A MASSIVE “CHALLIS” EPISODE HYDROTHERMAL SYSTEM IN SOUTH-CENTRAL BRITISH COLUMBIA: STABLE ISOTOPE EVIDENCE FROM THE CORYELL INTRUSIVE SUITE AND PENTICTON VOLCANICS

A well-exposed suite of Eocene alkaline volcanic and plutonic rocks associated with continental extension and related to the “Challis” epsode occurs in south-central British Columbia. Stable isotopes document a massive meteoric-hydrothermal system associated with the intrusion of the Coryell Intrusive Suite (CIS) into the shallow crust during the latest stages of extension in southern British Columbia and a cogenetic relationship between the CIS and the Penticton Volcanics (PV). The CIS is dominated by homogeneous plutons of porphyritic pink syenite with orthoclase megacrysts in a plagioclase, amphibole and biotite matrix. Miarolitic cavities suggesting shallow emplacement. Pervasive epithermal alteration (~350°C) is dominated by chlorite and epidote. Most δ¹⁸O values (–7.5 to +8.5‰) of mineral pairs are drastically out of equilibrium; this indicates ¹⁸O/¹⁶O exchange between rocks and meteoric-hydrothermal fluid. Sparse quartz δ¹⁸O values are homogeneous at +8.5‰ for one pluton and +6.0‰ for another, hinting a low-¹⁸O magma with a primary lower crustal source and secondary assimilated host rock. Mineral δD values cluster between ­–175 and –130 (just one sample preserves a primary δD value of –68). Together, these data fill out the horizontal limb of an inverted “L” pattern, indicating a high water-rock ratio system. The lower section of the cogenetic PV is alkaline, as indicated by sanidine and acmite phenocrysts, with compositions becoming more silicic up section as phenocryst assemblages become more plagioclase and hornblende rich, with quartz appearing towards the top. Zeolite-rich alteration and variable mineral δD values (­–76‰ to –161‰) indicate lower temperatures (< 200°C) and a less vigourous meteoric-hydrothermal system (lower W/R) than the CIS. Biotite and amphibole δ¹⁸O values (+3.5‰ to ~ +6.0‰) from samples with δD > –120‰ and wt.% H₂O < 3% are within the range of deep-crustal sourced alkaline magmas. This event, similar to the Challis-related system documented by Criss and Taylor (1983) for the Idaho Batholith, marks the final stage of massive regional-scale (>10⁵ km²) Eocene hydrothermal activity related to crustal extension, detachment faulting, and magmatism that affected south-central British Columbia.