Re Sulfides from Zhelos and Tokty-Oi Intrusions (East Sayan, Russia)

Abstract

Re sulfides were discovered in Cu–Ni–platinum-group elements (PGE) ores of the Zhelos and Tokty-Oi intrusions. These intrusions can be considered as products of the mantle superplume responsible for Rodinia’s break-up. The mineral compositions were determined in situ in polished samples. Electron microprobe analyses were mostly consistent with a general formula of (Cu,Fe,Mo,Os,Re)₅S₈, (Cu,Fe,Mo,Os,Re)₄S₇, and (Cu,Fe,Mo,Re)S₂. One of the major features of Re sulfide from the Zhelos intrusion is its high osmium content. The ΣMe/S ratio for a part of our data is consistent with that of the tarkianite. Re sulfides from the Tokty-Oi have a ΣMe/S ratio similar to those in rheniite or dzeskazganite, but differ from them by the presence of Fe and Cu and the metal-to-metal ratio. The localization of the Re sulfide within the chalcopyrite suggests its crystallization from the residual Cu-rich liquid.

1. Introduction

Rhenium is one of the rarest and most scattered elements; its average content in the Earth’s crust is ~0.7 ppb, in the primitive mantle −0.28 ppb. Strong Re enrichments are characteristic of marine sediments, due to their removal from sea water under anoxic conditions [1]. The content of rhenium in mafic and ultramafic rocks is characterized by a significant dispersion and changes in the range of 0.1–2 ppb [2], while the associated Ni–Cu (±PGE) ores are characterized by a high Re content (up to 286 ppb).

Individual rhenium minerals in nature are extremely rare. Due to their chalcophile properties, these occur in sulfides and selenides. A detailed history of the discovery of rhenium minerals is presented in [3,4]. The first rhenium mineral was discovered in copper-lead ores of the Dzheskazgan Deposit (Kazakhstan) in 1962 [5]. Since then, each discovery of rhenium minerals is considered unique and worthy of being discussed [6,7,8,9,10,11,12,13,14,15,16,17].

All known natural phases enriched in rhenium can be divided into three types: (1) rhenium disulfide—reniite [12,13,16], with a content of Re ~74 wt.%; (2) Re–Mo–Cu sulfides, with different stoichiometric ratios [3,6,7,8,9,10,11,15], containing ~46–60 wt.% Re; and (3) molybdenite, with a high content of rhenium ranging from 1–5 to 46 wt.% [17,18].

This study provides data on the composition of Re sulfides, which, in addition to Mo and Co, contain as much as 5 wt. % Os.

2. Materials and Methods

In this research, we studied the original core samples of rocks and ores selected from boreholes.

The composition of sulfides and minerals of platinum group and rhenium was determined in situ in polished samples. Previously, the samples were studied by scanning LEO1430VP (Carl Zeiss, Oberkochen, Germany) electron microscope with INCAEnergy 350 (Oxford instruments, Abingdon, UK) analitical system (Ulan-Ude). Then, polished samples were analyzed on a JEOL JXA8200 SuperProbe (GEOL, Tokyo, Japan), a high-resolution SEM and WD/ED combined Electron Prob Microanalyzer, equipped with 5 wavelength dispersive X-ray spectrometer, and an energy dispersive X-ray spectrometer with a 133 eV lithium drift type silicon semiconductor detector (Common Use Center of Isotope and Geochemical Studies in Vinogradov Institute of Geochemistry, Irkutsk). Elements were determined by their characteristic X-ray wavelengths on wave-dispersion spectrometers, at the following operation conditions: accelerating voltage 20 kV, beam current 20 nA, beam diameter 1 μm, count time 10 s along peak line plus 5 s for background on both sides of the peak line. The proprietary software was used for data processing. X-ray Kα lines were used to measure S, Fe, Ni, and Cu; Lα lines for Ir, Ru, Rh, Pt, Pd, and As; Lβ lines for Mo; and Mα lines for Os and Re. Metrological characteristics of the technique were determined on control samples of known composition.

