Summer work at Churchill turns up diamondiferous dyke system

The source of some highly compelling G10 indicator mineral chemistry that had eluded Shear Minerals (SRM-V, SRMUF-O) and Stornoway Diamond (SWY-T, SWYDF-O) has finally been located with the discovery of an extensive and significantly diamond-bearing kimberlite dyke system during the 2006 summer drilling campaign at their Churchill project, in eastern Nunavut.

Up to the start of this year’s exploration program, the joint venture had discovered some 43 kimberlite bodies in this new camp, 17 of which have proven diamondiferous, albeit weakly. Extensive till sampling identified and constrained two geographically distinct areas of elevated kimberlite indicator mineral counts, including “high interest” G10 sub-calcic pyrope garnet mineral chemistry whose source remained unexplained. These two corridors are called Josephine River and Sedna.

“We drilled over forty kimberlite pipes on the project; all of them have very low diamond counts and they never matched our very high potential G10 chemistry,” explains Pamela Strand, president of Shear Minerals.

The fine-grained kimberlite bodies have strong magnetic signatures but contain low indicator mineral counts, with rare garnet and more common ilmenite. More importantly, these kimberlites, with their poor mineral chemistry and unfavourable mantle geotherm, show a low diamond-carrying capacity.

The Churchill project covers a substantial package of mineral rights between the communities of Rankin Inlet and Chesterfield Inlet in the Kivalliq region of Nunavut. The land package was whittled down to 2 million acres in early 2006 from over 8.5 million acres held in 2005. Shear holds a 51% interest in the Churchill joint venture and is the project operator. The remainder is divided between Stornoway Diamond, with a 35% stake, and BHP Billiton (BHP-N) at 14%. The privately held Hunter Exploration Group retains a 2% gross overriding royalty and net smelter return royalty.

This year, the Churchill joint venture focused specifically on the areas where G10 pyrope garnets had been recovered, resulting in the discovery of four vertically placed, highly diamondiferous kimberlite dyke systems — PST003, Notch and Jigsaw in the Sedna corridor, and Kahuna in the Josephine River corridor. They range from half a metre to 4 metres in width.

The microdiamond counts are strong and show a good, healthy size distribution, suggesting the potential for larger, commercial-sized diamonds.

“The small samples are really too early to put a grade on, but from what we know, they are not low-grade kimberlites,” Strand says.

They are spatially distinct bodies that occur over a broad area of the project in multiple directions.

“There are multiple trends and I believe there is a swarm of these dykes crossing the project area,” Strand notes. “We have 17 unsourced G10 trains, in addition to the four kimberlite dykes found, and all of the chemistry appears to be the same. I believe we are looking at a unique age of kimberlite.”

The kimberlite in the dykes shows a moderate-to-high diamond carrying capacity and exhibits high indicator mineral counts. Pyrope garnet is common, with ilmenite being less common. The kimberlite is medium-grained, with two generations of olivine, including macrocrysts.

“Visually, it looks like the same kimberlite pulse,” Strand says. “There are some multiple phases that we have identified in a few areas, but we still don’t know that much about them yet. We are doing full petrography on them.”

An additional 20 tonnes of mini-bulk samples have been collected from outcropping exposures of each of the kimberlite dykes.

“The microdiamond counts point to a good population of macrodiamonds, which is what we are hoping to recover from this 20-tonne mini-bulk sample,” Strand explains. “The diamonds we have seen so far have been described as quite exceptional in quality, with good crystal shapes that are dominantly clear and colourless.”

The exploration budget for 2006 was set at $4.6 million, funded entirely by Shear and Stornoway. BHP Billiton elected not to participate in this year’s program and will dilute down to an 11.3% interest. Shear will increase to 52.6% and Stornoway will hold 36.1%. The season got under way in early May and the joint venture zeroed in on finding the source of the highly diamondiferous PST003 till sample.

The 20-kg till sample collected in 2005 yielded staggering grain counts of pyropes, ilmenites and chromites in the tens of thousands, as well as chrome diopsides and eclogitic garnets. In addition, the sample held kimberlite fragments and 162 microdiamonds exceeding a 0.075-mm cutoff, including three larger stones measuring 0.5 mm or better in two dimensions.

The till sample was taken in the Sedna corridor at the head of an anomalous G10 indicator mineral dispersion train, measuring 1.2 km long and 550 metres wide. An initial 116 garnets were confirmed by microprobe analysis as pyropes; 27% of these grains were classified as G10s, with high chromium content.

The oversize fraction of the promising till sample contained abundant altered rock fragments that displayed generally coarse-grained textures and contained rounded pyropes, with thick kelyphitic rims. Mineral Services Canada confirmed these fragments to be kimberlite, which show similarities to another small piece of unique kimberlite float found in the Josephine River corridor at the end of last year.

Seven shallow, angled holes were drilled from three different setups spaced 50 metres apart at the head of the G10 dispersion train. Each of the holes encountered narrow intervals of kimberlite, ranging from 0.15 to 1.52 metres in length. A composite 22.8-kg drill sample from the PST003 occurrence held 168 microdiamonds based on a bottom stone cutoff of 0.105 mm. Four of the larger stones measured 0.5 mm or better in at least two directions and included a white, clear octahedroid measuring 0.84 by 0.7 by 0.54 mm. A detailed look at the microdiamond counts is provided in Table 1 (see page 16).

“We are pleased that a bedrock source has been identified for the anomalous till sample and are encouraged that the small sample has returned significant diamond counts,” said Eira Thomas, Stornoway’s president, in a statement.

