Churchill kimberlite find shows significant grades

At the Churchill joint-venture project in eastern Nunavut, mini-bulk samples collected from isolated surface showings of four new kimberlite dyke discoveries show significant diamond grades running in the range of 0.39 to 2.04 carats per tonne.

This past summer, Shear Minerals (SRM-V, SRMUF-O) discovered four distinct kimberlite dyke systems while drilling and prospecting in different areas of the Churchill project holdings. PST003, Notch and Jigsaw were found in the Sedna corridor and Kahuna was discovered in the Josephine River corridor. They are spatially distinct, steeply-dipping bodies that occur over a broad area of the project in multiple directions. The kimberlite dykes range from 0.5 to 4 metres wide and are highly diamondiferous.

“We are extremely encouraged by the diamond content and initial diamond size distribution from all four kimberlites,” says Shear president Pamela Strand. “Having sample grades from PST003 at more than two carats per tonne and at Kahuna of more than one carat per tonne are very positive results. We haven’t seen many results this good coming out of Canada in quite a while.”

Based on preliminary observations, the majority of the recovered diamonds are described as clear, colourless to white and dominated by octahedrons, with some macles, dodecahedrons and twins.

The Churchill project covers a 2-million-acre package of mineral rights nestled between the northern communities of Rankin Inlet and Chesterfield Inlet in the Kivalliq region of Nunavut.

Shear is the operator and owns a 51% interest in the Churchill joint venture. The remainder is divided between Stornoway Diamond (SWY-T, SWYDF-O), with a 35% stake, and BHP Billiton (BHP-N), at 14%.

After discovering over 40 weakly diamondiferous kimberlite pipes on the Churchill holdings, the joint-venture partners began the 2006 exploration season by focusing on areas where highly promising G-10 pyrope garnets had been recovered. Extensive till sampling identified and constrained two geographically distinct areas of elevated kimberlite indicator mineral counts, including favourable “high interest” G-10 sub-calcic pyrope garnets whose source had remained unexplained. These two corridors are called Josephine River and Sedna.

The kimberlites discovered up to the start of 2006 all had very low diamond counts and, with their poor mineral chemistry, never matched the high potential till G-10 chemistry.

Last year’s exploration budget was set at $4.6 million, funded entirely by Shear and Stornoway. BHP elected not to participate in the 2006 program and will be diluted to a roughly 12% stake. Shear will increase to 51.7% and Stornoway will hold 36.3%.

The joint venture kicked off the 2006 exploration campaign by looking for the source of a highly diamondiferous till sample, known as PST003. 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 at least one direction.

The till sample was taken from the Sedna corridor at the head of an anomalous G-10 indicator mineral dispersion train, measuring 1.2-km long and 550 metres wide. The oversize fraction of the promising till sample contained abundant altered kimberlite rock fragments displaying rounded pyropes, with thick kelyphitic rims. These kimberlite fragments were similar in appearance to another small piece of unique kimberlite float found in the Josephine River corridor at the end of 2005.

Shear tested the PST003 target with seven shallow holes across a strike length of 150 metres. 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 0.15 mm, or 303 microdiamonds using a lower cutoff of 0.075 mm. The two biggest stones measured 0.5 mm or better in at least one direction and included a white, clear octahedroid measuring 0.84 by 0.7 by 0.54 mm.

The angled holes were positioned from three different setups spaced 50 metres apart on the head of the G-10 dispersion train. Geophysical surveys suggest the PST003 dyke extends for a distance of at least 500 metres in a northeast-southwest fashion.

In situ kimberlite was exposed in shallow hand-dug pits from which a little more than 3.5 tonnes of sample was collected for the analysis of larger, commercial-size diamonds and grade.

An additional 173 kg of kimberlite from the PST003 showing was separately analyzed for microdiamonds and yielded a staggering 1,300 stones using a higher cutoff of 0.15 mm, including the recovery of 81 diamonds greater than 0.5 mm in one or more dimensions.

