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12EARTH SCIENCES
ИССЛЕДОВАНИЕ ГЕОХИМИЧЕСКОГО ПОВЕДЕНИЯ РТУТИ В ПРОЦЕССЕ ОБРАЗОВАНИЯ ГИДРОТЕРМАЛЬНЫХ МЕТАЛЛОНОСНЫХ ОТЛОЖЕНИЙ РУДНОГО УЗЛА «ПОБЕДА»
СРЕДИННО-АТЛАНТИЧЕСКОГО ХРЕБТА
Лучшева Л.Н.
Кандидат биологических наук, научный сотрудник Лаборатории геологии и рудогенеза океанической литосферы Геологического института РАН, Москва
Габлина И.Ф.
Доктор геолого-минералогических наук, главный научный сотрудник Лаборатории седиментологии
и геохимии осадочных бассейнов Геологического института РАН, Москва Коновалов Ю.И.
Кандидат геолого-минералогических наук, научный сотрудник Лаборатории геологии и рудогенеза океанической литосферы Геологического института РАН, Москва
RESEARCH OF THE GEOCHEMICAL BEHAVIOR OF MERCURY IN THE PROCESS OF FORMATION OF HYDROTHERMAL METAL-BEARING SEDIMENTS OF THE ORE CLUSTER
"POBEDA" MID-ATLANTIC RIDGE
Luchsheva L.,
Candidate of biological sciences, researcher of the Laboratory of geology and ore genesis of oceanic lithosphere of the Geological Institute of the RAS, Moscow
Gablina I.,
Doctor of geological and mineralogical sciences, chief scientist of the Laboratory of sedimentology and geochemistry of sedimentary basins of the Geological Institute of the RAS, Moscow
Konovalov Yu.
Candidate of geological and mineralogical sciences, researcher of the Laboratory of geology and ore genesis of oceanic lithosphere of the Geological Institute of the RAS, Moscow
АННОТАЦИЯ
В металлоносных отложениях рудного узла «Победа» максимальное накопление ртути происходило в период наибольшей активности процесса гидротермального рудообразования. Установлена наиболее сильная корреляционная связь содержания ртути не с сульфидными минералами, а с содержанием серпентина и талька. Это может свидетельствовать о высокотемпературной стадии рудообразования, когда в зоне рудогенеза ртуть находится в основном в газообразном состоянии, а в минералах она может сохраняться преимущественно в изоморфной форме. ABSTRACT
In the metalliferous deposits of the ore cluster "Pobeda", the maximum accumulation of mercury occurred during the period of the highest activity of the hydrothermal ore formation process. The strongest correlation between the content of mercury is established not with sulfide minerals, but with the content of serpentine and talc. This may indicate a high-temperature stage of ore formation, when in the ore genesis zone mercury is mainly in a gaseous state, and in minerals it can be preserved predominantly in an isomorphic form.
Ключевые слова: ртуть, сульфиды, термоформы ртути, геохимические барьеры, этапы гидротермальной активности, рудный узел «Победа», Срединно-Атлантический хребет.
Keywords: mercury; sulfide ores; mercury thermoforms; geochemical barriers; stages of hydrothermal activity; ore cluster "Pobeda"; Mid-Atlantic Ridge.
At the bottom of sea areas with active underwater hydrothermal activity, massive sulfide ores are formed, as well as metalliferous sediments with an iron content of more than 10% [1]. This type of sediments represents the dispersion halos of ore-bearing hydrothermal fluids that form sulfide deposits on the seabed. Features of the chemical composition of metal-bearing deposits reflect the geochemical specialization of sulfide ores, as well
as the periodicity and intensity of hydrothermal ore formation processes [2].
Materials and Methods
The purpose of our study was to study the levels of content and the features of the geochemical behavior of mercury in metal-bearing sediments within the ore cluster "Pobeda" (17°07'-17°08'N) within Mid-Atlantic Ridge (MAR). This ore cluster is located within the
eastern flank of the rift valley and is confined to the western slope of the mountain massif (17°09'N). The ore cluster "Pobeda" includes the hydrothermal ore fields "Pobeda-1" and "Pobeda-2" as well as the ore occurrence "Pobeda-3" [2].
