Saskatchewan is the Rodney Dangerfield of diamond plays: it gets no respect.
After a decade of lethargic exploration and mundane results, which were eclipsed by the more exciting discoveries in the Lac de Gras area of the Northwest Territories, recent work by Shore Gold and the Fort la Corne joint venture has led to a much better understanding of the potential of some of these immense diamond-bearing bodies. The Fort la Corne kimberlite field of east-central Saskatchewan comprises some 73 kimberlite bodies, 65 km east of Prince Albert. The kimberlites occur along a northwest-oriented corridor 45 km long and 15 km wide.
The 225-sq.-km land holdings of the Fort la Corne joint venture covers 63 of these kimberlite bodies. The Canadian exploration division of
The Saskatchewan diamond play dates back to 1988 when Monopros, a De Beers affiliate, carried out work near Sturgeon Lake in a gravel pit that contained a greenish rock. The rock later proved to be large, rafted kimberlite blocks. Spurred on by rumours of kimberlite discoveries, a staking rush ensued and close to 2 million acres were snapped up near Prince Albert. Using aeromagnetic maps published by the Geological Survey of Canada (GSC), Uranerz Exploration & Mining interpreted the possible presence of kimberlite-type intrusions in and around the Fort la Corne provincial forest. An airborne magnetic survey completed over the main cluster of GSC anomalies revealed 29 discrete anomalies. Geophysical surveys would later identify 71 magnetic anomalies exhibiting kimberlite-type signatures in the Fort la Corne project area.
Thirteen years ago, the Fort la Corne joint venture was created between Uranerz and Cameco. A first pass of shallow exploratory drilling hit kimberlite in each of the seven targets drilled that year. In total, seven macrodiamonds were recovered from five of the seven kimberlites in drill-chip samples collectively weighing less than 100 kg. (A macro is defined here as exceeding 0.5 mm in at least one dimension.) The largest stone had a diameter of 1.27 mm and weighed 0.0035 carat.
De Beers joined the joint venture in 1992 under a 3-year option period. The joint venture was again amended in 1995 to include Kensington, which earned an initial 25% participating stake by spending $3.4 million on exploration over three years. The current ownership has resulted from dilution clauses in the joint-venture agreement. De Beers assumed operatorship of the project at the end of 1998.
By now, 69 of the original 71 geophysical targets have been tested by drilling and confirmed as kimberlite. The joint venture has trimmed its holdings by selling off some of the lower-priority satellite kimberlites. Forty-nine of the original discoveries are diamond-bearing, and 34 have yielded macros larger than 1 mm. The largest stone recovered weighed 1.53 carats.
The kimberlites are huge, ranging from 2.7 to 184 hectares in surface area. Individually, they are estimated to contain up to 675 million tonnes of kimberlite, as in the case of the 148 body. Kimberlites 140 and 141 were originally thought to be separate bodies; however, based on the results of the 2001 delineation drilling, the pair are now believed to form a single body containing some 932 million tonnes.
These are some of the biggest kimberlite bodies in the world. In terms of size, they compare to De Beers’ 50%-owned Orapa orebody in Botswana. At 117 hectares (1.2 sq. km), the Orapa kimberlite deposit consists of two pipes that coalesce near surface. Reserves and resources for the open-pit mine at the end of 2000 totalled 653 million tonnes grading 49 carats per 100 tonnes to a depth of 660 metres. The Orapa resource base, which incorporates four other smaller kimberlites, uses a bottom cutoff size of 1.65 mm and revenue of US$47 per carat.
Unlike the Orapa deposit, which has no overburden, the Fort la Corne kimberlites lie beneath 75-150 metres of unconsolidated overburden cover and have no surface expression.
The Fort la Corne project area lies near the northwestern rim of the interior platform of North America. The platform is covered by a series of sedimentary rocks over Precambrian basement in a 600-to-1,200-km-wide belt between the Rocky Mountains to the west, and the Canadian Shield, which crops out toward the northeast.
