Geological Survey of Denmark and Greenland Bulletin 17, 2009, 65-68 The search for diamonds in Greenland has resulted in the dis- covery of many new dykes of kimberlite and ultramafic lam- prophyre and, most importantly, in the acquisition of a wealth of chemical data on rocks and minerals representing mantle material entrained by the dyke magmas. The discov- ery of a diamondiferous sheet at Garnet Lake in southern West Greenland stimulated the research (Hutchison 2005). Over the past five to ten years, the Geological Survey of Denmark and Greenland together with the Bureau of Minerals and Petroleum in Greenland and international research groups have acquired, processed and interpreted data with the objective of identifying diamond-favourable regimes within the lithospheric mantle below the Archaean craton in West Greenland. Here we present mineral data from drift samples that allow us to iden- tify where mantle conditions in terms of lithology and depth may be favourable for the occurrence of diamonds. Neoproterozoic igneous province The province comprises the Sarfartoq carbonatite complex and abundant dykes and sills of carbonate-rich ultra- mafic silicate rocks (ultramafic lampro- phyre and kimberlite) that have been emplaced in late Neoproterozoic time into Archaean rocks of southern West Greenland between 65°N and 67°30´N (Larsen & Rex 1992; Nielsen et al. in press; Steenfelt et al. in press). Magma emplacement was con- centrated in the Sarfartoq and Maniitsoq regions (Fig. 1), and took place from c. 600 Ma to c. 555 Ma (Secher et al. in press). The first period of magmatism appears to be confined to the Sarfartoq region while later magma pulses affected the entire region (Fig. 1). A 568 ± 11 Ma age of the Garnet Lake sheet (Hutchison & Heaman 2008) places this intrusion in the younger part of the period. Ultramafic dykes (some of which are diamondiferous) of the same age in Labrador, Canada (Tappe et al. 2006) show that the magmatism ex - tended into the western part of Laurentia, the then contigu- ous continent comprising North America and Greenland. The magmatism is thought to have been triggered by incipi- ent continental rifting. At about the same time, more pro- Diamonds and lithospheric mantle properties in the Neo - proterozoic igneous province of southern West Greenland Agnete Steenfelt, Sven Monrad Jensen, Troels F.D. Nielsen, Karina K. Sand and Karsten Secher © GEUS, 2009. Geological Survey of Denmark and Greenland Bulletin 17, 65–68. Available at: www.geus.dk/publications/bull 65 Fig. 1. Neoproterozoic igneous province and localities for rocks tested positive for dia - monds (Jensen et al. 2004, with later updates from Intex Resources ASA, www.intex- resources.com). The large diamond symbol marks the Garnet Lake commercial diamond operation. The Sarfartoq carbonatite complex and dykes of kimberlite and ultramafic lamprophyre (UML) are emplaced into the Archaean craton, which suffered deformation in the northern part during a Palaeo - proterozoic collision. Southern boundary of Palaeoproterozoic deformation Maniitsoq region Sarfartoq region 50°W 66°N 67°N 65°N 52°W54°W Inland Ice Davis Strait Archaean craton Qaqarssuk, 165 Ma Age 554–572 Ma Age 577–604 Ma UML or kimberlite Carbonatite Diamond locality 50 km Garnet Lake Sarfartoq Greenland ROSA_2008:ROSA-2008 01/07/09 15:48 Side 65 66 nounced rifting took place at the northern margin of Laurentia and resulted in the intrusion of a prominent basaltic dyke swarm in North-West Greenland and northern Baffin Land (Dawes 2006). Exploration and diamond discoveries Exploration companies have used the so-called kimberlite indicator minerals in their search for host rocks for dia- monds. Samples of overburden or drift (mainly till) have been collected systematically over the entire Archaean craton of southern West Greenland and processed to obtain the non- magnetic heavy mineral fraction, from which grains of mantle- derived minerals including peridotitic garnet (pyrope), eclogitic garnet, chromite (chrome-spinel), picroilmenite and chrome-diopside have been picked under a microscope. Many grains were subsequently