The McArthur River Project: High Grade Uranium Mining

Brian W. Jamieson & Stan E. Frost

The McArthur River project is the world's largest known high-grade uranium deposit. It is presently being developed to allow the start of production in late 1999. Located in northern Saskatchewan, Canada, the deposit is estimated to contain 416 million pounds of U3O8 (160 000 tU) with an average grade of 15%. Provincial and federal government approvals were received in May 1997, and regulatory construction licences and permits have been issued or are in the process of being approved. Cameco Corporation owns 56% and is the operator of McArthur River. Cameco also operates and owns two-thirds of the world's two largest, highest grade and lowest cost producing uranium mines at Key Lake and Rabbit Lake, also located in northern Saskatchewan. Together these two operations will produce 26 million pounds of U3O8 (10 000 tU) in 1997. This represents about one third of the total Western World production. Within the next five years, reserves at both Key Lake and Rabbit Lake will be depleted. Nevertheless, Cameco's future is secured by its controlling interest in the McArthur River deposit and in the nearby high-grade Cigar Lake deposit. The uranium grades at McArthur River are roughly ten times those at Key Lake and Rabbit Lake and a hundred times higher than average grades elsewhere in the world (Ref 1). Planned production from the McArthur River underground mine is 18 million pounds of U3O8 (6900 tU) per year. The ore will be transported 80 km by road to be milled at the existing Key Lake operation. The successful mining of the McArthur River deposit, however, will not be accomplished without overcoming technical challenges. Not only is the high grade of the uranium ore challenging, but solutions must also be found to deal with groundwater at very high pressures, and ground conditions that vary substantially from excellent to wholly unconsolidated clays and gravels. The protection of workers and the environment has been the first priority in the design and development of the project. This paper will present a brief history of the McArthur River project, a description of the geology and the results of the underground exploration programme, followed by a summary of the proposed mining methods, radiation control procedures and ore processing facilities. Location The McArthur River deposit is located in the eastern part of the Athabasca Basin in northern Saskatchewan, Canada (see Figure 1), 80 km northeast of Key Lake and 40 km southwest of the Cigar Lake deposit. The site is approximately 620 km north of Saskatoon, a city with a population of 200 000 and the location of Cameco's corporate office. History Cameco, through one of its predecessor companies, Saskatchewan Mining Development Corporation, began operating the McArthur River exploration joint venture in 1980. After several changes in joint venture partners, the project is now owned by Cameco Corporation (55.844%), Uranerz Exploration and Mining Limited (27.922%), and Cogema Resources Inc (16.234%). The ore body was discovered in 1988 following eight years of systematic exploration in the area. Improvements to large-loop, time-domain electromagnetic methods allowed the definition of graphite in the basement fault structure that controls the location of the ore. Drilling confirmed this structure and discovered sub-economic mineralisation 5 km to the southwest of the McArthur River ore body. The recognition of favourable alteration patterns in drill holes helped guide the exploration drilling to the ore body. Several years of core drilling from the surface followed and resulted in the outlining of high-grade mineralisation over 1.7 km of strike length. By 1991 sixty holes were completed, of which thirty-seven intersected uranium mineralisation at a depth of 500 to 600 m. Based on this information a resource of 260 million pounds U3O8 at an average grade of 5% U3O8 was estimated. However, 70% of the estimated resource was based on only seven drill holes, and 18% was based on a single hole, which graded 43% U3O8 over 25 m (Ref 2). Following completion of the surface drilling it was decided to undertake an underground exploration programme, which would provide detailed information about the shape of the individual ore bodies. The project was referred to the Joint Federal-Provincial Panel on Uranium Mining Developments in Northern Saskatchewan in February 1991. Scoping meetings were held in nine northern and three southern communities in early 1992 to get public input into the guidelines for the environmental impact statement (EIS). The guidelines were issued later that year, after a public review of the draft. Environmental studies had already been started to develop the information necessary for the EIS. An EIS for underground development (Ref 3) was assembled and the panel conducted hearings on this subject at five northern and two southern communities in December 1992. After a favourable report from the panel (Ref 4) and licensing by both the federal Atomic Energy Control Board (AECB) and the Province of Saskatchewan, shaft sinking commenced in the spring of 1993. Under the current excavation licence, horizontal development on the 530 m level was undertaken to permit diamond drilling along a 300 m strike length of the mineralised