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Lehtonen et al. Results from a reconnaissance scale heavy mineral survey of kimberlitic indicator minerals… 4 RESULTS Sample locations, their indicator contents and compositional diagrams of indicator minerals are presented separately for each sample series in Appendices I-V. The analytical data obtained by electron microprobe is presented in Appendix VI. Indicator grains were identified and categorized according to widely used compositional classifications. The results are marked on the maps. KIM's were recovered throughout the survey area; however, the number of positive samples was distinctly higher in the southern area. Number of KIM's per sampling site is generally low, from 0 to 6 grains. However, case studies from two eastern Finland kimberlites (Lehtonen et al., 2005) demonstrate that indicator contents in till can be extremely low (< 10 grains) even within a kilometre distance from the kimberlitic body. The pyrope garnets were classified based on their major element composition into harzburgitic (G10), lherzolitic (G9) and megacrystal (Ti-pyropes = Ti-P) varieties according to Schulze (2003). The garnet data in the graphs below are presented in the CaO-Cr2O3-diagram redrawn after Gurney (1984) and Sobolev et al. (1973). The Cr-diopside analyses were studied using the Al 2O3-Cr2O3 discriminant plot by Ramsay (1992). The Cr-diopside grains with Cr2O3 content exceeding 0.5 wt-% were classified as indicators. However, they should be considered with reservations, since there are many rock types other than kimberlites that can contain Cr-diopside of this composition. Those Cr-diopside grains that coexist in the same samples as pyrope garnets or Mg-ilmenites should be given a higher importance. Potential kimberlitic ilmenites were recognized using the MgO-TiO 2 diagram by Wyatt et al. (2004). In addition to these indicators, a representative set of chromites, if present, were analyzed. Chromite analyses are presented in Cr2O3-MgO (Smith et al., 1991) and Cr2O3-TiO2 (Fipke et al., 1995) classifications. Kimberlitic chromites differ from other indicator minerals in that in general they cannot be readily distinguished in sediment samples under optical microscope or even based on composition from non-kimberlitic “background” chromites derived from other rock types. Some kimberlitic chromite grains have certain diagnostic morphological features that can be observed under optical microscope or SEM, such as resorbed grain surfaces (Muggeridge, 1995) and so called “corona texture”, i.e., the outer rim of the grain exhibits enrichment in Ti compared to the grain core (Lehtonen & Marmo, 2002). The recognition of kimberlitic chromites based on composition is not simple because they display a vast compositional spectrum (Griffin & Ryan, 1993). Fipke (1991) has recognized three compositional types of kimberlitic chromites useful in diamond exploration. These include (1) chromites with similar composition to the ones that exist as diamond inclusions and intergrowths (i.e. DI-chromites after Fipke’s terminology), (2) high-Cr, high-Ti chromites (Cr-Ti chromites) and (3) relatively low-Cr, high-Al and/or high- Fe Cr-spinels and chromites (Al-Fe chromites). The DI-group is characterized by high contents of MgO (>8.7 wt%) and Cr2O 3 (>57.8 wt%) and low contents of TiO2 (
Lehtonen et al. Results from a reconnaissance scale heavy mineral survey of kimberlitic indicator minerals… wt% TiO2 and plot within the field unique to lamproites/kimberlites in the Cr2O3-TiO2 classification, whereas the Al-Fe chromites plot within the overlap field along with chromites from other rock types. In this dataset we have used three compositional types for possibly kimberlitic indicator chromites: 1) Mg-chromite = Field 1 drawn on the Cr O 2 3-MgO-diagram, 2) Di-chromite = Field 2 drawn on the Cr O 2 3-MgO-diagram, and 3) Ti-chromite = Field 3 = kimberlitic/lamproitic field on the Cr O -TiO 2 3 2-diagram. (Corona texture chromites with rims composed of Ti-chromite were recognized only in few OMP-S series samples. Those grains are marked in the indicator results under the column "Ti-chromite"). 