Greens Creek keeps it clean.
Greens Creek mine in Alaska has managed to balance its position as the largest American silver producer with an outstanding record for stringent environmental protection. This is necessary, as the mine is surrounded by a designated wilderness area in a sensitive Arctic environment.
The only way to reach the mine site is by boat or float-plane, yet none of the work force lives on site. Instead, commuting has become a way of life for the operation's 250-strong team of miners, mill personnel, site administrators and engineers. The operating company, Greens Creek Mining Co., leases a ferry to transport its staff from their homes to the mine each day.
The 150-seat ferry, the Alaskan Dream, is a catamaran vessel, and takes just 35 minutes to travel the 24 km from its mainland terminal at Auke Bay, north of Juneau in Alaska's panhandle, to the landing dock on Admiralty Island. From there, buses take the staff 20 km up a dirt road to the mine itself, set in the spruce forest that covers the island, at an elevation of 280 m above sea level.
Greens Creek is owned by a joint venture between Kennecott (53.2%), Hecla Mining Co. (28.4%), CSX (12.6%) and Exalas (5.8%). Its daily production of sulphide ore is currently running at 1,050 mt, about 10% above design throughput, giving a mill head grade of approximately 10% zinc, 5% lead, 750 g/mt (22 oz/st) silver and 3.8 g/mt gold. The cost of bringing the property into production - in December 1989, and including exploration, development and construction - was $114 million. Based on initial reserves, the mine had a projected life of at least 10 years.
Exploration during 1990 revealed a massive increase in reserves. At the beginning of 1990, proven, probable, and possible reserves at Greens Creek totalled about 3.5 million st. After around two years of mining, proven and probably reserves now total in excess of 6.3 million st grading an average of 15.1 oz/st silver, 0.13 oz/st gold, 12.4% Zn, and 4.2% Pb. There is good potential for increasing reserves still further with exploration and development continuing through the rest of 1991.
In 1990 Greens Creek produced 7.635 million oz silver, 38,115 oz gold, 33,600 st zinc and 15,200 st lead.
Greens Creek ore is a complex mixed sulphide, which occurs in a number of discrete ore zones. A gossan above one of these zones gave the first clue to the existence of mineralization. The so-called "Big Sore" was spotted from a helicopter and led ground-based exploration crews to the discovery of this major deposit.
At the time, in 1977, the Pan Sound joint venture, consisting of Noranda, Martin Marietta, Exalas, Texas Gas, and the Bristol Bay Native Corporation, had claims on the property, and it was this group that undertook the preparatory work for the project. Geochemical surveying was followed by both drilling and the driving of nearly 1,500 m of exploration drifts from a portal at the 410 m elevation, on the hillside above today's main production adit.
In 1981, the orebody was first exposed, providing bulk samples, and by the beginning of 1983, the project's Environmental Impact Statement had been approved. This in itself was no mean feat, for Greens Creek lies in an enclave within the boundaries of the Admiralty Island National Monument - a designated wilderness area in which no development is permitted. In fact, exploration was already under way when the National Monument was established, leading to the exemption of the mine property from the wilderness area.
The first task facing the joint venture was the construction of the 13-km long access road from Hawk Inlet to the mine site. Work started in 1985, but conditions were so difficult that only 3 km were completed in the first two years. In the meantime, control of the property had been changing hands. Martin Marietta sold its interest in the joint venture to Anaconda, which also bought the Native Corporation's holding in conjunction with Noranda.
Noranda, however, was in the process of recording its first trading loss in 60 years, and as a result sold its interest, together with other properties, to Amselco (at that time, the North American minerals arm of BP). Amselco also bought Anaconda's holding, and sold part on to Hecla and the other companies currently involved. Amselco was then itself merged with Kennecott, which has been the project operator both before and after the sale of BP Minerals to RTZ.
With the priority now on completing the access road, rapid action was required. Over a two-day period in 1986, the project established a world record for air heavy-lift as earthmoving equipment was flown in to construction sites along the line of the road. By creating multiple work points from which the road could be driven in both directions, the remaining 10 km of road was pushed through that summer, as well as the 7 km stretch of road between the Hawk Inlet landing and the new ferry dock on the mainland side.
The road has been built to a constant grade, as it was originally intended to run a tailings pipeline alongside it down to a dam site below the mine. However, during the construction program, it was decided to minimize the impact of the tailings by using dry deposition rather than a slurry system, and the pipeline was never built.
