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Diamonds: Historical Longest Dimension based Micro Diamond Data
(From the archives - originally published in 2002)
Historical Longest Dimension based Micro Diamond Data
Definition: micro diamonds recovered from samples up to 1,000 kg through caustic or acid dissolution or fusion processes are divided into 2 groups called "micros" and "macros". The bottom cutoff is achieved through a sieve that usually has a 0.1 mm square mesh, but sometimes goes as low as 0.074 mm or as high as 0.15 mm. Companies do not always report the bottom cutoff, though they should because the frequency of micro diamonds often soars as size gets smaller. A "macro" is a micro diamond which is greater than 0.5 mm in its longest axis, or "dimension" as it is more commonly called. This measurement is made visually using a microscope. It does not distinguish between intact crystals and fragments, nor does it distinguish between flat crystals such as macles and heavier dodecahedra that have the same longest dimension measurement. The term "macro" in the context of small sample derived micro diamonds conflicts with the traditional definition of a "macro" as a diamond exceeding the minimum size recovered by a commercial mining operation. The minimum "commercial macro" size ranges from a 1.0 mm screen to 1.65 mm. When a grade (ie carats per tonne) is quoted for a kimberlite, it is understood to include only commercially recoverable diamonds. Sometimes companies report the total weight of their micro diamonds, which encourages people to calculate a diamond grade they extrapolate to the entire body. This extrapolation is inappropriate because the typical micro diamond sample is too small to be representative. However, a bigger mistake is to confuse total diamond grade with commercial macro grade. Unlike metals such as gold where particle size does not matter and all the recovered metal gets melted into a homogenous form whose value is defined purely by weight, diamonds must retain their original crystal form, whose clarity, colour, shape and size defines the value of each stone. Micro diamonds have no economic value, so their weight is irrelevant. Furthermore, it may surprise people to learn that the total diamond grade for a kimberlite that includes the weight of micro diamonds can be substantially higher than the "commercial macro" grade. In fact, simply switching from a 1.0 mm recovery screen to a 1.5 mm screen can result in significant variations in commercial grade. The paradox of micro diamond analysis is that micro diamonds are never mined; they provide only an oblique peek at the macro grade potential.
Analytical usage of longest dimension macro-micro data: the classification of micro diamonds into "macros" and "micros" based on a 0.5 mm measurement of the longest dimension does not contain any information that allows grade calculation. Analytical evaluation of longest dimension micro-macro data has thus been done on a comparative basis where the ratio of macros to micros or macros per weight unit (ie 10 kg) for new discoveries are compared to the micro diamond ratios of pipes for which a commercial macro grade has been established by bulk sampling. This evaluation approach is deeply flawed because 1) it ignores the fundamental nature of a diamond population as a continuum with a lognormal particle size frequency distribution, 2) the empirical record of longest dimension based micro diamond results published by Canadian companies since 1991 shows no correlation between commercial macro grade and longest dimension macro based ratios, and 3) it is blind to emplacement related physical factors such as resorption and aerial distribution that may negatively affect the smaller micro diamonds more than the larger ones.
Many public companies involved in diamond exploration publish tables which compare the longest dimension micro-macro count ratios with those of selected kimberlites with known commercially viable grades. The following tables include the longest dimension micro-macro results for kimberlites where bulk sampling has established uneconomic macro grades.
