U-Th-He Geochronology of Pyrite from the Uzelga VMS Deposit (South Urals)—New Perspectives for Direct Dating of the Ore-Forming Processes

Abstract

We report on the application of the U-Th-He method for the direct dating of pyrite and provide an original methodological approach for measurement of U, Th and He in single grains without loss of parent nuclides during thermal extraction of He. The U-Th-He age of ten samples of high-crystalline stoichiometric pyrite from unoxidized massive ores of the Uzelga volcanogenic massive sulfide (VMS) deposit, South Urals, is 382 ± 12 Ma (2σ) (U concentrations ~1–5 ppm; ⁴He ~10⁻⁴ cm³ STP g⁻¹). This age is consistent with independent (biostratigraphic) estimations of the age of ore formation (ca, 389–380 Ma) and is remarkably older than the probable age of the regional prehnite-pumpellyite facies metamorphism (~340–345 Ma). Our results indicate that the U-Th-He dating of ~1 mg weight pyrite sample is possible and open new perspectives for the dating of ore deposits. The relative simplicity of U-Th-He dating in comparison with other geochronological methods makes this approach interesting for further application.

1. Introduction

Isotope systems based on radiogenic helium are traditionally used in the field of low-temperature thermochronology mainly for evaluation of rock exhumation rates at shallow crustal levels [1], for dating volcanic eruptions [2] and regional weathering events [3]. Although, He can easily migrate from the crystal structures of most minerals over geological history [1], its preservation in native metals is very high [4]. Recent developments in understanding the behavior of He in minerals allowed Yu.A. Shukolyukov with co-authors to propose a new ¹⁹⁰Pt-⁴He method of isotope geochronology [5]. The method was successfully applied for dating Pt-Fe alloys from various deposits worldwide [6,7]. In addition, it was shown to be promising for dating sperrylite (PtAs₂) [8]. The ¹⁹⁰Pt-⁴He ages of this mineral from Norilsk (242 ± 12 Ma) and Kondyor (122 ± 6 Ma) deposits are consistent with biotite Ar-Ar ages, ~245–250 and 120 ± 1 Ma, respectively [9,10]. High retention of He in sperrylite suggests that pyrite, which has the same crystallographic structure, might also be a convenient mineral for geochronology. Recent experiments on helium thermodesorption from sulfides had confirmed this suggestion [11] and preliminary results of the U-Th-He dating of pyrite from the Uzelga deposit are promising [12].

Pyrite is abundant in the most of the ore deposits [13,14,15,16]) and is a common accessory mineral of igneous and sedimentary rocks [13,17]. Therefore, pyrite geochronology has a range of potential applications. In particular, the timing of ore-forming processes is a critical component in the development of genetic models for ore formation. In fact, tectonics, magmatism, and/or sedimentary basin evolution may exert a large-scale control on ore formation but this can be difficult to prove without robust age constraints [18].

Traditionally, sulfide mineralization is directly dated by the Re-Os method. Re-Os geochronology of molybdenite, pyrite and arsenopyrite has widely and successfully been used to date a large variety of ore types including orogenic gold, porphyry, sedimentary exhalative and Mississippi Valley-type deposits [19,20,21,22,23,24]. However, the results produced by pyrite Re-Os geochronology are complex and show a high degree of scatter and/or imprecise ages [23,25,26]. At present, it is clear that pyrite Re-Os geochronology should use only unaltered pyrite from a single paragenetic stage of mineralization [18], which is difficult to achieve when 0.4 g of material is required for the analysis.

The use of Rb-Sr, Sm-Nd and Ar-Ar isotope systems for dating pyrite is limited by a number of isotope-geochemical reasons related to low concentrations of parental isotopes. The application of these methods is complicated by the fact that in the vast majority of the cases, the age is obtained for minerals (inclusions) whose direct genetic relationships with the ore-formation process are often difficult to establish [27,28,29,30,31].

Herein, we present a novel approach for dating pyrite by the U-Th-He method. The potential of this technique is verified on example of the Uzelga volcanogenic massive sulfide (VMS) deposit, South Urals [32,33]. The deposit is considered to be a standard of the Uralian-type VMS deposits [34,35] and has been a key for the development of a convective-hydrothermal model of VMS formation in the arc-related complexes of the Urals [36,37].

Similar to other VMS deposits, the Uzelga deposit is closely associated with coeval volcanic, volcaniclastic and sedimentary rocks [38,39,40,41,42,43] and volcano-sedimentary Fe (±Mn)-silica jasperites [44,45]. Therefore, it has an obvious stratigraphic age that coincides with the age of the host series [35,46,47,48,49,50] and represents a relevant case study to validate the accuracy of geochronological data.

