Diamond Potential of Finland:
Thick lithospheric mantle, kimberlite/lamproite probes, and diamond indicator minerals; an example from Lentiira, eastern Finland.
Thick Lithospheric Mantle
By analogy with other shield areas, the potential exists for further and significant diamond discoveries in Finland. Furthermore, the late Archean terrain comprising much of eastern and northern Finland is contiguous with the Karelia and Kola–Kuloi cratons in Russia, which contain the major Archangelsk kimberlite province, at the easternmost edge of the shield, and diamond-bearing lamproites at Kostamuksha, near the Finnish border.
It is worth noting that the thick lithosphere area is not limited to the Archaean areas, but extends well into the Proterozoic terrain. However, for reasons not fully understood (e.g., preferential diamond formation in highly depleted, komatiite extracted?, chromite-bearing harzburgites and dunites) the other apparently critical factor for economic diamond deposits is an Archean age for the lithosphere to be prospective (Clifford's rule). A Proterzoic/Archean boundary is therefore demarcated in Fig. 1 and is based on Nd isotopic data where rocks to the NE of the line either are Archean or show evidence of an Archean component in their source region through negative ε Nd values. The area where all three of these factors overlap favorably is the area most prospective for diamond exploration in northern Europe.
In eastern Finland, kimberlite-hosted mantle peridotite xenoliths suggest that the lithosphere is at least 240 km (Kukkonen and Peltonen, 1999), one of the thickest localities known anywhere. Since diamonds are formed only at lithospheric depths exceeding 150 km (Kennedy and Kennedy, 1976) other diamondiferous pipes such as the Terskii kimberlites in the Kola, as well as at Kostamuksha (lamproite) and the Archangelsk pipes (Group I & II kimberlites; Mahotkin and Skinner, 1998) and Kemozero (kimberlite) attest to a very large region with anomalously thick lithosphere. Note that although much of Northern Russia is prospective using the above criteria, because of political and financial instability, the region within Russia is not highlighted as a preferred target area at this time.
Kimberlite and Lamproite Probes
As mentioned above, more than twenty-four intrusive bodies, mostly kimberlitic diatremes, but also including hypabyssal kimberlites, olivine lamproites and ultramafic lamprophyres have been discovered within the Archean Karelian Craton in Finland. Group I and II kimberlites and olivine lamproites are important because they are the only rock types known to contain economically significant amounts of diamonds worldwide.
The known kimberlites in Finland occur in two clusters at the edge of the Archean Craton, near the towns of Kuopio and Kaavi in northeastern Finland. Their mineralogy is typical of Group I kimberlite including abundant macrocrysts of olivine, picroilmenite, Cr-diopside, pyrope garnet; phenocrysts of olivine and microphenocrysts of monticellite, perovskite, kinoshitalite mica and spinel in a calcite + serpentine matrix. Notably phlogopite is not abundant, and there are no indications of Group II kimberlite (orangeite) affinities in these rocks. Major elements indicate that there is a crustal component in some of the samples even though only hypabyssal kimberlite material was used. Trace elements show that this assimilation has had mostly a dilutive effect and that the Finnish kimberlites in this study form a compositionally cohesive group of rocks more akin geochemically to the Koidu type kimberlites of West Africa than the Group I kimberlites from South Africa. Sr-Nd isotopic compositions either reflect derivation from a near bulk-earth mantle or represent assimilation and reaction of radiogenic Karelian Craton lithospheric mantle by kimberlite magmas that originally had Group I isotopic signatures (O'Brien and Tyni, 1999).
Farther into the Karelian Craton occur dikes of carbonate-rich ultramafic lamprophyre (aillikite) and a rock which appears to have both olivine lamproite and Group II kimberlite affinities. The latter contains most of the typomorphic minerals of lamproite including phlogopite, K-richterite, low-Al diopside and has a major element composition equivalent to average olivine lamproite. Despite this the rock has tetraferriphlogopite zoning profiles (Fig 2) and an accessory Ca-Zr silicate that differ from other reported lamproites and are in fact more typical of those seen in Group II kimberlites (orangeites).
Indicator Mineral Type Study – The Lentiira Area in Eastern Finland
Bedrock Geology – The Lentiira area is located in the western part of the Archean Karelian Province of the Fennoscandian Shield (Fig. 3). The province is composed of Archean, locally strongly deformed, granitoid-greenstone associations although metamorphic conditions rarely exceed amphibolite facies. U-Pb zircon ages show that the peak magmatic activity in the Karelian Province was between 2800–2650 Ma although zircon ages slightly over 3000 Ma have also been obtained in many places (Luukkonen, 1992).
