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Article

Geochemical Distribution of Platinum Metals, Gold and Silver in Intrusive Rocks of the Norilsk Region

by
Ludmila Canhimbue
* and
Irina Talovina
Department of Historical and Dynamic Geology, Saint Petersburg Mining University, 119106 Saint Petersburg, Russia
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(6), 719; https://doi.org/10.3390/min13060719
Submission received: 19 April 2023 / Revised: 18 May 2023 / Accepted: 22 May 2023 / Published: 24 May 2023
(This article belongs to the Special Issue Geochemistry and Mineralogy of Basic–Ultrabasic and Alkaline Intrusions and Related Magmatic Deposits)
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Abstract

:
The Norilsk ore district is one of the world leaders in the production of platinum metals. Long-term research focused on the detection of sulfide platinum-copper-nickel ores contributed to the accumulation of a large volume of scientific material on the geology and mineralization of the Norilsk area. Despite this, the issue of the composition of the initial melt for ore-bearing intrusive complexes and its degree of enrichment with noble metals remains open. Intrusive rocks of the Norilsk region are rarely analyzed for their ratio of noble metals. However, the analysis and comparison of geochemical parameters of different types of intrusions allows us to draw important conclusions not only about the composition of the initial magmas of ore-bearing complexes, but also about the formation conditions of the intrusions. This study demonstrates the distribution of platinum metals, gold and silver in the main petrographic differentiates of the Kharaelakh, Talnakh, Vologochan intrusions and Kruglogorsk-type intrusion. The regularities and variations of the distribution of metals depend on the host rocks. There are two series of rocks in the inner structure of the ore-bearing intrusions: 1. Picritic and taxitic gabbro-dolerites enriched in PGE-Au-Ag mineralization which forms disseminated ores at intrusion bottoms (ore-bearing rocks). 2. Olivine-, olivine-bearing, olivine-free gabbro-dolerites and leucogabbro with poor sulfide mineralization at the upper part of the intrusions (ore-free rocks). There is a distinct correlation between PGE, Cu, S and to a lesser extent correlation with Ni in the first rock group, which is a characteristic of sulfide PGE-Cu-Ni deposits. In the second group, correlations are also revealed, but the correlation coefficients are lower. The main element controlling the distribution of platinum metals is copper. The taxitic gabbro-dolerites of the Talnakh intrusion are the most enriched by noble metals. According to noble metal patterns the rocks of the Kharaelakh intrusion show the highest degree of melting of the initial mantle material during the formation of parental magmas chambers. Despite some differences, the geochemical features of the studied rocks indicate the similar characteristics of the accumulation of gold, silver and platinum metals in the intrusions of the Talnakh, Kruglogorsk and Zubovsk types, which allow suggesting the close conditions for the formation of ore mineralization of these intrusions.

1. Introduction

The Norilsk-Talnakh group of sulfide platinum-copper-nickel deposits located on the northwestern edge of the Siberian Platform is spatially and genetically related to clearly differentiated basite-hyperbasite intrusions [1,2,3,4].
A large number of intrusions of varying mineralization was identified and combined into the Norilsk intrusive complex. The observed clear differences in morphostructural features, internal structure, composition, degree of differentiation and content of the ore component allowed distinguishing of the Talnakh (also known as Norilsk or Norilsk-Talnakh), Zubovsk, Kruglogorsk and Lower Talnakh types of intrusions within the Norilsk intrusive complex. The silicate composition of rocks of all the intrusions is quite similar, despite their different positions in the stratigraphic section and a different mineralization [5,6] (excepting the latter type).
Two parts are distinguished in the structure of intrusions: the ore-bearing lower (from bottom to top: contact, taxitic and picritic gabbro-dolerites) and the upper (olivine, olivine-bearing and olivine-free gabbro-dolerites, leucogabbro), which probably crystallized from different portions of magma [7,8].
For a long time, the main attention of researchers was paid to the study of the behavior of platinum metals, gold and silver in massive and disseminated ores, which are the main source of extraction of these elements. In [9,10,11] and others, PGE-Au-Ag mineralization in ores confined to the bottom parts of intrusions is described in detail. The relationships of these metals with sulfide minerals, as well as the forms of their accumulation, have been studied thoroughly in [12,13,14,15]. Meanwhile, the ore-free parts of the intrusions remained insufficiently studied. A few works aimed at the study of ore-free horizons of intrusions and noble-metal mineralization in them have been published [16,17,18].
Of the problems in the formation of the Norilsk intrusions, one of the main is the composition of the initial melt. It remains unclear whether this melt was enriched with noble metals or not. Data on the compositions of ore-free rocks allow us to understand how platinum metals were concentrated in intrusions and whether the ratios of these metals differ in the ore and ore-free parts of the intrusions.
The purpose of this work is to identify geochemical distribution patterns of PGE, Au and Ag in ore-free rocks and compare them with lower ore-bearing rocks within three types of intrusions: Talnakh type (Kharaelakh and Talnakh intrusions), Zubovsk type (Vologochan intrusion) and Kruglogorsk type (unnamed intrusions in the north-western frame of the Kharaelakh intrusion and in the eastern flank of the Talnakh intrusion) at the modern level of excavation. To assess the content of platinum metals, gold and silver, we studied the full sections of intrusions of the Norilsk complex which differ in their mineralization. In this work, the authors adhere to the nomenclature of rocks developed in the Norilsk area and accepted in both Russian and English literature [1,4,19].