zone. This definition drilling increased the reserves and resources to 416 million pounds U3O8 at an average grade of 15%. A second EIS (Ref 5), assessing the impacts of construction, full underground production, ore transportation, and milling and tailings disposal at Key Lake, was submitted in late 1995, and the public hearings were conducted in late 1996. A favourable panel report (Ref 6) was issued in February 1997. Both provincial and federal government approvals were received in May 1997. The licensing of various details of the construction by the AECB and the Province of Saskatchewan is proceeding. Geology The large, high-grade Saskatchewan uranium deposits occur at or close to the unconformity which separates the generally flat-lying, unmetamorphosed, middle Proterozoic sandstones of the Athabasca Group from folded and metamorphosed lower Proterozoic and Archaean rocks beneath. At McArthur River this unconformity is at a depth of 500 to 600 m. The mineralisation at McArthur River is associated with a northeast trending, southeast dipping zone of reverse faulting, along which the unconformity is displaced vertically 60 to 80 m. This is referred to as the P2 fault. Locally the basement rocks include pelitic gneisses and significant quartzite units. Alteration is characterised by intense silicification of the sandstone with less intense clay alteration compared with other Athabasca deposits. The mineralisation is largely pitchblende without the associated cobalt-nickel-arsenic minerals that are present at Key Lake and Cigar Lake (Ref 2). Two distinctly different mineralised settings have been identified through both surface and underground diamond drilling. These mineralised zones, called Pod 1 and Pod 2, are shown on Figure 2 together with all existing underground development. In the first type, typified by Pod 1, mineralisation occurs in sandstone and is structurally controlled by the P2 fault. It is associated with a strong (150 to 200 MPa) but fractured zone of silicified sandstone and conglomerate. This mineralisation has been traced by surface drilling over a 1700 m strike length. Significant intersections in Pod 1 grade typically 10 to 30% U3O8. Dip varies from 45 to 90 degrees and the ore zone width is typically 10 m. Shaft sinking and diamond drilling from underground revealed the presence of groundwater associated with the sandstone and the conglomerate. The quantity of groundwater depends locally on the nature of flow pathways, hydrostatic pressure and pathway impedance. For Pod 1, groundwater is associated with sandstone bedding planes, joints and, most significantly, faulting and brecciation related to the P2 fault (see Figure 3). These water-bearing structures have generally responded well to pressure grouting techniques. Ground conditions are rated as good to very poor, largely depending on the spatial relationship with the P2 fault. Few restrictions exist as to development placement from a stability perspective. However, mine development in sandstone and conglomerate requires extensive water control measures. The second type of mineralised setting was identified primarily from underground diamond drilling. The large and high-grade Pod 2, or Pelite ore zone, is located in the basement rocks stratigraphically above a quartzite footwall unit (see Figure 4). The Pod 2 strike length is 100 m, its height varies from 30 to 90 m, and the width is typically 20 m. Average in situ grade is greater than 20% U3O8. Occasional drill intercepts with grades higher than 40% U3O8 were encountered over significant widths. The host rock consists of sheared and altered pelite (30 to 40 MPa) containing zones of massive and stringer pitchblende. Large groundwater flows associated with unconsolidated sand, clay and brecciated rock have been intercepted along the footwall of the Pod 2 ore zone. These areas have not responded well to pressure grouting techniques due to the difficulty in penetrating the fine-grained clays and sands in these areas. Ground freezing has been deemed necessary to consolidate this zone prior to mining. Drilling has also revealed ground water and brecciated sandstone above the ore zone. Acceptable locations for mine development for Pod 2 are, therefore, limited to the hanging wall basement rock and the quartzite below the mineralisation. Underground Exploration Programme In July 1995 underground development commenced to allow the detailed diamond drilling of the ore zones identified by surface drilling. This programme was aimed at determining the shape, grade and continuity of the central part of the ore body, on a strike length of 300 m. Once essential services were established for power, and the collection and pumping of mine water, development was extended to within 35 m of the ore zone. Development then progressed southwards and parallel to the strike of the ore zone for approximately 300 m. A total of 998 m of development was completed by June 1996. Diamond drill bays were created every 30 m along the strike. Diamond drilling commenced once development had adequately advanced, and holes were drilled on sections and fanned above and below the mineralised areas. In-fill drilling was conducted, as encouraging results and time permitted, to define the ore zones every 10 m along the strike (see Figure 5). During the 1995-96 underground drilling programme, 