The Millennium uranium deposit, discovered in 2000 by
Host to a resource totalling 57 million lbs. U3O8, the Millennium find represents the culmination of nearly 25 years of exploration in a relatively mature area of the basin.
Drill indicated resources are estimated at 449,000 tonnes grading 4.63% U3O8, while an additional 280,000 tonnes averaging 1.81% U3O8 are inferred. To the end of 2005, the Millennium deposit had been defined by 51 core holes drilled on nominal 50-metre sections, and down to 25 metres in the central core.
Charles Roy, Cameco’s director of advanced exploration, spoke at this year’s recent Mineral Exploration Roundup in Vancouver about the discovery.
“It’s arguably the most significant new basement discovery in more than 30 years,” he said.
The Athabasca basin of northern Saskatchewan is host to two of the world’s largest high-grade uranium deposits — McArthur River and Cigar Lake. They are both classified as unconformity type deposits as the uranium deposition is spatially related to an unconformity at the contact between Paleoproterozoic basement metamorphic rocks of the Wollaston Group and overlying sandstones of the Athabasca Group.
Although considered an unconformity type deposit, mineralization at Rabbit Lake, the first major uranium discovery in the basin in 1968, was hosted entirely in Paleoproterozoic basement metamorphic rocks, Roy explained. It was not until the discovery of the Deilmann and Gaertner deposits at Key Lake in 1975 and 1976 that the true unconformity type uranium deposit model was first recognized.
Over the last 25 years, subsequent discoveries of both basement-hosted and unconformity-related deposits have supported the concept that these deposits are genetically linked and that basement-hosted deposits are a derivative of the unconformity model.
Other significant basement-hosted deposits of the Athabasca basin include Eagle Point and Sue C.
The Millennium deposit, which is part of the Cree Extension project, occurs in the eastern portion of the basin, 35 km north of the former Key Lake mine. The eastern basin is comprised of a poly deformed and metamorphosed complex consisting of Archean granitoid gneisses and Paleoproterozoic metasedimentary rocks. The basement rocks are unconformably overlain by late Paleoproterozoic quartz-rich sandstones and conglomerates of the Athabasca Group. This area of the basin is considered a mature exploration region as activity has been ongoing since 1978.
The Cree Extension project is a uranium exploration joint venture held 42% by Cameco, 30% by
The Millennium deposit, a discovery first announced at the end of 2002, lies along a north-northeast trending structural corridor, which is defined by sophisticated electromagnetic and gravity geophysical surveys, plus diamond drilling. The interpretation of airborne magnetic data suggests that the basement rocks have been deformed into a complex foliated overprinted pattern. Two principal axial surface traces, F1 and F2, are interpreted.
The earliest F1 folds are inferred to be a relatively shallow-dipping recumbent structure that inverts to the northwest. The early structures have subsequently been refolded into more upright northeast-trending folds.
After taking over as project operator in late 1998, Cameco undertook an extensive review and compilation of the historic exploration data and drill core. This work suggested the southern portion of the corridor showed potential, which was best illustrated by drill hole CX-38, one of the final holes completed by previous operator Uranerz Exploration & Mining.
“The keys to the Millennium discovery were the observations and results from hole 38, specifically the uranium enrichment in the lower sandstone and the hydrothermal alteration of the basement rocks,” Roy explained.
The Millennium discovery hole CX-40, drilled in March 2000, was a vertical hole over 40 metres west of hole CX-38. This hole intercepted weak to moderate uranium mineralization 40 metres below the Athabasca unconformity and persisting over a core length of 153 metres from 608 to 761 metres down-hole. The best section of continuous mineralization was 29 metres averaging 1% U3O8.
“A significant uranium resource has been defined within the Main zone mineralization,” said Roy. The Main zone extends at least 230 metres along strike and 70 metres downdip, and is characterized by alternating higher and lower-grade intervals that can be traced section to section along strike. True width and grade of the intercepts is variable, with grades ranging from 1-5% U3O8 over true widths of between 20 and 30 metres. The top eight intercepts encountered to date range from 25-30 metres wide and carry a weighted average of nearly 4% to just under 7%. The best sample is 0.5 metre grading 60% U3O8.