Previous studies at the ore occurrence "Pobeda-3" (station 37L184k) [3] showed that the level of gross mercury in bottom sediments is relatively low: its maximum concentrations (0.28-0.37 ^g / g) in sediments are only 6-8 times exceed mercury clarke in sedimentary rocks of the earth's crust (0.045 ^g / g) [4]. In this study, the factual material is represented by a 53 cm long column of bottom sediments from station 37L245g, obtained during cruise 37 of the research vessel «Professor Logachev» of the Polar Marine Geological Expedition (PMGE). This station is located on the periphery of the active ore field "Pobeda-2" in the zone of dispersion of hydrothermal metalliferous sedimentary components [5].
The analysis of mercury was carried out by atomic absorption spectrophotometry on a RA-915+ mercury analyzer (OOO Lumex St. Petersburg). The chemical composition of the sediments was determined by the methods of chemical spectral and X-ray fluorescence analyzes. Sediments sampling was carried out along the entire length of the core sample. The sampling intervals of bottom sediments for the analysis of mercury were 3-5 cm, for other elements - 10 cm. The total number of precipitation samples analyzed for mercury was 13, for other elements - 6.
The ore cluster "Pobeda" is located in the southern part of the MAR segment at depths of 2200-3100 m, where rocks of the inner oceanic complex - apoperido-tite serpentinites and gabbroids - are widespread. This ore cluster includes two ore fields and one ore occurrence [5]. The hydrothermal ore field "Pobeda-2" is located at a depth of 2800-3100 m and is directly confined to the rocks of the gabbro-peridotite complex. The exact contours of this ore field have not yet been precisely established. The coordinates of the ore field center are taken as 17°07.45'N, 46° 24.5'W [5].
According to the sonograms of the profilograph GBO "MAK-1M" in the area of the ore field, the thickness of sediments on gentle slopes and in depressions reaches 4-4.5 m. In the near-bottom waters, hydrophysical stations recorded turbidity anomalies [5]. This indicates the presence of modern hydrothermal activity, presumably of a diffuse type, within this ore field.
The sediments of the ore cluster "Pobeda" are biogenic carbonate coccolith-foraminiferal oozes with in-terlayers of pteropod sands. The mineral composition of sediments is represented by three genetic groups: biogenic, hydrothermal, and lithogenic. The average CaCO3 content in sediments is 47%. They are significantly enriched in ore elements (Fe, Cu, Zn, As, V, Cr) and depleted in lithophilic elements (Si, Al, Ti) [6].
Results and Discussion
A.S. Beach and A.Yu. Petrov recorded the maximum average contents of the main ore elements (Fe, Mn, Cu and Zn) within the ore field "Pobeda-2" [7]. On the basis of correlation analysis, these authors identi-
fied groups of elements corresponding to different factors of sedimentation: hydrothermal factor (Fe, Cu, Pb, As, Zn); lithogenic factor, which combines terrigenous and edaphogenic factors (Al, Si, Ti, Mg, K), and bio-genic factor (Ca, Sr).
The maximum absolute ages of sediments in the lower part of the cores of the ore cluster "Pobeda" were determined by the radiocarbon method. For station 37L075k (ore field "Pobeda-1"), the absolute age of sediments is 31829 years ago, for station 37L075g (ore occurrence "Pobeda-3") - 29550 years ago [7]. In sedimentary deposits of the hydrothermal fields "Pobeda-1" and "Pobeda-2", the highest concentrations of ore elements (Fe, Cu) are observed in the lower and upper parts of the sedimentary layer. In the sedimentary layer of the ore cluster "Pobeda" of the A.S. Beach and A.Yu. Petrov [7] identified three main stages of hydrothermal activity: two stages of activation (1st and 3rd stages) and between them a stage of decline in activity (2nd stage), which separates the periods of activation. These stages were identified based on the vertical distribution of the concentrations of iron and copper - the main components of sulfide ores. For each stage of activity, the age limits corresponding to the absolute age of precipitation were also determined.
1st Stage of hydrothermal activity was recorded in the lowest part of the metal-bearing sediment columns for all ore objects of the ore cluster "Pobeda". This stage ended 29.8-28.2 thousand years ago.
2nd Stage was characterized by a decline and termination of hydrothermal activity.
3rd Stage began 22.1 thousand years ago and continues to the present time [7].
In our opinion, synchronization of the stages of hydrothermal activity with the processes of sediment accumulation can be allowed only with a certain degree of probability only in relation to 3rd Stage, that is, accumulation of sediments in the upper part of the sedimentary layer. For the lower part of the section (1st Stage), according to [7], such a mechanism for the formation of ore-bearing and metalliferous sediments cannot be applied.
