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GEOLOGICAL AND MINERALOGICAL SCIENCES
THE MERCURY TERMOFORMS AS INDICATORS OF GEOCHEMICAL CONDITIONS OF THE SULFIDE ORE GENESIS IN THE SEDIMENTARY ROCKS OF THE RIDGE OF JUAN DE FUCA
Luchsheva L.
Candidate of biological sciences, researcher of the Laboratory of volcanogenic-sedimentary and hydrothermal lithogenesis of the
Geological Institute of the RAS, Moscow Konovalov Yu.
Candidate of geological and mineralogical sciences, researcher of the Laboratory of volcanogenic-sedimentary and hydrothermal Lithogenesis of the
Geological Institute of the RAS, Moscow
Kurnosov V.
Doctor of geological and mineralogical sciences, head of the Laboratory of volcanogenic-sedimentary and hydrothermal Lithogenesis of the
Geological Institute of the RAS, Moscow
ABSTRACT
High variability of the content of thermoforms of mercury in the sedimentary thickness due to geochemical processes occurring in the zone of modern sulfide ore formation has been revealed. The indicator role of mercury associated with a change in the content of its thermoforms in various geochemical conditions has been established.
This paper presents the results of studying the distribution of concentrations of mercury and its ther-moforms in the zone of underwater hydrothermal sulfide ore genesis in the Middle Valley of the Juan de Fuca mid-ocean ridge. Sediment samples from the cores of two holes were studied (Leg 139 of the cruise of the drilling vessel "Joides Resolution" of Ocean Drilling Program) in Site 858, located above the active hydrothermal system, within the "Dead Dog" hydrothermal field with a big of heat flux and numerous sources with a high water temperature (up to 276°C). The precipitations is represented by Holocene-Upper Pleistocene hemipelagic deposits and turbidites [1].
Materials and Methods. We carried out a comparative analysis of the mercury content and its ther-moforms in the cores of the ore-bearing Hole 858B (at the depth of 38 m), which is located in the zone of the upward hydrothermal flow and the background Hole 855A (at the depth of 332 m), which was drilled outside the hydrothermal system.
Ore-bearing Hole 858B uncovered a sedimentary stratum with a thickness of about 40 m at the top of a sulfide hill; it is located directly in the zone of hydrothermal fluid rise at a distance of 10 m from the discharge zone of the active vent. In the upper layer of the sediments with a WSTP soil probe, a high groundwater temperature (196°C) was recorded. The temperature of the sedimental core sampled here was also high (an average of about 150°C).
Semi-massive sulfide ores and sulfide-rich breccias were discovered in this well, which were formed at relatively shallow depths corresponding to the discharge zones of the active hydrothermal vent. In this zone, the level of concentration of total mercury in sulfide deposits and in adjacent deposits is very high (up to 3-10 ^g / g). This is 60-200 times higher than the
value of clarke of mercury in sedimentary rocks (0.045 № / g) [2].
Background Hole 858A is located in the area with reduced heat flow. The distance from it to the Hole 858B is about 245 m, and to the mouth of the active hydrothermal vent - 100 m [1]. In the deposits of the upper part of the Hole 858A, the temperature was 92°C. These deposits are turbidites with interlayers of hemi-pelagic clays. The chemical composition of the silt waters in of these sediments is similar to the composition of sea water. This indicates active penetration of sea water into the sedimentary stratum in this area [1].
Different thermoforms of mercury found in the bottom sediments of this site have the characteristic temperature ranges at which peaks of the thermoforms appear on the thermograms during gradual heating of sediment samples [3; 4]. The thermoforms of the mercury we define include: elemental (SV), chloride (CL), physically sorbed (FS), chemically sorbed (CS), sulfide (SF) and isomorphic (IS) forms. The temperature ranges for the peaks of the individual mercury ther-moforms were determined by thermal atomic absorption spectrophotometry. For the elemental (SV) form of mercury, the temperature range is 150-160°C, for the chloride form (CL) - 170-200°C, for physically sorbed form (FS) - 190-290°C, for chemically sorbed form (CS) - 250-320°C, for sulfide form (SF) - 320-400°C, for the isomorphic form (IS) - 400-1000°C [3; 4; 5]. In all samples of the sediments in which mercury was analyzed, the content of about 50 chemical elements was also determined by X-ray structural analysis and ICP-MS.