The Fort la Corne kimberlites were emplaced into a Phanerozoic sedimentary sequence of shale and sandstones 95-100 million years ago. Little is known of the underlying metamorphic basement. Aeromagnetic and gravity data suggest the basement in the Fort la Corne area is geologically similar to the Glennie Domain, which is exposed farther north, in the vicinity of Lac La Ronge. The Glennie Domain is part of the Reindeer zone of the 1.8-billion-year-old Trans-Hudson Orogen and is composed of Paleoproterozoic island arc volcanogenic successions, separated by reworked Archean granitoids and granite gneisses. Based on a compilation of field mapping, radiometric dating and lithoprobe seismic data, the Glennie Domain is interpreted to blanket the apex of a largely buried Archean microcontinent, which has been dubbed the Saskatchewan Craton.
The Fort la Corne deposits are laterally extensive, pancake-shaped horizons of well-preserved, crater facies pyroclastic kimberlite. They were partially erupted and then capped by re-sedimented kimberlite, and range from simple single-eruptive bodies to large, multi-eruptive events consisting of a series of stacked horizontal kimberlite beds. Drilling has revealed that many of the kimberlite bodies were limited to 100 metres in thickness, with an irregular, peripheral apron narrowing to 30 metres thick. The roots, or feeder systems, to these large kimberlite bodies were not identified until quite recently. In early 2001, delineation drilling by Shore Gold on the Star kimberlite, at the southern end of the belt, returned a 539-metre-long intercept of kimberlite in hole 20, which was shut down while still in kimberlite at a depth of 627 metres. While further delineating the 141 kimberlite body in 2001, the Fort la Corne joint venture intersected 339 metres of kimberlite to a 450-metre depth in hole 13 before shutting down in kimberlite. A second deep intersection was seen in hole 9, which cut 258 metres of kimberlite before exiting into country rock at 363 metres of depth. Both holes are thought to have cut the main feeder vent of the 141 body.
“These kimberlites are similar, in many respects, to most others elsewhere in the world,” stated Kensington’s project manager, Brent Jellicoe, during a presentation earlier this year at the Cordilleran Exploration Roundup in Vancouver. “It’s just that here we preserved the large eruptive facies. They are uniquely well-preserved.”
Xenoliths
Shore’s hole 20 into the feeder zone of the Star kimberlite displayed uniform diatreme kimberlite lithologies from a depth of 279 metres downward, with abundant mantle-derived xenoliths, pyrope and eclogitic garnets.
The Fort la Corne kimberlites are dominated by subaerial olivine crystal tuffs and mixed olivine-lapilli tuffs, with rare-to-common country rock and mantle xenoliths, minor fine-grained interclast matrix, and rare amounts of mantle-derived garnet, ilmenite and chromite. Garnet geochemical data indicate a predominantly lherzolite population with lesser harzburgitic, websteritic, megacrystic components. Most of the kimberlites are dominated by G9 lherzolitic garnets, but peridotitic, eclogitic and macrocrystic garnets are also present. Prospective harzburgitic G10 garnets are present, but generally not abundant.
Chrome spinel is common in most of the kimberlites tested. Generally, Fort la Corne kimberlites may contain only a small percentage of chrome spinel grains that plot in the
diamond inclusion field, but a few bodies range up to 8%.
The thick overburden cover in the Fort la Corne area has made heavy-mineral till sampling an ineffective exploration tool; instead, companies have relied on magnetic and gravity geophysical surveys. The kimberlites are all magnetic to some degree. Apparent magnetic contents for the kimberlites range from 0.1% to 4%, in contrast to the surrounding non-magnetic Phanerozoic sediments. All of the known kimberlite bodies are represented by prominent magnetic anomalies in a GEOTEM aeromagnetic survey flown in 1996.
Six targets
In 1999 and the early part of 2000, De Beers conducted an extensive evaluation of each kimberlite body, utilizing diamond recoveries, previous diamond valuations and estimated body size. Kimberlites whose grade forecasts indicated the potential for commercial-size stone populations were given the highest priority. Based on those studies, six targets (122, 140, 141, 147, 148 and 150) were selected for continued evaluation of diamond content and overall economic value. The forecasted grades of these targets ranged from 10 to 19 carats per 100 tonnes. Gem-quality diamonds account for about 70% of the total stones recovered, based on bottom-size cutoff of 1 mm.
