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

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 (lgfS2 range from −12 to −11). And a high-sulfur assemblage (monoclinic pyrrhotite, pyrite and Ni-rich pentlandite) formed at higher sulfur activity (lgfS2 range from −10.5 to −10) due to the pyrrhotite-chalcopyrite 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 gabbro- dolerites (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.


Introduction
Platinum-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'sk-Kharaelyakh 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) and to the contact with siltstones has been established. The more common contact rock of the Noril'sk 1 intrusion is taxitic gabbro-dolerite. But the lower olivine gabbro-dolerire instead

Compositions of sulphides in picritic and olivine gabbro-dolerites 2.4.1. Fe(+Co) − S − Ni system
The compositions of sulfides in picritic and olivine gabbro-dolerites differ in sulfur and copper contents (figure 5). But in each unit they change with a depth. The pyrrhotite is close to troilite (FeS); pentlandite is Fe-rich (figure 5 a) and contains a minor Co (1.  5 a). In this case the pyrrhotite is most enriched in sulfur Fe 0.90 S 1.10 (monoclinic species), and contains Ni < 1 wt.%; troilite is absent.

Discussion
Thus, the following mineralogical-geochemical regularities are observed in the disseminated ores of the southern part of the Noril'sk 1 deposit. The Ni/Cu ratios are decreasing in the rocks with depth: Ni/Cu is more than 1 in picritic gabbro-dolerite, whereas Ni/Cu is less than 1 in olivine gabbrodolerite. The Ni/Fe ratio in the pentlandite increases with depth. Iron-nickel ratios in pentlandite reflect the activity of sulfur (lgfS 2 ) during its formation [14,15]. As the lgfS 2 increases, the concentration of Ni in the pentlandite increases as well. The variation of k = Ni/(Ni+Fe) in pentanedite varies in the range 0.33 − 0.47 in picritic gabbro-dolerite that corresponds to the lgfS 2 from −12 to −11. Whereas variation of k in pentlandites from olivine gabbro-dolerite varies from 0.49 to 0.57, therefore they crystallized under conditions of lgfS 2 corresponding to an interval from −10.5 to −9. Thus, two different sulphide assemblage are characteristic of picritic and olivine gabbro-dolerites. In the first case, the "low-sulfur" minerals sequence is due to the evolution of the sulphide Cu-rich melt fraction separated from Fe-rich melt in the pre-crystallization time [5]. The following parageneses are formed during fractionation of mss and iss: Po (h>tr) +Cp+Pn (Fe>Ni) → Po (h>tr) +Cp+Cb+Pn (Fe>Ni) → Cb±Cp+Pn (Fe>Ni) ±tr → Th+Cb+Pn±Bn [1]. This agrees with the investigated parageneses in picritic gabbro-dolerite of PH-14 borehole. But disseminated ore hosed by picritic gabbro-dolerite has a Ni/Cu ratio >1. It means that the Cu-rich melt fraction was enriched in Ni also largely because the Ni tends to be an incompatible element in all plausible sulfide magma compositions [16]. The evolution of mineral parageneses in olivine gabbro-dolerite is: Po (m>h) +Cp+Pn (Fe<Ni) → Cp+Po (m+h) +Pn (Fe<Ni) . It agrees with the experimental data on directional crystallization of melts [17] and is due to the pyrrhotite-chalcopyrite fractionation in conditions of increased sulfur fugacity.
A positive correlation between sulfur and copper for all types of rocks (figure 8 a) and a gradual increase in the concentrations of ore elements and sulfur towards the towards the bottom intrusion (see figure 2) is in agreement with the evolution of sulphide melt and fractionation of ore elements during its deposition [13]. So, there is a clear genetic relationship between the sulfide assemblages in disseminated ores and the type of the host gabbro-dolerites. A sulfur-poor and Cu-rich sulphide assemblage which has been formed at low sulfur activity is characteristic of picritic gabbro-dolerite. And a high-sulfur assemblage (monoclinic pyrrhotite, pyrite and Ni-rich pentlandite) formed at higher sulfur activity. Binary diagram of Cu versus Pd (figure 8 b) shows a difference in trends: a poorly defined linear for olivine gabbro-dolerite and indeterminate for picritic gabbro-dolerite.

Conclusions
Thus, on the basis of mineralogical and geochemical features it is established that: − Ni/Cu ratios are decreasing in rocks from picritic to olivine gabbro-dolerites and toward the massive ores, that agreed with the increase of the chalcopyrite in this direction; − Ni/Fe ratios in pentlandite are increasing from picritic to olivine gabbro-dolerites and to massive ores which indicates an increase in sulfur fugacity in mineral parageneses down the section of borehole; − There is a clear correlation between the sulfide assemblages and the type of the host gabbrodolerites: picritic gabbro-dolerites are characterized by a low-sulfur and Fe-and Cu-rich sulphide assemblage; olivine gabbro-dolerite is characterized by a high-sulfur assemblage of sulfides. − The change of the compounds of Pd with semimetals is observed along the section: Sn → Pb → As → Bi(Te) in the picritic gabbro-dolerite and Sn → Sb → As → Te(Bi) in olivine gabbro-dolerite. The difference in evolutionary trends is due to the different schemes of fractionation of sulphide melt.