North-central Quesnellia includes Late Paleozoic island arc and oceanic subterranes, the Lay Range and Slide Mountain, respectively; but its identity is based primarily on the voluminous deposits associated with the Early Mesozoic Quesnel arc, a fossil intraoceanic island arc.  The Quesnel arc had two phases of development, Late Triassic and Early Jurassic.  Its Late Triassic inception is represented by the Takla Group.  In the Takla Group, basal Carnian (to Norian?) basinal sedimentary rocks of the Slate Creek succession are overlain by late Carnian to Norian volcanic and volcaniclastic successions: the Inzana Lake, Witch Lake, Willy George and Plughat Mountain successions.  Considerable facies variability within the Late Triassic arc is reflected by the application of these local names, which depict discrete volcanic centres that overlie but also grade laterally into epiclastic deposits.  The Triassic early phase of arc development was dominated by augite-phyric basalt and alkalic basalt (shoshonite suite) volcanism, although rocks of more felsic compositions are also present.  The Triassic arc volcanic successions are overlain paraconformably by Early Jurassic volcanic suites, the Chuchi Lake and Twin Creek successions.  This depositional break corresponds to a volcanic hiatus but not to any discernible deformation.  The Early Jurassic suites show much greater compositional heterogeneity than the Late Triassic successions, and also dominance by plagioclase and plagioclase-augite phyric, subalkaline to shoshonitic lithologies.  They probably represent a more mature arc developed on thicker crust.  A sedimentary interval within the Chuchi Lake volcanic succession is of early to late Pliensbachian age.

The Quesnel arc is important from an economic point of view, due to a rich endowment of alkalic porphyry copper-gold mineral deposits.  The very large Mt. Milligan deposit, discovered in 1987, lies within the project area.  It centres on two minor intrusive bodies, the MBX and Southern Star stocks, which are crowded-porphyritic monzonites texturally similar to those that host all of the alkalic porphyry deposits of Quesnellia.  Mt. Milligan is part of a much more extensive local swarm of porphyry deposits and potassic-propylitic alteration zones, located south of the southeastern end of the Hogem batholith.  Early Jurassic zircon ages from intrusive bodies associated with porphyry-style mineralization in this area, clustered around 204 and 189-182 Ma, show that intrusive/metallogenetic episodes occurred during the later history of the Quesnel volcanic arc, near the ends of the two volcanic episodes described above.

Major transcurrent and related faults of Cretaceous and Early Tertiary age transect the Bulletin 99 project area, juxtaposing strata of widely varying ages including Early Tertiary graben fill.  Strands of the Manson-McLeod system, the Discovery Creek fault, and one splay of the Pinchi fault are recognized.  These faults have made geologic mischief in obscuring previous structural relationships between the Lay Range and Slide Mountain terranes and the Quesnel arc, and in truncating the eastern edge of the Mt. Milligan deposit.