A regional-scale reconnaissance bedrock mapping study was conducted in the Hotailuh batholith area, near the town of Dease Lake, as part of Geoscience BC's QUEST-Northwest Project. This mapping program is a partnership project between the BC Geological Survey and Geoscience BC. This study combines one summer of field bedrock mapping with detailed petrography, geochemistry of both altered/mineralized and unaltered samples, geochronological data and integration with airborne geophysical survey results.
The 2275 km2 composite Late Triassic to Middle Jurassic Hotailuh batholith occurs within the Stikine terrane of northern British Columbia. The investigation aimed to refine the temporal magmatic and geochemical evolution of the batholith, and specifically focused on relating mineralization with magmatic events.
The Late Triassic plutonic suite comprises the Gnat Lake ultramafic to mafic bodies, Cake Hill felsic pluton and Beggerlay Creek ultramafic to mafic pluton. The absence of equivocal crosscutting relationships, lack of absolute crystallization age determinations for the ultramafic to mafic intrusions, and the absence of detailed mineral chemistry data makes it difficult to determine the genetic and temporal relationships between the different Late Triassic intrusions. The Late Triassic intrusions are spatially associated with, and in places intrude, poorly exposed and poorly dated, intermediate-mafic volcanic and sedimentary rocks of the Triassic Stuhini group. The Middle Jurassic Three Sisters pluton comprises at least four phases, namely the finegrained mafic-intermediate phase, a mafic phase, central felsic phase, and a crosscutting potassic phase. U-Pb zircon crystallization ages and Ar-Ar cooling ages confirm that all phases formed during a relatively short time span in the Middle Jurassic. The northern and eastern margin of the batholith is nonconformably overlain by a succession of Early Jurassic basal sedimentary rocks grading upward into Early-Middle Jurassic volcanic rocks. The volcanic rocks were previously mapped as Triassic Stuhini group, but new geochronological data and mapping suggest they are part of the Jurassic Hazelton group.
Geochronology and geochemistry confirm the presence of at least two discrete magmatic events in the Late Triassic and Middle Jurassic. Both magmatic events have a strong calc-alkaline subduction-modified subcontinental mantle geochemical signature, and formed in a volcanic island arc to continental volcanic arc setting. At least two episodes of mineralization associated with Late Triassic and Middle Jurassic magmatism are found in the area. In addition to the known mineral occurrences within the batholith and its immediate surroundings, this study identified several new mineral occurrences. The intrusion-related magmatic-hydrothermal mineralization comprises predominantly copper, gold, silver, molybdenum and/or tungsten occurrences. Mineral occurrences occur peripheral to the main batholith and along contact zones between different magmatic phases within the batholith. The Gnat Pass ductile to brittle shear zone on the western margin of the batholith appears especially well endowed with mineral occurrences, and includes the newly dated (ca. 216.5 Ma) porphyry copper prospect at Gnat Pass.