OXIDATION. 93 



remained quantitatively unaffected during oxidation, so that merely the large rela- 

 tions can be glanced at. All the metals except silver and perhaps gold are present 

 in the oxidized ore in much diminished proportions. The lead, copper, and zinc are 

 present in small quantities. The manganese is now in the form of oxide, but very 

 little remains; the iron is in the form of oxide, with some residual or secondary 

 pyrite. There is much less gold in proportion to silver in the oxidized ore than in 

 the sulphide ore; but this may be fortuitous and depend on the specimen selected. 

 More than half the silver is in the form of sulphide, and from the very small quantity 

 of arsenic and antimony present this portion must be nearly all in the form of 

 argentite. The antirnonial silver sulphide is very probably pyrargyrite (ruby silver), 

 judging from microscopic observations. It is noteworthy that antimony and arsenic 

 are present in the same proportions to one another in both analyses. There is less 

 than a third as much selenium in the oxidized ore as in the sulphide ore, but the 

 discrepancy is not so great as in the case of lead, copper, manganese, zinc, arsenic, 

 and antimony; and this selenium seems to be still in the form of a silver selenide. 



Therefore it is probable that during the process of oxidation the primary 

 carbonates were attacked by surface waters, and the lime and magnesia, together 

 with most of the iron and manganese, removed in solution. Some of the iron and 

 manganese remain as oxides. No important change in the amount of gold and silver 

 is proved. The argentite has largely remained unaltered, but the polybasite (and 

 stephanite if present) has probably been attacked, and much of the silver selenide. 

 Part of this silver has been reprecipitated with little change of position as secondary 

 argentite, not distinguishable from the primary argentite, while a large portion has 

 been altered to chloride by the action of chlorine contained in the shallow 

 underground waters. Most of the arsenic and antimony in the original polybasite 

 and stephanite has been removed in solution; the rest goes to form the secondary 

 sulphide pyrargyrite, as indicated by numerous field observations. The pyrite and 

 the chalcopyrite have been attacked. Most of the iron in these sulphides has been 

 removed; a small part remains as oxide, or rarely as residual or secondary pyrite. 

 Nearly all the copper has been removed, a little remaining in the probable form of 

 oxide. 



It is thus seen that the so-called oxidized ore of the Tonopah district, like 

 that of man}' other deposits in desert regions, is really a modified ore consisting 

 of an intimate mixture of original sulphides (and selenides), together with 

 secondary sulphides, chlorides, and oxides. This case is without doubt character- 

 istic of the whole zone of oxidation from the outcrop downward, for the ores 

 throughout the zone are identical microscopically. 



As to the reprecipitation lower down of materials dissolved in the process 

 of oxidation there is little light. The plainly secondary sulphides within the 



