Gold exploration using heavy minerals in till and weathered bedrock in Petajaselka, northern Finland

As a part of a gold exploration project in Petajaselka, northern Finland, carried out by the Geological Survey of Finland, heavy mineral studies and detailed till geochemical and stratigraphical studies were used for mineral exploration. These studies covered c. 22 km(2) in the Petajaselka area. The objective of this research was to find out how heavy minerals and till geochemistry can be used in gold exploration, and how they reflect the glacial dispersion in shallow till areas in Petajaselka. Furthermore, an automated mineralogical identification method (SEM-EDS) was tested in the indicator mineral research. Samples from till and underlying, pre-glacially weathered bedrock were collected from the test pits and trenches. A total of 381 heavy mineral and 713 geochemical samples were collected. The heavy mineral samples were concentrated with a spiral concentrator and a Knelson concentrator. The geochemical samples were analysed by ICP-AES and GF-AAS following aqua regia decomposition. The elevated gold contents and distinct anomalies found in till and weathered bedrock indicate the whole study area to have potential for gold mineralization. Based on microscopic and automated mineralogical studies, native gold was the only useful indicator mineral present in the heavy mineral samples for gold exploration in Petajaselka. The detailed till geochemical results show that the glacial transport distance in Petajaselka is short, only some tens of meters. Differences in the Au-Ag contents of the gold grains were noted between the three gold mineralized targets in Kerolaki, Selka-Mantypaa and Ranta-Mantypaa. Arsenic, bismuth and tellurium proved to be pathfinder elements for gold mineralization both in till and weathered bedrock. The results of this study show that the heavy mineral studies, together with till geochemical studies, provide a useful method for identifying exploration targets in glaciated terrain. However, different methods of heavy mineral concentration affect the recovery of gold grains and thereby the final results of this study.