ISSN 1028334X, Doklady Earth Sciences, 2014, Vol. 458, Part 1, pp. 1077–1081. © Pleiades Publishing, Ltd., 2014. Original Russian Text © T.Yu. Tolmacheva, A.A. Ryazantsev, K.E. Degtyarev, O.I. Nikitina, 2014, published in Doklady Akademii Nauk, 2014, Vol. 458, No. 3, pp. 318–322.
GEOLOGY
Hydrothermal Barite Deposits in Upper Cambrian–Lower Ordovician Siliceous Successions of Southern Kazakhstan T. Yu. Tolmachevaa, A. A. Ryazantsevb, Corresponding Member of the RAS K. E. Degtyarevb, and O. I. Nikitinac Received April 14, 2014
DOI: 10.1134/S1028334X14090347
It is well known that lowtemperature hydrother mal seabed springs or cold seeps, which form local highly productive ecosystems in the poorly inhabited areas of presentday seas and oceans, were widespread in past geological epochs [1]. The sediments of hydro thermal systems or seeps are most frequently preserved within terrigenous strata in a form of carbonate or, less commonly, barite strata, lenses, or local anomalous accumulations of benthic fossils [2]. The carbonate cold seeps are now sufficiently well known [3], while barite seep deposits characterized by fossils are extremely rare despite the wide distribution of strati form barites within Precambrian and Lower Paleozoic sequence [4]. At present, the oldest deposits of barite cold seeps with remains of metazoans are known in the Lower Silurian of the Atlas Mountains in Morocco [5]. Unlike hightemperature vents, their lowtemper ature counterparts are not necessary accompanied by communities of chemoautotrophic organisms and their symbionts; they inhibit seeps only in deep sea areas with a reduced influx of nutrients produced by life activity of pelagic organisms [6, 7]. The lowtem perature hydrothermal system with the dominant bar ite deposition include cold seeps associated with fluid flows, which formation is induced by tectonic or hydrological activity and those related to volcanic activity. The latter stimulates also development of hightemperaturevents“black smokers,” where dep osition of barite is accompanied by sulfide mineraliza tion [8].
a
A.P. Karpinsky Russian Geological Research Institute (VSEGEI), St. Petersburg, Russia b Geological Institute, Russian Academy of Sciences, Moscow, Russia c Sarpaev Institute of Geological Sciences, Almaty, Kazakhstan email:
[email protected],
[email protected];
[email protected],
[email protected]
We have carried out detailed investigation of the structural and stratigraphic positions of barite lenses confined to the Upper Cambrian–Lower Ordovician siliceous successions widespread in the southwest from Lake Balkhash (southern Kazakhstan). The investiga tion also included analysis of the S and Sr isotope compositions in barite as well as the distribution and taxonomic diversity of fossils in the host rocks. The Upper Cambrian–Lower Ordovician siliceous successions, which are assigned to the Burubaital For mation in the southwestern Balkhash region, are the most peculiar strata in the Erementau–Buruntau zone, which extends for almost 2000 km in the subme ridional direction from the Erementau Mountains in the north to Lake Balkhash in the south [9]. The Burubaital Formation is represented by thin (up to 100 m) condensed successions composed of biogenic radiolarian cherts with rare intercalations of siliceous siltstones and sandstones. The siliceous strata were deposited during the Late Cambrian–Middle Ordovi cian (midDarriwilian), which is confirmed by con odonts particularly abundant in the basal part of the formation [10]. It was assumed that Cambrian–Lower Ordovician siliceous sediments of the Erementau– Buruntau zone were deposited on the continental slope and at the foot of the passive continental margin, the shelf part of which accumulated carbonate and carbonate–terrigenous deposits [11]. The Burubaital Formation was studied in two sec tions with barite lenses (Baritovyi Kar’er and Rak ovaya Gorka) and a section in which barite mineral ization is missing (Pamyuatnik Prirody) (Figs. 1, 2). The largest and best studied barite lens (Baritovyi Kar’er section) up to 30 m thick extends for over 1 km and is mined in the quarry located 22 km west of the Chiganak Settlement (45°07.410′ N, 73°44.707′ E) on the western coast of Lake Balkhash (Figs. 1, 2). In this area, the rocks of the Burubaital Formation are deformed together with overlying conglobreccias, silt stones, and sandstones of the Maikul Formation into
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Fig. 1. Schematic geological structure of the Baritovyi Kar’er area and cross section along line I–II. (1, 2) Maikul Formation (upper Darriwilian–lower Sandbian): (1) conglobreccia with cherty blocks, (2) sandstones, siltstones; (3–5) Upper Cambrian–Middle Ordovician (Darriwilian) Burubaital Formation: (3) siliceous siltstones, siltstones, (4) siliceous rocks, (5) barites; (6) Middle–Upper Cambrian Burultas Formation: phosphorite sandstones with sulfide lenses; (7) tectonic contacts; (8) quarry contour; (9) location of the examined sections of the Burubaital Formation in the inset: (1) Baritovyi Kar’er, (2) Pamyatnik Prirody, (3) Rakovaya Gorka.
