Carbonaceous substance of the Sukhoi Log gold deposit (East Siberia)

Russian Geology and Geophysics 49 (2008) 371–377 www.elsevier.com/locate/rgg

Carbonaceous substance of the Sukhoi Log gold deposit (East Siberia) E.A. Razvozzhaeva, V.K. Nemerov, A.M. Spiridonov, S.I. Prokopchuk * Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia Received 4 April 2007; received in revised form 11 July 2007; accepted 7 September 2007

Abstract Insoluble carbonaceous substance (ICS) at the Sukhoi Log gold deposit has been studied following the scheme: source shales — concentrates — residual substance of rocks. Examination by electron microscopy, transmission electron microscopy with electron microdiffraction, thermography, and mass spectrometry revealed several morphogenetic ICS varieties: dot-drop-like, honeycomb, single graphite crystals, and spherical graphite crystals. Study of source shales and concentrates has shown their similar carbon isotope compositions (δ13Cav = 18.03–17.54‰). Residual carbonaceous substance is characterized by a heavy carbon isotope composition (δ13Cav = 10‰). Its enrichment with heavy carbon isotope, as compared with the source rocks, calls for special geochemical studies. The ICS heterogeneity in the Sukhoi Log shales points to variations in the physicochemical settings of ore formation at the deposit. © 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Carbonaceous shale; insoluble carbonaceous substance; concentrate of insoluble carbonaceous substance

Introduction Investigation into the nature of carbonaceous substance and its role in the transition, concentration, and dispersion of ore-forming elements is one of the top-priority problems of modern geology and, in particular, ore genesis in black-shale sedimentary-metamorphic strata. The challenge of the research is a difficult analysis of metal contents in black shales. There are voluminous literature data on study of carbonaceous substance (CS) in black-shale deposits (Buryak and Khmelevskaya, 1997; Ermolaev et al., 1999; Kucha, 1982; Yudovich et al., 1990). The CS of metamorphic rocks is considered mainly as a sorbent of metals, including gold. The CS concentrates obtained from black shales in laboratory experiments are the product of transformation of marine-deposit CS during sedimentogenesis, diagenesis, catagenesis, and metamorphism. The CS of sediments was successively transformed into bio- and, then, geopolymers, finally passing into the insoluble carbonaceous substance (ICS) of black-shale deposits, which is a rock-forming component. Concentrates of ICS of shales consist mainly of carbon. For example, carbon in ash-free ICS concentrates from ores of the * Corresponding author. E-mail address: [email protected] (S.I. Prokopchuk)

Sukhoi Log deposit amounts to 95–96% (Razvozzhaeva et al., 2002). According to Fridman et al. (1982); Hausen and Kerr (1968), Varshal et al. (1991), and Volkova and Bogdanova (1980), ICS of shales is a sorbent of noble metals, including gold. But analyses of carbonaceous rocks for noble metals showed an unsatisfactory precision of results obtained by different methods. The contradiction and uncertainty of analytical data are most often due to the presence of carbonaceous substance in the rocks (Kurskii et al., 1995). Earlier, in analysis of carbonaceous shales for noble metals, the samples were preliminarily calcined at temperatures of up to 600 °C. But Varshal et al. (1991) experimentally established that thermal treatment of samples under these conditions does not ensure a complete oxidation of CS. Shales obviously contain both oxidizable and unoxidizable carbon, which testifies to the complex composition of their CS. Thus, the rocks have specific carbon species calling for nonstandard methods of analysis. Elaboration of such methods and study of the geochemical composition of black shales are impossible without investigation into the nature of carbonaceous substances, first of all, ICS, the main component of the dispersed carbonaceous substance (DCS) of black-shale deposits. The goal of this work was to study the geochemical composition of insoluble carbon of shales and its species, identify ICS compounds, elucidate their binding with metals,

1068-7971/$ - see front matter D 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2007.09.015

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etc. The main task was to carry out a complex study of carbon following the scheme: ICS in the source rocks–ICS concentrates (flotation method)–residual carbon after the extraction of floated carbon (ICS concentrate).

