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Natural electric fields in Siberian gold deposits: structure, origin, and relationship with gold orebodies
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ScienceDirect Russian Geology and Geophysics 58 (2017) 984–989 www.elsevier.com/locate/rgg
Natural electric fields in Siberian gold deposits: structure, origin, and relationship with gold orebodies L.Ya. Erofeev †, A.N. Orekhov *, G.V. Erofeeva National Tomsk Research Polytechnical University, pr. Lenina 30, Tomsk, 634050, Russia Received 23 March 2016; received in revised form 8 November 2016; accepted 6 December 2016
Abstract Characteristics of the constant natural electric field in the Siberian gold ore areas are given. The regularities of spatial variations in the electric-field potential and the parameters and properties of anomalies have been established. The cause of the natural electric field in deposits of major genotypes has been elucidated. It is shown that the electric field is induced mainly by physicochemical processes running in electron-conducting syn-ore metasomatites and by circulation of groundwaters. Orebodies do not influence significantly the structure of the observed electric fields. We give recommendations on application of the electric-field method at various gold ore objects. © 2017, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: electrical prospecting; natural electric field; gold deposits; Siberia
Introduction The self-potential (SP) method appeared in the geological practice in the first quarter of the 20th century and justly attracted the attention of both ore deposit prospectors and specialists in engineering and geological research. The peak of its use in Russia and abroad was in the second half of the 20th century. By that time, the physicomathematical and petrophysical basis of the method had been generally created, and the corresponding work practice and interpretation methods had been developed (Abdelrahman et al., 2006; Bigalke and Grabner, 1997; Ogil’vi, 1990; Ogil’vi et al., 1987; Parasnis, 1965; Revil and Jardani, 2013; Ryss, 1983; Semenov, 1974; Sveshnikov, 1967). By now, a large amount of factual material has been accumulated owing to the results of the practical SP method application. These data permit one to determine the structure and origin of the natural fields and assess the method potentialities in real physicogeologic conditions during the exploration for a definite type of mineral resources or the solution of particular standard geological problems. In this article, the above issues are considered with regard to gold deposits of the Siberian region.
†
Deceased.
* Corresponding author. E-mail address: [email protected] (A.N. Orekhov)
Gold deposits are localized mostly in the folded framing of the southern and southeastern areas of the West Siberian Platform (Kuz’min et al., 1999). The natural electric field was first discovered by V.V. Borodin during the prospecting for gold deposits in West Sayan (Ol’khovka–Chibizhek gold ore district) in 1934. Later, the SP method was widely applied at Siberian gold deposits (Chebakov and Roshchektaev, 2001; Erofeev et al., 2003; Krasnikov et al., 1967; Mozgolin, 1978; Narseev and Kurbanov, 1989; Nikiforov, 1945; Seifullin, 1965; Shatrov, 1979; Vovchenko, 1968a,b). To date, geophysical SP surveys (usually on a scale of 1:25,000 and larger) have been made within most of the ore fields of large gold deposits and gold ore districts in the Sayans, Yenisei Ridge, Lena gold ore province, and gold ore provinces of southeastern Siberia. Today, the SP method is often applied together with other geophysical methods during prospecting for gold (Saukov, 1975). The data of long-term field measurements of the SP parameters in different gold ore provinces of Siberia testify both to the partial similarity of the electric fields of deposits and to their considerably different relationships with the products of ore-forming processes in the regional gold ore areas.
