Haparanda Diamond Project

Geology of Diamonds

Before 1871, diamonds had only been mined from secondary deposits until it was discovered In South Africa, that primary diamonds are bound to kimberlitic pipes. Since then primary deposits have been found in nearly all old cratons. That led to a finding – called “Clifford’s rule” – which states that kimberlites or related rocks (lamproites/lamprophyres) of Archaean age are preferred for hosting diamonds. Following of this “rule” led to exploration rushes and discoveries in Botswana in the 1960’s, in Western Australia in the 1980’s and in Canada in the 1990’s.

An understanding of the conditions for diamond-formation gives the phase-diagram of elementary carbon in relation to the rise of temperature with depth (the geothermal gradient):

Fig.1 The geothermal gradient from Shirey &Shigley (2013)

According to this study, diamond is stable at pressures above 40000 atmospheres (4GPa) and temperatures between 950-1400°C and only the cratonic lithospheric keel is cold enough at high enough pressures to retain diamonds.

The 3D-Model in Fig.2 shows the spatial setting of diamond formation beneath Archean cratons within a peridotitic/eclogitic source rock. Kimberlites and similar rocks serve as a “diamond-elevator” – the breccia-structure with mainly olivine and dark mica (with xenoliths of peridotite and eclogite) testify the explosive eruption from a few 100km-depth in small pipes up to surface levels in very short time. This is the only mechanism that could prevent diamond from recalibrating back to graphite. So most of the diamond-occurrences are bound to nearly round kimberlitic-structures=pipes (100m-1km diameter), in Archean terranes with <40mW/m² heat flow and >200km lithospheric thickness.

Fig.2 Block diagram of the diamond-forming environments from Shirey &Shigley (2013)

Diamonds in Northern Europe

In Northern Europe the Archaean Karelian Craton is the only geological environment to fulfill these 3 main criteria for diamond potential. The overview map from Fig.3 shows that the prospective area is nearly the same size as the Slave Craton in Canada. And in fact some diamond-discoveries have been made in this region within the last decades, e.g. Kaavi/Kuopio-region (Finland) and Archangelsk (Russia) – but in relation to the other cratons on Earth the Karelian Craton is underexplored given its size and potential.

Fig.3 Overview Map of the Karelian craton. From the webpage of the Geological Survey of Finland (GTK)

In Sweden, the Karelian Craton includes only the very northeastern part of the country, at the boundary to Finland. Thereby, Goldore’s exploration permits are situated in an area of Archaean gneisses (>2.5 Ga) within Haparanda Municipality, just west of the border to Finland.

Fig.4 Geological Map of the permit areas
From the Swedish Geological Survey

The geological map in Fig.4 shows that the permit is situated in a relatively undifferentiated anticlinal structure. The flat surface is characterized by till-cover and peat bogs and is almost devoid of outcrops – a situation that is not uncommon in Archaean terranes hosting kimberlite-structures. Geochemical data which could give a reference to indicator minerals (e.g. Cr-diopside or pyrope garnet) is lacking so far.

But kimberlitic rocks are highly magnetic and a look on the Airborne Magnetic Map from the Geological Survey of Sweden (SGU) shows that there are a number of distinct, small and round, magnetic anomalies within the permit areas (Fig.5). These could possibly be caused by kimberlite. The nearest known kimberlite outcrops are small dykes in the environmentally protected Kalix Archipelago, 25km to the southwest.

The more extensive magnetic anomalies north of the permit areas are caused by mafic and ultramafic rocks according to the geological map.

So – all geological conditions for finding diamond are present within the permit area. Further exploration work has to rely heavily on geophysics (ground magnetic measurements) and geochemistry (moraine and stream sediment sampling).

Fig.5 Airborne Magnetic Map of the main permit area

Conclusions

The target of Goldore is simply to find the first Swedish diamond of quality. We intend to start that work with ground magnetic measurements, boulder tracing, and sampling of creeks for indicator minerals.

Hans Selbach

Dipl. Geolog

Literature References

Geological Survey of Finland Website: Diamond in Finland, http://en.gtk.fi/informationservices/commodities/diamond.html

Kresten,P. (1976): Chrome pyrope from the Alnö complex, GFF Vol.98 pp179-180,Stockholm

Kresten,P. & Holmquist,A. (1981): Diamanter I Sverige, SGU brap Nr.81585.

Kresten,P. & Nairis,H.J. (1982): Alnö diamonds, GFF Vol. 104, p.210, Stockholm

Larsson, J.O. (1997): Diamanter – en blivande bristvara? , Rapporter och meddelanden Nr.88, SGU Malå

Lilljequist,R. & Brundin,N.H. (1980): Förslag till program för prospektering efter diamanter i Sverige, SGU brap Nr.80009

Majka J. et al. (2014): Microdiamond discovered in the Seve Nappe (Scandinavian Caledonides) and its exhumation by the ”vacuum-cleaner” mechanism, Geology Vol.42, No.12

O’Brien,H. (2015): Kimberlite-hosted diamonds in Finland, Pages 345-375 in Mineral Deposits of Finland,Elsevier

Shirey, Steven B. & Shigley, James E. (2013): Recent Advances in Understanding the Geology of Diamonds, Gems&Gemology, Vol.49, No.4.

Stachel T. & Harris J.W. (2008): The origin of cratonic diamonds – constraints from mineral inclusions, Ore Geology Reviews Vol.34, Issues 1-2