Isomertieite and other platinum-group minerals from the Konttijaervi layered mafic intrusion, northern Finland

Outokumpu Oy, Espoo, Finland

Early Proterozoic layered mafic intrusions, 2,450 m.y. in age, occur in a zone several hundred kilometers long that trends west-east across northern Finland. They contain Cr and Fe-Ti-V deposits as well as subeconomic, but often persistent, Ni-Cu sulfide occurrences with frequently significant platinum-group element abundances. Pyrrhotite, pentlandite, chalcopyrite + or - pyrite are the predominant sulfides. The intensely metamorphosed Konttijrvi body is one of the layered mafic intrusions. It can be subdivided into three major units: the heterogeneous basal metagabbro, the overlying uralitized and chloritized ultramafic rocks, and an upper metagabbro which is the largest unit of the body. The sulfide mineralization is concentrated as medium or low-grade disseminations in the basal metagabbro and the overlying ultramafic rocks. The upper metagabbro, on the other hand, is almost sulfide free. The predominant sulfide assemblage in the basal metagabbro is pyrrhotite-chalcopyrite-pentlandite + or - pyrite. It grades upward from a pyrrhotite-rich to a more chalcopyrite and pentlandite-predominant association. In the upper portion of the lower mafic unit and in the ultramafic unit, an iron-poor sulfide assemblage, bornite-chalcopyrite-millerite-pentlandite-pyrite-violarite, is encountered as a low-grade dissemination. The abundances of Pd and Pt, and of Cu and Ni in the sulfide phase, increase gradually upward from the basal contact of the lower metagabbro to the upper part of the ultramafic unit; the maximum content of Pd is 3.7 ppm and that of Pt 1.2 ppm, the average Pd to Pt ratio being 2.9. The platinum-group elements, in the upper metagabbro are below the detection limit of 0.02 ppm.As in the other intrusions in the belt, the main carrier of platinum is sperrylite; in addition, a few grains of vysotskite have been encountered. The mineralogy of palladium, which is predominantly controlled by the system Pd-As-Sb, in which Cu sometimes substitutes for Pd, is more complicated. The corresponding mineral species are isomertieite--identified by microprobe and XRD--mertieite II, palladoarsenide, and arsenopalladinite or an unnamed Pd ₅ As ₂ mineral--recognized by microprobe and optical properties--and an unidentified phase close to the composition of Pd ₇ As ₂ . In chemical and optical properties the palladium nickel arsenide encountered resembles the mineral species majakite. Sporadic Pd bismuthotellurides have also been observed. In composition they all lie near the tie line Pd ₇ Bi ₈ -Pd ₇ Te ₈ , of which no species have been reported so far. Unidentified oxygen-bearing coatings, possibly palladium arsenates, occur as oxidation products on arsenopalladinite crystals.

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