page_title: Hatcher Pass
section_title: Geology
hierarchical_section_title: Hatcher Pass / Geology
caption_attribution_description: English: Hatcher Pass gold
context_page_description: Hatcher Pass is a mountain pass through the southwest part of the Talkeetna Mountains, Alaska. It is named after Robert Hatcher, a prospector and miner. The nearest cities are Palmer and Wasilla approximately 12 miles to the south, and Willow approximately 26 mi to the west. The communities are at an elevation of approximately 250 ft in the Mat-Su valley.
context_section_description: The Willow Creek mining district lies at the southwestern edge of a great mass of granitic intrusions that form much of the Talkeetna Mountains. The area is within the Wrangellia composite terrane 
  At Hatcher Pass a pervasively altered zoned 74 Ma (million years old) quartz diorite to tonalite pluton underlies the headwaters of Willow Creek and Fishhook Creek; most of the gold deposits occur in these rocks. A 67 Ma quartz monzonite pluton lies west and north of the older quartz diorite, a Cretaceous (?) quartz diorite pluton lies to the east.
  A high angle east-west fault passing through Hatcher Pass separates the plutonic rocks on the north side of the fault from the Hatcher Pass schist south of the fault. The Hatcher Pass schist consists mainly of metamorphosed and deformed sedimentary rocks, of Late Cretaceous to Paleocene age. The schist may represent subducted Valdez Group that was exhumed in the forearc region from beneath the Peninsular terrane. Both deformed and undeformed small felsic dikes occur in the schist. Several bodies of serpentinite are contained within the schist.
  West of Government Peak, the Arkose Ridge Formation lies to the south of the schist, the contact is a low-angle detachment fault. The Arkose Ridge Formation is unmetamorphosed but tilted and slightly folded, and consists of Tertiary (Late Paleocene to middle Eocene) sedimentary arkose, shale, sandstone, and conglomerate rocks that are clearly derived from the schists and intrusive rocks to the north. Locally, quartz diorite or gneiss bodies occur on the arkose-schist contact west of Government Peak. Minor basalt flows occur within the Arkose Ridge Formation.
  On Government Peak a 90 mya rock unit variously mapped as gneiss, or as intricately intermixed amphibolite and quartz diorite, or as a migmatite, and most recently as a mafic unit of gabbro and pyroxenite with local foliation and mylonitic textures structurally underlies the Arkose Ridge Formation and outcrops in between the Hatcher Pass schist and the arkose; south-dipping low angle detachment faults separate the units. East of Government Peak and the Little Susitna River the mafic rock unit is in contact with the Arkose Ridge Formation to the south and the felsic plutons to the north.
  Gold-bearing (+/- Ag, W, Sb, As, Cu, Mo, Pb, Te, Zn, Hg) veins and shear zones occur in the 74 Ma (million years old) quartz diorite, in small amounts in the schist and in the mafic unit, but not in the 67Ma quartz monzonite, and in only one instance in the Tertiary sediments. The gold occurrences in the schist, mafic unit, and sediments have much higher silver contents than the deposits in the quartz diorite. Most of the mineral deposits are close to the quartz diorite-schist contact.
  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 rockfall 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.
caption: Historical marker developed by Alaska State Parks
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