Phase equilibria constraints on relations of ore-bearing intrusions with flood basalts in the Noril'sk region, Russia

Abstract.

Phase equilibria analysis has been performed to elucidate whether ore-bearing Noril'sk- and poorly mineralized Lower Talnakh-type intrusions are co-magmatic with the extremely Ni-, Cu- and PGEs-depleted and not depleted flood basalts, respectively. The parental magma of the intrusions is classified as silica-undersaturated olivine basalt, whereas that of the flood basalts is assigned to silica-saturated tholeiite. The plutonic equivalent of the former parental magma is olivine gabbro or melagabbro, whereas the latter is gabbronorite. Phase equilibria relations clearly show that these two types of magmas cannot be co-magmatic as there is no way to derive one liquid from another by fractional crystallization. This provides no support for the current model considering the Noril'sk-type intrusions as exit conduits for a great volume of volcanic magma, which lost most of its chalcophile elements in the shallow settling chambers on its way to surface. The association of a large volume of chalcophile element-depleted basalts with the very sulfide-rich intrusions in the Noril'sk region is most likely coincidental and cannot, therefore, be used as a regional criterion when prospecting for the Noril'sk-type sulfides deposits in other flood basalts provinces.