Ancient hydrothermal mineralization

Vent fluids Vent sulphides Vent sedimcnE Basalt Anhydrite. Seawater 

Modem analogue—sulphide Modem analogue—sulphate 

a number of different types of hydrothermal deposit are briefly reviewed and classified according to their probable source of sulphur. 

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As pointed out by Seal et al. (2000), many studies of ancient hydrothermal systems have utilized equilibrium sulfate-sulfide sulfur isotope fractionation models, but these should be applied with great caution. As shown in Figure 9, seafloor hydrothermal vent fluid 5" Sh2S values do not conform to simple equilibrium fractionation models. Shanks et al. (1981) first showed experimentally that sulfate in seawater-basalt systems is quantitatively reduced at temperatures above 250°C when ferrous minerals like the fayalitic olivine are present. When magnetite is the only ferrous iron-bearing mineral in the system, sulfate-reduction proceeds to sulfate-sulfide equilibrium, but natural basalts contain ferrous iron-bearing olivine, pyroxene, titanomagnetite, and iron-monosulfide solid-solution (mss) (approximately pyrrhotite). It is the anhydrite precipitation step... 

The conservative behavior and chemical inertness of noble gases has led to their extensive use to provide information on the fluxes, movement and interactions of contemporary crustal fluids. A major advance has been in extending these studies to characterize ancient waters in fluid inclusions (Turner et al. 1993). The principal aims of these studies are to provide data on the noble gas geochemistry of inclusion fluids and understand the observations in terms of likely sources and interactions. This information provides novel insight into a palaeofluid s evolutionary history and can assist in discriminating between different models for the formation of hydrothermal mineral deposits. Because of the large differences in end-member isotopic and elemental compositions, He and Ar are the most informative and widely studied noble gases in... 

Stuart FM, Bumard PG, Taylor RP, Turner G (1995) Resolving mantle and crastal contribntions to ancient hydrothermal fluids He-Ar isotopes in fluid inclusions from Dae Hwa W-Mo mineralization, South Korea. Geochim Cosmochim Acta 59 4663-4673... 

Russell and Hall (this volume) adopt this premise and suggest that life first emerged where dissolved H2 from submarine hot springs redueed dissolved CO2 in the ancient ocean. They propose that iron-nickel sulfide minerals served as catalysts in the generation of essential biomolecules. Other important thermodynamic energy sources, Russell and Hall note, resulted from the strong pH and thermal gradients between the acidic ocean and alkaline hydrothermal fluids. 

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