2. Geology and Composition of Intrusions

The Norilsk area belongs to the activated peripheral region of the Siberian platform. The magma-controlling structures of the area are represented by a combination of swell-like uplifts, marginal deflections, rift depressions and systems of orthogonal and diagonal deep faults (Figure 1).
The extra-large deposits are localized at the intersection of hidden submeridional and sublatitudinal zones of deep faults [20,21,22,23]. The main plicative structures are the Tunguska syneclise, the Kharaelakh, Vologochan and Norilsk troughs filled with the Tunguska series and volcanogenic rocks, as well as the Dudinsky and Khantaysko-Rybinsky swells, etc.
The Norilsk intrusive complex unites a number of spatially converging intrusive bodies, among which several types are distinguished, differing in composition, structure and mineralization (Figure 2): (1) mesocratic fully differentiated ore-bearing intrusions of the Talnakh type (or in some publications Norilsk, Norilsk-Talnakh type); (2) differentiated leucocratic weakly ore-bearing intrusions of the Zubovsk and Kruglogorsk types; and (3) ore-free differentiated high-magnesian melanocratic intrusions of the Lower-Talnakh type [24]. Intrusive formations, represented by lopoliths, sills, dikes and irregularly shaped bodies, are localized in the section from the lower part of the sedimentary cover to the upper part of the tuff-lava strata. Many publications describe their internal structure, mineralogy and geochemistry [5,25,26]. In this work, we studied the Talnakh, Zubovsk and Kruglogorsk types of intrusions.
All industrial deposits and large occurrence of sulfide platinum-copper-nickel ores are associated with the Talnakh type of intrusions. Most of them are located in the central part of the Norilsk and Kharaelakh troughs and are controlled by the zone of the Norilsk–Kharaelakh fault. The stratigraphic level of localization of these intrusions covers the interval from the Upper Silurian to the Nadezhdinskaya formation of the Lower Triassic.
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Figure 1. Geological schema of the Norilsk area. Modified after Strunin [27]: (1) volcanic rocks of the trap association; (2) Tunguska group rocks; (3) terrigenous-carbonate rocks; (4) faults (1—Yenisey–Khatangsky; 2—Norilsk–Kharaelakh; 3—Mikchangdinsky; 4—Lamsko–Khatangsky; 5—Imangdinsky); (5) projections of the differentiated basite-hyperbasite intrusions: Kharayelakh (1), Talnakh (2), Norilsk-1 (3), Norilsk-2 (4), Chernogorsk (5), Imangda (6), Pyasino-Vologochan (7); 6—boreholes: red triangle—Talnakh type, green triangle—Kruglogorsk type, yellow triangle—Zubovsk type.
Figure 1. Geological schema of the Norilsk area. Modified after Strunin [27]: (1) volcanic rocks of the trap association; (2) Tunguska group rocks; (3) terrigenous-carbonate rocks; (4) faults (1—Yenisey–Khatangsky; 2—Norilsk–Kharaelakh; 3—Mikchangdinsky; 4—Lamsko–Khatangsky; 5—Imangdinsky); (5) projections of the differentiated basite-hyperbasite intrusions: Kharayelakh (1), Talnakh (2), Norilsk-1 (3), Norilsk-2 (4), Chernogorsk (5), Imangda (6), Pyasino-Vologochan (7); 6—boreholes: red triangle—Talnakh type, green triangle—Kruglogorsk type, yellow triangle—Zubovsk type.
Minerals 13 00719 g001
Minerals 13 00719 g002 550
Figure 2. Generalized sections of the Talnakh, Zubovsk and Kruglogorsk types of intrusions of the Norilsk intrusive complex. Modified after Krivolutskaya [28]: 1—gabbronorites; 2—gabbrodiorites; 3—leucogabbro; 4—10—gabbro-dolerites: 4—non-olivine, 5—olivine-containing, 6—olivine, 7—picritic, 8—taxitic, 9—taxite-like, 10—contact; 11—troctolites; 12—disseminated ores; 13—massive ores; 14—hybrid-metasomatic rocks; 15—eruptive breccias; 16—host rocks.
Figure 2. Generalized sections of the Talnakh, Zubovsk and Kruglogorsk types of intrusions of the Norilsk intrusive complex. Modified after Krivolutskaya [28]: 1—gabbronorites; 2—gabbrodiorites; 3—leucogabbro; 4—10—gabbro-dolerites: 4—non-olivine, 5—olivine-containing, 6—olivine, 7—picritic, 8—taxitic, 9—taxite-like, 10—contact; 11—troctolites; 12—disseminated ores; 13—massive ores; 14—hybrid-metasomatic rocks; 15—eruptive breccias; 16—host rocks.
Minerals 13 00719 g002
The massifs are tape-shaped and sheet-like deposits with a length of more than 15 km and a width of up to 3 km. The fully differentiated intrusions of the Talnakh type are characterized by a distinct stratification, a three-membered structure of the main stratified series (olivine-free, olivine, picritic gabbro-dolerites) and the development of the upper and lower stratified gabbro series corresponding to endocontact zones. There is a clearly defined chromite-bearing taxitic gabbro of the upper endocontact, which is characterized by the development of chromite, low-sulfide platinum mineralization [25,29,30,31].
Zubovsk-type intrusions are localized in the sediments of the Lower-Middle Devonian. The forms of intrusions are tape-shaped in plane and lenticular, less often sheet-shaped in cross-section. The thickness ranges from the initial tens of meters to 300 m. The Zubovsk-type intrusions are close to fully differentiated massifs in composition and characteristics of stratification, but differ from the latter primarily in the predominance of mafic leucocratic and mesocratic rocks in the section, a small volume of picritic gabbro-dolerites and troctolite horizons, weak development of taxitic gabbro-dolerites. In the structure of intrusions from top to bottom there following are distinguished:
  • Upper gabbro, gabbronorite series (gabbronorites, contact, non-olivine, olivine-containing, olivine gabbro-dolerites, gabbro-diorites);
  • The main differentiated series (olivine-free, olivine-containing, olivine, picrite gabbro-dolerites, troctolites);
  • Lower gabbro series (contact, olivine, taxitic gabbro-dolerites, gabbronorites) [5,32].
Intrusions of the Kruglogorsk type form sill-like or flat-sectioned bodies with a small thickness of 15–20 m, less often up to 40 m. This type of intrusions includes intrusions developed near to ore-bearing massifs at a distance of 10–15 km. The intrusions consist mainly of horizons of leucocratic gabbro and ferrogabbro that can compose up to 50% of the intrusive body. Furthermore, there are gabbro-dolerites from non-olivine to olivine varieties. Copper-nickel mineralization is spread unevenly, rarely reaching industrial concentrations in areas with the development of secondary changes such as taxitic texture [33,34,35].
Intrusions of the Norilsk complex are located at different stratigraphic levels and have different ages of formation. The dating of intrusion zircons shows a wide range of age values (Table 1).

