NovaGold reports capitalization costs for Donlin Creek

Exploring Donlin Creek in 1988. Photo of the author taken while mapping on a
ridge overlooking Snow Gulch (background)

The Donlin Creek gold deposit in the central Kuskokwim Basin in southwestern Alaska, is one of the largest, undeveloped, gold deposits on earth and contains as much gold as the Homestake mine. And it is likely to be larger with continued exploration of the deposit and future mining. The gold resource is more than 40 million ounces. To develop this property, it is estimated that capitalization will be as much as $7 billion in that the property is in the middle of nowhere in the Kuskokwim Mountains in southwestern Alaska. 

NovaGold reported proven and probable mineral reserves of 33.6 million ounces, measured and indicated mineral resources of 4.3 million ounces and inferred resources of 4.4 million ounces.

Rocks in the basin include Upper Cretaceous Kuskokwim Group, the most extensive rock unit in southwestern Alaska that consists of graywacke, siltstone, and shale. Additional rock types  include Proterozoic metamorphic rocks, Paleozoic clastic and carbonate rocks, and Mesozoic marine volcanic rock.

Late Cretaceous and Early Tertiary volcano-plutonic complexes in placers, intrude and overlie Kuskokwim Group sediments. The volcanic rocks are intermediate tuffs and flows that are  dominantly andesite, but also include dacite, rhyolite, and basalt. Plutons consist of calc-alkaline monzonite and granodiorite. Felsic to intermediate subvolcanic granite to granodiorite porphyry dikes, sills, and plugs are widely distributed and host gold mineralization at Donlin Gold.

NovaGold goes on to report, the Kuskokwim basin was formed by two continental-scale right-lateral strike-slip fault zones. The Denali−Farewell Fault bounds the southern flank of the basin, and the Iditarod−Nixon Fork Fault system limits the north.

Folding and thrusting followed deposition of Kuskokwim sediments. Eastward-trending folds and thrust faults are common in the central Kuskokwim basin, including Donlin Creek area.

Map from NovaGold.

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.

The Donlin Gold deposits comprise a northeast elongated cluster, roughly 5,000 feet wide x 10,000 feet long, that extends vertically over 3,100 feet. The deposits are hosted primarily in igneous rocks associated with a late Cretaceous hydrothermal system. Gold occurs in broad disseminated sulfide zones in rhyodacite and in vein networks.

Two primary deposits are recognized at Donlin: (1) ACMA and (2) Lewis. These 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 feet. 

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 feet 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%.