Background information for the excursion is available here (pdf, 14mb).
Utö is a geologically unique island, where the element Lithium was first discovered. During our visit we will see some of the typical rocks of Utö: the marbles, the volcanic rocks, and the greywackes with their structures. We will also see the old iron mine and the pegmatite that crosscut it. If time permits we will end up on one of the old tailing heaps at the mine, where we have the possibility to look for some of Utö's typical minerals.
Utö in the southern part of Stockholm Archipelago is, together with a few other islands, the remnants of a volcano active around 1900 million years ago (Ma). This volcano was typical for a setting where oceanic crust is sinking down in the interior of the Earth beneath another plate, much like the situation in Japan or Indonesia. The volcanos are explosive and produce thick deposits of volcanic debris. On Utö we find this kind of material as metavolcanic rocks in the center of the island.
The volcanism was infrequent and during quiet periods the volcano offered good habitats for photosyntesizing cyanobacteria. The bacteria also secreted carbonates, and on Utö we find this as limestone marble together with the volcanic rocks. On the flanks of the volcano, we find greywackes; sedimentary rocks of sand and mud, typically formed in deeper seas by erosional matter from land. On Utö some of these layers display beautifully preserved sedimentary structures.
Another type of rock on Utö are Banded Iron Formations (BIF). These deposits of interlayered silica and iron oxides formed by oxidation to insoluble ferric iron, either from solutions heated by the volcano, or precipitated directly from sea water.
Long after the volcano died, the continued movements of the tectonic plate led to the formation of a mountain chain at c. 1850 Ma. The rocks of Utö was folded, pressed down to depths of 20--30 km and heated to over 600°C;. This resulted in visible metamorphic alterations of the older rocks and metamorphic minerals such as garnet.
The last major geological event on Utö also makes Utö the most interesting. Just before 1800 Ma, dikes of a granitic pegmatite cut through the older rocks. Some of these pegmatites carried high quantities of lithium, cerium, tantalum and niobium. Lithium as an element was first discovered in material from Utö, and is present in a number of rare minerals such as petalite, lepidolite, spodumene, elbaite, and indigolite. What makes Utö unique in the World is that the lithium rich pegmatite also cut through the iron formation. This resulted in a peculiar natural laboratory with formation of many unusual and unique minerals, amongst other the lithium-bearing amphibole holmqvistit -- the landmark mineral of Utö.