115 holes were completed. The drilling of these holes provided both the ore geometry and grade, as well as geotechnical and hydrogeological information necessary to select mining methods and design material handling systems (Ref 7). As the high-grade ore was encountered, extremely high concentrations of radon (up to 8.9x109 Bq/m³) were found in the groundwater. The higher levels of radon were usually associated with low water flows; however, the water pressures were normally hydrostatic at a pressure of 51 bars. When the water flowed into mine openings, the radon was released to the mine atmosphere. Diamond drill bays were developed at right angles to the main drift and, therefore, created difficulties in designing a ventilation system that would ensure the capture of all air-borne contaminants. After some trial and error, plastic curtains and various sizes of flexible ventilation exhaust ducting were utilised to create a one-pass ventilation flow within these drill bays. This worked extremely well and prevented the contamination of working areas further downwind of the drillers. This careful design of ventilation systems and the diligent application of an appropriate radiation code of practice resulted in the underground drill programme being completed without any radiation excursions (see Figure 6). Reserves of Pod 1 and Pod 2 as identified by the underground exploration programme are presented in Table 1, along with the mineral resources identified by surface exploration. Mining Methods The controlling factors for mining method selection at McArthur River were: The high grade of the reserve. The wide range of ground conditions present,which limit acceptable locations for mine development. The presence of significant groundwater, mostly encountered within the sandstone and conglomerate geological units. All mining methods requiring workers to enter the mining area were immediately eliminated. Non-entry mining methods were required due to the high radiation fields from the ore. Other methods were eliminated if not compatible with grouting or freezing techniques for groundwater control. Strict adherence to the principles of limiting the time of exposure, maximising the distance between the workers and the ore, and placing shielding between the workers and the ore was necessary in order to limit worker gamma radiation exposures. Radon gas released from the ore and groundwater adjacent to the ore meant excellent ventilation practices were required. The need to capture radon gas at its source has also affected mining method selection. Seven potential mining methods were proposed in the EIS submitted for McArthur River, with final selection dependent upon ore grades and ground conditions. These methods are: raise boring; box-hole boring; remote box-hole stoping; blast-hole stoping, including vertical crater retreat; remote raise-bore stoping; jet boring; remote box-hole stoping with 'Viscaria' raise mining. The preferred options for the mining of the high-grade ore are raise boring, box-hole boring and remote box-hole stoping. Raise boring has been selected as the initial mining method at McArthur River and offers the following advantages: Improved productivity when compared with box-hole boring. Capability to extract the high-strength Pod 1 ore, in contrast to jet boring. Superior ability to limit the quantity of ore in process at any time, when compared with stoping methods. Ease of providing excellent ventilation control, in contrast to stoping methods. All mine planning to date has utilised this mining method. The high-grade Pod 2 ore zone will be the first zone to be mined. Freezing will be required to control groundwater and occasional unconsolidated ground conditions in this area, and must be implemented approximately nine months prior to mining in order to provide a frozen barrier sufficient to permit the safe extraction of the ore. A surface freeze plant of 800 t capacity will provide a chilled brine (-40°C) which will be circulated through a heat exchanger located on the 530 m level. A lower pressure brine at -30°C will then be circulated through freeze pipes surrounding the ore zones. Freeze holes will be at 2 m centres and drilled to approximately 100 m in depth. The 85 holes needed for the freezing of the first two mining areas of Pod 2 are planned to be drilled during 1998 to allow the freezing to start in early 1999. Raise Boring The raise-bore mining method as applied at McArthur River requires the establishment of mine openings of adequate size in surrounding non-radioactive rock both above and below the ore zone. Conventional drill-and-blast tunnelling methods will be used to develop these openings. Standard rock bolting, screening and a 75 mm application of shotcrete (a cement product sprayed onto the walls and roof of underground openings) will be used to provide long-term ground support. The raise-boring mining method is a four-step process (see Figure 7) (Ref 8). Firstly, the raise-bore machine is set up in the production chamber above the ore zone. The machine then drills a 300 mm pilot hole from the upper chamber, through the waste rock, the ore zone and the waste rock below the ore zone, into the lower extraction chamber. These pilot holes will be up to 125 m in length. Secondly, after breakthrough of the pilot hole into the extraction