The Donlin deposits are situated approximately 280 miles (450 km) west of Anchorage and 155 miles (250 km) northeast of Bethel up the Kuskokwim River. The closest village is the community of Crooked Creek, approximately 12 miles (20 km) to the south, on the Kuskokwim River.
Placer gold was first discovered at Snow Gulch, a tributary of Donlin Creek, in 1909. Intermittent small-scale placer gold production has continued to the present. Resource Associates of Alaska (RAA) carried out a regional evaluation for Calista in 1974 to 1975. This work included a soil grid and three bulldozer trenches in the Snow area immediately north of the current resource area. Calista followed up with prospecting activities between 1984 and 1986, and completed minor auger drilling in 1987.
The first substantial exploration drill program was carried out by Western Gold Exploration and Mining Co. LP (WestGold) in 1988 and 1989. WestGold completed geological mapping, trenching, rock and soil sampling, an airborne magnetic and VLF survey, and ground magnetic surveys. WestGold also tested biogeochemical sampling and ground penetrating radar with positive results. Based on this information, WestGold performed an initial Mineral Resource estimate.
Teck Exploration Ltd. (Teck) carried out a limited trenching and soil sampling program in the Lewis area in late 1993, and updated the Mineral Resource estimate.
Placer Dome US (Placer Dome) explored the property from 1995 to 2000. Placer Dome constructed an exploration camp and airstrip, undertook reconnaissance and geological mapping, aerial photography, completed rock chip and soil sampling, trenching, max-min (EM) geophysical surveys, airborne geophysical surveys, RC and core drilling, carried out detailed metallurgical test work, and prepared a series of Mineral Resource estimates and initial mining and engineering studies.
Placer Dome formed the Donlin Creek joint venture (DCJV) with NovaGold Resources, Inc. as operator in 2001. During the period of the DCJV, NovaGold undertook trenching, core and geotechnical drilling, updated Mineral Resource estimates, and completed a Preliminary Assessment.
Placer Dome reassumed management of the Project as operator in late 2002. From 2002 to 2005, work comprised additional core drilling, condemnation, geotechnical, and water drilling, geotechnical and hydrogeological studies, geological mapping and sampling of prospective calcium carbonate source areas, exploration and auger drilling program for sand and gravel resources, and updated Mineral Resource estimates.
Barrick Gold (Barrick) acquired the Placer Dome interest in the DCJV through a merger with Placer Dome in early 2006. Work completed in the period 2006-2007 included core drilling for resource infill, geotechnical, engineering, condemnation, waste rock, calcium carbonate exploration and metallurgical purposes, and updated Mineral Resource estimates.
The DCJV partners formed DCLLC in late 2007, with the subsequent name change to Donlin Gold occurring in 2011.
To 2011, work completed has consisted of soil and stream sediment sampling, core drilling for resource infill, geotechnical, engineering, condemnation, waste rock, and metallurgical purposes, and estimation of Mineral Resources and Mineral Reserves.
An initial feasibility study was completed on the Project in 2007, and updated in 2009. A second update was performed in 2011.
The Donlin gold deposits lie in the central Kuskokwim basin of southwestern Alaska, and is a northeast-trending basin that subsided between a series of amalgamated terranes. Rocktypes within the basin include Mesozoic marine volcanic rocks, Palaeozoic clastic and carbonate rocks, and Proterozoic metamorphic rocks.
The Kuskokwim basin is predominately underlain by the Upper Cretaceous Kuskokwim Group, a back-arc continental margin basin fill assemblage that formed in response to a change in the angle of convergence between the Kula oceanic plate and the Cretaceous North American continental margin. Sediments primarily consist of a coarse- to fine-grained turbidite comprising sandstone, siltstone, and shale with minor conglomerate.
Late Cretaceous and Early Tertiary volcano-plutonic complexes intrude and overlie the Kuskokwim Group sedimentary rocks. Volcanic components of these complexes consist of intermediate tuffs and flows. Subaerial volcanic tuffs, flows, and domes are regionally extensive and dominantly andesitic, locally include dacite, rhyolite, and basalt. Associated plutons are calc-alkaline in composition, ranging from monzonite to granodiorite. Felsic to intermediate hypabyssal granite to granodiorite porphyry dikes, sills, and plugs are also widely distributed and often intruded into northeast-striking extensional faults. Volumetrically minor Upper Cretaceous intermediate to mafic intrusive bodies are also common.