The kimberlite trends northeast-southwest and appears to dip steeply to the east. Overburden at the PST003 site ranges from less than 1 metre to 5 metres thick. Shallow hand-dug pits unearthed in situ kimberlite, from which 4.3 tonnes of samples were taken for the analyses of larger, commercial-sized stones and grade.

Tracing the source

Prospecting in the vicinity of unsourced, high-interest G10 indicator mineral anomalies led to the discovery of two new bedrock sources, referred to as the Notch and Jigsaw kimberlites. Notch was found in the MSC19-area, 3.5 km northeast of PST003. It has been trenched and tested with 11 widely spaced holes over a 3-km-distance to about a 50-metre depth. The Notch kimberlite strikes north-south and is 0.7- to 1-metre wide.

It has delivered 1,418 microdiamonds, including 31 stones exceeding 0.5 mm, from a composite 187.5-kg kimberlite sample derived from outcrop and drill core. The largest diamond measures 3 by 2.48 by 1.6 mm and was found resting on the +1.7-mm sized screen. An additional 5.9 tonnes of mini-bulk sample was taken from outcropping exposures.

The Jigsaw kimberlite dyke was found 20 km to the northwest along the Sedna corridor and occurs under a thin layer of till along a prominent east-west trending, grassy linear. A magnetic high geophysical signature is traceable for more than 1 km.

A 44.3-kg sample from the Jigsaw exposure held 157 microdiamonds, including seven stones of 0.5 mm or better. The diamonds are described as clear and colourless with good crystal shapes. The largest stone measured 1.98 by 0.75 by 0.53 mm. A 5.8-tonne mini-bulk sample is being processed for macrodiamonds.

The Kahuna kimberlite in the Josephine corridor yielded 342 stones, including 15 diamonds exceeding 0.5 mm, from a co
mposite drill-core sample weighing 57.6 kg. It was discovered earlier in the summer while drill-testing a linear magnetic trend. Hole 26A intersected an 11.4-metre-long interval of kimberlite, starting 27 metres down-hole. The hole was drilled at a minus 70 angle across the vertically dipping structure, so the intercept is not a true thickness. Described as coarse-grained, highly macrocrystic and pyrope-bearing, the kimberlite dyke has an estimated width of 3.5 to 4 metres.

A second hole, no. 26, was drilled from the same site as 26A but at a shallower angle of minus 50. It hit a near-surface 4.6-metre section of kimberlite, beginning 15 metres down-hole. The clay-dominated intersection was very crumbly, resulting in poor core recovery.

A composite 36.5-kg sample from the two holes yielded 252 microdiamonds exceeding 0.106 mm in size. The largest diamond measures 1.38 by 1.12 by 0.64 mm and was caught in the 0.85-1.18 mm sieve size fraction.

Prospecting in the area turned up two surface showings, 400 metres north of the discovery holes. A mini-bulk sample weighing 3.6 tonnes was collected from this site.

Kahuna

Shear stepped out on the north and southern extensions of the Kahuna trend, drilling additional holes 2 km south (hole 25) and 2.5 km north (holes 32/32A). The linear magnetic trend extends for over 5.5 km in length. Angled at minus 50, hole 25 intercepted a 1.9-metre-long kimberlite interval at 35 metres depth on the southern end.

At Kahuna North, hole 32 intersected three separate intervals of kimberlite for a total length of 3 metres (the single longest intercept being 1.9 metres) between 54.5 and 59.6 metres depth in a hole angled at minus 50. Steepened to minus 65, hole 32A encountered 9.7 metres of kimberlite in four distinct branches over a down-hole depth of 121 to 141 metres.

A composite 22-kg sample from the two holes held an impressive 90 microdiamonds; the two largest stones, measuring 1.04 by 0.92 by 0.61 mm and 0.82 by 0.8 by 0.48 mm, respectively.

The high diamond counts in the four dykes compare favourably to the early results from the Snap Lake kimberlite dyke project in the Northwest Territories, where De Beers is spending $975 million to achieve production. Winspear Diamonds discovered the Snap Lake dyke during a drill program in 1997. The program was focused on finding the source of diamond-bearing kimberlite boulders that had been discovered on the edge of Snap Lake in 1996. Those boulders returned 143 microdiamonds, including 33 stones exceeding 0.5 mm in at least one direction, from 34 kg of sampled material.

The 1997 drilling intersected shallow-dipping, hypabyssal-phase kimberlite dyke material in 13 core holes. Caustic fusion analysis returned 401 microdiamonds, of which 149 exceeded 0.5 mm in one direction and 25 exceeded 1 mm, from 137 kg of combined drill core.

The Snap Lake dyke is a gently dipping kimberlite sheet, generally 2-3 metres thick that extends at least 3.2 km north-south along strike and 3.1 km downdip. It’s estimated to contain ore reserves minable by underground methods of 18.3 million tonnes grading 1.46 carats per tonne, equal to 26.7 million carats valued at US$144 per carat.

Shear and Stornoway are now waiting for the results from the mini-bulk samples before making final preparations for the 2007 field season at Churchill.

“The results from the 20-tonne sample are going to give us preliminary diamond grades and a look at the initial diamond quality,” Strand says. “Remember, we are only sampling four surface areas on very extensive dyke systems.”

Next year, exploration will be two-fold. The grade and tonnage potential of the kimberlite dykes will need to be assessed in terms of overall economics. There also remain 17 unsourced anomalies that require follow-up drilling.

Table 1: Churchill Microdiamond Results