3. Results

3.1. Geological Background

The Zhelos and Tokty-Oi Intrusions located in the central part of the Eastern Sayan (South Siberia, Russia). Ultramafic intrusions are widespread in this region. These intrusions are thought to be produced by the mantle superplume responsible for Rodinia’s break-up [19]. Ultramafite rocks are lens- and arc-shaped bodies (Figure 1), with a visible width of 20 to 700 m and a length of 0.1 to 7 km, divided into separate blocks by series of faults. Intrusions, which previously represented dike-like and sill-like bodies, together with the host rocks of the volcanogenic-sedimentary complex, were involved in folding and underwent metamorphic transformations [20]. Host rocks are various gneisses, marbles, and orthoamphibolites. Contacts with host rocks are mostly obscured by plicative deformations.

The Zhelos and Tokty-Oi intrusions are differentiated bodies, composed of rocks varying from dunites to lherzolite and characterized by gradual transitions between species. Rocks of both intrusions have a composition, with major oxides ranging within (wt. %): SiO₂ 38–46, TiO₂ 0.08–0.9, Al₂O₃ 1–8, MgO 25–49, CaO 0.4–12. The composition of the initial melt was close to picritic [19].

In both intrusions, there are four types of sulfide ores: (1) disseminated with a sulfide content of 5–15 vol. %; (2) net-textured (15–30 vol. % sulfides); (3) massive with sulfide content of up to 80 vol. %; and (4) vein disseminated and massive in the host rocks [20]. The rhenium sulfide was found in the Zhelos intrusion in disseminated sulfide ores, and in the Tokty-Oi intrusion in vein disseminated ores.

The highest Re content (up to 480 ppb) is typical for vein disseminated and massive ores, while the total content of the PGE varies from 2 to 12 ppm. In the net-textured and massive ores of the Zhelos intrusion the content of Re is 280 ppb, total PGE contents up to 15 ppm, with a ratio Pd/Pt = 3–8. In the disseminated ores of the Zhelos intrusion the content of Re is 200–350 ppb, total PGE contents 1–7 ppm. In the Tokty-Oi intrusion, disseminated ores are characterized by a content of Re 150–450 ppb, the total PGE being 1.6–3.3 ppm.

3.2. Composition of Sulfide Ores

The most common sulfide assemblage is pyrrhotite (troilite) + pentlandite ± chalcopyrite in both intrusions. The vein ores in the host amphibolites are composed of pyrrhotite or chalcopyrite. Troilite forms thin lamellae in hexagonal pyrrhotite and anhedral polycrystalline aggregates. Monoclinic pyrrhotite is present only in vein ores. Chalcopyrite and pentlandite occur mostly as anhedral polycrystalline aggregates, and pentlandite is present as exsolved flames in pyrrhotite. Pentlandite lamellae and flames are common in chalcopyrite, and mackinawite "worms" are ubiquitous in pentlandite. Cobaltite–gersdorfite are common in all types of ores. Most of the PGM identified are sperrylite, palladium bismuthotelluride phases, irarsite, osarsite, and omeiite [20].

Pyrrhotites from the Tokty-Oi intrusion are characterized by a generally higher Ni content (up to 1.5 wt. %). The Fe/Ni atomic ratio in pentlandite ranges from 1.4 to 0.7. In pentlandite from sulfide ores of the Zhelos intrusion, the value of the Fe/Ni ratio is higher than that of the Tokty-Oi intrusion. Pentlandite from vein ores of the Tokty-Oi intrusion and in all ore types of the Zhelos intrusion has a high cobalt content up to 2 wt. %. The compositions of pentlandite and iron monosulfides are in good agreement with each other. Pentlandite with a ratio of Fe/Ni > 1.2 is associated with troilite, whereas pentlandite with Fe/Ni ~0.7 is associated with pyrrhotite. The composition of chalcopyrite is generally stoichiometric.

3.3. Samples with Re Sulfide

The analytical results are presented in

Table 1. Electron-microprobe data (wt. %) on sulfide associated with rhenium minerals.