The centre of the Kuskokwim basin lies between two continental-scale, dextral slip-fault zones: the Denali-Farewell Fault system to the south and the Iditarod-Nixon Fork Fault system to the north. Fold-and-thrust-style deformation formed the earliest structures in response to subduction-related compression shortly after deposition of the Kuskokwim sediments. Eastward-trending folds and thrust faults are common in the central Kuskokwim basin, including the Donlin Creek area. Younger north–northeast-trending folds are dominant near the Iditarod-Nixon Fork Fault and Denali-Farewell Fault but also formed throughout the region in response to basin-scale dextral movement. Most of the folds predate emplacement of the volcano-plutonic complexes. Pre-, syn-, and post-(?) intrusion, northeast-striking normal and oblique slip faults formed during subsequent late compressional and extensional events and focused intrusive igneous rocks and hydrothermal systems across the basin.
Regional Geology of Central Kuskokwim Area
Because outcrop is limited and of generally poor quality, property-scale geology is largely interpreted from trenches, drill holes, aeromagnetic surveys, and soil geochemistry.
The Project area is underlain by a 5 mile (8.5 km) long x 1.5 mile (2.5 km) wide granite porphyry dike and sill swarm hosted by lithic sandstone, siltstone, and shale of the Kuskokwim Group.
The oldest igneous rocks at Donlin Creek are intermediate to mafic dikes and sills. They are not abundant but occur widely throughout the property as generally thin and discontinuous bodies. The younger and much more voluminous granite porphyry intrusive rocks vary from a few feet to 200 ft (60 m) wide and occur as west–northwest- trending sills in the southern resource area and north–northeast-trending dikes farther north. The granite porphyry dikes and sills all have similar mineralogy, and the porphyry texture indicates relatively shallow emplacement. Although these rocks belong to the regionally important granite porphyry igneous event, geologists working on the Project classify them into five textural varieties of rhyodacite. These units are chemically similar, temporally and spatially related, and probably reflect textural variations of related intrusive events.
The Project is located in a structurally complex area about 15 miles (25 km) southeast of the Iditarod–Nixon Fault. Sedimentary bedding generally strikes northwest and dips 10° to 50° to the southwest. Overall, sedimentary structure in the northern resource area is monoclinal, while sedimentary rocks in the southern resource area display open eastward-trending folds. East–southeast-trending and plunging folds or monoclinal warps are the oldest recognized structures and are associated with north-vergent thrust faults. Thrust faults are generally southwest-dipping, parallel to the bedding plane, and account for imbrication of the sedimentary rocks and locally moderate to steep southwest and northeast dips. Younger, low-amplitude north–northeast-trending folds crop out in the airstrip exposures along American Creek and are recorded on historic trench geology maps. Lack of cleavage or other evidence of dynamic recrystallization suggests that folds and thrust faults formed at relatively shallow depths.
A northeast, elongated, roughly 5,000 ft (1.5 km) wide x 10,000 ft (3 km) long cluster of gold deposits has an aggregate vertical range that exceeds 3,100 ft (945 m). The deposits are hosted primarily in igneous rocks, and are associated with an extensive Upper Cretaceous gold–arsenic–antimony–mercury hydrothermal system. Gold occurs primarily in sulphide and quartz–carbonate–sulphide vein networks in igneous rocks and, to a much lesser extent, in sedimentary rocks. Broad disseminated sulphide zones formed in igneous rocks where vein zones are closely spaced. Submicroscopic gold, contained
primarily in arsenopyrite and secondarily in pyrite and marcasite, is associated with illite–kaolinite–carbonate–graphite-altered host rocks.
Interpreted Property-Scale Igneous Rocks
Note: RDA = Aphanitic Porphyry; RDX = Crowded Porphyry; RDXL = Lath-Rich Porphyry; and RDXB = Blue Porphyry. Figure courtesy Donlin Gold.
