SYNOPSIS OF PART I. 
7 
The ores contain on an average only about 1 ounce of silver to 10 ounces of gold. 
In some mines the proportion is very much less. Small shipments have been 
made of tetrahedrite ore rich in silver. 
The tenor of the ore as mined has decreased somewhat in the last ten years, 
but this is probably mostly due to lowered operating and reduction expenses. 
CHAPTER IX.-PROCESSES OF ALTERATION. 
Metasomatism in connection with vein formation .—During the vein-forming 
processes the rocks adjacent to veins have undergone a certain amount of alteration, 
and in case of the porous breccia this alteration has spread over large areas. The 
alteration is, on the whole, slight and involves a transformation of the dark silicates 
to carbonates, pyrite, and fluorite and a change of the feldspars and feldspathoid 
minerals into sericite and adularia. The latter mineral is characteristic of all the 
altered rocks, but especially of the so-called “granitic ore” of the Elkton, Ajax, 
Stratton’s Independence, and other mines. Calaverite develops in places by meta- 
somatic processes in breccia, gneiss, phonolite, and basic dikes, but its appearance 
in this form is rare, compared to its universal occurrence as fissure filling. Galena, 
zinc blende, and other sulphides sometimes appear as metasomatic products, but 
are not common. Chemically the process, when carried to completion, involves 
an almost entire loss of soda and a corresponding gain of potash. Small amounts 
of sulphur, carbon dioxide, and fluorine are introduced. The percentage of silica 
is slightly reduced, and lime and magnesia have often been partially removed. 
The course of the alteration, which is shown to have been produced by hot alkaline 
waters, is illustrated by several analyses. 
Oxidation .—The oxidation away from the vicinity of veins has not penetrated 
to great depths. Globe Hill is an exception, for here complete oxidation has affected 
the breccia to a depth of at least 800 feet below the surface. Along the veins 
oxidation extends far deeper, in some cases down to 1,200 feet below the surface. 
The water level originally stood at an elevation of about 9,500 feet, rising to 9,600 
or 9,700 feet under Bull Hill. In a general way the depth of oxidation along veins 
coincides with the depth of the water level. There are many exceptions; in the 
western part of the district unaltered tellurides may be found close to the surface, 
while in other cases complete oxidation extends for 200 or 300 feet below the water 
level. 
Oxidation tends to destroy the original structure of the vein and changes the ore 
to a brown, soft, and homogeneous mass. Oxidation tends also to transform tellu¬ 
rides into brown, spongy gold and tellurites, while sulphides are altered to various 
oxy-salts. The silicates are changed to kaolin, quartz, manganese dioxide, and 
limonite. The agent producing oxidation is atmospheric water percolating down¬ 
ward along the vein and the process continues until this water has lost its oxygen. 
The waters are acid, due to the sulphuric acid derived from pyrite. The results 
of oxidation are illustrated by several analyses. No evidence favoring the deposi¬ 
tion of secondary sulphides has been found. The tetrahedrite supposed by some 
to be of such origin is a primary vein mineral. 
I 