1068-7971/$ - see front matter D 201 7, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 7.07.009
L.Ya. Erofeev et al. / Russian Geology and Geophysics 58 (2017) 984–989
The structure of the natural electric field and the parameters and origin of its anomalies The amplitude of the SP anomalies at gold deposits usually varies within several hundred mV, locally reaching 1 V and more (for example, at the Olimpiada deposit). The period of the spatial SP variations ranges from few tens to few hundreds of meters. By the spectral composition, these variations are conventionally divided, in a first approximation, into two types: low-frequency low-amplitude (usually few tens of mV) anomalies and high-frequency large-amplitude ones, relatively isomeric or linearly elongate. These types of SP variations exist not at all fields. Only “low-frequency” SP variations are always observed (Fig. 1a, c). Both types of variations exist only at some deposits, with “high-frequency” perturbations being predominant (Fig. 1b) and distorting or concealing the “low-frequency” background. Spatially large low-amplitude EF anomalies are caused mostly by groundwater circulation (usually at a velocity of about 20–40 m/day) in the subsurface zones of geologic environment (so-called filtration fields) owing to a double electric Layer in the capillaries. These SP variations are in an inverse correlation with the day surface relief (Fig. 1). Another group of local SP anomalies (high-amplitude ones) at gold deposits is typical of rock sites enriched in electronconducting minerals, such as graphite and metal sulfides (most often, pyrite). The nature of SP anomalies in the areas with
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such sites is generally known (Bigalke, 1997; Ogil’vi, 1990; Parasnis, 1965; Revil and Jardani, 2013; Ryss, 1983; Semenov, 1974). They are caused by physicochemical processes running at the boundaries between electron conductors and the enclosed watered rocks with ionic conductivity. The SP intensity is determined both by the difference in the redox potentials of waters circulating at different depths (Bigalke, 1997; Parasnis, 1965; Semenov, 1974) and by the electrode potential of electron conductors, which is not constant at the deposits but lies nearly in the same range of values (200–500 mV) and follows the near-normal distribution law (Fig. 2). The electrode potential of graphite is, on average, higher than that of sulfides. In the study area, including the gold ore fields with a permafrost rock bed, the shape and amplitude of the “highfrequency” EF variations depend neither on the season (Fig. 3a) nor on the year of observation (Fig. 3b) significantly governing the physicochemical conditions in the active (in terms of electrical conductivity and water circulation) horizon. This indicates that the SP anomalies under study, in contrast to the “low-frequency” ones, form with the participation of pellicular (loosely bound) and capillary waters freezing at temperatures much below 0 ºC. For example, pellicular waters freeze at –78 ºC (Saukov, 1975). The regional gold deposits can be divided into three significantly different groups according to the type of relationship between the SP morphology and gold orebodies.
Fig. 1. Self-potential at gold deposits: a, Aprelkovo; b, Sarala; c, Tsentral’noe. 1, granodiorite; 2, basic effusive rocks; 3, argillaceous-carbonaceous shales; 4, ore vein; 5, trench and its number.
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Fig. 2. Statistical distribution of the electrode potentials of pyrite (I) and carbonaceous substance of shales (II) at the Sarala gold deposit (Mozgolin, 1978).
Deposits with dielectric pre-ore metasomatites In the ore fields of gold deposits where the natural electric field manifests itself only as “low-frequency” variations, the orebodies are not related to the SP. They are not accompanied by local anomalies, i.e., do not induce a SP, though they contain up to 10% and more electron-conducting metal sulfides. Such deposits occur in almost all gold ore districts of Siberia (Kuz’min et al., 1999). The absence of local SP anomalies is observed at the gold deposits assigned (according to the geological and mineralogical classification (Benevol’skii and Vartanyan, 2002)) to a vein gold–sulfide–quartz association. Their orebodies are an aggregate (vein) of en-echelon lenses of sulfide and gold clusters in quartz veins, where they are deposited in narrow (few mm) quartz cracks.
The veins are usually several decimeters or, seldom, up to 1–2 m (in swells and bonanzas) in thickness and extend for several tens of meters along the lens strike and dip. The lenses in the vein plane are separated from each other by a barren rock, and the veins in the contact zones are rimmed with altered enclosing metasomatic rocks: syn-ore listwaenites developed after gabbroids, beresites developed after granodiorites, or quartzites in terrigenous sedimentary strata. These are thin light rock zones several decimeters to 1–2 m in thickness. They are of variable composition (carbonates, quartz, sericite, and chlorite). All these objects are dielectrics. If this physicogeologic setting is not significantly disturbed by post-ore tectonic impacts that make a vein crushed and ground, intense redox reactions are impossible in it, because the electron and ionic conduction of the enclosing geologic environment is hot high enough for current flow. A classical example of such objects is the Tsentral’noe deposit in Kuznetsk Alatau and the Aprelkovo deposit in the gold–molybdenum belt of eastern Transbaikalia (Erofeev and Orekhov, 2014), which have been studied in detail and have been in the prolonged commercial use. No noticeable EF anomalies are observed at the deposit sites with quartz–goldore veins containing up to 10–15% sulfides (Fig. 1a, c). Both deposits are entirely localized in granodiorite massifs, have almost the same structure, and comprise orebodies similar in morphology, composition, and gold contents but formed at different times: The Tsentral’noe deposit formed during the Paleozoic tectonomagmatic cycle, whereas the Aprelkovo deposit, during the Cimmerian cycle (Fogel’man, 1968; Timofeevskii, 1952). Analysis of the SP of these deposits shows that the significant difference in the time of their formation and their spatial location do not affect the preservation of their orebodies.
Fig. 3. Comparison of the SPs in the gold ore areas of eastern Transbaikalia measured at different times: a, December 1959 (1) and September 1961 (2); b, 1936 (1) and 1941 (2) (Seifullin, 1965); 3, granodiorites; 4, quartz diorites; 5, pyritization zones; 6, gold ore vein.
