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Geochemistry of silver in Permo-Triassic traps of the Siberian Platform
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Russian Geology and Geophysics 53 (2012) 669–674 www.elsevier.com/locate/rgg
Geochemistry of silver in Permo-Triassic traps of the Siberian Platform A.Ya. Medvedev *, A.I. Al’mukhamedov Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia Received 4 February 2011; accepted 15 July 2011
Abstract The first data on the silver content in volcanics of the West Siberian Plate are presented, and data on basalts of the Siberian Platform are supplemented. The silver contents in all studied rocks do not depend on the fractionation of initial melts and contamination of the host rocks and average 0.07–0.10 ppm. The high silver contents can be associated only with sulfide formation. © 2012, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: volcanics; silver; Siberian Platform; West Siberian Plate
Introduction Metallic silver has been long known to the mankind. It is less oxyphile, more chalcophile, and more siderophile than copper; therefore, much of it was trapped by the metallic and sulfide phases during the Earth’s core formation (Ryabchikov et al., 1999). The geochemistry of silver in igneous rocks has been poorly studied. The main difficulty of its study, especially at the clarke level, is caused by the presence of silver in different species in mineral phases and by its problematic analysis. The available literature data are concerned mainly with the metallogeny of silver and its concentration in ore deposits and provide no information about its behavior at different stages of evolution of magmatic systems (Grigor’ev, 2007; Orlova et al., 1983; Ryabchikov et al., 1999; Sidorov et al., 1989). Earlier we considered the distribution of silver in platform flood basalts (Medvedev and Al’mukhamedov, 1995). Since that time we have obtained a lot of new data on the Siberian Platform basalts. We were the first to determine silver in the volcanics of the West Siberian Plate. The sampling localities are shown in Fig. 1. In this work we consider the geochemistry of silver from the Permo-Triassic volcanics of East and West Siberia. These rocks were chosen for study for particular reasons. We established that the buried volcanics of the West Siberian Plate and exposing effusions of the Siberian Platform form the world-largest igneous province (LIP); at least in the Cambrian these regions were a single territory (Bochkarev et al., 2010; Kontorovich et al.,
* Corresponding author. E-mail address: [email protected] (A.Ya. Medvedev)
2008). On the North Asian craton, Permo-Triassic volcanics occupy an area of more than 2.6 × 106 km2. The volume of the effused volcanics is estimated at ~2.3 × 106 km3. The interest to this LIP is due to several factors. First, volcanism occurred in all areas of the North Asian craton nearly at the same time (Reichow et al., 2009). Second, during the large-scale volcanism (at ~251 Ma), mass extinction of live organisms took place, which was probably related to the effusion of huge batches of magma. And third, the trap formation of the Siberian Platform bears unique Cu–Ni deposits, and the West Siberian Plate has unique hydrocarbon pools. It was proved earlier that volcanism was intraplate in both regions (Al’mukhamedov et al., 2004; Medvedev et al., 2003) and was caused by the activity of superplume or two plumes (Dobretsov, 1997). Basites in traps and rift zones are rich in incompatible elements and are similar in chemical composition to oceanic-plateau basalts (Simonov et al., 2004). The Siberian superplume was 2000–3500 km across (Dobretsov, 2010) and probably had two head projections. One of them was beneath the central part of the West Siberian Plate and caused mass rifting, and the other was likely beneath the western part of the Yenisei–Khatanga basin, where a rift zone is assumed to exist. The magmatism in West Siberia and that in East Siberia have much in common. In both regions, basaltoids are predominant rocks. In the Tunguska syneclise area, however, their lithologic composition evolved in time and space. We recognized three types of volcanic sections: primitive (monotonous), normal (with periodic changes in composition), and anomalous (sandwich-like) (Al’mukhamedov et al., 2004; Sharapov et al., 2003). The first-type sections are predominant
1068-7971/$ - see front matter D 201 2, V . S. S o bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2012.05+.005
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A.Ya. Medvedev and A.I. Al’mukhamedov / Russian Geology and Geophysics 53 (2012) 669–674
Fig. 1. Schematic occurrence of Permo-Triassic traps of the Siberian Platform and paleorift structures of the West Siberian Plate and the study regions. Scale 1 : 20,000,000. 1, Phanerozoic sedimentary cover; 2, fold belts and uplifts of the Precambrian basement; 3, effusions; 4, basaltic tuffs and tuffites; 5, areas of occurrence of intrusive traps; 6, surface projections of revealed and predicted rift structures of the pre-Jurassic basement of the West Siberian Plate; 7, main disjunctions; 8, sites of detailed studies of the Tunguska syneclise volcanogenic strata; 9, localities of boreholes stripped a Triassic volcanic complex in West Siberia.
