Dispersed organic matter characteristics as an indicator of rock alteration degree of the Safyanovskoe copper-sulphide deposit (Middle Urals)

Annotation Relevance of the study is due to the importance of composition and maturity of the dispersed organic matter (DOM) as indicators of rock forming conditions, which may contribute to the paleoecological reconstruction of sedimentation conditions for rocks in the ore-bearing stratum of the Safyanovskoe deposit. Objective: to analyze the composition and characteristics of DOM in carbon-siliceous rocks of the ore-bearing stratum of the Safyanovsky copper-pyrite deposit. The obtained characteristics of the DOM make it possible to reveal the source of original accumulation and the level of its transformation under the influence of various geological processes. Methods. A detailed study of DOM was carried out by the EPR method (electron paramagnetic resonance) for powder preparations. EPR spectra were recorded at room temperature on X-band spectrometers DX-70, ESR 70-03 DX/2, and SE/X-2547 RadioPAN. The analytical procedure for studying organic matter (OM) included: determination of insoluble residue and Corg content in the rock, extraction of the chloroform bitumoids (CB) and the alcohol-benzol bitumoids (ABB), humic acids (HA), determining the group composition of CB and hydrocarbons (HC), chromatographic fractionation with determination of the sum of methane-naphthenic and aromatic fractions of hydrocarbons, GC-MS analysis of n-alkanes, cyclic and polyaromatic hydrocarbons (PAHs). GC-MS analysis was performed on the complex Hewlett Packard 6850/5973 with a quadrupole mass detector and analytical information processing software. Results. Analysis of the EPR spectra of carbon-siliceous rocks of the Safyanovskoe deposit showed the presence of the two types of carbon signal (C-org), characteristic of the plant and the animal residues. It was also found that the DOM has never been exposed to temperatures greater than 300 °C. Geochemical analysis of OM indicates that DOM is at a high maturation stage (residual organic matter (ROM) > 99%). But the type of distribution of polyaromatic hydrocarbons (PAHs) suggests that the primary OM was specifically altered at elevated temperatures. Conclusions. DOM of carbon-siliceous rocks of the Safyanovsky deposit is genetically the same, its type is mainly sapropelic and accumulation is associated with marine conditions. A characteristic feature is, on the one hand, its high polymerization, which is typical for DOM of the stage of late mesocatagenesis, and, on the other hand, its molecular composition does not allow us to talk about the maturation of DOM in the process of natural regional metamorphism.


Introduction
Safyanovskoe deposit is located within the East Urals rise in the southern part of the Rezhevskaya structural-formational zone (Fig. 1). It is localized in the altered volcanic and volcano-sedimentary rocks of acid-medium composition, which opened in the career of the Main ore deposit.
Main ore body of massive pyrite ores has a length of 400 m and a width of up to 140 m. The southern flank of the ore body rapidly tapers out, and the northern passes in a series  (southwestern flank of the Safyanovskoe ore field). Here they are in contact with Devonian amphipora bituminous limestone and serpentinite of the Rezhevsky massif.
Carbon-bearing-siliceous rocks are represented by psephite-psammitic tuffites, crystal-lithocrystalloclastic and pelitolitic. The structure consists of fragments of quartz, plagioclase crystals, angular-rounded fragments of rhyodacites, shells of foraminifera, radiolarians, and accumulations of carbon-bearing organic matter (COM), carbonates as well as chlorite, mica, kaolinite [3]. Sandstones are found in the ore-hosting mass of series and have virtually identical composition, graded bedding and, sometimes, wavy contacts with siliceous-carbonaceous rocks (Fig. 2, a).
The composition and characteristics of dispersed organic matter (DOM) is an important indicator of rock facies and genesis of ore formation. With that in mind we have been studied carbon-bearing-siliceous rocks of ore-hosting strata of the Safyanovskoe copper-sulfide deposit (Middle Urals).

Materials and methods
Carbon-bearing siliceous rocks of the ore-hosting strata exposed by mine workings at the contact of limestone and  (Fig. 2, b). It was determined that in the south-east direction they taper out and grade to carbon-bearing-siliceous rocks (Fig. 2, c). Out of fault zone limestone are micro granular argillaceous-detrital broken by cracks with calcite and carbon-bearing-siliceous material. In a limestone it is observed that amphipora interlayer capacity of 0.1-0.2 m occurred through 0.5-1.0 m. In order to determine the genesis of DOM we studied limestone opened by drift and carbon-bearing-siliceous rocks drift and quarry.
