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Magma-country rock interaction and the genesis of Ni-Cu deposits in the Vammala Nickel Belt, SW Finland

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Summary

Ultramafic cumulate bodies in the Vammala Nickel Belt, some of which host small magmatic Ni-Cu sulphide deposits, are remnants of synorogenic intrusions that were emplaced into the early Proterozoic Svecofennian arc terrane and became progressively boudinaged by continuing tectonic movements.

Mineral and sulphide compositions of mineralised Svecofennian intrusions require that sedimentary sulphides (0.5 wt.% Zn; Se/S ≈ 100 x 10−6) played an important role in ore genesis. It is proposed that when ascending magmas encountered sedimentary formations containing abundant sulphidic black schists, they assimilated external sulphur, which led to the formation of an immiscible sulphide phase in the magma. The high Zn contents of interstitial sulphides (280-450 ppm) and liquidus chrome spinels (0.7–1.0 wt.%) indicate that the parental magma contained much more Zn than conventional assimilation processes would allow. Probably, S and Zn were selectively transferred by C-O-H-S fluids from the black schists into the cooling magma. Desulphidisation (involving conversion of pyrite to pyrrhotite) in the country rocks was driven by thermal energy provided by both the intrusions themselves and the concomitant regional metamorphism. Magma-country rock interaction during ore genesis is also indicated by the presence of minor phases such as graphite, ZnS, PbS, MoS2, an unknown Re-Mo-Cu-Os sulphide and numerous tellurides among the Fe-Ni-Cu sulphides.

During the peak of regional metamorphism small felsic dykes intruded the cumulate bodies and remobilised the interstitial Fe-Ni-Cu sulphides into thin massive ore veins. Compared to interstitial ore, vein sulphides are depleted in Cu, Se, and Zn. Some Fe-Ni-Cu sulphides also migrated, probably due to regional strain effects, into the country rocks and mixed with sedimentary sulphides.

Those magmas that formed unmineralised intrusions had already intruded sulphidic black schists and assimilated external S and Zn prior to final emplacement, and had thus become depleted in chalcophile elements and Zn by the segregation of sulphides and chrome spinel, respectively.

Zusammenfassung

Die Genese der Ni-Cu Lagerstätte Vammala, SW Finnland Die ultramafischen Kumulatkorper im Vammala Nickel Belt sind Reste einer synorogenen Intrusion, die innerhalb des friihproterozoischen svecofennischen Inselbogens Platz genommen hat and wahrend fortschreitender Tektonik boudiniert worden ist. Einige dieser Körper führen kleine magmatische Ni-Cu-Sulfid-Vererzungen.

Die Mineralzusammensetzung dieser mineralisierten svecofennischen Intrusionen zeigen, daß sedimentare Sulfide (0.5 Gew.% Zn; Se/S 100 x 10 - 6) eine wichtige Rolle bei der Erzgenese gespielt haben. Es wird vermutet, daß die Magmen durch sedimentare Formationen, reich an sulfidischen Schwarzschiefern, aufgestiegen sind, und dabei externen Schwefel assimiliert haben. Dies fiihrte zur Entmischung einer SulfidPhase im Magma. Die hohen Zn-Gehalte der disseminierten Sulfide (280-450 ppm) und der Gehalt an liquidus Chromspinell (0.7–1.0 Gew.%) deuten an, daß der Zn-Gehalt im Stammagma weit höher gewesen sein muß, als es konventionelle Prozesse der Assimilation zulassen. Möglichweise wurden S and Zn selektiv durch C-O-H-S Fluide aus den Schwarzschiefern in das abkühlende Magma eingebracht. Die Desulfidisierung (mit Umwandlung von Pyrit zu Pyrrhotin) im Nebengestein wurde durch thermische Energie angetrieben, die sowohl von den Intrusionen selbst wie auch von der gleichzeitigen Regionalmetamorphose stammt. Die Magma-Nebengestein-Interaktion wahrend der Erzgenese ist auch durch untergeordnete Mineralphasen wie Graphit, ZnS, PbS, MoS2, einem unbekannten Re-Mo-Cu-Os Sulfid and einigen Telluriden, zusammen mit den Fe-Ni-Cu Sulfiden, dokumentiert

Während des Höhepunktes der Regionalmetamorphose intrudierten dünne felsische Gänge die Kumulatkorper, wobei es zu einer Remobilisierung der disseminierten Fe-Ni Sulfide in dunne, massive Erzgänge gekommen ist. Verglichen mit den disseminier ten Erzen sind die Gang-Sulfide an Cu, Se and Zn verarmt. Einige Fe-Ni-Cu Sulfide migrierten, wahrscheinlich aufgrund von regionalen Strain-Effekten, in die Nebengesteine und vermischten rich mit den sedimentären Sulfiden.

Jene Magmen, die nicht-mineralisierte Intrusionen gebildet haben, hatten schon vor der eigentlichen Platznahme die Schwarzschiefer intrudiert and externen S and Zn assimiliert; sic verloren durch die Segregation von Sulfiden and Chromspinell chalkophile Elemente and Zn.

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  1. Geological Survey of Finland, FIN-02150, Espoo, Finland

    P. Peltonen

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Peltonen, P. Magma-country rock interaction and the genesis of Ni-Cu deposits in the Vammala Nickel Belt, SW Finland. Mineralogy and Petrology 52, 1–24 (1995). https://doi.org/10.1007/BF01163124

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