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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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