Tracing fluid–rock reaction and hydrothermal circulation at the Saldanha hydrothermal field

Abstract

The Saldanha hydrothermal field is positioned on the top of a seamount located in a non-transform offset (NTO5) on the Mid-Atlantic Ridge (MAR). This hydrothermal system was first described as a low-temperature diffuse field, driven by peridotite–seawater reactions following the detection of high concentrations of CH 4 and H 2 in the water column, and the occurrence of serpentinite outcrops in the vent area. We have studied the geochemistry and isotopic composition of sediment and rock samples collected across the area and show that hydrothermal circulation at Saldanha is complex and spatially variable, comprising areas of low-temperature diffuse flow but also more focused higher-temperature venting zones. While most sediment samples have an isotopic composition that is similar to normal pelagic sediments, one core (SCD7) show significant hydrothermal influence, sulphide mineralization, non-radiogenic Pb and radiogenic Nd isotope ratios and positive Eu anomalies. This is best explained by mineral precipitation from high-temperature hydrothermal fluids that have circulated through mafic rocks. The host rock lithology and alteration is also highly variable and comprises both fresh basalts, serpentinites and hydrothermally altered rocks (metabasalts, metagabbros and steatites). Serpentinites have REE patterns and ε Nd(0) values that fall between seawater and mantle peridotite reference values, resulting from extensive interaction of seawater with the original peridotite. This process was probably favoured by the deeply penetrating and long-lived faults occurring at this NTO. Steatites have a positive Eu anomaly and non-radiogenic Pb isotopic values. These signatures, together with the sulphide mineralisation and the extensive Si input necessary for steatization of serpentinites, imply that higher-temperature hydrothermal fluids reacted with gabbroic intrusions at depth. The more hydrothermally altered sediment and rock samples appear to be associated with the Saldanha fault network that promotes a more focused fluid flow and thus enhances hydrothermal alteration within a region of low-temperature diffuse flow.