MANGANESE IN GOLD DEPOSITS 17 



laws and physical conditions controlling secondary enrichment have 

 been reviewed in several reports more recently published. The 

 papers of Lindgren, Ransome, Spencer, Boutwell, Irving, Graton, 

 McCaskey, Spurr, and Garrey and Ball are particularly valuable. 

 Such work has shown that the secondary enrichment of pyritic 

 copper deposits is a very inportant process; that many silver 

 deposits are enriched by superficial agencies; but that many gold 

 deposits do not show deep-seated secondary enrichment. 



T. A. Rickard 1 has brought out clearly the processes by which 

 gold deposits may be enriched relatively near the surface in the 

 oxidized zone by the removal of valueless minerals which are more 

 readily dissolved than gold. On the problem of deeper-seated 

 precipitation of gold below the zone of oxidation there is less evi- 

 dence. In some mines, however, the transportation and deep- 

 seated precipitation of gold is clearly shown, as was pointed out 

 long ago by Weed. 



While engaged in the investigation of certain auriferous deposits 

 in the Philipsburg quadrangle, Montana, for the U.S. Geological 

 Survey, I was confronted by evidence gained in two important 

 mines, which seemed to be conflicting on this point. In one of 

 them, the Cable mine, there was no evidence that gold had been 

 concentrated by cold solutions below the zone of oxidation, but in 

 the Granite-Bimetallic Lode there was enrichment of both gold and 

 silver below the zone of leached oxides. 



Although the ores of the two deposits differ in other respects, 

 the most striking difference is in the manganese content. The 

 use of manganese in the chlorination process to give free chlorine, 

 which dissolves gold, is well known. Le Conte 2 said as early as 

 1879 that free chlorine is the most important natural solvent of 

 gold, and Pearce, in 1885, recorded experiments in which gold 

 had been dissolved in hot sulphate solutions with common salt 

 and manganese dioxide. 3 Don obtained similar results with more 

 dilute solutions. 4 It appeared desirable, therefore, to ascertain 

 whether these reactions are carried on in cold dilute solutions 



1 Op. cit. 2 Elements of Geology, p. 285. 



3 Proceedings of the Colorado Scientific Society, II, 3 (1885-87). 



4 Trans., XXVII, 654 (1897). 



