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Metamorphism of Ni-Cu Sulfides in Mafic-Ultramafic Intrusions: The Svecofennian Sääksjärvi Complex, Southern Finland

By
Franco Mancini
Franco Mancini
Mineral Resources Department, Geological Survey of Japan, 1-1-3 Higashi, 305 Tsukuba, Ibaraki, Japan
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Heikki Papunen
Heikki Papunen
Department of Geology and Mineralogy, University of Turku,Turku 20014, Finland
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Published:
January 01, 1998

Abstract

The Sääksjärvi ultramafic-mafic complex is an example of a nickel-copper sulfide deposit in upper amphibolite to lower granulite facies metamorphic rocks. Prograde metamorphic events at 650° to 750°C and 4 to 6 kb have recrystallized the host rocks to olivine-magnesiohornblende-orthopyroxene ± chromite assemblages (metaperidotite) and to magnesiohornblende ± clinopyroxene ± plagioclase ± quartz assemblages (hornblendites and gabbros).

These assemblages were overprinted subsequently by retrogressive serpentinization. Three distinct metamorphic effects have modified the sulfides, namely recrystallization, solid-state flow during deformation, and low-temperature alteration, leading respectively to the following types of mineralization: (1) disseminated sulfides; (2) sulfide veins; and (3) sulfides in serpentinite. The first type has retained primary interstitial structures and has undergone multiple exsolutions during the slow retrogressive cooling. The second type originated by the injection of the ductile sulfides into fracture and shear zones, where mechanical mixing with the more brittle gangue silicates formed sulfide-matrix breccias.

The last type of mineralization originated from the alteration of the primary sulfides during serpentinization. Analyses of the compositions of the sulfides have led to a general model describing the metamorphic effects in such low-grade mineralization in which: (1) disseminated sulfides received some addition of nickel due to reequilibration with olivine; (2) the preferential remobilization of copper sulfides in veins and breccias increased the Cu/(Cu + Ni) ratios; and (3) the low-temperature alteration effectively redistributed the sulfide elements in serpentinites, leading to highly variable Cu/(Cu + Ni) values.

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Contents

Reviews in Economic Geology

Metamorphic and Metamorphogenic Ore Deposits

Frank M. Vokes
Frank M. Vokes
Volume Editor
Department of Geology and Mineral Resource Engineering Norwegian University of Science and Technology Trondheim, Norway
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Brian Marshall
Brian Marshall
Volume Editor
Department of Applied Geology University of Technology Sydney NSW 2007 Australia
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Paul G. Spry
Paul G. Spry
Volume Editor
Department of Geological and Atmospheric Sciences Iowa State University Ames, Iowa 50011 USA
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Society of Economic Geologists
Volume
11
ISBN electronic:
9781629490182
Publication date:
January 01, 1998

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