2. Isotope-Geochemical Constrains

The U-Th-He method is based on the accumulation of He atoms produced by the alpha decay of U and Th. In the U-Th-He system, the chemical behavior of parent and daughter isotopes contrasts most significantly compared to other radioisotope systems. This provides unique opportunities for dating of processes not accessible by other techniques [51].

2.1. Uranium and Thorium in Sulfides

The concentration of U in sulfides varies over a wide range from few ppb to tens of ppm [17,52,53,54]. In uranium deposits, the concentration of U in pyrite can be as high as 2–4 wt.% [55]. The distribution of U in this mineral is highly heterogeneous [52]. In submarine hydrothermal sulfides, U usually strongly prevails over Th (Th/U = 0.001–0.04; [56]).

Pyrite can uptake U from aqueous solutions due to redox reaction with U (VI) on its surface [57,58]. The very low solubility of U under dominant reducing conditions of ore forming processes ensures that it remains fixed within the sulfide [52].

Inclusions of U-Th-bearing minerals can be another source of U and Th in sulfides [59,60]. Submicron inclusions of uraninite, brannerite, monazite, apatite, zircon and xenotime are reported for copper-zinc VMS ores [32,34,61,62,63]. The size of these inclusions, as a rule, does not exceed a few microns [64]. Alpha particles produced from the decay of U and Th travel relatively long distance away from their parents [65]. The mean U decay energy is ~5.4 MeV [65], which means the average alpha-stopping distance in uraninite is ~10 µm (calculations in SRIM; [66]). Therefore, the radiogenic He produced in these inclusions mostly remains incorporated in the crystal lattice of the host pyrite.

Microradiographic studies have shown that for high-U sulfides, the inclusions of uraninite are more common. In low-U sulfides (<0.01%), neutron-induced fission tracks do not form radial patterns, which indicates a more homogeneous distribution of U, likely resulting from its adsorption during mineral growth [54].

2.2. Hydrothermal Fluid-Derived ⁴He

Pyrite is known to be an ideal mineral for preserving He from an ore-forming fluid [67,68,69,70]. Typically, the concentration of trapped He in sulfides from black smokers in modern seafloor hydrothermal sulfide fields does not exceed 10⁻⁸–10⁻¹⁰ cm³ STP g⁻¹ [71,72,73,74,75]. The amount of radiogenic ⁴He depends on the U and Th content in pyrite and its age. The concentration of radiogenic ⁴He in a 10-Ma old pyrite grain, which has 1 ppm U (Th/U = 0.001), is more than 10⁻⁶ cm³ STP g⁻¹. Thus, the amount of fluid-derived ⁴He may be considered insignificant for samples older than tens of millions of years. The younger grains of pyrite or those with lower U content, might require corrections for trapped He.

2.3. Retentivity of U-Th-He System in Pyrite and Mineral Stability

High-crystallinity pyrite has a particularly strong thermal retentivity for ⁴He [11]. Step-heating experiments have shown that He begins to migrate from pyrite crystal lattice at temperatures of ~450–600 °C, when the mineral begins its transformation into pyrrhotite (under anoxic conditions). This indicates that there is likely no loss of He at temperatures of pyrite stability. High retention of He in pyrite is indirectly supported by the results of successful ¹⁹⁰Pt–⁴He dating of sperrylite [8].

Pyrite is relatively fast oxidized under atmospheric or aquatic conditions [76]. However, under anoxic conditions pyrite has been shown to remain robust for billions of years even through post-mineralization thermal and metamorphic disturbances [77,78,79].

3. Object of Study

3.1. Geological Setting

The Uzelga deposit (54°07′ N, 59°20′ E) is one of the largest massive sulfide deposits of the Urals and contains about 4 Mt of Cu+Zn reserves. The deposit is located within the Verkhneuralsk ore district in the northern part of the East Magnitogorsk paleovolcanic belt (Figure 1) [32,37,80,81,82]. This belt comprises intra-oceanic arc-related complexes of Ordovician to Early Carboniferous ages [35,36,83,84,85].

The ore bodies of the deposit are mostly hosted by felsic, locally, altered rocks which form a large structure reminiscent of a volcanic dome (Figure 2). Primary volcanic structures are weakly deformed by small-amplitude faults. The ore-bearing basalt-rhyolite series belongs to the Upper Eifelian Karamalytash Formation and roughly corresponds to the australis-kokelianus zones [41], ca. 389 Ma [86]. The rocks are overlain by Givetian limestones, Lower Frasnian chert-siltstone horizon and Middle-Upper Frasnian volcano-sedimentary basalt-basaltic andesite unit [83]. The group of parallel WNW-trending subvolcanic mafic dikes of Givetian (?) age, which form thick (up to 20–30 m) steeply dipping bodies, cut the rocks of the Uzelga deposit [33,34].

