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UBC Theses and Dissertations

The geology and petrology of the Averill Alkaline Plutonic Complex, near Grand Forks, British Columbia Keep, Myra


The Averiil Alkali Plutonic Complex is an informal name given to a suite of alkalic plutonic rocks that occur in southern British Columbia, approximately 80 kilometres north of Grand Forks. The suite comprises five plutonic members, ranging in composition from pyroxenite to syenite, which have been intruded by two later coeval dyke swarms. The first four members of the plutonic suite include pyroxenite, monzogabbro, monzodiorite and monzonite, and define a concentrically zoned intrusion with pyroxenite at the centre and monzonite at the edge. The fifth member, a syenite, was intruded through the centre of this concentric zonation, causing brecciation of the pyroxenite and monzogabbro. The first four members of the alkali suite have the same mineralogy, including pyroxene, amphibole, biotite, alkali feldspar, plagioclase feldspar, apatite, sphene, oxides and sulphides, with occasional epidote. The only difference between the four lithologies is the modal proportions of each mineral present. The syenite differs in mineralogy comprising essentially alkali feldspar with interstitial mafic phases and poikilitic garnet. Apatite and sphene are also present. Electron microprobe analysis demonstrates that the constituent pyroxenes are uniformally augite/salite, and some zoning is present. The amphibole is hornblende, that shows no variation in chemical composition. Biotite has a uniform composition also. Poikilitic garnet in the syenite is andraditic in composition. Alkali feldspars in the pyroxenites to monzonites range from orthoclase (100) to albite (100). Plagioclase compositions range from albite (100) to anorthite (50). The syenites contain only alkali feldspar ranging from orthoclase 70 to orthoclase 100. There is good field, petrographic and geochemical data to propose that the pyroxenite to monzonite sequence of rocks is cogenetic. Furthermore, Pearce element ratio diagrams suggest that the syenite can be related to the monzonite through simple fractionation of alkali feldspar. Rare platinum group element mineralization in the map area is associated with copper rich mineralization derived from the intrusion of the syenite.

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