chemically analysed to verify the visual identification and allow chemical classification. Numerous samples with diamond-indicative, high-pressure mineral compositions indicate that the Neoproterozoic province has a high prospective potential, and subsequent diamond tests have confirmed that the carbonate-rich ultra- mafic magmas brought up diamondiferous mantle at several localities within the province (Fig. 1). The huge amount of mineral analyses were compiled and quality controlled by Jensen et al. (2004). In 2004 Hudson Resources Inc., guided by drift samples with an abundance of garnets derived from the diamond-sta- ble mantle, discovered a significant amount of diamonds hosted in carbonatite-rich ultramafic rocks at Garnet Lake (Fig. 1) in the Sarfartoq region (Hutchison 2005). Con - tinued exploration has established the presence of a 4 m wide, shallow-dipping sheet of kimberlitic rock with a promising diamond grade and diamond crystals up to 4 carats (0.8 g; Hutchison & Heaman 2008; www. hudsonresources.ca). Harzburgite G10 400 600 800 1000 Estimated garnet equilibrium temperature °C La tit ud e °N La tit ud e °N La tit ud e °N 1200 1400 1600 A B C Graphite stable Diamond stable 65 65.5 66 66.5 65 65.5 66 66.5 65 65.5 66 66.5 67 Lherzolite G9 High-Ti lherzolite G11 Fig. 2. Estimated temperatures of three classes (A: G9, B: G11 and C: G10) of mantle-derived garnet grains picked from the non- magnetic, heavy mineral fraction of drift samples (mainly till). Northern latitudes on vertical scale: upper group Sarfartoq region (north of 66°N), lower group Maniitsoq region. Orange, open triangles are grains from the Garnet Lake area. Red, vertical line marks the temperature of graphite-diamond phase transition (900°C). Blue line (1200°C, c. 180 km depth) marks the cut-off value for deeply derived grains plotted in Fig. 3. Squares represent lithologi- cally sorted temperature estimates based on mantle xenoliths (Larsen & Rønsbo 1993; Garrit 2000; Bizzarro & Stevenson 2003; Jensen et al. 2004; Sand et al. in press); lines between ×-symbols are ranges in Ni-in-garnet temperatures of grains in garnet concentrates (data from Garrit 2000). Large, filled, red triangles are temperatures determined on xenoliths from Garnet Lake (Hutchison & Heaman 2008). ROSA_2008:ROSA-2008 01/07/09 15:48 Side 66 The Neoproterozoic lithospheric mantle Many mineralogical and chemical investigations of mantle xenoliths hosted by the kimberlites and ultramafic lampro- phyres have demonstrated that the lithospheric mantle com- prises an upper section of peridotitic rock types (lherzolite, harzburgite, dunite) depleted in elements such as Ca, Fe and Ti relative to asthenospheric mantle because of extraction of large portions of basalt. The section of depleted mantle is underlain by a section with a predominance of Fe-Ti-rich, so- called fertile garnet lherzolite (references in Fig. 2). It has also been demonstrated that some xenoliths from both regions have been derived from depths clearly within the high-pres- sure regime where diamond is stable (references in Fig. 2). The constraints for the Neoproterozoic geotherm have recently been improved to 38–41 mW/m2, and the thickness of the Neoproterozoic lithosphere has been estimated to be at least 215 km over the entire province (Sand et al. in press). Studies by Hutchison & Heaman (2008) indicate that the diamonds at Garnet Lake probably formed at great depths within the fertile lherzolite, i.e. at temperatures above 1200°C, and within a period of 50 mil- lion years before the transporting mag - ma brought them to the surface. The deep lithospheric mantle provenance of the xenoliths is also stressed by Grütter & Tuer (in press), who found an unusu- ally high proportion of high-T perido - titic garnets in drift samples from the immediate surroundings of Garnet Lake. Garnets from deep lithospheric mantle Garnet is the mineral that has been used most extensively in lithosphere studies and diamond exploration to reflect the temperature and pressure conditions as well as the lithology at the site where it equilibrated. The 15 000 available ana - lyses of