Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1934-05-19

hinheradke streak, with every
second PenObseot' which farno'
In reply to inquiries about the
geology OF the district, G. ,.
Ropes. prominent Helena mining
engineer, wrote the Mining De
nartment OF the Journal the FOL
lowing interesting discussion OF
the occurance of gold In the
MarysVille district:
You ask about the geological
factors which have influenced
the naturalization of the veins
and deposition OF gold in the
MarygvfIie district7 The gener-
ally accepted origin Of our metal
qiferous ores lies in the deep
seated, fluid magma from which
the igneous rocks intrude into
the solid crust Of the earth,
or break out in volcanic erup-
tons at the surface; that It Is
from the magma and in associa-
ton with the igneOus rocks that
the metallfferous as well as many
non metaIliferous, commercial
ores and minerals we're derived.
The central portion OF the
Marysville district lies In an
amphitheater followed by erosion.
in the heart OF the Marysvtlle
batholith, an intrusion of quartz
4nonzonite into one of the ear
Hest, stratigraphic dispositions,
the Belt formation of Algonkian
age. The ''hOrnstones, meta
morpnosed Helena limestone and
Empire shale, upper members OF
the Belt formation rim the amphi-
theater and It is into these, and
the granite OF the batholith that
the gold veins have penetrated
In this, southern belt,-Bald
observation-disclosures Bell
Boy, in order from east to west.
we are constrained because OF
their close association herewith,
to attribute the introduction OF
the rich gold veins to the pre-
vious intrusion OF the igneous,
Belmont diorite porphyry dixie
along which all these mines are
situated. The diorite porphyry
dikes OF this type, in the district
are subsequent TO the intrusion
OF the granite (monz0nite) ana
lie in a series OF parallel dikes,
the most n0rtherly intruding
the granite, the others to the
south, cutting the Helena lime
stone and Empire shale-horn-
stones.
Consequent upon the dikes, the
gold bearing veins invaded the
formations. intersecting rocks
and dikes alike. The strike OF
the veins Is at a wide angle from
that OF the dikes, Of the latter
north with south
west dip. The strike OF the
Bald Butte system OF veins aver
age S.S5sE.. dipping south where
as that OF the Channel ana |
PenoMscOt veins approximately |
N. 7OSE., the veins dipping north
erly. Fissuring was extensive
upon the introduction OF the gold 4
hearing veins; nine distinct veins
or branches profitably worked in
the Bald Butte mine, at least
four known veins occur on the
Channel property and two branch
es we're mined in the PenoDseot
mine.
Comment has been made on
the conspicuous, black manganese
associated with the vein gangue
along the outcrops Of the VEINS
what Is its significancel
40 a rule, we find that this
oxidIzed. frequently leached, out
croD carries but a low gold con
tent say 800 to $500 per Ton
where the manganese IS In eVl-
dence. but that in depths per
manent water level Is approached
the rold tenor rises and We
antar fhA bonanza zone IN
,vbich the value OF the gold N
tbe ore may run into thousanos
of dollars per ton. This conai-
lion AZ attributed to a simple
chemical reaction, one which was
annlied IN the earlier metalurgy'
viz. that manganese and hydro
chloric acid react to release
free chloric which is q solvent
Of gold. The gold in solution
a. Il passes downward in the vein
comes in contact with sulphide
ores or other precipitant causing
the deposition Of the gold, thus