Pebble

Overview

The Pebble copper-gold-molybdenum project is located in the Bristol Bay region of Alaska,U.S.A.  

Pebble porphyry deposit and associated precious metals prospects were discovered by Cominco Alaska in the 1980s.  Northern Dynasty acquired its initial interest in the project in 2001 from Teck Cominco and Hunter Dickinson. In 2007, Anglo American acquired 50% interest in the project by committing $1.5 billion to advancing the project toward permitting and operations. 

The project is to be mined as an open pit by utilizing conventional drill, blast and truck-haul methods, with an initial life of 25 years and potential for mine extensions to 78 years and beyond. 

The process plant will have a nominal mill throughput of 200,000 tons per day, utilizing conventional crush-grind-float technology and equipment, and secondary gold recovery. 

Copper-gold concentrate produced on-site at Pebble will be transported via a slurry pipeline to a new deep-water port onCook Inlet. There it will be de-watered and bulk shipped to offshore smelters. Other products of the process plant are: gold dore, which will be flown to market from Iliamna; and molybdenum concentrate, which will be bagged and trucked to the port for shipment. 

The 45-year Reference Case produces 31 B lb copper, 30 M oz gold, 1.4 B lb molybdenum, 140 M oz silver, 1.2 M kg rhenium and 907,000 oz palladium while mining only 32% of the mineral resource. 

A preliminary estimate of the construction cost was $5,757 million in 2011.

Location

The Pebble mining project lies 200 miles southwest of Anchorage,Alaska and 60 miles west of tidewater on Cook Inlet.

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 ofAlaskafrom a bankrupt Russian Empire for $7.2 million.

The project area is characterized by tundra, gently rolling hills – as a result of glacial abrasion - and the absence of permafrost. Elevations over the mineral deposit range from 800 to 1,200 ft asl, while the surrounding mountains reach 3,000 to 4,000 ft asl. 

Climate is transitional. Mean monthly temperatures range from -17 deg C during the winter to 13 deg C over the summer. 

Wildlife in the project area is rich and characteristic for Alaska’s lands and waters. 

Alaska’s economy is resource based being dependent on oil and gas for 90% of its revenue; the balance is made by mining, transportation, fishing, seafood processing and tourism. 

Bristol Bay communities practice a sustenance lifestyle based on salmon fishing and hunting of terrestrial mammals and birds.

Geology and Mineralization

Southwest Alaska is composed of an assemblage of northeast-trending tectono- stratigraphic terranes that amalgamated southward in response to long-lived, northeast- to northwest-directed subduction, beginning in the Late Paleozoic.  

The Pebble property comprises Jura-Cretaceous to Eocene igneous and sedimentary rocks , flysch and interbedded mafic to intermediate volcanic flows and tuffs that were intruded by subalkalic diorite and granodiorite sills along with alkalic intrusives. 

The district was subjected to extensive erosion, and sedimentary and volcanic strata were deposited so they overlie the older units. Low-lying parts of the district are covered by thin fluvio-glacial sediments. 

The Pebble deposit is a calc-alkalic copper-gold-molybdenum porphyry deposit which formed in association with granodiorite intrusions emplaced at about 90 Ma. The deposit comprises the contiguous Pebble West and Pebble East zones. Mineralization is associated with several coalescing hydrothermal centres formed around small granodiorite stocks which intruded the older rocks. Pebble West extends to surface. Mineralization in the Pebble East zone occurs within a granodiorite stock, and in surrounding flysch cut by granodiorite sills, and is overlain by east-thickening, post-mineralization volcanic and sedimentary strata. The mineralization at Pebble East is deeper and higher grade than that in Pebble West. 

Mineralization is dominated by hypogene pyrite, chalcopyrite and molybdenite. Bornite is also an important component in some parts of the Pebble East zone. The Pebble West zone has a thin, volumetrically subordinate zone of supergene mineralization and a very minor zone of oxide mineralization. The Pebble East zone contains only hypogene mineralization.

Mining & Operation 

Seven pit phases were optimized for the first 25 years of mining. In general, mining will be initiated with progression through two pits: East and West. The plan will continue within both pits until the final West pit bottom is reached; thereafter, mining will continue in the East pit only. Standard 100 ft high benches and 35 ft wide berms were adopted within the overall slope guidelines. 

In year 16 of the open pit mine life, a bench on 400 ft elevation provides an opportunity for the installation of an IPCC (In Pit Crushing and Conveying) station. 

Main Equipment include 19 Drills 12.25"; 8 Shovels 73 yd3; 3 Hydraulic Shovels 53 yd3; 3 53 yd3 Wheel Loader; 158 Autonomous Trucks 400 ton. 

Ancillary Equipment consists of 16 Dozers; 9 Wheel Dozers; 15 Graders; 10 Water Trucks. 

The operations workforce is projected to average 1,120 over the initial 25-year life of the mine. 

Mine development at Pebble following the initial 25 years of open pit production at Pebble could be undertaken via underground block caving. 

Underground production schedules have been prepared using a mining rate of 150,000 tons per day – i.e. 50,000 tpd per block. Load Haul Dump (LHD) units working on the extraction level would direct tip into 63-inch x 75-inch jaw crushers with conveyors transporting the crushed ore to the production shafts. 