Massive to semi-massive sulfide ores are located at two levels from the present surface, 130–380 m (bodies Nos. 1, 5, 6, 9) and 420–640 m (bodies Nos. 2–4, 7, 8) with 300–350 m between the levels (Figure 3). Ores are commonly slightly metamorphosed, retain relict syngenetic fabrics and they are considered as being close to primary ores that were deposited in seafloor [49] or seafloor/subseafloor [32,33,34,81,87] environments. The upper sulfide-bearing horizon is significantly enriched in tennantite-tetrahedrite, whereas the lower level is characterized by only a minor occurrence of tennantite and local enrichment in pyrrhotite (pyrite-pyrrhotite assemblage; ore body No. 4; Figure 3) [32,33,80,87].

The Uzelga deposit is an example of a long-lived, volcanic-related, seafloor/subseafloor ore-forming system [32,36,81,87,88,89]. The formation of main sulfide masses was synchronous with the Late Eifelian basalt-rhyolitic magmatism and was coeval with the terrigenous-carbonate sedimentation, ca. 389 Ma [34,35,40,41,44]. Hydrothermal processes remained active in the area throughout the Givetian stage, which is supported by—(i) the presence of disseminated sulfide mineralization in the Givetian limestones above the ore bodies of Uzelga deposit [35,37]; (ii) the position of ore body No 5, which is controlled by overlying limestones (Figure 3); (iii) the presence of large sulfide ore bodies (XIX Parts’ezda deposit) at 600 m distance within the Givetian rocks of the Ulutau Formation [80,90]. Sulfide mineralization is absent in the overlaying rocks, starting from the puncata zone [41,83], ca. 380 Ma [86]. Thus, the biostratigraphic age of ore mineralization of the Uzelga deposit is in a range of 389 to 380 Ma.

The VMS deposits of the Urals are subjected to varying degrees of later metamorphic events [34,91,92,93]. Ores of the Uzelga deposit mainly have massive textures. The common occurrence of colloform and breccias-like textures is evident of a low grade of post-depositional metamorphism [32,34,81] of prehnite-pumpellyite facies (~150–300 °C [93]). Zones of local recrystallization occur along rare steeply-dipping small-amplitude faults as well as in narrow (cm-dm thickness) contact zones of the mafic dikes. The largest area of ore recrystallization is located in the central part of the ore body 4 at the lower sulfide-bearing level corresponding to the occurrence of pyrrhotite-bearing ore on Figure 2 and Figure 3.

The age of regional metamorphism is consistent within the Magnitogorsk zone and is significantly distinct from the age of island-arc volcanism and massive sulfide formation [33,85,93]. The Ar-Ar ages of sericite from the Babaryk (Alexandrinka district, Figure 1) and Barsuchi Log (~380 km to the South) VMS deposits are 340 ± 7 and 345 ± 3 Ma, respectively [94]. The same range of values ~350 Ma is measured for sericite from the Gai VMS deposit (~350 km to the SSW) by K-Ar isochron method [32].

Re-Os ages of sulfides from VMS ores of Urals are typically ~30 Ma younger than the ages of host rocks and show some consistent values ~360–365 Ma, that at the moment do not have generally accepted explanation [94,95]. Re-Os ages for sulfides of the Alexandrinka deposit (Alexandrinka district, Figure 1), which is the nearest to the Uzelga deposit dated by this method, is 355 ± 15 Ma [96]. No Re-Os data are available for the Uzelga deposit. The Pb-Pb isotope study of galena from VMS ores of the Urals was shown to be more useful for determining the source of the metals, rather than for the model Pb-Pb dating [97].

3.2. Ore Geochemistry and Mineralogy

Cu-Zn massive ore at the Uzelga deposit is dominant with following average range of contents: 1–1.5 wt% Cu; 2.5–5 wt% Zn, 0.8–2.5 ppm Au, 15–80 ppm Ag, 20–150 ppm Se; and 30–200 ppm Te [33,100,101].

Pyrite is the major ore mineral (50–95 vol%). Chalcopyrite (1–5, up to 10 vol%) and sphalerite (1–10, up to 30 vol%) are the major economic minerals. Tennantite-tetrahedrite is a common mineral in both the lower (0.1–1 vol%) and upper (0.5–5 vol%) sulfide lenses. Galena occurs widely but in smaller amounts (0.1–0.5 vol%). Pyrrhotite is abundant in the lower ore-bearing level and comprises the considerable fragments in the central zone of ore body No. 4 (Figure 2 and Figure 3). Two massive ore types are dominant at the lower ore-bearing level—pyrite-sphalerite-chalcopyrite (minerals listed in decreasing abundance order) and a pyrrhotite-pyrite-sphalerite-chalcopyrite. The latter is dominant by pyrite-pyrrhotite (±sphalerite, ±chalcopyrite, ±siderite) assemblage.

