TRAP MAGMATISM AND ORE-BEARING NORILSK-TYPE INTRUSIONS IN THE NORTHWEST OF THE SIBERIAN PLATFORM: PRESSING PROBLEMS

A voluminous literature on the effusions and intrusions of the northwestern Siberian Platform has recently appeared. This has regenerated interest in the relationship between the Permo-Triassic trap niagmatism and ore-bearing Norilsk-type intrusions and in corresponding models for this magmatism. Geochemical data revealed that the thickest series of the latest (Morongo-Samoed) lavas is similar to ultrabasic-basic ore-bearing intrusions. Of the four recognized types of magma, three early types contain both basaltic and picritic lavas, and the latest type is represented only by tholeiitic lavas. The data available suggest that the fourth-type of magma also contains picritic melts although in the intrusion facies only. The ore-bearing Norilsk intrusions are just of this kind. Separation of subultrabasic mantle magma into picritic and tholeiitic parts, obviously, proceeded during the crystallization differentiation in a deep-seated intermediate chamber with the subsequent separate transfer of these parts to the upper layers of the Earth’s crust owing to a difference in their densities. Sulfides in these commercial Norilsk intrusions are rich in heavy sulfur isotope, which unambiguously indicates the crustal nature of these sulfides. It is also established that there were no sulfides in the differentiated intrusions at the stage of formation of the layered series. Some of the intrusions became ore-bearing only at the stage of formation of horizons of basic pegmatoids (taxitic gabbro-dolerites and taxitic gabbros) at endocontacts. This was due to different mechanisms of formation of ultrabasic-basic intrusions at the final stage of magma evolution. Fluids coming from the deep-seated chambers of intrusions along the above horizons interact with exocontact rocks and intrusion rocks adjacent to pegmatoids (picritic and contact gabbro-dolerites) to produce sulfides of sulfurization-metasomatic genesis.