Geology of the western flanks of the Oktyabr’skoe deposit, Noril’sk district, Russia: evidence of a closed magmatic system

The Oktyabr’skoe deposit in the Northern Siberian Platform is one of the world’s largest deposits in terms of platinum-group elements and nickel reserves. The genesis of this deposit has been inferred from the inner geological structure of its central portion and remains a matter of long-lasting and heated discussions. The current dominant hypothesis is that the ores were produced in an open magmatic system. This study is devoted to only one aspect of the complicated problem of ore formation, i.e., the geological structure of the Kharaelakh intrusion to which the huge orebodies are related. Recently, new data have been acquired on the structure and composition of the western flanks of the Kharaelakh intrusion, which pinches out in this direction and does not coincide with basalts as has been suggested in many publications. A distinctive feature of this part of the deposit is that it is cut by numerous relatively thin intrusions of contrasting compositions (from picritic gabbro-dolerite to leucogabbro) that are spatially separated in contrast with the central portion of the deposit, where these rocks occur as discrete layers of a single intrusion. These bodies taper off westward and are “blind” apophyses of the intrusion to which various ore types are related. The inner structure of this part of the vertical section of the intrusion is analyzed based on detailed data on the core material recovered by a number of boreholes and on the chemical and mineralogical characteristics of the ores, which differ from those in the main orebodies in that they possess lower Cu and PGE concentrations (up to 3.5 wt% and 9 ppm, respectively) and hence result in the dominance of pyrrhotite and limited amounts (and smaller grain sizes) of platinum-group minerals. Peripheral sills of the Kharaelakh intrusion were formed by portions of initial magma with different melt/cumulus minerals proportions, forming separate bodies of picritic and olivine-bearing gabbro-dolerite. The last rock varieties comprise disseminated ore, in contrast with the central parts of the intrusion, where these ores are located in the picritic and taxitic horizons. No peripheral sills or dikes connected with lava flows have been found inside the studied area. These facts evidence ore formation in a closed magmatic system.