Lower Talnakh Type Intrusions of the Norilsk Ore Region

Abstract

Troctolites, olivine and picrite gabbrodolerites account for up to 75% of the Lower Talnakh type intrusions in places of their increased thickness whereas reduced thickness sections consist of olivine-free and olivine-bearing gabbrodolerites. Differentiation is not obvious within these high-Mg cumulates, although the content of TiO2 and alkalis increases towards the upper endocontacts. The transitions between the rock types are gradational, and the composition of low Ni olivine in different rocks (Fo70–83, 0.01–0.2 wt % NiO) overlap significantly. Clinopyroxene (Fs7–13, Mg# 68–89) is characterized by both the lowest contents and variation ranges of Cr2O3 (0.01–0.5 wt %) and TiO2 (0.05–1.0 wt %) among all types of the intrusions of the Norilsk complex that is consistent with the Cr-depleted (0.002–0.051 wt % Cr2O3) bulk rock compositions. Later orthopyroxene (Fs15–30) is crystallised by the reaction of the residual melt with early olivine. Plagioclase forms porphyritic phenocrysts and their intergrowths along with ophitic laths as well as dominates in schlieren and fragments of leucocratic rocks in taxitic and picritic gabbrodolerites with a poorly sorted layered texture. In olivine-rich rocks, sulfides are represented by the association of troilite ± hexagonal pyrrhotite + Fe- and Co-rich pentlandite + Fe-enriched chalcopyrite (± putoranite, talnakhite) ± cubanite. In the upper and lower parts of the intrusions, the association of hexagonal pyrrhotite + chalcopyrite + pentlandite occurs, while monoclinic pyrrhotite + chalcopyrite + Ni-enriched pentlandite are formed in the endo- and exocontacts. The concentration of base (0.077–0.21 wt % Ni, 0.05–0.38 wt % Cu) and platinum metals (0.03–0.26 to 0.40 g/t total PGE) in mineralized rocks is very low. Upon small amounts of sulfides and extremely low base and platinum metal tenors, the heterogeneous S isotopic composition of Lower Talnakh type sulfides (mainly 3.8–8.6‰, but up to 11.8%) most likely reflects the achievement of repeated sulfide saturation during the assimilation of sulfate S by magma that has previously experienced loss of chalcophile metals into a coexisting sulfide fluid at depth. The Sr-Nd isotopic compositions of the Lower Talnakh intrusions (Sri – from 0.7073 to 0.7087 and εNd(Т) from –1.8 at –5.9 recalculated to 250 Ma) show the predominant contamination with Proterozoic material, in contrast to the ore-bearing intrusions, which Sr-Nd isotope compositions indicate contamination with upper crustal sedimentary matter of the Paleozoic age.