268 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



The quartz croppings carry pyrite and oxides of manganese. They are always 

 argentiferous, with- an appreciable amount of gold. They may be divided into two 

 zones, one overlying the other. The upper is composed of oxides (red ores) and the 

 lower of sulphides (black ores). The upper contains, besides iron oxide (always 

 auriferous), oxides of manganese and chlorides and bromides of silver; it has a 

 maximum downward extent of nearly 1,000 feet. The lower zone contains sul- 

 phides of lead, silver, etc. The lower limit of the upper zone corresponds to the 

 ground-water level. 



Calcite is found only in small quantities. Of the sulphides, pyrite, galena, 

 and argentite were in most cases deposited simultaneously with the quartz. The 

 abundant manganese oxide in the upper zone is replaced in the sulphide zone by 

 a lesser quantity of the silicate, rhodonite. Pyrite is frequent in the mineralized 

 parts of the veins, and is also abundant in the country rock, but in the country 

 rock it does not contain even traces of the precious metals. Native silver has been 

 found at all depths; ruby silver has not been observed at Pachuca, but is found 

 at Real del Monte. a 



Rich ores occur in certain parts of the veins called bonanzas, which are of 

 irregular form, frequently nearly elliptical. The bonanzas of the different veins 

 group themselves in a northeast-southwest zone nearly normal to the vein strike. 

 Some are in the oxidized, some in the sulphide zone; the former are more 

 numerous. In some cases bonanzas were encountered at the surface; in others 

 they were found in depth, where the vein was barren at its outcrop. The size 

 of the bonanzas varies; one of the largest was encountered at a depth of over 300 

 feet and was elliptical, the greatest axis being over 3,000 and the smaller 1,300 

 feet, with a thickness of 8 feet. 



The veins become impoverished at great depths. At the bottom they change 

 to barren galena and blende, too poor to repay working. However, certain 

 developments lead to the belief that at still greater depth new bonanzas might 

 be found. Most of the mines are only about 1,300 feet or less deep; in only 

 one has a little work been done as deep as 1,800 feet. 



This district is similar to Tonopah in the character and age of the wall rocks 

 (Miocene andesites); in the nature of the alteration of the rock near the veins 

 (silicification near the veins, propylitic alteration farther away); in the structural 

 characters of the veins, which form a splitting and reuniting group; in the 

 general character of ores (both oxide and sulphide), and of gangue, though 

 adularia as a gangue material and selenides as ores have not been recognized at 

 Pachuca; and in the occurrence of the rich ores in bonanzas, which seems to be 

 due to the intersection of transverse fractures with the main vein zone. 



" Be<;k, Richard, Erzlagerstiittcn, 2d ed., p. '277. 



