The Shamrock property is situated on the steep north bank of the Similkameen River, known locally as Holmes Mountain, about 8 kilometres east of Princeton. This area north of the Similkameen River is underlain to the east by intrusive rocks of the Early Jurassic Bromley batholith, and to the west by volcanics and minor sediments of the Upper Triassic Nicola Group.
The numerous areas of mineralization comprising this occurrence lie in the western margin of the Bromley batholith, near the contact with Nicola Group rocks. Here, various north-trending bodies and dyke-like masses (roof pendants?) of dark green to greenish grey porphyritic andesite and basalt are engulfed in grey to pink, coarse- grained granodiorite, granite and quartz diorite. The volcanic bodies are 8 to 30 metres wide and extensively altered and fractured.
Chalcopyrite, bornite and pyrite occur along fractures and shears in a gangue of quartz and brecciated country rock within the volcanic masses. This mineralization is strongest at or near intrusive contacts. The zones trend north and exhibit extensive malachite staining over widths of up to 15 metres and occur in the cliffs just below the plateau-like summit of Holmes Mountain. Small inclusions of skarn-altered sediments, containing chalcopyrite and pyrite in a gangue of epidote and garnet, are sometimes found in the volcanics and intrusives. Samples from various old workings assayed 2.3 to 6.6 grams per tonne gold, 34 to 1030 grams per tonne silver and 3.5 to 16 per cent copper (Minister of Mines Annual Report 1908, page 130).
One zone of fracturing and shearing on the Blue Ridge claim, on the south side of Holmes Mountain, has been traced northward by tunnelling for 37 metres. One prominent shear, up to 0.9 metre wide, contains malachite, azurite, chalcopyrite and pyrite over a width of 25 centimetres and a length of 6 metres in a gangue of brecciated volcanics cemented with quartz and calcite. A sample of sorted ore from this shear assayed trace gold, 27 grams per tonne silver and 7.2 per cent copper (Minister of Mines Annual Report 1915, page 242). Nine tonnes of ore mined from this zone and shipped to the Granby smelter at Grand Forks averaged 35 grams per tonne silver and 15 per cent copper (Minister of Mines Annual Report 1919, page 172). A crosscut indicates the adjacent unfractured volcanics are mineralized with disseminated chalcopyrite and pyrite for at least 23 metres from the zone of fracturing and shearing.
This occurrence has been explored by a number of trenches and adits excavated between 1908 and 1919. A total of 9 tonnes were mined in 1913.