The Millennium deposit is hosted almost entirely in the Wollaston Group metasediments, close to the unconformity boundary. The uranium mineralization exhibits apparent stratigraphic control and extends more than 150 metres below the unconformity. The Main zone uranium occurs in a pelitic to semi-pelitic stratigraphic assemblage of gneisses and schists. This unit is marked by a strongly graphitic (5-10%) pelitic gneiss along its upper margin that corresponds with the main geophysical conductor.
Various geophysical surveys have been undertaken in the vicinity of the Millennium deposit both pre- and post-discovery, which have aided in further defining the discovery, including ground electromagnetic, resisitivity and gravity, plus airborne magnetic and radiometric surveys. The three-dimensional inversion of pole-to-pole resistivity data collected over Millennium indicates the presence of a significant resistivity low extending well into the sandstone and centred over the mineralization. This low is interpreted to be a result of an increase in the clay content of the sandstone and hydrothermal alteration.
Millennium model
The most striking feature of the Millennium deposit is the extensive hydrothermal alteration overprinting both the lower sandstone and basement lithologies. Athabasca Group alteration is characterized by basement leaching and an increase of clay content in the lower sandstone.
The alteration of the basement rocks is more intense and includes a distal halo of saussurite and sericite, through to a more proximal zone of chlorite, illite and sericite, and into a central zone of increasing argillite alteration dominated by illite and dravite. The main area of uranium mineralization is coincident with the proximal alteration assemblage, commonly associated with dark chlorite and illite.
“Although a distinct alteration envelope surrounds the deposit, the mineralization itself is hosted by altered, yet competent, lithologies,” Roy said. “Therefore if sufficient resources can be identified to allow this deposit to be put into production, the need for expensive ground utilization by freezing is not considered necessary.”
The ore mineralogy in the Main zone consists of pitchblende, the primary uranium mineral, with lesser amounts of coffinite and uraninite. The mineralization occurs in a variety of styles, including massive foliation-controlled replacement, pitchblende matrix in hydraulic breccias, irregular fracture-controlled fillings and redox accumulations. Massive replacement mineralization is the dominant style suggesting mineralizing fluids infiltrated the rocks using the permeability of the dominant penetrative foliation, and to a lesser extent, lithological contacts and fractures.
“The Millennium model is relatively simple,” Roy said.
Mineralization, he explained, is attributed to
downward filtration of uranium-bearing oxidized fluids from the overlying sandstones into the more reduced basement rocks via a major reverse fault.
The most significant structural feature in the deposit area is a major fault zone at the base of a lower calc-silicate assemblage. This fault zone, which is interpreted to be reverse thrust, is 10 metres thick, strikes north and dips moderately east. The fault structure is interpreted to represent the main hydrothermal conduit for uranium-bearing fluids that then moved out along foliation planes before precipitating in structural traps.
This year, work is expected to begin on a prefeasibility study for the deposit involving engineering and environmental components. Engineering studies will assess underground design, mining methods, surface infrastructure and capital operating costs. The program will also include further diamond drilling. Several holes will be drilled in the deposit, while the majority of the drilling will evaluate the limits of the deposit along strike.
“The discovery of the Millennium deposit has reinforced the prospectivity of the Athabasca basin, but more importantly, it has increased the significance for the potential of basement-hosted deposits and the significance of associated argillic alteration as an exploration guide,” Roy said.
In related exploration news, Cameco continues to encounter “promising” results from drilling at the Collins Creek zone, about 15 km northwest of the Rabbit Lake mill. Six of the eight holes completed in 2005 returned significant uranium mineralization, with the best intercept averaging 5.62% U3O8 across 7.8 metres. Recent exploration at Collins Creek has outlined mineralization over a strike length of 650 metres and at depths of about 200 metres. However, the wide drill spacing doesn’t allow a resource estimate at this time. Cameco has planned an aggressive infill program of 20-25 holes in 2006.
Further drilling on the Centennial discovery at the Virgin River project in Saskatchewan has expanded the known boundaries of this zone. Four of six holes drilled during the summer of 2005 intersected significant grades and widths of uranium mineralization, with the best intercept being 8.39% U3O8 over 3.9 metres.
Cameco has budgeted $32 million in uranium exploration for 2006. The company has expanded its exploration activities by acquiring several new land positions, including projects in Nunavut, the Northwest Territories, Quebec and Australia.
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