Donlin Creek is a 32,600 ha mineral property located 480 km north of Anchorage, Alaska, U.S.A. on the western flank of the Kuskokwim Mountains. The Kuskokwim River is a regional transportation route and is serviced by commercial barge lines.
Gold was discovered in the Donlin Creek area in 1909 and placer production of about 30,000 ounces of gold occurred between 1909 and 1956. From 1956 to 1988,
exploration was focused on trying to determine the lode source of the placer gold.
Barrick (formerly Placer Dome) acquired a 20 year lease from Calista, an Alsakan Native corporation, in 1995. Subsequently, in 2002, NovaGold joint ventured into the property.
Donlin Creek boasts reserves of 33.6 million ounces of gold that could support a mining operation extending over a stretch of 25 years by producing in average 1.25 million ounces of gold per year at an average total cash cost of $444/oz. These production levels would make Donlin Creek one of the world's largest gold producing mines.
Based on the feasibility study, the Donlin Creek mine has been designed as a year-round, truck and shovel open-pit operation with plant start-up anticipated for 2015.
It is expected that the Donlin Creek ores will be processed at a rate of 53,000 tpd by crushing and milling followed by flotation, pressure oxidation and CIL recovery. The process plant design uses the most current technology for both the process systems and equipment selection. Particular attention was paid to incorporating state-of-the-art technology for safety and environmental protection.
The Donlin Creek mine is expected to draw an average of 127 megawatts of electrical power sourced from a combination of on-site combined cycle gas turbine generators and wind co-generation.
Key infrastructure for the mine includes a port on the Kuskokwim River, a 123 km access road connecting the port to the mine site and barge terminals at the Port of Bethel - the northernmost medium-draft port in the United States.
The total estimated cost to design and build the Donlin Creek Project is $4,481 million, including an owner-provided mining fleet and self-performed pre-development.
Life-of-mine operating costs, including allocations for mining, processing, administration and refining are estimated at $30.03/t milled, $4.60/t mined, and approximately $440/oz overall.
Donlin Creek is proud to maintain a high local hire rate at the project, with the majority of contractors and employees coming from local communities. At its peak the mine would provide employment for almost 850 people.
Alaska, or alaxsxaq meaning mainland in Aleut language, is the largest state and at the same time the least populated state in the U.S. In 1867, the United States bought it from Russia for about 2 cents per acre.
Alaska represented the first leg of the journey for humans that migrated from Asia over the Bering land bridge sometimes between 16,000 an 10,000 years B.C.
The first European to land in Alaska was Vitus Bering, a Danish navigator in the service of the Russian Navy, who in 1741 collected fine specimens of sea otter pelts from the rich coast of the new land. In 1784, Russians established the first permanent settlement in Alaska.
Increasing presence of Russian fur traders, incursions of Spanish galleons and a competing British presence that started with Captain James Cook's 1778 voyage marked the history of the territory for the next 100 years. In 1867 the U.S. Congress approved the purchase of Alaska from a bankrupt Russian Empire for $7.2 million.
In 1898, J. E. Spurr, an American geologist and explorer, led the first U.S.G.S. expedition in the interior of the new territory. He identified, mapped and charted the new country (including the Kuskokwim Mountains), its geology and minerals including gold deposits. Gold was discovered in the Donlin Creek area in 1909, and placer production of about 30,000 ounces of gold occurred between 1909 and 1956.
The Donlin Creek property is located in southwest Alaska, approximately 20 km north of the village of Crooked Creek on the Kuskokwim River – the "big slow moving thing" in native Yupik language. The river is a regional transportation route and is serviced by commercial barge lines. There is no road or rail access to the site and therefore all personnel and supplies are transported by air.
The principal economic activities along the river have historically been fur trapping and fishing. Economic deposits of placer gold were discovered in 1901 and mineral production in the region has mainly been from scattered placer gold deposits, with a total of 3.5 million troy ounces of gold produced.
The project area is one of low topographic relief on the western flank of the Kuskokwim Mountains. Elevations range from 152 m to 640 m. Ridges are well rounded and easily accessible by all-terrain vehicle. Hillsides are forested with black spruce, tamarack, alder, birch and larch. Soft muskeg and discontinuous permafrost are common in poorly-drained areas at lower elevations.
