The exploration company VR Resources [LON:VRR] has stumbled across a geological structure known as a kimberlite pipe.
Late last year, VR Resources were running a drill program on their Hecla Kilmer rare earth project in Ontario, Canada, and once completed they decided to send the rig 12km northeast to their Northway tenement to test a prominent magnetic anomaly located on a regional structure. What they discovered in drill hole 001 was the top of an intact kimberlite pipe buried under 240m of cover.
Kimberlite pipes are the host to diamonds
For non-geologists, the excitement about the discovery is due to the fact that kimberlite pipes are the host to diamonds. On realising the implications of their discovery and knowing that kimberlites often form in clusters, VR Resources set about staking 20 other potential targets in the area. They may have just secured pole position in a potential new kimberlite field.
The map on the left shows the tenements held in Ontario prior to drill hole 001 at Northway. The map on the right shows the additional ground staked after the discovery.
A second hole drilled in May this year confirmed the kimberlite find, but the hole had to be abandoned due to excessive caving within the hole after drilling only 117m into the pipe. Frustratingly, minerals of interest for diamond exploration, like the wonderfully named eclogite, were showing an ‘increase in abundance downwards in hole 002.’
More clues about the find would have to wait.
Ontario has been explored for diamonds before
This month, the company are drilling a third hole deep into the heart of the kimberlite target, which might provide further information about whether the pipe is diamondiferous or not.
But Ontario has been explored for diamonds before. Notably by De Beers who discovered the Attawapiskat kimberlite cluster in the north of Ontario in 1987. Eighteen pipes were found, sixteen of which were diamondiferous. From this cluster. The Victor pipe was developed into a successful diamond mine, opening in 2008 and eventually closing in 2019. The diamond grade at Victor was low by global standards, but not the value; the diamonds had a reputation worldwide for their flawless character and extremely high value.
It was long suspected that Ontario was prospective for further diamond discoveries, so why hasn’t Northway and this potential kimberlite field been discovered sooner?
The answer lies partly in the results of geophysical surveys known as magnetic surveys. In simple terms, the earth’s magnetic field can, in some cases, induce magnetization in susceptible rocks when they are formed. And this magnetization can be detected by magnetometers and studied by exploration geologists for clues about what lies below the earth’s surface.
Kimberlites pipes usually contain a mineral of iron oxide known as magnetite. Which, in this region of North America, would usually register in a magnetic survey as a positive magnetic anomaly, known as a magnetic high.
A magnetic survey of Northway produced the following images in plan view (i.e. looking down):
The blue colour represents a negative magnetic signature or a magnetic low. The warmer reds and purples represent a positive magnetic signature or a magnetic high.
The circular blue blob in the centre of the Northway tenement (see magnetic image above) appears to be a pipe of some sort but it wasn’t expected to be a kimberlite for two reasons. Firstly, it was a magnetic low and other kimberlite pipes in the region tend to show up as magnetic highs. Secondly, the pipe appeared to be huge (about 1.2km across) and kimberlite pipes generally tend to be significantly smaller.
What could account for Northway’s kimberlite pipe having a magnetic low?
Timings of the kimberlite eruptions
To understand this, we need to consider the age of the rocks at the time that the kimberlite eruptions happened.
De Beers’ Attawapiskat kimberlite cluster, which the Victor mine is a part of, erupted and formed in the Jurassic Period, between 145.0 to 201.3 million years ago. Back then, the world is believed to have looked something like this:
Notice the ancient supercontinent of Gondwana beginning to break apart into South America and Africa, and North America taking shape above that.
The kimberlite eruption at Northway however, is believed to have take place much earlier. In a news release, VR Resources stated that ‘Northway is older than the Paleozoic Strata (410 million years ago) which cover it.’
Approximately 425 million years ago, during a time known as the Silurian Period, the world is believed to have looked something like this:
Notice the tectonic plate known as Laurentia that eventually becomes North America. During the Silurian Period (425 million years ago) North America resides BELOW the equator. The tectonic plate has broken free from Gondwana but has a long way to travel to its position in the northern hemisphere during the Jurassic Period, about 275 million years later.
This is the key point to understand. As I mentioned earlier, the earth’s magnetic field can, in some cases, induce magnetization in susceptible rocks when they are formed. But the magnetization of the rock at the time of formation will be affected by the location of the rock relative to the earth’s magnetic field.
Kimberlite pipes formed during the Silurian Period while in the southern hemisphere would likely produce a magnetic low, and kimberlite pipes formed during the later Jurassic Period once the tectonic plate had drifted into the northern hemisphere would likely produce a magnetic high.
This is what has wrong-footed previous diamond explorers in Ontario. They were assuming that other kimberlites would be of similar age to the Attawapiskat kimberlite cluster.
VR Resources describe the issue as follows:
“The area west of James Bay was explored extensively for diamonds from the 1960s through 1990s, culminating in the discovery and development by DeBeers of the Victor mine located 300km to the north of Northway. Northway, however, is different from the anomalies targeted during that exploration:
Northway is a magnetic low, not a magnetic high;
Northway is large, exceeding 1 km in size, compared to the small magnetic highs typically a few hundred metres across, and;
Northway is older than the Paleozoic strata (410MA*) which cover it, as opposed to the younger, Mesozoic kimberlite pipes (170MA) targeted previously in the region which come through the Paleozoic limestone to the base of the glacial till cover.”
VR Resources to rejuvinate the industry with Northway?
VR just announced (22nd June 2023) that Hole 003 has started and released the following plan image known as a magnetic amplitude map. As the name suggests, this image takes the amplitude of the signal only, ignoring whether the signal is a high or a low. Amplitude processing of the data can help focus on the contacts between the various magnetic rock bodies (ie boundaries of the pipe against country rock). The highest amplitude areas in light purple suggest that hole 003 will cut into the meat of the matter at Northway and should provide enough rock to send for caustic dissolution, the process for micro-diamond evaluation.
Caustic dissolution is where the core is left in a bath of caustic soda. After several months, all of the rock dissolves away leaving only diamonds, if there are any.
It’s too early to tell if Northway’s kimberlite pipe is diamondiferous but I’m watching drill hole 003 closely. Exploration is highly risky but the returns if successful can be significant, especially in this case if one understands the sheer value created by the high-grade kimberlite pipes discovered in Canada’s Northwest Territories in the mid-1990’s and the high value diamonds found at the Victor Mine in Ontario around the same time. These discoveries founded a new and robust diamond industry in Canada that, lacking any major discoveries since, is waning after some 30 years of production. Mines come to their natural end as shown by the closure of the Victor mine in 2019. Could Northway be about to rejuvenate the industry?
Disclosure: I own shares in VR Resources.
