Initial results from underground bulk sampling on the Star kimberlite in Saskatchewan’s Fort la Corne field indicate the recovery of coarse-size diamonds of up to 3.5 carats.
The implied sample grade compares reasonably well to the neighbouring kimberlites on the Fort la Corne (FalC) joint venture of
As part of an underground bulk-sampling program designed to recover at least 3,000 carats for valuation purposes, Shore has sunk a shaft into the throat of the Star eruptive centre to a targeted depth of 250 metres below surface in order to collect a sample of about 25,000 tonnes. The top of the kimberlite was encountered at a depth of 107 metres. Most of the material will be extracted from 2.4-metre diameter drifts driven out laterally by as much as 100 metres into the kimberlite in northeasterly and southeasterly directions from the 235- and 175-metre levels.
The Star kimberlite occurs at the southern end of the main Fort la Corne trend, 60 km east of Prince Albert. Shore owns all of the 96-sq.-km project, which ties on to the southern boundary of the joint-venture area shared by De Beers, Kensington and Cameco. The 4.5-metre diameter vertical shaft is less than 100 metres east of the FalC property boundary. The shaft was sunk into the heart of the pipe’s eruptive centre, close to a large-diameter (61-cm) reverse-circulation hole that was drilled in 2002, next to core hole 26. The RC hole intersected 192 metres of kimberlite to a depth of 296 metres, before stopping as a result of mechanical failure. From the 82.7 tonnes of extracted kimberlite chips, 183 diamonds larger than 1.1-mm sq. mesh sieve were recovered; their total weight was 8.52 carats.
The mini-bulk sample was marred by a recovery of only 59%, based on hole-diameter and expected rock-chip returns. Granulometry studies on the chips concluded that there had been over-grinding of the kimberlite, resulting in smaller chips than anticipated. The kimberlite in the top 200-metre portion of the hole was found to be much less competent than below this level, generating a much finer rock-chip sample. All material smaller than 1.2 mm was lost in the fines. Using the theoretical 140 tonnes that should have been recovered, the mini-bulk drill sample shows a diamond content of 6.1 carats per 100 tonnes.
Overall, the results of the mini-bulk sample showed a fairly even distribution of larger diamonds throughout the hole. An increase in stone density with depth, especially for microdiamonds, was thought to be a condition of rock competency and sample recovery. Two zones within the hole, at 175-190 metres and 258-295 metres, showed significantly higher microdiamond counts in more competent kimberlitic units.
Kimberlite material collected during the current underground campaign is being processed in batches in order to assess the different phases and components of the Star complex. The first batches of samples were extracted from progressively deeper levels of the Star body during the sinking of the shaft. Initial results show the diamond content or sample grade improving with depth (see accompanying table for a breakdown of the initial bulk-sampling results). The first sample batch of 826 tonnes represents the top 24.4 metres of the kimberlite from 107-131.4 metres of depth. It returned a diamond content of just 1.58 carats per 100 tonne but included a 1.3-carat stone. Batch 1 was taken from the sediment-kimberlite interface. “We knew it would have a lower-grade, due to a significant dilution by mudstone xenoliths,” says George Read, Shore’s vice-president of exploration. This material was used to test and commission an on-site modular dense media separation (DMS) kimberlite processing plant.
The next batch of processed samples came from the area around the proposed lateral drift on the 175-metre level. Batch 5 was sampled from a 9.5-metre section of the shaft immediately above the 171-metre level. The 374-tonne sample yielded a grade of 4.95 carats per 100 tonnes. The largest stone weighs 2.61 carats.
Batch 6 was mined from the 171-to-190.5-metre level of the shaft and includes kimberlite material extracted during construction of the 175-metre-level sub-station. This larger sample batch of 1,878 tonnes shows a diamond content of 6.96 carats per 100 tonnes. The three largest diamonds recovered to date, at 3.5, 3.31 and 3.19 carats apiece, were found in this batch of sample.
Of note is a 5.7-metre section of highly altered kimberlite breccia isolated in batch 7A at 190.5-196.2 metres of depth in the shaft. A 266-tonne sample of this kimberlite unit delivered a grade of 12.36 carats per 100 tonnes, including a 2.87-carat stone.