Fluid inclusion studies and field and drill hole observations define three distinct styles of gold mineralization that are locally telescoped and cross-cut one another. The earliest is a porphyry-style stockwork vein system at the Dome prospect.
Dome is located within the same dike-and-sill swarm that hosts the ACMA–Lewis resource, but the Kuskokwim sedimentary rocks are thermally metamorphosed to a siliceous hornfels. Quartz veins have a Au–Ag–Cu–Zn–Bi ± Te trace metal signature (Ebert et al., 2003c; Drexler, 2010) with up to 3% arsenopyrite–pyrite–chalcopyrite–pyrrhotite ± Fe-rich sphalerite and trace amounts of electrum, native bismuth, and bismuth tellurides and selenides. Veins cut both the hornfels and porphyry dikes.
ACMA–Lewis-style mineralization post-dates the Dome veins and consists of sparse Au–Ag–As–Sb–Hg ± W (Ebert et al., 2003c; Drexler, 2010), trace metal-bearing quartz-Fe-dolomite veins with <3% auriferous arsenopyrite-pyrite ± stibnite ± late realgar, native arsenic, and graphite. Veins and related disseminated sulphide zones are primarily hosted in illite-carbonate-kaolinite-altered rhyodacite dikes and sills but also occur in Kuskokwim Group sedimentary rocks near igneous contacts.
Variations between Dome and ACMA–Lewis vein habits, vein mineralogy, wall rock alteration, geochemical signatures, stable isotope variations (Drexler, 2010), and fluid inclusion chemistry (Ebert et al., 2003c) indicate that hydrothermal fluids were sourced at depth northeast of the Dome prospect, precipitated the base metal assemblage at Dome from metals sequestered in the vapour phase, and then migrated southwestward to the more distal ACMA–Lewis environment, where gold-bearing minerals were precipitated due to mixing with meteoric waters and boiling.
The last event consists of gold-bearing quartz–stibnite veins up to 3 ft (1 m) thick with variable carbonate, pyrite, and arsenopyrite found mainly around the margins of Dome and partially overlapping ACMA–Lewis. Quartz–stibnite veins also contain anomalous Au–As–Cu–Zn–Bi and have fluid chemistry and temperatures intermediate between Dome and ACMA–Lewis (Ebert et al., 2003). In the opinion of Donlin Gold, these veins do not contain significant gold mineralization.
The stratigraphy in the deposit area consists of basin margin clastic rocks (MacNeil, 2009) dominated by greywacke (lithic sandstone) units with complex transition zones of interbedded siltstone, shale, and greywacke. Marker beds are not yet recognized, so absolute stratigraphic breaks are difficult to identify. Greywacke is dominant in the northern part of the resource area (Lewis, Queen, Rochelieu, Akivik), whereas shale–siltstone-rich units are common in the southern part (South Lewis, ACMA).
Interpreted Surface Geology of Resource Area
Mafic Dikes/Sills - oldest. Intermediate to mafic dikes and sills; locally host high-grade gold; generally less than 10 ft thick. In the transition area between Akivik and ACMA, mafic sills are extremely abundant within the Lower
Greywacke, immediately below the Main Shale. Code - MD.
Fine-Grained. Earliest rhyodacite intrusions recognized. grey, typically fine-grained, felsic porphyries. RDF intrusives occur as two main northeast-striking, 16.5 to 32.8 ft (5 to 10 m) wide dikes in the Lewis zone and possible discontinuous bodies in early eastward-trending compressional faults, e.g., the Lo Fault. Code - RDF
Porphyry Crowded Porphyry. Volumetrically the most significant intrusive phase. Grey, characterized by a uniformly crowded feldspar porphyry texture. Present as two 164 to 328 ft (50 to 100 m) wide dike zones in the eastern edge of the north to north–northeast mineralized trend of Lewis/South Lewis. RDX is also found as sills throughout ACMA near the basal part of the sill sequence. Code - RDX.
Lath-Rich Porphyry. Characterized by sparse, elongate plagioclase laths; significant coarser-grained biotite. occurs as two important dikes in the Akivik area that strike south into the centre of the ACMA deposit. In Akivik and ACMA, RDXL occurs as a significant sill immediately below the RDX sill. The RDXL sill continues to the west but pinches out to the east. RDXL dikes are also present within the main Lewis area RDX dike trend, but here they are volumetrically insignificant. Code - RDXL.