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Fig. 4. Distribution of orebodies within the natural electric field at the Darasun deposit: 1, quartz–gold–sulfide veins (I, Glavnaya (Major); II, Novo-Kuznetsovskaya; III, Svintsovaya (Lead); IV, Elektricheskaya-2; V, Karpaty; VI, Sergo); mineral assemblages in ore veins: 2, copper–antimony; 3, quartz–pyrite; 4, quartz–tourmaline; 5, SP isolines (mV).
Deposits with electron-conducting pre-ore metasomatites (sulfides) The electric field is of different morphology within the ore fields of gold deposits where pre-ore metamorphism processes of different intensities took place and formed large sites with electron-conducting rocks. At the deposits localized in intrusive or terrigenous carbon-free rock complexes, these are sites significantly enriched in metal sulfides, and at the deposits formed in black-shale strata, these are sites enriched in sulfides and graphite. An example of a deposit with sites of sulfidized rocks is the well-known Darasun gold deposit in the gold–molybdenum belt in eastern Transbaikalia. It has been comprehensively studied in terms of geology and geophysics (Lokotko and Shadrin, 1972; Zvyagin and Sizikov, 1971). Figure 4 schematically shows the location of major ore veins within the deposit and the SP isolines. The Darasun deposit is composed of Early Proterozoic basic intrusive bodies and Middle Paleozoic granodiorites. Its orebodies (gold-quartz–sulfide veins) are not genetically related to these intrusive complexes but formed in the Mesozoic. Ore formation here proceeded in several stages, with temporal breaks. The first stage (pneumatolithic-hydrothermal) was the appearance of sites with pyritized rocks. At the following stages (solely hydrothermal), veins of clear shape with up to 40% sulfides formed, which were few tens to few hundreds of meters long and up to few decimeters thick in the swells (Timofeevskii, 1957; Zvyagin and Sizikov, 1971).
Comparison of the scheme of orebody localization with the plan of the SP isolines clearly shows that some ore veins (e.g., Glavnaya and Karpaty) lie beyond the SP anomalies, whereas several veins cut the SP anomaly zones. This is best seen for the Novo-Kuznetsovskaya and Svintsovaya veins. Many veins localized within the SP anomalies do not manifest themselves in this field and do not even coincide in strike with the SP anomaly zone. Ore veins of various hydrothermal mineral assemblages, from quartz–tourmaline to arsenopyrite ones, are randomly distributed within the natural electric field (Fig. 4). All this unambiguously indicates that the electric field at the Darasun deposit is induced by the sites with pyritized rocks, whereas quartz–gold–sulfide veins do not contribute to its origin. A similar situation is clearly seen in the Maiskoe gold deposit, whose orebodies formed in the Triassic terrigenous sedimentary strata at the sites with syn-ore sulfide mineralization (Mezentseva and Esipenko, 2014) (Fig. 5). Deposits with electron-conducting pre-ore metasomatites (sulfides and graphitoids) The most intense natural electric fields are detected at the largest gold deposits of Siberia, such as Olimpiada, Sukhoi Log, Chertovo Koryto, Zun-Kholba, and others, and at the smaller ones, e.g., Sarala in Kuznetsk Alatau and the deposits in the Ol’khovka–Chibizhek ore zone, Bodaibo region (eastern Transbaikalia), Yenisei Ridge, etc. These electric fields are induced by the carbonaceous horizons (or their sites) of a terrigenous sedimentary rock complex, which underwent in-
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Fig. 5. Self-potential (isolines, mV) at the Olimpiada gold deposit (compiled by V.I. Klimenko and L.V. Polyakov). 1, orebodies.
tense metamorphism at the pre-ore stage of the deposit formation. The orebodies of these deposits are localized either in carbonaceous horizons or, more often, in near-contact zones, where carbonaceous rocks were subjected to sulfidization under the impact of metasomatism processes and carbonaceous substance transformed into an electron-conducting mineral (graphite or schungite (graphitoid)) during metamorphism. In this case, the electric field is also related to redox processes running at the sulfide boundaries. These processes are favored by a polarized current conductor, namely, an ordered carbonaceous substance (graphite), which creates a low-resistivity medium intensifying the oxidation, transition, and deposition of sulfide components and increases the amplitude of the SP anomalies (Parasnis, 1965; Ryss, 1983; Semenov, 1974; Sveshnikov, 1967). The gold orebodies localized in metasomatites inducing the electric field do not seriously affect its structure (Fig. 5). As seen from the figure, the gold orebodies of the Olimpiada deposit are generally confined to the gradient zone of a large intense negative anomaly of the natural electric field but do not manifest themselves. A similar physicogeologic pattern is observed in many gold ore areas of the Bodaibo region. Here, in the pre-ore mineralization zone of carbonaceous rocks, the SP also varies considerably, but the orebody does not cause any anomalous effects (Orekhov, 2015).
