164 BASTIN AND HILL: ORE DEPOSITS OF GILPIN COUNTY 



carried down below the oxidized zone is probably for the most 

 part in balance with the sulphate radicle SO4. It is redeposited 

 mainly in the form of silver-arsenic sulphides, as replacements 

 of the primary ore minerals or as filhngs of small fissures in the 

 primary ore. The chemistry of the process is not well under- 

 stood, but observations in this region fix certain limiting con- 

 ditions. It is notev/orthy that in the secondary minerals as in 

 the primary minerals arsenic greatly predominates over antimony. 

 Silver enrichment is practically confined to the veins of the 

 galena-sphalerite type and is absent from those of the pyritic 

 type. The most important cause of this restriction is believed 

 to be the presence of carbonates in the galena-sphalerite veins. 

 These carbonates neutralize the descending oxidizing waters and, 

 in the case of siderite, ferrous sulphate, an active silver precipi- 

 tant, is generated by reaction with sulphuric acid. 



Nishihara- has compared the neutralizing effect of various 

 minerals on sulphuric acid and also their efficiency in reducing 

 ferric sulphate to ferrous sulphate. Pyrite, quartz, and chalco- 

 pyrite, minerals characteristic of the pyritic type of ores, are 

 ineffective in both respects. Galena, sphalerite, and, of course, 

 the carbonates are effective neutralizers and fairly active in 

 reducing ferric to ferrous sulphate. Moreover, galena and sphaler- 

 ite in contact with solutions carrying sulphuric acid or ferric 

 sulphate generate hydrogen sulphide. The composition of the 

 galena-sphalerite ores favors, therefore: (a) rapid neutraliza- 

 tion; (b) reduction of ferric to ferrous sulphate; (c) generation 

 of ferrous sulphate from siderite; (d) generation of hydrogen 

 sulphide. All these actions are beheved to favor the precipita- 

 tion of silver. The pyritic ores, on the other hand, have a min- 

 eral composition which favors persistence of acidity and per- 

 sistence of the iron in the ferric state. Dissolved silver de- 

 scending through such ore-bodies is likely to remain in solution, 

 to enter the general groundwater circulation, and to be lost so 

 far as the ore deposits in question are concerned. 



2 Nishihara, G. S., The rate of reduction of acidity of descending waters by 

 certain ore and gangue minerals and its bearing upon secondary sulphide enrich- 

 ment. Econ. Geol., 9: 743-757. 1914. 