Aphanitic Porphyry. Rhyodacite rock with a salt-and-pepper texture of fine biotite phenocrysts and variable quartz and potassium feldspar phenocrysts. Numerous (up to eight) RDA dikes strike south from the Vortex/Rochelieu (Lewis) area into the East ACMA/ACMA area. The dikes are typically found west of the Vortex Fault but are also present between the Lo and Vortex faults and below the Lo Fault. An extensive sill package of RDA lies immediately above the RDX sills in the ACMA area. In West ACMA, the RDA sills are buttressed against,
and locally cross-cut, RDX sills. Another package of RDA sills is found south of the AC Fault, in the Aurora domain. Code - RDA.
Blue Porphyry. Final intrusive event; coarsely porphyritic with large blocky feldspars set in a graphite- and sulphide-rich matrix. locally hosts important high-grade disseminated sulphide material in addition to gold-bearing veins. RDXB occurs as two major dikes, the Lewis Blue Porphyry dike and the Vortex Blue Porphyry dike.
Extensive thin RDXB sills are found in the uppermost part of the sill sequence in the South Lewis and ACMA areas, and RDXB sills are present as both distinct sills and co-mingled with RDA in the core of ACMA and in the Aurora domain. Code - RDXB
The morphology of intrusive rocks in the deposit is largely governed by the rheology of sedimentary rocks and pre-intrusion faults and folds. Faults in the geological model (from earliest to youngest) are the American Creek (AC) Fault, Lo and Rochelieu faults, Vortex Fault, and ACMA Fault.
Bench Level Geology
Note: Oblique view looking north-eastward of the 3D geological model projected on the 328 ft (100 m) pit bench level and the Mineral Reserves (DC9) pit outline. Note: Figure courtesy Donlin Gold.
Lewis Area Section
Note: Shows intrusive rocks, faults, drill holes, and Mineral Reserves (DC9) pit outline, looking northeast. Note: Figure courtesy Donlin Gold
Note: Shows intrusive rocks, faults, drill holes, and Mineral Reserves (DC9) pit, looking southeast. Note: Figure courtesy Donlin Gold.
The Donlin deposits include eleven mineralized areas that exhibit slightly different geological settings but generally fall into two geologically similar deposit areas: ACMA and Lewis. ACMA, or the intrusive sill and shale–siltstone sedimentary setting, includes the Aurora, 400, Akivik, ACMA, and East ACMA mineralized zones. Lewis, or the massive greywacke-hosted intrusive dike setting, includes the South Lewis, Lewis, Vortex, Rochelieu, Queen, and North Akivik mineralized zones.
Veins in north–northeast-striking, east- or west-dipping faults and fracture zones are the primary control on gold distribution and are ubiquitous in all mineralized areas. Northwest- and northeast-striking veins occur locally but are relatively rare. Veins are narrow (typically <1 cm wide), highly irregular, discontinuous, and generally sparsely distributed, although vein density can locally range up to 2 to 8 per meter in higher-grade zones. Vein zones vary from 6.5 to 100 ft (2 to 35 m) wide and 300 to 1,150 ft (100 to 350 m) long. Individual vein zones generally display limited lateral and vertical continuity; however, swarms of many anastomosing vein zones form larger mineralized corridors characterized by extensive lateral and depth continuity.
Vein corridors are more apparent in the north–northeast-trending dikes of Lewis than in the west–northwest-trending ACMA sill zone. The greater width of the sill-hosted ACMA mineralized zone makes discreet corridors less obvious (but still present). 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).
Veins are best developed in relatively more brittle intrusive rocks and massive greywacke. Small, irregular, carbonate-altered mafic bodies often host very high grade gold as sulphide dissemination, replacement, and breccia fill. Structural breccias in sedimentary rocks are also favourable sites for high-grade gold. Gold distribution in the deposit closely mimics the intrusive rocks, which contain about 80% of the resource. Structural zones in competent sedimentary units account for the remaining 20%. The more steeply dipping sills in the ACMA sill sequence host the highest-grade and most continuous igneous-hosted mineralized zones, particularly where intersected by northeast-striking “feeder” dikes and faults. Gold grade is directly proportional to vein density and intensity of overlapping disseminated sulphide vein aureoles. The dike-dominant Lewis deposit areas consist of sheeted veins with limited disseminated sulphide in the wall rocks and are characterized by lower-grade and less continuous mineralized zones.
