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Buschmann B., Maslennikov V.V. , Bergakademie T.U.
The palaeoceanographic record of South Urals vms deposits

Buschmann¹, V.V. Maslennikov²

¹TU Bergakademie, Freiberg, Germany,

Bernd.Buschmann@mineral.tu-freiberg.de,

²Institute of Mineralogy, Urals Branch of RAS, Miass, Russia,

mas@ilmeny.ac.ru

THE PALAEOCEANOGRAPHIC RECORD OF SOUTH URALS VMS DEPOSITS

Many volcanogenic massive sulphide (VMS) base metal deposits were generated by focussed exhalation of hydrothermal fluids at the sea-floor. This is particularly recorded by relics of hydrothermal vent chimneys, erosive burial by volcanic and/or sedimentary rocks, and hydrothermal vent fauna in some Phanerozoic examples. In the South Urals Palaeozoic VMS district, abundant VMS deposits of different age display one of these features at least [Maslennikov, 1999; Herrington et al., 2005; Zaykov, 2006].

These deposits may provide an as yet underrated record of palaeoceanographic conditions that refers to oceanic mixing and related deep-sea bottom water aeration in the first line. Such conditions are poorly known for the Palaeozoic Era because Palaeozoic ocean-floor is largely subducted and relics usually metamorphosed. The record of Early to Mid Palaeozoic marine sediments that were deposited on continental lithosphere and physical oceanographic models indicate widespread dysoxic-anoxic bottom water conditions during periods of equable greenhouse climatic conditions (black shale facies) but deep oceanic circulation and ventilation during icehouse intervals. However, we poorly know wether this really applies to deep oceans of this period. This has not only consequences for understanding of oceanic energy pathways and hydrocarbon source rock generation during the Palaeozoic, but also of potential long-term effects of enhanced global warming on circulation and aeration of rising epicontinental seas. The latter will be briefly outlined for the aftermath of the Late Ordovician Hirnantian icehouse period.

Several South Urals VMS deposits reveal five important features necessary for assessment of deep ocean circulation in Palaeozoic time: (1) narrow age constraints for correlation with the global record, (2) formation on oceanic lithosphere, (3) palaeodepths beyond oceanic wind-mixing ventilation depth (> 500 mbsl), (4) evidence of bottom water aeration conditions, and (5) proxies to degrees of oceanic mixing. Age constraints are predominantly provided by conodont biostratigraphic data whereas formation on oceanic lithosphere is revealed both by geochemical features of host rocks and VMS ores and general stratigraphic patterns.

Particularly crucial is the assessment of a minimum palaeodepth and this can be offered only by relics of high-temperature hydrothermal venting in VMS deposits. Water depth (hydrostatic pressure) exerts control on maximum temperatures of hydrothermal fluids during exhalation at the sea-floor defined by the boiling curve of seawater. Empirical and experimental studies show that Cu is effectively transported at fluid temperatures in excess of 300 °C in widespread modern sea-floor hydrothermal systems. Exhalation of such fluids is confined to water depths in excess of ca. 840 mbsl, i.e. below the depth range of oceanic ventilation by turbulent wind-mixing. Hence, chalcopyrite (CuFeS₂) linings of fossil hydrothermal vent chimneys record high-temperature hydrothermal exhalation at deep-sea floors. They predominantly occur in Late Ordovician and Eifelian VMS deposits in the South Urals. In case of destruction of primary ore textures by tectonic overprint, enhanced contents of Cu in bulk VMS ore provide an indicator of high-temperature sea-floor hydrothermal venting provided that the stratigraphy excludes ore formation by replacement. This is the case for many Late Emsian VMS deposits in the South Urals. Modern high-temperature sea-floor hydrothermal vent complexes are concentrated in the lower bathyal to upper abyssal depth range (1000–4000 mbsl) [Hannington et al., 2005] and this obviously applies to formation of abundant Palaeozoic VMS deposits in the South Urals.

Deep-sea bottom water aeration is evidenced by fossil hydrothermal vent fauna and sea-floor oxidation patterns in tops of VMS ore bodies that represent fossil sea-floor gossans. Sulphur isotope values of hydrothermal barite (BaSO₄) from South Urals deep-sea VMS deposits provide a proxy to mixing of oceanic water masses. Dissolved barium in hydrothermal fluid exhalations is precipitated by bonding to sulphate from ambient seawater. Anoxic water columns experience withdrawal of light sulphur from seawater sulphate by bacterial sulphate reduction and burial as pyrite (FeS₂). In view of low sulphate concentrations in Early to Mid Palaeozoic oceans, this should have caused shifts to higher δ³⁴S values in anoxic water columns compared to the ventilated sea surface layer. However, δ³⁴S values of hydrothermal barite from South Urals VMS deposits match those of carbonates and sulphate from contemporaneous shallow marine deposits. This indicates oceanic mixing below the depth of turbulent wind-mixing and argues against presence of long-term stratified anoxic water columns.

Late Ordovician deep-sea floor aeration and oceanic mixing evidenced by South Urals VMS deposits may coincide with the Late Ordovician climatic icehouse period. It supports suggestions of Late Ordovician climate-driven thermohaline ocean circulation and represents the first direct record of these conditions from deep-sea deposits on oceanic lithosphere. By contrast, Late Emsian and Eifelian deep-sea floor aeration and oceanic mixing shown by South Urals VMS deposits contradicts models of sluggish ocean circulation during this period. These case studies highlight the unique potential of weakly overprinted Palaeozoic deep-sea VMS deposits for records of deep-sea environmental conditions and global palaeoceanic patterns.

References

Hannington M. D., de Ronde C. & Petersen S. Sea-floor tectonics and hydrothermal systems. In: Hedenquist J. W., Thompson J. F. H., Goldfarb R. J. & Richards J.P. (eds.). Economic Geology. 100^th Anniversary Volume // Society of Economic Geologists, Littleton, Colorado, 2005. P. 115–157.

Herrington R., Maslennikov V., Zaykov V., Seravkin I., Kosarev A., Buschmann B., Orgeval J.-J., Holland N., Tesalina S., Nimis P. & Armstrong R., Classification of VMS deposits: Lessons from the South Uralides. Ore Geol. Rev., 2005. V. 27. № 6. P. 203–237.

Maslennikov V. V. Sedimentogenesis, halmyrolysis and ecology of massive sulfide-bearing paleohydrothermal fields (examples of the South Urals). Urals Branch of Russian Academy of Science, Miass, 1999. 348 p. (in Russian).

Zaykov V. V. Volcanism and Sulphide Mounds of Paleooceanic Structures. Moscow: Nauka, 2006. 429 p. (in Russian).