Object of study We studied carbonaceous shales from the Sukhoi Log deposit localized at the center of the Lena gold-bearing area, in the Bodaibo synclinorium composed of Riphean carbonaceous-terrigenous and terrigenous-carbonate rocks (Buryak and Khmelevskaya, 1997; Wood and Popov, 2006). The terrigenous carbonaceous rocks enclosing mineralization were metamorphosed under greenschist facies conditions. They compose the Upper Riphean Khomolkho Formation. According to data of microscopic studies, their mineral composition is as follows: quartz (10–62%), plagioclase-sericite-chlorite aggregate containing plagioclase (10–25%), chlorite (5–20%), sericite (up to 10%), and biotite (1–5%). The bulk content of carbon varies within 0.5–7%. There are also minor amounts of rutile, ilmenite, tourmaline, leucoxene, sphene, zircon, epidote, and apatite. A specific feature of the formation deposits is the presence of clastics of siliceous rocks (2–3%). According to Neelov’s (1980) classification, the formation shales correspond in petrochemical parameters to silt pelites of low to medium alkalinity. Their Mg content is considerably higher than that of Ca, i.e., these are highly magnesian shales. They are also characterized by high Fe/(Fe + Mg) values (Nemerov, 1988). The samples of carbonaceous quartz-sericite-chloritic shales provided by V.V. Kotkin (VostSibNIIGGiMS, Irkutsk) were taken in the central part of the ore zone of the Sukhoi Log deposit (BH-292, BH-180, BH-286) and on its eastern flank (BH-671) (Fig. 1).

Analytical technique Insoluble CS was extracted from preliminarily debituminized carbonaceous shales in aqueous medium without heating. The extraction was performed by the flotation method with the use of petroleum ether. Concentrates of ICS were united into a single sample and repeatedly washed with water. With the subsequent centrifugation, the substance was settled onto an X-ray film cleaned from emulsion. The dried substance was taken from the upper part of the film and studied (Razvozzhaeva, 1978, 1983). The content of carbon in the initial rocks and ICS concentrates was determined by the classical combustion method (Korchagina and Chetverikova, 1976). The content of gold in shales was determined by the extraction–atomic-absorption method. Its detection limit was n⋅10−7% (Men’shikov et al., 1977). Analysis of ICS concentrates for noble metals was carried out by the direct atomicemission method, which also permitted estimating their distribution between the mineral and carbonaceous components of the flotation products of the shales (Vasil’eva et al., 1997, 2005). The isotopic analysis of carbon (13C/12C) was made on a VARIAN-VFN-203 mass spectrometer, using the international RDV standard sample; the instrumental error was ±0.2–0.3% (analyst M.P. Bogacheva, Institute of Geochemistry, Moscow). The CS of ores and ICS concentrates was studied on a Labirbux Carl Zeiss Jena (Tesla-620) electron microscope, and thermal analyses of shales and ICS concentrates were carried out on a MOM derivatograph designed by F. Paulick, I. Paulick, and L. Erdey (Hungary) (analyst V.V. Kotel’nikov, VostSibNIIGGiMS, Irkutsk).

Fig. 1. Schematic map of the Sukhoi Log deposit. 1 — Quaternary deposits; 2–5 — formations: 2 — Aunakit, 3 — Imnyakh, 4 — Middle Khomolkho Subformation, 5 — Upper Khomolkho Subformation; 6 — thrust zone, 7 — faults; 8 — zone of intense sulfide mineralization; 9 — boreholes.

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E.A. Razvozzhaeva et al. / Russian Geology and Geophysics 49 (2008) 371–377 Table 1 Contents of bulk carbon (%) and gold (ppm) in initial rocks from the Sukhoi Log deposit Borehole (sample), depth range, m

Rock

Cbulk

Au

292, 88–93

Carbonaceous quartzchlorite-sericitic shale

0.93

0.96

286, 74–117

The same

0.85

0.96

671, 63–65

The same

0.66

0.49

180, 88–185

The same

0.70

0.60

Note. Atomic-absorption analysis for gold was carried out by G.A. Vall (VostSibNIIGGiMS, Irkutsk).

