Vol. 4] 



Laicson. — The Robinson Mining District. 



333 



Another sample of the mine water, collected by the writer 

 and analyzed in the mine laboratory as expeditiously as possible 

 after collection, by Mr. Herbert Ross, gave the following results, 

 for the ferrous and ferric sulphates : 



From this it would appear that the iron of the mine water 

 is nearly all in the ferrous state and that the high proportion 

 of ferric sulphate in Mr. Miller's analysis is ascribable to oxida- 

 tion in the interval between the taking of the sample and the 

 time of his receiving it. If this be so, it clearly indicates a 

 deficiency of oxygen in the mine waters, thus promoting the 

 maintenance of a supply of S0 2 which, as has been so well shown 

 by Winchell,* is favorable for the reduction of copper sulphate 

 to cuprous sulphide and the precipitation of the latter as chal- 

 cocite upon pyrite. That the chalcocite is secondary upon the 

 pyrite is very apparent in several samples of the ore that were 

 examined, the chalcocite being frequently a shell encasing, or 

 partially encasing, pyrite. 



From what has thus far been stated it appears certain that 

 the copper ore of the Ruth Mine, porphyry impregnated with 

 chalcocite and pyrite in sporadic crystals, owes its value in part 

 to secondary enrichment by downward leaching from the zone 

 of oxidation. The fact that the oxidized zone might easily be 

 robbed of its copper ore by the leaching action of sulphate of 

 iron, was experimentally verified by placing a mixture of pulver- 

 ized chalcocite and porphyry in a burette and causing a 10 per 

 cent, solution of ferrous sulphate to pass through it in presence 

 of the atmosphere. The chalcocite was taken into solution and 

 passed from the burette as copper sulphate, CuS0 4 , at a fairly 

 rapid rate. While this secondary enrichment of the porphyry 

 below the zone of oxidation is unquestionable, two important and 

 significant facts remain to be noted. The first of these is that 

 the products of this enrichment are not localized at any partic- 

 ular horizon in the porphyry, so far as the development of the 

 mine has yet gone. The copper values are by no means evenly 



* Bull. G. S. A., Vol. 14, pp. 269-276, 1903. 



FeSO, 

 Fe 2 (S0 4 ) 3 



9.36 

 .4 