Earlier, we showed [6] that in the studied zone of the Mid-Atlantic Ridge, the accumulation of ore components at the base of sedimentary sections occurred as a result of intense metasomatic processes. These processes occurred as a result of the impact of diffuse flows of high-temperature hydrothermal solutions, which penetrated into the sedimentary layer from the bedrock during periods of increased hydrothermal activity. At the same time, the biogenic carbonate material was dissolved and replaced by hydrothermal minerals, including ore minerals. This was reflected, in particular, on the total abundance and species composition the shells of nano- and micro-fossil [8].
To reconstruct the stages of hydrothermal activity within the ore field "Pobeda-2" in the sedimentary layer (core station 37L245g), vertical profiles of the concentrations of lithogenic and ore elements, as well as mercury, were constructed (Fig. 1). In addition, we studied the distribution of the percentage of minerals (pyrite, marcasite, serpentine, talc and magnetite), which are present in noticeable amounts in all reference horizons
of the sedimentary layer. Based on the distribution of the concentrations of the studied elements on vertical profiles in the core of station 37L245g, we identified their distinct geochemical zoning.
The lower part of the sedimentary layer. In the lower part of the sedimentary core (interval 34-53 cm), we recorded a layer of metalliferous sediments enriched with rock fragments, smaller of which are represented by fragments of individual minerals. This sedimentary layer is significantly enriched in Fe (up to 61%), Cu (up to 6.62%), Zn (up to 0.28%), Pb (up to 0.024%), Co (up to 0.015%), as well as P, S, and Ag.
In the sediments of this layer, the average mercury content is 1.27 ^g / g, which is 28 times higher than its clarke. The formation of this sedimentary layer is associated with the destruction of the altered underlying rocks of the gabbro-peridotite complex. The enrichment of the sedimentary layer with ore elements is apparently associated with the presence of rock fragments containing disseminated ore minerals, as well as with the ore-forming effect of high-temperature diffuse hydrothermal fluids seeping into the sediments from the bottom rocks.
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According to geochronological dating 1st Stage, the formation of this metal-bearing layer apparently took place during of the activation, which is characterized by high-temperature hydrothermal activity [7]. In this sedimentary layer, the maximum mercury content (2.32 ^g / g) was recorded at a depth of 41 cm of the sedimentary core in a narrow layer of sediments that are significantly enriched in nickel and magnesium. In this layer the maximum content of minerals was also recorded (serpentine and talc), which are products of destruction of serpentinized and talcouzed ultrabasic rocks.
The concentrations of mercury have the strongest correlation with the content of these minerals, especially with the serpentine (r = 0.98). It turned out that the correlations between mercury and the components of sulfide minerals are much weaker than with the above minerals. Thus, the correlation between mercury and iron is 0.54, with zinc - 0.39, and with copper, there is no bond with mercury at all.
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Fig. 1.
Distribution of main ore components in metalliferous sediments of the ore field "Pobeda-2" (station 37L245g)
This may be due to the fact that the high-temperature isocubanite-chalcopyrite-marcasite-pyrite association is predominant in the massive sulfide ores of the ore field "Pobeda-2" [9]. These sulfide ores are characterized by high temperatures of mineral formation (more than 350°C), which is not favorable for the formation of mercury sulfide. As you know, the general chemical formula of serpentine is X2-3Si2O5 (OH) 4,
where X = Mg, Fe2 +, Fe3 +, Ni, Al, Zn, Mn. The serpentine is formed in the weathering crusts of ultrabasic rocks and belongs to magnesium-ferruginous hydrosilicates. At high temperatures (above 400-450°C) serpentine transforms into talc [10].
We do not quite understand the anomalous enrichment of serpentinite in mercury, since it is known that sulfur and hydrogen sulfide are the best precipitants of mercury in low and medium temperature conditions.
Obviously, significant enrichment of sulfur-free minerals with mercury is possible only under extreme conditions, in particular at very high temperatures and pressures.
In our opinion, under such conditions a significant enrichment of minerals with mercury is possible only if
it is present in them in a high-temperature isomorphic form. Indeed, it is known that within the MAR, bottom sulfide edifices are characterized by a high content of mercury (up to 12 ^g / g), which is predominantly in isomorphic form [11].