By the beginning of the 21st century, the reserves of the Uzelga deposit were about 84 billion tons at an average Cu content of 1.36 wt.% and Zn content of 3.4 wt%, corresponding to about 1.1 Mt Cu and 2.8 Mt Zn [33].

3.3. Sample Location and Description

Pyrite grains for U-Th-He dating were manually extracted from a large (25 × 25 cm) specimen of pyrrhotite-pyrite-sphalerite-chalcopyrite ore of the lower horizon (sample Uz-190-5, level –550 m) in the southern part of ore body No. 4 (Figure 2 and Figure 3). The ore is characterized by a porphyry-like texture, marked by large, up to 5–8 cm, pyrite porphyroblasts enclosed in a fine-grained ground mass of pyrrhotite, minor sphalerite and rarer chalcopyrite (Figure 4). According to instrumental neutron-activation analyses (INAA), pyrite (sample Uz-190-5) is rich in Se (498 ppm), Co (357 ppm) and Te (117 ppm) and has lower contents of As (<1 ppm), Ag (<1 ppm), Sb (<0.3 ppm) and Au (0.2 ppm), comparing to typical pyrite of this deposit [33,101].

4. Analytical Methods

Prior to U-Th-He dating, pyrite grains in polished sections were studied on a Hitachi S-3400N scanning electron microscope equipped with an AzTec Energy 350 detector (“Geomodel” Research Centre, Saint-Petersburg State University). Grain crystallinity was studied using a Bruker D2 PHASER powder diffractometer, (CuK_α1+2 radiation, 30 kV/15 mA, 2theta degree step 0.02, count time for each step 0.6 s) (Research Centre for X-ray Diffraction Studies, Saint Petersburg State University). The data were processed using Rietveld refinement in TOPAS.

Thermal analysis of pyrite was undertaken using a NETZSCH STA 449 F3 Jupiter thermal analyzer (Department of Geochemistry, Saint-Petersburg State University, St Petersburg, Russia), which allows simultaneous measurement of the mass loss (thermogravimetric analysis, TG and DTG) and thermal effects (differential scanning calorimetry, DSC).

The fragments of pyrite grains 0.5–1.5 mg in weight were selected for U-Th-He dating. Following the recommendations of Reference [18], part of the grains were washed in 0.2 N HCl in an ultrasonic cleaner for 30 min to remove possible alteration phases. The concentrations of U and Th in the leached material were monitored with an ELEMENT XR ICP mass-spectrometer (Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences). Prior and after leaching, the samples were weighted on a Sartorius RC 210 P balance.

4.1. Measurement of Radiogenic ⁴He Contents

The ⁴He contents were measured at the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences (IPGG RAS) with a high-sensitivity MSU-G-01-M mass spectrometer. For extraction of He, we used original methodological approach, that excludes U and Th loss during annealing. The pyrite grains and their fragments ~0.5–1 mg in weight were placed in a quartz ampoule (~1 cm long) and sealed under vacuum conditions (10⁻³ torr) (Figure 5A). Using a special lock, the ampoule was transported into a Re cylinder and heated in several steps up to temperatures of ~1100 °C. When heated, He easily diffuses through thin walls of the ampoule, while U and Th remain inside. This approach avoids contamination of the sample with the material of the extractor of the mass spectrometer and also saves the sample for further determination of U and Th. Details of the He measurement technique and the design of the instrument are described in References [5,102]. The complete procedural blank determined by heating the empty ampoule in the Re cuvette to 1100 °C corresponds to ~5 × 10⁻¹⁰ cm³ STP, whereas the detection limit of the instrument is ~5 × 10⁻¹³ cm³ STP of ⁴He. After the extraction of He, the ampoule was removed from the mass spectrometer for subsequent separation of U and Th (Figure 5b).

The possibility of simple wrapping of pyrite grains in Ta foil for the extraction of He was also tested. The procedure is easier and the blank level is around five times lower that using quartz ampoule. In this case, if the U and Th are mainly preserved in the form of inclusions, then evaporation of S during heating in the extractor should not lead to loss of parent nuclides. The methodology of He extraction and U and Th analysis remained the same.