garnet grains from the Neoproterozoic province therefore provide excellent material with which to locate dykes that have incorporated material with deep mantle provenance similar to that recorded at Garnet Lake. Using a chemical discrimination system devised by Grütter et al. (2004) we have selected garnets derived from depleted lherzolite (G9; Fig. 2A), fertile lherzolite (G11, Fig. 2B) and harzburgite (G10, Fig. 2C), and determined their equi libration temperatures using MnO concentrations (Grüt - ter et al. 1999). The results shown in Fig. 2 are plotted against the latitude of the sample sites in order to reveal any regional differences. Temperatures of Garnet Lake grains and pub- lished temperature estimates based on other minerals are shown for comparison. The diagrams show that a majority of the garnet grains derive from depths where diamond is the stable carbon phase, i.e. where the temperature is above 900°C. It appears that G11 garnets (from fertile lherzolite) mainly come from greater depths and have large populations over the entire lat- itude interval. This enforces the validity of current models invoking the ubiquitous presence of fertile lherzolite in the 67 Fig. 3. Neoproterozoic igneous province with localities of drift samples and results of screening garnet grain analyses belonging to classes G9, G10 and G11. Deep garnets (green dots) have T-Mn above 1200°C. The red symbols marking samples (drift or rock) with more than 10 deep garnets in the picked populations of peridotitic garnet grains show spatial correlation with diamond occurrences. Southern boundary of Palaeoproterozoic deformation Maniitsoq region Sarfartoq region 50°W 66°N 67°N 65°N 52°W54°W Inland Ice Davis Strait Drift sample all sites with garnet with deep garnet with many deep garnets Rock sample with many deep garnets Carbonatite Diamond locality 50 km Garnet Lake Sarfartoq K ROSA_2008:ROSA-2008 01/07/09 15:48 Side 67 68 lower lithospheric mantle section. A tendency for relatively more G11 grains above 1200°C in the Maniitsoq region is observed. Lherzolitic (G9) and harzburgitic garnets (G10) display origins in wider depth intervals, and very deep grains occur in both regions. The Garnet Lake garnets do stand out in reaching higher temperatures than many grains in the Sarfartoq region. However, grains from several localities in the Maniitsoq region have also yielded temperatures above 1400°C. It should be mentioned that the temperature is inversely correlated with MnO concentrations, so that tem- perature data above 1600°C are uncertain owing to low ana- lytical precision at low concentrations. The range in garnet T–Mn temperature estimates is in good agreement with estimates using other methods, and the advantage of having the many additional data to establish a more statistically reliable, regional picture of mantle prove- nance is obvious. In addition, the drift-derived garnets pro- vide information from areas where dykes have not been located or sampled. Distribution of sites with high diamond potential Figure 3 shows the extensive coverage of drift sample sites and the clusters of garnet-bearing samples (any mantle- derived kind) where dykes are common (compare Fig. 1). The deep (high-T) garnets have a narrower distribution, yet they are abundant in both regions. In order to highlight localities with a high proportion of deep garnets, an arbitrary lower limit of ten grains has been applied. Rock-sample local- ities with a high proportion (more than ten grains) of deep garnets have been identified and are added as supplementary information. They outline additional localities with diamond potential in the Sarfartoq region. The distribution of localities rich in deep garnet exhibits spatial correlation with that of diamond-bearing rocks and thus supports the observation made at Garnet Lake that an abundance of deep, lower lithospheric mantle material is characteristic of diamond-bearing dykes. However, the data also demonstrate that Garnet Lake is not unique in the province in this respect and the potential for making equally promising diamond discoveries elsewhere appears to remain. One