Results and Discussion. In the 858B well, sulfide ores were recorded at the depths of 11-12 m, 32 m and 38 m in three mineralized layers, which apparently formed in different periods of ore formation. Ore mineralization is the richest at the depth of 11-12 m in the
However, the mercury content in the sediments these two layers is significantly lower than in the 1st sulfide layer: 0.24 ^g / g in the 2nd sulfide layer and 0.51 ^g / g in the 3rd sulfide layer. In the two lower sulfide layers the reduced content of the mercury is apparently due to the influence of brecciation and ther-mometamorphism on sedimentary rocks. As a result of the impact of these processes, mercury can intensively evaporate from precipitation (figure).
The sedimentary thickness that was discovered by the Hole 858B is a zone of sulfide ore formation. Within this sedimentary thickness, we identified several geochemical barriers, according to the classification developed by A.I. Perelman [6]. These are the reduction sulfide hydrogen barrier, the gley reduction barrier, the hydrolytic alkaline barrier, the carbonate sorption barrier and the evaporation barrier. At these geochemical barriers, there is a slowdown in the geo-chemical migration of mercury and its accumulation as a result of changes in its forms of finding.
The reducing hydrogen sulfide barrier is usually formed in those sedimentary layers where the formation of sulfide minerals with a high content of ore elements occurs. In the 1st and 3rd sulfide layers, which are located at the depths of 11-12 m and 38 m, respectively, the content (52-64%) of the isomorphic form (IS) of mercury, which is characteristic of sulfide formation zones, predominates significantly [3]. This allows us to assume that these ore-bearing sedimentary layers are isolated from aggressive groundwater containing sea water.
In the 2nd sulfide layer, which is located at a depth of 32 m, the isomorphic form of mercury is completely absent, and the dominant forms are chemically sorbed (CS; 44%) and sulfide (SF; 26%) forms of mercury. The 2nd sulfide layer is located at the depth of 27-32 m, within the brecciated rock stratum which is well permeable to silt waters and sea water. As a result of this, apparently, processes of sulfate reduction and the formation of nitrogen-alkaline waters actively occur in the 2nd sulfidized sedimentary layer, which contributes to the intensive hydrolytic leaching of silicon from the enclosing rocks [7].
A number of anionic elements are intensively carried out from this layer. This in particular uranium which intensively accumulates on the reducing gley ge-ochemical barrier. This barrier is localized in the overlying sedimentary layer located above the 2nd sulfide layer.
The alkaline hydrolytic barrier is above the 2nd sulfide layer (at the depth of 27-29 m). The formation of this barrier is obviously associated with a sharp decrease in the alkalinity of nitrogen-alkaline waters, which leak out of this layer when mixing them with the surrounding silt waters. Anionic elements (V, Cr, Zr, Nb, Mo), as well as hydroxides of Fe, Ca, Mg and some amount of the uranium are actively deposited on this geochemical barrier. In the zone of this barrier, the sulfide form of mercury sharply dominates (60-80%) which may be due to its active synthesis as a result of the bacterial sulfate reduction process [7].
Figure. Distribution of concentrations of the total mercury and iron in the profile of the Hole 858B.
The gley reducing geochemical barrier is at the depth of 13-18 m in the contact zone of alkaline and acidic gley waters. In this zone, alkaline waters rising from below are neutralized, and carbonate salts of a number of elements (Fe2+ Ca, Mg, Mn, Cu, Zn, Pb) precipitate from them, and also uranium is actively precipitated (up to 38 ^g / g). In the contact zone (at the depth of 15-18 m) of alkaline waters with acidic gley waters, the chemically sorbed form of mercury predominates (60-62%). Its formation can be explained by the active growth of newly formed crystals of carbonates and, in particular, sulfates and phosphates [7].
In the acidic environment of the gley barrier and intense silicification of the rocks occurs due to polymerization and coagulation of the silicic acid together with the humic acids, the source of which is nitrogen-alkaline waters [7]. In the gley barrier zone a characteristic feature of the mercury temperature spectrum is a significant increase in the proportion (up to 28-40%) of the physically sorbed form (FS) of mercury and the chloride form (CL) of mercury (up to 17-22%). The increase in the content of these forms of mercury can be associated with the active sorption of physically sorbed mercury by amorphous silica, as well as with the selective capture of undissociated molecules of mercury chloride by amorphous silica.
