68 STUDIES IN GEOLOGY, No. i 



are producing sulphide ores, but the depths to which they 

 extend are not known. 



Ore deposition has taken place principally in arenaceous 

 and pebbly beds and only in the ramos occasionally in 

 shales. Typical native copper ore consists of nearly white 

 to light-greenish sandstone irregularly mottled with specks 

 of copper. A thin section of this ore is made up of rounded 

 to subangular grains of quartz, and a little plagioclase 

 feldspar, ranging from .15 to .60 mm. in diameter and 

 averaging .3 mm. The matrix of these grains is feldspar, 

 chlorite, and a little calcite. The native copper occurs 

 chiefly as a replacement of the matrix, penetrating the 

 boundaries of the quartz grains to a very limited extent, 

 in grains and flakes varying from .I-.3 mm. in diameter 

 and averaging .15 mm. In 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 variations 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 mineralizing solutions and 

 ore deposition has taken place by the replacement of the 

 matrix, a process which has proceeded much further than 

 is usually the case in the native copper ores, even to the 

 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 



