Fe–Si-oxyhydroxide deposits at a slow-spreading centre with thickened oceanic crust: The Lilliput hydrothermal field (9°33′S, Mid-Atlantic Ridge)

Diffuse and focused low-temperature fluids emanate at 9°33′S (Mid-Atlantic Ridge) and precipitate Fe–Si-oxyhydroxides that form chimneys, mounds and flat-lying deposits. This extensive vent field, named Lilliput, lies at the axial zone of a spreading segment with a significantly thickened crust (~ 11 km). Theoretically much more heat needs to be removed from a thick-crust spreading center compared to a spreading center with typical thickness of ~ 6 km. Therefore, settings with thickened crust should be favourable for supporting very powerful hydrothermal systems capable of producing large mineral deposits. This is the first report on the composition of seafloor hydrothermal deposits at abnormally thickened oceanic crust due to hotspot–ridge interaction. Our studies revealed that generally the Lilliput hydrothermal deposits are very similar in morphology, structure, composition and lateral extent to other low-temperature hydrothermal deposits of mid-ocean ridges and intraplate volcanoes. Deposits at the Lilliput vent field are composed of Si-containing goethite and ferrihydrite, have very low contents of a number of transition and rare earth elements and show REE distribution patterns with negative Ce and Eu anomalies. The speciation and precipitation of the main deposit-forming elements, Fe and Si, at the hydrothermal field appear to be partially controlled by live microbes and exuded organic compounds. The δ 18O values of the precipitated silica-containing Fe-oxyhydroxides point to low-temperature formation and Sr–Nd–Pb–isotope variations suggest that the hydrothermal precipitates scavenged metals predominantly from the ambient seawater. These findings are in agreement with the biogeochemical scenario for their precipitation. ►Thickened oceanic crust does not fuel vent systems different from their thin-crust counterparts. ► Lilliput deposits do not differ principally from the other oceanic low-T hydrothermal deposits. ► Fe speciation, precipitation and preservation at the Lilliput field is organically-controlled. ► Low-T, diffuse venting seems to be the most important cooling agent at the hotspot-affected MOR.