Gold-bearing zones are coincident with quartz–carbonate–sulphide veins and related disseminated sulphide aureoles in hydrothermally altered rhyodacite bodies and, to a lesser extent, in sedimentary rock near igneous contacts. Continuity and grade of mineralized material within the rhyodacite host rocks varies directly with vein spacing and the amount of vein and disseminated arsenopyrite, the principal gold-bearing mineral. Gold in sedimentary rocks and minor mafic igneous bodies is generally limited to small and discontinuous vein and breccia fill occurrences.
Vein and Disseminated Mineralization
Veins in the ACMA–Lewis area are subtle in appearance and vary from <1 mm to 20 cm wide, averaging <1 cm. They formed in brittle fractures and are typical of open- spaced fillings with vugs, drusy quartz-lined cavities, vein wall-banded and cockscomb quartz, and bladed carbonate. Veins are composed of gray to clear quartz, white to tan carbonate, and as much as 3% sulphides.
100 m Bench Level Gold Distribution (>1 g/t Au grade blocks)
V1. Thin, irregular, and discontinuous sulphide (>50%) veins with pyrite and trace arsenopyrite, little or no quartz (<30%) or carbonate (<50%). Broad disseminated selvage of pyrite and poorly crystalline illite and Fe–carbonate alteration. Barren or very low grade.
V2. Thin, discontinuous quartz (>30%) sulphide veins contain variable pyrite and arsenopyrite. May have broad, often pervasive selvages of fine-grained, needle- like arsenopyrite. Broad pyrite aureole may surround the arsenopyrite selvage. Open-space vuggy textures common. Trace amounts stibnite. Have moderate gold grade and strong illite alteration aureoles with variable Fe–carbonate replacement of the host rock.
V3a. Higher-grade veins. Thicker, more planar and continuous, open-space quartz veins with Fe-dolomite, pyrite, arsenopyrite, native arsenic, and variable amounts of stibnite. Commonly show broad arsenopyrite-rich selvages with little to no Fe–carbonate as wall rock alteration.
V3b. Thicker, more continuous, and planar quartz veins with open-space textures and complex mineralogy, including pyrite, arsenopyrite, stibnite, native arsenic, realgar, and trace other sulphides in intensely illite altered material. Gold grades are commonly much higher than the average grade of the deposit.
V4 Latest vein phase. Barren carbonate-quartz (>50% and <50%, respectively) vein sets that post-date mineralized veins. Primarily barren white and clear quartz veinlets and calcite ± ankerite veinlets with no sulphides.
Mineralized zones are consistently oriented sub-parallel to the main 01 axis (024) of the compressive structural regime (Piekenbrock and Petsel, 2003). Veins in the ACMA–Lewis resource evolved through a continuum (V1 through V3) of changing mineralogy and increasing gold grade while maintaining a generally consistent NNE strike and SE dip. The final carbonate–quartz vein set (V4) has a broader range of orientation.
MacNeil (2009) found that the average vein orientation for all veins is 024/71. This orientation is generally consistent across all domains and vein types, which indicates that veins at Donlin formed during the same mineralizing event.
A comparison by host rock shows that veins in igneous rocks strike more easterly and dip more steeply than veins in sedimentary rocks, probably due to refraction across lithologic contacts.
Several quartz and carbonate phases have been recognised, including pre-gold-stage Mn–calcite veins and wall rock replacement and cockscomb quartz veins; Fe–dolomite in main gold stage veins; and post-gold-stage clear quartz veins and ankerite stringer veins.
Euhedral and porous replacement pyrite are the earliest sulphide phases, followed in order by marcasite, arsenopyrite, realgar, and native arsenic. Stibnite is most abundant in later veins. Most accessory sulphides are relatively early, while boulangerite is relatively late. Arsenopyrite occurs as both coarse (up to 1 cm) crystals and very fine (0.1 to 0.2 mm) euhedral grains. Fine-grained arsenopyrite contains five to 10 times more gold than the paragentically earlier coarse-grained phase.
