Kontoniemi, Olavi & Nurmi, Pekka A. (eds.) 1998. Geological setting and characteristics of tonalite-hosted Paleoproterozoic gold deposit at Osikonmäki, Rantasalmi, southeastern Finland. Geological Survey of Finland, Special Paper 25. 119 pages. 53 figures, 27 tables and 5 appendices.
Abstract
The results of the Osikonmäki gold exploration project at Rantasalmi, SE Finland, carried out by the Geological Survey of Finland during 1986-1991 are reported in this Special Paper. Seven articles cover the following topics: geology of the Osikonmäki area and the granitoid host rock, geological setting, characteristics and fluid inclusion data of the gold deposit, and beneficiation properties of the ore.
The epigenetic Paleoproterozoic Osikonmäki gold deposit, is hosted by the synorogenic (1887+5 Ma) Osikonmäki tonalite pluton. The deposit exhibits a strong structural control, being closely associated with E-W trending shear zones that represent oblique ductile second order structures within the NW-trending crustal scale Raahe - Ladoga deformation zone. The granitoid is clearly I-type, calc-alkaline to subalkaline, metaluminous to marginally peraluminous, and is similar to calc-alkaline granitoids in modern collision zones.
The most common sulfide minerals associated with the ore are pyrrhotite, arsenopyrite, löllingite and chalcopyrite, which typically occur as irregular to somewhat banded disseminations. Gold and electrum, together with a number of Bi-Te-Se minerals, occur both as inclusions and at grain boundaries within and between arsenopyrite and silicate grains. Ore mineral parageneses indicate that gold mineralization took place under metamorphic conditions of amphibolite facies following the intrusion of synorogenic granitoids, but nevertheless prior to the peak of regional metamorphism. Fluid inclusion studies indicate that the main stage of mineralization was followed by influx of aqueous fluids of variable salinity, at least locally in the eastern part of the deposit, which transported gold and associated elements to higher structural levels during continued deformation within the shear zone.
Relative to the tonalite, As is the most enriched element in the deposit followed by Au, Se, S, Bi, Ag, Te, Sb, Cu, Pb, and Mo in decreasing order of enrichment. High concentrations of As, Au, and Se are characteristic of the Osikonmäki gold deposit. Gold is geochemically associated with Bi and Te whereas As-S-Se and Cu-Ag are distinct geochemical associations. The associations are interpreted as representing a temporal continuum of spatially overlapping episodes of precipitation.
In conventional bulk-flotation, gold recovery was only 70%, and the arsenic content in the bulk concentrate was 10-25%. Depending on the grinding fineness, the maximum recovery of gold by cyanide and bromide (GeobromTM3400A) leaching was 75-80%. Pressure oxidation increased gold recovery from the cyanide leaching by 10-25%. In kinetic tests, maximum gold recovery was achieved in only 8 hours by bromination as compared to the cyanidation which required nearly 24 hours to complete.
The publication provides new and detailed information on Osikonmäki tonalite and Osikonmäki gold deposit. Geological setting, mineralogy and geochemistry of this mainly premetamorphic deposit resemble other Paleoproterozoic (2000-1750 Ma) Svecofennian gold deposits.
Key words (GeoRef Thesaurus, AGI): gold ores, tonalite, mineralogy, geochemistry, fluid inclusions, beneficiation, Proterozoic, Paleoproterozoic, Osikonmäki, Rantasalmi, Finland
Olavi Kontoniemi
Geological Survey of Finland,
P.O. Box 1237, FIN-70211 Kuopio, Finland
E-mail: Olavi.Kontoniemi@gsf.fi
Pekka Nurmi
Geological Survey of Finland,
P. O. Box 96, FIN-02151 Espoo, Finland
E-mail: Pekka.Nurmi@gsf.fi
ISBN 951-690-704-0
ISSN 0782-8535
Price: FIM 135 (incl. 8% VAT)
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