Kimberlites 122 and 141 were chosen for further work in 2000. The 122 body contains about 540 million tonnes of kimberlite, whereas 141, originally estimated at 395 million tonnes, is now believed to join with 140. Combined, 140 and 141 host 932 million tonnes. The primary objective of the 2000 program was to obtain large mini-bulk drill samples. Working from a $2.3-million budget, three large-diameter (24-inch) reverse-circulation (RC) holes were drilled into the 122 body, and two were placed into 141.
A 328-tonne sample collected from kimberlite 122 yielded 212 diamonds with greater than a 1-mm cutoff; the combined weight was 17.31 carats. The sample has an implied grade of 5.3 carats per 100 tonnes. Six diamonds larger than half a carat were recovered from the 122 body.
A 21.06-carat parcel of 278 diamonds was recovered from a 312.6-tonne kimberlite sample of body 141, for an implied grade of 8.4 carats per 100 tonnes. Eight of the stones were larger than half a carat, with the two biggest stones weighing 1.53 and 1.08 carats.
“The recovery of these larger stones supports the high confidence forecasts by De Beers for distribution of stones in the 1-to-10-carat range,” said Jellicoe. “The more large stones we get, the better we’re able to get more accurate grade forecasts.”
There are concerns that the small parcels and lack of larger stones recovered to date may cause some to underestimate the potential of the Fort la Corne kimberlites; this potential is based on diamond distribution curves which predict better recoveries of commercial-size stones. In evaluating the kimberlites, De Beers has considered three scenarios: “conservative,” “best-fit” and “optimistic.” Based on the initial mini-bulk sample work and a bottom-size cutoff of 1 mm, the company has modeled a grade of 14-19 carats per 100 tonnes for the 141 body, with a predicted value ranging from US$88 to US$179 per carat for the three scenarios. The best-fit scenario yielded a value of US$148 per carat.
“Presuming values are up in the best-fit or optimistic range, this [means we are] a world-class deposit,” stated Jellicoe. “We’re in the right ballpark.” Revenue models for the best-fit and optimistic scenarios indicate values of US$28-33 per tonne, based on a grade forecast of 19 carats per 100 tonnes. The model for the conservative scenario is not as rosy, with a projected value of under US$17 per tonne. Jellicoe assumes operating costs for a large-scale mining operation would be in the range of US$8-10 per tonne. “We need to increase the number of carats so that this is not a low-confidence estimate but a much higher one,” he said.
He added that results from the 122 target are also encouraging. The body has a modeled grade of 9-13 carats per 100 tonnes and a best-fit-to-optimistic carat value of US$133-136. No values for the lower-end conservative scenario are available.
The main thrust of the 2001 program, budgeted at $4.8 million, was to acquire a further 80 carats of diamonds from the 141 target in order to improve the confidence level surrounding the modeled diamond values of that body. Following a 16-hole program of core drilling to delineate the 140, 141 and 150 bodies, the joint venture extracted an additional 769 tonnes of kimberlite from nine large-diameter RC holes drilled into the 141 target and an initial 121 tonnes from a single hole drilled into the 150 target.
The bulk samples were processed last fall at De Beers’ dense-media-separation plant in Grande Prairie, Alta. The heavy-mineral concentrates were then sent to South Africa for final diamond recovery, which was completed during the first quarter of this year. Kensington originally expected that De Beers would have completed the final report (including breakage studies, valuation, grade prediction and revenue-modeling) by the end of April. Kensington’s manager of investor relations, William Callahan, says they are awaiting full results of the revenue-modeling before releasing any of the data to the public. An update on the joint venture is expected shortly.
Star
Shore Gold owns 100% of the Star kimberlite project, at the southern end of the Fort la Corne belt. Acquired in 1995, the project area covers 46 sq. km in a burned area of the Fort la Corne provincial forest. The company holds a further 226 sq. km of mineral claims in the immediate area.
