V. V. Zaykov

The fossil record of hydrothermal vent communities

Abstract There are 19 known fossiliferous volcanogenic massive sulphide (VMS) deposits which range in age from the Silurian to the Eocene. Most of these are in the Ural Mountains, Russia. The deposits contain assemblages of inarticulate and rhynchonellid brachiopods; gastropod, bivalve and monoplacophoran molluscs; and a small diversity of worm tube morphologies, some of which may be attributable to alvinellid polychaetes and vestimentiferans. The fossils are preserved mainly as external moulds of pyrite, which is consistent with biomineralization processes occurring at modern vent sites. Most of the fossil taxa are new to science, but the lack of original shells and organic tubes makes placement in existing phylogenetic schemes difficult. A comparison between modern vent communities and fossil vent assemblages shows that vestimentiferans, alvinellid polychaetes, bivalves, gastropods, monoplacophorans and perhaps brachiopods are shared at higher taxonomic levels, but that arthropods are found only in the modern communities. There are no direct ancestor-descendant relationships between the fossil and modern vent molluscs and brachiopods. This demonstrates that the modern vent environment is not a refuge for the known Palaeozoic and Mesozoic shelly vent taxa. Hence, taxonomic groups have moved in and out of the vent ecosystem through time. These findings are discussed in relation to alternative hypotheses for the origins of modern vent communities.

Origin of chromite in mafic–ultramafic-hosted hydrothermal massive sulfides from the Main Uralian Fault, South Urals, Russia

Abstract Mafic–ultramafic-hosted hydrothermal Fe–Cu–(Ni–Co) sulfide ores from the Main Uralian Fault Zone (MUFZ), South Urals (Ivanovka and Ishkinino ore fields), contain a relatively large (up to 3%) proportion of chromite. This association is common for magmatic Fe–Ni–Cu sulfides, but definitely unusual for hydrothermal sulfides. Textural, morphological and compositional data are used here to gain an insight into the origin and significance of this unusual chromite–sulfide association. The studied chromites occur both as broken fragments and as euhedral or subhedral crystals, which are included in the sulfides or scattered in their talc±chlorite±saponite±quartz±carbonate matrix. They are characterized by high Cr/(Cr+Al) ratios (0.58–0.85) and range in composition from magnesiochromite to chromite sensu stricto. Textural, morphological and compositional features, as well as the occurrence of relatively high-silica, low-Ti, low-K melt inclusions in some of the crystals, indicate that the ore-associated chromites (i) are a mixed population of grains derived from mafic–ultramafic mantle and crustal magmatic rocks and mantle peridotite melting residua, (ii) have no genetic relation with the host sulfides and (iii) represent relicts derived from the hydrothermally altered country rocks. The compositions of the chromites and of the melt inclusions denote a clear supra-subduction zone signature. The melts parent to the cumulitic chromites had an arc tholeiitic to, possibly, boninitic affinity. These data suggest that the host mafic–ultramafic complexes formed in an early arc or forearc setting and do not represent obducted portions of MORB oceanic lithosphere. Hence, contrary to previous interpretations, the associated massive sulfides could not originate on a mid-ocean ridge, but rather in an early arc or forearc environment. Given the relatively short life of the western Uralian arc system, the most probable time window for sulfide ore deposition is confined to Early to Middle Devonian time.

Ancient vent chimney structures in the Silurian massive sulphides of the Urals

Abstract Exceptionally well preserved volcanogenic massive sulphide ores at Yaman Kasy in the Silurian of the southern Urals have yielded not only well-preserved sulphidized vent macrofauna but also fragments of vent chimneys. All fragments show a broad 3-fold mineralogical zonation. An outer zone which forms the chimney/conduit wall comprises largely pyrite and marcasite which is laminated or collomorphic and is commonly porous. In one fragment this zone is characterized by a honeycomb-like structure in the pyrite, infilled by barite, sphalerite and chalcopyrite. Dendrite growth textures branch outwards towards the chimney wall. The middle of the three zones comprises largely pyrite with chalcopyrite and sphalerite as thin veinlets and infillings. The innermost zone is dominated by chalcopyrite. Minor gold and bismuth tellurides occur at the boundary between the middle and the inner zones. The inner zone is interpreted as the hydrothermal conduit lining and in all cases is defined by bladed chalcopyrite, which shows a texture consistent with growth toward an inner open space. The central part of the conduit is now infilled with sphalerite or in one case pyrite, chalcopyrite, silica and minor barite. Fluid inclusion studies indicate the presence of high-temperature (> 350°C) fluids of around 3.5 0.000000e+00quivalent NaCl in the basal parts of the massive sulphide mound with cooler temperatures (< 100°C) recorded in barite from the upper part of the mound. Barite is associated with chimney fragments, as worm tube infillings and as later cross-cutting veins. Preliminary S and Sr isotope data from sulphides and sulphates supports both igneous and seawater sources for sulphur in the hydrothermal system with evidence for seawater circulation and sulphate precipitation beneath the sulphide mound. The results are consistent with a similar model of chimney growth to that proposed for modern vent sites. It is proposed that a high temperature fluid flux through open conduits fed black smoker activity accompanied by lateral fluid diffusion through the chimney wall which mixed with seawater. The result of this is a combination of conductive cooling and fluid mixing leading to precipitation of distinctively zoned mineral assemblages across the vent conduit wall. The occurrence of tellurium, bismuth and precious metal-bearing phases indicates some similarities with the geochemistry of sulphides from other Palaeozoic massive sulphide deposits associated with felsic volcanic centres.

