Diamonds and their mineral inclusions from unconventional diamond deposits

Team: Banas A, Chinn I (De Beers), Czas J, Hunt L, Kong J (De Beers), Kurszlaukis S (De Beers), McCandless T (now MCC Geoscience Inc.), Morton R, Read G (Shore Gold), Seller M (De Beers), Stachel T, Walker E (Petrologic) with support of Ashton Mining of Canada , De Beers Canada, Pele Mountain Resources, Shore Gold Inc., Sola Diamond Corp.

Diamonds from the Buffalo Hills in Alberta, the Victor Mine in Ontario and the Fort a la Corne field in Saskatchewan occur in “unconventional” settings as the last tectonothermal events affecting the the Buffalo Head Terrane and the Sask Craton date back to only about 1.9-1.8 Ga. Similarly, the diamondiferous Carolina kimberlite in northwestern Brazil (Rondônia) is located on the Proterozoic Amazon Craton (<1.8 Ga). The Victor Mine (Attwapiskat field in northern Ontario) is located on a part of the Superior Craton showing clear evidence for a thermal event related to the 1.1 Ga Midcontinent Rift. Such occurrences contradict the conventional expectation that primary diamond deposits are restricted to Archean Cratons (Clifford’s Rule in the sense of Janse, 1994). Diamond occurrences in the Wawa area of Ontario represent an unusual setting since diamond emplacement in the Neoarchean seems to have coincided with volcanic activity in an active continental margin setting.

Diamonds as windows into the sub-lithospheric mantle

Team: Brenker F (Frankfurt), Brey GP (Frankfurt) , Cartigny P (Paris), Harris JW (Glasgow), Ickert R,  Kiseeva K (Oxford U), McCammon C (Bayreuth) , Meyer N, Motsamai T, Palot M, Pearson DG, Seitz M (Frankfurt), Stachel T, Stern R, Tappert R with support from De Beers, Lucara Diamonds and Petra Diamonds

Ultra-deep inclusions in diamonds from Kankan (West Africa), South Africa (Jagersfontein, Koffiefontein), Botswana (Karowe), Tanzania (Mwadui),
Canada (DO27, Ekati, Diavik) and Brazil (Juina area) are studied to obtain a more detailed picture (i) on the chemical and mineralogical composition of the sublithospheric upper mantle, the transition zone and the lower mantle and (ii) on the distribution of trace elements between ultra high pressure phases. Chemical (δ13C, δ15N and total nitrogen content) and physical characteristics (shape, surface characteristics and nitrogen aggregation state) of the host diamonds are evaluated to obtain additional information on these in part ultra deep sources. SIMS analyses of δ18O in inclusions are employed to reveal the contribution of crutal components to the generation sublithospheric diamond sources. Micro-Mössbauer determinations of ferric iron ratios in lower mantle phases (ferropericlase and MgSi-perovskites) are used to constrain the fO2 conditions during diamond formation in the lower mantle. Synchrotron Mossbauer analyses on majoritic garnet inclusions reveal the redox state of subducted diamond stubstrates in the asthenosphere and transition zone. TEM and in situ XRD studies constrain the structural state of ultra deep inclusions.

The micro-/macro-diamond relationship

Team: Armstrong J (now Lucara Diamond Corp.), Carlson J (Ekati Diamond Mine), Johnson CN, Krebs M, McCandless T (MCC Geoscience Inc.), Melton G, Nowicki T (Mineral Services Group), Pearson DG, Stachel T, Stern R with support of Ashton Mining of Canada , Ekati Diamond Mine, Stornoway Diamond Corp.

Micro-diamond (<0.5mm) based size-frequency distributions are a standard tool for the prediction of macro-diamond grade during diamond exploration. Through a detailed assessment of various diamond characteristics (δ13C, δ15N, nitrogen content and aggregations state, and platelet and 3107 cm-1 centres) spanning a range of sieve classes we are assessing if the underlying assumption of a common origin of micro- and macro-diamonds is valid. So far it has emerged that the mix of various components (diamonds from distinct sources and lithologies in the mantle) that make up an overall production varies considerably with diamond size, but nevertheless micro- and macro-diamonds are not fundamentally distinct. Our studies to date include micro- and macro-diamonds from Artemisia (northern Slave), Panda and Misery (Ekati Mine, central Slave)

New diamond prospects

Team: Armstrong J (now Lucara Diamond Corp.), Banas A, Hunt L, Johnson CN, Nichols K, McCandless T (now MCC Geoscience Inc.), Peats J, Pell J (Peregrine), Smit K, Stachel T, Stern R with support of Ashton Mining of Canada , Metalex Ventures Ltd., Peregrine Diamonds Ltd. and Stornoway Diamond Corp.