Carbon-siliceous formations are with carbon-bearing veinlet opened by mining, black and consist of quartz, chlorite, mica and plagioclase. There are impurities of calcite, magnesite, siderite, pyrite and COM in the rock matrix. They are identical on mineral composition to overlying formation rocks exposed by quarry. An earlier thermal analysis of the silica-carbonaceous rock samples, breccias and circum-ore metasomatites showed slightly metamorphosed organic matter (OM) of plant origin in amounts up to 6%, which burns at 200-330 °C [6] in studied samples.
The paramagnetic properties of the Safyanovskoe deposit rock containing DOM were studied by EPR (electron para- Note. The mineral composition of the rocks is defined by X-ray phase analysis on diffractometer XRD-7000 (Shimadzu) (operator O. L. Galakhova), organic content determined by the thermal method (analyst V. G. Petrishcheva). Analyses are carried out in physical and chemical test laboratory of the Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences the city of Ekaterinburg. magnetic resonance). Spectra of paramagnetic carbon radicals are relatively simple and have been described for many solids and liquid natural OM as well as products of their thermochemical transformations.
Samples of carbon-bearing-siliceous rocks of quarry were studied in the Physics of minerals laboratory of KFU by A. A. Galeev and laboratory of physical and chemical test laboratory of the Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences (IGG UB RAS) by Yu. V. Shchapova. In the Physics of minerals laboratory of KFU EPR spectra were recorded from a powder weighed quantity of 20-40 mg of initial and heated samples at temperatures of 350 and 600 °C for 30 minutes. The recording was made at room temperature in the automatic mode on a DX-70 portable spectrometer with operating frequency 9.272 GHz. In the IGG UB RAS laboratory ESR spectra were recorded on spectrometer 70-03 DX/2 at room temperature (initial test) and after samples heating up to 300 and 600 °C for 30 min. Studies of limestone and carbon-bearing-siliceous rocks exposed by mine workings were carried out in the laboratory of mineralogy of the Institute of Geology of Komi Science Centre UB RAS by V. P. Lyutoev on X-band spectrometer SE/X-2547 RadioPAN. Recording mode is identical. The results are shown in table 2.
Modern geochemical studies at the reconstruction of facial-genetic conditions commonly use biomarker analysis. The amount of hydrocarbons (HC) in the composition of the DOM and their structural features serve to characterize  Note: I. R. (ir) -insoluble residue; A chl -chloroform bitumoid; А alb -alcohol-benzene bitumoid; HA -humic acid; β -bituminosity coefficient; HC -hydrocarbons; Me-Nf -methane-naphthenic fractions of hydrocarbons; Ar -aromatic hydrocarbon fraction; ROM (residuel organic matter) -degree of polymerization; CPI -oddity coefficient. Weighed quantity of sample D16/12 to 104.5 g sample weight D14/12 100 g. Sample data L.K. IV-th of the Safyanovsky quarry is given according to citation 2 [2]. the genetic type of OM and its catagenetic transformation degree.
Earlier geochemical OM analysis of carbon-bearing-siliceous rocks of the Safyanovskoe deposit (height 157 m; quarry depth 50 m) showed that it belongs to the sapropelic type [2]. We have recently carried out a geochemical OM study of rocks exposed by mine workings at a depth of 285 m. The OM component composition is shown in Table 3.
The analytical procedure used in the OM study included determination of the rock insoluble residue and content С org , chloroform extract (A chl ) and alcohol-benzene (А alb ) bitumens, humic acids (   Results and discussion Fig. 3 shows the typical ESR spectra of carbon-bearing-siliceous rocks from the Safyanovskoe deposit quarry. Analysis of the spectra showed the presence of two types of signal С org specific to plant and animal remains [7]. The spectra, as in the initial samples as well as after heating at 300 °C, reveals a signal in the С org area with a broad spectrum line (Fig. 4). The signal characteristics after annealing at 300 °C: g ~ 2,0031 ± 0.0001 and line width ∆B ~ 0.5-0.7 mT, indicating the presence of OM plant residues metamorphosed at relatively low-temperature conditions (not exceeding 300 °C) [8]. Once heated to 600 °C С org , a signal with g ~ 2.0027 ± 0.0001 appeared as a narrow spectrum line (∆B = 0.15-0.2 mT). The signals from these parameters are peculiar to the OM remains of protein series [7]. Perhaps, this OM formed as a result of spoilage microorganisms on the stage of sedimentation and early diagenesis.
