Geochemical structure of the Kivakka layered olivinite-norite-gabbronorite intrusion, North Karelia: Distribution of chalcophile elements

Abstract

The distribution of elements and their correlations were analyzed throughout the section of the Kivakka intrusion to formulate the geochemical tendencies in the behavior of elements of different groups during formation of layered basic-ultrabasic complexes. (1) It was shown that the distribution of trace elements in the layered series is controlled by their ability or inability to enter isomorphically the cumulus minerals. The distribution of trace elements that occur as isomorphic admixtures in cumulus minerals (Cr, Mn, Zn, Co, Ni, and Ti), regardless of their geochemical type, is correlated with the crystallization order and distribution of minerals in the silicate matrix of the rocks. Elements that practically are not involved into silicates (S, Cu, Ag) show no any systematic variations; i.e., they are not controlled by fractionation of major rock-forming mineral. Their behavior is driven by the appearance of individual phase—sulfide melt, whose localization, formation time, and scales of fractionation are determined by saturation of parental melt in sulfide sulfur and general degree of its fractionation. (2) The comparison of cross-section variations of elements that isomorphically substitute for major elements in the Fe-Mg silicates, but differ in chalcophile affinity (in order of increase of chalcophile affinity: Mn → Zn → Co → Ni) reflects the contribution of unmixing and fractionation of sulfide melt during intrusion solidification. This is quantitatively defined primarily by partitioning coefficients of elements between cumulus silicates and magmatic melt, on the one hand, and between magmatic and sulfide melts, on the other hand. (3) The absence of simple correlation between local sulfide-rich horizons and silicate matrix of the rocks and signs of independent fractionation of sulfide melt prevent any attempts to predict the localization and scales of local sulfide mineralization within the layered series on the basis of petrochemical and related criteria. Only tendencies in the distribution of ore elements and sulfur across the section can be used for these purposes.