Results from a reconnaissance scale heavy mineral ... - arkisto.gsf.fi

Lehtonen et al.
Results from a reconnaissance scale heavy mineral survey of kimberlitic indicator minerals…
A common diamond exploration method in recently glaciated terrains, and one employed heavily
in the Fennoscandian Shield, is to track kimberlitic indicator mineral grains (KIM) dispersed in
Quaternary till and stream sediments. This prospecting method has been applied worldwide especially
during reconnaissance and regional stages of exploration programs. The classic KIM
suite used in diamond exploration includes Cr-pyrope, Ti-pyrope and eclogitic garnet, Crdiopside,
Mg-ilmenite, and chromite. Exploration based on KIM's requires understanding of the
glacial history in the region as well as sensitive processing methods, as the indicator contents in
glacial sediments can, for various reasons, be extremely low even relatively close to the kimberlite.
Major portions of the diamond prospective area in Finland (Fig. 1) has been subjected to reconnaissance
scale KIM surveys, initially by Malmikaivos-Ashton during the mid 80's to mid 90's
(Tyni, 1997) followed by several junior companies after 1994 when Finland became first a member
of the EEA and then the EU. The earlier survey work resulted in the discovery of approximately
25 kimberlitic bodies in eastern Finland, mostly in the Kaavi-Kuopio area. The more recent
discoveries of kimberlites, in Kuhmo-Lentiira and Kuusamo have also been dependent upon
or facilitated by Quaternary till sampling programs.
GTK’s role in diamond exploration is to provide services mainly to junior companies in terms of
sampling, laboratory work, geophysical surveys and expert consultancy. GTK has, for instance,
developed a KIM processing system by modifying and augmenting 3" and 4.5" Knelson Concentrators
that accomplishes a nearly complete recovery of moderately heavy minerals (>0.25 mm)
from till (Chernet et al., 1999). GTK does not explore for diamonds, but to raise interest in new
prospective areas, it has carried out limited reconnaissance scale heavy mineral surveys in areas
where companies have not been recently active. Initially, a case study was carried out in eastern
Finland to test the methodology for KIM work (Lehtonen & Marmo, 2002), and two eastern
Finland kimberlites have been selected as targets of detailed KIM surveys (Lehtonen et al.,
2005). These case studies provide key information for diamond explorers in Fennoscandia.
In 2001- 2005 GTK conducted a reconnaissance scale heavy mineral survey in till in a diamond
prospective area in eastern Finland. To improve the quality of the GTK survey the sampling was
carried out using excavators in contrast to earlier sampling programs that were restricted to hand
shoveling.
Results from this survey are presented in Appendices I-V, separately for each of the five sample
series. The results include maps over the sampling areas, coordinates and KIM contents of the
sampling sites, and, electron microprobe analyses of KIM's presented in compositional classification
diagrams. The original microanalytical data is given in Appendix VI. KIM's were recovered
throughout the survey area; however, the number of positive samples was distinctly higher
in the southern area. The number of KIM's per sampling site is generally low, from 0 to 6 grains,
which, based on long term experience, is a typical result in reconnaissance scale sampling in
Finland.