3. Materials and Methods

The study of the distribution pattern of metals in rocks is based on the results of geochemical analyses of borehole samples that were uncovered in the western part of the Kharaelakh intrusion, the central part of the Talnakh intrusion, the northern part of the Vologochan intrusion, as well as Kruglogorsk-type unnamed intrusions in the north-western frame of the Kharaelakh intrusion and in the eastern flank of the Talnakh intrusion. In total, we studied a selection of 5 wells and 151 samples. Intervals between sampling points along the boreholes ranged from 0.4 to 5.0 m, core diameter—47.6 mm. The samples were processed using a multi-stage crushing–grinding cycle. The quantity weights for Pt, Pd, Rh, Ru, Ir, Os, Au and Ag analysis were 300 g, the quantity weights for Cu, Ni and S analysis were 50 g.
Laboratory and analytical studies were carried out in the Central Laboratory of Russian Geological Research Institute (Saint Petersburg). All samples were analyzed for their platinum, palladium, rhodium, ruthenium, iridium, gold, nickel, copper and sulfur content, and some for silver and osmium. The content of non-ferrous metals was measured by atomic emission spectrometry with inductively coupled plasma on an atomic emission spectrometer using the “IRIS Advantage” model (ThermoElemental, Waltham, MA, USA) with detection limits at 0.002–0.003 ppm. The content of sulfur was determined by infrared spectrometry using the “SC-144DR” IR analyzer model (LECO Corporation, St. Joseph, MO, USA) with sensitivity of 1 ppm.
Platinum, palladium, rhodium, ruthenium, iridium, osmium and gold content was determined by atomic emission and mass spectrometric methods with inductively coupled plasma after concentration into nickel matte using an ELAN DRC mass spectrometer (Perkin Elmer, Waltham, MA, USA) with detection limits at 0.1–5 ppt. The silver content was measured by atomic absorption method after concentration in nickel matte using the “SOLAAR S-2” atomic absorption spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).
Transparent-polished and polished thin sections were prepared from rock samples. A detailed petrographic description of all the rock varieties was carried out in Saint Petersburg Mining University using Axio Imager A2m (Zeiss, Oberkochen, Germany) and Leica DM750 M (Leica Camera, Wetzlar, Germany) reflected- and transmitted-light microscopes. Mineral compositions were investigated using a “JSM-6460LV” scanning electron microscope (JEOL, Akishima, Japan) equipped with a field emission gun and an energy dispersive X-ray silicon drift detector (Oxford Instruments, Abingdon, UK).
In sum, the distribution of platinum metals, gold and silver were studied in 150 samples of leucogabbro, olivine gabbro-dolerites, picritic gabbro-dolerites and taxitic gabbro-dolerites. In addition to variations in the composition of rocks, we analyzed such ratios as Pd/Pt, ƩPGE/S, Cu/Ni, Cu/S and (Rh + Pt + Pd)/(Ru + Ir + Os).

