The formation of an ancient, gold-rich volcanogenic massive sulphide deposit : a study of the Balta Tau deposit in the Southern Urals of Russia
The formation of an ancient, gold-rich volcanogenic massive sulphide deposit : a study of the Balta Tau deposit in the Southern Urals of Russia
Balta Tau is a gold-rich VMS deposit located in a well preserved Palaeozoic island-arc in the Southern Urals, Russia. Host sequences are andesitic tholeiites of the Irendyk Formation with regional prehnite/actinolite-pumpellyite metamorphism. Massive sulphides comprise pyrite-chalcopyrite-sphalerite and upper sphalerite-barite ores overlain by barite containing tennantite-chalcopyrite-galena-electrum. Ore textures are consistent with deposition at the seafloor and indicate a seafloor hydrothermal origin for electrum. Marginal compositionally layered sulphides represent clastic sulphides that recrystallized during late-stage ore formation. The footwall contains extensive vein-hosted and disseminated sulphides, with central chalcopyrite-pyrite grading to marginal sphalerite-rich ores.
Palaeofluid and isotope analyses indicate the derivation of ore-forming fluids from seawater interacting with volcanic arc rocks, with estimated fluid δ18O of -2.5 to + 1‰, 87Sr/86Sr of 0.7045 and salinities of 2.4-6.4 eq. wt% NaCl. Sphalerite-barite ores formed at 130-170°C, while solubility constraints suggest chalcopyrite-rich ores to have formed at 250-300°C. Mineral assemblages suggest low/moderate sulphidation and moderate/high oxidation conditions. Barite 87Sr/86Sr (0.7050-0.7069) and δ18O (+8.3 to + 13.7‰) indicate significant conductive cooling of hydrothermal fluids and 5-50% seawater mixing during ore formation. Sulphides (δ34S - 3.3 to +2.4‰) precipitated from hydrothermal H2S derived from underlying volcanics, while sulphate (δ34S + 17.6 to +21.6‰) was derived from seawater. Sulphur isotopic equilibrium was not attained between H2S and SO42-, however SO42- and H2O probably approached oxygen isotopic equilibrium. Footwall quartz-Mg-chlorite and hanging-wall sericitization formed at 100-150°C during waning stages of hydrothermal activity.
The Au-Ag-Pb-Ba enrichment at Balta Tau reflects source rock composition, however deposit-scale factors were responsible for the characteristics of specific ore facies. Sulphide compositional zoning reflects differences in the saturation temperatures of base metal chloride complexes. The gold-barite-rich ores most likely formed in a barite structure that trapped hydrothermal fluids emerging from the sulphide mound and limited seawater mixing.
University of Southampton
Holland, Nicola Grace
c255403f-af70-4a9d-bc85-79c850fa4d6c
2004
Holland, Nicola Grace
c255403f-af70-4a9d-bc85-79c850fa4d6c
Holland, Nicola Grace
(2004)
The formation of an ancient, gold-rich volcanogenic massive sulphide deposit : a study of the Balta Tau deposit in the Southern Urals of Russia.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
Balta Tau is a gold-rich VMS deposit located in a well preserved Palaeozoic island-arc in the Southern Urals, Russia. Host sequences are andesitic tholeiites of the Irendyk Formation with regional prehnite/actinolite-pumpellyite metamorphism. Massive sulphides comprise pyrite-chalcopyrite-sphalerite and upper sphalerite-barite ores overlain by barite containing tennantite-chalcopyrite-galena-electrum. Ore textures are consistent with deposition at the seafloor and indicate a seafloor hydrothermal origin for electrum. Marginal compositionally layered sulphides represent clastic sulphides that recrystallized during late-stage ore formation. The footwall contains extensive vein-hosted and disseminated sulphides, with central chalcopyrite-pyrite grading to marginal sphalerite-rich ores.
Palaeofluid and isotope analyses indicate the derivation of ore-forming fluids from seawater interacting with volcanic arc rocks, with estimated fluid δ18O of -2.5 to + 1‰, 87Sr/86Sr of 0.7045 and salinities of 2.4-6.4 eq. wt% NaCl. Sphalerite-barite ores formed at 130-170°C, while solubility constraints suggest chalcopyrite-rich ores to have formed at 250-300°C. Mineral assemblages suggest low/moderate sulphidation and moderate/high oxidation conditions. Barite 87Sr/86Sr (0.7050-0.7069) and δ18O (+8.3 to + 13.7‰) indicate significant conductive cooling of hydrothermal fluids and 5-50% seawater mixing during ore formation. Sulphides (δ34S - 3.3 to +2.4‰) precipitated from hydrothermal H2S derived from underlying volcanics, while sulphate (δ34S + 17.6 to +21.6‰) was derived from seawater. Sulphur isotopic equilibrium was not attained between H2S and SO42-, however SO42- and H2O probably approached oxygen isotopic equilibrium. Footwall quartz-Mg-chlorite and hanging-wall sericitization formed at 100-150°C during waning stages of hydrothermal activity.
The Au-Ag-Pb-Ba enrichment at Balta Tau reflects source rock composition, however deposit-scale factors were responsible for the characteristics of specific ore facies. Sulphide compositional zoning reflects differences in the saturation temperatures of base metal chloride complexes. The gold-barite-rich ores most likely formed in a barite structure that trapped hydrothermal fluids emerging from the sulphide mound and limited seawater mixing.
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Published date: 2004
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Local EPrints ID: 465455
URI: http://eprints.soton.ac.uk/id/eprint/465455
PURE UUID: d1207746-dd2d-47e3-b211-e6931b139f17
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Date deposited: 05 Jul 2022 01:08
Last modified: 16 Mar 2024 20:11
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Nicola Grace Holland
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