DISTRIBUTION OF BONANZAS IN THE PACHUCA SILVER 
DISTRICT OF MEXICO. 
BY CHARLES R. KEYES. 
{Abstract.) 
There is one feature of mining that has long been of especial interest. 
It is the distribution of the particularly rich ore mass or bonanza. In a 
recent visit to the celebrated silver mines of Pachuca, Mexico, unusual 
opportunity was offered me to carefully examine into some of the most 
instructive phases of this subject. The facts obtained throw light upon 
many vexed problems connected with many abandoned silver mines of 
our own country, still certainly containing large amounts of easily avail- 
able ores. 
The Pachuca and Real del Monte mining districts lie in the Sierra 
de Pachuca about 60 miles northeast of the City of Mexico. The rocks 
composing the mountain range are mainly Teritiary andesites, rhyolites 
and basalts, the first mentioned type being the oldest and containing the 
principal ore-bodies. These silver mines are among the most famous in 
the world. Geologically they are of great interest on acount of the large 
number of very rich ore-bodies which have been opened up in them-^ 
by the Mexicans termed ‘Bonanzas.” 
The ores are disposed chiefly in east and west lodes which are in the 
main nearly parallel to one another and hade about 75 degrees to the 
southward. The gangue is principally quartz ; and quartz-reefs form 
conspicuous ridges on the surface of the ground. In this quartz the 
silver in the sulphide form is disseminated in a finely divided state. 
Much has been made of the observation that in the parallel systems of 
veins the bonanzas of contiguous lodes alternate; that is, the richest 
parts of one vein are opposite the lean portions of the adjoining one. I 
knoAV of no published explanation of this phenomenon. In my own ex- 
amination of the mines and of the geologic structures of the country 
about I was greatly impressed Avith the significance of a series of great 
joint-planes, or faults, which traverse the mountain range nearly parallel 
Avith its axis and at an angle Avith the trend of the lodes. I am inclined 
to associate the localization of the bonanzas Avith the intersections of the 
fault-planes Avith the mineral veins. As the faults cut the latter at a 
