MINERALOGICAL AND GEOCHEMICAL FEATURES AND THE MECHANISM OF FORMATION OF PICRITIC GABBRO-DOLERITE HORIZON IN THE NORTHEASTERN BRANCH OF THE TALNAKH INTRUSION ( Norilsk region )

Norilsk type intrusions are characterized by unique reserves of disseminated Cu–Ni mineralization localized in picritic and taxitic gabbro-dolerites. Picritic and taxitic gabbro-dolerites are rocks of different genesis, but the mechanism of formation of picritic gabbro-dolerites is still debatable. Most often, they are regarded as a cumulative part of a layered series. In this work we show a well-pronounced geochemical contact between a layered series and picritic gabbro-dolerites. In the section of the latter, we have established reverse geochemical zoning expressed as regular accumulation of major elements, which do not form a single trend of crystallization differentiation with the rocks of the main layered series. The rocks are discrete within the horizon of picritic gabbro-dolerites; we have identified two intervals: the lower one with low Cr contents and stable Eu and Sr anomalies and the upper one with anomalous high chromium contents, reduced LILE contents, and no positive Eu anomaly specific to the lower section of picritic gabbro-dolerites. Based on the known models of formation of reverse zoning in the marginal zones of layered massifs, we present a new genetic scheme for the formation of picritic gabbro-dolerites as products of pulsed intrusion. This scheme implies that the lower part of picritic gabbro-dolerites formed from hybrid magma, and the upper one, from primitive magma as a result of the pulsed filling of the magma chamber. In our opinion, this is the cause of the reverse zoning and the accumulation of chromium in the upper section of picritic gabbro-dolerites.