore mineral to be 

 deposited, as it is often enclosed in wolfram, or intergrown with 

 the mica at the edges of the veins. It was followed by wolfram, 

 cassiterite, bismuth, bismuthinite and then the majority of the 

 sulphides like pyrite, chalcopyrite, arsenopyrite, pyrrhotite, galena 

 and blende. Pyrite belongs to several stages of the crystallization 

 processes and is not always confined to its place in the general 

 order given above. Flourite was probably one of the latest minerals 

 to be formed. It is usually found in crystals coating otter minerals. 

 It should be pointed out that all these compounds do not occur 

 in every vein; the sulphides of load, bismuth and zinc are rarely 

 found, but the order given is the best that can be made out from 

 specimens which we have seen, and it may of course be modified 

 by future discoveries, though we do not think that is likely. Dr. 

 Morrow Campbell has listed most of the following examples from 

 the district and we have noted similar occurrences and added others. 



Molybdenite enclosed in wolfram, wolfram inside a quartz 

 crystal, cassiterite enclosed in wolfram, cassiterite enclosed in 

 bismutite derived from bismuthinite, cassiterite enclosed m mica, 

 mica in wolfram and cassiterite, quartz enclosed in soh>elite, quart/. 



1 J. Morrow Campbell (1), p. 17. 



