UBC Undergraduate Research

Characterizing the Bennett Batholith : implications for Early-Middle Jurassic magmatism in Southwest Yukon Pillsbury, Joshua Nash Phillips

Abstract

Late Triassic to Early Jurassic magmatic belts which extend from southern British Columbia into the Yukon-Tanana Terrane of Yukon are known to collectively host the most important porphyry Cu-Au and Cu-Mo deposits in the Northern Cordillera. Late Triassic to Early Jurassic magmatism in Yukon is currently divided into four plutonic suites, which from oldest to youngest are: Stikine (228-210 Ma), Minto (204-195 Ma), Long Lake (192-180 Ma), and Bryde (178-167 Ma). In addition to hosting the Minto and Carmacks Copper deposits in Yukon, these plutonic suites also represent the northern extension of paired plutonic belts which host significant copper-gold and copper-molybdenum deposits in British Columbia. The purpose of this study is to determine the potential of Early-Middle Jurassic intrusion-related mineralization in southwest Yukon by providing a comprehensive characterization of the Bennett batholith, the largest pluton of the Bryde plutonic suite. At over 50 km in length and 15 km in width, the intrusion is a weakly foliated, coarse to medium grained, leucocratic granodiorite to quartz monzonite, characterized by large (up to 5 cm) locally aligned alkali feldspar phenocrysts. To facilitate creating a tectono-magmatic framework for the Bennett batholith, three other plutons are sampled for comparison: the Aishihik batholith of the Long Lake suite, the Fourth of July batholith from the Bryde suite, and the Mt. Bryde pluton of the Bryde suite. This characterization and comparison includes modal mineralogy using the image-processing program ImageJ, petrography, major and trace element geochemistry, uranium-lead zircon dating, and plagioclase-hornblende geothermobarometry. The Bennett batholith is an Early Jurassic (178.20 ± 0.05 Ma), medium grained, leucocratic, equigranular to megacrystic, (rarely) biotite-bearing hornblende granite to granodiorite which may have been emplaced at depths of 4.5 ± 1.5 km below the surface. In addition to having similarities in composition, texture, mineralogy and tectonic setting with rocks of the Aishihik batholith of the Long Lake plutonic suite, the Bennett batholith is at the very least 2 Ma older than the Fourth of July batholith or Mt. Bryde pluton. Preliminary constraints on age, emplacement depth, oxidation state and hydrothermal activity suggest the calc-alkaline Bennett batholith appears to have been a fertile system for copper porphyry generation.

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