4. Results

4.1. Petrographic Characteristic of Rocks

The structure, petrology and mineralogy of the Norilsk intrusions were described in detail in the [33,38,39,40] works. The following is a description of the Kharaelakh intrusion section along the V-6 borehole, where the varieties of the studied rocks are presented in the best possible form. The composition of minerals measured by SEM is presented in Table S1.
Leucogabbro are large-crystalline rocks with a gabbro and prismatic-ophitic texture, usually containing 50%–80% protoplagioclase (more basic An70–100 plagioclase of the first generation) (Figure 3a). Rocks may contain large olivine grains of up to 5%–7%, prismatic clinopyroxene, and quartz is often noted at the bottom of the horizon, sometimes up to 5%–7%. Areas of pegmatite accretions of second-generation plagioclase and clinopyroxene are also observed. Ilmenite, titanomagnetite and sulfide minerals are present up to 5 vol.%.
Olivine gabbro-dolerites are rocks of massive structure, medium-fine-grained appearance and consist of: plagioclase (An40–85)—55%–60%, clinopyroxene—25%–30%, olivine—10%–20% and orthopyroxene up to 5%. The texture is poikilophitic, prismatic-granular and ophitic (Figure 3b). In olivine gabbro-dolerites, pyroxene forms large grains (up to 4 mm) and contains small euhedral inclusions of plagioclase, and less often grains of olivine or another pyroxene (Figure 3c).
The rocks are weakly subjected to secondary changes, rare substitutions of biotite and chlorite by pyroxene, single interstitial sulfide releases, as well as reaction rims of hornblende around ore minerals are observed. The content of ore minerals does not exceed 5 vol.%.
Picritic gabbro-dolerites are fine-grained rocks of massive, rarely spotted structure. The texture is panidiomorphic-granular, poikilitic and sideronitic (Figure 3d, e). Microrhythmic gravitational stratification is established in the rocks due to the alternation of varieties with different amounts of cumulus olivine and plagioclase. The content of plagioclase (An55–66 and An72–84) is 30%–35%, augite (Fs9–14) is 30–35%, olivine (Fo78–82) is 25%–30% and orthopyroxene (Fs20–23) is up to 5%. Biotite (5%–7%), sulfide minerals (2%–15%) and oxides (magnetite, chromite) are common in rocks (Figure 4b, d–f). Secondary minerals are represented by prenite, saussurite, hornblende, biotite, serpentine, saponite and talc. Apatite, zircon and baddeleyite are found as the accessory minerals.
Magnetite is present in large quantities (up to 10%) and cements idiomorphic olivine and pyroxene crystals, clearly manifesting a xenomorphism in the rocks (Figure 3e). Within the horizon of picritic gabbro-dolerites, there is a variable composition of rocks and an unrestrained ratio of rock-forming minerals.
Taxite gabbro-dolerites compose the lower endocontact zone. They are uneven-grained rocks with an ataxitic structure (Figure 3f,g). They contain xenoliths of picrite and contact gabbro-dolerites, which indicates the later formation of taxite gabbro-dolerites with the probable participation of the fluid phase [41,42]. The mineral composition of rocks varies widely, but the main rock-forming minerals are plagioclase (An66–57, An83–76) and clinopyroxene (Fs10–12). Schlierens and xenomorphic inclusions of sulfide minerals (up to 10%–15%) are widespread. Magnetite and chrome spinels are also present. Postmagmatic modifications are insignificant: plagioclase is weakly sossuritized and clinopyroxene is partially replaced by hornblende, biotite and chlorite.
Iron and titanium oxides are often formed in rocks. In all rock variations the sulfide minerals are mainly represented by pentlandite, chalcopyrite, minerals of the pyrrhotite group, cubanite and sometimes by pyrite, millerite, sphalerite and galena (Figure 4c–f). Mineral forms of platinum metals are present, but the role of solid solutions of platinum metals in ore minerals is quantitatively more considerable.

4.2. PGE, Au and Ag Distribution in Ore-Bearing Intrusions

Our research has shown that in all types of intrusions, the lower horizons are more enriched in noble metals than the upper ones. Studied rocks are characterized by a platinum-palladium geochemical specialization of mineralization, as well as by the distribution of noble metals according to a Pd > Ag > Pt > Au > Rh > Ru > Ir, Os scheme.