Sample	Figure	Point	Fe	Co	Ni	Cu	S	Total
09-6	2(a)	1	64.05	bdl	bdl	bdl	36.33	100.38
09-6	2(a)	2	63.64	bdl	bdl	bdl	36.22	99.86
09-6	2(a)	3	38.34	0.6	27.97	bdl	33.52	100.43
09-6	2(a)	4	39.2	0.98	25.77	bdl	32.98	98.93
09-6	2(a)	5	35.65	0.9	30.16	bdl	33.09	99.8
09-6	2(b)	1	35.2	0.91	30.85	bdl	33.34	100.3
09-6	2(b)	2	35.52	0.78	31.02	bdl	33.67	100.99
09-6	2(b)	3	34.91	0.9	30.9	bdl	33.35	100.06
09-6	2(b)	4	49.88	0.96	11.87	0.83	35.36	98.9
09-6	2(b)	5	49.17	0.99	11.34	1.65	35.04	98.19
09-6	2(b)	6	44.62	1.06	17.66	bdl	35.01	98.35
06-2	2(с)	1	27.51	1.97	37.27	bdl	33.08	99.83
06-2	2(с)	2	31.5	bdl	bdl	33.56	34.55	99.62
06-2	2(d)	1	30.47	bdl	bdl	34.45	34.4	99.32
09-6	3(b)	1	31.09	bdl	bdl	33.66	34.68	99.43
09-6	3(b)	2	34.02	1.49	31.01	bdl	33.46	99.98
09-6	3(b)	3	31.16	bdl	0.84	32.74	34.99	99.73
09-6	3(b)	4	33.68	0.93	31.41	bdl	32.61	98.63
06-2	3(d)	4	31.3	bdl	bdl	34.2	34.28	99.78
06-2	3(d)	5	31.13	bdl	bdl	33.57	34.66	99.36

Note: “bdl” means “below detection limit”.

and

Table 2. Electron-microprobe analyses (wt. %) of the Re sulfide.

Sample	Figure	Point	Fe	Cu	Os	Mo	Re	S	Total	ΣMe/S1	Re/Mo1	Re/Cu1
09-6	3b	5	3.54	4.92	4.48	13.05	47.00	27.91	100.90	0.63	1.86	3.28
09-6	3b	6	3.17	4.87	3.98	12.14	48.27	27.67	100.10	0.63	2.05	3.41
09-6	3b	7	3.42	7.22	4.93	9.38	49.76	25.32	100.03	0.72	2.73	2.37
09-6	3b	8	5.43	8.51	3.24	7.34	49.45	26.22	100.19	0.72	3.47	2.0
09-6	3b	9	2.75	5.75	3.65	9.35	51.38	27.18	100.06	0.63	2.83	3.07
09-6	3b	10	5.56	6.15	3.39	11.84	46.60	26.49	100.03	0.71	2.03	2.60
09-6	3b	11	5.07	6.39	4.75	12.24	45.57	26.24	100.27	0.72	1.92	2.45
09-6	3b	12	15.69	13.12	2.88	6.62	32.53	28.76	99.60	0.77	2.53	0.85
06-2	3c	1	3.82	6.99	bdl	bdl	57.61	31.24	99.66	0.50		2.83
06-2	3c	2	3.41	6.2	bdl	bdl	57.81	31.89	99.32	0.47		3.20
06-2	3c	3	4.74	6.7	bdl	bdl	58.50	30.04	99.99	0.54		3.00
06-2	3d	1	1.77	6.36	bdl	5.85	56.48	29.32	99.77	0.54	4.97	3.05
06-2	3d	2	1.37	5.59	bdl	4.89	58.39	29.13	99.37	0.52	6.15	3.59
06-2	3d	3	1.83	6.39	bdl	bdl	60.20	30.97	99.39	0.47		3.24

Note: ¹atomic ratio. “bdl” stands for “below detection limit”.

