Geology & Mineralization

The Donlin Gold deposit includes 11 mineralized areas.

Geology

The Donlin Gold deposit lies in the central Kuskokwim basin of southwestern Alaska.  The basin fill predominately consists of the Upper Cretaceous Kuskokwim Group, the most extensive rock unit in southwestern Alaska covering an area of 90,000 square kilometers, consisting mainly of sandstone, siltstone, and shale.  Additional rock types in the basin include Proterozoic metamorphic rocks, Paleozoic clastic and carbonate rocks, and Mesozoic marine volcanic rock.

Late Cretaceous and Early Tertiary volcano-plutonic complexes intrude and overlie the Kuskokwim Group sedimentary rocks. The volcanic rocks consist of intermediate tuffs and flows that are regionally extensive and dominantly andesitic, but may include dacite, rhyolite, and basalt. Associated plutons are calc-alkaline in composition, ranging from monzonite to granodiorite. Felsic to intermediate subvolcanic granite to granodiorite porphyry dikes, sills, and plugs are widely distributed and host gold mineralization at Donlin Gold.

The Kuskokwim basin was formed by two continental-scale right-lateral strike-slip fault zones.  Initial movement on these faults was early and long lasting, and these faults dominate the structural setting of southwest Alaska.  The Denali−Farewell Fault system bounds the southern side of the basin, and the Iditarod−Nixon Fork Fault system limits the north.

Fold-and-thrust-style deformation followed shortly after deposition of the Kuskokwim sediments. Eastward-trending folds and thrust faults are common in the central Kuskokwim basin, including the Donlin Gold area.

Younger north–northeast-trending folds formed throughout the region in response to the basin-scale movement. Most of the folds predate emplacement of the volcano-plutonic complexes. Northeast-striking normal and oblique slip faults formed during the late compressional and extensional events that resulted in mineralizing hydrothermal systems across the basin.

Deposit

The Donlin Gold deposits comprise a northeast elongated cluster, roughly 5,000 ft (1.5 km) wide x 10,000 ft (3 km) long, that can extend vertically over 3,100 ft (945 m).  The deposits are hosted primarily in igneous rocks and are associated with an extensive late Cretaceous hydrothermal system.  Gold occurs in broad disseminated sulphide zones in rhyodacite and in vein networks.

The two primary deposits at Donlin, ACMA and Lewis, have different geological settings.  The ACMA deposit is comprised of dikes and sills intruded into folded shale and siltstone rocks.  The Lewis deposit consists of dikes intruded into massive greywacke.  Mineralized material in the ACMA deposit tends to be higher grade and more continuous compared to Lewis and other dike dominant areas of the deposit.  The most extensive and highest-grade mineralized zones in ACMA are located where “feeder” dikes intersect the sill sequence.  Mineralized zones follow steeply dipping dikes and sills beyond the depth limits of current drilling, or over a vertical range of at least 3,100 ft (945 m). 

North-northeast mineralized corridors are made up of similar striking, high-angle fracture zones that are the primary control of gold-bearing veins.  These mineralized corridors of veins range up to 98 ft (30 m) wide and hundreds of meters long.  Intrusive rocks and competent massive greywacke are the favored host rocks.  Gold distribution in the deposit closely mimics the intrusive rocks, which contain about 75% of the resource.  Structural zones in competent sedimentary units account for the remaining 25%.  

Maps

 

Footnotes