The area has a relatively dry interior continental climate with typically less than 50 mm total annual precipitation. Summer temperatures are relatively warm and may reach nearly 30°C. Minimum temperatures may fall to well below -40°C during the cold winter months. Work is possible on the project year-round.
The Kuskokwim region of southwestern Alaska is predominantly underlain by rocks of the Upper Cretaceous Kuskokwim Group that filled a subsided northeast-trending strike-slip basin between a series of amalgamated terranes.
Overall, sedimentary structure in the northern resource area is monoclinal, whereas sedimentary rocks in the southern resource area display open easterly-trending folds.
Gold deposits are associated with an extensive Late Cretaceous gold-arsenic-antimony-mercury hydrothermal system. Gold-bearing zones exhibit strong structural
and host rock control along north-northeast-trending fracture zones and are best developed where those zones intersect relatively competent host rocks. Mineralized material is most abundant in the igneous rocks, but sedimentary rocks are also mineralized within strong fracture zones.
Two distinct styles of gold-rich mineralization (ACMA-Lewis style and Dome-Duqum style) occur within the Donlin Creek trend. The ACMA-Lewis style of mineralization, a
later low-temperature, low-sulphidation epithermal system, constitutes the main mineralizing system within the Donlin Creek property. This is the sole style of mineralization within the current resource area. The ACMA-Lewis style consists of sheeted quartz, quartz-carbonate and sulphide-only veins characterized by a gold-arsenic-antimony-mercury geochemical signature. The bulk of the gold occurs in the lattice structure of arsenopyrite. Disseminated gold-bearing arsenopyrite can also be found typically adjacent to veins and vein zones.
There is considerable potential for additions to the Mineral Resources at Donlin Creek. The project remains open along the Donlin trend to the north. The discovery potential
in the remaining 6 km geologic trend is high.
Mining & Operation
The mine will be an open pit operation, and is proposed to be mined by a combination of bulk and selective mining methods using owner-operated large-scale equipment.
Two pits are planned, at ACMA and Lewis. A set of 14 mining phases were designed, eight in the ACMA pit and six in the Lewis pit.
This sequence aims to deplete ACMA as early as possible to maximize use of the waste backfill dump designed inside the pit while minimizing deviation from the optimal.
Donlin Creek is envisaged to be mined by a conventional truck-and-shovel operation. Initial pioneering and pit development will be undertaken to remove overburden,
develop mine access roads suitable for large mining equipment, and "face-up" the initial pit into productive set-ups for the large shovel and mining equipment.
Large hydraulic shovels mining the full 12 m benches will be the primary loading equipment in zones of waste and steeply dipping ore. The same primary shovels will be used on the 6 m split benches, thereby avoiding the need for a mixed fleet of hydraulic shovels. Large 360 t capacity haul trucks will be used for transporting both ore and waste out of the pit.
Haul roads are designed at 10% maximum grade for uphill loaded haulage and at a maximum of 8% for downhill loaded haulage. The final road width design is 40 m.
Blasting will be required. Blast hole drilling in predominantly waste areas will be performed with nominal 251 mm diameter production drills. Ore zones will be drilled
on a single 12 m bench with 200 mm diameter holes or a single 6 m bench with 140 mm diameter holes, depending on the size and continuity of the ore blocks outlined by grade control drilling. All blasting will be based on 70% emulsion/30% ANFO, which will be manufactured on site.
Support equipment will be used for road, bench, and dump maintenance and miscellaneous projects. Track dozers and rubber-tired dozers will spot loads and maintain the waste spoil dumps. A fleet of graders will maintain the roads. Crushed rock will be provided to help maintain good roads and improve truck tire life. Water trucks will spray roads and working areas during dry and dusty periods. Small
backhoes will be used for ditch work and other dewatering projects. Dozers will be used on larger construction projects such as re-contouring waste dumps and spreading reclamation materials.
Waste rock from open pit mining will be placed in an ex-pit waste rock facility (WRF), in the American Creek valley, east of the pit area, or in a backfill dump in ACMA.
