The Weiser signal. [volume] (Weiser, Idaho) 1890-1904, 1900-04-19
PROCESS OF EARLY FORMATION
Article Of VIM. Besls, Jr. IL Engineering and
Mining Journal and Repulished In
Idaho Mining journal.

The Seven Devils Mining District
is situated in the central western part
of Idaho, in Washington and Idaho
counties. The Snake river 5 miles
away, borders it on the west; and it
is about 100 miles north from the
town of Weiser. On all sides the
district is surrounded by deep cut
and narrow gulches, the Characters
tic topography Of the Lower Snake
country At the close Of the Terti-
ary basalt flows the Seven Devils and
the greater portion Of the copper dis
trict projected as rugged islands well
above the resultant lava plain. Since
that period the Snake River and
many of its tributaries have deeply
dissected and uncovered the old pre-
lava flow topography and in part
stripped off the basalt from the re-
gion lying close to the copper dis
trict, so that today, while most of
the valleys have been cut down below
the lava, the radiating buttresses are
often capped with basalt. The char
acteristic rocks of pre- Tertiary times
were extensive areas OF fine grained
quartz porphyries or rhyolites, in
which the old river systems had al
ready developed q topography Of
sharp relief. A general uplift OF con-
siderable magnitude occurred, which
resulted in much faulting and fold
ing and regional shearing, so that
today the porphyries show all phases
between thoroughly massive rocks
and we'll Illuminated schists. Associ-
Ated with them are scattered areas
OF we'll crystallized marble. Pre-
vious to the advent of the basalt, and
subsequent to the shearing, these
porphyries we're penetrated by i'm
mense bosses and dykes Of coarse
quartz diorite, which sent innumer-
able apophyses in every direction in
10 the porphyrles as we'll as into the
limestone. Contact phenomena are
we'll marked; the limestone was
changed to a coarsely crystalline mar
BILE with the formation of secondary
minerals, and many inclusions of the
metamorphosed porphyry are seen in
the dlorite.
The copper deposits of the Seven
Devils district occur at the contact OF
the diorits and porphyry, or the d.
orite and marble. These copper de.
posts or veins present some peculi-
arities, not only as to size but a. re-
CARDS composition. The veins them
selves have q width ct times of near
" 100 feet, and the character of the
vein minerals 10 the same whether
the contact be with the porphyry or
marble. The main gangue minerals
In part are iron garnet, generally
massive, but frequently crystallized
- exposed in the northern part of
the camp, in and around the Old
Peacock workings. Epidote is also
present as prominent gangue min
eral, and lesser amounts of quartz,
granular magnetic, tubular hema-
lite, calcite, and rarely molybdenite.
Powellite was first discovered in the
Old Peacock. 1t is a remarkable fact
that the garnet-epidote veins have
sharp lines Of demarkation, separat-
atlng them on the one hand from the
diorite and on the other from the
marble or porphyry. II the veins
veins are the result of contact meta
morphism we should expect to see
transition of the garnets and cpl
dotes into the lime, but only In few
instances were garnets noted as small
crystals in the marble. The vein
matter, with it's included ores, be-
haves in most cases like molten rocks
injected along the contacts and To-
cally sending massive apophyses in
great profusion into the neighboring
marble. As all contacts noted in
this district show the garnet-epldote
vein, it is reasonable to suppose that
these conditions will remain the same
in depth. The associated minerals
Tins far met with SOW IN n dont, ~