Tonopah daily bonanza. [volume] (Tonopah, Nev.) 1906-1929, 1909-10-10


the rhyolite. In some instances, how
ever, the veins pass from the lode
porphyry into the rhyolite without
! material change in the breadth or
richness of the ore.
The favorable nature of the earlier
andesite as a wall-rock is shown by
the production; more than 80 per
cent of the ore extracted from the
mines of Tonopah has been stopped
from veins in that formation. In this
/ the ore-deposits conform to the gem
eral law of a selectiVe deposition
namely, that some geological forma-
lions appear to be everywhere barren
of ore; others occasionally carry
small deposits, workable where the
conditions are exceptionally favor
able but in each mining region GER
tain strata are ore-bearing in a de-
gree exceeding all other formations
combined
The earlier andesite appears at the
surface in limited area, in ground
owned by the Midway and by the
Tonopah Mining Co., where it ex-
tends to depth of 500 to 600 feet
elsewhere it is capped by the dark
colored later andesite, or by flows of
rhyolite. Beneath these Gaping for
nations, the earlier andesite has
much greater areal extension and
| stretches east and west in a broad
belt, extending from the Belmont
mine, at the extreme east of the dis
trict, to the Tonopah Extension and
Golden Anchor properties on the
west, distance, as now explored, of
nearly mile. In the Tonopah and
Midway workings, the belt is devel-
oped oyer breadth of 1000 feet
In prospecting the Tonopah mines,
the problem is largely the explora-
ton of the lode-porphyry, or earlier
andesite. Owing to the fact that all
the ore-deposits known in the dis
trict are subsequent in formation to
the intrusion of the youngest rock in
the series, the rhyolite, in follow
ing the veins to the deep, certain
places are found where conditions
have been such that the ore departs
from the earlier andesite and cuts
through the underlying rhyolite; or,
| in other words, that in particular
| places, the highly heated waters and
vapors that deposited the ore have
broken through the sheet of rhyolite
in their upflow from the depths, and
have penetrated and laid down the
ore in the earlier andesite resting
upon it. In their ascent toward the
surface, the veins usually terminate
on reaching the cap-rock; from this
cause, many of the productive veins
are blind and do not appear at the
surface. Only in the area mentioned,
on the Tonopah and Midway proper
ties, where the earlier andesite-fiow
covers the surface, do the veins out
crop in pay-ore. Other ore bodies
spring from the contact of the rhyo-
lite with the earlier andesite (Which
forms the footwall or south bound
airy of the lode-porphyry belD, and
extend upward in the andesite. In
the West End and MacNamara mines,
sheet-deposits of high-grade ore oc-
cur on this contact. Some of the lar
ger veins, when followed on the dip,
go down in a series of flats and
pitches (like flight of giant steps, !,
in which the horizontal travel of the
vein to the north is greater than the
vertical descent on the normal dip.
This is the result of faulting, and
tends to carry the ore-bearing ifs
sures, In their descent TO the deep.
towards the north side of the earlier
andeslte belt.
There remains to describe pecu-