in basalt areas and are composed mainly of low-K tholeiites. The second-type sections are dominated by a differentiated series of alkaline and subalkalic rocks in the lower horizons. The third-type sections are formed by irregularly alternating subalkalic (alkaline) and tholeiitic rocks. Study of the spatial distribution of different types of rocks showed that normal and anomalous sections are specific only for the northern and northwestern margins of the Tunguska syneclise and are confined to the shoulders of paleorift structures, whereas primitive sections occupy most of the studied area. Based on all the above data, we recognized two stages of magmatic activity in the region: initial (rift formation) and final (beyondrift blanket formation). According to statistical data, the rift formation took place somewhat earlier than the blanket formation (normal sections), but locally, the eruption of magmas of different alkalinity might have proceeded almost synchronously with the rift formation (anomalous sections). Thus, all volcanics of the Tunguska syneclise are basic rocks with different degrees of alkalinity and basicity. The situation in West Siberia is somewhat different. Volcanics in this area vary widely in composition: from basalts to rhyolites, including alkaline rocks—trachybasalts and phonolites (Medvedev et al., 2003). All Permo-Triassic volcanics in the region are considered to be products of rift magmatism. This agrees with the presence of a huge system of rift structures in the Precambrian basement, which completed their evolution in the Triassic (Surkov et al., 2003). It was established that all West Siberian volcanics occur either in rift zones or on interrift uplifts and were generated at the rift
formation stage. Remind that despite the great diversity of volcanics, basalts prevail. No zoning in the distribution of volcanics throughout the West Siberian area has been revealed because of lack of factual data. It is of crucial importance to determine the content of Ag in similar rocks generated in both regions at the rift and blanket formation stages. Some basalt samples from the Noril’sk region which bear visible sulfide minerals contain up to 86 ppm Ag; therefore, we studied samples free of visible sulfides. As a rule, the Ag-richest samples have the maximum content of Pb. Probably they contain galena grains. To eliminate the influence of sulfides, we chose samples with the maximum Ag content of 0.31 ppm. The content of silver was determined by atomic-emission spectroscopy (AES) with the use of a discharge arc (Smirnova et al., 1993), following a special technique (Kuznetsova et al., 2007), which ensures results consistent with ICP MS data and a detection limit of 0.01 ppm. The contents of Ag in different rocks are listed in Table 1. Despite the wide scatter of values for each type of rocks, the average contents are close, 0.06–0.10 ppm. In most types of volcanics from both regions, the Ag contents are below the element clarke in basites (after Vinogradov’s theory). On the Siberian Platform, the rocks produced at the rift stage have somewhat higher contents of Ag than all other volcanics. We cannot state the same for West Siberia, since all studied rocks there were generated at the rift stage. The basalts from both regions show no difference between tholeiitic and subalkalic
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A.Ya. Medvedev and A.I. Al’mukhamedov / Russian Geology and Geophysics 53 (2012) 669–674 Table 1. Content of silver in different varieties of volcanics from the Siberian Platform and West Siberian Plate Rock varieties
Number of samples
Content, ppm min
max
average
Siberian Platform Tholeiitic basalts
360
0.01
0.31
0.07
Subalkalic basalts
72
0.02
0.26
0.08
Picrobasalts
20
0.05
0.25
0.10
Rift stage basaltoids
93
0.02
0.26
0.09
Blanket stage basaltoids
369
0.01
0.31
0.07
Tholeiitic basalts
20
0.03
0.30
0.07
Subalkalic basalts
38
0.03
0.24
0.06
Trachybasalts
6
0.04
0.15
0.09
Phonolites
5
0.04
0.09
0.06
Rhyolites and rhyodacites
17
0.04
0.16
0.10
Rift stage basaltoids
58
0.03
0.30
0.07
West Siberia
varieties. Note that the Siberian Platform rocks are somewhat richer in Ag. Probably, this is due to the regional difference between the parental melts or the metallogenic specialization of the Siberian Platform.
We tried to find a correlation between the Ag content and the contents of major and trace elements in the volcanics from the Siberian Platform and West Siberia but failed. Figure 2, a shows a MgO–Ag correlation in the studied rocks. We have
Fig. 2. Ag content vs. MgO (a), Pb (b), and Cu (c) contents in basalts from the Siberian Platform and West Siberian Plate. 1, Siberian Platform basalts produced at the rifting stage; 2, Siberian Platform basalts produced at the blanket formation stage; 3, West Siberian Plate basalts.