Conclusions Thus, there are constant natural electric fields at the gold ore deposits in Siberia, with their potential varying, on average, within a few hundred mV. The most intense EFs are observed at the deposits localized in carbonaceous rocks. The
EF potential variations are divided into two types: low-frequency low-amplitude anomalies negatively correlated with the day surface relief and high-frequency large-amplitude anomalies. These anomalies are of different nature: The former are caused by the groundwater circulation, and the latter are related to the physicochemical processes at the boundaries of sulfides of pre-ore genesis and to polarized graphitoid. Gold deposits are clearly divided into two groups according to the SP structure: (1) with only low-frequency SP anomalies and (2) with both low- and high-frequency anomalies. The orebodies of all deposits do not contribute to the SP origin but are often spatially confined to sites with pre-ore metasomatites causing SP anomalies. Hence, during the exploration of gold ore objects, the SP method should be applied (in complex with other geophysical methods) mainly at the deposits with electron-conducting pre-ore metasomatites. There is no sense to use the SP method at deposits localized in intrusive massifs (dielectrics). In any case, however, there will be a significant relationship between the SP morphology and gold mineralization.
References Abdelrahman, E.M., Essa, K.S., Abo-Ezz, E.R., Soliman, K.S., 2006. Self-potential data interpretation using standard deviations of depths computed from moving-average residual anomalies. Geophys. Prospect. 54, 409–423. Benevol’skii, B.I., Vartanyan, S.S. (Eds.), 2002. Methodical Guide for the Assessment of Predicted Diamond and Noble- and Nonferrous-Metal Resources. Issue “Gold” [in Russian]. Izd. TsNIGRI, Moscow. Bigalke, J., Grabner, E.W., 1997. The Geobattery model: a contribution to large scale elrctrochemistry. Electrochim. Acta 42 (23–24), 3443–3452. Chebakov, G.I., Roshchektaev, P.A., 2001. A set of geophysical methods for search for primary gold deposits in East Sayan, in: Proceedings of the International Science and Technology Conference “Mining and Geological Education in Siberia. A Hundred Years in the Service of Science and
L.Ya. Erofeev et al. / Russian Geology and Geophysics 58 (2017) 984–989 Production”, Tomsk, 9–13 April 2001 [in Russian]. Izd. TPU, Tomsk, pp. 171–175. Erofeev, L.Ya., Orekhov, A.N., 2014. Geophysical and petrophysical studies of low-sulfide quartz-vein gold deposits in Siberia. Geofizika, No. 3, 56–61. Erofeev, L.Ya., Nomokonova, G.G., Orekhov, A.N., 2003. Petrophysical conditions for the localization of gold deposits in carbonaceous rocks, in: Geophysical Methods for Mineral Exploration and Ecological Research. Proceedings of the All-Russian Science and Technology Conference [in Russian]. Izd. TPU, Tomsk, pp. 207–212. Fogel’man, N.A., 1968. Tectonics of the Mesozoic Arched Uplift in Transbaikalia and Regularities of Localization of Gold Deposits within Its Area (TsNIGRI Transactions, Issue 84) [in Russian]. TsNIGRI, Moscow. Krasnikov, V.I., 1967. The influence of ore-hosting environment on self- and induced-potential anomalies, in: Issues of Ore Geophysics in Siberia [in Russian]. SNIIGGiMS, Novosibirsk, Issue 53, pp. 84–88. Kuz’min, M.I., Zorina, L.D., Spiridonov, A.M., Amuzinskii, V.A., Borisenko, A.S., Mitrofanov, G.L., Polyakov, G.V., Sotnikov, V.I., 1999. Major types of gold deposits in Siberia (composition, genesis, and exploration problems), in: Gold of Siberia. Abstracts of the First Siberian Symposium with International Participation. Krasnoyarsk, 1–3 December 1999 [in Russian]. Izd. KGA MiZ, Krasnoyarsk, pp. 14–15. Lokotko, V.V., Shadrin, A.I., 1972. The regularities of localization of mineralization in the Darasun region (from geophysical data), in: Issues of Geology of Cisbaikalia and Transbaikalia [in Russian]. ChitGTU, Chita, Issue 9, pp. 64–66. Mezentseva, A.E., Esipenko, A.G., 2014. Physical properties of metasomatic wallrocks in gold and silver deposits of northeastern Russia: comparison, problems, and tasks. Razvedka i Okhrana Nedr, 31–36. Mozgolin, Yu.A., 1978. The regularities of variations in the physical properties of black-shale rocks of the Sarala ore field, in: Geophysical Investigations of Gold-Bearing Regions of Central Siberia [in Russian]. Izd. KNIIGiMS, Krasnoyarsk, pp. 55–58. Narseev, V.A., Kurbanov, N.K. (Eds.), 1989. Prediction and Search for Gold Deposits [in Russian]. TsNIGRI, Moscow. Nikiforov, N.A., 1945. Geophysical methods as a tool for studying and searching for gold deposits. Izvestiya AN SSSR 9 (4), 78–82. Ogil’vi, A.A., 1990. Fundamentals of Engineering Geophysics [in Russian]. Nedra, Moscow.