4.2. Measurement of U and Th Contents

The U and Th contents were measured by isotope dilution method. The quartz ampoule was opened and spiked with a combined ²³⁰Th–²³⁵U tracer. The samples were dissolved in closed Teflon vials for 24 h at a temperature of 130 °C in concentrated nitric acid. The products of the thermal decomposition of pyrite (Figure 5b) include sulfur, alpha-iron, pyrrhotite and troilite (X-ray diffraction (XRD) analysis). All these components are dissolved in concentrated nitric acid [103]. As was discussed previously, some part of U and Th in sulfides might occur in submicron-sized mineral inclusions, which probably does not dissolve in nitric acid. The dissolution of pyrite yields sulfuric acid, which enhances the dissolution of the submicron-sized inclusions of U and Th-bearing minerals [103]. The completeness of dissolution was monitored using an optical microscope. The ²³⁵U/²³⁸U and ²³⁰Th/²³²Th isotope ratios were measured on an ELEMENT XR ICP mass-spectrometer (Vernadsky Institute of Geochemistry and Analytical Chemistry).

4.3. Calculation of U-Th-He Age

Because of dating the fragments of larger grains, the calculation of U-Th-He ages requires no alpha-recoil corrections [65]. The amount of radiogenic ⁴He accumulated in the mineral over the time t is:

$${}_{ }{}^{4}He=8 {}_{ }{}^{238}U \left(e^{{\lambda }_{238}t}-1\right)+7{}_{ }{}^{235}U \left(e^{{\lambda }_{235}t}-1\right)+6{}_{ }{}^{232}Th \left(e^{{\lambda }_{232}t}-1\right),$$

(1)

where λ₂₃₈, λ₂₃₅ and λ₂₃₂ are the decay constants of ²³⁸U, ²³⁵U and ²³²Th, respectively. This equation has no analytical solution relative to t but easily be solved iteratively [104] and several options for approximate calculations exist that are remarkably accurate for all applications [105].

The average U-Th-He age can be calculated in different ways [104]—(1) arithmetic mean of the single-grain ages; (2) the “pooled” age; (3) the U-Th-He isochron age; and (4) the central age (HelioPlot; [106]). From the mathematical viewpoint, the latter method is the most accurate [104].

The accuracy of complete dating procedure (measurement of He, U and Th) was assessed by simultaneous experiments on the Durango apatite, which is the international standard for U-Th-He method. The calculated central age for five grains of Durango apatite is 32.2 ± 1.9 (2σ) Ma (MSWD 1.57), which is in good agreement with published data of 32.3 ± 0.9 Ma [107] (

Table 1. Results of U-Th-He dating of pyrite from Uzelga deposit and Durango apatite.

Sample ID	Weight, (mg)	4He, cm3 STP g−1	U, 1010 at	σ	Th, 1010 at	σ	4He, 1010 at	σ	T, Ma	σ
Uzelga pyrite
599	0.84	1.06 × 10−4	510	75	1.6	1.1	239	7	350	50
600	0.35	4.01 × 10−5	70.0	1.4	1.7	1.1	37.4	1.2	399	15
601	0.81	1.55 × 10−4	680	120	1.1	0.7	336	9	370	60
602	0.49	2.07 × 10−4	550	44	0.8	0.4	271	7	370	30
603	0.28	1.20 × 10−4	178	13	1.5	1.1	90.6	2.5	380	29
604	0.69	1.70 × 10−4	580	50	2.3	1.2	316	8	410	36
631	–	–	154	8	1.5	0.7	75	3	366	24
632	–	–	58	3	60	30	266	6	2370	170
633	–	–	1340	70	4	2	667	17	375	20
676 1	–	–	13.85	0.08	5.0	2.7	27	1	1240	58
677 1	–	–	10.52	0.03	3.3	1.7	26	1	1550	58
433	–	–	880	50	12.9	0.8	476	2	404	22
435	–	–	38	2	10.3	0.6	20	1	371	21
437	–	–	15	1	5.2	0.3	10	1	446	27
							Central age 2:		382	6
Durango apatite
660	–	–	1700	55	39,500	1200	412	8	29.6	0.6
661	–	–	2030	170	43,800	3400	434	9	27.8	0.6
662	–	–	850	60	17,000	1300	187	4	30.2	0.6
663	–	–	1240	60	29,000	1400	334	7	32.4	0.7
666	–	–	554	17	12,300	300	160	4	36.6	0.9
							Central age:		32.2	1.0
Qu blank	–	–	1.26		0.52		1.23
Ta blank	–	–	1.52		1.56		0.31

¹ in Ta foil; ² samples 676, 677, 632 and 437 were excluded from the calculation of the central age; at—atoms; T— age; “–” – has not been measured.

).

In total, we dated 14 fragments of pyrite with weights ranging from 0.5 to 1.5 mg (including nine earlier values [12]). Two pyrite grains were wrapped in Ta foil and others were sealed in the quartz ampoule (Table 1). The U-Th-He ages of each grain were calculated in HelioPlot software [106].

