The POTOSI showing is located on a low ridge between Rendell and Waterloo creeks, approximately 3.65 kilometres northwest of Lightning Peak.
The showing is hosted by limestone and schist of the Devonian- Triassic Harper Ranch Group. Metavolcanic rock forms a large part of the Harper Ranch Group to the north of the showing. A large mass of granodiorite and diorite, which outcrops to the north and west, is part of an unnamed Middle Jurassic intrusion. Quartz porphyry dikes intrude the Harper Ranch Group and are associated with quartz veining.
The POTOSI showing consists of quartz veins exposed at several locations on a broad-topped ridge, informally referred to in the 1920s as the Baby Range. The POTOSI 1-4 claims were located by J. Graham, of Greenwood, in 1921. Intermittent stripping and trenching was carried out through the 1920s and 1930s. Most work focused on shear zones parallel to that of the WATERLOO mine (082ENE017), 600 metres to the south.
The POTOSI LOC. 6 occurrence, included in this showing, consists of 2 parallel quartz veins, which strike a few degrees east of north and lie about 50 metres apart. The westernmost of the two veins has been exposed by a number of open-cuts for about 90 metres. The vein width varies from 60 to 90 centimetres, and it is mineralized with disseminated pyrite and minor galena. Another exposure, 250 metres to the north, reveals a 1.2-metre wide, barren quartz vein.
Approximately 660 metres to the west-southwest, a trench has exposed a small vein containing minor pyrite and calcite. The vein is associated with a quartz porphyry dike. Several trenches, 720 metres to the north-northwest of POTOSI LOC. 6 occurrence, expose a shear zone containing some quartz veining, pyrite, and iron and manganese oxides. The shear is thought to trend to the northeast. A grab sample from one of the trenches assayed 1.36 grams per tonne gold and 1197 grams per tonne silver (Minister of Mines Annual Report 1927, page C227). Another report refers to a 2.4-metre wide vein that carried silver, galena, and tetrahedrite in a gangue of calcite. All of these occurrences are now collectively known as the POTOSI showing.
From 1968 to 69, International Mine Services Ltd. carried out geochemical and geological surveys over the area around the WATERLOO mine. Anomalies associated with shear zones were identified. In 1977, the POTOSI area was staked as the JON claim; and in 1979, a program of soil geochemistry and a magnetometer survey was carried out by Lightning Minerals Inc. Several spot anomalies were found; however, a review of the data by Sawyer Consultants Inc. concluded that no significant geochemistry-magnetic coincidence was encountered.
The JON claim was optioned by Mohawk Oil Co. Ltd. and during the period 1981-1984, they carried out a several programs of mapping, prospecting, trenching and induced polarization. Their prospecting and mapping identified over 60 hand-dug trenches dating from the 1920s and 1930s. In 1984, Mohawk Oil excavated 15 trenches, for a total length of approximately 500 metres. Trench 84-1, located in the vicinity of the POTOSI LOC. 6, exposed a 60-centimetre wide quartz vein over a strike length of 85 metres. Mineralization consists of pyrite, hematite, limonite, minor galena and a trace of sphalerite. A 38-centimetre channel sample assayed 2.05 grams per tonne gold, 197 grams per tonne silver, 0.15 per cent lead and 0.05 per cent zinc (Assessment Report 13319).
Approximately 500 metres to the southwest, trench 84-9 exposed a 3-metre wide zone of kaolinite and sericite alteration on the hangingwall of an east-west shear zone. This trench is located in a small body of intensely limonite altered granodiorite. The alteration zone hosts small lenses of chalcedony containing minor galena and sphalerite mineralization. Analysis of the lenses returned only traces of precious metals. The induced polarization survey, carried out in 1984, was inconclusive.
In 2011, Tower Resources optioned the property and completed a program of geological mapping and sampling from outcrops, trenches, and dump piles located near old workings. New areas of precious metal enriched rocks were also identified during this work. The 39 samples, collected over an area of 4 by 1.3 kilometres, average 112 grams per tonne silver and 0.9 grams per tonne gold. Highlights from this sampling program included four samples which assayed up to 303.0 grams per tonne silver and 4.75 grams per tonne gold (http://www.towerresourcesltd.com).
