Metals push up through Archaean bedrock

Photo: Jari Väätäinen, GTK

Finland’s bedrock is of interest to geologists researching how metallic ores are formed in the Earth’s crust. Theoretical and empirical models of mineral deposit formation emphasise the importance of fractures (faults) in the rocks, particularly the border areas around Archean bedrock. Finland is one of the countries where such areas are the focus of study.

The more geologists understand the complicated processes underway in the Earth’s interior, the better they can construct models of how magma moves through the Earth’s crust and how different mineral deposits are formed. Based on these models, geologists can evaluate the potential of a region to host certain types of mineral deposits.

“Within geology, analytical techniques are improving all the time, and we are gaining access to more exact, reliable data. Over the last ten years, we have also acquired technology to measure the thickness of the bedrock, making it easier to understand the geological structure of the Earth,” comments Wolfgang Maier, PhD, from the University of Western Australia in Perth.

Maier is a specialist in the formation of and exploration for magmatic mineral deposits . This autumn, he was a guest lecturer for a month at the Geological Survey of Finland and participated in a project to construct a geological model of how metallic ores develop in the Fennoscandian Archaean rock. The scientists are attempting to use the theoretical model to link the appearance of various types of ore with different phases of the development of the Earth’s crust.

“My area of research is to try and understand the development of so-called mafic and ultramafic magmatic rocks, dark rocks that tend to be particularly common in ‘greenstone belts’. These rocks often host deposits of nickel and metals from the platinum group,” says Maier. He has primarily conducted research in various areas of southern and central Africa. Over the last few years, he has also investigated similar areas in Canada, Australia, India and Finland.

“I am particularly interested in applied research, that is understanding how ore deposits are formed. But firstly, we need to theoretically work out the basic principles of how quite ordinary magmatic rocks, such as granite or basalt, are formed. We don’t even know that, and the processes of how minerals and ores form are even more complicated,” says Maier.

Finland has a high potential for platinum metals

Dr Maier confirms that geologists do not yet understand where in the Earth’s mantle platinum metals are located and what chemical compounds they form. The most useful information has been gained by investigating fragments of the mantle found in certain eruptive rocks such as kimberlites or basalts. Partly based on this information, geologists now understand that particularly large magmatic events have an enhanced potential to form nickel and platinum group element deposits. The magmas also need deep fractures to ascend to the surface of the Earth.

“Nickel deposits are particularly found in the border zones between very old, stable Archaean rocks – forming ‘cratons’ – and the surrounding younger, thinner parts of the Earth’s crust. These border zones cannot usually be seen on the surface, and thus, modern seismic technology that can measure, the thickness of the Archaean rock is required.

The cratons are the oldest, innermost parts of the continental plates in the Earth’s crust, blocks that survived the fusions and divisions of the continents during several billions of years. A craton is very thick: the Archaean rock can stretch down to depths of 200 kilometres. The Fennoscandian shield contains several cratonic blocks, namely Karelian and Kola cratons, in the east and northeast of the country.

“Most of Europe consists of relatively young bedrock with no nickel or platinum metal deposits. However, a belt from northern Norway and Sweden across Finland to the Kola Peninsula, Russian Karelia and down to the Ukraine contains very old cratons. That is why these areas offer the most promising areas in Europe for deposits and prospecting of platinum metals and nickel,” confirms Maier. There are of course also similarly prospective areas elsewhere in the world, e.g. in Siberia, southern Africa and northern Canada, but they are more difficult to prospect in practical terms.

Platinum belongs to the same group of metals as palladium, rhodium, iridium and ruthenium. The demand for platinum and palladium, in particular, on the global market is increasing, as they are used as catalytic converters in vehicles and in the chemical industry, as well as within the electronics industry.

Eighty per cent of the global production of platinum comes from South Africa. Palladium is almost entirely produced by Russia and South Africa. If you look at the prospecting potential for platinum metals, Finland is among the five most interesting countries. Besides Russia and South Africa, the others are Zimbabwe and Canada. Platinum is not currently mined in Finland, but deposits at Suhanko, Portimo, and Penikat, south of Rovaniemi, are being thoroughly investigated.

TEXT: HARRIET ÖSTER