The Lentiira area occurs between the Kuhmo and Kostamuksha greenstone belts in an area characterized by TTG migmatites but contain a minor component of tholeiitic volcanics and rare examples of strongly deformed serpentinites enclosed in amphibolites. The latter could represent fragmented and boudinaged relicts of komatiitic flows possibly related to the major ca. 2785 Ma komatiitc volcanic pulse in the Kuhmo and Suomussalmi greenstone belts (Luukkonen, 1992).
Quaternary Geology – The study area is characterized by sandy till deposited during the drift of a glacier flowing from direction 280–290º during the youngest glaciation (Weichsel). In some places a 0.5–1.0 m thick washed layer of sand or gravel overlies the till. The till itself ranges in thickness from 1–3 m in topographic highs but may be thicker in swampy areas. Based on grain size analysis sandy till is the correct classification with fine grained material (<0.06 mm) and clay fractions (<0.002 mm) representing 20% and 1–2%, respectively. Based on Quaternary interpretation, transport distances of the basal till were short, less than 1 km, but material in drumlins might have come from longer distances, 1–5 km.
Sampling – The GTK sampled the Kuhmo area, using 20 kg samples, as part of a regional heavy mineral survey in 1994, but those results were negative. In May 1997, twelve additional samples of 40 kg each (-20 mm material) were taken from surficial till and processed and picked for indicator minerals. Three of the samples contained indicators, including chromite, picroilmenite and orange garnet in the 0.5–2.0 mm size fraction. In October a further 39 sites were sampled using excavator to reach the deepest till horizons. These samples were processed using the same procedure and picked for all indicators in the 0.5–2.0 mm fraction, along with chrome pyrope and chrome diopside from the 0.25–0.5 mm fraction. These grains were then analyzed at the GTK Cameca SX-50 electron microprobe facility using standard procedures.
During till sampling two subvertical 335º striking olivine lamproite dikes about one meter wide were discovered, one less altered and exposed in a road cut (dike 1) and the other (dike 2) from gravel used in road construction some 20 years ago. Closer inspection of the gravel pit revealed a highly altered lamproite dike covered by ca. 1 m of till. Wall rock of dike 2 consists of chemically altered fragments of country rock, mostly diabase, coarse grained pyroxenite and Archean gneiss embedded in a clay-rich matrix.
Results – Kimberlitic indicator minerals were discovered in 15 of the basal till samples. The bulk of the indicators came from the fine-grained fraction, with only a few recovered from the coarser grain fraction. Two hundred kg of the dike 1 olivine lamproite were processed producing a concentrate extremely rich in chrome spinels of lherzolitic origin. One hundred kg of dike 2 was processed, and in addition to lherzolitic spinels, the sample produced chrome pyropes, chrome diopsides and a few chromites in the diamond inclusion field and one picroilmenite. Also a few kg sample of the interstitial clay of the dike 2 wall rock was processed and studied revealing the same heavy minerals as in the dike.
A CaO versus Cr2O3 plot of the chrome pyrope grains from till and dike 2 is presented in Fig. 4. The garnets from the dike produce a distinctive group from those in the till (lherzolite and two harzburgite/dunite grains, i.e. G10). In fact the dike garnets are compositionally indistinguishable from kimberlite megacryst compositions. Cr-poor Ti pyrope megacryst garnets and picroilmenite do not occur in olivine lamproites (Mitchell and Bergman, 1991) and are exceedingly rare in Group II kimberlites (e.g., Moore and Gurney, 1991). Further inspection reveals that one grain of megacryst pyrope composition was also found in till.
A MgO versus Cr2O3 plot for chromite grains shows that five grains dikes and one from till (Fig. 5) plot inside the diamond inclusion field but lherzolitic compositions by far predominate.
Conclusions – Kimberlitic and diamond indicator minerals have been found in eighteen till samples and lamproitic dikes in the Lentiira Area. The existence of olivine lamproites in the area is no surprising given the information on the Russian side of the border where there is an abundance of such dikes. Based on the indicator compositions from dike 2, it is unlikely that this dike is an olivine lamproite as we first thought and more logically belongs to the Group I kimberlite description. However, this inference is based solely on xenocrysts from dike 2 samples and should be backed up by petrologic and geochemical study of fresher dike material if such becomes available.
The sensitivity of the till sampling method is quite good and able to recover extremely low concentrations of diamond indicator minerals, one or two grains totally 0.01 to 0.02 mg per 80 kg of -20 mm sample (2 x 10-10 or 200 ppt).
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Last updated: 14.11.2014
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