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A.Ya. Medvedev and A.I. Al’mukhamedov / Russian Geology and Geophysics 53 (2012) 669–674
Fig. 3. Average contents of silver over formations in different regions of the Siberian Platform. 1, predominantly lavas of different compositions; 2, alternation of lavas and tuffs; 3, predominantly tuffs, tuffites, and, more seldom, lavas; 4, Paleozoic (C2–P2) sedimentary deposits of the Siberian Platform cover; 5, formations of weided-tuff strata (from bottom to top): A, Northwestern region: iv, Ivakino, sw, Syverma, gd, Gudchikha, hk, Khakanchan, tk, Tuklon, nd, Nadezhda, mr, Morongo, mk, Mokulai, hr, Kharaelakh, km, Kumga, sm, Samoed; B, D, Central Tunguska region (B, middle reaches of the Lower Tunguska River; D, Putorana plateau): v tt, Tutochany; kr, Korvunchana; ndm, Nidym; kc, Kochechum; jm, Yambukan; vr, Vodorazdel’noe; an, Ayan; hm, Khonnamakit; nr, Nerarkar; C, Maimecha–Kotui region: pb, Pravaya Boyarka; on, Onkuchan; tv, Tuvankit; dl, Del’kan; and mm, Maimecha. R, Rifting stage; B, blanket formation stage.
A.Ya. Medvedev and A.I. Al’mukhamedov / Russian Geology and Geophysics 53 (2012) 669–674
established that a change in the Mg-number of rocks, determining the degree of fractionation of parental magmas, virtually does not affect the Ag content. No correlation between the Ag content and the alkalinity of rocks was revealed. Though silver in deposits of all types is intimately associated with lead (Antonov, 2009), this is not specific for volcanics. Our examination of all samples from the studied regions did not reveal a regular Ag–Pb correlation (Fig. 2, b). The only element correlating with Ag is Cu (Fig. 2, c). We studied the areal distribution of silver only on the Siberian Platform. In the West Siberian area such studies are impossible because of lack of data. The average Ag contents over formations in different regions of the Siberian Platform are shown in Fig. 3. One can see that there is no serious difference between them in most of regions. Somewhat higher Ag contents were found in the Noril’sk region, where the maximum element contents are observed at the boundaries of cycles 2 and 4. But the reason for this phenomenon is still unclear. Probably, this is related to the influence of ore-bearing Cu–Ni–PGE intrusions widespread in the region. Analysis of all available data permitted us to draw particular conclusions. The diversity of volcanics in the studied regions is determined by many factors. The main factor is fractionation (in the broad sense of this term) of parental basic magmas, probably, of varying alkalinity (Zolotukhin and Al’mukhamedov, 1990), in chambers localized at different depths (Al’mukhamedov et al., 1991, 1993). According to the above data, magma differentiation does not lead to concentration of silver, and the same is true for crustal contamination, since the Ag content in felsic igneous rocks and most of sedimentary ones is lower than that in basites. The above-mentioned specific behavior of silver is due to the difficulty of Ag+ incorporation into the structures of main rock-forming minerals by the isomorphism scheme (Nesterenko and Al’mukhamedov, 1973) because it has more pronounced chalcophile properties. Thus, the behavior of silver in the magma evolution processes is similar to the behavior of copper and gold. Earlier we determined the coefficients of gold partition among sulfide and silicate melts (Al’mukhamedov and Medvedev, 1978; Mironov et al., 1978). Most likely, the partition coefficient of silver is intermediate between those of gold and copper. It is the chalcophile properties of silver that explain the wide variations in its contents, up to ultrahigh ones. This is also confirmed by findings of argentoplumbite, Ag-containing galena, and intermetallic compounds of silver in sulfide ores from the deposits associated with the basalts of the northwestern Siberian Platform (Genkin et al., 1981). It is of great interest to consider the sulfide formation process. According to literature data (Begg et al., 2010; Zhang et al., 2008), sulfides are produced under the interaction of “plume” magma ascending to the surface with lithosphere enriched in Ni, Cu, and PGE in the regions of Archean cratons, when the magma becomes saturated with these elements. In the presence of sulfur-containing phases, the magma undergoes sulfidization, and a sulfide melt is produced, as we showed earlier
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(Al’mukhamedov and Medvedev, 1977). In this case, owing to the high partition coefficients of the above elements, which concentrate mainly in the sulfide melt, the latter becomes enriched in Ni, Cu, PGE, and, probably, Ag. The possibility of this mechanism is pointed by the positive correlation between copper and silver contents (Fig. 2, c).
