Graptolitic argillite

Although the name dictyonema argillite is widely used instead of graptolitic argillite, this name is now considered a misnomer as the graptolite fossils in the rock, earlier considered dictyonemids, were reclassified during the 1980s as members of the genus Rhabdinopora.

In mainland Estonia, it occurs at the foot of the North-Estonia Klint, ranging from the Pakri Peninsula to Narva in an area covering about 11,000 square kilometres (4,200 sq mi). When findings in the western Estonian islands are included, its extent increases to about 12,200 square kilometres (4,700 sq mi). The thickness of the layer varies from less than 0.5 metres (1 ft 8 in) to a maximum of 8 metres (26 ft) in western Estonia, as does its subsurface depth, which ranges from 10 to 90 metres (33 to 295 ft). It formed some 480 million years ago.

The composition of graptolitic argillite varies by location within Estonia. Its organic content ranges from 10 to 20% and its sulfur content is 2–4%. Correspondingly, its calorific value is 5–8 megajoules per kilogram (MJ/kg) and Fischer Assay oil yield is 3–5%. In northeastern Estonia its carbon-to-hydrogen atomic ratio is about nine. Graptolitic argillite from the northeastern region contains up to 9% pyrite (generally between 2.4 and 6%), heavy metals such as uranium (up to 1,200 parts per million (ppm) or 300 grammes per tonne (g/t)), molybdenum (up to 1,000 ppm or 600 g/t), vanadium (up to 1,600 ppm or 1,200 g/t), and nickel, and other minerals including K-feldspars, quartz, clay minerals, light-brown phosphatic ooids, and accessory amounts of zircon, tourmaline, garnet, rutile, chalcopyrite, and glauconite. Graptolitic argillite from the northwestern region contains fewer metals than are present in deposits in the northeast; it also contains more clay minerals. Rock in these deposits contains corundum, amphiboles, and kyanite.

Geological reserves of graptolitic argillite in Estonia have been estimated at 60–70 billion tonnes. Although the amount of graptolitic argillite exceeds that of kukersite, attempts to use it as an energy source have been unsuccessful due to its low calorific value and high content of sulfur. However, it contains potentially about 2.1 billion tonnes of oil. In addition, graptolitic argillite in Estonia contains 5.67 million tonnes of uranium which makes it one of the main potential future uranium sources in Europe. Graptolitic argillite in Estonia also contains 16.53 million tonnes of zinc and 12.76 million tonnes of molybdenum; however, there is not yet any environmentally friendly and economically feasible technology to extract those metals.

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