Noble Metal Contents in Ferropicritic Magmas and Associated Ni-Sulfide Deposits of the Pechenga Complex, Kola Peninsula

G. E. Brügmann Max-Planck-Institut für Chemie, Becherweg 25, Postfach 3060, 55020 Mainz, Germany


E. J. Hanski Geological Survey of Finland, P.O. Box 77, SF-Rovaniemi, Finland

A. J. Naldrett Department of Geology, University of Toronto, Toronto, M5S 3B1, Canada

V. F. Smolkin Geological Institute, Academy of Sciences, SU-184200 Apatity, Russia

The Proterozoic Pechenga Complex of the Kola Peninsula consist of 4 volcanic-sedimentary units. The sedimentary part of the youngest unit, the Pilgujärvi suite, hosts layered gabbro-wehrlite intrusions with Ni-sulfide deposits which have been exploited since there discovery in the 1920's. The intrusions represent the crystallisation product from FeO- and TiO2-rich ferropicritic parental magmas which also occur as massive flows, pillow lavas, differentiated flows and tuffs. Some petrographical and geochemical features, such as the occurrence of olivine and clinopyroxene spinifex-textures in the differentiated lava flows and the ultramafic composition of the parental ferropicritic magma (15-17 wt.% MgO), are similar to those observed in komatiites. However, the alkaline affinity, the high FeO content (>14 wt.%) and the strong enrichment in incompatible elements clearly distinguish ferropicrites from komatiites. For example, ferropicrites with 15 wt.% MgO contain 2.0-2.5 wt.% TiO2, whereas komatiites contain only 0.5-0.7 wt.% TiO2.

Primitive massive lavas and chilled margins (MgO>10 wt.%) from layered flows and intrusions contain <3-7 ppb Pd and Pt and 0.1-1.0 ppb Ir. The Pd/Ir ratios range from 5-11; such low values are typical for ultramafic magmas. Evolved ferropicritic liquids from the top of differentiated bodies have less than 10 wt.% MgO, high S-concentrations (1000-3000 ppm) but low concentrations of chalcophile elements such as Ni and Cu (<50 ppm) and the Platinum-group-elements (<3 ppb Pd or Pt, <0.1 ppb Ir). The olivine cumulates at the base of the differentiated bodies contain elevated PGE abundances up to 10 ppb Pd and Pt. A similar pattern is displayed in mineralised layered intrusions, such as the Kammikivi and Pilgujärvi intrusions. The olivine cumulates at the base of these bodies contain massive and disseminated Ni-Cu-sulfides with up to 2 ppm Pd and Pt, but the PGE concentrations in the overlying clinopyroxene and gabbro cumulates are below detection limits. This distribution pattern of the chalcophile metals suggests that the silicate liquid became sulfide saturated during the emplacement. The segregation of the sulfide liquid depleted the parental ferropicritic magma in Ni, Cu and Platinum-group-elements, but these metals became concentrated together with the sulfides at the base of the differentiated magma bodies.

Assimilation of S- and SiO2-rich sediments during the emplacement of the ferropicritic magmas in the sedimentary part of the Pilgujärvi suite probably caused sulfide saturation in the silicate magma. Although the isotopic composition of Sm/Nd (eNd=1.4±0.4) of flows and intrusions preclude significant contamination of the parental magma, the high and radiogenic 187Os/186Os values in sulfide ores and the lower Th/U in the Pilgujärvi intrusion compared to that of ferropicritic volcanic rocks (Hanski, 1992) indicate some assimilation of country rock.