Conclusions Independently of the fractionation processes or contamination of the host rocks by basaltic melt, the latter does not accumulate silver. Basalts of the Siberian Platform and volcanics of the West Siberian Plate do not differ significantly in Ag contents. We have shown that the high Ag contents can be related only to sulfide formation processes, during which silver concentrates together with other chalcophile elements. This work was financially supported by State Contract 02.740.11.0324 from the Federal Science and Innovation Agency.
References Al’mukhamedov, A.I., Medvedev, A.Ya., 1977. Sulfurization as one of the possible mechanisms of formation of Cu–Ni sulfide deposits. Dokl. Akad. Nauk SSSR 236 (4), 965–968. Al’mukhamedov, A.I., Medvedev, A.Ya., 1978. Partition of chalcophile elements among sulfide and silicate melts (from experimental data). Dokl. Akad. Nauk SSSR 240 (2), 717–721. Al’mukhamedov, A.I., Zolotukhin, V.V., Al’mukhamedov, E.A., 1991. Cenozoic Deccan traps. 2. Geochemical characteristics. Geologiya i Geofizika (Soviet Geology and Geophysics) 32 (10), 58–67 (49–56). Al’mukhamedov, A.I., Zolotukhin, V.V., Al’mukhamedov, E.A., Sandimirova, G.P., Pakhol’chenko, Yu.A., 1993. Geochemical model of basalt magma hybridization exemplified by the Tulai-Kiryaka intrusion (Taimyr). Geologiya i Geofizika (Russian Journal of Geology and Geophysics) 34 (4), 50–59 (42–50). Al’mukhamedov, A.I., Medvedev, A.Ya., Zolotukhin, V.V., 2004. The spatial and temporal lithologic evolution of Permo-Triassic basalts of the West Siberian Platform. Petrologiya 4, 330–360. Antonov, A.E., 2009. Foreign Silver Deposits [in Russian]. GEOS, Moscow. Begg, G.C., Hronsky, J.A.M., Arndt, N.T., Griffin, W.L., O’Reily, S.Y., Hayward, N., 2010. Lithospheric, cratonic, and geodynamic setting of Ni–Cu–PGE sulfide deposits. Econ. Geol. 105 (6), 1057–1070. Bochkarev, V.S., Brekhuntsov, A.M., Larichev, A.I., Mashchak, M.S., Olennikova, E.V., 2010. The western boundary of the occurrence area of Siberian traps and their geodynamic nature. Gornye Vedomosti, No. 11, 6–26. Dobretsov, N.L., 1997. Permo-Triassic magmatism in Eurasia as reflection of a superplume. Dokl. Akad. Nauk 354 (2), 220–223. Dobretsov, N.L., 2010. Global geodynamic evolution of the Earth and global geodynamic models. Russian Geology and Geophysics (Geologiya i Geofizika) 51 (6), 592–610 (761–784). Genkin, A.D., Distler, V.V., Gladyshev, G.D., Filimonova, A.A., Evstigneeva, T.L., Kovalenker, V.A., Laputina, I.P., Smirnov, A.V., Grokhovskaya, T.L., 1981. Copper–Nickel Sulfide Ores of Noril’sk Deposits [in Russian]. Nauka, Moscow. Grigor’ev, N.A., 2007. Distribution of silver in rocks in the upper continental crust. Ural’skii Geologicheskii Zhurnal, No. 3, 35–54. Kontorovich, A.E., Varlamov, A.I., Emeshev, V.G., Efimov, A.S., Klets, A.G., Komarov, A.V., Kontorovich, V.A., Korovnikov, I.V., Saraev, S.V., Filippov, Yu.F., Varaksina, I.V., Glinskikh, V.N., Luchinina, V.A., Novozhilova, N.V., Pegel, T.V., Sennikov, N.V., Timokhin, A.V., 2008.