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Ogil’vi, A.A., Ostrovskii, E.Ya., Ruderman, E.N., 1987. Self-Potential Exploration in Modern World Research [in Russian]. VIEMS, Moscow. Orekhov, A.N., 2015. Informative geophysical methods for the search for gold mineralization in black-shale strata. Zapiski Gornogo Instituta 212, 117–121. Parasnis, D.S., 1965. Principles of Applied Geophysics [in Russian]. Mir, Moscow. Revil, A., Jardani, A., 2013. The Self-Potential Method: Theory and Applications in Environmental Geosciences. Cambridge University Press, Cambridge. Ryss, Yu.S., 1983. Geochemical Prospecting Methods [in Russian]. Nedra, Leningrad. Saukov, A.A., 1975. Geochemistry [in Russian]. Nauka, Moscow. Sveshnikov, G.B., 1967. Electrochemical Processes at Sulfide Deposits [in Russian]. Izd. LGU, Leningrad. Seifullin, R.S., 1965. Application of the self-potential method to the search for sulfide deposits. Razvedka i Okhrana Nedr, No. 6, 53–55. Semenov, A.S., 1974. Self-Potential Exploration [in Russian]. Nedra, Leningrad. Shatrov, B.B., 1979. Specifics of the natural electric field of ore–quartz deposits and its practical application, in: Prospecting Methods and Techniques [in Russian]. ONTI VITO, Leningrad, pp. 27–34. Timofeevskii, D.A., 1952. The Tsentral’noe gold deposit, in: Geology of Major Gold Deposits of the USSR [in Russian]. Nedra, Moscow, pp. 131–148. Timofeevskii, D.A., 1957. Geological and structural mineralogical characteristics of the Darasun field and its periphery, in: Gold (TsNIGRI Transactions, Issue 24) [in Russian]. TsNIGRI, Moscow, pp. 17–23. Vovchenko, R.G., 1968a. The geoelectric filtration field and its regard during the interpretation of materials on the natural electric field at ore objects, in: Proceedings of the Fifth Scientific Conference on the Geology of Baikal and Transbaikalia [in Russian]. Izd. ZabNII, Chita, pp. 12–14. Vovchenko, R.G., 1968b. Specifics of the natural electric fields of the Klyuchevskoe deposit, in: Proceedings of the Third Scientific Conference of the Transbaikalian Research Institute [in Russian]. Izd. ZabNII, Chita, pp. 28–31. Zvyagin, V.G., Sizikov, A.I. (Eds.), 1971. Geology and Metallogeny of the Darasun Gold Field [in Russian]. Izd. Zabaikal’skogo Filiala Geofizicheskogo Obshchestva SSSR, China, Issue 52.
Editorial responsibility: M.I. Epov
ScienceDirect Russian Geology and Geophysics 58 (2017) 984–989 www.elsevier.com/locate/rgg
Natural electric fields in Siberian gold deposits: structure, origin, and relationship with gold orebodies L.Ya. Erofeev †, A.N. Orekhov *, G.V. Erofeeva National Tomsk Research Polytechnical University, pr. Lenina 30, Tomsk, 634050, Russia Received 23 March 2016; received in revised form 8 November 2016; accepted 6 December 2016
Abstract Characteristics of the constant natural electric field in the Siberian gold ore areas are given. The regularities of spatial variations in the electric-field potential and the parameters and properties of anomalies have been established. The cause of the natural electric field in deposits of major genotypes has been elucidated. It is shown that the electric field is induced mainly by physicochemical processes running in electron-conducting syn-ore metasomatites and by circulation of groundwaters. Orebodies do not influence significantly the structure of the observed electric fields. We give recommendations on application of the electric-field method at various gold ore objects. © 2017, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: electrical prospecting; natural electric field; gold deposits; Siberia
Introduction The self-potential (SP) method appeared in the geological practice in the first quarter of the 20th century and justly attracted the attention of both ore deposit prospectors and specialists in engineering and geological research. The peak of its use in Russia and abroad was in the second half of the 20th century. By that time, the physicomathematical and petrophysical basis of the method had been generally created, and the corresponding work practice and interpretation methods had been developed (Abdelrahman et al., 2006; Bigalke and Grabner, 1997; Ogil’vi, 1990; Ogil’vi et al., 1987; Parasnis, 1965; Revil and Jardani, 2013; Ryss, 1983; Semenov, 1974; Sveshnikov, 1967). By now, a large amount of factual material has been accumulated owing to the results of the practical SP method application. These data permit one to determine the structure and origin of the natural fields and assess the method potentialities in real physicogeologic conditions during the exploration for a definite type of mineral resources or the solution of particular standard geological problems. In this article, the above issues are considered with regard to gold deposits of the Siberian region.
