Abstract

The ∼3.5 km thick Kivakka complex (2.45 Ga) in northern Karelia hosts a zone of disseminated Pd-Pt-Ni-Cu mineralization near its base. The zone is related to a special or “critical horizon” and displays relative maxima in bulk-rock concentrations of Cu, Ni, Co, and Cr, in the ratios Cu:Ni, Cu:Zr, and 100Fe2O3/(FeO + Fe2O3), and elevated levels of Cr in ortho- and clinopyroxene, along with higher levels of incompatible Ti, V, and Zr. Early crystallizing minerals (highly magnesian pyroxenes and An-rich plagioclase) are associated in this zone with 1–10% (commonly 3 to 5%) base-metal sulfides (BMS) and interstitial phases [nearly pure Ab, chlorapatite, zircon, Fe-(Ti) oxides, etc.]. Apatite is characterized by extensive variations in Cl (both intragranular and intergranular), which attain the composition of chlorapatite (>6 wt.% Cl), not observed below the zone. The compositional trend of the apatite series is extensive; the content of F generally increases upward, whereas Cl decreases, with the observed maximum (3.3% F) near the top of the intrusion. The PGE-rich zone is inferred to have formed at the “critical level” as a consequence of a combination of several factors. Factor 1 represents an abrupt change in the sequence and character of cumulus phases; a massive crystallization of cumulate An75–81 is accompanied by a sharp decrease in the amount of orthopyroxene with cessation of crystallization of olivine. As a likely result, the remaining melt became substantially enriched in Cr. Alternatively, the Cr enrichment is related to an influx of a new portion of magma. Factor 2 represents a rise in levels of oxygen fugacity, f(O2). Factor 3 is displayed by the sharp increase in Cl in the intercumulus apatite; the fractionated melt well above this zone appears to be depleted in Cl. The sudden drop in Cl is attributed to degassing of the melt, from which a Cl-rich volatile phase exsolved, causing fractionation of Cl from F. Factor 4 involves an increase in levels of sulfur fugacity, f(S2), resulting in the deposition of the disseminated BMS in association with tiny grains of Pd-Pt bismuthotellurides and sperrylite. Thus, the formation of PGE mineralization appears to have proceeded in Kivakka as a sequence of processes and events, rather than as a result of a single factor or process during fractional crystallization of the complex.

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