Title: Saldanha hydrothermal field: the role of the sediment-cover in the preservation of ore-forming minerals
Authors: Ágata Sofia C. M. Alveirinho Dias and Fernando J. A. S. Barriga
E-mail: ascdias@fc.ul.pt or agata.dias@usj.edu.mo
Abstract: In spite of the importance of lower temperature and diffuse hydrothermal systems for ore forming processes, deep-sea exploration has been more focused in the study of higher temperature and more vigorous hydrothermal fields. This is probably because diffuse hydrothermal fields do not have a comparable biodiversity and are harder to be detected by conventional physical-chemical prospecting tools. Moreover, fluids emanating from focal and discrete vents are easier to sample and both the minor fluid-rock-sediment reactions at recharge zones and the less-pronounced dilution by the background seawater at these systems allow a more direct access to the end-member fluid chemistry.
Saldanha hydrothermal field is one example of a diffuse hydrothermal system where transparent and low-temperature fluids emanate from discrete orifices on the seafloor, triggered by ultramafic and mafic heat sources.
By the study of the mineralogy and geochemistry of hydrothermal deposits from Saldanha field, two distinct hydrothermal signatures were identified, one related with the diffuse hydrothermal fluid flow (Type I) and the other related with more discrete up-flow zones (Type II). Type I is “diluted” within the pelagic carbonate ooze cover that overprints almost completely the hydrothermal signature. Type II, collected directly at vent orifices, has an evident enrichment in metals (Cu, Zn, Fe and Co). These are incorporated in sulfide phases precipitated during earlier periods of hydrothermal fluid contact with interstitial seawater, followed by precipitation of Mn and of the remaining Fe as oxyhydroxide phases, as consequence of the more pronounced seawater-fluid interactions taking place closer to the surface. Type II deposits are also characterized by the presence of hydrothermal calcite in a matrix of serpentinite + talc ± chlorite. Effective separation of Cu, Zn, Fe and Mn occurs, with decreasing ratios of Cu/Zn, Cu/Fe, Zn/Fe and increasing ratios of Mn/Fe from the bottom to the upper layers, as consequence of the decrease in temperature and Eh and increase in pH conditions of the hydrothermal fluid during its ascent. As a result, ore-forming minerals precipitate before reaching the surface, justifying the transparency of the fluids that are released at the orifice vents. In contrast to hydrothermal systems where focused vents release metal-rich fluids directly to the water column thus promoting their oxidation, as it happens for example in the nearby Rainbow hydrothermal field, at Saldanha the presence of a sediment cover preserves metals as sulfides, promoting the formation of ore deposits.