4.2.1. Ore-Bearing Rocks

According to the results of the study, picritic and taxitic gabbro-dolerites can be attributed to ore-bearing rocks.
The highest concentration of platinum group metals occurs in taxitic gabbro-dolerites of the Talnakh intrusion—up to 5.1 ppm (Table 2). They also note the maximum background gold content.
With the estimation of the relative concentration, expressed as ΣPGE (ppm)/S (wt. %), it was noted that this value is higher in picrite gabbro-dolerites than in taxitic varieties in all studied intrusions of the Talnakh and Zubovsk types. In the Kruglogorsk-type intrusions the relative concentration of PGE prevails in leucogabbro.
Pd/Pt ratio in the rocks of the Talnakh intrusion has a relatively stable character: in leucogabbro—2.0–3.1, in olivine gabbro-dolerites—2.3–2.5, in picritic gabbro-dolerites—2.5–3.6 and in taxitic gabbro-dolerites—2.9–3.4.
In the Kharaelakh intrusion the Pd/Pt ratio shows a large variation within each rock horizon. Thus, in leucogabbro this ratio changes from 1.5 to 3.6, in olivine gabbro-dolerites from 0.18 to 3.7, in picritic gabbro-dolerites from 2.2 to 3.9 and in taxitic gabbro-dolerites from 1.9 to 4.9.
In the Vologochan intrusion of the Zubovsk type the concentration of noble metals in the rocks is significantly less than in the rocks of ore-bearing intrusions; the average content of ∑PGE is 1.47 ppm. The relative concentration of platinum metals in picritic gabbro-dolerites is 1.7–2.7 and in taxitic gabbro-dolerites it is 1.3–1.8. At the same time, the Pd/Pt ratio differs from 2.0 to 6.9 in both picritic and taxitic horizons.
Both Kruglogorsk-type intrusions have a ΣPGE/S ranging from 2 to 3.4 in leucogabbro, 0.2–0.7 in olivine gabbro-dolerites and 0.6–2.1 in taxitic gabbro-dolerites. The Pd/Pt ratio reaches 5.3 in olivine gabbro-dolerites of the Kruglogorsk-type intrusion in the northwestern frame of Kharaelakh and 8.8 in taxitic gabbro-dolerites of the Kruglogorsk-type intrusion in the eastern flank of Talnakh.
In general, there is a high correlation between platinum metals, copper, nickel and sulfur in picritic and taxitic gabbro-dolerites of Talnakh and Zubovsk types of intrusions (Figure 5).
The distribution of platinum metals, depending on the type of host horizon, retains the trend observed mainly for Cu, and to a lesser extent for Ni and S. There is a noticeable decrease in correlation from the picritic horizon to the taxitic ones in the Kharaelakh and Vologochan intrusions. Meanwhile, the opposite is observed in the Talnakh intrusion: the dependence of platinum metals on copper, nickel and sulfur increases from the picritic to the taxitic layers. There is almost no correlation between metals in Kruglogorsk-type intrusions.
The geochemical pattern of the distribution of Au and Ag is closely related to and sometimes similar to the behavior of PGE (Figure 6). Often the correlations between noble metals are close to a value of 1.

4.2.2. Ore-Free Rocks

Leucogabbro and olivine gabbro-dolerites can be classified as ore-free rocks.
The Talnakh-type intrusions are characterized by the predominance of platinum metals, Au and Ag in leucogabbro horizons compared to olivine gabbro-dolerites, whereas in the Kruglogorsk-type intrusions, leucogabbro is significantly more depleted than olivine gabbro-dolerites.
In the Talnakh intrusion and in the Kruglogorsk-type intrusions, there is an increase in the dependence of platinum metals on copper, nickel and sulfur from leucogabbro to olivine gabbro-dolerites (Figure 7). In the Kharaelakh intrusion, on the contrary, the correlations between elements are stronger in leucogabbro. This highlights slightly different mechanisms of the intrusion’s establishment. The strong dependence of the content of platinum metals on the concentration of copper in ore-free rocks is consistently observed.
The role of sulfur is quite difficult to assess due to large variations in correlation. Thus, in the Kharaelakh intrusion, there is a high dependence of platinum metals on sulfur in the leucogabbro horizon and a low dependence in olivine gabbro-dolerites. In the Talnakh intrusion, on the contrary, the influence of sulfur on platinum metals in olivine gabbro-dolerites is stronger than leucogabbro. In the intrusions of the Kruglogorsk type this dependence is practically absent.
Nickel shows a strong link with PGE in the ore-free rocks of the Talnakh intrusion, in olivine gabbro-dolerites of Kruglogorsk-type intrusions and in the leucogabbro of the Kharaelakh intrusion. In other cases, the influence of nickel on the distribution of platinum metals is insignificant.