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New type of Cambrian section in eastern part of West Siberian Plate (based on Vostok-1 stratigraphic well data). Russian Geology and Geophysics (Geologiya i Geofizika) 49 (11), 843–850 (1119–1128). Kuznetsova, A.I., Zarubina, O.V., Sklyarova, O.A., 2007. Determination of trace elements (Ag, B, Ge, Mo, Sn, W, Tl) in reference samples of soils and bottom sediments by atomic emission spectrometry: a traceability and fitness-for-purpose study. Geostand. Geoanal. Res. 31 (3), 251–259. Medvedev, A.Ya., Al’mukhamedov, A.I., 1995. Geochemistry of silver in platform flood basalts. Dokl. Akad. Nauk 344 (1), 101–105. Medvedev, A.Ya., Al’mukhamedov, A.I., Kirda, N.P., 2003. Geochemistry of Permo-Triassic volcanic rocks of West Siberia. Geologiya i Geofizika (Russian Geology and Geophysics) 44 (1–2), 86–100 (82–98). Mironov, A.G., Al’mukhamedov, A.I., Medvedev, A.Ya., Krendelev, F.P., 1978. Geochemistry of gold in basaltic melts (from experimental data). Geokhimiya, No. 11, 1639–1652. Nesterenko, G.V., Al’mukhamedov, A.I., 1973. Geochemistry of Differentiated Traps (Siberian Platform) [in Russian]. Nauka, Moscow. Orlova, G.P., Ryabchikov, I.D., Volchenkova, V.A., 1983. Partition of gold among granitic melt and fluid. Geologiya Rudnykh Mestorozhdenii 25 (3), 34–43. Ryabchikov, I.D., Orlova, G.P., Babanskii, A.D., Magazina, L.O., Tsepin, I.A., 1999. Chalcophile metals in magma and Earth’s core formation processes. Rossiiskii Zhurnal Nauk o Zemle 1 (6), 7–15. Reichow, M.K., Pringle, M.S., Al’mukhamedov, A.I., Allen, M.B., Andreichev, V.L., Buslov, M.M., Davies, C.E., Fedoseev, G.S., Fitton, J.G., Ingel, S., Medvedev, A.Ya., Mitchell, C., Puchkov, V.N., Safonova, I.Yu.,
Scott, R.A., Saunders, A.D., 2009. The timing and extent of the eruption of the Siberian Traps large igneous province: implications for the end-Permian environmental crisis. Earth Planet. Sci. Lett. 277 (1), 9–20. Sharapov, V.N., Vasil’ev, Yu.R., Al’mukhamedov, A.I., Medvedev, A.Ya., 2003. Local and regional variability in composition of Permian–Triasic effusive traps of the Siberian Platform. Geologiya i Geofizika (Russian Geology and Geophysics) 44 (8), 741–752 (709–721). Sidorov, A.A., Konstantinov, M.M., Eremin, R.A., 1989. Silver: Geology, Mineralogy, Genesis, and Regularities of Deposit Localization [in Russian]. Nauka, Moscow. Simonov, V.A., Kovyazin, S.V., Al’mukhamedov, A.I., Medvedev, A.Ya., 2004. Petrogenesis of basaltic series of the Ontong Java–Nauru submarine plateau. Petrologiya 12 (12), 191–205. Smirnova, E.V., Kuznetsova, A.I., Chumakova, N.L., 1993. Atomic-Emission Analysis in Geochemistry [in Russian]. Nauka, Novosibirsk. Surkov, V.S., Kuznetsov, V.L., Latyshev, V.I., 2003. The structure of the deep-level Earth’s crust in petroliferous provinces of Siberia. Razvedka i Okhrana Nedr, No. 1, 6–8. Zhang, M., O’Reily, S.Y., Wang, K-L., Hronsky, J.A.M., Griffin, W.L., 2008. Flood basalts and metallogeny: the lithospheric mantle connection. Earth Sci. Rev. 86, 145–174. Zolotukhin, V.V. Al’mukhamedov, A.I., 1990. Fractionation and alkalinity in the evolution of the source magmas of platform basites (by the example of the northwestern Siberian Platform). Geologiya i Geofizika (Soviet Geology and Geophysics) 31 (10), 15–21 (13–18).
