«XA0201938 IAEA-SM-362/29 Mining the high grade McArthur River uranium deposit B.W. Jamieson Cameco Corporation, Saskatoon, Saskatchewan, Canada ...»
Mining the high grade McArthur River uranium deposit
Saskatoon, Saskatchewan, Canada
Abstract. The McArthur River deposit, discovered in 1988, is recognized as the world's largest, highest grade
uranium deposit, with current mineable reserves containing 255 million 1b U308 at an average grade of 17.33% U3O8.
In addition the project has resources of 228 million pounds U3O8 averaging 12.02% U308. Mining this high-grade ore body presents serious challenges in controlling radiation and in dealing with high water pressures. Experience from the underground exploration programme has provided the information needed to plan the safe mining of the massive Pelite ore zone, which represents the most significant source of ore discovered during the underground drilling programme, with 220 million pounds of U3O8 at an average grade in excess of 17%. Non-entry mining will be used in the high-grade ore zones. Raise boring will be the primary method to safely extract the ore, with all underground development in waste rock to provide radiation shielding. Water will be controlled by grouting and perimeter freezing. The ore cuttings from the raise boring will be ground underground and pumped to surface as slurry, at an average daily production of 150 tonnes. The slurry will be transported to the Key Lake mill and diluted to 4% before processing. The annual production is projected to be 18 million lb U3Og. The paper focuses on the activities undertaken since discovery, including the initiation of the raise bore mining method utilized to safely mine this high- grade ore body. Radiation protection, environmental protection and worker health and safety are discussed in terms of both design and practical implementation.
1. I N T R O D U C T I O N
1.1. Location The McArthur River deposit is located in the eastern part of the Athabasca Basin in northern Saskatchewan, Canada (Fig. 1), and is located 80 kilometres northeast of Key Lake and 40 kilometres southwest of the Cigar Lake deposit. The site is approximately 620 kilometres north of Saskatoon, a city with a population of 220 000, and the location of Cameco's corporate office.
1.2. 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 (69.805%), and Cogema Resources Inc.
(3 0.195%). In 198 8 the ore body was discovered following eight years of systematic exploration in the area.
Improvements to large-loop time-domain electromagnetic methods allowed the definition of graphite conductors in the basement fault structure which controls the location of the ore. Drilling confirmed this structure and discovered sub-economic mineralization five kilometres 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 surface followed and resulted in the outlining of high-grade mineralization over 1.7 kilometres of strike length. By 1991, 60 holes were completed of which 37 holes intersected uranium mineralization at a depth of 500 to 600 metres. Based on this information a resource of 260 million pounds at an average grade of 5% U3Og was estimated. Seventy per cent 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 metres . Following completion of the surface drilling it was decided to undertake an underground exploration programme which would provide the detailed information about the shape of the individual ore bodies.
. 1. Location of Me Arthur River and other uranium projects in northern Saskatchewan.
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 necessaiy for the EIS. An EIS for underground
Under that excavation license, horizontal development on the 530 metre level was undertaken to permit diamond drilling along a 300 metre strike length of the mineralized zone. This definition drilling increased the reserves and resources to 416 000 000 pounds U3O8 at an average grade of 15% . A second EIS to proceed to underground production was submitted in late 1995, and the public hearings were conducted in the fall of 1996. A favourable Panel report was issued in February, 1997. Both provincial and federal government approvals were received in May, 1997.
In August 1997 all licenses and permits had been received by both federal and provincial agencies to allow the two years construction of the project to proceed. The main license issued being the 'License to Construct' by the AECB. Construction was completed within the feasibility cost estimate and on schedule.
Operating licenses were received in October, 1999 for McArthur River, and in November, 1999 for Key Lake to receive and process the high grade McArthur River ore. Production commenced, as scheduled, in early December, 1999 following the commissioning of process equipment with waste rock and low grade ore.
The large and 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 Archean rocks beneath. At McArthur River this unconformity is at a depth of 500 to 600 metres. The mineralization 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 metres. This is referred to as the P2 fault. Locally the basement rocks include pelitic gneisses and significant quartzite units. Alteration is characterized by intense silicification of the sandstone with less intense clay alteration compared to other Athabasca deposits. The mineralization is largely pitchblende without the associated cobalt-nickel-arsenic minerals which are present at Key Lake and Cigar Lake .