5. Discussion

The analysis of the ratio of noble metals in the ore-free intrusion horizons makes it possible to understand whether the initial melt was enriched with these metals. Furthermore, a comparison of different types of intrusions allows us to answer the question of whether they are products of a single magma or different ones.
Our results show that the paragenetic associations of platinum group minerals in the rocks of the Norilsk intrusions are largely related to the paragenetic associations of sulfide minerals and petrographic rock types. The amount of PGE and sulfide minerals does not depend on the degree of secondary changes in rocks. Spatially, platinum group minerals of all varieties are located inside sulfide grains or along the boundaries with magnetite or nonmetallic minerals. Often, PGMs have correct crystallographic limitations, others show the development of facets and inclusions of sulfides, i.e., signs of metacrystals (Figure 8a, b).
The palladium mineral phases are strongly predominated in the composition of the PGM. Furthermore, palladium is present in the form of solid solutions. Among the minerals of platinum metals, there are complex aggregates, which most likely arose as decomposition products of a solid solution (Figure 8c,d) [43,44]. The study of polished sections revealed a wide distribution of intermetallic alloys of Pd-Pt-Sn, Pd-Sb-Bi-Te and Pt-Bi-Te types. Gold occurs mainly as a high-fineness Au–Ag alloy in pyrrhotite-rich samples. Native silver is widely spread, as well as solid solutions of silver in chalcopyrite.
As given in [45] and confirmed by our research, a high Pd/Pt ratio is fixed in ore-free and low ore-bearing rocks as well as in ores. The content of PGE and gold in economic ore-bearing Talnakh and Kharaelakh intrusions of the Talnakh type decrease from taxitic and picritic gabbro-dolerites up to the section and increase again in leucogabbro. Olivine gabbro-dolerites contain the minimum number of sulfides and show the lowest Pd/Pt ratio = 2.4–2.6 (Table 2). This value probably characterizes the palladium-to-platinum ratio close to the true value in ore-free horizons intrusions of the Norilsk-Talnakh type. The opposite results are observed in Kruglogorsk-type intrusions, where the maximum palladium–platinum ratios are recorded precisely in olivine gabbro-dolerites. First of all, it occurs due to a different sequence of petrographic varieties in the intrusive section: inherent olivine gabbro-dolerites are above the taxitic ones. It can be assumed that palladium, which demonstrates chalcophile properties, concentrated in the sulfide melt of the intrusion and enriched the lower horizons of the magma chamber.
The ratio ∑Pt/∑Ir = (Rh + Pt + Pd)/(Ru + Ir + Os) reflects the degree of melting of the initial mantle material during the formation of parental magma chambers [46]. According to our results, the maximum values correspond to taxitic gabbro-dolerites horizons, especially in the Kharaelakh intrusion. Most likely, the formation of taxite gabbro-dolerites was the result of processes occurring in the sulfide-silicate system and including gravitational deposition of sulfide liquid. Perhaps there was a process similar to remelting, in which the increased pressure of the gas phase played a significant role. This is particularly confirmed by the fact that the Kharaelakh intrusion is characterized by the largest halos of contact-metasomatic rock in the lower part [45,47].
Our arguments confirm Ryabov’s conclusions [48] that the taxitic gabbro-dolerites are a later formation compared to the formation of the main differentiates of the intrusion, which were subjected to hornfelsing with further melting.
Noteworthy are the high values of ∑Pt/∑Ir in olivine gabbro-dolerites from the Kruglogorsk-type intrusions. This fact requires further study.
Chondrite-normalized traces of element distribution (noble metals, Ni, Cu) result in a positive slope of the graph (Figure 9). It indicates a high degree of fractionation of low-melting platinum metals relative to high-melting ones. In general, the distribution pattern of the metals in the rocks of the studied intrusion types has a similar topology, which suggests a close composition of the melts that formed these intrusions. The differences are only noted in the content of the elements. Apparently, the low metal content in the ore-free rocks is the reason for the absence of an obvious pattern in their distribution.
Furthermore, the multi-element diagrams for ore-free and ore-bearing rocks are similar. This allows us to conclude that the ratio of noble metals is the same throughout the whole section of the intrusion and does not depend on the ore mineralization.
As a result of our research, we concluded that the initial magmatic melt of the intrusions of the Norilsk region was similar for all studied types of intrusions and, probably, was initially enriched in platinum minerals, gold and silver.

6. Conclusions

We analyzed the distribution of PGE, gold and silver in ore-free rocks (leucogabbro, olivine gabbro-dolerites) and in ore-bearing rocks (picritic and taxitic gabbro-dolerites) of three types of intrusions of the Norilsk area: Talnakh type (Kharaelakh and Talnakh intrusions), Zubovsk type (Vologochan intrusion) and Kruglogorsk type (unnamed intrusions in the north-western frame of the Kharaelakh intrusion and in the eastern flank of the Talnakh intrusion).
The conducted research allowed us to draw the following conclusions:
  • Intrusive rocks are characterized by a high concentration of PGE. Palladium prevails over platinum; the role of rhodium is significant, rare platinum group metals are contained in limited quantities. The distribution of noble metals has a Pd > Ag > Pt > Au > Rh > Ru > Ir, Os scheme.
  • Platinum group metals are present mainly in the form of Pd-Pt-Sn, Pd-Sb-Bi-Te, Pt-As and Pt-Bi-Te alloys. The amount of PGE and sulfide minerals does not depend on the degree of secondary changes in rocks
  • There is a high degree of correlation of the content of PGE, Au and Ag with the concentration of Cu, S and Ni both in ore-bearing and ore-free rocks. There are different patterns of changes in the correlations of elements in the section of intrusions, which indicates different mechanisms of the establishment of these intrusions.
  • The geochemical features of the leucogabbro, olivine gabbro-dolerites, picritic gabbro-dolerites and taxitic gabbro-dolerites indicate the similar nature of the accumulation of noble metals in the intrusions of the Norilsk-Talnakh, Kruglogorsk and Zubovsk types. Most likely, these intrusions are products of similar mantle melts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min13060719/s1, Table S1: Chemical composition of minerals from the Kharaelakh intrusion.