5. Results

5.1. Mineralogical Features

Scanning electron microscopy (SEM) analysis of the pyrite grains revealed the presence of micron-sized inclusions of uraninite, which are spatially associated with chalcopyrite veinlets (Figure 6). Rare inclusions of sphalerite, hematite, altaite, chromite and native gold are also observed within the grains.

XRD analysis showed that pyrite is characterized by high crystallinity—the size of the regions of coherent scattering (crystallite size) is more than the resolution of the diffractometer, which is 9000 nm.

Thermal analysis showed no thermal effects (the absence of oxidation or sulfur release) within the temperature range 25–400 °C, which indicates the high degree of stoichiometry of pyrite (Figure 7). No thermal effects that might be associated with the decrepitation of fluid inclusions were also observed. Three exothermic effects at the DSC curve at 480.3, 552.2–568.5 and 612.0 °C, were accompanied by mass loss, as indicated by the TG and DTG curves. The first two effects correspond to the breakdown of pyrite to pyrrhotite and magnetite [108,109]. The third effect may correspond to the decomposition of sphalerite inclusions [110].

5.2. Results of Leaching Experiments

Fragments of pyrite grains lost 0.2%–0.3% of their weight after 30 min washing in 0.2 N HCl in an ultrasonic cleaner (

Table 2. U and Th contents of the solution after washing of pyrite grains from the Uzelga deposit in 0.2 N HCl for 30 min in ultrasonic cleaner.

Num	Weight, mg	σ,%	Δ, mg	σ,%	Th, ng	Th, ppm	±	U, ng	U, ppm	±	Th/U
1	1.217	0.2	0.003	100	b.d.l.			0.23	80	80
2	0.751	0.4	0.003	100	b.d.l.			0.03	10	10
3	32.247	0.0	0.057	5	0.037	0.65	0.03	56.96	1000	50	0.0006
4	1.954	0.2	0.006	50	0.001	0.16	0.08	11.38	1900	950	0.0001
5	3.307	0.1	0.023	13	b.d.l.			6.50	280	40
6	1.358	0.2	0.002	150	b.d.l.			0.39	200	300
bl					0.001			0.001

Num – sample number; bl—blank; Δ —difference in sample weight before and after leaching.

). The Th content of the solution was low and typically did not exceed three times the blank level. The amount of U leached to the solution was remarkably higher, up to 57 ng. The leached grains were considered unsuitable for U-Th-He dating, as they have lost part of U.

5.3. U-Th-He Dating Results

The concentrations of ⁴He in pyrite grains were ~4 × 10⁻⁵–2 × 10⁻⁴ cm³ STP g⁻¹. From all samples, He was released at temperature of their decomposition into pyrrhotite and sulfur (~500 °C). The concentrations of U varied from 0.8 to 5 ppm. The average Th/U ratio was ~0.007.

Most samples ages in a range 350–410 Ma (Table 1). Two samples of pyrite (#676 and #677) that were wrapped in Ta foil gave remarkably older ages (1240–1550 Ma). They also showed very low concentrations of U (~50 ppb) and higher Th/U ratios (~0.3). This suggests loss of U during annealing of grains from an unsealed Ta envelope. Thus, these grains were ignored from calculation of the average (central) U-Th-He age. Pyrite grain #632 also gave an older age of ~2370 Ma and showed a very low concentration of U (~200 ppb) and atypically high Th/U ratio of 1. This anomalous grain was also excluded from the calculations, as well as grain #437 with very low ⁴He content.

The central age calculated for ten grains of pyrite is 382 ± 12 (2σ) Ma. The arithmetic mean is 386 ± 18 (MSWD 0.42) and the geometric mean is 387 ± 18 (MSWD 0.42). Ternary and log-ratio plots are not informative due to the low concentration of Th. Therefore, for visualization of the data, we plotted an U-Th-He isochron (Figure 8).

6. Discussion

The central age of 382 ± 12 Ma for pyrite from the Uzelga deposit is consistent with independent (biostratigraphic) estimations of the age of ore formation (ca, 389–380 Ma) and is remarkable older than the age of regional prehnite-pumpellyite facies metamorphism (~340–345 Ma; see Section 3.1). The pyrite U-Th-He ages are quite reproducible but show scattering in a range 350–410 Ma, which can have different explanations: (i) the presence of excess ⁴He; (ii) the loss or input of U, Th, He during geological history; (iii) methodological imperfections.

6.1. Excess ⁴He

There are several reasons of ⁴He excess in minerals: (i) trapped hydrothermal ⁴He; (ii) inclusions-related excess of ⁴He; (iii) implanted radiogenic ⁴He.