Two distinctly different mineralized settings have been identified through both surface and underground diamond drilling. These mineralized zones are called Pod 1 and Pod 2 (Fig. 2).
In the first type, typified by Pod 1, mineralization 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 mineralization has been traced by surface drilling over a 1700 metre strike length.
Significant intersections in Pod 1 grade typically 10 to 30% UsOg. Dip varies from 45 to 90 degrees and the ore zone width is typically 10 metres.
Shaft sinking and diamond drilling from underground revealed the presence of ground water associated with the sandstone and the conglomerate. The quantity of ground water depends locally on the nature of flow pathways, hydrostatic pressure and pathway impedance.
For Pod 1, ground water is associated with sandstone bedding planes, j oints, and most significantly, faulting and brecciation related to the P2 fault (Fig. 3). These water bearing structures have generally responded well to pressure grouting techniques.
1 Atomic Energy Control Board (AECB) was renamed Canadian Nuclear Safety Commission (CNSC) on May 31, 2000.
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 mineralized 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 (Fig. 4). The Pod 2 strike length is 100 metres, its height varies from 30 to 90 metres, and the width is typically 20 metres. 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 ground water 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 was 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 mineralization.
3. UNDERGROUND EXPLORATION PROGRAMME
In July of 1994 underground development commenced to allow the detailed diamond drilling of the ore zones identified by surface drilling. This program was aimed at determining the shape, grade and continuity of the central part of the ore body, on a strike length of 300 metres.
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Once essential services were established for power, and the collection and pumping of mine water, development was extended to within 3 5 metres of the ore zone. Development then progressed southwards and parallel to the strike of the ore zone for approximately 300 metres. A total of 998 metres of development was completed by June 1996.
Diamond drill bays were created every 30 metres along strike. Diamond drilling commenced once development had adequately advanced, and holes were drilled on sections and fanned above and below the mineralized areas. Infill drilling was conducted, as encouraging results and time permitted, to define the ore zones every 10 metres along strike.
During the 1995/1996 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 . As the high-grade ore was 276 encountered, extremely high levels of radon (up to 8.869 billion2 Bq/m3) were found associated with the ground water. 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.
Reserves of Pod 1 and Pod 2 as identified by the underground exploration program are presented in Table 1 along with the mineral resources identified by surface exploration.
4. MINING METHODS 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:
(1) Raise boring (2) Boxhole boring (3) Remote boxhole stoping (4) Blasthole stoping, including vertical crater retreat (5) Remote raise bore stoping (6) Jet boring (7) Remote boxhole stoping with "Viscaria" raise mining The preferred options for the mining of the high-grade ore are raise boring, boxhole boring, and jet boring.
Raise boring was selected as the initial mining method at McArthur River and offers the following
(a) Improved productivity when compared with boxhole boring.
(b) Capability to extract the high strength Pod 1 ore, in contrast to jet boring.
(c) Superior ability to limit the quantity of ore in process at any time, when compared to stoping methods.
(d) Ease of providing excellent ventilation control, in contrast to stoping methods.
All active mine planning to date has utilized this mining method. The high grade Pod 2 ore zone is the first zone being mined. Freezing has been introduced to control ground water and occasional unconsolidated ground conditions in this area, and was 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 tonne capacity provides a chilled brine (-40 degrees Celsius) which circulates through a heat exchanger located on the 530 metre level. A lower pressure brine at -30 degrees Celsius is then used to circulate through freeze pipes surrounding the ore zones. Freeze holes are at two metre centres and drilled to approximately 100 metres in depth. There are 78 holes in use for the freezing of the first two mining areas of Pod 2.
4.1. 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 are used to develop these openings. Standard rock bolting, screening and a 75 millimetre application of shotcrete (a cement product sprayed onto the walls and roof of underground openings) are utilized to provide long term ground support. The raise boring mining method is a four-step process (Fig. 5) .
Firstly, the raise bore machine is set up in the production chamber above the ore zone. The raise bore machine then drills a 300 millimetre pilot hole from the upper chamber, through the waste rock, the ore zone and the waste rock below the ore zone and into the lower extraction chamber. These pilot holes are up to 125 metres 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.