BOLIVIAN COPPER DEPOSITS 
369 
some places the copper is regularly distributed through the rock; 
in other places it occurs in small streaks and patches between 
which the sandstone is nearly barren. The barren places are some¬ 
times thoroughly bleached but often they are more or less red in 
color. The alteration products of the native copper ores are 
cuprite, malachite, and azurite. The ores show wide local varia¬ 
tions in richness. The run of mine ores usually range from 2.5 
to 3.5 per cent copper. The ore from the Capilla mine averages 
6 per cent and the San Augustin is said to have produced ore 
with 15 per cent copper. 
Typical sulphide ore is more highly mineralized than the native 
copper ore, and the rock is more uniformly impregnated with 
chalcocite. A thin section of this ore shows that the quartz grains 
of the original sandstone have been little affected by the mineraliz- • 
ing solutions and ore deposition has taken place by the replace¬ 
ment of the matrix, a process which has proceeded much further 
than is usually the case in the native copper ores, even to the 
point of almost complete replacement of the matrix by chalcocite. 
The color of the sulphide ore unaffected by oxidation is a uniform, 
metallic-looking gray. Its average tenor is 7 to 8 per cent copper. 
In the zone of oxidation it alters chiefly to the basic sulphate bron- 
chantite, but also to malachite and azurite; and especially in frac¬ 
tures and along bedding planes, cuprite forms. Hand-sorted 
mixed oxidized and sulphide ores carry 18 to 25 per cent copper. 
The charque, or sheets and arborescent forms of native copper, 
occurs in fractures, along bedding planes, and in the principal 
fault planes. The thickness of the sheets varies with the width 
of the fracture and according to whether gangue minerals are 
associated with the copper or not. In some cases the native metal 
occurs as a single sheet completely filling the opening; in other 
cases it is enclosed in gangue, or is interwoven with gangue. The 
commonest gangue mineral is gypsum, but celestite occurs in con¬ 
siderable quantity. Celestite also occurs in tabular crystals lin¬ 
ing druses in which there has been no copper deposition. 
No additional data can be given concerning the occurrence of 
silver ores, as such ores are no longer produced and silver is a 
very subordinate constituent of the copper ores now worked. The 
native copper concentrates, with a tenor of 85 per cent in copper, 
contain only 6 ounces of silver; and the sulphide concentrates. 