†
Deceased.
* Corresponding author. E-mail address: [email protected] (A.N. Orekhov)
Gold deposits are localized mostly in the folded framing of the southern and southeastern areas of the West Siberian Platform (Kuz’min et al., 1999). The natural electric field was first discovered by V.V. Borodin during the prospecting for gold deposits in West Sayan (Ol’khovka–Chibizhek gold ore district) in 1934. Later, the SP method was widely applied at Siberian gold deposits (Chebakov and Roshchektaev, 2001; Erofeev et al., 2003; Krasnikov et al., 1967; Mozgolin, 1978; Narseev and Kurbanov, 1989; Nikiforov, 1945; Seifullin, 1965; Shatrov, 1979; Vovchenko, 1968a,b). To date, geophysical SP surveys (usually on a scale of 1:25,000 and larger) have been made within most of the ore fields of large gold deposits and gold ore districts in the Sayans, Yenisei Ridge, Lena gold ore province, and gold ore provinces of southeastern Siberia. Today, the SP method is often applied together with other geophysical methods during prospecting for gold (Saukov, 1975). The data of long-term field measurements of the SP parameters in different gold ore provinces of Siberia testify both to the partial similarity of the electric fields of deposits and to their considerably different relationships with the products of ore-forming processes in the regional gold ore areas.
1068-7971/$ - see front matter D 201 7, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 7.07.009
L.Ya. Erofeev et al. / Russian Geology and Geophysics 58 (2017) 984–989
The structure of the natural electric field and the parameters and origin of its anomalies The amplitude of the SP anomalies at gold deposits usually varies within several hundred mV, locally reaching 1 V and more (for example, at the Olimpiada deposit). The period of the spatial SP variations ranges from few tens to few hundreds of meters. By the spectral composition, these variations are conventionally divided, in a first approximation, into two types: low-frequency low-amplitude (usually few tens of mV) anomalies and high-frequency large-amplitude ones, relatively isomeric or linearly elongate. These types of SP variations exist not at all fields. Only “low-frequency” SP variations are always observed (Fig. 1a, c). Both types of variations exist only at some deposits, with “high-frequency” perturbations being predominant (Fig. 1b) and distorting or concealing the “low-frequency” background. Spatially large low-amplitude EF anomalies are caused mostly by groundwater circulation (usually at a velocity of about 20–40 m/day) in the subsurface zones of geologic environment (so-called filtration fields) owing to a double electric Layer in the capillaries. These SP variations are in an inverse correlation with the day surface relief (Fig. 1). Another group of local SP anomalies (high-amplitude ones) at gold deposits is typical of rock sites enriched in electronconducting minerals, such as graphite and metal sulfides (most often, pyrite). The nature of SP anomalies in the areas with
985
such sites is generally known (Bigalke, 1997; Ogil’vi, 1990; Parasnis, 1965; Revil and Jardani, 2013; Ryss, 1983; Semenov, 1974). They are caused by physicochemical processes running at the boundaries between electron conductors and the enclosed watered rocks with ionic conductivity. The SP intensity is determined both by the difference in the redox potentials of waters circulating at different depths (Bigalke, 1997; Parasnis, 1965; Semenov, 1974) and by the electrode potential of electron conductors, which is not constant at the deposits but lies nearly in the same range of values (200–500 mV) and follows the near-normal distribution law (Fig. 2). The electrode potential of graphite is, on average, higher than that of sulfides. In the study area, including the gold ore fields with a permafrost rock bed, the shape and amplitude of the “highfrequency” EF variations depend neither on the season (Fig. 3a) nor on the year of observation (Fig. 3b) significantly governing the physicochemical conditions in the active (in terms of electrical conductivity and water circulation) horizon. This indicates that the SP anomalies under study, in contrast to the “low-frequency” ones, form with the participation of pellicular (loosely bound) and capillary waters freezing at temperatures much below 0 ºC. For example, pellicular waters freeze at –78 ºC (Saukov, 1975). The regional gold deposits can be divided into three significantly different groups according to the type of relationship between the SP morphology and gold orebodies.
