PROCESSES OF ALTERATION-OXIDATION. 
201 
The altered rocks in the veins contain orthoclase, sericite, chlorite, dolomitic 
carbonates, and magnetite, together with a small portion of biotite, augite, and 
hornblende which have resisted the alteration due to vein-forming action. The 
most prominent product of oxidation is kaolin, which often becomes segregated 
in large, pure-white masses. The iron-bearing minerals are converted to limonite, 
and the manganese in the silicates, and especially in the carbonates, separates as 
earthy pyrolusite or as wad. During these processes some silica becomes con¬ 
verted to the soluble state, and there is reason to believe that the oxidizing waters 
in places are rich in this constituent. This was particularly observed in the Zenobia 
mine down to the 500-foot level, where a slimy opaline'silica seems to be deposited 
at the present time. Another evidence is in the occurrence of residual pseudo- 
morphs of gold after tellurides, the spongy gold showing a coating of opal. In 
places the silica will be deposited as chalcedony or jasperoid colored red in various 
tints by ferric oxide. 
The reaction between free sulphuric acid and the earthy carbonates produces 
gypsum, which has been noted at many points in the oxidized ores. The large 
mass of this mineral in the Deerhorn mine is, however, probably not formed during 
oxidation, but is more likely a primary deposit by hot waters. By the same reaction 
epsomite (hydrous sulphate of magnesia) and mallardite (hydrous sulphate of 
manganese) will be formed, and, being very soluble, frequently form a coating or 
effervescence on the walls of the mine workings in the upper levels. Zinc sulphate 
occasionally enters into these complex salts. Hydrous sodium sulphate and 
aluminum sulphate appear in the same manner and are evidently formed by com¬ 
bination of sulphuric acid with the soda set free during processes of kaolinization. 
A little potassium is contained in these sulphates, but the greater mass of this 
substance is tenaciously held in the rocks. The vein-forming processes involve a 
development of potash micas and potash feldspars from silicates containing sodium, 
and also from the orthoclase. The same tendency seems to persist in the oxidized 
ore to some extent, though well-defined micas are not known to develop. It is a 
well-known fact that some phonolites upon weathering are apt to become richer 
in potassium and poorer in sodium . a At any rate, the sericite due to vein-forming 
action tenaciously resists oxidation. 
A black or dark-green hydrous ferric sulphate is sometimes encountered as a 
soft filling of cracks and fissures of the oxidized zone. When dry it assumes a yellowish 
brown color, and is in all probability identical with chloropal or with the mineral 
morencite^ recently discovered in the Clifton copper mines of Arizona., 
Phosphates are sometimes found in considerable quantities in oxidized ores, 
indicating local concentrations during the process. 
In 1894 Dr. Richard Pearce, of Denver, contributed some important notes 
to the subject of oxidation of Cripple Creek ores. c He examined a specimen of 
ore from the Moose mine, evidently a partially replaced volcanic rock, one half 
of which was oxidized while the other half had remained fresh. 
a Roth, Allgemeine Geologie, vol. 2, p. 255. 
!> Am. Jour. Sci., 4th ser., vol. 18, December, 1904, p. 455. 
c Proc. Colorado Sci. Soc., vol. 5, 1894-1896, pp. 11-16. 