Fig. 2. Distribution on vertical profiles of the concentrations of Hg (pg / g), S, Ca and serpentine (%) in the column of metalliferous sediments at the station 37L245g of the ore field "Pobeda-2"
The formation of a predominantly isomorphic form of mercury can also be facilitated by the nonequi-librium crystallization conditions of underwater pyrites due to the nature of the kinetics of hydrothermal ore formation processes, which are often very intense. One of the typomorphic features of pyrites that form under water is also the inclusion of a large amount of impurities from the host and underlying rocks, which enter the ore masses during the formation of hydrothermal massive sulfide structures [12, 13]. Therefore, we believe that particles of serpentine, which is a product of destruction of the ocean floor rocks - serpentinized gab-broids and gabbro-peridotites, can be a significant source of mercury in metalliferous sediments of the ore field "Pobeda-2" (Fig. 2).
Middle part of the sedimentary layer. Sediments in this part of the sedimentary core (interval 26-34 cm) were formed during the 2nd Stage of hydrothermal activity on the ore field "Pobeda-2", which was characterized as a decline in volcanic activity [7]. In this interval of the sedimentary layer, in our opinion, geochemical barriers were formed: carbonate barrier - at a depth of 26 cm and sulfide barrier - at a depth of 34 cm.
These geochemical barriers correspond to local zones of the sedimentary layer with the maximum concentrations of calcium carbonate and sulfur, respectively (Fig. 2). The sulfide geochemical barrier appears to have formed in the sulfate reduction zone created by the interaction of ore-rich hydrothermal fluids that diffuse from seabed rocks with sulfate-rich seawater.
The carbonate geochemical barrier is localized in the layer of low-altered biogenic carbonate sediments,
consisting of calcite shells of nano- and microplankton. The mercury content in the sediments of this layer varies from 0.62 ^g / g at the sulfide barrier to 0.16 ^g / g at the carbonate barrier. In this case, the level of mercury content in sediments at the carbonate geochemical barrier corresponds to its background level for metalliferous sediments, which on average is 0.15 ^g / g [11].
The upper part of the sedimentary layer. The chemical composition of the upper part of the sedimentary core (interval 4-26 cm) characterizes 3rd Stage of hydrothermal activation in the study area [7]. During this period, the formation of metalliferous sediments is probably associated mainly with the deposition of ore material from the hydrothermal plume. The plume was formed as a result of the intensification of hydrothermal activity in the period that began 22.1 thousand years ago and continues to the present day [7].
The absolute age of sulfide ores from the upper part of the sedimentary layer of the ore field "Pobeda-2", determined by us by the 230Th / U method using 5 samples, ranges from 5.1 ± 0.2 to 11.2 ± 0.4 thousand years [9]. Only one dating is 134.5 ± 11.4 / 10.1 thousand years for a sample of fine-crystalline pyrite ores, which can be a fragment of more ancient formations from the underlying rocks [9].
During this period of sedimentation, the influence of hydrothermal activity was expressed in a gradual but significant upward increase in the concentrations of Fe, Cu, Zn, Pb and Co in sediments in the interval of 4-26 cm of the sedimentary core. At the same time, the begun hydrothermal activation did not have a noticeable effect on the concentration levels in the sediments of
some other elements, for example, Mg, S, Ni, Ag. Regarding mercury, it can be noted that directly in the near-surface layer of the sediments at a depth of 4 cm, the mercury content (0.34 ^g / g) increased threefold, compared with its average content (0.12 ^g / g) into the middle part of the sedimentary layer.
Conclusion
The results of our study revealed the features of the process of mercury migration in the sedimentary strata of the ore cluster "Pobeda" (MAR). This local process is one of the links in a single global process of mercury degassing of the Earth. Presumably, the generation of mercury-containing fluids causing global mercury degassing occurs in the Earth's mantle [11]. Therefore, one can assume very high levels of mercury content in igneous ultramafic rocks and in their weathering crusts.
Serpentine, talc, and MgFe-silicates are known to be one of the main components of ultrabasites. Therefore, the strong correlations we have established between the concentrations of mercury and the content of serpentine (as well as the concentrations of nickel and magnesium that are included in its composition) can serve as confirmation of this assumption. It is known that in stony meteorites, which usually consist of iron-magnesian silicates with nickel iron, anomalously high concentrations of mercury are also recorded [14].
Thus, it can be concluded that the joint study of the distribution of mercury concentrations and their correlations with the concentrations of other components of the chemical composition of metalliferous deposits, carried out within the ore cluster "Pobeda" (MAR), makes it possible to better understand the role of individual chemical elements, as well as various factors affecting hydrothermal ore formation.
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