Analysis of the EPR spectra of DOM rocks exposed by mining revealed its identity to quarry samples. It was metamorphosed under temperature not higher than 300 °C ( Table  2). A sample of limestone SH10/12 revealed two types of signals С org , which may indicate similar depositional conditions for carbon-bearing-siliceous formations as well as limestone. Judging by the intensity of the spectrum in sample 1346 of quarry [6] and SH16/12 (Table 2) carbon-bearing-siliceous rocks, the concentration of paramagnetic centers С org -1.3 • 10 18 spin/g, and in samples of limestone SH11/12 -2,6 • 10 18 (Table 2), which is typical of some types of coal [8]. Scanning electron microscopic images of carbonaceous streaks in siliceous-carbonaceous rock obtained using a REMMA-202M microscope (operator E. V. Nuzhdin) show a structurally uniform black surface with a shell fracture, which may be characteristic of vitrinite (Fig. 4). It can be assumed that the carbonaceous material was at a low (brownish) stage of metamorphism and is now represented by fusenized (inertinite) and vitrenized (gelified) plant residues as well as soluble compounds that were originally part of the lipoid microcomponents of resins.
In addition, in the spectra of carbon-bearing-siliceous rocks (SH16/12 and 1346) the signal E' in the center of quartz are observed (Fig. 3). Preservation of this center throughout the history of rocks also indicated the low-temperature transformation of the rock. Additional centers were identified in the EPR spectra of limestone (sample SH10/12): center SO 2 -(g = 2,005) and axial CO 2 -(g = 1,999; 2,003), which indicates the low temperature effects on limestone not exceeding 250 °C.
The EPR spectrum of the sample SH15/12 (g = 2.0026, Table 2) is typical for OM series of bitumen, which g-factor value can be attributed to oxykerit with the line width and the presence of a weak signal after heating at 600 °C. Except for the value of the g-factor, oxykerites similar to some shungites of Karelia (g = 2,0024) [9]), which are on the regressive stage of metamorphism.
As a result of chemical analysis OM of carbon-bearing-siliceous rocks of the Safyanovskoe deposit it was found that in spite of a fairly high content of С org (4.19%) and chloroform bitumoid (A chl = 0.011%), DOM is at the advanced stage of transformation (ROM > 99%) [10]. This is consistent with the absence of humic acid.   [12].
Polyaromatic hydrocarbons. The distribution of PAHs is shown in Fig. 5. Based on the above data, a feature of the test sample is a high content of PAH. As part of the aromatic hydrocarbons fraction they represent approximately 65% (MPI -0.40 -transformation level index of non-substituted phenanthrene and its methyl homologues; Fl/202 -0.25 fluoranthene share in the total content of molecular group 202). А alb = 2.9), and the value of β (0.25) is much lower than 1 (Table 3), which indicates synergetic nature OM. It is known that β -is the main indicator at the identification of the genetic type bitumoid its sinergeticity or epigeneticity in relation to the host rocks. In the case of syngenetic bitumoid its value does not exceed 5-10 [11].
The hydrocarbon content of COM and OM in the rock is relatively low, but in their group composition dominates aliphatic compounds.
Normal alkanes and isoprenoids. Paraffin hydrocarbons are genetic markers that allow determining the origin of the initial organic material. The parameters characterizing the distribution of hydrocarbon alkane series (С [17][18][19] ) and (С 27-31 ) make it possible to indicate the fraction of aquatic and terrigenous biota in the formation of OM bottom sediment [12]. The molecular structure of n-alkanes and isoprenoids shows mostly hydrobionts genesis of the original OM, as indicated by the ratio of the markers of sapropel and humic compounds (С 17 /С 29 = 10.88) as well as the mono-modal nature of the n-alkanes distribution with the overwhelming dominance of low molecular weight compounds (С 15-19 ) (Fig. 5). The oddity coefficient CPI, close to 1 indicates the high degree of n-alkanes conversion and OM as a whole. Isoprenoids pristane and phytane ratio (Pr/Ph < 2 and Pr/C 17 = 0.45) is typical for marine deposition of sediments [12,13]. Terpanes and steranes. Distribution of cyclane hydrocarbons (terpanes, hopanes, cyclanes) in the studied sample also indicates mainly sapropelic genesis of OM. This is evidenced by the value of tricyclanes coefficients (C 19-20 T ric / C 23-26 ; C 23-26 T ric /C 28-31 ) and the ratio of norgopan, hopane and C 31 -homohopane that can serve as indicators of the origin of the initial organic material [14]. A data presented in Table 4 clearly demonstrate the significant role of the "sea" hydrobiont OM in the original organic material.