Editorial responsibility: A.S. Borisenko
Russian Geology and Geophysics 53 (2012) 669–674 www.elsevier.com/locate/rgg
Geochemistry of silver in Permo-Triassic traps of the Siberian Platform A.Ya. Medvedev *, A.I. Al’mukhamedov Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia Received 4 February 2011; accepted 15 July 2011
Abstract The first data on the silver content in volcanics of the West Siberian Plate are presented, and data on basalts of the Siberian Platform are supplemented. The silver contents in all studied rocks do not depend on the fractionation of initial melts and contamination of the host rocks and average 0.07–0.10 ppm. The high silver contents can be associated only with sulfide formation. © 2012, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: volcanics; silver; Siberian Platform; West Siberian Plate
Introduction Metallic silver has been long known to the mankind. It is less oxyphile, more chalcophile, and more siderophile than copper; therefore, much of it was trapped by the metallic and sulfide phases during the Earth’s core formation (Ryabchikov et al., 1999). The geochemistry of silver in igneous rocks has been poorly studied. The main difficulty of its study, especially at the clarke level, is caused by the presence of silver in different species in mineral phases and by its problematic analysis. The available literature data are concerned mainly with the metallogeny of silver and its concentration in ore deposits and provide no information about its behavior at different stages of evolution of magmatic systems (Grigor’ev, 2007; Orlova et al., 1983; Ryabchikov et al., 1999; Sidorov et al., 1989). Earlier we considered the distribution of silver in platform flood basalts (Medvedev and Al’mukhamedov, 1995). Since that time we have obtained a lot of new data on the Siberian Platform basalts. We were the first to determine silver in the volcanics of the West Siberian Plate. The sampling localities are shown in Fig. 1. In this work we consider the geochemistry of silver from the Permo-Triassic volcanics of East and West Siberia. These rocks were chosen for study for particular reasons. We established that the buried volcanics of the West Siberian Plate and exposing effusions of the Siberian Platform form the world-largest igneous province (LIP); at least in the Cambrian these regions were a single territory (Bochkarev et al., 2010; Kontorovich et al.,
* Corresponding author. E-mail address: [email protected] (A.Ya. Medvedev)
2008). On the North Asian craton, Permo-Triassic volcanics occupy an area of more than 2.6 × 106 km2. The volume of the effused volcanics is estimated at ~2.3 × 106 km3. The interest to this LIP is due to several factors. First, volcanism occurred in all areas of the North Asian craton nearly at the same time (Reichow et al., 2009). Second, during the large-scale volcanism (at ~251 Ma), mass extinction of live organisms took place, which was probably related to the effusion of huge batches of magma. And third, the trap formation of the Siberian Platform bears unique Cu–Ni deposits, and the West Siberian Plate has unique hydrocarbon pools. It was proved earlier that volcanism was intraplate in both regions (Al’mukhamedov et al., 2004; Medvedev et al., 2003) and was caused by the activity of superplume or two plumes (Dobretsov, 1997). Basites in traps and rift zones are rich in incompatible elements and are similar in chemical composition to oceanic-plateau basalts (Simonov et al., 2004). The Siberian superplume was 2000–3500 km across (Dobretsov, 2010) and probably had two head projections. One of them was beneath the central part of the West Siberian Plate and caused mass rifting, and the other was likely beneath the western part of the Yenisei–Khatanga basin, where a rift zone is assumed to exist. The magmatism in West Siberia and that in East Siberia have much in common. In both regions, basaltoids are predominant rocks. In the Tunguska syneclise area, however, their lithologic composition evolved in time and space. We recognized three types of volcanic sections: primitive (monotonous), normal (with periodic changes in composition), and anomalous (sandwich-like) (Al’mukhamedov et al., 2004; Sharapov et al., 2003). The first-type sections are predominant
1068-7971/$ - see front matter D 201 2, V . S. S o bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2012.05+.005
670
A.Ya. Medvedev and A.I. Al’mukhamedov / Russian Geology and Geophysics 53 (2012) 669–674
Fig. 1. Schematic occurrence of Permo-Triassic traps of the Siberian Platform and paleorift structures of the West Siberian Plate and the study regions. Scale 1 : 20,000,000. 1, Phanerozoic sedimentary cover; 2, fold belts and uplifts of the Precambrian basement; 3, effusions; 4, basaltic tuffs and tuffites; 5, areas of occurrence of intrusive traps; 6, surface projections of revealed and predicted rift structures of the pre-Jurassic basement of the West Siberian Plate; 7, main disjunctions; 8, sites of detailed studies of the Tunguska syneclise volcanogenic strata; 9, localities of boreholes stripped a Triassic volcanic complex in West Siberia.