small area near Kangerlussuaq (K in Fig. 3) has not yet proved positive for diamonds, but is considered a target for further exploration. Subsurface exploration methods would be needed in that area, though, because poor exposure impedes surface recognition of significant dykes. References Bizzarro, M. & Stevenson, R.K. 2003: Major element composition of the lithospheric mantle under the North Atlantic Craton: evidence from peridotite xenoliths of the Sarfartoq area, southwestern Greenland. Contributions to Mineralogy and Petrology 146, 223–240. Dawes, P.R. 2006: Explanatory notes to the geological map of Greenland, 1:500 000, Thule, Sheet 5. Geological Survey of Denmark and Green - land Map Series 2, 97 pp. + map. Garrit, D. 2000: The nature of the Archaean and Proterozoic lithospheric mantle and lower crust in West Greenland illustrated by the geo- chemistry and petrography of xenoliths from kimberlites, 289 pp. Unpublished Ph.D. thesis, University of Copenhagen, Denmark. Grütter, H. & Tuer, J. in press: Constraints on deep mantle tenor of Sarfartoq-area kimberlites (Greenland), based on modern thermo- barometry of mantle-derived xenocrysts. Lithos. Grütter, H.S., Apter, D.B. & Kong, J. 1999: Crust-mantle coupling: evi- dence from mantle-derived xenocrystic garnets. In: Gurney, J.J. et al. (eds): Proceedings of the VIIth International Kimberlite Conference 1, 307–313. Cape Town: Red Roof Design. Grütter, H.S., Gurney, J.J., Menzies, A.H. & Winter, F. 2004: An updated classification scheme for mantle-derived garnet, for use by diamond explorers. Lithos 77, 841–857. Hutchison, M.T. 2005: Diamondiferous kimberlites from the Garnet Lake area, West Greenland: exploration methodologies and petrochemistry. Danmarks og Grønlands Geologiske Undersøgelse Rapport 2005/68, 33–42. Hutchison, M.T. & Heaman, L.M. 2008: Chemical and physical charac- teristics of diamond crystals from Garnet Lake, Sarfartoq, West Green - land: an association with carbonatitic magmatism. The Cana dian Mineralogist 46, 1063–1078. Jensen, S.M., Secher, K., Rasmussen, T.M. & Schjøth, F. 2004: Diamond exploration data from West Greenland: 2004 update and revision. Dan - marks og Grønlands Geologiske Undersøgelse Rapport 2004/117, 90 pp. + 1 DVD. Larsen, L.M. & Rex, D.C. 1992: A review of the 2500 Ma span of alkaline- ultramafic, potassic and carbonatitic magmatism in West Greenland. Lithos 28, 367–402. Larsen, L.M. & Rønsbo, J. 1993: Conditions of origin of kimberlites in West Greenland: new evidence from the Sarfartoq and Sukkertoppen regions. Rapport Grønlands Geologiske Undersøgelse 159, 115–120. Nielsen, T.F.D., Jensen, S.M., Secher, K. & Sand, K.K. in press: Regional and temporal variations in the magmatism of the diamond province of southern West Greenland. Lithos. Sand, K.K., Waight, T., Pearson, D.G., Nielsen, T.F.D., Makovicky, E. & Hutchison, M.T. in press: The lithospheric mantle below southern West Greenland: a geothermobarometric approach to diamond poten- tial and mantle stratigraphy. Lithos. Secher, K., Heaman, L.M., Nielsen, T.F.D., Jensen, S.M., Schjøth, F. & Creaser, R. in press: Timing of kimberlite, carbonatite and ultramafic lamprophyre emplacement in the alkaline province located 64°– 67°N in southern West Greenland. Lithos. Steenfelt, A., Jensen, S.M., Nielsen, T.F.D. & Sand, K.K. in press: Provinces of ultramafic lamprophyre dykes, kimberlite dykes and car- bonatite in West Greenland characterised by minerals and chemical components in surface media. Lithos. Tappe, S., Foley, S.F., Jenner, G.A., Heaman, L.M., Kjarsgaard, B.A., Romer, R.L., Stracke, A., Joyce, N. & Hoefs, J. 2006: Genesis of ultra- mafic lamprophyres and carbonatites at Aillik Bay, Labrador: a conse- quence of incipient lithospheric thinning beneath the North Atlantic Craton. Journal of Petrology 47, 1261–1315. Authors’ addresses A.S, T.F.D.N. & K.S., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: ast@geus.dk S.M.J., Intex Resources ASA, Munkedamsveien 45A, N-0250 Oslo, Norway. K.K.S., Nano-Science Center, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark. ROSA_2008:ROSA-2008 01/07/09 15:48 Side 68