Olivine and Cr-spinel from the Noril'sk-1 deposit: compositions and petrological implications

Differentiated intrusions of the “Norilsk type” are among the world's largest deposits of Cu, Ni and platinum group elements (PGE). They are composed of gabbro-dolerites (gabbros with a pronounced ophitic structure), which mineral composition ranges from picritic and troctolitic varieties to leucogabbro and gabbro -diorites . Economically significant reserves are concentrated in three types of ores: (1) massive sulfide ores that form deposits in the bottom parts of the intrusions, (2) disseminated sulfide ores located at the lower parts of the intrusions, and (3) rocks with rich chromite mineralization and high ratios of PGE/Cu-Ni-sulfides - low-sulfide ores that form discontinuous horizons (LS-horizons) in the upper endocontact zones of the intrusions. Among the hypotheses about the origin of the Norilsk type intrusions, the most acknowledged is the "flow chamber model", within which the intrusions and accumulations of sulfide ores were formed during a long flow through the near-surface chambers of basaltic melts, comagmatic to the basalt formations of the Norilsk region [Naldrett et al., 1992; Rad'ko, 1991]. However, a number of studies show that (1) the intrusions of the Norilsk type have been forming over a long period of time from 290 to 230 Ma [Malitch et al., 2010]; (2) they were formed later than a significant part of the trap basalts [Krivolutskaya, 2016] and (3) the parental magmas for these intrusions differed significantly from the trap ones, and in fact, there are no bona fide co -magmatic rocks of the Norilsk intrusions among the represented basalt formations [Latypov, 2002]. In this work, we consider compositions of olivine and Cr-spinels from disseminated sulfide ores and the LS-horizon of the Norilsk-1 intrusion as petrological indicators, compare them with these minerals in trap basalts, and estimate redox conditions of formation of these types of ores. It was noted that after crystallization, olivine and chrome spinel re-equilibrated with each other and the environment (melt, other minerals). Olivine in disseminated sulfide ores is characterized by narrow Mg# range and re -equilibration with high Ni sulfides, which is manifested in the growth of Ni and Fe (reverse Ni-Mg # trend) [Barnes et al., 2011; Barnes et al., 2013]. The composition of Cr-spinels varies over a very wide range, especially in Mg # (from 5 to 55) values and TiO2 (up to 18 %) content, and in Fe²⁺/Fe³⁺ variations. Evolution of the Cr-spinel compositions towards low Mg# values region is characteristic of Cr-spinel inclusions in olivine and clinopyroxene, as well as in altered minerals, while plagioclase hosts more magnesian Cr-spinel grains. Based on the obtained results and data of [Kamenetsky, Crawford, Meffre, 2001] Cr-spinels with Mg # > 25 can be considered relatively primitive (weakly re-equilibrated). Comparison of the composition of the studied minerals with their counterparts from trap basalts showed that the compositions of olivine, especially in terms of Ni, differ between disseminated ores, LS-horizon and trap basalts. In general, the NiO concentrations in the analyzed olivines lie between the picrobasalts of the Gudchikhinskiy Formation and the basalts of the Tuklons kiy and Nadezhdinsky Formations. However, compositions of olivines of disseminated ores are close to the olivines from picro-basalts of the Gudchikhiy Formation, as well as several obtained analyzes from the Mokulaevskiy Formation. Cr -spinels of the Norilsk-1 intrusion are generally richer in Ni than the chromites of the Tuklonskiy and Nadezhdinskiy formations and, according to this feature, correspond to the picrobasalts of the Gudchikhinskiy Formation. Based on the data obtained, we assume that the ore-bearing rocks of the Norilsk-1 intrusion were formed from magmas enriched in Ni and similar to the effusive rocks of the Gudchikhinskiy or Mokulaevskiy formations. At the same time, different magmatic impulses and, accordingly, different melt compositions could be responsible for the formation of disseminated sulfide ores and the MS horizon. Since strong variations in Fe²⁺/Fe³⁺ ratio in Cr-spinels can result from variations in the redox conditions the parental media, we estimated f(O2) for the rocks of the studied series. As soon as application of olivine-spinel thermometer and oxybarometer was considered as limited due to strong re-equilibration of Cr-spinel [Nikolaev et al., 2016], the olivine-chromite pair was used to calculate the T-f(O2) parameters for only the most primitive Cr-spinels from the picrobasalts of the Gudchikhinskiy Formation and the LS-horizon of the Norilsk-1 intrusion. The temperature calculated using the Al-in-olivine oxybarometer [Coogan, Saunders, Wilson, 2014] was 1150-1240 °C, and the log10f(O2) calculated using the Mg-Fe olivine-chromite oxybarometer [Nikolaev et al., 2016] was -9 - -9.8 for picrobasalts, and LS-horizon -8 - -9. To estimate f(O2) for the rocks of the LS-horizon that do not contain fresh olivine and for disseminated sulfide ores, in which all olivine - chromite pairs were strongly reequilibrated, we adopted a semi-quantitative method based on the conversion of Fe²⁺/Fe³⁺ in Cr-spinel to Fe²⁺/Fe³⁺ in the melt [Maurel, Maurel, 1984] with further estimation of f(O2), based on the empirical correlation for basaltic magmas [Fudali, 1965] which was calibrated to f(O2) for Cr-spinels of the Gudchikhinskiy Formation, obtained using the olivine-spinel oxybarometer. According to the estimations, disseminated sulfide ores of the Norilsk-1 intrusion crystallized at log10f(O2) from -7.5 to -9, which at 1175 ^oC is ~ NNO ± 1 and slightly increased relative to the picrobasalts of the Gudchikhinskiy, Nadezhdinskiy and Tuklonskiy formations. For the LS-horizon, estimates of log10f(O2) showed very different values for different samples. In most cases, log10f(O2) at 1175oC was about -9, which is close to the picrobasalts of the Gudchikhinskiy Formation. Nevertheless, some samples showed more oxidized conditions (up to log10f(O2) = -7 or ~ QFM + 2), and some samples, on the contrary, down to log10f(O2) = -12. The latter are close to the IW (Fe-FeO) buffer and are extremely reduced conditions for igneous rocks. Such broad range of f(O2) is unique to the Norilsk-1 intrusion and is not typical for the rocks of the upper endocontact with the LS-horizon of the Talnakh intrusion, for which the log10f(O2) estimates do not fall below the WM (FeO-Fe3O4) buffer. We assume that the variations in redox conditions in the LS-horizon of the Norilsk-1 intrusion are associated with the intense assimilation of coal shales of the Tunguska Formation.

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