Author Contributions

Conceptualization, data analysis, visualization, writing—original draft preparation, geochemical study of rocks, L.C.; project administration, review and editing, I.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Acknowledgments

The study was carried out with the involvement of the laboratory base of the Centre for Collective Use of Saint Petersburg Mining University.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 3. Photomicrographs in transmitted light of the main petrographical differentiates of the Kharaelakh intrusion: (a) leucogabbro; (b,c) olivine gabbro-dolerite; (d,e) picritic gabbro-dolerite; (f,g) taxitic gabbro-dolerite. Ol—olivine, Pl—plagioclase, Cpx—clinopyroxene, Opx—orthopyroxene, Ser—serpentine, Mt—magnetite, Sul—sulfide minerals.
Figure 3. Photomicrographs in transmitted light of the main petrographical differentiates of the Kharaelakh intrusion: (a) leucogabbro; (b,c) olivine gabbro-dolerite; (d,e) picritic gabbro-dolerite; (f,g) taxitic gabbro-dolerite. Ol—olivine, Pl—plagioclase, Cpx—clinopyroxene, Opx—orthopyroxene, Ser—serpentine, Mt—magnetite, Sul—sulfide minerals.
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Minerals 13 00719 g004 550
Figure 4. Photomicrographs in reflected light of sulfide aggregates in taxitic (a,c) and picritic (b,d–f) gabbro-dolerite: (a) interstitial inclusions of magnetite in fusion with ilmenite and sulfide aggregates; (b) euhedral chromite grains in olivine and orthopyroxene; (c) dissolution texture of pyrrhotite in chalcopyrite; (d) superimposed mineralization of pyrite over pyrrhotite and chalcopyrite; (e) decomposition of a solid solution of cubanite-chalcopyrite in pyrrhotite and pentlandite; (f) grain of sperrylite on the border of pentlandite. Po—pyrrhotite, Ccp—chalcopyrite, Pn—pentlandite, Cbn—cubanite, Mt—magnetite, Ilm—ilmenite, Chr—chromite, Ol—olivine, Ser—serpentine, Spy—sperrylite PtAs2.
Figure 4. Photomicrographs in reflected light of sulfide aggregates in taxitic (a,c) and picritic (b,d–f) gabbro-dolerite: (a) interstitial inclusions of magnetite in fusion with ilmenite and sulfide aggregates; (b) euhedral chromite grains in olivine and orthopyroxene; (c) dissolution texture of pyrrhotite in chalcopyrite; (d) superimposed mineralization of pyrite over pyrrhotite and chalcopyrite; (e) decomposition of a solid solution of cubanite-chalcopyrite in pyrrhotite and pentlandite; (f) grain of sperrylite on the border of pentlandite. Po—pyrrhotite, Ccp—chalcopyrite, Pn—pentlandite, Cbn—cubanite, Mt—magnetite, Ilm—ilmenite, Chr—chromite, Ol—olivine, Ser—serpentine, Spy—sperrylite PtAs2.
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Figure 5. Binary plots of (Pt + Pd) vs. Cu (wt.%), Ni (wt.%), S (wt.%) for picritic and taxitic gabbro-dolerites of the Kharaelakh, Talnakh, Vologochan and Kruglogorsk-type intrusions.
Figure 5. Binary plots of (Pt + Pd) vs. Cu (wt.%), Ni (wt.%), S (wt.%) for picritic and taxitic gabbro-dolerites of the Kharaelakh, Talnakh, Vologochan and Kruglogorsk-type intrusions.
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Figure 6. Binary plots of Au vs. (Pt + Pd) and Au vs. Ag for the Kharaelakh, Talnakh, Vologochan and Kruglogorsk-type intrusions.
Figure 6. Binary plots of Au vs. (Pt + Pd) and Au vs. Ag for the Kharaelakh, Talnakh, Vologochan and Kruglogorsk-type intrusions.
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Figure 7. Binary plots of (Pt + Pd) vs. Cu (wt.%), Ni (wt.%), S (wt.%) leucogabbro and olivine gabbro-dolerites of the Kharaelakh, Talnakh and Kruglogorsk-type intrusions.
Figure 7. Binary plots of (Pt + Pd) vs. Cu (wt.%), Ni (wt.%), S (wt.%) leucogabbro and olivine gabbro-dolerites of the Kharaelakh, Talnakh and Kruglogorsk-type intrusions.
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Figure 8. BSE images of noble metal minerals in taxitic gabbro-dolerite of the Kharaelakh intrusion. Pn—pentlandite, Po—pyrrhotite, Cbn—cubanite, Ccp—chalcopyrite, Spy—sperrylite PtAs2, Plv—paolovite Pd2Sn, Tmy—taimyrite (Pd, Cu, Pt)3Sn, Plr—polarite Pd2BiPb, Cbr—cabriite Pd2SnCu, Hes—hessite (AgPd)2Te.