. Rhenium sulfide was found in two samples. Sample 09-6 is an ultramafiс rock from the Zhelos intrusion with disseminated sulfides. In the sample, sulfides are presented by aggregates of troilite, pentlandite, pyrrotite, and chalcopyrite (Figure 2a,b). Magnetite occurs as veins crosscutting pentlandite grains and is located on the border of pentlandite and troilite grains (Figure 2a). Pentlandite (Fe_4.83–5.09Ni_4.31–4.05Co_0.10–0.12S₈, Figure 3b, point 1–3) sometimes contains numerous worm-like grains of mackinawite (Fe_0.83–0.75Ni_0.28–0.18Co_0.02Cu_0.01–0.02S, Figure 2b, point 4–6). Pentlandite contains up to 1 wt. % Co, the value of Fe/Ni ratio is 1.22–1.44 (Figure 2a, point 3–5). Re sulfide is hosted by chalcopyrite that also contains pentlandite lamellae (Figure 3a,b). The composition of chalcopyrite is characterized by a very small deviation from the stoichiometry in the direction of increasing Fe and the presence of 0.82 wt. % Ni. The chalcopyrite formula is consistent with Fe_1.02Cu_0.94Ni_0.04S₂. Pentlandite in "lamellae" contains 1.5 wt. % Co, and the value of the Fe/Ni ratio is 1.12–1.15. Platinum group minerals in this sample are represented by michenerite, zonal grain of hollingworthite–irarsite and sobolevskite. All minerals are localized in the marginal parts of chalcopyrite.

Sample 06-2 is vein chalcopyrite ore with a small amount of pentlandite in the amphibolite. Chalcopyrite occurs as anhedral patches (Figure 2c,d). In the marginal parts, it is replaced by limonite. Pentlandite mainly occurs as small grains associated with chalcopyrite. In this sample, Pyrrhotite is absent. The composition of chalcopyrite corresponds to the formula Fe_1.02Cu_0.98S₂. Pentlandite contains 1.97 wt. % Co and is characterized by a Fe/Ni ratio of 0.77. The pentlandite formula is Fe_3.82Ni_4.92Co_0.26S₈. Platinum group minerals are represented by testibiopalladinite, which is also closely associated with chalcopyrite.

3.4. Re Sulfide Composition

In sample 09-6 Re sulfide was found inside the grain of chalcopyrite (Figure 3a,b) in the form of three idiomorphic crystals of size 12 μm × 12 μm, 8 μm × 7 μm, and 5 μm × 2 μm. In all points, there is up to 5 wt.% Os. Rhenium content varies in the range of 46–51 wt.%. The composition of the largest grain is characterized by changes in the ΣMe/S ratio, with variations in the contents of Re, Mo, and Cu. In points 5, 6, and 9, the mineral formula, as calculated on the basis of S = 8 atoms, can be written as (Cu_0.71Fe_0.52–0.58 Mo_1.25–1.17Os_0.22–0.19Re_2.32–2.4)_5.01S₈. In points 7 and 8, the mineral formula, as calculated on the basis of S = 7 atoms, which can be written as (Fe_0.54Mo_0.87Os_0.23Re_2.37)₄S₇ and (Cu_1.13Fe_0.83Mo_0.66Os_0.15Re_2.27)_5.04S₇, respectively.

These compositional variations were confirmed by repeated analyses at neighboring points. The composition of the second grain in terms of the ratio ΣMe/S is close to that in points 7 and 8. It is characterized by higher contents of Fe and Mo; for points 10 and 11, they can be written as (Cu_0.81Fe_0.84Mo_1.05Os_0.15Re_2.12)_4.97S₇ and (Cu_0.85Fe_0.77Mo_1.09Os_0.21Re_2.09)_5.03S₇, respectively.

