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Lehtonen et al. Results from a reconnaissance scale heavy mineral survey of kimberlitic indicator minerals… 2 SAMPLING AREA The bedrock in the study area consists mainly of Archean granodioritic to tonalitic felsic plutonic rocks and remnants of 2.7 Ga greenschist belts. Subordinate serpentinized ultramafic bodies are present as well as late Archean granites. Paleoproterozoic SE-NW oriented mafic dike swarms of 2.2-1.6 Ga are ubiquitous. The bedrock has been overprinted by Svecofennian orogeny, which culminated at 1.9-1.8 Ga. There are no kimberlite discoveries reported from the survey area. Quaternary features include extensive basal till exposure, esker formations, and glaciolacustrine or marine sediments. Loose ablation till, lakes and swamps limit access to the basal till in some places. The till formations sampled for this work date back to the deglaciation at the end of the Weichselian when large glacial lobes developed in the southern and central parts of Finland. Most often, only one basal till bed is present forming a layer that seldom exceeds 3 m in thickness. General glacial transport was from NW-NWW to SE-SEE, but local variations exist. The southern part of the sampling area is triangular in shape, located roughly between the towns of Kiihtelysvaara, Ilomantsi and Värtsilä and delimited in the east by the Finnish-Russian International border. The southern area consists of sample series OMP-S (Appendix I). The northern sampling area extends from Nurmes in the south through Kuhmo to Lentiira in the north. The western extend reaches Valtimo and the eastern extent is along the Russian border. The northern sampling area consists of sample series OMP-N, VEP, IIV and LAA (Appendices II-V, respectively). The sampling lines in the southern area totalled 190 km and in the northern area 170 km. The entire program consisted of 501 sampling sites. 3 METHODOLOGY Samples were collected along the local and logging road network by excavator with a maximum depth of ca. 5 m. Sampling site spacing was typically 0.5 to 1 km along lines perpendicular to the local main ice flow direction. The number of samples taken from each excavator pit was dependent on the thickness of the basal till unit. The first sample (so called PIN) was taken from the uppermost layer of the basal till, i.e., the C-horizon usually at ~1 m depth, and the following in 1–2 m intervals (so called KES samples). The last sample was taken from just above the bedrock (POH) or, if the bedrock was not reached, from the deepest part of the pit (KES). Information for each pit was recorded on a modified GTK Quaternary sample card. Glacial striations were recorded on outcrops wherever discernible. A constant 45 kg of wet screened –1.0 mm till was processed and all processing and analytical work was done following standardized laboratory protocols at the GTK Research Laboratory. Preconcentrates were prepared using a GTK modified 3"Knelson Concentrator (Chernet et al., 1999). The concentrator includes an attached semi-automatic pulp feeder that enables near quantitative extraction of moderately heavy minerals from till samples. The process is regularly moni-
Lehtonen et al. Results from a reconnaissance scale heavy mineral survey of kimberlitic indicator minerals… tored by processing of blank till samples each spiked with 0.25–0.5 mm fraction Cr-pyrope grains (Lehtonen and Marmo, 2002). The preconcentration process reduced the till samples to 700–1000 g. Final heavy mineral concentrates were liberated by heavy medium separation (HMS) using methylene iodine (d =3.3 gcm -3 ). A dry low intensity drum magnetic separator (LIMS) was applied to the HMS concentrates to remove magnetite. Finally, the 0.25–1.0 mm size indicator grains were hand picked under binocular microscope from the non-magnetic fraction. In order to confirm optical identification, all indicator grains were analyzed using an EDS equipped Jeol JSM-5900LV scanning electron microscope. Quantitative mineral compositions from selected indicator grains were determined by Cameca Camebax SX50 and SX100 electron microprobes. For garnet, ilmenite and chromite analyses, an acceleration voltage of 25 kV, probe current of 48 nA (or 55 nA) and beam diameter of 1 µm were applied. The parameters for clinopyroxene analyses were 15 kV, 30 nA and 5 µm, respectively. A simplified schematic flow sheet for sample processing is presented in Figure 2. >1 mm Optical microscopy or stored Overflow stored d < 3.3 stored Magnetic fraction stored
Page 1 and 2: M41.2/2009/55 22.10.2009 Results fr
Page 3 and 4: Lehtonen et al. Results from a reco
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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
Page 39 and 40: 7120000 7110000 7100000 7090000 708
Page 45 and 46: Cr 2O 3 wt-% 4 3 2 1 VEP cpx Grt-pe
Page 47 and 48: Cr 2O 3 wt % 75 70 65 60 55 50 45 4
Page 49 and 50: %U %U %U %U %U %U %U %U %U %U %U %U
Page 51 and 52: %U %U %U %U %U %U %U %U %U %U %U %U
Page 55 and 56: Cr 2O 3 wt% 14 13 12 11 10 9 8 7 6
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Cr 2O 3 wt% 70 65 60 55 50 45 40 35
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%U %U %U %U %U %U %U %U %U %U %U %U
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%U %U %U %U %U %U %U %U %U %U %U %U
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Lehtonen et al. Results from a reco
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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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