Sorption carbonate geochemical barrier is located in the upper part of the sedimentary sequence at a depth of 6 - 9 m in the area of the "iron hat". Here, at present, precipitation of carbonates and oxidation of ferrous iron. These substances are supplied by hydrothermal solutions that drain the 1st sulfide layer. In the zone of this geochemical barrier, the chemically sorbed form of mercury sharply dominates (48-65%), apparently due to the formation of energetically unbalanced surfaces of newly formed crystals of carbonates. The high content (15-35%) of the isomorphic form of mercury is also obviously due to external causes of the isomorphism process: temperature, pressure, and concentrations of substances that caused the high intensity of mineral formation in this zone.
The evaporative geochemical barrier in bottom sediments was recorded at the depth of 0.6 m from the surface of the seabed. This barrier was formed due to the intense evaporation of hot hydrothermal solutions mixed with seawater which are seeped out of the sediments. The chemically sorbed form of the mercury prevails at this barrier (93%), which is formed due to the intensive evaporation of water from silt waters containing sea water as a result of the separation and crystallization of salts from them. An increased content of gross mercury (3.1 ^g / g) and gold (0.04 ^g / g) was recorded in this zone, which is typical for zones where hydrothermal solutions are boiled for a long time [8].
In the core of background Hole 858A, which is located in the area with reduced heat flow, all sedimentary thickness available at Site 858 are most fully represented. These sediments are subject to slight hydrothermal changes. Therefore, the sedimentary stratum in this region and its elemental composition are a "background" stratigraphic record of the features of the hem-ipelagic and turbidite deposition processes. In sediments of core of the Hole 858A the average content of
total mercury is 0.22 ^g / g, which is 5.5 times lower than this (1.21 ^g / g ) in the sediments of the core of Hole 858B. However, the content of mercury in core sediments of the Hole 858A is almost 5 times higher of the clarke of mercury in sedimentary rocks.
In our opinion, the main source of mercury in sediments of this zone are hemipelagic clays containing weathering products of hydrothermally altered basalts, which are abnormally enriched in mercury (up to 1623 ^g / g) in this zone [9]. From these clays the mercury can be leached by aggressive solutions containing sea water as a result of the water-rock interaction. It was established that the levels of mercury in the Hole 858A sediments depend on the degree of hydrothermal change in the rocks [9].
Unchanged precipitation is at the depths of 0.1-34 m; the mercury content in them is the highest (from 0.21 to 0.58 |ig / g). At the depths of 52-256 m, slightly altered lithified sediments are located, in which the average mercury content is 0.21 ^g / g. The minimum mercury content (0.10 ^g / g) was in significantly altered silicified sediments located at the depths of 315331 m. This, obviously, is associated with the ther-mometamorphism of sedimentary rocks and the intense evaporation of mercury from them.
The correlation analysis of geochemical data showed that in this place mercury has the strongest significant bonds with halogens (Cl, Br, J, F) and with the association of other elements (Fe3+, Ca, K, P, Cu, Zn, Ga, Ge, Rb , Nb, Ba, Th, U), which is closely associated with halogens. Based on these data, we concluded that mercury accumulates mainly in the CS, CL, FS and SV forms in saline sediment thickness that were formed, obviously, on the evaporation geochemical barrier at different time periods. Therefore, the previously noted alternation of sediments layers with a contrasting mercury contents in the sedimentary cover section is similar to a "layered cake" [9] and, most likely, is the result of mercury accumulation in brines and salt deposits.
Conclusion. Our studies revealed a high variability in the concentrations of total mercury and its ther-moforms in the sulfide deposits of the studied Site 858 located above the active hydrothermal system, within the Dead Dog hydrothermal field the Middle Valley of the Juan de Fuca mid-ocean ridge. This high variability in the concentrations of mercury and its thermoforms depends on the characteristics of the natural geochemi-cal processes that currently occur at geochemical barriers in the modern sulfide ore formation zone (Hole 858B) and the adjacent background region (Hole 858A). Our studies confirm the high indicator properties of mercury in geological processes, which are caused not only by its total content, but also by the high variability of the content of its thermoforms.
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