Mineralogical and geochemical feature of the disseminated ores of the southern part of the Noril’sk 1 deposit

Abstract

Abstract.The new data on the composition of disseminated sulphide ore in the southern part of the Noril'sk 1 deposit are obtained. It was found that a sulfur-poor assemblage: as Fe-rich (cubanite, mackinawite) and Cu-rich (talnakhite, bornite, chalcocite, native copper) minerals due to the evolution of two different fractions of sulphide melt is characteristic of picritic gabbro-dolerite. It has been formed at relatively low sulfur activity (lgfS 2 range from −12 to −11). And a high-sulfur assemblage (monoclinic pyrrhotite, pyrite and Ni-rich pentlandite) formed at higher sulfur activity (lgfS 2 range from −10.5 to −10) due to the pyrrhotitechalcopyrite fractionation of the sulphide melt is characteristic of olivine gabbro-dolerite. The ratios of Ni/Cu in the rocks decrease from picritic (Ni/Cu = 1.24 average) to olivine gabbrodolerites (Ni/Cu = 0.69 average). But the Ni/Fe ratios in pentlandite are increasing down the cross-section due to increase of sulfur fugacity during the evolution of ore-forming system. The Pd-parageneses vary along the cross-section top down as follows: Sn → Pb →As →Bi(Te) in picritic gabbro-dolerite and Sn →Sb →As →Te(Bi) in olivine gabbro-dolerite. There is a clear correlation between the sulfide and PGM assemblage and the type of the host rocks. IntroductionPlatinum-copper-nickel deposits of the Noril'sk region are located in the northwestern part of the Siberian platform and are associated with flood-basalt of Perm-Triassic age [1][2][3]. The Noril'sk 1 deposit is located in the northern part of the same name intrusion, confined to the Noril'skKharaelyakh fault in the Noril'sk Trough. Intrusion of Noril'sk I breaks the Carbon-Permian terrigenous deposits of the Tungusska Series and basalts of the Upper Permian-Lower Triassic age [2].Numerous works are devoted to the ores of the Noril'sk 1 deposit [5,[7][8][9][10]. Most of the works devoted to the massive ores or low-sulphide horizons, and only some of them are considering the disseminated ore of the Main Ore Horizon composed of picritic, taxitic, and lower olivine gabbrodolerites [7,11,12]. The features of sulfide mineralization in each unit have been characterized previously [2], however the task of establishing a correlation between the types of the host rocks and the chemical and mineral composition of ores is still relevant [3]. The new data on the composition of disseminated sulphide ore in the southern part of the Noril'sk 1 deposit are obtained on the basis of the samples study of the PH-14 borehole (figure 1). This borehole crosses all the rocks, alternating in a certain stratigraphic sequence which have been described earlier [1,2]. We studied samples of the lower part of the intrusion Noril'sk 1, composed of picritic and lower olivine gabbro-dolerites containing the disseminated Cu-Ni sulphide with the minerals of platinum group elements (PGM). The zoning (changes of geochemical feature, mineral parageneses and compositions of sulfides) from picritic gabbro-dolerite (821.2-902.0 m) to olivine gabbro-dolerite (902.0-938.1 m)...

The evolution of the ore-forming system in the low sulfide horizon of the Noril'sk 1 intrusion, Russia

“…A similar ‘low-temperature association’ (chalcopyrite + millerite + pyrite) is characteristic of the low-sulfide horizon of the Talnakh intrusion (Sluzhenikin et al , 1994). This has also been described in disseminated ores of olivine gabbro-dolerites in the lower part of layered section of the Noril'sk 1 deposit (Tolstykh et al , 2017).…”

“…In general, it can be concluded that the captured sulfide droplet-like inclusions are composed of a low-sulfur association (troilite, Fe-enriched pentlandite and chalcopyrite with Fe > Cu). A similar association is characteristic of disseminated ores from picritic gabbro-dolerites of the Noril'sk 1 (Tolstykh et al , 2017). Sulfides from the sulfide segregations belong to a high-sulfur association, which include chalcopyrite, pyrite, monoclinic pyrrhotite, millerite and Ni-rich pentlandite.…”

Mineralogical zoning of the PGE–Cu–Ni orebodies in the central part of the Oktyabr'sky Deposit, Norilsk District, Russia

“…The southern and northern orebodies differ in mineralogical composition. The southern C-4 orebody is characterized by a high-sulfur association and zoning, expressed by pyrrhotitechalcopyrite fractionation (Distler et al, 1975), and the northern C-3 orebody contains minerals typical of a low-sulfur association (troilite, talnakhite, sugakiite) which is a characteristic of disseminated ores in the picritic gabbro-dolerite only (Distler et al, 1999;Tolstykh et al, 2017).…”

“…In our case, it is also unlikely that massive ores, in this case, could be a result of permanent settling of sulfide droplets to the bottom of the formation chamber and the penetration of the accumulated sulfide melt through the layers of taxitic and contact gabrodolerites into the underlying rocks. The arguments are: i) disseminated ores in picritic gabbro-dolerites and massive ores have completely different sulfide phase and PGM compositions; ii) there is a distinct geochemical and mineralogical contrast between picritic and underlying taxitic ores (Tolstykh et al, 2017(Tolstykh et al, , 2020b(Tolstykh et al, , 2021, rather than gradual changes in composition, as would occur with sequential droplet deposition; iii) there is a gap between disseminated and massive ores -the presence of a layer of barren rocks. There is also a hypothesis that disseminated ore in the gabbro-dolerites are enriched in "fractional sulfide fluid coming from underlying massive sulfide ores" (Naldrett, 2004).…”