In the fall of 1996, Shore encountered multiple kimberlite horizons while sinking three widely spaced holes into the higher magnetic portions of a large, complex magnetic anomaly measuring 2 by 1.7 km. A uniform sequence of up to four separate horizons of kimberlite and kimberlitic sediments was intersected in each of the holes, starting at a depth of 85 metres from surface. The property is covered by 80-125 metres of unconsolidated glacial overburden.
A visually distinct, higher-grade kimberlite unit near the top of the sequence ranged between 20 and 58 metres in thickness. Microdiamond analysis yielded 162 micros and seven macros from 383.6 kg of sample collected from the upper unit. An additional 81.4 kg of material, primarily taken from hole 96-3 (outside of the Main zone), returned seven micros and three macros. That fall, Shore tested another distinct anomaly, 1 km east of Star, with two holes that intersected 200 metres of kimberlite but few diamonds.
Shore returned the next year and further tested the Star discovery with two larger diameter PQ-size (8.5-cm-diameter) core holes near the site of one of the previous holes. A 983-kg sample of kimberlite returned a total of 682 micros and 67 macros. The upper main zone accounted for 637 of the micros and 65 of the macros in 818 kg of core. Between January and May 2000, Shore completed the first phase of delineation drilling on the Star kimberlite, completing 16 core holes. A total of 2,245 kg of split core returned 403 micros and 120 macros weighing 0.62 carat. The largest diamond recovered was a 2-mm fragment weighing 0.04 carat.
Feeder zone
The follow-up drilling of seven holes in late 2000 and early 2001 resulted in the discovery of the first-ever feeder zone in Fort la Corne, with hole 20 intersecting 539 metres of vertically continuous kimberlite. A 625-kg sample taken from upper 312 metres of that hole returned 125 micros and 46 macros. The two largest stones measured 1.49 by 1.41 by 1.29 mm and 1.52 by 1.25 by 1.15 mm. In total, 585 micros and 197 macros weighing 1.24 carats have been recovered from 3,422 kg of split core sampled from 21 core holes drilled in 2000 and 2001.
A further seven delineation holes have since been drilled, with micro results pending. Two of these holes intercepted kimberlite thicknesses of 200 and 236 metres. Preliminary estimates have the Star kimberlite covering almost 4 sq. km and exceeding 500 million tonnes, based on a minimum cutoff thickness of 30 metres. Much of the kimberlite is medium-to-coarse-gra
ined with abundant olivine macro-crysts. Indicator minerals include garnet and ilmenite.
Last fall, Shore completed a large-diameter (24-inch) hole into the Star kimberlite to a depth of about 300 metres and recovered 82.7 tonnes of kimberlite rock-chips. The mini-bulk sample was processed at De Beers’ Grande Prairie plant; then, half of the heavy mineral concentrate was sent to its South African lab, while the other half went to Lakefield Research for diamond recovery. In total, 184 diamonds larger than 1.1-mm sq. mesh screen were recovered, with a total weight of 8.52 carats; the implied sample grade was 10.3 carats per 100 tonnes. The two largest stones recovered were fragments weighing 0.64 and 0.4 carat.
Shore has enlisted De Beers to carry out grade and value-modeling studies, including breakage and luminescence studies, in preparation for a major bulk-sampling program on the Star kimberlite. The junior recently beefed up its board of directors with the addition of Hugo Dummett.
Skeena
Other companies active in the Fort la Corne area include
More recently, Skeena has optioned seven properties northwest and northeast of the De Beers and Kensington joint venture. Skeena can earn up to a 90% interest in three properties held by General Resources, a private, Saskatoon-based company. The 467-sq.-km properties cover several regional magnetic lows. Skeena can also earn up to a 70% interest in four other properties, including the Foxford property, held by Shore. The Foxford property covers the closely spaced 179 and 180 kimberlite bodies, discovered by Uranerz in 1989.
Since 1989, about $60 million has been spent on diamond exploration in Saskatchewan.
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