GEOLOGICAL SETTING, HISTORY OF DISCOVERY AND EXPLORATION ECONOMICS OF PRECAMBRIAN GOLD OCCURRENCES IN FINLAND

The study covers 39 gold occurrences in Finland where gold is the only or most important ore metal. They occure as clusters in three geologically different environments: in eastern Finland 2.9-2.7 Ga Archaean greenstone belts, early Proterozoic (2.5-1.9 Ga) greenstone belts in northern Finland, and the 2.0-1.75 Ga Svecokarelian complex. The occurrences in greenstone belts are characterized by mafic-ultramafic host rocks, greenschist-grade metamorphism, extensive alteration, and association with regional shear zones; they are thus analogous to mesothermal gold deposits in other shields. Svecokarelian gold mineralization usually occurs in the contact zones of tonalitic intrusions and are typified by amphibolite grade metamorphism, a vein-type mode of occurrence, small-scale alteration, lack of carbonatization, and the presence of arsenopyrite and chalcopyrite. Exploration for gold in the Svecokarelian areas has almost invariably resulted from the finding of mineralized boulders and exposures (70% of the occurrences were outcropping), whereas in the greenstone belts, it has mainly been based on indications provided by regional geochemical till surveys. Local till geochemistry has contributed more than any other method, since about 20% of occurrences were discovered with this method alone and over 50% in conjunction with other indications. Two occurrences were discovered by drilling geophysical anomalies, and Slingram or IP anomalies contributed to discovery in 20% of cases, magnetic anomalies in 12% and VLF anomalies in 5%. Fifteen percent of the occurrences were discovered on the basis of mineralized outcrops and 5% on the basis of boulders. One occurrence was found directly due to geological analogy. The total budget for gold exploration in Finland was about C$ 38 million in 1984-1988, during which time most of the occurrences were found. About 29% of the budget was allocated to local till geochemistry (100 000 samples analysed), 31% for drilling (103 km drilled), 31% for mapping, trenching and overall expenses and 9% for ground geophysics (650 km2 surveyed). The present study suggests that the gold potential is fairly high in Finland and that large areas remain underprospected. Considering the starting point of gold exploration and the results obtained so far, the abundant use of geochemical methods has been fully justified. In the future, however, successful exploration will require a higher priority for drilling.