Figure 8. BSE images of noble metal minerals in taxitic gabbro-dolerite of the Kharaelakh intrusion. Pn—pentlandite, Po—pyrrhotite, Cbn—cubanite, Ccp—chalcopyrite, Spy—sperrylite PtAs2, Plv—paolovite Pd2Sn, Tmy—taimyrite (Pd, Cu, Pt)3Sn, Plr—polarite Pd2BiPb, Cbr—cabriite Pd2SnCu, Hes—hessite (AgPd)2Te.
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Figure 9. Chondrite C1 normalized multi-element variation diagrams of rocks of the Kharaelakh, Talnakh and Vologochan intrusions and Kruglogorsk-type intrusion in the Eastern flank of the Talnakh intrusion: (a) for ore-free rocks (leucogabbro and olivine gabbro-dolerites); (b) for ore-bearing rocks (picritic and taxitic leucogabbro).
Figure 9. Chondrite C1 normalized multi-element variation diagrams of rocks of the Kharaelakh, Talnakh and Vologochan intrusions and Kruglogorsk-type intrusion in the Eastern flank of the Talnakh intrusion: (a) for ore-free rocks (leucogabbro and olivine gabbro-dolerites); (b) for ore-bearing rocks (picritic and taxitic leucogabbro).
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Table
Table 1. U-Pb age data of rocks of industrial ore-bearing and ore-bearing intrusions of the Norilsk region after [36,37].
Table 1. U-Pb age data of rocks of industrial ore-bearing and ore-bearing intrusions of the Norilsk region after [36,37].
IntrusionRocksAge, Ma
TalnakhLeucogabbro
Gabbrodiorites
Olivine, olivine-bearing, olivine-free gabbro-dolerites
Taxitic gabbro-dolerites		262.4 ± 0.9
258.2 ± 2.8
256.4 ± 1.3; 229.3 ± 3.4
256.7 ± 1.4; 224.3 ± 3.0
KharaelakhOlivine gabbro
Leucogabbro
Picritic gabbro-dolerites		347 ± 16
265 ± 11
253.9 ± 1.7; 235.9 ± 6.1
VologochanPicritic gabbro-dolerites
Gabbro-troctolites		331.6 ± 4.1
246.2 ± 3.1; 226.7 ± 1.3
Table
Table 2. Average content (in ppm) of PGE, Au and Ag in rocks of different intrusions of the Norilsk region.
Table 2. Average content (in ppm) of PGE, Au and Ag in rocks of different intrusions of the Norilsk region.
IntrusionRocksn *PtPdRhRuIrOs∑PGEAuAg∑NMPd/Pt∑Pt/∑IrCu/Ni
TalnakhLeucogabbro60.330.930.0470.0200.011-1.30.045-1.382.8381.1
Olivine gabbro-dolerites60.170.420.020.0110.010-0.630.0270.280.942.4281.1
Picritic gabbro-dolerites111.053.30.120.0380.0160.0114.50.172.77.43.2571.4
Taxitic gabbro-dolerites71.13.80.170.0500.0200.0145.20.1938.33.5701.6
KharaelakhLeucogabbro140.140.480.0120.0060.0030.0050.70.051.82.52.8582.4
Olivine gabbro-dolerites380.110.320.0130.0070.0040.0050.490.0230.671.12.4362.2
Picritic gabbro-dolerites110.561.60.0760.0270.0090.012.40.152.75.23.01621.4
Taxitic gabbro-dolerites230.431.60.0640.0200.0080.0082.30.102.64.93.8852.7
Kruglogorsk-type intrusion in the Eastern flank of the Talnakh
intrusion		Leucogabbro30.130.310.0130.010.01-0.470.024-0.502.222.1.04
Taxitic gabbro-dolerites40.352.00.0390.0130.01-2.450.07-2.524.9981.8
Kruglogorsk-type intrusion on the N-W edge of Kharaelakh
intrusion		Leucogabbro60.130.390.0170.010.01-0.560.026-0.492.8261.6
Taxitic gabbro-dolerites30.250.970.0360.0120.01-1.20.056-1.23.3562.01
Olivine gabbro-dolerites50.241.10.210.040.021-1.60.044-2.53.6321.9
VologochanPicritic gabbro-dolerites50.110.370.0140.010.01-0.760.028-0.543.3221.04
Taxitic gabbro-dolerites90.221.010.0290.0110.011-2.10.07-1.34.2981.7
* n—number of samples; NM—noble metals; ∑Pt/∑Ir = (Rh + Pt + Pd)/(Ru + Ir + Os); “-”—the value has not been determined.
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Canhimbue, L.; Talovina, I. Geochemical Distribution of Platinum Metals, Gold and Silver in Intrusive Rocks of the Norilsk Region. Minerals 2023, 13, 719. https://doi.org/10.3390/min13060719

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Canhimbue L, Talovina I. Geochemical Distribution of Platinum Metals, Gold and Silver in Intrusive Rocks of the Norilsk Region. Minerals. 2023; 13(6):719. https://doi.org/10.3390/min13060719

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Canhimbue, Ludmila, and Irina Talovina. 2023. "Geochemical Distribution of Platinum Metals, Gold and Silver in Intrusive Rocks of the Norilsk Region" Minerals 13, no. 6: 719. https://doi.org/10.3390/min13060719

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