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The timing and structural evolution of the Donlin Creek gold deposit, southwest Alaska

The >30 million ounce Donlin Creek deposit in the Kuskokwim Mountains of southwestern Alaska (Fig 1.1) has characteristics of epithermal, reduced granite, Carlin style and orogenic gold deposits. It has
been classified as alkalic-rock-related (Bundtzen and Miller, 1997), low sulphidation epithermal (Ebert et al, 2000a), shallow reduced intrusion related or high level intrusion related (Ebert et al, 2002; Hart et al., 2002), and orogenic gold mineralization (Goldfarb et al., 2002). The structural evolution of this gold deposit and host rocks as well as the timing of those events with respect to the geologic history is critical to any of these models.
Donlin Creek is a high tonnage, low grade gold deposit that has measured and indicated resource potential of 371.7 Mt @ 2.46 g/t Au (29.38 Moz Au) with an additional inferred resource of 46.5 Mt @ 2.46 g/t Au (3.46 Moz Au) (NovaGold Resources inc. press release February 7, 2008). Gold mineralization is refractory and is associated with arsenopyrite. The highest gold grades are located at intersections between mineralized fracture zones and granite porphyry intrusions.
This MSc project was sponsored by Barrick.

Project Information

Commodity:
Research Themes:
Location:
Alaska, USA
Project Status:
End Date: 2009

Lithologic and structural data from more than 220 structurally oriented diamond drill holes are used to delineate the subsurface geology, 3-D geometry and to constrain the structural evolution of the >30 million ounce Donlin Creek gold deposit. The geology of the Donlin Creek gold deposit is characterized by north-northeast – south-southwest shortening that produced structural fabrics similar to those formed in thin-skinned fold and thrust belts. This deformation consists of dominantly southwest dipping thrust faults, subordinate northeast dipping back thrusts, and associated thrust ramp anticlines in the Kuskokwim Group sedimentary rocks. The fold and thrust style deformation is locally overprinted by northeast trending, low amplitude open folds interpreted to be related to regional dextral strike-slip tectonics. Low displacement, north-northeast striking, steeply southeast dipping normal faults cut the thrust faults and both generations of folds. The normal faults, northeast trending folds, and older fold and thrust geometry are intruded by a porphyritic dyke and sill swarm. Bedding discordant intrusions are emplaced along the north-northeast striking, steeply southwest dipping normal faults. Bedding concordant intrusions are most common in the folded sedimentary rocks in the hanging walls of thrust faults. The intrusions are cut by tensional gold being quartz ± carbonate + sulphide veins, which represent the youngest structural fabric. The distribution of veins is dictated by the rheology of the host rocks. For example, the brittle intrusions are the best host to gold mineralization; however veins are also present in coarse grained sedimentary rocks. U-Pb and ⁴⁰Ar/³⁹Ar geochronology provide absolute ages for deposition of the Kuskokwim sediments (~ 88 Ma), emplacement of the felsic intrusions (~71 Ma) and timing of gold mineralization (~ 71 Ma). These data indicate approximately 14.9 million years between initial sedimentation, emplacement of intrusions, and precipitation of gold mineralization. Geochronology constrains the timing of fold and thrust style deformation, and overprinting north-northeast trending folds to between ~ 88 Ma and ~ 71 Ma. The available geologic and geochronologic data also indicates that intrusion of the post-kinematic Late Cretaceous

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