Inclusion in diamond based studies and micro-diamond studies (SIMS based stable isotopic analyses combined with infra-red spectroscopic analyses of nitrogen characteristics and platelet and hydrogen related absorption features) are conducted to determine the source and residence history of diamonds in the subcratonic lithospheric mantle beneath new diamond prospects. In Canada alone, over the past decade we studied diamonds from newly discovered kimberlites at Artemisia (Slave Craton), Aviat (Churchill Province), Buffalo Hills (Buffalo Head Terrane of Alberta), Chidliak (Baffin Island), T1 and U2 (James Bay area, central Superior Craton), and Renard (eastern Superior Craton)

The internal structure of diamond – constraints on diamond forming reactions

Team: Chacko T, Chinn I (De Beers), Howell D (DDL, Netherlands), Harris JW (Glasgow U), Hunt L, Palot M, Pearson DG, Petts D, Smart K, Stachel T, Stern R, Wescott P with support from De Beers and Diavik Diamond Mine

The tools available to us in the Canadian Centre for Isotopic Microanalysis (SEM based micro-imaging of diamond growth structure based on cathodoluminescence, in situ SIMS analyses of δ13C, δ15N and nitrogen content with 15 µm spatial resolution) and the De Beers Laboratory for Diamond Research (micro-FTIR for in situ analysis of nitrogen content and aggregation, and platelet and 3107 cm-1 peak intensities) allow us to gain unprecedented new insights into the internal structure of diamond. Plates of gem diamond, coated diamond and fibrous/polycrystalline diamond (from e.g., Diavik, Finsch, Jericho, and Victor) are used to constrain modes of diamond formation from co-variations among δ13C, δ15N and nitrogen content, analyzed across growth profiles.

Metasomatic re-enrichment of depleted cratonic peridotites

Team: Aulbach S, Banas A, Brey GP (Frankfurt), Cartigny P (Paris), Harris JW (Glasgow U), Hunt L, Motsamai T, Pearson DG, Smit KV, Stachel T, Tappert R, Viljoen KS (Johannesburg U) with support from De Beers, Diavik Diamond Mine, Ekati Diamond Mine, Stornoway Diamond Corp, Lucara Diamonds

Characterization of the initial chemical depletion and subsequent metasomatic enrichment of the lithospheric upper mantle beneath several cratons world-wide based mainly on trace element studies (SIMS). Inclusions in diamonds and mantle xenoliths studied so far are from kimberlites in Canada (Attawapiskat, Diavik, Ekati, Renard) South Africa (De Beers Pool, Venetia and Roberts Victor), Botswana (Orapa, Jwaneng and Karowe), Ghana (Birim), Guinea (Kankan), Tanzania (Mwadui) and Brazil (Boa Vista, Canastra and Arenapolis).

Variations in the oxidation state of the subcratonic lithospheric mantle

Team: Brey GP (Frankfurt), Creighton S, Höfer H (Frankfurt) , Luth R, Matveev S (now Utrecht U), Stachel T, Viljoen KS (Johannesburg) with support from De Beers and Diavik Diamond Mine

Peridotite xenoliths from the Finsch and Bultfontein kimberlites in South Africa and the A154 South and North kimberlites (Diavik, Lac de Gras) in Canada reflect variations in oxygen fugacity with depth within the subcratonic lithospheric mantle. The oxidation state of iron in xenolith minerals can be determined using a new method based on the shift and deformation of the Fe L lines in X-ray emission spectra using EPMA. These data provide constraints on the origin and the character (methane-water or carbonate-water bearing) of the metasomatic agents infiltrating the cratonic lithosphere from beneath and precipitating diamonds they percolate upwards.

Composition and thermal state of cratonic peridotites (xenolith studies)

Armstrong J (Lucara Diamonds), Chislett K, Creighton S, Czas J, Eichenberg D (Diavik), Hanrahan M, Heaman L, Hunt L, Johnson A, Kurszlaukis S (De Beers), McLean H (Rio Tinto), Pearson DG, Read G (Shore Gold), Seller M (De Beers), Smit KV, Stachel T, Verigeanu D, Whiteford S (Rio Tinto), with support from De Beers Canada, Diavik Diamond Mine, Northwest Territories Geoscience Office, Southernera, Stornoway Diamond Corp.

Combined studies on the thermal state and the major and trace element chemistry (EPMA and LA-ICP-MS) of the lithospheric mantle are carried out to evaluate variations in the style of metasomatic modification in different lithologies and depths. Recent and current projects include the Attawapiskat and Renard kimberlites on the Superior Craton, the A154 South and North kimberlites (Diavik Mine) on the Slave Craton, and the Fort a la Corne kimberlites on the Sask Craton.

Cratonic eclogites and eclogitic diamonds

Team: Czas J, Harris JW (Glasgow U), Ickert R, Hunt L, Kobussen A (Rio Tinto), Kurszlaukis S (De Beers), Read, G, Smit KV, Stachel T with support from De Beers, Rio Tinto and Shore Gold

Eclogite xenoliths and eclogitic diamonds and their inclusions are studied to examine possible links between ancient subduction processes and the generation of diamond source regions. Techniques employed involve stable (δ18O for garnet, δ13C and δ15N for diamond) and radiogenic isotopes (Re-Os, Lu-Hf, Rb-Sr, U-Pb on xenolith minerals), and trace and major element analyses. Recent detailed studies focussed on eclogite xenoliths from the Victor Mine (Attawapiskat, Superior Craton), diamondiferous eclogites from Fort a la Corne, and eclogitic diamonds and their inclusions from the Argyle Mine (Kimberley Craton, W-Australia) and the former Damtshaa Mine (Kalahari Craton, Botswana)