THE STRUCTURE OF THE LUKKULAISVAARA INTRUSION, OULANKA GROUP, NORTHERN KARELIA: PETROLOGICAL IMPLICATIONS

Abstract

The Lukkulaisvaara intrusion (U-Pb age: 2442 ± 1.9 Ma), in northern Karelia, Russia, belongs to the Oulanka plutonic group. The intrusion cuts rocks of the Archean granite - migmatite - gneiss basement and is disconformably overlain by Proterozoic metavolcanic rocks. The layered body does not exceed 4600 m in thickness; data obtained on the structural profile of the intrus ion suggest a complete section. Its weighted-mean composition is equivalent to a magma of the marianite-boninite series, but whethe r or not this composition corresponds to the parental magma is uncertain owing to possible multiphase features of the intrusion. It is quite possible that large and small lenticular bodies of fine-grained gabbronorite whose texture suggests rapid crystallizat ion are associated with injections of fresh magma. Crystallization to fine-grained gabbronorite in the process of magma chilling is related, in turn, to decompression. Chilling would be equally feasible in the case of the injection of residual melts, squeezed from lower horizons and already carrying cumulus minerals in the process of compaction. Structures in which fine-grained rocks occur do not differ from "potholes" in morphology and structural setting. Two genetic types of sulfides are distinguished: (a) sulfid es of magmatic stage are present in fresh rocks, commonly with magmatic quartz and biotite; (b) a metasomatic sulfide assemblage contains the richest sulfide and platinum mineralization and is related to potholes. Amounts of sulfide in the metasomatic rock s are very variable (1-30 vol.%). Forty platinum-group minerals have been documented in sulfide-bearing metasomatic rocks; the concentration of noble metals (Pt + Pd) covers a wide range (0.3-10 ppm Pt, 0.42-66 ppm Pd). Sulfides in the cumulates and iron - bearing magmatic minerals altered by metasomatism are considered to be the source of the ore-bearing metasomatic assemblages. These assemblages were formed under the action of a reducing hydrothermal fluid. A high content of chlorine was observed in biotite (up to 0.55 wt%), amphibole (up to 2.5 wt%), chlorite (up to 0.1 wt%), and scapolite (up to 2.3 wt%). Examination of th e metasomatic quartz reveals the presence of different types of micro-inclusions. Some are filled with liquid only, others are ga s- liquid, aqueous - salt inclusions with a gas bubble, and hydrocarbon inclusions with various amounts of liquid. The highest T h recorded is 370°C at a pressure of 1.5 kbar, as estimated using aqueous - salt inclusions. These data agree well with thermobarometric results calculated using estimated equilibration states. Nd and Sr isotopic data suggest that the metasomatic assemblages formed simultaneously with the layered intrusion (2442 Ma) during an autometasomatic process due to reworking of intrusive rocks by a mantle-derived fluid with an Nd of +2.1 and an initial 87Sr/86Sr value of 0.7028.