5 REFERENCES Calcagnile, G., 1982. The lithosphere-astenosphere system in Fennoscandia. Tectonoph. 90, 19- 35. Chernet, T., Marmo, J., Nissinen, A., 1999. Technical note. Significantly improved recovery of slightly heavy minerals from Quaternary samples using GTK modified 3WKnelson preconcentrator. Min. Eng. 12, 1521–1526. Fipke, C.E., 1991. Significance of chromite, ilmenite, G5 Mg-almandine garnet, zircon and tourmaline in heavy mineral detection of diamond bearing lamproite. In: Fifth International Kimberlite Conference, Brazil,1991. CPRM Spec. Pub. 2/91, pp. 97-100. Fipke, C.E., Gurney, J.J., Moore, R.O., 1995. Diamond exploration techniques emphasising indicator mineral geochemistry and Canadian examples. Geol. Surv. Can., Bull. 423 (86 pp.). Griffin, W.L., Ryan, C.G., 1993. Trace elements in garnets and chromites: Evaluation of diamond exploration targets. In: Diamonds: Exploration, Sampling and Evaluation. Prospectors and Developers Assoc. Canada, Short Course Notes, pp. 187-211. Griffin W.L., Ryan C.G., 1995. Trace elements in indicator minerals: area selection and target evaluation in diamond exploration. In: W.L. Griffin (edit.), Diamond Exploration: Into the 21st Century. J. Geochem. Explor. 53: 311-337. Gurney, J.J., 1984. A correlation between garnets and diamonds. In: Glover, J.E., Harris, P.G. (Eds.), Kimberlite Occurrence and Origin: A Basis for Conceptual Models in Exploration, Geol. Dept. and Univ. Ext., Univ. of WA, Publ., vol. 8, pp. 143– 166. Korhonen, J.V., Kivekäs, L., 1997. Petrophysical properties of kimberlites and rocks of Archaean basement of central Fennoscandian Shield. In Papunen, H. (ed.) Mineral Deposits: Research and Exploration; where do tey meet? Proceedings of the fourth biennial SGA Meeting held in Turku, Finland, 11-13 August 1997. Balkema Rotterdam, 771-774. Kukkonen, I.T., Jõeleht, A., 1996. Geothermal modelling of the litosphere in the central Baltic Shield and its southern slope. Tectonoph. 255 (1-2): 25-45. Lehtonen, M.L., 2005. Kimberlites in Finland: Information about the mantle of the Karelian Craton and implications for diamond exploration . Geological Survey of Finland, 109 p. Lehtonen, M.L., Marmo, J.S., 2002. Exploring for kimberlites in glaciated terrains using chromite in quaternary till — a regional case study from northern Finland. J. Geochem. Explor. 76 (3), 155– 174. Lehtonen, M.L., Marmo, J.S., Nissinen, A.J., Johanson, B.S., Pakkanen, L.K., 2005. Glacial dispersal studies using indicator minerals and till geochemistry around two eastern Finland kimberlites. Journal of Geochemical Exploration 87 (1), 19-43. Marmo, J.S., Vilpas, L., Nenonen, K. 1997. Petrophysical properties of kimberlites and rocks of Archaean basement of central Fennoscandian Shield. In Papunen, H. (ed.) Mineral Deposits:
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Page 13 and 14: Cr-pyrope Cr-pyrope Total number Pr
Page 21 and 22: Cr 2O 3 wt% 14 13 12 11 10 9 8 7 6
Page 23 and 24: Cr 2O 3 wt-% 4 3 2 1 OMP CPX S Grt-
Page 25 and 26: Cr 2O 3 wt % 75 70 65 60 55 50 45 4
Page 27 and 28: 6970000 6960000 6950000 6940000 693
Page 29 and 30: 6970000 6960000 6950000 6940000 693
Page 31 and 32: Cr-pyrope (G9) 0.25- 1.0 mm Cr-pyro
Page 33 and 34: Cr 2O 3 wt-% 4 3 2 1 OMP-N CPX Grt-
Page 35 and 36: Cr 2O 3 wt % 75 70 65 60 55 50 45 4
Page 37 and 38: 7120000 7110000 7100000 7090000 708
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Page 45 and 46: Cr 2O 3 wt-% 4 3 2 1 VEP cpx Grt-pe
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Cr 2O 3 wt-% 4 3 2 1 LAA cpx Grt-pe
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Cr 2O 3 wt % 75 70 65 60 55 50 45 4
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7090000 7080000 7070000 7060000 353
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EPMA DATA wt% Cr2O3-MgO Cr2O3-TiO2
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EPMA DATA wt% Sample series Mineral
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EPMA DATA wt% MgO-TiO2 Sample serie
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