Construction of the mine, the mill and the concentrator started in 1987, with the first concentrate being produced in February 1989. The first shipment was made in May of that year through the newly built shiploading facility at Hawk Inlet, which is capable of handling the ocean-going vessels needed to transport Greens Creek's production to smelters in countries such as the United Kingdom, South Korea, Belgium, Italy and France.
HIGHLY COMPLEX OREBODIES
The mine originally produced around 1,100 mt of ore and 350-400 mt of waste each day. Production was increased to 1,225 mt/d this year, and the concentrator was modified to accept this. Of 13 defined mineralized zones within the property, only three are currently in production. The North, Central and South orebodies are each split into two or more zones, and no less than eight different ore types have been identified.
Compounding the problems, the ore zones themselves are highly irregular, varying widely in dip (from horizontal to vertical), strike, and thickness over relatively short distances. The ore occurs at the junction of two rock units; the younger argillite appears as the footwall with the older phyllite above the contact, indicating that the complete structure has been overturned.
Minimum ore thicknesses of 600 mm in the North orebody increase to over 6 m elsewhere. Massive sulphides can be either high-pyrite, low-base-metals, or the converse, the former being the principal component of the upper portions of the Central and South orebodies, while the high-base-metals massive sulphide is found to predominate in the North orebody.
The massive sulphides contain microscopic gold, tetrahedrite, freibergite and chalcopyrite but, unlike many such deposits, the gold is unusually coarse at 10-15 microns, even reaching 75-80 microns in places. The next most common ore is the "white ore" type, which contains clots of tetrahedrite, tennantite, freibergite, galena, sphalerite and chalcopyrite in a barite, carbonate or siliceous gangue; in the last case, gold particles as large as 2 X 9 mm have been discovered in places. Overall, local gold values can be extremely high - examples of 1,100 g/mt (32 oz/st) over 300 mm and 310 g/mt over 3.6 m have been found - while just a few meters away, the value can be less than 2 g/mt.
Such a complex system requires the tight control given by intensive exploration. Underground drilling on rings 60 m apart has been followed by detail drilling at 30- and 15-m horizontal and vertical intervals. At the time of the E&MJ visit, over 115,500 m of core drilling had been completed, mostly N and B sizes, with some H core taken from surface. Of this, in 1990 the mine completed 6,000 m of surface drilling, using two contractor's drilling rigs, plus 18,000 m of core from its own three underground rigs. The surface drilling has been designed to prove down-dip extensions to the existing orebodies, while the underground drilling has been for increased orebody definition. Much work, however, remains in this area.
Data from the 15- X 30-m grid drill program is composited to give a minimum 1.8 m thickness, and is then kriged to give 15- X 30-m blocks in both plan and section. Diluted grades and tonnages are computed separately for each block, using Lynx Geosystem's software. A stope panel length of 120 m is used.
Access to the orebodies is gained by a series of ramps from the main 280 m elevation haulage level. The North, Central and South orebodies are being mined individually, being separated by 250 m and 120 m of waste respectively. Up to 40 headings are in use at one time, with all the mine's production being generated from development ends which measure from 2.4 X 2.4 m to 3.6 X 3.6 m in section. The main haulage is 4 m high by 4.5 m wide, and is used by the mine's Wagner trucks to transport both ore and waste from development headings to the surface.
Each orebody is accessed from the hanging wall, starting at a point that divides the 120-m panel length into 45- and 75-m sections. A 3.6-m high access decline is driven into the orebody from the zig-zag ramp that serves it, the vertical distance between decline entries depending on the local dip of the structure. Once the limits of the orebody have been proved, a strike drive is mined the entire length of the panel, following the footwall contact. If the orebody is wider than the strike drive, the side is usually slashed to reveal the hanging wall contact.
The next stage of extraction is to take a 2.4-m bench along the length of the stope, keeping the access ramps as steep as possible. Cemented fill is then placed to within 2.1 m from the stope back, a height that allows trucks to place the fill. The mine has a fleet of four Paus (Germany) backfill trucks, originally equipped with a slinger system, but now used only for transport, the fill being emplaced with LHDs and compacted with a Caterpillar D3B tractor.
After the fill has been allowed to set for four days, the next 3-m slice is taken out of the stope back, and the cycle is repeated another two times until a 13-m vertical height has been taken from the length of the stope. Where the orebody dip is lower, the basic system is modified in that the ore boundaries are first defined by drilling, using a Sitec nuclear detector to determine the position of the orebody limit in the hole. Once this has been done, the stope drive can be mined, and a bench and roof cut extracted before the next level access is put in.
