The Koillismaa Intrusion, northeastern Finland – evidence for PGE reef forming processes in the layered series

Tiivistelmä

The 2440 Ma old Koillismaa Intrusion, located in northeastern Finland, forms the westernmost part of the Koillismaa-Näränkävaara Layered Igneous Complex. The intrusion originally formed an extensive sheet-like body that has been later tectonically dismembered into several blocks. The cumulate stratigraphy of the intrusion is divided into Marginal Series and Layered Series. The Marginal Series is up to 200 m thick and consists of gabbroic cumulates at the base that are overlain by progressively more magnesian rocks including pyroxenites and peridotites. The Layered Series is composed mostly of gabbroic cumulates. Its stratigraphic thickness in different intrusive blocks ranges from 1000 to 3000 m.

The Marginal Series includes contact-type base metal sulphide- and platinum group element mineralization, which contains up to 0.9 wt. % Cu, 1.1 wt. % Ni and 4.3 ppm Au+Pd+Pt. Palladium contents are significantly higher than Pt contents, resulting in a Pd/Pt ratio of 2.8. The average Cu/Ni ratio in the sulphide fraction of the Marginal Series is 1.3.

At present, only one PGE-Cu-Ni sulphide mineralized layer that is referred to as the Rometölväs Reef is known from the Layered Series. In the stratigraphic sequence of the Koillismaa Intrusion, this layer is marked by the appearance of cumulus clinopyroxene and inverted pigeonite and the disappearance of cumulus bronzite. The reef shows a close spatial relationship with small, 1-6 m thick, noncumulus-textured gabbronorite bodies. These bodies can be considered as essential part of the reef, because the highest PGE grades (up to 0.3 wt. % Cu and 0.3 wt. % Ni and 1 ppm of Au+Pd+Pt) are found in sulphide-bearing portion of one noncumulus-textured gabbronorite body. The mineralized cumulates surrounding these bodies typically have a mottled appearance due to small, up to 3 cm sized clusters of sulphide and low-temperature minerals that host a significant portion of the mineralization. The mottled cumulates of the reef contain up to 0.9 wt. % Cu, 0.3 wt. % Ni and 0.6 ppm of Au+Pd+Pt. The Pd/Pt ratios in both cumulates and noncumulate bodies are close to 1.0. The average Cu/Ni ratio in the sulphide fraction of the mottled cumulates is 4.1.

It is concluded that the origin of the mineralization of the Rometölväs Reef was triggered by emplacement of dykes of a new magma pulse into partially consolidated cumulates of the Koillismaa Intrusion. These dykes are presently represented by the noncumulus-textured gabbronorite bodies of the reef. The stratigraphic level of the Koillismaa Intrusion at which bronzite gave way to inverted pigeonite and augite was the most favorable place for these dykes. The intruding magma was more primitive than that of the parental magma of the Koillismaa Intrusion and was saturated with ore-bearing fluid phase escaped from the magma upon emplacement in the form of late magmatic fluids, introducing PGE-Cu-Ni into host cumulates and recrystallizing them into mottled varieties. Sudden loss of volatiles from the intruding magma resulted in its supersaturation and related rapid crystallization to form the noncumulus-textured gabbronorite bodies.

Keywords (GeoRef Therasaurus, AGI): layered intrusions, lithostratigraphy, igneous rocks, mineralogy, geochemistry, platinum group, copper, nickel, isotopes, Sm/Nd, genesis, magmatism, Paleoproterozoic, Koillismaa, Finland

Tuomo Karinen
Geological Survey of Finland, P.O. Box 77, FI-96101 Rovaniemi, Finland

E-mail: tuomo.karinen gtk.fi