Fig. 1. Self-potential at gold deposits: a, Aprelkovo; b, Sarala; c, Tsentral’noe. 1, granodiorite; 2, basic effusive rocks; 3, argillaceous-carbonaceous shales; 4, ore vein; 5, trench and its number.
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Fig. 2. Statistical distribution of the electrode potentials of pyrite (I) and carbonaceous substance of shales (II) at the Sarala gold deposit (Mozgolin, 1978).
Deposits with dielectric pre-ore metasomatites In the ore fields of gold deposits where the natural electric field manifests itself only as “low-frequency” variations, the orebodies are not related to the SP. They are not accompanied by local anomalies, i.e., do not induce a SP, though they contain up to 10% and more electron-conducting metal sulfides. Such deposits occur in almost all gold ore districts of Siberia (Kuz’min et al., 1999). The absence of local SP anomalies is observed at the gold deposits assigned (according to the geological and mineralogical classification (Benevol’skii and Vartanyan, 2002)) to a vein gold–sulfide–quartz association. Their orebodies are an aggregate (vein) of en-echelon lenses of sulfide and gold clusters in quartz veins, where they are deposited in narrow (few mm) quartz cracks.
The veins are usually several decimeters or, seldom, up to 1–2 m (in swells and bonanzas) in thickness and extend for several tens of meters along the lens strike and dip. The lenses in the vein plane are separated from each other by a barren rock, and the veins in the contact zones are rimmed with altered enclosing metasomatic rocks: syn-ore listwaenites developed after gabbroids, beresites developed after granodiorites, or quartzites in terrigenous sedimentary strata. These are thin light rock zones several decimeters to 1–2 m in thickness. They are of variable composition (carbonates, quartz, sericite, and chlorite). All these objects are dielectrics. If this physicogeologic setting is not significantly disturbed by post-ore tectonic impacts that make a vein crushed and ground, intense redox reactions are impossible in it, because the electron and ionic conduction of the enclosing geologic environment is hot high enough for current flow. A classical example of such objects is the Tsentral’noe deposit in Kuznetsk Alatau and the Aprelkovo deposit in the gold–molybdenum belt of eastern Transbaikalia (Erofeev and Orekhov, 2014), which have been studied in detail and have been in the prolonged commercial use. No noticeable EF anomalies are observed at the deposit sites with quartz–goldore veins containing up to 10–15% sulfides (Fig. 1a, c). Both deposits are entirely localized in granodiorite massifs, have almost the same structure, and comprise orebodies similar in morphology, composition, and gold contents but formed at different times: The Tsentral’noe deposit formed during the Paleozoic tectonomagmatic cycle, whereas the Aprelkovo deposit, during the Cimmerian cycle (Fogel’man, 1968; Timofeevskii, 1952). Analysis of the SP of these deposits shows that the significant difference in the time of their formation and their spatial location do not affect the preservation of their orebodies.
Fig. 3. Comparison of the SPs in the gold ore areas of eastern Transbaikalia measured at different times: a, December 1959 (1) and September 1961 (2); b, 1936 (1) and 1941 (2) (Seifullin, 1965); 3, granodiorites; 4, quartz diorites; 5, pyritization zones; 6, gold ore vein.
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Fig. 4. Distribution of orebodies within the natural electric field at the Darasun deposit: 1, quartz–gold–sulfide veins (I, Glavnaya (Major); II, Novo-Kuznetsovskaya; III, Svintsovaya (Lead); IV, Elektricheskaya-2; V, Karpaty; VI, Sergo); mineral assemblages in ore veins: 2, copper–antimony; 3, quartz–pyrite; 4, quartz–tourmaline; 5, SP isolines (mV).
Deposits with electron-conducting pre-ore metasomatites (sulfides) The electric field is of different morphology within the ore fields of gold deposits where pre-ore metamorphism processes of different intensities took place and formed large sites with electron-conducting rocks. At the deposits localized in intrusive or terrigenous carbon-free rock complexes, these are sites significantly enriched in metal sulfides, and at the deposits formed in black-shale strata, these are sites enriched in sulfides and graphite. An example of a deposit with sites of sulfidized rocks is the well-known Darasun gold deposit in the gold–molybdenum belt in eastern Transbaikalia. It has been comprehensively studied in terms of geology and geophysics (Lokotko and Shadrin, 1972; Zvyagin and Sizikov, 1971). Figure 4 schematically shows the location of major ore veins within the deposit and the SP isolines. The Darasun deposit is composed of Early Proterozoic basic intrusive bodies and Middle Paleozoic granodiorites. Its orebodies (gold-quartz–sulfide veins) are not genetically related to these intrusive complexes but formed in the Mesozoic. Ore formation here proceeded in several stages, with temporal breaks. The first stage (pneumatolithic-hydrothermal) was the appearance of sites with pyritized rocks. At the following stages (solely hydrothermal), veins of clear shape with up to 40% sulfides formed, which were few tens to few hundreds of meters long and up to few decimeters thick in the swells (Timofeevskii, 1957; Zvyagin and Sizikov, 1971).