Hydrothermal activity and segmentation in the Magnitogorsk-West Mugodjarian zone on the margins of the Urals palaeo-ocean

Abstract The Magnitogorsk-West Mugodjarian zone is interpreted to have developed as a marginal rift to the main Urals palaeo-ocean. In this zone, seven distinct tectonic segments each with a strike extent between 140 and 220 km have been identified. These segments can be distinguished on the basis of volcanic facies and features of hydrothermal activity. In the three southernmost segments the volcanics comprise basaltic and rhyolite-basalt complexes. The former contain massive sulphides of Cyprus-type and cherty iron-formation whilst the latter contain massive sulphides of ‘Uralian’ type, cherty iron-formation and manganese oxide deposits. In the two central segments rhyolite-basalt complexes are developed associated with giant massive sulphide deposits (Sibay > 100 million tonnes massive sulphides). In the two northernmost segments rhyolite-basalt and andesite-basalt complexes are developed, both associated with cherty iron-formation and manganese oxide deposits. It is proposed that the variations in frequency, size and composition of the metalliferous deposits between the segments are a result of differences in the sequence of Palaeozoic oceanic development. Rifting started during the Eifelian in the south (current position) and in Late Givetian times in the north. This diachronous rifting and tectonic development has influenced the major geological features of the segments.

Geoarchaeological research into the historical relics of the South Urals: problems, results, prospects

Abstract A description is given here of the major geoarchaeological research undertaken in the Southern Urals region between 1991 and 1998. General petrographic characteristics of the stone material used for the manufacture of tools in the settlements of Arkaim and Alandskoe are discussed, together with their possible raw material sources. Research has indicated that Bronze Age settlements in the region used at least nine types of copper ore. One of these, the ancient mine ‘Vorovskaya Yama’, is described. Chemical analysis of metal objects from the settlements of Arkaim, Sintashta and Kuisak show that three types of copper (pure, arsenical and argentiferous) and three kinds of bronze (arsenical, stanniferous, and nickeliferous) are used for the artefacts. Buried objects show evidence of corrosion with formation of the minerals atacamite, paratacamite, nantokite, malachite, cuprite, and tenorite. The process of corrosion in the presence of organic material has been studied and malachite, azurite and sampleite have been shown to form. Microprobe analysis of gold objects from eight burial mounds shows that, in the Bronze Age, pure native gold was used to make jewellery. The use of binary artificial alloys of gold and silver is characteristic of the Early Iron Age; in the Early Middle Ages ternary artificial alloys of gold, silver and copper were used. The composition of lead wire found in the Kuisak settlement has also been determined.

Lead isotopic systematics of Urals massive sulphide deposits

The isotopic composition of lead from a total of 53 samples of galena from 18 VHMS deposits shows a range between 17.437 and 18.111 for 206Pb/204Pb; 15.484 and 15.630 for 207Pb/204Pb and 37.201 — 38.027 for 208/204Pb. The results show a systematic trend with the leads of the Sibay, Barsuchiy Log and Djusa deposits being most radiogenic by comparison with those of Bakr-Tau and Oktiabrskoe which are the least radiogenic deposits. The Bakr-Tau and Oktiabrskoe deposits occur within most primitive fore-arc rocks at the lower part of the Baymak-Buribay formation, which contain lavas of boninitic affinity. The Sibay, Barsuchiy Log and Djusa deposits are found in intra- and back-arc setting and are hosted by a sequence of bimodal tholeiites. The deposits in “arc” setting such as the Balta- Tau, Gai and Alexandrinka deposits occupy an intermediate position. This trend is explained in term of mixing between mantle wedge and continental blocks.