in basalt areas and are composed mainly of low-K tholeiites. The second-type sections are dominated by a differentiated series of alkaline and subalkalic rocks in the lower horizons. The third-type sections are formed by irregularly alternating subalkalic (alkaline) and tholeiitic rocks. Study of the spatial distribution of different types of rocks showed that normal and anomalous sections are specific only for the northern and northwestern margins of the Tunguska syneclise and are confined to the shoulders of paleorift structures, whereas primitive sections occupy most of the studied area. Based on all the above data, we recognized two stages of magmatic activity in the region: initial (rift formation) and final (beyondrift blanket formation). According to statistical data, the rift formation took place somewhat earlier than the blanket formation (normal sections), but locally, the eruption of magmas of different alkalinity might have proceeded almost synchronously with the rift formation (anomalous sections). Thus, all volcanics of the Tunguska syneclise are basic rocks with different degrees of alkalinity and basicity. The situation in West Siberia is somewhat different. Volcanics in this area vary widely in composition: from basalts to rhyolites, including alkaline rocks—trachybasalts and phonolites (Medvedev et al., 2003). All Permo-Triassic volcanics in the region are considered to be products of rift magmatism. This agrees with the presence of a huge system of rift structures in the Precambrian basement, which completed their evolution in the Triassic (Surkov et al., 2003). It was established that all West Siberian volcanics occur either in rift zones or on interrift uplifts and were generated at the rift
formation stage. Remind that despite the great diversity of volcanics, basalts prevail. No zoning in the distribution of volcanics throughout the West Siberian area has been revealed because of lack of factual data. It is of crucial importance to determine the content of Ag in similar rocks generated in both regions at the rift and blanket formation stages. Some basalt samples from the Noril’sk region which bear visible sulfide minerals contain up to 86 ppm Ag; therefore, we studied samples free of visible sulfides. As a rule, the Ag-richest samples have the maximum content of Pb. Probably they contain galena grains. To eliminate the influence of sulfides, we chose samples with the maximum Ag content of 0.31 ppm. The content of silver was determined by atomic-emission spectroscopy (AES) with the use of a discharge arc (Smirnova et al., 1993), following a special technique (Kuznetsova et al., 2007), which ensures results consistent with ICP MS data and a detection limit of 0.01 ppm. The contents of Ag in different rocks are listed in Table 1. Despite the wide scatter of values for each type of rocks, the average contents are close, 0.06–0.10 ppm. In most types of volcanics from both regions, the Ag contents are below the element clarke in basites (after Vinogradov’s theory). On the Siberian Platform, the rocks produced at the rift stage have somewhat higher contents of Ag than all other volcanics. We cannot state the same for West Siberia, since all studied rocks there were generated at the rift stage. The basalts from both regions show no difference between tholeiitic and subalkalic
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A.Ya. Medvedev and A.I. Al’mukhamedov / Russian Geology and Geophysics 53 (2012) 669–674 Table 1. Content of silver in different varieties of volcanics from the Siberian Platform and West Siberian Plate Rock varieties
Number of samples
Content, ppm min
max
average
Siberian Platform Tholeiitic basalts
360
0.01
0.31
0.07
Subalkalic basalts
72
0.02
0.26
0.08
Picrobasalts
20
0.05
0.25
0.10
Rift stage basaltoids
93
0.02
0.26
0.09
Blanket stage basaltoids
369
0.01
0.31
0.07
Tholeiitic basalts
20
0.03
0.30
0.07
Subalkalic basalts
38
0.03
0.24
0.06
Trachybasalts
6
0.04
0.15
0.09
Phonolites
5
0.04
0.09
0.06
Rhyolites and rhyodacites
17
0.04
0.16
0.10
Rift stage basaltoids
58
0.03
0.30
0.07
West Siberia
varieties. Note that the Siberian Platform rocks are somewhat richer in Ag. Probably, this is due to the regional difference between the parental melts or the metallogenic specialization of the Siberian Platform.
We tried to find a correlation between the Ag content and the contents of major and trace elements in the volcanics from the Siberian Platform and West Siberia but failed. Figure 2, a shows a MgO–Ag correlation in the studied rocks. We have
Fig. 2. Ag content vs. MgO (a), Pb (b), and Cu (c) contents in basalts from the Siberian Platform and West Siberian Plate. 1, Siberian Platform basalts produced at the rifting stage; 2, Siberian Platform basalts produced at the blanket formation stage; 3, West Siberian Plate basalts.
