SECONDARY ENRICHMENT. 253 



related changes are undergone by the natural tungstates in other 

 climates or when accompanied by other series of mineral associa- 

 tions one in which easily decomposed sulphide ores are absent 



being prominent in our minds — we must leave for those better 

 acquainted with them than ourselves to decide. We are inclined 

 to think that the sulphides of molybdenum and bismuth are also 

 oxidised, dissolved and leached away on their passage through 

 a soil cap from their parent veins towards the valley deposits in 

 moist tropical climates. We conclude by pleading for a stricter 

 use of the word " alluvial " in descriptions of these and similar 

 processes, than is the case in most modem literature, for it does not 

 appear correct to us to assign residual desert deposits, or detrital 

 deposits of hill slopes, which may have been to some extent moved, 

 but not definitely arranged, by water, to this class. 



SECONDARY ENRICHMENT. 



Although it is possible to imagine theoretical conditions under 

 which tungsten ores in the upper part of a vein should be dissolved 

 to form solutions which, percolating downwards, might have their 

 tungsten contents precipitated in a lower region, the fact remains 

 that in Burma there are no zones of any economic importance in 

 wolfram mines, which have been enriched by solution and reprecipita- 

 tion in any way comparable to those of. say, copper and silver in 

 other parts of the world. 



According to Gannett, after tungsten is dissolved it is very 

 easily precipitated by ferric salts, but this precipitate is colloidal 

 and difficult to filter. If such a precipitate should form in nature, 

 it might be carried long distances and be widely distributed through 

 the rocks without being lodged in a definite deposit. In such a case 

 the original source of the tungsten would become impoverished. 

 Precipitated ferric tungstate or the hydrated tungstio oxide, which 

 is often seen in Tavoy, are both compounds that it is practically 

 impossible to recover by ordinary concentrating devices in any 

 case. Secondary scheelite is not unknown in the district, but it 

 is a mineralogical curiosity and possesses no economic importance. 



PROSPECTING FOR WOLFRAM VEINS. 

 The granites of Tavoy and indeed of all other parts of the pro- 

 vince where we have examined wolfram and cassiterite-bearing 

 veins in association with them, have a very uniform composition 



