Native gold and ore-forming fluid characteristics of the Rajalampi ore occurrence, Hautavaara structure, South Karelia

The results of the study of the native gold characteristics and fluid inclusions in quartz from the Rajalampi ore occurrence, South Karelia, are reported. The gold analyzed displays a relatively high grade (934 to 964 ‰, an average of 948 ‰), a particle size of 0.05 to 0.5 mm and a uniform golden-yellow colour. The intergrowths are dominated by bismuth minerals (tellurojoseite and tetradimite), suggesting precipitation of gold with high-temperature bismuth compounds. Thermocryometric data for ore occurrences in the central Hautavaara structure were obtained for the first time. Analysis of the fluid inclusions shows that the Rajalampi rocks were formed as a result of multi-stage fluid evolution: a transition from slightly saline aqueous fluid containing Mg chloride and carbon dioxide to chloride brine and then again to hydrous-carbon dioxide fluid was observed. The presence of three-phase inclusions with highly saline fluid - up to 40 wt. % eq NaCl in quartz from Rajalampi ore-bearing rocks can be interpreted like evidence of magmatic source of ore-bearing fluid existence and the consistency of Rajalampi Au-Bi mineralization to an intrusion related type deposits.