Results The shales under study (samples 292, 180, 286, and 671) have a rather homogeneous chemical composition. Their bulk contents of carbon vary within 0.66–0.99% (Tables 1 and 2). The ICS of carbonaceous rocks is the main component of their DCS, because the contents of bitumoid (0.00⋅n%) and gas components are minor. We revealed two kinds of ICS (Tesla-620) in the shales. One form — dot-drop-like (free form) — consists of 1–3 µm long particles, which regularly penetrate through the hosted minerals. But their dispersion density in the rock is different. In places, the CS density increases so that the rock becomes black, nontransparent in transmitted light. The fine-banded distribution of CS coincides with the rock layering. The drop-like variety seems to be the primary CS form syngenetic to the host rock. The other CS variety consists of 20–30 µm long honeycomb particles (bounded form) connected with each other and forming thin intergrowths with the terrigenous material. Besides the above morphologic forms of CS, we also found (with electron microscopy and electron microdiffraction) occasional graphite single crystals in the Sukhoi Log ores, which evidence that the ICS undergoes the initial stage of graphitization. Small graphite crystals yield dot-ring patterns on electron-microscopic images, and larger crystals, dotted reflexes (Fig. 2). Next, we studied ICS concentrates (Razvozzhaeva, 1983) extracted from the deposit ores. These are highly dispersed black substances consisting mainly of carbon. Carbon enrichment sometimes yielded ash-free ICS concentrates. According to X-ray diffraction analysis (DRON-3 diffractometer, CuKα, Ni filter), ICS of concentrates (of different degrees of carbon

enrichment, including ash-free concentrates) from the studied shales is composed of irregular-structure graphites. We successively extracted ICS concentrates (I and II) from the shales (Table 1, sample 292). Concentrate I (93.3% carbon) was obtained by the flotation method (rock treatment with water and petroleum ether). Concentrate II (87.7% carbon) was extracted from the same shale sample after the preparation of concentrate I through the dissolution of carbonate-silicate mass of the rock in HCl and HF according to the technique of Razvozzhaeva (1983). The ICS of concentrate I is likely a free dot-drop-like form easily separable from the mineral part of the rock (the substance floats in the aqueous medium). The ICS of concentrate II might be tentatively assigned to the bound (honeycomb) form. With a POOS-1 X-ray diffractometer, we measured reflection dispersions for the ICS of these concentrates. As seen from Fig. 3, concentrate II shows a slightly higher reflection power than concentrate I. According to the SEM analysis with electron microdiffraction (JEM-100 C electron microscope), the ICS of concentrates from the Sukhoi Log ores is mainly amorphous (Distler et al., 2003). In sample 292, amorphous ICS occurs as two forms of aggregates of ultrafine oval particles (600–1000 Å) and films. Comparing the CSs of ores and ICS concentrates, we assumed that the free dot-drop-like form corresponds to aggregates of ultrafine oval particles and the bound honeycomb form, to films on mineral phases. In addition to amorphous CS, the concentrates also contain graphite single crystals in the form of fine (∼600 Å) scales, which are scarce in the ores of the initial rocks. These crystals yielded the ring diffraction patterns of the ICS, showing only three clear reflexes. Reflex 100 was the most distinct, and following reflex 101 was not detected. This indicates that the graphite crystals are strongly “corrugated” along plane {001}. In samples 292 and 180, Distler et al. (2003) revealed (for the first time for Sukhoi Log shales) large curled graphite spherules. These particles are of great interest as a new morphogenetic kind of ICS and thus call for further study. Note that the deposit ores contain occasional graphite crystals, which accumulate during the concentration of substance. Thus, we have recognized the following morphologic varieties of insoluble carbon in ores and ICS concentrates: dot-drop-like, honeycomb (600–1000 Å), graphite single crystals (∼ 600 Å), and curled graphite spherules (∼1 µm). We carried out a thermal analysis of ICS concentrates from the sampled shales (Table 3). The temperatures of the beginning of exothermal reactions (T0) for these samples do not exceed 500–570 °C. According to the temperature scale of the degree of ICS metamorphism, based on the temperatures of the

Table 2 Chemical composition of carbonaceous shales from ores of the Sukhoi Log deposit (wt.%) Sample SiO2

