The structure of the Lukkulaisvaara intrusion, Oulanka group, northern Karelia: Petrological implications

The Lukkulaisvaara intrusion (U-Pb age: 2442 +/- 1.9 Ma), in northern Karel ia, 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 exce ed 4600 m in thickness; data obtained on the structural profile of the intr usion suggest a complete section. Its weighted-mean composition is equivale nt to a magma of the marianite-boninite series. but whether or not this com position corresponds to the parental magma is uncertain owing to possible m ultiphase features of the intrusion. It is quite possible that large and sm all lenticular bodies of fine-grained gabbronorite whose texture suggests r apid crystallization are associated with injections of fresh magma. Crystal lization to fine-grained gabbronorite in the process of magma chilling is r elated, in turn, to decompression. Chilling would be equally feasible in th e 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 morphol ogy and structural setting. Two genetic types of sulfides are distinguished : (a) sulfides 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 rocks are very variable (1-30 vol.%) . Forty platinum-group minerals have been documented in sulfide-bearing met asomatic rocks; the concentration of noble metals (Pt + Pd) covers a wide r ange (0.3-10 ppm Pt, 0.42-66 ppm Pd). Sulfides in the cumulates and iron-be aring magmatic minerals altered by metasomatism are considered to be the so urce of the ore-bearing metasomatic assemblages. These assemblages were for med under the action of a reducing hydrothermal fluid. A high content of ch lorine 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-inclu sions. Some are filled with liquid only. others are gas-liquid, aqueous - s alt inclusions with a gas bubble, and hydrocarbon inclusions with various a mounts of liquid. The highest Th recorded is 370 degreesC at a pressure of 1.5 kbar, as estimated using aqueous - salt inclusions. These data agree we ll with thermobarometric results calculated using estimated equilibration s tates. Nd and Sr isotopic data suggest that the metasomatic assemblages for med simultaneously with the layered intrusion (2442 Ma) during an autometas omatic process due to reworking of intrusive rocks by a mantle-derived flui d with an epsilon (Nd) of +2.1 and an initial Sr-87/Sr-86 value of 0.7028.