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Pebble Mine

Undeveloped copper-gold-molybdenum mineral deposit in Alaska, United States

  • 33 min read
  • 5 chapters

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Through Mining, Fault (geology) and Sedimentary rock

As of November 2020, the mine developer, Northern Dynasty Minerals, still sought federal permits from the United States Coast Guard and the Bureau of Safety and Environmental Enforcement. State permitting would then follow, which the developer expected to take up to three years. In November 2020, the U.S. Army Corps of Engineers (USACE) denied a permit for the proposed mine discharge plan.

On September 9, 2021, it was reported that the United States Environmental Protection Agency had "asked a federal court to allow for Clean Water Act protections for parts of the bay." On January 31, 2023, the EPA effectively vetoed the project, using a rarely invoked power to restrict development to protect watersheds.

Background

Geography

The Pebble prospect is in a remote, wild, and generally uninhabited part of the Bristol Bay watershed in Southwest Alaska. The nearest communities, about 20 miles (32 km) distant, are the villages of Nondalton, Newhalen, and Iliamna. The site is 200 miles (320 km) southwest of Anchorage, Alaska.

Pebble is approximately 15 miles (24 km) north of, and upstream of, Lake Iliamna, and near Lake Clark. The deposit area is characterized by relatively flat land dotted by glacial ponds, interspersed with isolated mountains or ranges of hills rising one or two thousand feet above the flats. Pebble is under a broad flat valley at about 1,000 feet (300 m) above sea level dividing the drainages of Upper Talarik Creek and the Koktuli River.

Upper Talarik Creek flows into Lake Iliamna, which flows through the Kvichak River into Bristol Bay. Waters in the Koktuli River drain into the Mulchatna River, a tributary of the Nushagak River which empties into Bristol Bay at Dillingham. Water from Lake Clark, approximately 20 miles (32 km) east of Pebble, flows down the Newhalen River to Lake Iliamna.

Geology

Regional

The Kahiltna terrain is interpreted to represent a sediment trough formed on the landward (Alaska) side of the Wrangellia volcanic arc terrane, prior to collision of Wrangellia with Alaska. The Wrangellia and Kahiltna terrains docked to Alaska in the Cretaceous Period. This part of the Kahiltna terrane is dominated by Late Triassic basalt, andesite and sedimentary rocks overlain by Jurassic-Cretaceous andesitic turbidites. Cretaceous granitic intrusive activity was widespread in the Kahiltna terrane. Tertiary volcanic and sedimentary rocks, and Quaternary glacial deposits, developed over the older rocks.

The Lake Clark fault, or a splay, probably lies within twenty miles (32 km) of the Pebble deposits and possibly much closer. The fault is a major right-lateral strike-slip crustal feature, considered to be a westward expression of the Castle Mountain fault. The actual ground trace of the fault and its splays are unknown in the Pebble area, due to extensive ground cover. A 2007 report indicates that magnitude 7.1 quakes occur on the fault on a 700-year cycle. The Lake Clark fault several hundred miles to the north is sub-parallel to the Denali fault and considered to be of similar nature. A magnitude 7.9 quake struck the Denali fault in 2002. The subduction zone of the Aleutian Trench lies approximately 125 miles (200 km) south of Pebble. This zone was the source of the 1964 Good Friday earthquake of magnitude 9.2. The Augustine Volcano, which lies 25 miles offshore, last erupted in 2006.

The ore body

A contiguous body of ore is known as Pebble West where mineralization locally extends to the surface and as Pebble East where it is deeply buried. Pebble holds mostly low-grade ore, requiring a large-scale operation to economically recover it.

The Pebble deposit is hosted in porphyritic granodiorite to tonalite of Upper Cretaceous age intruded into deformed sedimentary rocks of the Jurassic to Cretaceous Kahiltna flysch terrane.

The copper ore is a calc-alkali porphyry copper-gold-molybdenum deposit. The ore body extends from the surface to at least 1,700 metres (5,600 ft) depth. In the western part of the orebody, mineralization occurs in a complex of several small granodiorite cupolas, diorite sills, older intrusions, breccias, and sediments. The western part of the deposit is locally exposed at the surface; thin gossans are developed and oxidation reaches 100 feet (30 m) in depth. The orebody extends eastward across a fault contact, at depth. East of the fault mineralization occurs in abundant sills and in the intruded sediments. Farther east, and deeper, the sills coalesce into a deeply buried granodiorite pluton. Mineralization and ore continue into the pluton. The eastern part of the deposit was eroded when it was exposed at the surface millions of years ago. It has since been buried by a thickening-to-the-east wedge of post-mineralization-age Tertiary sedimentary and volcanic sedimentary rocks.

Mineralization at Pebble is aged from 90.4 ±0.6 MYA to 89.5 ±0.3 MYA.

Metallic minerals identified at Pebble include pyrite, chalcopyrite, molybdenite, and bornite, along with minor covellite, chalcocite, digenite and magnetite.

Resources

In 2008, Pebble was estimated to be the second-largest ore deposit of its type in the world in terms of the value of the contained metal, slightly smaller than Indonesia's Grasberg Mine, which contains more metal in a smaller amount of ore than Pebble.

The last resource estimate for Pebble, as at December 2017, was a measured and indicated resource of 6.46 billion tonnes of ore grading 0.40% copper, 0.34 g/t gold, 1.7 g/t silver and 240 ppm molybdenum. Contained metals in the resource (at 100% recovery) were 56.9 billion pounds of copper, 70.6 million troy ounces of gold, 3.4 billion pounds of molybdenum and 344 million ounces of silver. The estimate used a 0.3% copper equivalent cutoff.

As of August 2020, gross value of the contained metals, 57% was from copper, and 27% from gold and 14% from molybdenum. Co-products including silver, rhenium, and palladium might also be recoverable.

Fisheries

All five Eastern Pacific salmon species spawn in Bristol Bay's freshwater tributaries. The bay is home to the world's largest commercial sockeye salmon fishery. The Kvichak River has the world's single largest sockeye run. The Kvichak drains from Lake Iliamna, which is downstream of the deposit. Salmon, herring and other fisheries account for nearly 75% of local jobs.

Sport fishing is another important local industry. Many lodges cater for sport fishermen exploiting the salmon and trout populations in the freshwater tributaries. Freshwater species include humpback whitefish (Coregonus pidschianpp), Dolly Varden trout (Salvelinus malma), arctic grayling (Thymallus arcticus) and rainbow trout (Oncorhynchus mykiss).

Seasonal harvesting of salmon and year-round harvesting of freshwater fish is a critical part of life for rural residents, most of whom live downstream of the mine site.

Human populations

The Pebble site is within Lake and Peninsula Borough, about 1,600 inhabitants as of 2010 United States census, adjacent to the Bristol Bay Borough of about 1000 inhabitants and the Dillingham Census Area, 4,800 inhabitants. Some 7,500 people live largely rural lifestyles within or near the area downstream of the Pebble site. The populations of Lake Clark National Park and other parts of the Bethel Census Area are upstream of the site or in a different watershed.

The populations in the area rely heavily on wild resources for subsistence, harvesting moose, caribou and salmon. Wild resources play an important part in the region's cultural heritage. There are more than 30 Alaskan native tribes in the region that depend on salmon to support their traditional subsistence ways of life, in addition to other inhabitants and tourists in the area.

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