In the flattest zones, a type of room and pillar mining is used, taking 9-13 m wide rooms, while elsewhere a shortwall retreat system, with simultaneous filling, provides the ore. Structurally, only a few areas of the mine require filling, but its use ensures that over half the total daily production of tailings can be disposed of underground, and not in the environmentally sensitive surface dump.
A problem not commonly experienced elsewhere in North America is sulphide dust ignition. The mine has overcome this through the use of a different stemming system, and by ensuring that susceptible areas underground are thoroughly washed down on a regular basis.
Greens Creek follows up its high-definition exploration drilling with a very thorough grade-control policy. Some 70% of all rounds are mapped by the grade control team, the boundaries between the various rock units are painted on the face. A Polaroid photograph of the face is taken, and the face is channel sampled.
The areas shown on the scaled photograph are digitized and input into the grade-control computer. Information from subsequent rounds provides the area of influence of the samples taken, allowing the computer to calculate grades and tonnages.
Run-of-mine ore is brought out of the mine in 22 mt loads, and is dumped on a pad close to the mill feed hopper. The ore is blended with a dozer, and is loaded into the 65-mt capacity hopper with a wheel loader. A grizzly with a 380-mm aperture screens out any oversize material, which is broken on the bars using a boom-mounted Teledyne hydraulic hammer.
A 1.5- X 4.3-m apron feeder draws the coarse ore from the hopper at a rate of 44 mt/hr, loading it on to the 1,200-mm wide mill feed conveyor. There is no crusher at Greens Creek; the primary ore reduction takes place in a 450-kw, 4.9- X 1.5-m Marcy semi-autogenous grinding mill. Load cells are used to monitor the SAG mill load, so ensuring that a steady feed rate is maintained.
The minus-1-mm mill product is immediately treated in a unit cell to remove any carbon present - values can range up to 8% C - which would affect later stages of the recovery process. Reagents used are just fresh water, a frother, and sodium cyanide and sodium sulphite depressants, otherwise any gold present will be removed as well.
The tailing from the unit cell, comprising the bulk of the pulp, is screened, the oversize reporting back to the SAG mill, and the undersize going on to further grinding, to 80% minus-37-microns, in a 670-kw Marcy 3.3- X 4.0-m ball mill. An alternative route, not currently in use, is the recovery of a gravity concentrate on Deister tables prior to ball milling; development work on this section of the plant is still under way at the Hazen Research laboratories in Denver, Colorado.
Ball-mill products are cycloned, the overflow reporting to the lead flotation circuit, while the underflow is treated in a second unit cell that recovers a high-grade lead concentrate, together with any gold particles that have been liberated in the ball mill. Reagents used at this stage are Aero 343 xanthate and minimal amounts of Aero 3477 promoter, which both prepares the pulp for normal lead flotation as well as acting as a lead collector in the unit cell.
The first flotation stage consists of two Outokumpu [1.5-m.sup.3] rougher cells, followed by two [1.5-m.sup.3] cleaners, which produce the first lead concentrate. Cleaner tails are scavenged in a bank of six [3.1-m.sup.3] Outokumpu cells, the scavenger concentrate being cleaned once in three [3.1-m.sup.3] cells before forming the feed for the bulk flotation circuit. Tails from both parts of the lead scavenger circuit form the zinc flotation circuit feed.
This consists of two [1.5-m.sup.3] roughers, two [1.5-m.sup.3] cleaners and nine [3.1-m.sup.3] scavengers. The scavenger tails form the final mill tails, while the scavenger concentrate is cleaned twice in two banks of three [3.1-m.sup.3] cells, from which the tails are used as bulk flotation feed, joining the concentrate from the lead circuit scavenger cleaner cells.
The bulk flotation circuit consists of six [3.1-m.sup.3] roughers and three [3.1-m.sup.3] cleaners. Both the lead and bulk concentrates are thickened in separate Denver thickeners - the former 6-m and the latter 9-m dia - before reporting to a common Sala 16-plate, [36-m.sup.2] pressure filter. The filter cake is transported by conveyor to the concentrate stockpiles, a tripper feeds each product to the correct area of the storage building.
