Indium is a critical metal for the world’s electronic industry due to its use in flat panel devices such as liquid crystal displays, plasma display panels and OLED displays. Thus, indium is used in televisions, mobile phones, laptops and notebooks. The current supply and demand has led to an increased interest for indium exploration globally. This thesis describes features of two different rapakivi-related In-rich mineralisations in southeastern Fennoscandia; polymetallic veins in Sarvlaxviken, Lovisa, southeastern Finland and skarn ore in Pitkäranta, Ladoga-Karelia, westernmost Russia.

The polymetallic veins in Sarvlaxviken occur in two rapakivi granite varieties and can be divided into five metal associations: Li-As-W-Zn-Mn, Pb-Zn and Cu-As-In in wiborgite bedrock and As-Sn-Cu and Mo-Bi-Be in even-grained granite. These veins form complex systems, with multiple vein generations. The first three associations are strongly controlled by NNW-trending structures and evolved in two main stages (generation 1 and 2). The two latter associations exist in alteration zones of the Marviken granite but have only been observed in glacier transported boulders and thus, the timing of these are uncertain (generation x). The fluid inclusion data indicate that the depositional conditions of the wiborgite hosted veins were almost identical in all generations. The chlorine and sulphur contents in the fluids affected the formation of metals. The F-rich ore forming fluids emanated most likely from the Marviken granite.

The skarn ore in the historic mining district of Pitkäranta is composed of four end member ore types, dominated by the metals Fe, Cu, Sn and Zn. However, several ore types exist in one and the same mine with gradational borders between each ore type, most likely reflecting variations in the depositional conditions. Some metals are, however, more common in certain areas and associated with certain trace elements, e.g. Ag, Bi, In and Li.

Indium is in both study areas commonly associated with Zn –rich ores with sphalerite as the main carrier. Yet, the indium mineral roquesite has been observed in both study areas. The presence of indium minerals can be predicted by calculating the ppm In / % Zn ratios. If the ppm In / % Zn ratio exceeds 50, it is likely that the sample contains indium minerals.

Over 2000 till samples were collected at 100-m or 20-m intervals for geochemical investigations in the Sarvlaxviken area. Numerous anomalous samples indicate several metal sources. The clear connection between anomalies and the already discovered polymetallic veins were not detected. These anomalies strongly suggest undiscovered veins under the soil cover.

This thesis provides important information about rapakivi-related In-rich mineralisations in the Fennoscandian Shield. It includes mineralogical, geochemical and fluid inclusion studies, as well as detailed mapping of the bedrock and the till anomaly patterns, and thus provides a tool for exploration of metals in the rapakivi areas.