Geology
At Hatcher Pass the southwestern margin of the Cretaceous to Tertiary age Talkeetna Mountains batholith is in intrusive contact with an older pelitic schist unit. The Talkeetna Mountains batholith in this area consists of a 74 Ma (million years old) tonalite body to the east and a 67 Ma quartz monzonite to the west. The schist consists mainly of metamorphosed and deformed sedimentary rocks, probably of Jurassic age. Plutonic bodies of Jurassic age, including dikes, occur within the schist; some are deformed indicating they were intruded before deformation of the schist, whereas some are not and so must postdate deformation. Unmetamorphosed Tertiary terrestrial sediments of the Chickaloon and Arkose Ridge Formations lie to the south of the schists and intrusives.
Gold-bearing (+/- Ag, W, Sb, As, Cu, Mo, Pb, Te, Zn, Hg) veins occur in the 74 Ma tonalite, the schist, and the Jurassic intrusives, but not in the 67 Ma quartz monzonite or in the Tertiary sediments.
The Castle Mountain fault is a major tectonic feature. It strikes ENE and passes a few miles south of Hatcher Pass. It can be clearly seen crossing the Hatcher Pass road where concrete barriers and fencing protect the road from landslides of the incompetent rocks on the fault trace. Studies show that magnitude 7 earthquakes can be expected to occur on this fault with approximately a 700-year recurrence interval. The last big earthquake was probably 650 years ago.
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