Rhyodacite bodies are ubiquitously altered to an illite–carbonate–kaolinite–chlorite / smectite ± quartz ± graphite assemblage.
Mafic igneous rocks are strongly altered by carbonate ± fuchsite and contain locally high-grade gold with disseminated, massive replacement or breccia filling sulphide.
Altered sedimentary rocks consist of relict quartz grains in a matrix of illite, kaolinite, carbonate, hematite, and <1% pyrite and trace sphalerite (Drexler, 2010).
Pyrite is widespread in all altered rocks (0.5% to 2%) but is more abundant (1% to 4%) in mineralized zones. Alteration is most intense near veins and is typically zoned outward from iIlite ± kaolinite to kaolinite ± illite and then to a distal zone of chlorite ± smectite ± quartz.
Silica is dominantly restricted to veins in the ACMA–Lewis area and is not generally expressed as pervasive silicification. Vein relationships show an increase in quartz content from early sulphide-dominant veins to late silica-dominant veins. Some increased silicification has been noted in the Queen area (Ebert, 2003b).
Short-wave infrared reflectance (SWIR) spectroscopy data, collected between 2007 and 2011, are interpreted by Donlin Gold show that higher gold is most strongly correlated with an alteration suite dominated by NH4–illite (ammonia–illite), whereas kaolinite-bearing zones contain lower-grade gold.
The most abundant minor elements associated with gold-bearing material are iron (Fe), arsenic (As), antimony (Sb), and sulphur (S). These are contained primarily in the mineral suite associated with hydrothermal deposition of gold, including pyrite (FeS2), arsenopyrite (FeAsS), realgar (AsS), native arsenic (As), and stibnite
(Sb2S3). Minor hydrothermal pyrrhotite (Fe 1-x S) and marcasite (FeS2), and syngenetic or sedimentary pyrite, also account for some of the Fe and S.
Much less abundant elements such as copper (Cu), lead (Pb), and zinc (Zn) are contained in relatively rare or accessory hydrothermal mineral species observed in the deposit, including chalcopyrite (CuFeS2), chalcocite (Cu2S), covellite (CuS), tennantite (Cu12As4S13), tetrahedrite (Cu12Sb4S13), bornite (Cu5FeS4), native copper (Cu), galena (PbS), sphalerite (ZnS), and boulangerite (Pb5Sb4S11). Small amounts of silver (Ag) in the deposit are most likely accommodated within the crystal structures of tetrahedrite and galena, and to a lesser extent in some of the other sulphides. Molybdenum (Mo) occurs in rare molybdenite (MoS2). Very minor nickel (Ni) has been observed in the secondary sulphide mineral millerite (NiS) and minor cobalt (Co) in various secondary minerals in sedimentary rocks. The Ni and Co probably have a sedimentary origin.
Three elements of particular processing significance are mercury (Hg), chlorine (Cl), and fluorine (F). Graphitic carbon and carbonate minerals also have the potential to negatively affect the metallurgical process.
According to Donlin Gold, the Donlin gold deposits share characteristics of several gold deposit genetic models. It has been classified as:
- Granite porphyry-hosted gold polymetallic (Bundtzen and Miller, 1997)
- Distal or high-level epizonal intrusion-related (Hart et al., 2002)
- Low-sulphidation epithermal (Ebert et al., 2003a)
- Orogenic- or intrusion-related (Goldfarb, 2004)
- Reduced porphyry to sub-epithermal Au–As–Sb–Hg (Ebert et al., 2003c; Hart, 2007).
Hart (2007) classifies the deposit as a high-level, reduced intrusion-related vein system to account for the reduced ilmenite series intrusions, near contemporaneous age of mineralization, and the apparent genetic relationship to the higher-temperature hydrothermal system at Dome (Drexler, 2010).
The Lewis–ACMA part of the district is clearly a low sulphidation, reduced intrusion related, epizonal system with both vein and disseminated mineral zones and conforms most closely to the Hart (2007) classification.