Fluid Evolution and Mineralization of Two Distinct High-Sulfidation Epithermal Occurrences in the Toodoggone District, British Columbia, Canada

Abstract

At the Toodoggone District of northern British Columbia, the Ranch area and the Silver Pond zone within the Lawyers area represent two distinct high-sulfidation epithermal occurrences that show some broad similarities but many significant differences. Similarities include some overlaps in terms of gangue mineralogy and alteration style, but pronounced differences include ore mineralogy (relatively simple, comprising few common sulfides at Silver Pond, versus more complex, comprising an array of sulfides and sulfosalts at Ranch) and especially textural styles of mineralization (with common open-space filling textures of veins and breccias at Silver Pond, versus mostly disseminated/replacement style at Ranch with the only open space being sparse vugs). Here, we explore the mineralogy, paragenesis, and fluid evolution of these two nearby yet disparate occurrences to deduce factors that influenced their different styles. Fluid inclusion results clearly indicate persistent boiling throughout the whole paragenesis of both systems. However, microthermometric analyses reveal distinct trends, with evidence of progressive cooling and dilution at Silver Pond, versus apparently isothermal mixing between saline and dilute fluids at Ranch. Specifically, the well-constrained paragenetic sequence of open-space vein minerals at Silver Pond shows a clear and monotonic progression from an early, ~325 °C fluid of ~15 wt.% NaCl eq., towards a later fluid of ~100 °C and nearly zero salinity, consistent with progressive incursion of cold meteoric water. In contrast, the fluids at Ranch show little cooling but span an array of salinities from ~25 wt.% NaCl eq. down to nearly zero, suggesting mixing between a saline magmatic fluid and hot, deeply-circulated groundwater. Hence, these two occurrences seem to represent interesting case studies that document different hydrothermal processes and fluid evolutions, leading to distinct mineralization styles in the epithermal environment.