chamber, the pilot hole drill bit is removed and replaced with a reaming head. The reaming head will initially be 2.4 m in diameter, but may, if geotechnical conditions permit, be increased to a diameter of 3.0 m. Expandable reamer heads may also be utilised to minimise waste dilution and to improve productivity. By applying upward thrust and rotation, the raise-bore machine then reams the waste rock immediately above the extraction chamber. This barren waste rock is removed with a conventional mobile mine vehicle and hoisted to surface for appropriate disposal. Prior to the reaming head entering the ore, reaming is stopped and an ore handling structure is placed beneath the raise opening. Reaming then recommences with all reamed ore product passing through a sizing screen and crusher. The undersize material will be pumped to the underground ore grinding area. All over-size ore product will be collected in a closed container and transported to the same grinding area. The raise-bore reaming typically produces a fine material with few large pieces. Tests conducted at the Eagle Point mine at Rabbit Lake showed that 91.4% of the reamed product is less than 19 mm in size. Larger pieces are expected to arise from the structure and jointed nature of the ore zone, and are likely to originate within the raise after the reamer head has passed. Reaming continues upward until the top of the ore zone has been reached. At this point the reaming head is lowered to the extraction chamber and removed. The raise-bore machine then raises the pilot drill rods and removes them within the upper chamber. In the third step of the process, the bottom of the raise is covered, and the empty raise is filled with a rapidly curing, high-strength concrete, introduced from below into the lower part of the raise. This concrete plug is designed to support the placement of the next, much larger, concrete pour. Finally, once this first concrete application has cured, the remainder of the raise is filled from the upper chamber with a lower-strength, fast curing concrete. After curing of the concrete fill, extraction of adjacent ore by repeating the sequence described above is possible. By overlapping the raises, a high percentage extraction of the ore zone is achieved. After mining and filling a series of rows, the upper and lower chambers are widened to permit the mining of subsequent rows of bored raises. The chambers above and below the completed raises are filled with concrete to provide ground support as mining progresses with the completion of each row of bore holes. Box-hole Boring The box-hole boring method only requires the lower mining chamber. The machine pushes the reamer upwards through the ore, adding stabilisers to the drill rods as mining progresses. The ore falls down the raise to a chute above the box-hole machine, and is then diverted to the sizing screen and crusher. All further ore processing is as described for the raise-boring method. The method is shown in Figure 8, with an expandable reamer head being utilised to improve productivity. Remote Box-hole Stoping This mining method combines the productivity improvements offered by stoping with the control and containment provided by box-hole boring. The raise is reamed as described in the box-hole boring method. Once completed, blast holes are drilled from the security of production drill drifts placed lateral to the raise and above the mining chamber. These blast holes intersect the raise, and are loaded with explosive and blasted, as required, to provide broken ore to the box-hole boring unit. The reamer head is kept within the lower section of the raise and functions to control the size and regulate ore flow to the sizing screen and the crusher below (see Figure 9). On average, each raise will produce approximately 190 000 pounds of U3O8 (73 tU) from within initial mining areas of Pod 2, with this zone providing most of the production planned during the first years of mining. Due to the high grade of the ore, an average of only 125 t of ore is required to be mined each day. The total time necessary to mine and fill a raise is expected to be about 10 to 11 days. During this time the raise bore machine will likely be reaming in ore for one day. A total of four raise-bore machines are planned to be in operation during full production. Waste Rock Management Waste rock will be generated both by mine development and by ore production. The production of waste rock will be minimal due to the low tonnages of ore required to be mined each day. Potentially problematic material (waste rock containing >0.03% U3O8 or with a net neutralising potential of less than 3, ie. neutralising potential to acid-generating potential of <3:1, despite the low pyrite content) will be hoisted conventionally via the main service shaft and stored on lined pads at McArthur River. This material will either be used for back-fill underground at the mine site, or transported to Key Lake for final placement in existing approved storage areas. During the development phase, 140 000 t of potentially mineralised non-ore material, and 75 000 t of potentially mineralised sandstone are expected. The extensive use of cement grout will likely mediate any residual pyrite content of the rock. A total of 900 000 t of inert waste rock is expected from underground development, including