200 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
and of the acids set free, as well as to the increasing mobility of substance near the 
surface. Oxidation tends to thorough change of composition and also to oblitera¬ 
tion of structure of the original rock or vein. 
Of the metallic minerals the tellurides form the most important division. 
The bond between the tellurium and the gold is not a strong one, and direct 
oxidation very easily produces residual pseudomorphs of free gold and tellurium 
dioxide (the mineral tellurite), which in presence of iron oxides easily changes to 
various tellurites such as the yellowish-green emmonsite. In the Bonanza King 
lode of the Midget mine occur veinlets of pyrite with tellurides. Within 30 or 40 feet 
of the surface oxidation has changed the tellurides to specks of native gold, while 
pyrite remains practically unaltered. The tellurides in fissures and joints, which 
form the most common mode of occurrence, will be most easily oxidized, while 
those occurring as metasomatic products in the rocks are not so readily reached 
by the surface waters. An instance of this occurred on the rich pocket of the 
generally oxidized W. P. IT. vein, 250 feet below the suface. Some of the tellurides 
are found here as replacement in schist fragments and remain partially fresh at 
this relatively shallow level. 
Pyrite also is attacked without much difficulty, especially in the vein fissures, 
and yields sulphuric acid and ferrous sulphate, which is easily oxidized to ferric 
sulphates and finally into ferric hydrate and free acid. The sulphuric acid attacks 
kaolin and probably also sericite, forming segregations of such impure alunite as 
is encountered, for instance, in the Last Dollar veins (p. 377). 
Small quantities of basic ferric sulphates are fairly common in the oxidized 
zone, but can rarely be referred to definite mineral species (p. 125). 
The rapid oxidation of pyrite is well illustrated by bowlders of breccia, con¬ 
taining 2 per cent of finely distributed pyrite, such as were found in surface pits 
near the C. O. D. mine. The rock consists of a normal breccia of granite, and 
phonolite, with much fine material. Angular fragments of phonolite are up to 
2 cm. long. A dark-brown skin covers the surface of the bowlders and evidently 
consists of limonite. Underneath it lies a light-gray zone of breccia, up to 1 cm. 
thick, while the center consists of the normal dark-gray breccia. Examination 
under the microscope shows that the only difference between these zones is that 
in the outer light-gray layer the pyrite has completely disappeared without leaving 
any limonite. No other changes are apparent in the rock. The pyrite has evi¬ 
dently been oxidized to sulphates and these have been carried to the surface by 
capillary action, there to be changed to limonite. 
Galena and zinc blende are probably somewhat less easily attacked than 
pyrite, but eventually become oxidized, and when changed to insoluble lead sulphate 
and zinc silicate may remain or even be somewhat concentrated into the altered mass. 
The molybdenite oxidizes very readily to yellow and blue molybdite and 
ilsemannite, and these products do not seem to be easily carried away. Tetrahedrite 
yields various forms of oxidized copper compounds, which, as usual, show considerable 
mobility. No oxidized products containing antimony have been recognized. The 
fluorite upon exposure to oxidation loses its deep-purple color and becomes dis¬ 
integrated. Eventually a part of it is dissolved in surface waters. 