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Fig. 3. Average contents of silver over formations in different regions of the Siberian Platform. 1, predominantly lavas of different compositions; 2, alternation of lavas and tuffs; 3, predominantly tuffs, tuffites, and, more seldom, lavas; 4, Paleozoic (C2–P2) sedimentary deposits of the Siberian Platform cover; 5, formations of weided-tuff strata (from bottom to top): A, Northwestern region: iv, Ivakino, sw, Syverma, gd, Gudchikha, hk, Khakanchan, tk, Tuklon, nd, Nadezhda, mr, Morongo, mk, Mokulai, hr, Kharaelakh, km, Kumga, sm, Samoed; B, D, Central Tunguska region (B, middle reaches of the Lower Tunguska River; D, Putorana plateau): v tt, Tutochany; kr, Korvunchana; ndm, Nidym; kc, Kochechum; jm, Yambukan; vr, Vodorazdel’noe; an, Ayan; hm, Khonnamakit; nr, Nerarkar; C, Maimecha–Kotui region: pb, Pravaya Boyarka; on, Onkuchan; tv, Tuvankit; dl, Del’kan; and mm, Maimecha. R, Rifting stage; B, blanket formation stage.
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established that a change in the Mg-number of rocks, determining the degree of fractionation of parental magmas, virtually does not affect the Ag content. No correlation between the Ag content and the alkalinity of rocks was revealed. Though silver in deposits of all types is intimately associated with lead (Antonov, 2009), this is not specific for volcanics. Our examination of all samples from the studied regions did not reveal a regular Ag–Pb correlation (Fig. 2, b). The only element correlating with Ag is Cu (Fig. 2, c). We studied the areal distribution of silver only on the Siberian Platform. In the West Siberian area such studies are impossible because of lack of data. The average Ag contents over formations in different regions of the Siberian Platform are shown in Fig. 3. One can see that there is no serious difference between them in most of regions. Somewhat higher Ag contents were found in the Noril’sk region, where the maximum element contents are observed at the boundaries of cycles 2 and 4. But the reason for this phenomenon is still unclear. Probably, this is related to the influence of ore-bearing Cu–Ni–PGE intrusions widespread in the region. Analysis of all available data permitted us to draw particular conclusions. The diversity of volcanics in the studied regions is determined by many factors. The main factor is fractionation (in the broad sense of this term) of parental basic magmas, probably, of varying alkalinity (Zolotukhin and Al’mukhamedov, 1990), in chambers localized at different depths (Al’mukhamedov et al., 1991, 1993). According to the above data, magma differentiation does not lead to concentration of silver, and the same is true for crustal contamination, since the Ag content in felsic igneous rocks and most of sedimentary ones is lower than that in basites. The above-mentioned specific behavior of silver is due to the difficulty of Ag+ incorporation into the structures of main rock-forming minerals by the isomorphism scheme (Nesterenko and Al’mukhamedov, 1973) because it has more pronounced chalcophile properties. Thus, the behavior of silver in the magma evolution processes is similar to the behavior of copper and gold. Earlier we determined the coefficients of gold partition among sulfide and silicate melts (Al’mukhamedov and Medvedev, 1978; Mironov et al., 1978). Most likely, the partition coefficient of silver is intermediate between those of gold and copper. It is the chalcophile properties of silver that explain the wide variations in its contents, up to ultrahigh ones. This is also confirmed by findings of argentoplumbite, Ag-containing galena, and intermetallic compounds of silver in sulfide ores from the deposits associated with the basalts of the northwestern Siberian Platform (Genkin et al., 1981). It is of great interest to consider the sulfide formation process. According to literature data (Begg et al., 2010; Zhang et al., 2008), sulfides are produced under the interaction of “plume” magma ascending to the surface with lithosphere enriched in Ni, Cu, and PGE in the regions of Archean cratons, when the magma becomes saturated with these elements. In the presence of sulfur-containing phases, the magma undergoes sulfidization, and a sulfide melt is produced, as we showed earlier
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(Al’mukhamedov and Medvedev, 1977). In this case, owing to the high partition coefficients of the above elements, which concentrate mainly in the sulfide melt, the latter becomes enriched in Ni, Cu, PGE, and, probably, Ag. The possibility of this mechanism is pointed by the positive correlation between copper and silver contents (Fig. 2, c).
Conclusions Independently of the fractionation processes or contamination of the host rocks by basaltic melt, the latter does not accumulate silver. Basalts of the Siberian Platform and volcanics of the West Siberian Plate do not differ significantly in Ag contents. We have shown that the high Ag contents can be related only to sulfide formation processes, during which silver concentrates together with other chalcophile elements. This work was financially supported by State Contract 02.740.11.0324 from the Federal Science and Innovation Agency.
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Editorial responsibility: A.S. Borisenko