Mineralogical and geochemical hydrothermal evidences on sediments from the serpentinite-hosted Saldanha hydrothermal field

Abstract

Mineralogical and geochemical data of hydrothermal sediments collected from the Saldanha site are presented. The Saldanha hydrothermal field is located at the top of a serpentinized massif Mount Saldanha (MS), at a non-transform offset (NTOS) along the Mid-Atlantic Ridge (MAR), south of the Azores. It is one of the rare known sites where direct evidence of low-temperature hydrothermal activity has been provided by direct observtion of hydrothermal fluid venting through small orifices in the ocean floor sedimentary cover. This study reveals that these hydrothermal sediments are highly “diluted” within dominant foraminiferal nanofossiliferous ooze and that the mineral assemblage of the hydrothermal component is characterized by sulphides, poorly crystallized Mn oxihydroxides, Mn brucite and hydrothermal clays. Sediments with a stronger hydrothermal component are located at the top of the mount, where hydrothermal activity was observed. Here the sediments are enriched in Cu, Zn and Fe sulphides, Mn-Mg oxy-hydroxides and putative manganobrucite. Total geochemical analyses show that all sediments are enriched in elements such as Mn, Fe, Cu and Zn, derived from hydrothermal fluids, Mg, P and V, scavenged from seawater, and Ni, Cr and Co derived from ultramafic rocks. Sediments collected next to the vents are particularly enriched in Mn. The geochemical data together with the observed mineral assemblage and the geological setting suggest that the hydrothermal fluids have much higher temperature than those measured at the escape orifices (max. 9°C), and a strong enrichment in Mg, mainly at the top of the mount, agrees with extensive mixing of the hydrothermal fluid with unmodified seawater.

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