TiO2

Al2O3 Fe2O3 FeO

MnO

MgO

CaO

Na2O

K2O

Li2O

Rb2O

Cs2O

292

56.75

0.88

17.42

2.20

3.95

0.08

3.10

0.60

1.51

3.28

0.014

0.013

180

55.75

0.81

16.44

2.40

3.17

0.09

3.40

0.80

1.64

3.15

0.016

286

57.01

0.79

17.05

2.10

3.80

0.11

3.00

1.00

1.70

3.01

0.015

671

59.01

0.88

15.41

3.17

2.24

0.12

3.10

1.10

1.88

2.86

0.014

P2O5

F

Stot

LOI

Total

0.0004 0.14

0.50

1.20

10.11

100.75

0.011

0.0005 0.10

0.47

2.10

9.65

100.00

0.010

0.0004 0.12

0.42

1.30

8.96

100.40

0.012

0.004

0.50

0.89

8.52

100.63

0.14

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Fig. 2. Electron diffraction pattern of graphite crystals (sample 292 from the Sukhoi Log deposit).

beginning of exothermal effects, the studied ICS concentrates are assigned to graphitoids and cryptocrystalline graphites of muscovite-chlorite and biotite-chlorite greenschist subfacies (Ivanova et al., 1974; Kotkin and Titkova, 1982). All shale thermograms have two exothermal maxima evidencing the presence of two CS forms oxidizing at different temperatures. These data point to the inhomogeneous composition of ICS from the Sukhoi Log ores. The content of carbon in residual ICS (IS remained after the extraction) is 0.08–0.19%. The carbon isotope composition of the shales was studied following the scheme: initial rock–ICS concentrate (kerogen)–

residual carbon (Table 4). It varies from 17.27 to 18.00‰. The same range of isotopic composition is typical of carbon of ICS concentrates (17.30–17.90%). The carbon isotope composition of the residual ICS is 9–11%. Thus, two types of carbon are recognized according to δ13C values. One type is shales and ICS concentrates with δ13C = 18.03–17.54‰, and the second type is carbon of residual ICS with δ13C = 10‰. Thus, the residual carbon is enriched with heavy isotope by more than 7‰ as compared with the carbon of the initial ores and ICS concentrates (Fig. 4).

Discussion

Fig. 3. Dispersion reflection power of ICS concentrates. I — dot-drop-like concentrate I (93.3% carbon), II — honeycomb concentrate II (87.7% carbon). R — Reflection power, λ — wavelength.

Insoluble carbonaceous substance, the main component of the dispersed CS of the Sukhoi Log shales, is mainly finely dispersed and distributed throughout the rock. The CS of the deposit ores underwent not only metamorphism but also hydrothermal-metasomatic transformations (Nemerov et al., 2005). The results of complex study of the shale ICS point to the inhomogeneous composition of CS. This is well seen in studying CS forms (free, bound, and graphite microcrystals). The substance inhomogeneity is also confirmed by thermographic data (thermograms have two CS oxidation maxima) and the heavier isotopic composition of residual carbon, which is most firmly bound to the shale minerals.

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E.A. Razvozzhaeva et al. / Russian Geology and Geophysics 49 (2008) 371–377 Table 3 Thermal analysis of ICS concentrates from ores of the Sukhoi Log deposit Sample

C, %

0

max

Tend

Type of ICS by Krasavina’s (1973) and Panyak’s (1973) T0 scales

°C 292

50.05

500

600, 680

740

Graphitoid

180

47.11

570

650, 690

750

Graphitoid + cryptocrystalline graphite

286

62.05

510

610, 690

740

Graphitoid

671

71.70

520

600, 680

760

The same

Note. C — Carbon content in ICS concentrates; temperature of: T0 — beginning of the reaction, Tmax — maximum exothermal effect, Tend — end of the reaction. Table 4 Carbon isotope composition of shales, ICS concentrates, and residual ICS Sample

