Abstract
The Archean bedrock of the Ilomantsi area consists of narrow greenstone belts between dome-like granitoids. The main tectonic phases are isoclinal folding, dragging of isoclinal folds around subvertical axes and regional faulting. Conspicuous anomalies of As, Au, Mo and W were revealed in the course of regional geochemical mapping of till, which led to more detailed studies in Ilomantsi. The data presented in this study are based mainly on the results of 2960 till samples collected by light percussion drill at local scale (16 samples/km 2), but also on the results of regional (1 sample/4 km 2) and detailed (100–400 samples/km 2) sampling and on rock samples collected from outcrops. The analytical methods most commonly used were emission spectrometry, atomic absorption spectrophotometry and spectrometry. The results indicate that the lowest part of the basal till is dominated by local debris and that the geochemistry of the till reflects well the underlying bedrock. The highest Au values exist near the contacts of tonalite and supracrustal rocks. Arsenic follows intimately the directions of faulting. The anomalous contents of W and Mo are restricted to the southern part of the 15-km 2 Kuittila tonalite. Potassium, Na and Ca form conspicious anomalies in till, cutting both the supracrustal rocks and the granitoids. Local studies revealed in the Kuittila tonalite a notable network of Mo- and W-bearing quartz veins restricted to tensional zones of main fault directions, which are cut by gold-bearing shear zones and quartz vein nets. These shear zones are of en echelon type and they are silicified, sericitized, tournalinized, Na-depleted and partly carbonitized. The hosting tonalite is intensely biotitized. These metasomatic features of the bedrock can be correlated to the wider anomalies of K, Na and Ca in till. The implication is that significant migration of mobile elements has taken place in the bedrock of the study area and that, in places, gold has participated in these processes.
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