Mafic layered intrusions in northern Finland

Mutanen, T. 1997. Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara layered complex, northern Finland. Geological Survey of Finland, Bulletin 395, 233 pages, 88 figures, 11 tables and 5 appended maps.

Abstract

The Akanvaara and Koitelainen intrusions, ca 2440 Ma old, represent a group of cratonic mafic layered intrusions (aged from 2500 to 2440 Ma) of the Fennoscandian (Baltic) Shield. These intrusions, which show all the standard types of igneous layering, host deposits of chromite, vanadium, titanium, platinum-group elements (PGE) and gold.

The cumulate stratigraphy in Akanvaara and Koitelainen reflects both normal tholeiitic fractionation and superimposed intermittent selective and wholesale contamination by anatectic salic melt (now granophyre) from the floor and the roof, and by refractory residual phases after anatexis of the roof rocks. Cumulus phases separated mainly in a hybrid melt between the mafic main magma and the overlying buoyant acid melt. The contaminants are found in cumulates as fossil melt and magma inclusions in olivine, chromite, orthopyroxene and plagioclase, and as "granitic" intercumulus material. The composition of the intercumulus material in the most heavily contaminated cumulates deviates strongly from what one would expect of the mafic main magma. Unusual minerals (loveringite, chlorapatite. zircon, zirconolite, baddeleyite, thorite, allanite, galena, perrierite, a Ti-Th-REE-P silicate) are common in the intercumulus of the ultramafic cumulates; ilmenite, chlorapatite and zirconolite occur as daughter and occluded minerals in melt and magma inclusions in olivines and orthopyroxene. Chlorapatite, ilmenite and loveringite are the most common as Doppelgänger phases (Mutanen, 1992), of which only the apatite phase appears (as a fluorapatite) appears as a late cumulus mineral.

The melting occurred both early and late in the crystallization history, corresponding to the water-saturated and dry melting of the granitic constituents of the surrounding crustal rocks. There is no evidence for, nor need of, multiple magma pulses; the reversals can readily be explained by contaminants transmitted by two-phase convection. Contamination of the magma by crustal material shifted the liquidus phase boundaries, resulting in excessive separation first of olivine and, subsequently of orthopyroxene. Refractory aluminous phases after partial melting of pelitic roof rocks were responsible for the anorthositic cumulates.

Chromitite layers occur from near the base of the intrusions to the 86 PCS level. Almost all the chromitites have elevated concentrations of PGE. The gangue is generally rich in biotite-phlogopite. All chromites are very low in MgO (from 0.0% to 1.2%). These features imply that chromite crystallized and was equilibrated in a melt poor in magnesium and rich in potassium. It is suggested that the chromium of the uppermost chromitites (UC layers) was exotic: it was liberated from melted high-aluminous schists averaging 500 ppm Cr. Exotic chromium may well have contributed to the genesis of some other anorthosite-associated chromitites, too (e.g., Fiskenaesset, Sittampundi, Soutpansberg, Bushveld, Stillwater).

In the Akanvaara and Koitelainen intrusions the PGE show little respect for sulphides, and are typically associated with oxides (chromite, ilmenomagnetite, ilmenite). Evidently the PGM phases crystallized from the silicate liquid direct.

The Keivitsa intrusion (age 2057 Ma) is located south of the Koitelainen intrusion. Isotopic (Sm-Nd, S-isotopes, Re-Os), geochemical (Ni/Co, Ni/S, PGE/S, S/Se, S/Te, Re/PGE, among others) and mineralogical data all point to significant exotic contamination by surrounding carbonaceous, sulphide-rich sediments and by solid refractory debris from disaggregated komatiitic rocks. The addition of exotic sulphur (as anatectic immiscible sulphide melt) and olivine debris was decisive for the genesis of the low-grade Keivitsansarvi Cu-Ni-PGE-Au deposit, which is located far above the basal contact of the intrusion. Other contaminants acquired from surrounding rocks were water, reduced carbonaceous material and chlorine, all of which affected the crystallization of the magma. Mineralogical evidence from Keivitsa and from other PGE-bearing magmatic sulphide deposits together with our experimental work (Mutanen et al., 1996) suggest strongly that immiscible sulphide melt either acted as a phase boundary collector for platinum-group mineral (PGM) particles already crystallized from melt or that the supersaturated PGM nucleated on sulphide droplets.

Key words (GeoRef Thesaurus, AGI): layered intrusions, contamination, degassing, petrology, chlorapatite, Cl, ultramafics, chromitite, gabbros, geochemistry, copper ores, vanadium ores, nickel ores, platinum ores, gold ores, Proterozoic, Finland, Lappi Province, Savukoski, Akanvaara, Sodankylä, Koitelainen, Keivitsa, Keivitsansarvi, Satovaara.

Tapani Mutanen
Geological Survey of Finland
P.O. Box 77
FIN-96101 ROVANIEMI, FINLAND

ISBN 951-690-652-4
ISSN 0367-522X

Price: 392 FIM (incl. VAT)

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