Initial rock

ICS concentrate

Residual ICS

Corg, %

δ13C, ‰

Carbon content, %

δ13C, ‰

Carbon content, %

δ13C, ‰

292

0.99

–17.27

20.56

–17.56

0.15

–10.00

180

0.70

–17.49

31.30

–17.30

0.08

–10.00

286

0.85

–18.00

71.70

–17.90

0.19

–9.00

671

0.66

–17.75

62.05

–17.40

0.17

–11.00

Average

0.8

–18.03

46.40

–17.54

0.15

–10.00

Special attention must be given to noble-metal analysis of carbonaceous shales from gold deposits. On calcination of shales at 600 °C, their CS is destroyed incompletely (Varshal et al., 1994). This is the reason for the discordance and nonreproducibility of the results of analyses for noble metals (gold). By the example of ores from the Kumtor (Kirghizia) and Bakyrchik (Kazakhstan) deposits, we have experimentally established that most part of CS oxidizes, and the residual substance, being activated on heating and under the effect of acid, is responsible for gold loss. Hence, the residual carbonaceous substance in the sample might possess specific properties different from those of CS oxidizing at these temperatures. The oxidation of CS is drastically intensified in the presence of catalysts, such as lead nitrate (Varshal et al., 1994). We have established that part of CS (residual substance) is preserved in the sample at 600 °C and is oxidized only at 750 °C in the presence of catalysts. These data also evidence that the CS of shales is a mixture of heterogeneouscarbon compounds. The CS heterogeneity is confirmed by the carbon isotope compositions of initial ores, ICS concentrates, and residual carbon not extracted on flotation and remained in the shale minerals. Indeed, carbon isotope analysis of the residual ICS showed its enrichment in heavy carbon (δ13Cav = 10‰) as compared with the ICS of initial ores and concentrates (δ13Cav = 18.03–17.54‰) (Table 4). The ICS of the Sukhoi Log rocks is inhomogeneous (data of electron microscopy, thermography, carbon isotopy). This explains the discrepancy (Kurskii et al., 1995) in results of noble-metal analyses. Comprehensive high-resolution electron-spectroscopic studies of highly carbonaceous ICS concen-

trates from the Sukhoi Log ores (Distler et al., 2003; Pavlova et al., 2005) revealed microparticles of native metals, in particular, platinum (Fig. 4). Thus, native-metal particles found in highly carbonaceous concentrates (Corg = 50–90%) occur in insoluble-carbon matrix. It is probably this natural screen that hinders ores from being decomposed and analyzed for noble metals. High contents of platinum in the ICS concentrates and its particle

Fig. 4. SEM images of platinum particles 2×4 µm in size.

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Fig. 5. Distribution of platinum particles in concentrates. a — Sample 292 (Corg = 91.78 wt.%, Pt = 500 ppm), b — sample 180 (Corg = 53.31 wt.%, Pt = 1000 ppm).

size are confirmed by scintillation analysis (Razvozzhaeva et al., 2002) (Fig. 5). In addition to Au and Pt, being in paragenesis with the ICS, we discovered a series of other native metals and minerals in the CS concentrates, namely, molybdenite, rhenium bisulfide, and compositionally unusual PtTlCl3, many of which have been preserved owing to the carbonaceous matrix of the Sukhoi Log ores (Distler et al., 2003). The coexistence of noble-metal and carbon nanoparticles in ore-bearing black shales, most likely, evidences the in situ separation of the metals at the initial metamorphogenic stage of ore formation under the greenschist facies conditions (420–380 °C, 5–6 kbar) rather than their external supply. Moreover, these metals are separated from the naphthide component of CS (Nemerov et al., 2005). These anoxic conditions provide a highly reducing medium, in which metals pass into a native form and rare minerals are produced. The minerals form parageneses with carbon typical of the Sukhoi Log ores. At the same time, reduced compounds with light carbon isotope (e.g., methane) are removed; thus, residual carbon probably becomes enriched in 13C. The participation of CS in ore formation processes is partly confirmed by the fractionation of carbon isotope composition. The established existence of at least two or three ICS varieties points to a stage-by-stage change of the physicochemical conditions of the ore formation (Nemerov et al., 2005) and/or the existence of an additional source of CS at one of the stages (Razvozzhaeva et al., 2006). It is not ruled out that both factors influence the genesis of both CS and ores of the Sukhoi Log deposit localized in black-shale formations.

Conclusions Insoluble carbonaceous substance is the main component of dispersed carbonaceous substance. Three morphologic types of ICS have been recognized: dot-drop-like, honeycomb, and graphite crystals. The difference in the determined δ13C values of CS provides additional information on the genesis of carbon of the Sukhoi Log deposit. The data obtained give an insight into the nature of carbon and its role in ore formation processes. This work was supported by grants 05-05-64466 and 05-05-97301-r-Baikal from the Russian Foundation for Basic Research.

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