Abstract Significant lithium (Li) resources have been discovered in sedimentary basin brines across North America, including the Western Canada Sedimentary Basin. Although the basin’s geology is well characterized through decades of hydrocarbon development, the origin and distribution of Li remain understudied, complicating exploration efforts. The Triassic Montney Formation, a prolific unconventional hydrocarbon play in northeastern British Columbia and west-central Alberta, has been identified as a potential target for brine-hosted Li resources due to elevated concentrations and high volumes of fluid processed related to hydraulic fracturing. In this work, integrated geochemical and isotopic (δD, δ 18 O, 87 Sr/ 86 Sr) data are used to elucidate the Li source and enrichment mechanisms in formation waters of the Montney Formation. Lithium concentrations range from 3.5 to 99.7 mg/L and have little correlation with total dissolved solids (TDS), Mg, K, or Ca. Salinity is interpreted to be derived predominantly from seawater evaporation, and δD vs δ 18 O and 87 Sr/ 86 Sr ratios indicate that Li enrichments are influenced by water–rock interactions. Whereas there was no direct correlation between δ 18 O or 87 Sr/ 86 Sr and Li content, there are isotopic values above which elevated Li concentrations are observed. We interpret that Li is sourced from volcanically derived clay minerals that have been affected by a series of diagenetic reactions including dolomitization, albitization, and illitization which may be linked to other factors such as seawater evaporation, time, basin hydrogeology and stratigraphy, and tectonic setting. We present a framework to describe the generation of Li-rich basinal brines that contributes to the development of a mineral systems approach to assist in the discovery of new brine-hosted Li resources in sedimentary basins.