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Silver Cup mine, Lardeau : regional frame-work and structural ore control Trettin, Hans Peter


The Silver Cup mine is about 10 miles east of Trout Lake in the Central Lardeau. From 1895 to 1915 it produced about 1.5 million oz. of silver and some lead, zinc, and gold. The problem of the paper is to study its geological setting and structural ore controls. Eastwood has shown that the mine is close to the axial plane of a major isoclinal anticline that is overturned to the southwest and plunging to the northwest. He has correlated the greenstones in the core of the anticline with the top of the Bunker Hill Group and has named the overlying black slates and phyllites Triune Formation. These two stratigraphic units were divided into three and four members respectively. The repetition of certain horizons and the trend of contacts indicates that the major anticline here has two apices separated by a tightly compressed syncline. Ninety five per cent of the production of the mine came from a zone that has a maximum length of 300 feet, a maximum width of 200 feet and has been stoped down to 1200 feet below its outcrop. Geological mapping shows that the ore is contained in openings of a structure that is a combination of a drag fold and a compressional bulge which is dipping with the host horizon to the northeast and raking steeply to the northwest. Host is the basal member of the Triune formation, a siliceous graphitic slate. Three other ore zones in the vicinity of the main zone have a similar lenticular shape and steep rake and are contained in the same member, but they are not all in the same structural position with respect to the two apices of the major anticline. These observations suggest that the mechanical properties of the host rock rather than a continuous structure such as a fault or a shear zone are responsible for the localization of ore. It is shown how the texture of the host rock, the thickness of the host member and the texture of the overlying rocks facilitate the formation of lens like openings if differential stresses are applied. Due to the steep rake of the structures, these stresses cannot be related to relative movement of outer layers towards the apices of the anticline. Their origin is not known but two hypotheses based on field evidence are offered.

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