6.1.1. Trapped Hydrothermal ⁴He

The concentrations of He in submarine hydrothermal fluids are 10⁻⁶ to 10⁻⁵ cm³ STP ⁴He g⁻¹ [112]. Modern submarine hydrothermal sulfides typically yield 10⁻⁸–10⁻¹⁰ cm³ STP ⁴He g⁻¹ [71]. Helium concentrations in them do not show any relationship with geodynamic settings (fast/slow spreading mid ocean ridge; back-arc basin) or water depth [113]. There is no correlation between concentration of trapped He and its isotopic composition [113,114]. The study of He in modern massive sulfides from Middle Valley which are mainly deposited at the subseafloor environment (Ocean drilling program Leg 139, Site 856) show the same range of He concentrations [114].

The measured concentrations of ⁴He in pyrite from the Uzelga deposit are several orders of magnitude higher (10⁻⁴–10⁻⁵ cm³ STP g⁻¹) than the concentrations of He that is trapped by modern hydrothermal sulfides. Thus, if we assume that the pyrite had trapped the fluid with the same proportion as modern ocean sulfides the contribution of fluid-derived helium can be considered insignificant.

Studied pyrite porphyroblasts is a product of interaction of primary ores of the lower level with later high-temperature hydrothermal fluids, that might be enriched in He. By the moment the highest measured concentration of trapped He in sulfides is that in arsenopyrite from the Panasqueira deposit (1.2 10⁻⁶ cm³ STP g⁻¹; [69]). The Panasqueira deposit is not a VMS type deposit, however, if we assume that concentration of trapped He in the pyrite-pyrrhotite assemblage of the Uzelga deposit is within the same range as in arsenopyrite from the Panasqueira deposit, the contribution of trapped component in the total balance of ⁴He would not exceed 3%.

6.1.2. Inclusions-Related Excess of ⁴He

Primary ores were precipitated in the seafloor environment as well as in subseafloor position – beneath weakly lithified calcareous, siliceous and silty sediments inside of felsic volcanic unit [81,87,115,116,117,118]. Thus, they had a chance to trap some relatively large (>30 µm) minerals inclusions (zircon, rutile, etc.), that had already accumulated some ⁴He. The studied samples were derived from the pyrrhotite + pyrite (±sphalerite, chalcopyrite, siderite) ores that were formed at the latest stage of hydrothermal process at 300–400 °C (up to 520 °C [32]). This temperature is far above than closure temperature of U-Th-He system in most U and Th-bearing minerals (70–250 °C) [51]. Thus, it is very unlikely that any ⁴He remains inside the trapped mineral inclusions.

6.1.3. Implanted Radiogenic ⁴He

It is difficult to estimate the amount of alpha-particles generated within the fragments of pyrite grains that have been ejected and implanted as the distribution of U and Th in pyrite appears to be heterogeneous (see Section 2.1) [65]. The diameter of the studied pyrite grains (~300–800 µm) are remarkably larger than alpha-stopping distance in pyrite, which is ~18 µm (SRIM [66]). Thus, the fraction of implanted ⁴He, relative to retained ⁴He might be considered as insignificant [65].

6.2. U-Th-He System Behavior in Pyrite

The open behavior of the U-Th-He system can be related to the He loss, U, Th input or U and Th loss. The first two processes lead to relatively younger U-Th-He ages, when the last one tends to increase the age value.

6.2.1. Uranium and Thorium in Pyrite

The concentrations of U and Th in pyrite grains from the Uzelga deposit (0.8 to 5 ppm) are within the range typical of sulfides from modern black smokers (0.00–11 ppm; [17,52,53,54]). Uranium strongly prevails over Th, which is consistent with its inferred hydrogenic origin [52]. SEM analysis of pyrite grains show that part of U is associated with micron-sized inclusions of uraninite that are spatially related to chalchopyrite veinlets (Figure 6b). The size of the inclusions is small enough (<10 µm) so that all He produced in them is implanted into pyrite due to alpha-recoil effect (see Section 2.1).

The apparent ‘aging’ of pyrite grains that were annealed in the Ta foil indicates a loss of U during heating. Uraninite is a highly refractory compound. At 1300 °C, the UO₂ vapor pressure is negligible (~1.6 × 10^–8 torr). Thus, it is unlikely that heating the sample to 1100 °C could lead to significant loss of U from uraninite inclusions. More likely, part of U occurs in absorbed form within pyrite grains.