Says Read: “These breccias have been shown to contain significant grade, and, as we can see from our results, the grade of this breccia is the highest we have encountered.”
Drifting
The results of two other breccia sample batches, 7B and 7C, are pending. Shore considered establishing the first sub-station in this breccia unit, but unstable ground conditions forced the company to establish the heading in a safer, more competent setting, on the 175-metre level. Proposed drifting along this level is expected to intersect the prospective breccia out to the east. “We think these breccias will yield good results,” Read says.
Kimberlite recovered from the construction of a sub-station at the 235-metre level showed a diamond content of 9.35 carats per 100 tonnes for the 983-tonne batch 9 sample. The single largest stone recovered from this sample batch weighs 2.14 carats.
A 586-tonne sample mined from the bottom of the shaft, at 237-250 metres depth, returned a grade of 9.73 carats per 100 tonnes, including a 1.1-carat stone, in batch 10.
In all, 88 diamonds weighing more than half a carat were recovered from the first six processed sample batches, including 33 stones exceeding 1 carat and 10 diamonds greater than 2 carats. “The colour of the stones is impressive,” says Read. “There are some wonderful looking stones — very clear, white stones. There are stones with inclusions, but the overall colour of the parcel, and the clarity of the parcel, is good. I think the parcel will have a high value.”
More than 80% of the 338-carat diamond parcel is described as white in colour, with a further 12% classified as off-white. A number of coloured stones were recovered, including 24 yellows, 21 pinks, and three greens. The pink and green stones are relatively small and less than half-a-carat.
“We’ve had a cursory look at these diamonds,” says Kenneth MacNeill, president of Shore. “We will get more involved and have third parties actually get in to look at each and every one of these diamonds as we move forward. We will have further evaluations once our larger parcels start to come in. [So far,] we have only stated the information we have received from the laboratories.”
Results from eight kimberlite batches mined during the sinking of the shaft are pending, as bulk samples from the 175-metre and 235-metre levels were prioritized so that “diamond abundances and qualities” could be confirmed before proceeding with lateral drift development. Shore wanted to make sure that thes
e were the correct areas in which to drift. The two levels had been highlighted from previous surface drilling, based on a combination of diamond results, geology (kimberlite type) and rock mechanics. “The results confirm we are in the right place,” says Read.
“We have covered a spectrum of kimberlite types within the Star kimberlite in these results, but we must emphasize that these are early days,” cautions Read. “These are six batches from a potential 80 to 100 batches of kimberlite that will be ultimately processed in order to give us the 25,000 tonnes we aim to recover.”
Northeast drift
Shore is mining on the 235-metre level. Seven crosscuts have been developed off the main southeast drift, which has advanced some 60 metres into the kimberlite. “We will go a bit farther than that, but we don’t want to go as far as the kimberlite-wall rock interface, which could be, perhaps more than 100 metres out,” says Read. A northeast drift on that level has been diverted to the north to follow more favourable mining conditions. This drift has encountered kimberlite breccias that are similar in geologic appearance to those sampled in batch 7A. “The breccias certainly seem to be the areas that contain the real goods of the Fort la Corne,” says Read.
He says they won’t be as constrained on the 175-metre level, with “almost unlimited room to move to the east.”
To the end of June, a total of 10,500 tonnes of kimberlite had been mined from the Star body, including 2,000 tonnes from drifting on the 235-metre level. The on-site processing plant has treated more than 9,000 tonnes of wet kimberlite since it was commissioned in February. The plant consists of a 30-tonne-per-hour crushing circuit and a 10-tonne-per-hour DMS plant, including a 250-mm separating cyclone and diamond recovery circuit that utilizes an X-ray diamond sorting machine and a grease table. Plant concentrates from 20 sample batches have been shipped to SGS Lakefield Research for final diamond recovery.
The company hopes to have the entire 25,000 tonnes of kimberlite mined by the end of August, with the completion of on-site processing by mid-October and the evaluation of the total parcel before year-end.
