Summary:
The Donlin mineralization model is a high-level, reduced intrusion-related vein system. The ACMA (ACMA is named after the American Creek magnetic anomaly)-Lewis part of the district is a low sulphidation, reduced intrusion related, epizonal system with both vein and disseminated mineral zones.
The Donlin deposits lie in the central Kuskokwim basin of southwestern Alaska, which contains a back-arc continental margin basin fill assemblage of the Upper Cretaceous Kuskokwim Group, and Late Cretaceous volcano-plutonic complexes. The Project area is underlain by a 8.5 km long x 2.5 km wide granite porphyry dike and sill swarm hosted by lithic sandstone, siltstone, and shale of the Kuskokwim Group.
The deposits are hosted primarily in igneous rocks and are associated with an extensive Upper Cretaceous gold–arsenic–antimony–mercury hydrothermal system. The northeast, elongated, roughly 1.5 km wide x 3 km long cluster of gold deposits has an aggregate vertical range that exceeds 945 m. These areas consist of the ACMA and 400 Zone, Aurora and Akivik mineralized areas (grouped as ACMA) and the Lewis, South Lewis, Vortex, Rochelieu and Queen mineralized areas (grouped as Lewis).
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. Sub-microscopic gold, contained primarily in arsenopyrite and secondarily in pyrite and marcasite, is associated with illite– kaolinite–carbonate–graphite-altered host rocks.
In the opinion of the QPs, knowledge of the deposit settings, lithologies, and structural and alteration controls on mineralization is sufficient to support Mineral Resource and Mineral Reserve estimation. The mineralization style and setting of the Donlin deposits are also sufficiently well understood to support Mineral Resource and Mineral Reserve estimation.
Mineralization
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.
Vein Stages:
- Vein 1 (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;
- Vein 2 (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;
- Vein 3a (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;
- Vein 3b (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;
- Vein 4 (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 d1 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 degrees. This orientation is generally consistent across all domains and vein types, which indicates that veins in the Donlin deposits 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 recognized, 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 paragenetically earlier coarse-grained phase.