Surprisingly, washing of pyrite grains in 0.2 N HCl led to leaching of a significant amount of U (Table 2). Simple calculations allow us to estimate that the concentration of U in dissolved material reaches 1–2%, with a Th/U ratio of ~0.003. The 0.2 N HCl solution is a very weak acid that can mainly dissolve alteration phases or probably some carbonates [103]. However, it is highly unlikely that the concentration of U in any associated carbonate could be as high as 1000 ppm, as oceanic carbonates have up to 2 ppm U [119]. Among U-rich minerals, amorphous uranium oxides are the most soluble. Thus, due to the radiation damage, the zones around U-rich inclusions in pyrite and uraninite itself probably became highly defective, metamict and therefore easily dissolved. Mobilization of U is crucial for retentivity of U-Th-He system in pyrite as it indicates that even at the room temperatures U can be washed out from pyrite grains by weak acid solutions. This phenomenon requires additional study.

6.2.2. Radiogenic ⁴He in Pyrite

Radiogenic He released from the studied samples at the temperatures of pyrite decomposition (>450 °C). All grains gave U-Th-He age that is remarkably older than the age of a regional metamorphic event (~340–345 Ma; see Section 3.1). This indicates high retention of radiogenic He in pyrite during the prehnite-pumpellyite facies metamorphism (150–300 °C) which is in a good agreement with kinetic experiments [11]. There is still some probability that He loss occurred simultaneously with U loss, which may explain the absence of younger ages. Taking into account the reproducible U-Th-He ages and contrast geochemical behavior of He, U and Th, it seems to be very unlikely.

6.3. Methodological Imperfections

The main methodological disadvantages that could lead to scattering of U-Th-He ages are incomplete release of He from pyrite grains and its incomplete decomposition. The temperature of ⁴He release from the grains was ~1100 °C. That is more than enough to release all He from pyrite grains [11]. The pyrite grains were dissolved in no HF acid (see Section 4.2), as we tried to avoid the dissolution of fragments of the quartz ampoule. Although we registered no undissolved mineral inclusions in the vials after the treatment the sample by HNO₃ acid, some small silicate U,Th-bearing inclusions might remain undissolved. Likely it is the case of the grain #632 that gave remarkably older age.

6.4. Comparison of Pyrite U-Th-He and Geological Ages

The age value of 382 ± 12 Ma without error for pyrite from the Uzelga deposit tend to the lower boundary of the biostratigraphic age interval 389–380 Ma.

The pyrrhotite-bearing ore (ore body No 4; Figure 3 and Figure 4) with studied pyrite porphyroblasts is a product of interaction of early ores of the lower level with high-temperature hydrothermal fluids that formed upper sulfide-bearing horizon (ore body No 4 is situated just beneath ore body No 5; Figure 2 and Figure 3) [32,34]. Pyrite porphyroblasts were formed due to retrograde desulfurization of pyrrhotite [115,120]. Spatial association of uraninite with chalcopyrite veinlets, which cross-cut the pyrite porphyroblasts, indicates that the age likely reflects the final stage of synvolcanic ore formation.

7. Conclusions

The possibility of U-Th-He dating of pyrite is shown on example of high-crystallinity stoichiometric pyrite from the Uzelga VMS deposit, South Urals. The U-Th-He age without error for ten pyrite fragments from a large specimen of pyrite-pyrrhotite massive ore of 382 ± 12 Ma (2σ) is in agreement with biostratigraphic age estimations of ore-bearing hydrothermal activity (389–380 Ma).

In general, single-grain U-Th-He ages tend to suffer from overdispersion with respect to the formal analytical precision [121]. Thus, reliable U-Th-He dating require a set of analyses (multi single aliquot dating). The U-Th-He dating has certain obvious limitations. Very small grains (less than 200 µm) will lose a large percentage of He due to the alpha-recoil effect. This problem is solved by introducing an additional mathematical correction but it greatly increases the uncertainty of measurements [121]. Grains that are younger than tens of millions of years or have relatively low U content (<100 ppb) should require additional corrections on trapped fluid-derived ⁴He. Sample preparation seems to be very important for accurate He dating. Routine prewashing of grains in 0.2 N HCl is not suitable for the removal of possible alteration phases, as it can also mobilize U. Development of new protocols are required. Nevertheless, the U-Th-He dating technique is rather promising, as it allows to date single pyrite grains (~500 µm).

Several theoretical problems should be solved to make this technique widely acceptable. Will it be possible to date non-stoichiometric pyrite, since it is much less stable? Does recrystallization completely reset the U-Th-He system? It is unclear how trace elements (i.e., As, Co, Ni, Au, etc.) affect the retentivity of He in the pyrite. It is also unclear if the zonation of pyrite affects the age values and whether the rims can differ significantly in age. The problem of the behavior and preservation of U in pyrite remains open. The relative ease of U-Th-He dating in comparison with other geochronological methods and small amount of the material (~0.5–1 mg) required for this dating technique makes this approach interesting for further development.