Diamond-bearing kimberlites in the Fort la Corne region consist primarily of laterally extensive pyroclastic kimberlite deposits interstratified with Lower Cretaceous marine, marginal marine and continental sediments. The intrusive bodies occur as stacked, sub-horizontal lenses of crater facies volcaniclastic kimberlite. They tend to have broad, champagne-glass forms lying under some 100 metres of glacial overburden.
Shore has completed 37 widely spaced core holes in the Star kimberlite since discovering it in the fall of 1996. A July 2003 technical report by ACA Howe International concludes that “the Star kimberlite is a major, laterally continuous zone with a surface area of crater facies in excess of 1.5 sq. km.” Shore previously reported that the Star kimberlite exceeds 500 million tonnes in mass, based on a 30-metre cutoff.
Eruptive history
The surface topography of the kimberlite body interpreted from drilling shows a raised cone/crater-like shape around the eruptive centre. The Star kimberlite was one of several Fort la Corne kimberlites selected by the government-sponsored Targeted Geoscience Initiative program for analysis regarding its emplacement history. All available drill core was logged and lithologically analyzed. Geochemical investigations, including uranium-lead age-dating of perovskite fractions and whole-rock geochemistry, have been used to reconstruct and refine the eruptive history of Star.
These results, along with high-resolution, two- and three-dimensional shallow seismic studies, aided by borehole geophysics, confirm that the Star body is an eruptive complex with at least four phases of kimberlite and two or more possible feeder vents.
The initial kimberlite volcanism resulted in the formation of a pipe-shaped feeder vent and overlying maar complex. Subsequent eruptions, dominated by sub-aerial deposits, formed a positive-relief cone over-top of the feeder vent/maar. The shape of this tephra cone was modified during marine transgression, resulting in re-worked kimberlite sand along the fringes and kimberlite event deposits, such as turbidites and debris flows, in more distal settings.
The shaft has cut through two main eruptive phases that are interpreted to be equivalent with stratigraphically defined horizons in the Saskatchewan sedimentary basin, namely the Early and Late Joli Fou formations of the Lower Colorado Group. “Through whole-rock geochemistry, we have managed to sub-divide our kimberlite into the various eruptive phases that were synchronous with sediments that were being deposited in this basin at the same time,” explains Read. “The kimberlite and sediments are really a volcano-sedimentary pile.”
The Upper part of the shaft, from 107 to 171 metres (represented by batches 1 through 5), is in the Late Joli Fou stratigraphic-equivalent kimberlite phase. The lower part of the shaft, from 190.5-250 metres (batches 7A through 10), is in the equivalent of the Early Joli Fou. Batch 6, at the 175-metre sub-station, is on the boundary between the two kimberlite zones. “The lion’s share of the kimberlite is made up of the Early and Late Joli Fou,” Read tells The Northern Miner.
Grade difference
Based on the partial results to date, there is a distinct grade difference between the Early and Late Joli Fou equivalent kimberlite, with better grades evident in the lower part of kimberlite or the Early Joli Fou phase. However, large carat stones are being found in both kimberlite horizons. “It points toward a good distribution of size of diamonds,” says Read. The proposed 175-metre level drift is expected to sample the top of the Early Joli Fou phase.
But what happens to grade and stone-size distribution as you move away from the pipe-shaped feeder vent into the more extensive overlying pyroclastic and reworked volcaniclastic kimberlite horizons?
“If we start at a 150-metre depth and take the kimberlite to a depth of 250 metres, and assume a 200-metre radius around the shaft, we end up with a 30-million-tonne disk, half to a third of which is presumably on the Kensington-De Beers side of the property boundary,” writes John Kaiser, publisher of the Kaiser Bottom-Fishing Report. “For the Star kimberlite to become economically interesting, Shore Gold will have to demonstrate that the grades and values obtained within a 100-metre radius of the shaft between the 150-to-250-metre interval hold up at a 200-metre radius and to a depth of 350 metres, where the tonnage footprint approaches 60-70 million tonnes.
Ideally, Shore needs to demonstrate a several-hundred-million-tonne minable resource to justify a “lucrative” bulk mining scenario, according to Kaiser, who assumes a grade of 10-15 carats per 100 tonnes and an average carat value of US$150 or better would translate into a rock value of US$15-23 per tonne.
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