isoclinal folds with subvertical axial planes. The folds are disturbed by longitudinal reversed faults and steep thrusts with multiply repeated section fragments (Fig. 1). In the quarry, the barite lens up to 20 m thick rests upon thin pyritized phosphorite sandstones and is overlain by an 80mthick sequence of cherts and sili ceous siltstones of the Burubaital Formation. The lower part of the formation is composed of gray rib bonbanded semitransparent cherts, which alternate with highly kaolinized siltstones. Conodont and rare brachiopod (lingulids) remains which are practically missing from the cherts, are confined to siltstone bed ding surfaces. The upper part of the formation is rep resented by red and brown cherts alternating with sili ceous siltstones with conodonts scattered throughout the entire section. The time of formation of the largest barite lens in the Baritovyi Kar’er section is estimated by age of the overlying cherts and cherts alternating with barite in marginal parts of the lens. It has been established that the lens was formed during a relatively long period from the Late Cambrian (Eoconodontus notchpeakensis Zone) to the Tremadocian of the Early Ordovician (Cordylodus angulatus Zone). This section also contains thin barite lenses documented at the
level of the Tremadocian–basal Floian Paroistodus proteus Zone of the Lower Ordovician. In the Rakovaya Gorka section, barite lenses up to 1 m thick occur among banded gray to black cherts containing conodonts of the Floian Prioniodus elegans Zone of the Lower Ordovician (Fig. 2). No fossils are found in barites, although host rocks adjacent to them contain abundant and diverse faunal assemblages. They include mass accumulations of benthic (rhabdopleurids) and planktonic ptero branchs (graptolites) (Figs. 3a, 3e), pelagic arthropods (caryocaridids) (Fig. 3f), and ostracods (Fig. 3d), in addition to conodonts and radiolarias. Conodont remains of fecal origin occur in cherts surrounding barites (Fig. 3g). The beds of of siliceous sandstones located slightly above barite are composed of con odont elements sorted by size (Fig. 3b). It should be noted that the mass accumulations of ostracods observed in these rocks represent a first find of this fau nal group in deepwater siliceous sediments [12]. The faunal remains including chitin matter of pelagic graptolites and arthropods appear to be well preserved near barite bodies owing to the formation of specific facies around them such as carbonaceous phtanites (Rakovaya Gorka section), which are miss DOKLADY EARTH SCIENCES
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Fig. 2. Sections of the Burubaital Formation and generalized reconstruction of the barite seep. The dark color designates coeval lower part of the formation that was deposited during activity of the cold seep: (1) barite; (2) conglobreccias, sandstones; (3) red cherts with knobby bedding surfaces; (4) gray and green cherty siltstones; (5) red cherty siltstones; (6) siliceous sandstones; (7) red translucent and hemitranslucent cherts; (8) gray, white, yellow translucent and hemitranslucent cherts; (9) red clayey opaque cherts; (10) alternating thinbedded white cherts and carbonaceous black phtanites; (11) black carbonaceous phtanites; (12) sil iceous sandstones; (13) stratigraphic hiatus; (14) sites of conodont sampling; (15) conodonts scattered through the section; (16) conodonts on bedding surfaces; (17) fecal pellets; (18) benthic pterobranchs, graptolites; (19) pelagic arthropods; (20) sponges; (21) brachiopods; (22) ostracods. Open intervals designate gaps in observations.
