VARIABILITY IN DIAMOND POPULATION CHARACTERISTICS ACROSS THE SIZE RANGE 0.2 – 3.4 MM – A CASE STUDY BASED ON DIAMONDS FROM MISERY (EKATI MINE)

In diamond exploration size frequency distributions are widely used for predicting the macro-diamond grade of new kimberlite discoveries. The purpose of this study is to investigate the relationship between the different diamond size fractions extracted from microdiamond samples used for grade evaluation of the Misery kimberlite; this will allow us to assess the validity of this approach.

Of an initial representative suite of 1627 diamonds, ranging in size from 0.075 to 3.35 mm, 558 diamonds were sub-sampled (ranging in size from 0.212 to 3.35 mm) and analyzed for their nitrogen characteristics using FTIR spectroscopy. Of this sub-suite, a random sample of ~ 50% was taken for determination of carbon isotope (δ¹³C) systematics by secondary ion mass spectrometry (SIMS).

The acquired nitrogen data are very variable, ranging in concentration from below the limit of detection (<10 at.ppm; Type II) to over 2500 at.ppm with a median abundance of ~ 846 at.ppm. A slight decrease in median N abundance with increasing size fraction is visible. The diamonds of sieve classes < 0.85 mm show median values between ~ 790 and 915 at.ppm while coarser diamonds have a median of ~ 600 at.ppm.

The interrelationship between N abundance and aggregation state allows the calculation of time averaged mantle residence temperatures (T_N) based on an assumed residence time (3 Ga). Misery diamonds show two distinct subpopulations in T_N, one ≤ 1125 °C and a second ≥ 1175 °C. Projected on a central Slave geotherm, the two temperature groups correspond to diamonds coming from depth less than ~160 km and greater than ~170 km, respectively. This bimodal distribution is visible in all size fractions. The relative proportion of ‘shallow’ to ‘deep’ diamonds, however, decreases with sieve size.

Carbon isotopic (δ¹³C) characteristics have been determined for 260 diamonds (640 spots analyzed). Our results show that > 95% of the data have δ¹³C values in the range between - 8 to - 2‰ with a symmetrical distribution and a median of −4.4‰. Distinct subpopulations exist within the interval, but no discernible differences between the individual size fractions are immediately apparent.

Overall the Misery diamonds show broadly similar nitrogen characteristics and δ¹³C compositions indicating a strong genetic relationship between diamonds of different size fractions.