Late cretaceous tectonic evolution and metallogeny of southwestern alaska
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Late Cretaceous Tectonic Evolution and Metallogeny of Southwestern Alaska. M arti L. Miller 1 Dwight C. Bradley 1 Thomas K. Bundtzen 2 Richard J. Goldfarb 3. U.S. Geological Survey, Anchorage Pacific Rim Geological Consulting U.S. Geological Survey, Denver.

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M arti l miller 1 dwight c bradley 1 thomas k bundtzen 2 richard j goldfarb 3
M Southwestern Alaskaarti L. Miller1Dwight C. Bradley1Thomas K. Bundtzen2Richard J. Goldfarb3

  • U.S. Geological Survey, Anchorage

  • Pacific Rim Geological Consulting

  • U.S. Geological Survey, Denver


Tectonic setting of the kuskokwim mineral belt
Tectonic setting of the Southwestern AlaskaKuskokwim Mineral Belt

  • Lies within a broad zone of dextral strike-slip faults

  • Occupies a backarc position ~400 km inboard of the present subduction zone

  • Occurs at the western end of a curved, continental-scale, strike-slip system


Southwestern alaska tectonostratigraphic terranes
Southwestern Southwestern AlaskaAlaska—tectonostratigraphic terranes

After Decker and others, 1984


Geology central kuskokwim mineral belt
Geology--central Kuskokwim Mineral Belt Southwestern Alaska

Kuskokwim Group


Geology central kuskokwim mineral belt1
Geology--central Kuskokwim Mineral Belt Southwestern Alaska

Volcanic-plutonic complex

Felsic porphyritic dike


Deposits of the central kuskokwim mineral belt
Deposits of Southwestern Alaskathe central KuskokwimMineral Belt

  • Epizonal Hg-Sb and Au

  • Precious metal-bearing intrusion related


Shotgun
Shotgun Southwestern Alaska

Kuskokwim Group

Granodiorite stock

Mineralized qtz-feldspar porphyry

  • 70 Ma qtz-feldspar porphyry

  • Veins, breccias, stockworks

  • Au, As, B ± Cu, Mo, Bi, Te


Epizonal hg sb deposits
Epizonal Hg-Sb deposits Southwestern Alaska

Red Devil

Cinnabar Creek


Epizonal au bearing deposits
Epizonal Au-bearing deposits Southwestern Alaska

Donlin


Late cretaceous tectonic evolution and metallogeny of southwestern alaska


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

Along strike-slip faults: Southwestern Alaska

  • Fortyseven Creek

  • Nixon Fork

Dike-bearing ridge

west of Fortyseven Cr


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

Red Devil Southwestern Alaska

Between master faults:

  • Donlin

  • Red Devil


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

What we know Southwestern Alaska

  • Dextral strike-slip motion was taking place at the time of ~70 Ma deposit formation

  • Faults focused the fluids and accompanying mineralization

  • Some of the deposits are spatially associated with the master faults and others lie between these faults


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

What we don’t know Southwestern Alaska

  • Why was there voluminous ~70 Ma magmatism over a wide area?

  • Why was the regional thermal gradient elevated across a broad region?

  • Why are both mantle- and flysch-derived intrusive rocks present?

  • What got the fluids and melts moving?

  • Are mineralization and magmatism both products of the same tectonic event?

  • What changed in the tectonic regime at ~70 Ma?


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

Escape


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

Plate ?


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

At ~60 Ma three possibilities for identity of subducting plate

--Resurrection Plate preferred

Preferred:

Miller et al.,2002

Haeussler et al., 2003

Engebretsenet al., 1985

Bradleyet al., 1993



Late cretaceous tectonic evolution and metallogeny of southwestern alaska

A witches brew: to ridge subduction

  • Curved margin

  • Oblique subduction

  • Escape to free face


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

Possible tectonic scenarios to ridge subduction

Low angle subduction

Slab break off

Ridge subduction

Escape tectonics


References cited
References cited to ridge subduction

  • Bradley, D.C., Haeussler, P.J., and Kusky, T.M., 1993, Timing of early Tertiary ridge subduction in southern Alaska: U.S. Geological Survey Bulletin 2068, p. 163-177.

  • Decker, J., Bergman, S.C., Blodgett, R.B., Box, S.E., Bundtzen, T.K., et al., 1994, The geology of southwestern Alaska, in Plafker, G., and Berg, H.C., eds., The geology of Alaska: Geological Society of America DNAG Series, v. G-1, p. 285-310.

  • Ebert, S., Miller, L., Petsel, S., Dodd, S., and Kowalczyk, 2000, Geology, mineralization, and exploration at the Donlin Creek project, southwestern Alaska: British Columbia and Yukon Chamber of Mines Special Volume 2, p. 99-114.

  • Engebretsen, D.C., Cox, Allan, and Gordon, R.G., 1985, Relative motions between oceanic and continental plates in the Pacific Basin: Geological Society of America Special Paper 206, 59 p.

  • Haeussler, P.J., Bradley, D.C., Wells, R.E., and Miller, M.L., 2003, Life and death of the Resurrection plate: Evidence for its existence and subduction in the northeastern Pacific in Paleocene-Eocene time: Geological Society of America Bulletin, v. 115, p. 867-880.

  • Miller, M.L., Bradley, D.C., Bundtzen, T.K., and McClelland, W., 2002, Late Cretaceous through Cenozoic strike-slip tectonics of southwestern Alaska: Journal of Geology, v. 110, p. 247-270.