ing from the background sections of the Burubaital Formation. The Pamyatnik Prirody (Fig. 2) and other sections of the Burubaital Formation, where barite bodies are absent, contain only rare radiolarians, sponge spicules, conodonts, and lingulids.
bottom water around seeps. The abundance of sili ceous sandstones including that one with conodonts (Fig. 3b) implies the local presence of paleomounds, which occurrences could be related to activity of barite seeps.
The sediments in marginal parts of barite bodies enclose intercalations of red ferruginous jaspers and siliceous sandstones, which is illustrated well by the Tremadocian interval of the Pamyatnik Prirody sec tion. All the sections coeval to barite lenses and located near them are characterized by more diverse lithology as compared with background sections that is explained by the different chemical composition of
Despite the fact that rocks surrounding stratiform barite bodies are barren of fossils associated with che mautotrophic organisms characteristic of hydrother mal vents, the increased concentration of organic remains and the development of specific facies (car bonaceous phtanites, hematitized cherts) indicates their formation due to the activity of synsedimentation hydrothermal system at the bottom.
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Fig. 3. (a) The lower part of the Baritovyi Kar’er section with the barite lens at its base; (b) “sandstone” bed composed of con odont elements of the Oepikodus evae (upper Floian) in the siliceous matrix; (c) fine alternation of barite with Upper Cambrian dark gray cherts; (d) mass accumulation of ostracods Burultalina nikitinae Melnikova [12]; (e) graptolite; (f) pelagic arthropod; (g) fecal pellet composed of conodont elements.
The 87Sr/86Sr and 36S/34S ratios widely used as the geochemical indicators of barite formation environ ments [4, 8]. These ratios in barite from the main lens in the Baritovyi Kar’er section were measured on the DELTAplusXL and FinniganMat Triton2 mass spec trometers, respectively, at the Center for Isotopic Investigations of VSEGEI. The δ34S values in three samples are 37.6, 43.3, and 54.7‰, being higher as compared with its average values in barite cold seeps in the presentday and relatively young basins and indi cating barite precipitation by sulfatereducing organ isms [13]. The 87Sr/86Sr values in two samples are 0.708560 ± 6 (85Rb/86Sr = 0.000044) and 0.708421 ± 6 (85Rb/86Sr = 0.000044), which are slightly lower than the average 87Sr/86Sr value in coeval oceanic waters [14]. These data indicate the dominant role of the crustal source of barite in fluid flows and subordinate contribution of the juvenile mantle source. At the same time, the confinement of barites to condensed siliceous successions of the passive continental margin and sulfide mineralization in sandstones underlying the barite body implies its volcanic nature. At present, barites of the Burubaital Formation represent one of the oldest paleontologically charac
terized deposits of lowtemperature hydrothermal sys tems. Although the nature of these seeps is not quite clear, they are obviously strongly influenced the rela tively impoverished Late Cambrian and Early Ordovi cian ecosystems of open oceans significantly increas ing the productivity of waters surrounding them and were responsible for the paleolandscape heterogeneity of the oceanic bottom and formation of lateral succes sion of paleofacies with specific taphonomic parame ters that favored preservation of faunal groups atypical of siliceous sections. ACKNOWLEDGMENTS The authors are grateful to chief surveyor of LLP “Vos tochnoe Rudoproyavlenie” (Kazakhstan) V. P. Grishin. This work was supported by the Division of Earth Sciences of the Russian Academy of Sciences (pro gram “Geodynamic evolution of lithotectonic com plexes of the Earth in the Neogean”), by the Russian Foundation for Basic Research (project nos. 1205 00844 and 130400629) and by grants from the Royal Swedish Academy of Sciences (KVA) and the Swedish DOKLADY EARTH SCIENCES
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Translated by I. Basov