The zinc circuit has its own thickener and filter system, consisting of a 9-m-dia Denver thickener and a Sala eight-plate, 18-m2 filter. Theoretical outputs are 180 mt/d of 12% Pb and 28% Zn bulk concentrate, 94 mt/d of 53% Zn concentrate and 28 mt/d of 50% Pb concentrate. Some 30% of the lead concentrate is recovered from the first unit cell, and the precious metals in the ore report to either the lead or bulk concentrates.
Tailings from each of the flotation sections are transported to an 18.3-m-dia thickener before being dewatered in twin 32-plate, [72-m.sup.2] Sala pressure filters. Filtration was an area of major concern during the early days of the Greens Creek operation. However, the problems that were encountered were rectified by the end of 1989, and the tailings filters have now achieved a throughput of approximately 25 mt/hr, which is over 50% higher than their theoretical capacity, when working on a 6-min filtration cycle.
Of the 750 mt/d tailings production, about 450 mt/d are returned underground after the addition of cement and water in an automatically controlled batching plant, operated by the fill truck driver. The remaining 300 mt/d is hauled by a contractor approximately 10 km to the tailings disposal area, where it is compacted and graded before being hydroseeded to stabilize the surface and minimize the environmental impact.
Plant control is achieved through the use of a Proscon control system, fed information by Outokumpu on-stream analyzers placed at critical points throughout the plant. Nine of these units are in place, supported by sampling stations located on all the main pulp streams. Immediate plant control is still manual, as it has been found that the ore is too complex for the automatic system without gaining extensive operating experience beforehand. While the flotation system in particular is designed to run as indicated above, there are many alternative options available to the operators, depending on the ore type and grade that is being processed at any particular time.
Greens Creek is a self-contained unit - given its location, this is essential. The mine offices, welfare facilities, workshops, mill and stockpiles of both concentrates and tailings are all situated in one compact development area of approximately 1.3 [km.sup.2] (0.5 [mi.sup.2]) cut out of the surrounding forest. The complex provides its own power - three 2.2-Mw Ruston diesel-generator sets are installed, of which two are running at any time. Water is drawn from Greens Creek, and what water the plant uses is cleaned and recycled to minimize make-up requirements.
A clean house is essential, and because of the possibility of attracting the unwanted attentions of some of the island's substantial population of bears, all rubbish is collected and shipped back to the mainland for disposal. Water runoff from the site is monitored, as is the marine cycle in Hawk Inlet. Permitting for the project has demanded discharge contamination limits lower than naturally occurring background levels.
All roadbuilding material is brought in from the mainland, not only because the local stone is of poor quality, but to obviate the need for a quarry. The sides of the access roads have been seeded with clover and grass, a move seemingly much appreciated by the local deer, and fish ladders have been installed in Greens Creek to enhance the salmon spawning habitat.
One particular area of concern has been the runoff from the muskeg that fills the valleys around. Inactive in winter, the muskeg produces an alkaline runoff, which changes to acidic once the spring arrives. This can cause remobilization of metals in the settling ponds, and a water runoff treatment plant has now been installed as a contingency against possible problems in the future.
Greens Creek is unique, and it has shown that even in a highly sensitive environment, adequate compromises can be achieved between protection and production. Its existence - which required permitting from no fewer than 22 different government agencies - is a testimony to the cooperation that was achieved between the company, the government, the community and the environmental lobby. Greens Creek has been faced with having to draw its labor force from an area that had largely lost its old mining tradition, together with the high costs of both establishment and operation in complex orebodies. However, it is heartening to suggest that if it is possible at Greens Creek, it should also be possible elsewhere.
PHOTO : Location map of Greens Creek on Admiralty Island off the Alaska panhandle north of Juneau.
PHOTO : Wagner ST2 LHD is put through its field trials at Greens Creek. Flexibility is a key factor in the mining operation, since as many as 40 production headings may be in use at any time.
PHOTO : The fill plant is attached to the concentrator building, allowing filtered tailings to be loaded directly into the batching plant hopper. Cment is added, and the fill is measured into the trucks that transport it to the stopes.
PHOTO : Greens Creek has eight jumbos - two Gardner-Denver electro-hydraulic machines, three Atlas Copco diesel-hydraulics, and three Atlas Copco electro-hydraulic rigs.
* Simon Walker, former editor of International Mining, is an independent journalist and engineer specializing in the international minerals industry. He can be contacted at 16 Gathorne Road, Oxford OX3 8NF, England; phone and fax +44 865 61875.
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|Title Annotation:||Green Creek Mining Co.'s environmental care in Juneau, Alaska minesite|
|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Nov 1, 1991|
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