GOLD IN SURFACE ALTERATIONS OF ORE BODIES 323 



changed for one sensitive to o.ooooi g. which was used from that time 

 on. The whole time w^as 34 days. In experiment 4 it will be seen 

 that the losses in {a) and {h) are approximately proportional to the 

 areas of the plates. 



The results of these experiments, as given above, show conclusively 

 that, of the conditions under consideration, the most favorable for 

 the solution of gold involve the presence of manganese dioxide and 

 chlorides. Although it is frequently stated that gold is readily soluble 

 in ferric sulphate solutions,' no loss of gold was detected after 34 days' 

 contact with a tenth normal solution of that salt.^ 



In order to reproduce more nearly the conditions in nature experi- 

 ment 5 was prepared as follows: a solution was made N/io with 

 respect to ferric sulphate and sulphuric acid, and N/25 with respect 

 to sodium chloride. To 50 c.c. of solution i g. of powdered man- 

 ganese dioxide was added and the experiment was carried on as 

 before.'^ The loss is comparable to that found in experiment 4, 



5. Fe,(S04)3+H,S04+NaCl + MnO, + Au 



Loss of Au . 00505 g. Time 14 days. 



allowing for the shorter time and the greater dilution of the chloride. 

 The same experiment without MnOj showed no loss of gold. 



After it had been shown that chlorides and manganese dioxide were 

 necessary under these conditions, the next point to be determined was 

 whether the free acid or the ferric chloride is the active agent in bring- 

 ing about the solution. In experiment 6, 50 c.c. of N/io HCl was 

 used with i g. of powdered MnO^. In experiment 7, sodium hydrox- 

 ide was added to 50 c.c. of N/io ferric chloride solution until the pre- 

 cipitate formed barely redissolved on shaking,^ after which i g. of 

 powdered MnOj was added. 



6. HCl+MnO,+Au 



Loss of Au .01369 g. Time 14 days. 



7. FeCl3 + MnO,-t-Au 



Loss of \n .00062 g. Time 14 days. 



1 E.g., Genesis of Ore Deposits, 478, 481, and elsewhere. 



2 This agrees with Stokes, loc. cit. 



3 This is essentially the experiment of Rickard, Trans. A.I.M.E., XXVI, 798. 

 From experiments 6 and 7 it appears that the ferric salt is unnecessary. 



4 Even then the solution was somewhat acid owing to the hydrolysis of the ferric 

 chloride. 



