Sammendrag
The Gállojávri ultramafic intrusion is part of a cluster of small-sized PGE-mineralised bodies in the Karasjok Greenstone Belt (KGB). Although the Gállojávri body has received some attention from prospecting companies, the formation of the intrusion and associated sulphide mineralisation, and thus its regional significance, are still poorly constrained. This study provides the first holistic geological model for the Gállojávri intrusion that considers its petrogenesis, associated ore-forming processes and relation to the regional tectonomagmatic evolution of the KGB. Uranium-Pb geochronology yields an age of 2051 ± 8 Ma, whereas radiogenic isotopes exhibit fluctuations over short distances—e.g., bulk initial εNd ranges from -15 to +4 over a four-metre interval. The mineral chemistry of silicates supports a polybaric evolution, ranging from 400 to 700 MPa. In addition, spinel analyses exhibit a Fe-Ti trend characteristic of subvolcanic intrusions in intracontinental environments. The best explanation for the variation in bulk geochemistry, phase equilibria and mineral chemistry is a transcrustal conduit system, and according to thermodynamic modelling, the Gállojávri intrusion formed from a contemporary komatiitic parental melt. The system remained open after partial solidification, as demonstrated by the wide variation in radiogenic isotope signatures, suggesting local assimilation in the cumulate pile. The zircon age and parental komatiite support that the Gállojávri intrusion formed as part of a c. 2.05 Ga mantle plume event that resulted in vast ultramafic magmatism over much of the Fennoscandian Shield. The magmatism is regionally associated with mineralised conduits, such as the Kevitsa and Sakatti deposits in Finland. However, most komatiites in the KGB are uniformly depleted in chalcophile elements, which was previously interpreted as the result of sulphide sequestration en route, supposedly hindering their potential for forming magmatic sulphide deposits. Though, our results show that the Gállojávri intrusion is S-poor, with a δ34S(median) of 2.4 ‰ and Cu/Pd ratios ranging from depleted to primitive mantle. Moreover, unmineralised samples are enriched in chalcophile elements relative to primitive mantle. The mantle-like δ34S and Cu/Pd ratios support only cotectic sulphide removal at low silicate-to-sulphide mass ratios (R-factor). Combined with high Ni concentrations in olivine and chalcophile-enriched unmineralised rocks, our results support no early sulphide saturation in the Gállojávri conduit at or before emplacement. Therefore, the conduit system, with associated KGB komatiites, exhibits concurrent depletion and enrichment trends characteristic of a prospective mineral system, different from previous interpretations. For instance, the Porsvann and Karenhaugen intrusions are two prospective analogues in the region. Cu/Pd ratios are compatible with early S saturation and R-factors allowing for economic metal concentrations. Our findings show that dynamic conduit-like systems in the KGB should be considered potential exploration targets for magmatic sulphide deposits.
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