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PORTER IDAHO MINING COMPANY, LIMITED

The holdings of the Porter Idaho Mining Company, Limited, are
about 4,000 feet above sea-level on the east slope of the mountain between
Portland canal and Kate Ryan creek (the north fork of Marmot river)
and adjoin the Prosperity group on the south.

The Porter Idaho group of claims was staked in 1919 and has been
under development on a small scale ever since. The group became the
property of the Porter Idaho Mining Company, Limited, a few years
after it was staked. It passed into the control of the Premier Gold Mining
Company in 1928.

The country rocks are tuffs, breccias, rhyolites, and andesites of the
Bear River formation. On the mountain side above the Porter Idaho
group the rocks strike northeast along the mountain side and dip north-
westward into the mountain at moderate angles. On the lower part of the
group the attitude of the beds is the same. The upper part of the group,
however, is covered with loose slide rock, and, therefore, the attitude of
the rocks beneath is not known, but is probably the same as higher or
lower on the mountain side. The rocks on the lower part of the Porter
Idaho group and on the adjoining property to the west are sheared along
a west-northwest direction. The shearing on the Porter Idaho claims has
produced a foliated rock, but farther west the rocks have been changed
locally to fissile schists. Several vertical lamprophyre dykes striking
north-northwest intrude the volcanic rocks.

A fault, known locally as the Big Rig fault (See Figures 13 and 14),
striking east and dipping 75 to 80 degrees north, occurs on the lower part
of the Porter Idaho property. The surface expression of the fault is a
long, shallow depression. In most places in the underground mine work-
ings, the fault is a shattered and crushed zone 30 feet or more in width;
in one place, 250 feet below the surface, it is a fracture, 6 feet wide, full
of hard gouge. A narrow shear zone occupied by an ore-bearing vein
known as the Winze vein lies north of the fault and as it approaches the
fault, swings from a southerly to a southwesterly strike. The rock where
the shear zone and the fault zone come into contact is crushed to such an
extent that the relation of one to the other is not evident. They may have
originated at the same time, in which case the shear zone may be con-
sidered as a branch of the fault. The fault may, however, be later than
the shear zone. That the shear zone is not later than the fault seems
fairly certain, as this shear zone and several other similar ones are known
north of the fault, but none is known to cross the fault. A lamprophyre
dyke crosses the fault, has not been offset by it, and is consequently
younger. Underground, however, the dyke is broken and crushed where
it crosses the fault, showing that slight movement continued along the
fault even after dyke intrusion.

Five parallel, narrow, shear zones north of the Big Rig fault are
occupied by ore-bearing veins. Ore is present in the shear zone, that is
in the Winze vein, which comes in contact with the fault, and similar ore
occurs locally in the fault zone a short distance west. The whole of the
fault zone except where it consists of gouge appears to contain dissemi-
nated sulphides. ‘The crushed rock, however, is everywhere covered with

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