Impressive grade

The 3.5-tonne mini-bulk sample held an impressive 7.24 carats of rough diamonds better than a 0.85-mm cutoff, giving a preliminary grade of 2.04 carats per tonne. The three largest diamonds are described as 0.55-carat (5.1 by 4.4 by 2.8 mm) white octahedroid crystal, a 0.29-carat (5.6 by 4.6 by 2.2 mm) white twinned octahedron and a 0.16-carat (3.7 by 2.84 by 2.6 mm) white broken octahedron.

Prospecting in the vicinity of unsourced, high-interest G-10 indicator mineral till anomalies led to the discovery of the Notch and Jigsaw showings. Notch was found 2 km north of PST003. It has been trenched and tested with 11 widely spaced holes to about a 50-metre depth along a 3-km-long geophysical trend. The Notch dyke system strikes north-south and is 0.7 to 1.5 metres wide.

An initial 187.5-kg composite sample derived from outcrop and drill core held 692 diamonds, including 31 stones exceeding 0.5 mm, based on a cutoff of 0.15 mm, or 1,418 stones using a lower cutoff of 0.075 mm. In total, 1,633 diamonds down to a 0.15-mm sieve size classification were recovered from 397.6 kg of core.

A 4.9-tonne mini-bulk sample of Notch produced 3.4 carats of diamonds for a grade of 0.69 carat per tonne, including a 0.13-carat (3.6 by 3.4 by 1.92 mm) white broken aggregate, a 0.11-carat (3.64 by 2.4 by 1.8 mm) colourless broken crystal and a 0.11-carat (3.1 by 2.1 by 2 mm) white coloured stone.

A smaller sample of 500 kg, or 0.5 tonne, was retrieved from a second showing at Notch North, 1.5 km north of the main Notch outcrop. It delivered a 0.4-carat parcel of stones for an implied grade 0.8 carat per tonne.

The Jigsaw kimberlite dyke is a 1.3-metre-wide showing found 20 km northwest of Notch 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. No drilling has been conducted on the Jigsaw trend.

Microdiamond analysis on 326.9 kg of sample yielded 962 stones using a cutoff of 0.15 mm, including 57 larger diamonds exceeding 0.5 mm.

A 5.1-tonne mini-bulk sample collected from the Jigsaw exposure shows a grade of 0.39 carat per tonne based on the recovery of 1.99 carats of rough diamonds. The largest diamonds are a 0.16-carat (3.08 by 2.9 by 2.6 mm) black octahedroid, a 0.14-carat (2.2 by 1.82 by 1.44 mm) colourless broken crystal and a 0.12-carat (3.72 by 2.32 by 2.3 mm) colourless broken aggregate.

Shear discovered the Kahuna kimberlite in the Josephine corridor, 15 km northeast of Notch, while drilling a linear magnetic feature that extends for more than 5.5 km. The 3.5- to 4-metre-wide vertically emplaced dyke was intersected in five holes drilled from three setups as well as trenching at two sites. A composite 203.8-kg sample taken from surface sampling and drill core returned 909 diamonds down to a sieve size fraction of 0.15 mm, including a 0.3-carat stone measuring 3.55 by 2.95 by 2.9 mm.

A 3.1-tonne sample taken from the Kahuna surface showing produced a grade of 1.09 carats per tonne based on the recovery of 3.4 carats of rough di
amonds. The three largest diamonds are described as a 0.27-carat (4.78 by 3.94 by 2.66 mm) white stone, a 0.18-carat (4.08 by 3.58 by 2.56 mm) white broken aggregate and a 0.1-carat (2.4 by 2.2 by 2.14 mm) pink cubic crystal.

“These initial high diamond grades provide justification for an aggressive program in 2007 that will include the collection of larger, more definitive mini-bulk samples and further exploration for additional kimberlites associated with this potentially extensive diamondiferous dyke system,” Strand says. “Our modelling says there is room for improvement as we take larger samples, especially for Kahuna and Notch.”

The joint-venture partners are finalizing budgets for the 2007 field program, which is expected to include an assessment of the project’s tonnage and grade potential in terms of overall economics. “We want to get a good parcel of at least 200 carats from each of the kimberlites,” Strand says.

There remain another 17 unsourced G-10 trains, in addition to the four kimberlite dykes found.