Genesis and preservation of metalliferous sediments

Goulding, Helen Claire (1998) Genesis and preservation of metalliferous sediments. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Major, trace and rare earth element data are presented on a series of Fe-oxide sediments from the active TAG mound (26°N, Mid Atlantic Ridge) and the Cretaceous Semail Ophiolite of Oman. The data are used to investigate the genesis of the sediments and form the basis of a comparative study of sediment formation and post-depositional processes in the two tectonic settings.

A 25cm push core from the southern periphery of the TAG mound displays marked mineralogical and geochemical gradients, which are consistent with sharp redox contrasts in the subsurface. This study demonstrates that Mn concentrations of up to 16% occur in proximal sediments, which has not previously been observed at TAG. The sediment geochemistry provides evidence for a low temperature, oxidising fluid with a distinct REE pattern, which is responsible for direct precipitation of sediments.

Ocean Drilling Program material recovered from near the surface of the TAG mound shows textures and mineralogy typical of mound crusts and immature sulphide ledges. These samples show evidence for the sub-surface circulation and zone refining inferred from TAG drilling results. For the first time, this study documents REE patterns for individual siliceous Fe oxide grains rather than bulk sediments. These patterns show marked positive Eu anomalies and reflect derivation from a hydrothermal source. Evidence is seen for silicification of at least parts of the system by direct precipitation of silica from low temperature fluids. Weathering of sulphide material is intense west of the black smoker complex, but insignificant in the southern areas of the mound where low temperature flow dominates the mineralogy and geochemistry.

Whilst sediments from the Semail ophiolite are associated with TAG-like ore deposits and have settings and sedimentological textures reminiscent of TAG samples, the REE inventory for most of these is completely overprinted by seawater scavenging that occurs on the seafloor prior to obduction. However one sample from Yanbu and two from the Zuha gossan show REE patterns with a remnant hydrothermal signature.

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