23
ALTERATION OF ORE AND COUNTRY ROCK

The country rock, on the whole, has not been greatly altered in the
vicinity of mineral deposits. The permeating ore solutions have deposited
pyrite, quartz, and chlorite in the wall-rocks, but except for pyrite
impregnations the alteration is not in general noticeable except on micro-
scopic examination. Alteration in argillaceous wall-rocks consists almost
entirely of the deposition of pyrite; in harder rocks, chlorite and quartz
as well as pyrite have been found.

The ores are mostly primary. No secondary enrichment has been
proved and secondary action is of slight importance except in certain
places. At the Porter Idaho mine oxidation is known to extend 450 feet
beneath the surface. The oxidation at this place is fairly thorough, so
that the ore stripped consists partly of rust containing residual nodules
of quartz and sulphide. This is the only place in the map-area where
deep oxidation has been proved, but even in this place underground
development has not reached sufficient depth to show whether or not
secondary enrichment has taken place. This example, however, serves to
prove that deep secondary enrichment in the map-areas is quite possible.

DISTRIBUTION OF ORE IN THE DEPOSITS

Most of the larger silver-lead-zine deposits of the area are too lean to
be mined completely. In most instances the only ore is in the form of
shoots within the mineralized body. A few deposits are rich enough to
mine as a whole and these also contain shoots of richer ore. Individual
shoots of ore in some deposits are big enough to occupy the whole width
of the deposits for over 100 feet in length and depth, but most are smaller.

In a few lead-zinc-silver veins the chief value lies in gold or the gold
content is the factor that raises the grade of the mineral deposits to that
of commercial ore. In such veins the gold, so far as can be learned, occurs
sporadically and the vein matter is ore only where the gold content is
greater than a certain amount. In other veins of the silver-lead type, silver
decides the value of the ore. In these veins the silver is contained usually
in silver-rich tetrahedrite, and consequently the abundance and distribu-
tion of tetrahedrite determines the richness and the size of the ore-shoots.
In some of these veins the silver-bearing minerals are part of the general
mineralization, but in others they occur in fractures traversing the vein,
appear to be later, and the richness of the ore is governed by the size and
number of the fractures. In veins where the chief value is in lead and
zinc, ore-shoots are simply under parts of the veins or parts of the veins
where lead and zinc minerals are relatively more abundant.

Veins formed chiefly by replacement contain shoots of ore which origin-
ated perhaps through replacement of more easily attacked parts of the
rock and hence are more thoroughly replaced. The replacement deposits
are mostly of the copper-bearing type, but those containing silver-lead and
zine hold ore-shoots that are simply parts of the mineralized body where
replacement has been more thorough. In the replacement deposits ease of
replacement and not richer mineral has apparently controlled the size and

richness of the ore-shoots.