Comparison of the scheme of orebody localization with the plan of the SP isolines clearly shows that some ore veins (e.g., Glavnaya and Karpaty) lie beyond the SP anomalies, whereas several veins cut the SP anomaly zones. This is best seen for the Novo-Kuznetsovskaya and Svintsovaya veins. Many veins localized within the SP anomalies do not manifest themselves in this field and do not even coincide in strike with the SP anomaly zone. Ore veins of various hydrothermal mineral assemblages, from quartz–tourmaline to arsenopyrite ones, are randomly distributed within the natural electric field (Fig. 4). All this unambiguously indicates that the electric field at the Darasun deposit is induced by the sites with pyritized rocks, whereas quartz–gold–sulfide veins do not contribute to its origin. A similar situation is clearly seen in the Maiskoe gold deposit, whose orebodies formed in the Triassic terrigenous sedimentary strata at the sites with syn-ore sulfide mineralization (Mezentseva and Esipenko, 2014) (Fig. 5). Deposits with electron-conducting pre-ore metasomatites (sulfides and graphitoids) The most intense natural electric fields are detected at the largest gold deposits of Siberia, such as Olimpiada, Sukhoi Log, Chertovo Koryto, Zun-Kholba, and others, and at the smaller ones, e.g., Sarala in Kuznetsk Alatau and the deposits in the Ol’khovka–Chibizhek ore zone, Bodaibo region (eastern Transbaikalia), Yenisei Ridge, etc. These electric fields are induced by the carbonaceous horizons (or their sites) of a terrigenous sedimentary rock complex, which underwent in-
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Fig. 5. Self-potential (isolines, mV) at the Olimpiada gold deposit (compiled by V.I. Klimenko and L.V. Polyakov). 1, orebodies.
tense metamorphism at the pre-ore stage of the deposit formation. The orebodies of these deposits are localized either in carbonaceous horizons or, more often, in near-contact zones, where carbonaceous rocks were subjected to sulfidization under the impact of metasomatism processes and carbonaceous substance transformed into an electron-conducting mineral (graphite or schungite (graphitoid)) during metamorphism. In this case, the electric field is also related to redox processes running at the sulfide boundaries. These processes are favored by a polarized current conductor, namely, an ordered carbonaceous substance (graphite), which creates a low-resistivity medium intensifying the oxidation, transition, and deposition of sulfide components and increases the amplitude of the SP anomalies (Parasnis, 1965; Ryss, 1983; Semenov, 1974; Sveshnikov, 1967). The gold orebodies localized in metasomatites inducing the electric field do not seriously affect its structure (Fig. 5). As seen from the figure, the gold orebodies of the Olimpiada deposit are generally confined to the gradient zone of a large intense negative anomaly of the natural electric field but do not manifest themselves. A similar physicogeologic pattern is observed in many gold ore areas of the Bodaibo region. Here, in the pre-ore mineralization zone of carbonaceous rocks, the SP also varies considerably, but the orebody does not cause any anomalous effects (Orekhov, 2015).
Conclusions Thus, there are constant natural electric fields at the gold ore deposits in Siberia, with their potential varying, on average, within a few hundred mV. The most intense EFs are observed at the deposits localized in carbonaceous rocks. The
EF potential variations are divided into two types: low-frequency low-amplitude anomalies negatively correlated with the day surface relief and high-frequency large-amplitude anomalies. These anomalies are of different nature: The former are caused by the groundwater circulation, and the latter are related to the physicochemical processes at the boundaries of sulfides of pre-ore genesis and to polarized graphitoid. Gold deposits are clearly divided into two groups according to the SP structure: (1) with only low-frequency SP anomalies and (2) with both low- and high-frequency anomalies. The orebodies of all deposits do not contribute to the SP origin but are often spatially confined to sites with pre-ore metasomatites causing SP anomalies. Hence, during the exploration of gold ore objects, the SP method should be applied (in complex with other geophysical methods) mainly at the deposits with electron-conducting pre-ore metasomatites. There is no sense to use the SP method at deposits localized in intrusive massifs (dielectrics). In any case, however, there will be a significant relationship between the SP morphology and gold mineralization.
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Editorial responsibility: M.I. Epov