CATHODOLUMINESCENCE AND GEOCHEMICAL PROPERTIES OF KIMBERLITIC AND LAMPROITIC ZIRCONS: APPLICATION TO DIAMOND EXPLORATION

Belousova, E.A1, Griffin, W.L1,2 and O'Reilly, S.Y.1

1 GEMOC Macquarie, 2 CSIRO Exploration and Mining

Zircon is a minor mineral in kimberlites, and is recognised as a member of the low-Cr suite of mantle-derived megacryst minerals. The frequent occurrence of zircon in kimberlites suggests that, by finding specific characteristic features of kimberlite zircons, it may be used as an indicator mineral during diamond exploration. However, the morphology of the kimberlitic zircon grains does not show clearly defined features peculiar to the zircon of kimberlites. Therefore, cathodoluminescence (CL) microscopy and laser ablation microprobe (LAM) ICPMS analysis were used to study the internal structure and chemical composition of zircon crystals from kimberlites of South Africa, Russia, Botswana and Australia.

Zoning revealed by CL ranges from fine oscillatory to broad homogeneous cores and overgrowths predominantly in bluish colours; yellow CL colours are much less common. Samples or zones with yellow CL have higher U, Th, Y, and REE than those with blue-violet CL. We suggest that variations in the concentrations of a range of trace elements lead to different amounts of lattice defects, creating the possibility for different levels of direct excitation of luminescence centres, and therefore different CL colours. All crustal zircons studied so far exhibit only dull yellow CL colours.

LAM-ICPMS data show that kimberlite zircons have distinctive trace element patterns, with well defined ranges for REE, Y, U, Th and P. Low U contents (commonly 6-20 ppm) and REE contents (_ REE < 50 ppm), as well as chondrite-normalised REE patterns with low and flat HREE, are characteristic of kimberlite zircons and distinguish them from crustal zircons.

Yakutian kimberlitic zircons (Russia) are represented by 39 grains from 19 kimberlite pipes in 8 kimberlite fields. The trace element signatures of the Yakutian zircons divide them into two well defined groups, belonging to on-craton and off-craton kimberlite fields. On-craton zircons originate from areas with thick Archaean lithosphere and low geotherms required for high diamond prospectivity. The trace element patterns of on-craton zircons are similar to those of South African and Australian kimberlitic zircons. Furthermore, the trace element abundances and the slope of the trace element patterns decrease towards the inner part of the Archaean craton. Zircons from the off-craton fields, in contrast, have higher concentrations of almost all trace elements.

South African kimberlitic zircons are the best represented group; 67 grains from 13 different kimberlite pipes have been studied. Their averaged abundances of heavy REE, Y, Sn, Hf, Mn, Ti and Pb are slightly higher than those of kimberlitic zircons from on-craton fields of Yakutia and Australia. However, the averaged trace element pattern of Southern African zircons is distinct (lower and flatter) from those of zircons from off-craton kimberlite pipes of Yakutia and old Jwaneng zircons (Botswana).

Australian kimberlitic zircons are represented by 20 grains from two small kimberlite bodies: Orroroo, South Australia and Pteropus, Kimberley. The Orroroo kimberlite pipe has a low diamond content, while Pteropus is barren. Cathodoluminescence colours range from yellow through pink to bluish and dark violet. Trace element data are very similar for both pipes and are similar to data obtained for other kimberlite localities.

Because lamproites are also known to be potentially diamondiferous rocks, lamproitic zircons from Argyle, Australia and the Kirovograd Block of the Ukrainian Shield have been used for comparison. Trace element data indicate that all studied zircons from lamproitic rocks appear to be crustal-derived.

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