512 THE ORE-UEPOSITS OF BORAH CREKK, 



" 2. The free gold-auriferous pyrite zone, with coarse quartz 

 gaiigiie. 



" 3. The cupriferous pyrite zone. 



" 4. The galena-blende (galena usually argentiferous) zone. 



" 5. The zone of silver and also much gold, usually associated 

 with metals which combine with them to make substances which 

 are undoubtedly highly mobile, and account for the relatively 

 elevated position of the zone. These associated metals include 

 antimony, bismuth, arsenic, tellurium, and selenium. Charac- 

 teristic minerals of this zone are tellurides, argentiferous tetra- 

 hedrite and tennantite, polybasite, stephanite, and argentite. 



*' 6. The zone of earthy gangues, barren of valuable metals." 



Here we see that the order which he ascribes to ore-formation 

 from magmatic differentiation is, (1) tin and allied ores, (2) free 

 gold-auriferous pyrite zone, (3) cupriferous pyrite zone, (4) galena- 

 blende zone. 



In connection with the Borah Creek Mine, it has been shown — 



l.The Mine is situated in the same country-rock (the "acid 

 granite ") as the adjacent tin-dejDosits. 



2. Arsenopjrite is the earliest deposited mineral in the lode. 



3. This is followed by a sphalerite-chalcopyrite zone. 



4. And further by a galena-zone (argentiferous). 



5. And finally by a quartz-filling. 



The order of deposition in the Borah Creek Mine, and its 

 association wiih the tin-deposits, is thus strikingly in accord 

 with the generalised order suggested by Mr. Spurr. 



The arguments in favour of the formation of the Borah Creek 

 Lode from a deep-seated, magmatic extraction are briefly as 

 follows : — 



1. The relationship of the deposits to the adjacent tin-deposits 

 — a complete gradation existing between the two types. This 

 relationship is not unique, but occurs in other parts of the world. 



2. The absence of ore in the joints adjacent to the lode is 

 evidence against the lateral secretion theory. 



3. The compact nature of tlie country-rock is also against the 

 lateral sectetion theory. 



