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

Abstract

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.