Abstract

The metamorphosed Palaeoproterozoic Orivesi gold deposit in southern Finland is located within the Tampere Schist Belt, which belongs to the Svecofennian domain. The Orivesi mine, run by Outokumpu Mining Oy, was in production from 1994 to 2003, during which time a total of approximately 1.7 million tons of ore was extracted, with an Au content of 9.31 g/t, implying a total output 13.115 tons of gold in concentrate.

The hydrothermal alteration halo can be divided successively into chlorite-dominant, sericite-dominant and quartz-dominant rocks from the outer zone inwards. The host rocks of the ore are quartz rocks with andalusite-rich quartz rocks. Topaz-bearing rocks also occur in the inner part of the alteration halo. In addition to Au, the elements Ag, Te, Bi, Sb, S, As, Se, Cu, Zn, Pb, Sn and Mo are enriched to varying degrees within the alteration halo.

The main ore minerals include base metal sulphides, sulphosalts and tellurides. Pyrite is the most common sulphide. The sulphosalts are represented by tetrahedrite, bournonite, boulangerite and meneghinite. The most common gold, gold-silver and silver tellurides are calaverite, montbrayite, petzite, kostovite, sylvanite and hessite. Other known tellurides include tellurobismuthite, altaite, melonite, frohbergite, tsumoite, tetradymite and rucklidgeite.

Gold occurs mostly in fine-grained native grains containing an average of 5% Ag. The native gold is usually of very small grain size, generally < 20 µm. Most of the gold grains in the deposit occur as intergrowths with tellurides.

The adjacent hypabyssal intrusion is an obvious source of both hydrothermal fluids and metals. A comb quartz layering has been discovered in the transition zone between the intrusion and the alteration halo.

The Orivesi deposit is thought to belong to the high-sulphidation epithermal type. Soon after its formation the deposit encountered deformation and metamorphism that amounted to lower amphibolite facies conditions. The subsequent retrograde metamorphism caused the reappearance of some hydrothermal minerals typical of high-sulphidation epithermal deposits.