Longest Dimension Based Reporting for Micro Diamond Results
Sorted by macros per 10
kg
Pipe
Sample kg
<0.5 mm
>0.5 mm
Macro/Micro
Macro/10 kg
Grade ct/t
DMM - Misery - 61 kg -
4.26 ct/t
61
259
155
0.6
25.6
4.260
Ashton K14C w/frag -
0.12 ct/t
89
422
94
0.2
10.6
0.120
MPV5034 - 6027 kg - 1.64 ct/t
6027
11064
5458
0.5
9.1
1.640
Snap Lake - 214 kg -
1.78 ct/t
214
325
169
0.5
7.9
1.780
Tahera-JD/0D1 - 2415 kg
- 0.84 ct/t
2415
2589
1764
0.7
7.3
0.840
Bandore-Area E - 250 kg
- 0.05 ct/t
250
9094
141
0.0
5.6
0.050
DMM 97/C - 57 kg
57
45
32
0.7
5.6
Aber - A154S - 1352 kg -
4.8 ct/t
1352
1406
728
0.5
5.4
4.800
Aber A418 - 3.7 ct/t
997
994
503
0.5
5.0
3.700
DMM 92/I - 122 kg
122
132
55
0.4
4.5
Aber - A154N- 1067 kg - 2.4 ct/t
1067
1023
424
0.4
4.0
2.400
DMM-Phoenix - 240 kg -
1.41 ct/t
240
158
95
0.6
4.0
1.410
ACA - Artemisia - 332 kg
- 0.11 ct/t
332
1121
120
0.1
3.6
0.110
DMM 92/F - 177 kg
177
198
64
0.3
3.6
Twin Torngat 1 - 322 kg
- 0.04 ct/t
322
214
112
0.5
3.5
0.040
DMM 93/L - 123 kg
123
143
40
0.3
3.3
DMM 92/D - 71 kg
71
155
23
0.1
3.2
DMM 92/B - 72 kg
72
117
23
0.2
3.2
DMM Wildebeest - 35 kg
35
41
11
0.3
3.1
DHK - DO27 Pyro - 314 kg
- 0.36 ct/t
314
299
96
0.3
3.1
0.360
DMM 97/A - 67 kg
376
166
106
0.6
2.8
DMM-Bear Tooth - 376 kg
- 1.2 ct/t
376
166
106
0.6
2.8
1.200
DMM Point Lake - 59 kg -
0.63 ct/t
59
65
16
0.2
2.7
0.630
Tanqueray-Torrie - 198
kg - 0.026 ct/t
198
181
53
0.3
2.7
0.026
DMM 97/B - 407 kg
407
167
102
0.6
2.5
Tanqueray - Sue - 71 kg
71
52
17
0.3
2.4
DMM 93/A - 86 kg
86
60
19
0.3
2.2
Aber - A21 - 1385 kg -
3.0 ct/t
1385
789
300
0.4
2.2
3.000
DMM Pegasus - 295 kg
295
291
62
0.2
2.1
DMM 97/D - 232 kg
232
78
47
0.6
2.0
DMM 92/A - 108 kg
108
95
21
0.2
1.9
DMM-Koala North - 268 kg
- 0.63 ct/t
268
38
51
1.3
1.9
0.630
DMM 99/A - 235 kg
235
266
39
0.1
1.7
DMM 92/E - 70 kg
70
36
11
0.3
1.6
DMM 98/B - 73 kg
73
12
11
0.9
1.5
DMM 93/M - 111 kg
111
73
13
0.2
1.2
Ashton Renard 3 - 156 kg
156
21
17
0.8
1.1
ACA Potentilla - 208 kg
208
230
22
0.1
1.1
Ashton K91 - 117 kg -
0.14 ct/t
117
180
12
0.1
1.0
0.140
DHK - DO27 Diat - 160 kg
- 0.013 ct/t
160
14
13
0.9
0.8
0.013
Ashton K252 - 0.55 ct/t
107
135
8
0.1
0.7
0.550
Ashton K14 - 153 kg -
0.12ct/t
153
139
11
0.1
0.7
0.120
DMM 92/G - 122 kg
122
45
8
0.2
0.7
DMM 92/C - 161 kg
161
28
10
0.4
0.6
DMM Kodiak - 301 kg
301
74
17
0.2
0.6
Shore-Star - 2,245 kg -
0.065 ct/t
2245
403
120
0.3
0.5
0.065
Candle Lake #29 - 65 kg
- 0.04 ct/t
65
14
3
0.2
0.5
0.037
Ashton K14C no frag -
0.12 ct/t
89
152
4
0.0
0.4
0.120
DMM 98/A - 195 kg
195
11
8
0.7
0.4
Twin Freightrain 1026 kg
- 0.2 ct/t
1026
385
40
0.1
0.4
0.200
Tahera Ranch Lake - 208
kg - 0.22 ct/t
208
38
6
0.2
0.3
0.220
Winspear CL25 - 350 kg
350
212
9
0.0
0.3
Ashton K6- 117 kg - 0.072 ct/t
154
33
3
0.1
0.2
0.072
SUD 92-2 - 1104 kg
1104
35
21
0.6
0.2
DMM 92/H - 60 kg
60
7
1
0.1
0.2
Longest Dimension Based Reporting for Micro Diamond Results