Sufficient allocation was made in the WRF design to accommodate non-acid generating (NAG) and potentially acid generating (PAG) rock from the ACMA and Lewis pits. An engineered rock dam is planned downstream of the WRF
to support higher water discharge events.
The operating life-of-mine (LOM) is estimated to be 25 years based on a nominal processing rate of 53.5 kt/d. Mine start-up is assumed to commence in 2014, and cease in 2039.
The projected total labour force complement for mine operations, maintenance, engineering, and contractors is 442 in 2015 (Year 1), peaks at 646 in Year 11, and decreases to 83 in the final full year of pit operation.
A total of 1.69 Gt of waste will be stored in the WRF and another 404 Mt in the ACMA backfill dump.
A total of 38 Mt of in-pit overburden will be mined at Donlin Creek, of which 7.7 Mt of peat and loess and 9.6 Mt of colluvium/terrace gravel will be stockpiled over the LOM to meet site reclamation requirements. The remainder will be stored within the WRF. Some 17.3 Mt of overburden will stored in overburden stockpiles.
The main objectives of the water management plan for the Donlin Creek project are to minimize or eliminate the need for treatment and discharge of contact water during
mine construction, operations, and closure; to achieve the pit-slope depressurization requirements; and to provide adequate quantity and quality of water supply to the mill. Contact water will be stored behind a dam in American Creek, and tailings will be stored in the adjacent Anaconda Creek basin.
Gold is recovered in two areas in the proposed plant:
- Gold recovered from the flotation circuit to the flotation concentrate.
- Gold recovered through leaching/adsorption (CIL) of the pressure oxidized (autoclaved) flotation concentrate.
The processing rate is variable from period to period as a function of sulphur grade and ore hardness. To maximize plant utilization, long-term ore stockpiling is required to balance sulphur feed grades. Short-term stockpiling will also be required to handle crusher downtime and production fluctuations in the pit.
Key features of the plant are:
- Gyratory crusher feeding a covered stockpile. Design operating times are 65% for the primary crusher and 93% for the process plant. The covered stockpile is planned to have a capacity of 38,000 t, representing 16 hours of process plant operation, and a total capacity of approximately 174,000 t, representing 3.2 days of process plant operation.
- Mill-chemical-float-mill-chemical-float (MCF2) grinding and flotation circuit. A single semi-autogenous grind (SAG) mill will operate in closed circuit with parallel
cone crushers, followed by a primary ball mill in closed circuit with cyclones. Primary ball mill product reports to primary rougher flotation. Rougher flotation tailings report to the secondary ball mill circuit, while in closed circuit with cyclones. Secondary ball mill product at P80 50 µm reports to secondary rougher flotation.
Secondary rougher flotation concentrate reports to cleaner flotation. A cleaner scavenger flotation circuit treats the cleaner flotation tailings.
- Combined flotation concentrates from primary rougher and cleaner flotation are dewatered in a thickener before acidulation and counter-current decant (CCD) washing to remove solubilised ions from the concentrate. The flotation circuit has a concentrate storage option.
- The autoclave circuit includes two autoclaves operating in parallel. Thickened flotation tailings slurry is used as a cooling medium in the autoclave letdown circuit.
- Flotation tailings are combined with the flotation concentrate wash solution product to neutralize the acidic solution before discharge to the tailings storage facility. The carbonate in the flotation tailings slurry will provide primary neutralization. The final pH level will be adjusted by adding slaked lime.
- Flashed and cooled autoclave discharge slurry is cured before POX discharge to CCD. The acidic solution recovered by CCD is recycled to acidulation and flotation feed conditioning.
- POX CCD product slurry is neutralized with lime ahead of cyanidation.
- The CIL circuit would consist of tanks that are fully enclosed and vented to a caustic scrubbing system to recover cyanide and recycle it back to the CIL circuit. Carbon will be handled with in-tank revolving screens.
- The carbon-handling area for the loaded carbon consists of an acid wash circuit and a modified pressure Zadra circuit for stripping carbon. Carbon is reactivated in
an electric kiln.
- Gold is recovered in an electrowinning circuit. The electrowinning sludge is treated in a retort before being melted in an induction furnace. The final product is doré
- Mercury that evolves in the process plant will be captured in a number of mercury abatement systems.