ventilation shaft sinking. Inert waste rock will be placed on surface at approved unlined sites. Radiation Control The control of radiation has been the primary factor in determining the designs for mine and plant layout, equipment selection and the processing of the ore at McArthur River. In order to minimise exposures the following criteria were applied: Radon gas is controlled by a dual ventilation system; a primary fresh air flow is always maintained in all active work areas, with a secondary exhaust system to remove contaminated air from particular sources. Radon is also controlled by the freezing and grouting techniques used to control ground water. During all mining and processing stages the ore is fully contained. Gamma radiation is controlled by utilising the principles of shielding, distance and time. The use of heavy-wall steel pipes, thick vessel walls, concrete and sometimes lead sheeting is standard practice. Mining and ore handling and processing will be accomplished remotely with computer control. Due to the low tonnages required to be mined, there is a long period between scheduled maintenance work. A total of three shafts will be utilised to provide 455 m³/s of air at full production. Two shafts will supply fresh air (the main service shaft #1, and shaft #2), while a third shaft will exhaust the mine workings. Every operation has been analysed for exposure time, and distance and shielding calculations have been done to ensure that radiation doses are acceptable. Design calculations have been confirmed by doing physical measurements of radiation fields around pipes filled with high grade ore from the test mining at Cigar Lake, and at the existing Key Lake and Rabbit Lake mills. The radiation exposure calculations included estimates of exposures arising from equipment maintenance and spill clean-up. As a result of these design criteria, the predicted annual doses for the workers are well under the regulatory dose limit (see Figure 10) (Ref 5). Actual experiences with radiation control, the treatment of radium-rich mine water, and waste rock management during the underground exploration phase have shown that the techniques used provided excellent results with very minimal exposures. This knowledge and extensive public and regulatory review have resulted in a fairly broad-based consensus that the project can be developed while meeting current radiation protection and environmental objectives. Ore Processing Once mined, the ore is screened and/or crushed prior to being pumped to a grinding circuit located underground. It was decided to process the ore to a slurry suitable to be pumped directly to the surface. This eliminates the need to hoist the high-grade ore within the shaft used to move men and material and to supply fresh air. The grinding circuit is fairly conventional and includes a ball mill fed from a crushed ore surge tank. The mill has been sized to grind ore that has a Bond work index of 17 kWh/t to 80% passing 200 microns. A classifying screen is operated in closed circuit with the ball mill. Classifying screen underflow, the final ground slurry, is pumped to the two underground ore thickeners. Overflows are recycled back to the mill. Thickener underflow slurry (controlled at 50% solids by weight) is pumped from the underground ore thickeners to an agitated thickener underflow tank, which feeds the ore slurry hoisting pumps. The two ore slurry hoisting pumps are of positive displacement type, and are each connected to a dedicated pipeline to convey the ore slurry directly to the surface. On the surface the ore slurry is pumped through a U3O8 on-stream analyser. Depending on the indicated ore grade, the slurry is placed into one of four ore storage tanks. When container loading begins, the ore is then re-slurried for grade blending, thickened to 50% solids and placed into purpose designed containers. Once all four containers are filled, washed and successfully scanned, the truck will depart for Key Lake. Each truck is designed to carry four containers and results in the transportation of 18 t of ore (21.2 m³ of slurry) per trip. Approximately eight trips per day will be required to transport ore to the Key Lake mill at average grades. At the Key Lake operation, the ore will be diluted to 4% U3O8 by blending it with special waste material prior to milling. All tailings will be placed in the existing Deilmann pit tailings management facility at Key Lake. Conclusion The McArthur River deposit has been proven to be a major uranium discovery during the past few years. As a world-class ore body, this deposit will help secure Cameco's position as a competitive uranium producer for decades to come. The McArthur River Joint Venture will have spent approximately C$450 million (US$330 million) during the eleven years from discovery to production. While this may seem a long project lead time, it is in reality representative of the normal investment of time required to bring a uranium mine into production in Canada. The underground exploration programme has proven mineable, high-grade ore zones. Through well-focused engineering design and extensive project review, it has been shown that this deposit can be mined by non-entry methods, such as raise boring, to achieve the goal of high-grade ore extraction safely and with acceptable radiation exposures.