Hydrothermal sulphides

2 Future perturbations related to resource extraction 8.4.2.1 Hydrothermal sulphides 

However, little is known about the possible effects of mining seafloor methane on cold seeps. Any disturbance could potentially cause a release of pressure, which could then trigger a massive release of methane through destabilisation of the hydrates. This could have important climate implications because of methane s strong global warming potential, which is estimated to be 23 times that of C02 over a 100-year time horizon (IPCC, 2001). 

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The first indication of a possible connection between geological processes occurring at the boundaries between tectonic plates of the mid-oceanic ridges and the biogenesis problem was provided by J. B. Corliss (1981). He considered the hydrothermal conditions to be ideal reactors for abiotic synthesis these ideal conditions were the water temperature gradients, the pH, and the concentrations of solutes in the hot springs. The presence of certain minerals which could act as catalysts, such as montmorillonite, clay minerals, iron oxide, sulphides etc., was also very important. The initial model presented for the hydrothermal synthesis of biomolecules (Corliss, 1981) was modified, particularly by Russell (1989) and Wachtershauser (see Sect. 7.3). 

An example of a hydrothermal deposit is the New Rambler deposit, described by McCal-lum et al. [4] in the Medicine Bow Mountains in south-western Wyoming, USA, which contains a significant amount of PGM. The ore occurs in irregular pods that are hydro-thermally decomposed into metadiorite and metagabbro zones. Pyroxenite and peridotite are reported to be intersected at a depth beneath the ore zone. All have been affected by supergene alteration. The main sulphides in the ore include pyrite, chalcopyrite, pyrrhotite, covellite and marcasite with associations of electrum, pentlandite and PGM. 

There is no evidence that the depth may be a result of an alteration in the original concentration of magmatic sulphides. It may be a result of concentration of hydrothermal solutions. 

The most important tin deposits are hydrothermal deposits (hypothermal and mesothermal). The magmatic deposits do not often contain tin mineralization. Tin may also be present in pegmatitic ore bodies. However, tin found in pegmatitic deposits can be classified into two basic types (a) quartz-cassiterite lenses in granite, when cassiterite is associated with topaz, beryl and, to a lesser degree, sulphides (b) sulphide deposits, where tin is mainly cassiterite associated with arsenopyrite, pyrite, chalcopyrite and pyrrhotite. Such deposits are common in South America (Peru, Bolivia). 

Plimer, I.R. 1979. Sulphide rock zonation and hydrothermal alteration at Broken Hill, Australia. Transactions of the Institution of Mining and Metallurgy Section B Applied Earth Science, 88, 161-176. 

The VMS sulphides define a broadly conformable sheet 5-50 m below the base of the BLUC. This synformal surface of mineralisation generally coincides with the transition from ultramafic cumulates and strongly foliated metavolcanic rocks to an underlying sequence of metavolcanic rocks and interflow metawacke and metasiltstone rocks (Fig. 2). Sphalerite-pyrrhotite mineralisation in hydrothermal veins was also observed within the BLUC several km west of this study area. 

Hydrothermal alteration involved Na depletion and Mg addition, and visibly altered rock is restricted to within a few metres above visible sulphide mineralisation, but not below ... 

The size of the reactors is quite variable. In length, the biggest reactor has dimensions of 12 x 18 m and has a thickness of 20 to 50 cm (Fig. la). The core of the reactors consists of a 5 to 20 cm thick layer of uraninite embedded in clays (illite and chlorite). Clays around the reactors result from the hydrothermal alteration of the host sandstone during the fission reactions. This alteration occurred at a temperature close to 400 °C in the core. Temperature decreased drastically toward the vicinity with a thermal gradient of 100 °C/m (Pourcelot Gauthier-Lafaye 1999). The uranium content of the core ranges between 40 and 60%. Accessory minerals are mainly sulphides (pyrite and galena), hematite and phosphates (mainly hydroxyapatite). 

O Brien, S.J., Dube, B. and O Driscoll C.F. (1999) High-Sulphidation, Epithermal-Style Hydrothermal Systems in Late Neoproterozoic Avalonian rocks on the Burin Peninsula, Newfoundland Implications for Gold Exploration Report 99-1, Current Research Newfoundland Department of Mines and Energy Geological Survey, pp. 275-96. 

Figure 10.1 Hydrothermal vents, so-called black smokers, carry high concentration of sulphur compounds with traces of methane and hydrogen together with suspended sulphide particles. (Picture by courtesy of Professor Verena Tunnidiffe, University of Victoria, Canada)...

Edmond J. M., Campbell A. C., Palmer M. R., KlinVhammer G. P., German C. R., Edmonds H. N., Elderfield H., Thompson G., and Rona P. (1995) Time-series studies of vent-fluids from the TAG and MARK sites (1986, 1990) Mid-Atlantic Ridge a new solution chemistry model and a mechanism for Cu/Zn zonation in massive sulphide ore-bodies. In Hydrothermal Vents and Processes, Geol. Soc. Spec. Publ. 87 (eds. L. M. Parson, C. L. Walker, and D. R. Dixon), The Geological Society Publishing House, Bath, UK, pp. 77-86. 

Spooner E. T. C. and Bray C. J. (1977) Hydrothermal fluids of seawater salinity in ophiolitic sulphide ore deposits in Cyprus. Nature 266, 808-812. 

It is considered that in the primary (hypogene) environment, Hg is transported principally as complexes in hydrothermal solutions, although the relative importance of sulphide, bisulphide, chloride, hydroxy and organic complexes remains controversial (Bames et al., 1967 Barnes, 1979). At temperatures of over about 200°C vapour-phase transport may also be important, but very little evidence exists to indicate the possible scale and practical significance of any such transport (Krauskopf, 1964 Bames et al., 1967 Khodakovsky et al., 1975). 

The Drake Ag-Au deposits in northern New South Wales occur in Permo-Carboniferous volcanic rocks mineralisation is considered to result from hydrothermal activity associated with the acid volcanism (Bottomer, 1986). At one of these deposits. Lady Hampton, stratabound mineralisation occurs as disseminations in a volcaniclastic unit exhibiting sericite-clay-quartz alteration. Pyrite is the major sulphide present with minor sphalerite, galena, chalcopyrite, Ag sulphosalts, tennantite and native alloys. 

Igumnov, S.A., 1976. Experimental study of isotope exchange between sulphide and sulphate sulphur in hydrothermal solution (in Russian). Geokhimiya, 4 497—503. 

Heinrich, C. A., and Eadington, P. J., 1986a, Thermodynamic predictions ofthe hydrothermal chemistry of arsenic and their significance for the paragenetic sequence of some cassiterite-arsenopyrite-base metal sulphide deposits Economic Geology, v. 81, p. 511-529. 

The existence in the Archaean of sulphate-reducing bacteria, producing a spread in S isotopes around b S = 0 5%o has long been accepted (e.g. Goodwin et al. 1976 Shen et al. 2001). Sulphate would have come from ambient water, with a component supply of metals and other nutrients from hydrothermal sources, as shown by the REE (Table 1). In modem microbial mats, sulphate-reducing bacteria extract sulphur from sea water, fractionating it by an amount that depends on the efficiency and rate of extraction. Conversely, sulphide-oxidizers reverse the process. 

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