A total of 1,459 million tons of ore can be mined over a 35-year underground mine life. Over 800 people would be on the underground operations payroll. 

Pebble will be a greenfield project and as such requires the development of supporting infrastructure.

In addition to requisite infrastructure at the mine site, such as maintenance facilities, offices, utilities, and worker accommodation, several key primary infrastructure components must be constructed. These include a tidewater port for concentrate off-loading and trans-shipment of operating supplies; an access road to connect the mine site with the port and the existing airport at Iliamna; pipelines for concentrate, reclaim water, diesel fuel, and natural gas transport; and electrical power generation facilities . 

The proposed port, Port Site 1, is located on the western shore of Iniskin Bay on Cook Inlet. Port Site 1 is adjacent to deep water and does not require dredging for port access or mooring.

Process 

The process plant consists of a conventional circuit, similar to many existing designs for large-scale copper porphyry operations around the world. 

A 200,000 ton per day processing plant has been selected as the design criteria for downstream plant capacity. A target of 80% passing 200 μm, determined from the grind recovery work, has been selected as the appropriate feed size for the flotation circuit. The resulting comminution design features two 40 ft x 25 ft @ 29 MW SAG mills and four 26 ft x 40 ft @ 16.4 MW ball mills in SABC-A configuration, in which the crushed pebbles are recycled to the SAG mill. 

The process is based on conventional grind-crush-float technology, using proven plant equipment of the largest sizes that have been industrially installed. 

Run-of-mine (ROM) ore from the open pit will be crushed and conveyed to the concentrator. The ore feed will be ground to liberate the mineral values from the host rock, and then separated by industrystandard flotation processes. A bulk copper/molybdenum sulphide concentrate will pass through the molybdenum separation circuit to produce a molybdenum sulphide concentrate, which will be bagged and trucked to port facilities onCook Inlet. The molybdenum separation generates a copper concentrate (containing most of the recovered gold) that will be pumped through a pipeline to the port site. The pyrite concentrate will report to a secondary gold recovery circuit where gold doré and bagged carbon-bearing fines will be produced and shipped off-site. Recovered gravity gold will be sent to the copper concentrate. 

The 25-year IDC Case will process 1.99 billion tons of ore with a strip ratio of 1.5: 1 and average grades of 0.38% Cu, 0.012 oz Au/ton, 182 ppmMo.Over the life-of-mine, the 25-year IDC Case will produce some 12.9 Blb of copper, 16.4 Moz of gold and 616 Mlb of molybdenum, as well as 67 Moz of silver, 502,000 kg of rhenium and 385,000 oz of palladium. Production rate is estimated at 80 Mt/y. Average annual copper recovered: 518 Mlbs.; Gold = 518 koz.

The 45-year Reference Case will process 3.8 billion tons of ore, with a strip ratio of 2.1:1 and average grades of 0.46% Cu, 0.011 oz Au/ton and 214 ppmMo.Over the life-of-mine, the 45-year Reference Case will produce 30.5 Blb of copper, 30.3 Moz of gold and 1.4 Blb of molybdenum, as well as 140 Moz of silver, 1.2 Mkg of rhenium and 907,000 oz of palladium. Production rate was estimated at 84 Mt/y. Average annual copper recovered: 678 Mlbs.; Gold = 673 Koz.

The 78-year Resource Case will process 6.5 billion tons of ore, with a strip ratio of 2.6:1 and average grades of 0.46% Cu, 0.011 oz Au/ton and 243 ppmMo.Over the life-of-mine, the 78-year Resource Case will produce 53.4 Blb of copper, 50.1 Moz of gold and 2.8 Blb of molybdenum, as well as 242 Moz of silver, 2.3 Mkg of rhenium and 1.59 Moz of palladium. Production rate estimated at 84 Mt/y. Average annual copper recovered: 685 Mlbs; Gold = 643 Koz.

Environment & Community

The tailings facility will contain both non-pyritic and pyritic tailings produced separately in the process plant. 

At closure, the waste rock will be processed through the mill to extract metal content and sulphides. The tailings from this process will be pumped to the open pit. 

Following closure, the open pit will be permitted to fill with groundwater. The level of the pit lake will be maintained at an elevation that ensures the pit remains a groundwater sink. 

The underground workings will be allowed to flood and will eventually connect to the water in the open pit. Some of the waste rock will have been placed over the subsidence zone during operations; this area will be covered with growth medium and vegetated prior to the time at which subsidence begins. An earthen berm will be placed around the subsidence zone to restrict entry. 

Some 70-80% ofBristol Baysockeye production is hydrologically isolated from any potential effects of the Pebble Project. The three watersheds that could be affected by the Pebble project produce less than 0.5% of the total Bristol Bay sockeye run. 

Other wildlife using the deposit area includes various species of raptors and upland birds, brown bear, caribou and moose. Four species listed under the Endangered Species Act – Steller’s Eider, Sea Otter, Steller’s Sea Lion, and the Cook Inlet Beluga Whale – have been observed near the proposed port site, as have harbour seals, a species protected under the Marine Mammals Protection Act. 

The mining companies invested in local communities. It lalso offered grants, developed  new water wells, had geothermal exploration initiatives and developed community incinerators and recycling programs. Local people are trained to become future Pebble employees.