In the geological objects hopanes are present in the form of homologous series of compounds of С 27 -С 35 . At the stage of sedimentogenesis and early diagenesis, biogenic hopanes (ββ-hopanes) are formed, at the postdiagenetic stage, take the structural transformation of hopanes occurs with formation αβ-and βα-hopanes. For hopanes of composition of С 31 -С 35 , enantiomers of the configuration R in S are isomerized. Absence in the composition of terpanes, biohopanes and moretanes (ββ-and βα-isomers) indicates the postdiagenetic transformation of DOM. At the same time the ambiguity of hopanes maturity level of DOM (Ts/Ts + Tm, 22 S/ 22 S + 22 RC 31 ) does not allow to characterize the maturity levels of DOM as reached the thermodynamic limit. . This assumption is also supported by the distribution of sterans (Table 4), whose maturity indices are far from thermodynamic limit and do not exceed the values characteristic of the early stage katagenesis (МК 1 ).
In a homologous series of steranes (С 27 -С 30 ) dominated cholestanes (С 27 ), which reflect the contribution of aquatic organisms in the composition of initial OM. The ratios of regular and rearranged isomers in the homologous series of steranes are also aimed at determining the genetic type of the initial DOM. These data compare favorably with the above mentioned values for alkanes and terpanes.
Unlike conventional sedimentary rocks [12], PAHs are dominated by unsubstituted condensed polyaromatic compounds with four to seven rings (MM 178,202,228,252,276,278). Thus the pyrogenic compound, the formation of which is associated with a high impact on the DOM, constitutes 94% of the PAH amount (Fig. 6). Naphthidogenic PAH are typical of sedimentary rocks, being formed by the transformation of DOM during lithogenesis. However, in this case it is a minor component. This type of PAH distribution suggests that initial OM undergone specific transformation under high temperatures.
As a result of geochemical researches it can be concluded that the OM carbon-bearing-siliceous rocks of the Safyanovskoe deposits genetically the same type, mainly sapropel and its accumulation is due to sea conditions. A characteristic feature is, on the one hand, its polymerization, which is typical for the DOM of the stage of late mesokatagenesis. On the other hand, its molecular structure cannot be indicative of the maturation of DOM during natural regional metamorphism. The specifics of the geochemical parameters of DOM can be due, for example, to the effect of high temperatures associated with the intrusive processes (contact metamorphism) or post volcanic hydrothermal activity.
As noted above, in some physical and chemical parameters (sample D 15/12,  [15]). Temperature of metamorphism of Shungovskoe deposit is estimated at 300-330 °C [16]. However, unlike shungites, the HC of the Safyanovskoe massive sulfide deposit contains aromatic compounds with a high content of PAH, which means a lower degree of metamorphic transformation. A geochemical study of OM of modern sulfide hydrothermal sediments of the Mid-Atlantic Ridge (i. e., in the Ashadze, Lost City, Rainbow and Broken Spur fields) showed its mixed genesis, the specificity of which is due to accelerated OM maturation processes under extreme environmental conditions [17,18]. Pyrogenic compounds in PAH composition were identified in the OM bottom sediments within the Ashadze-1 hydrothermal field [17]. This pyrolytic component is a narrow range of compounds (Fl/202 -0.7). Given the presence in the composition of cyclanes, cheilanthanes, diasteranes and geogopanes as sufficiently pyrogenic indicators of aromatic hydrocarbons, being indicators of the maturity level of the n-alkanes substance (CPI-1), the authors [7] concluded that the accelerated thermal catalytic maturation of OM in sediments of the Ashadze-1 field under the influence of the hydrothermal conditions of the environment was "expressed in the seal and the polycondensation of geochemically less stable structures. " Hydrocarbon distribution in the bottom sediments of the hydrothermal fields of Lost City and Rainbow (MAC) is char-acterized by the presence of even-numbered homologues in the low-and high-molecular-weight portions of the area, as reflected in the CPI parameter values, which average 0.86 and 0.89, respectively [19]. This is a result of reduction processes that have occurred in the hydrothermal transformation of OM. Moreover, in the sulfide sediments of the Rainbow and Broken Spur fields there are n-alkanes of abiogenically thermo-origin and sediments of active smokers characterized by high levels of long-chain n-alkanes ΣС 23 -С 35 -85.7% [18].
According to data from our research, the OM of the Safyanovskoe deposit was exposed to the low temperature -not higher than 300 °C. Since, according to the EPR data, it presedved paramagnetic tracers of plant and animal origin (Sample SH10/12, Table 2), as well as radiation centers in minerals, for some samples, in particular, limestone, temperature exposure limit was not higher than 250 ° C. Given the data [20] on the forming conditions of barite veins in the formation of ore facies of the Safyanovskoe deposit as well as in the formation temperature of chlorite wall rock [21], we can assume that the temperature range of wall-rock alteration, in the presence of rocks containing OM, ranges from 130° to 260 °C.