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Diamonds: Micro Diamond Charts

(From the archives - originally published October 10, 2002)

Micro Diamond Charts - John Kaiser

Below are a series of micro diamond size frequency distribution charts based on published results for various kimberlites. For those interested in evidence why the single longest dimension reporting method is worthless in predicting grade potential, check out Longest Dimension Historical Data. For a discussion on micro diamond reporting check out Micro Diamond Analysis.

These charts have been constructed from a database of sieve based micro diamond results compiled by John Kaiser from public disclosures by juniors. Results have been normalized to a per 1,000 kg basis. Be very careful when the sample size is less than 200 kg, especially when the sample size is less than 50 kg.

There are two types of charts: a) the 3 data point square mesh system of 0.1-05, 0.5-1.0, and greater than 1.0 mm, and, b) the 8 data point sieve system used by two independent Canadian labs that provide caustic fusion services for kimberlite samples -Saskatchewan Research Council (SRC) and Lakefield. The latter is the most useful in evaluating the commercial macro grade potential on the basis of micro diamond results for small kimberlite samples. It is noted below the chart when companies have only published results on the basis of the two longest dimensions. Results reported only according to the Lakefield/SRC system have been converted to the 3 data point system on the following basis: stones caught by the 0.1-0.425 sieves, 0.6-0.85 sieves, and stones caught by the 1.18 mm sieve corresponding to the 0.1-0.5, 0.5-1.0 and >1.0 mm size categories. Such curves will look slightly weaker than curves for data reported in the 3 data point format.

The following charts will be periodically updated as indicated by the date. Because the micro diamond curves often overlap and can create too much clutter, an attempt has been made to keep down the number of curves per chart. Most of the charts will include size frequency curves for kimberlites for which a macro grade has been established by bulk sampling. The data as presented does not allow for mathematical calculation of macro grade potential, but it is very helpful on a comparative basis. Use these charts only as a guide - there are a number of factors that can make the curves be misleading either negatively or positively!

In general do not pay too much attention to the curve at the small end of the size spectrum. Instead, focus on how the curve behaves as the size category increases. The shallower the slope of the curve and the farther to the right it extends, the better the macro grade potential. For a sense of the grade potential compare the curve's behaviour in the larger size categories to that of pipes for which macro grade has been established by bulk sample.

Normalization of a small sample to 1,000 kg often produces a curve that floats high in the chart and appears to project nicely toward the right, leading one to anticipate that a larger sample will extend the curve as it picks up bigger diamonds. But in low macro grade pipes with abundant micro diamonds this will not happen. The curve for a small sample becomes interesting when it extends into the 0.85-1.0 mm sizes as is the case with the King and Sand Piper samples.

Micro diamond curves can make a kimberlite's grade potential look weaker than it is if the grade is low and the abundance of the smaller micro diamonds has been negatively impacted by resorption during the magma's ascent or aerial dispersal during eruption. They can also make a kimberlite's grade potential look exceptionally better than it is because the sample may have included a xenolith of the original diamond host rock (harzburgite or eclogite) that made it to the surface intact. Normally the diamond host rock, which can be extraordinarily rich, is completely disaggregated during emplacement and thoroughly mixed within the kimberlite magma. In both cases caustic fusion or acid dissolution of a larger sample will eliminate the positive or negative distortion. In the case of the "weak" grade the larger sample will deliver the bigger stones that allow extension of the curve toward the right while the curve for the truly weak grade will not improve much. In the case of the "strong" grade the larger sample will not repeat the high counts in the larger size categories and the curve will drop downwards and its slope steepen if the original sample was distorted by a lucky nodule.

Ashton's Renard Kimberlites from the Foxtrot Project in the Otish region of Quebec

The micro diamond curves for the Renard 2-5 kimberlites look very similar to each other. The Renard 2 curve is tricky to interpret because of the dip in the 0.5-1.0 mm size range, but what counts is the greater than 1.0 mm data point which is similar to Renard 4 and 5. A mini bulk sample for Renard 2 has yielded a grade of 0.7 ct/t. The Renard 3 curve scores higher in the 1.0 mm plus size, suggesting a grade in excess of 1 ct/t. Ashton has run several samples for these kimberlites with similar results in each sample, decreasing the risk that luck may have skewed initial results upwards. Renard 6, which Ashton chose not to mini bulk sample, suggests a lower grade, perhaps similar to that of Twin's Freightrain kimberlite. The Freightrain curve is a good benchmark because it is based on a 1,026 kg sample from a kimberlite body for which bulk sampling has established a recovered grade of 0.2 ct/t with the presence of large, gem quality diamonds confirmed. The Snap Lake, King and 5034 kimberlite curves are clearly superior at this stage, representing kimberlite grading 1.5-2.0 ct/t and containing large, quality stones. Off the scale is the curve for the Sand Piper kimberlite discovered on Victoria Island by Diamonds North. The indicated grade is in the range of 5-10 ct/t, but trusting this result remains very risky until Diamonds North repeats these results with a larger sample from this particular kimberlite unit.

Note: the Renard counts are based on measuring the two longest dimensions

Pele Mountain's "breccia bodies" in the Wawa region of Ontario

Most of the samples from the various showings are less than 30 kg and consequently do not extend past the 0.425 mm sieve. Note that the curves for the Salon, Krug and Pommery outcrops look very similar to the pink curve for the Cristal outcrop. A 100 tonne bulk sample from the Cristal outcrop recovered only 0.02 ct/t and was modeled by De Beers at 0.06 ct/t, a grade too low to be interesting. The Cristal sample is almost ten times bigger and yet the curve does not get past 0.425 mm, a sure sign that this particular unit is very low grade. The curve for the Cristal25 sample at 21 kg is about the same as the that for the 171 kg sample. A bigger sample from Salon, Krug and Pommery might produce a curve that extends beyond the Cristal curve, but that is not Pele's priority because much more interesting are preliminary curves for the Mumm, Dom Perignon and Genesis outcrops. In fact, Pele Mountain has gone ahead with a 4.5 tonne mini bulk sample using an inefficient processing technique that recovered 0.12 ct/t. AMEC has combined the mini bulk sample data with the micro diamond data and modeled a grade of 0.3 ct/t. For this part of the world in a setting where this rock could represent a hundred million tonnes such a grade is very interesting. Cabo's 9 kg Cobalt sample literally screams for a larger sample and investigation of this rock unit's tonnage potential.

Diamonds North Results for Victoria Island

The chart below presents recently reported results from the Blue Ice project of Diamonds North. On September 18 Diamonds North reported the results for two different facies encountered in the Sand Pipe kimberlite. A 4.1 metre interval of hypbassyal kimberlite provided spectacular micro diamond results from a 6.49 kg sample. A second hole 75 metres to the southeast encountered 51.3 metres of tuffisitic kimberlite which yielded mediocre results from 31.74 kg presented by the curve marked "diat". On October 10 Diamonds North reported results for two distinct phases labeled "upper" and "lower" for the Carina kimberlite located 7.5 km from the Sand Piper kimberlite. The results for the olivine rich "Lower Carina" are considerably poorer than the Sand Piper, and pale even in comparison to the Wawa curves reported by Pele Mountain. The goal is to find higher grade units on Victoria Island with a diamond content similar to that of the hypabyssal Sand Piper unit.


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