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

A cluster of low volcanic peaks
mark the site of the mining camp of
Tonopah. 1t has in the past been an
area of concentrated volcanic activ-
ity; of repeated eruptions and out
pourings of lava, one succeeding an
other, beginning in the early Ter
lively recent geologic time. With
the dying out of the eruptive phase
of volcanic activity, there followed a
period of intense dynamic action; of
assuring and faulting of strata, ac-
companied by the formation of de-
posits of gold and silver ore. So re-
cent has been the cessation of VOL
canic outbursts in this district that
the rocks, like the eruptive forma-
tons of the Comstock, still retain a
portion of their original heat Not
withstanding the comparatively late
deposition of the ore, there are evil
dences in the mines that the forma-
ton of the deposits extended over a
long period, with many disturbances
and Interruptions.
Tonopah conforms to the general
law of areal distribution of mining
districts, namely, that lore deposits
have been formed only in local areas
of disturbance and faulting and in
direct association with eruptions of
the igneous rocks" So intense and
concentrated has been the formation
of the ore in this basin among the
volcanic hills that the deposits are
of richness and size seldom equal
led
THE ORE-BEARING FORMATION.
Examined more particularly, it is
seen that the rocks of the district are
all of igneous origin; the successive
eruptions of lava, of varying compo-
sition, having been outpoured from a
number of volcanic vents, in sheets
Superimposed one upon another. In
places the structure is not so simple,
and the successive fiow-sheets of lava
are broken through by later intru-
sions of volcanic rock. Further, the
strata, after the sheets were laid
down, have been extensively faulted
OF the many igneous formations in
this area, it is only necessary to de-
scribe those which directly influence
the occurrence of ore. These, in the
order of their eruption, are the ear
lier andesite, the later andesite and
the rhyolite. The earlier andesite is
white or light gray in color, SOFT
breaking with a rough fracture; the
later andesite is darker in color
weathering near the surface with a
deep brown or purple tint, so that it
is usually readily distinguished from
the earlier andesite; the rhyolite is
largely composed of quartz, breaks
with fine-grained, splintery frac-
ture. and is commonly light greenish-
gray in color. The general dip of
the andesites is to the north, at an
giles usually from Is to as degrees;
the rhyolite sheet, being intrusive in
the older lavas, often does not con-
form to this dip. and in some areas
is nearly horizontal.
1t is. however. when the ore-bear-
ing character of these different rocks
is considered. that this distinction
between the several flows becomes
important The earlier andesite is
the great ore-bearing formation of
the district, and for this reason is
called by the miners the "lode-por-
phyry" the rhyolitein places carries
workable deposits of ore; while the
later andesite, known as 'cap-rock"
is usually barren. Owing to causes
which 1t is not necessary to here dis
cuss. the veins are larger and more
regular where traversing the earlier
andesite. and often contract in width,