In sample 06-2, there is one large grain (7 μm × 20 μm) (Figure 3c) and a few small. The first and the three largest of the last (Figure 3d), with the size 3 μm × 5 μm, 3 μm × 2 μm, 2 μm × 2 μm, have been analyzed. Rhenium sulfide in this sample does not contain Os and in some cases Mo. Rhenium content varies in the range of 56–60 wt.%. For the first grain (Figure 3c, points 1–3) of the mineral formula, as calculated on the basis of S = 2 atoms, can be written as (Cu_0.22Fe_0.07Mo_0.13Re_0.66)_1.08S₂, (Cu_0.19Fe_0.05Mo_0.11Re_0.69)_1.04S₂ and (Cu_0.21Fe_0.07Re_0.67)_0.95S₂, respectively. The composition of fine grains (Figure 3d, points 1–3) is consistent with the formula (Cu_0.19–0.21Fe_0.12–0.18Re_0.62–0.67)_0.95–1.08S₂.

4. Discussion and Concluding Remarks

One of the major features of Re sulfide from the Zhelos intrusion is its high osmium content, similar to the mineral from the Ekojoki Ni–Cu (–PGE) deposit in Finland [11]. All rhenium-bearing minerals contain some radiogenic Os, which was formed as a result of the radioactive decay of the ¹⁸⁷Re by emission of β-particles. Since the half-life of ¹⁸⁷Re is 42.3 billion years, only very rich Re and geologically ancient minerals can contain enough ¹⁸⁷Os to be determined by the electron microprobe [11]. If we assume that all Os in the Re-sulfide crystal is radiogenic ¹⁸⁷Os, then the Re–Os model age will exceed 5 billion years.

Such an ancient model age indicates that not all Os in this sulfide is radiogenic and that Re–Mo–Cu–Fe–Os sulfide contained a significant amount of “common” Os at the time of crystallization.

The Cu and Fe concentrations measured in Re sulfides are realistic because they are substantially higher than can be expected due to the fluorescence from associated chalcopyrite and characterize steadily all grains, regardless of their location and size.

Re sulfides from the Zhelos and Tokty-Oi, closely resemble some of the Cu-containing rhenium sulfides reported previously, although there are certain differences. The ΣMe/S ratio for a part of our data (Figure 3b, points 5, 6, and 9) is consistent with that of the tarkianite with the composition corresponding to the ideal formula (Cu,Fe)(Re,Mo)₄S₈ [4]. A mineral of similar composition was identified in a Ni-rich sulfide from the Precambrian ultrabasic rocks (in Sweden) [7], Hitura Ni–Cu deposit (Finland) [6] and Lukkulaisvaara layered intrusion (Karelia, Russia) [3]. The sulfide from Zhelos is distinct from these by its high content of Os, but it does have a similar metal-to-metal ratio to that of the mineral from Lukkulaisvaara (Figure 4). Some of the data obtained (Figure 3b, points 7, 8, 10, and 11) have no known analogues in the metal–sulfur ratio.

Re sulfides from the Tokty-Oi have a ΣMe/S ratio as in rheniite (ReS₂) [13] or dzeskazganite (Re,Mo)S₂) [21], but differ from them by the presence of Fe and Cu and the metal-to-metal ratio. The composition of the Tokty-Oi minerals is closer to that of the Re sulfide from the Voronov Bor deposit (Russia) [14] in the (Cu + Fe + Ni + Co)–(Re + Os + Mo)–S ternary diagram and in the (Cu + Fe)–(Re + Mo + Os) binary diagram (Figure 4).

The sulfide mineral assemblages observed in the Zhelos and Tokty-Oi sulfide ores may be interpreted as the result of fractional crystallization of orthomagmatic sulfide liquids. The observed association of the rhenium sulfides with magmatic Fe–Ni–Cu sulfides suggests that it was formed from an immiscible sulfide liquid. It is well-known that Os, Ir, Ru, Rh (IPGE), Re, and Co are partitioned into MSS [22,23,24], however the localization of the Re sulfide within the chalcopyrite allows to speculate its crystallization from a residual Cu-rich liquid. The fractionation of original sulfide liquids resulted in a Cu-rich residue enriched in Re and some other chalcophile elements (Ag, Bi, Te). However, since high levels of rhenium are observed in all types of ores of the studied intrusions, it can be assumed that the initial magma was enriched in rhenium. So far, there is no information on the rhenium contents in the host rocks. Therefore, the question of what caused the high content of rhenium in the parental magma is open to debate.