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UBC Theses and Dissertations

Genesis of the sulphide mineralization at the Big Ledge Property, B.C. Morris, Arthur


The Big Ledge property is a group of claims in the Upper Arrow lake area, opposite St. Leon hot springs and about 7 miles west of the lake itself. The property is underlain by rocks of the Hamill series of late Precambrian age. These rocks are composed of schists, paragneisses, and crystalline limestone. Intimately associated with these rocks are pegmatite, and banded gneissic and granitic rocks of the pegmatite-gneiss complex of the Nelson batholith. The structure of the Precambrian rocks along the ledge is regular. The beds strike approximately east-west and their average dip is about 40 degrees to the south. The mineralized zone appears to be conformable with the enclosing rocks. However, instead of swinging to the south as one would expect with a decrease in elevation of 3000 feet to the east, the zone continues to outcrop in roughly the same east-west direction throughout its length. This seems to indicate a structural control of some sort if the mineralization is to be attributed to hydrothermal solutions. The mineralization of the Big Ledge is confined to a zone remarkable for its continuity and uniformly low grade. Pyrrhotite is the most abundant sulphide, sphalerite and pyrite occur in about equal quantities, and galena is present in minor amounts. The mineralization does not extend across the entire width of the ledge but is confined to bands or lenses a few inches to several feet thick within the zone. In these bands the sulphides occur either as disseminations in or as massive replacements of silicate minerals that are the products of metamorphism of impure dolomitic limestones. Commonly massive bodies of sulphides appear to be associated with pegmatitic intrusives in the mineralized zone. Geological reconnaissance and laboratory investigations indicate at least two distinct kinds of metamorphism. The first type, regional metamorphism, is indicated by the prevalence of garnet-mica schist throughout the Upper Arrow lake area. The second kind, thermal metamorphism, is apparently restricted to the mineralized zone. It is thought that the thermal metamorphism was the first of a sequence of events, including injection of the pegmatitic material and circulation of hydrothermal solutions that resulted in the low-grade mineralization of the Big Ledge.

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